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Electronic Theses and Dissertations, 2004-2019

2011

Characterization Of Composite Broad Band Absorbing Conjugated Characterization Of Composite Broad Band Absorbing Conjugated

Polymer Nanoparticles Using Steady-state, Time-resolve And Polymer Nanoparticles Using Steady-state, Time-resolve And

Single Particle Spectroscopy Single Particle Spectroscopy

Maxwell Scotland Bonner University of Central Florida

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STARS Citation STARS Citation Bonner, Maxwell Scotland, "Characterization Of Composite Broad Band Absorbing Conjugated Polymer Nanoparticles Using Steady-state, Time-resolve And Single Particle Spectroscopy" (2011). Electronic Theses and Dissertations, 2004-2019. 1828. https://stars.library.ucf.edu/etd/1828

CHARACTERIZATION OF COMPOSITE BROAD BAND ABSORBING

CONJUGATED POLYMER NANOPARTICLES USING STEADY-STATE,

TIME-RESOLVE AND SINGLE PARTICLE SPECTROSCOPY

by

MAXWELL SCOTLAND BONNER

B.S. University of Southern Mississippi, 2005

A dissertation submitted in partial fulfillment of the requirements

for the degree of Doctor of Philosophy

in the Department of Chemistry

in the College of Sciences

at the University of Central Florida

Orlando, Florida

Fall Term

2011

Major Professor: Andre J. Gesquiere

ii

©2011 Maxwell S. Bonner

iii

Dedicated to Brandi, James, Doc, Lulu and Chris

for their love and support,

and never leading me astray.

iv

ABSTRACT

As the global economy searches for reliable, inexpensive and environmentally friendly

renewable energy resources, energy conservation by means of photovoltaics has seen near

exponential growth in the last decade. Compared to state-of-the-art inorganic solar cells, organic

photovoltaics (OPVs) composed of conjugated polymers are particularly interesting because of

their processability, flexibility and the potential for large area devices at a reduced fabrication

cost. It has been extensively documented that the interchain and intrachain interactions of

conjugated polymers complicate the fundamental understanding of the optical and electronic

properties in the solid-state (i.e. thin film active layer). These interactions are highly dependent

on the nanoscale morphology of the solid-state material, leading to a heterogeneous morphology

where individual conjugated polymer molecules obtain a variety of different optoelectronic

properties. Therefore, it is of the utmost importance to fundamentally study conjugated polymer

systems at the single molecule or nanoparticle level instead of the complex macroscopic bulk

level.

This dissertation research aims to develop simplified nanoparticle models that are

representation of the nanodomains found in the solid-state material, while fundamentally

addressing light harvesting, energy transfer and interfacial charge transfer mechanisms and their

relationship to the electronic structure, material composition and morphology of the nanoparticle

system. In preceding work, monofunctional doped nanoparticles (polymer-polymer) were

fabricated with enhanced light harvesting and Fӧrster energy transfer properties by blending

Poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV)

and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at various MEH-

PPV doping ratios. While single particle spectroscopy (SPS) reveals a broad distribution of

v

optoelectronic and photophysical properties, time-correlated single photon counting (TC-SPC)

spectroscopy displays multiple fluorescence lifetime components for each nanoparticle

composition, resulting from changing polymer chain morphologies and polymer-polymer

aggregation. In addition, difunctional doped nanoparticles were fabricated by doping the

monofunctional doped nanoparticles with PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester)

to investigate competition between intermolecular energy transfer and interfacial charge transfer.

Specifically, the difunctional SPS data illustrated enhanced and reduced energy transfer

mechanisms that are dependent on the material composition of MEH-PPV and PC60BM. These

data are indicative of changes in inter- and intrachain interactions of BPPV and MEH-PPV and

their respective nanoscale morphologies. Together, these fundamental studies provide a thorough

understanding of monofunctional and difunctional doped nanoparticle photophysics, necessary

for understanding the morphological, optoelectronic and photophysical processes that can limit

the efficiency of OPVs and provide insight for strategies aimed at improving device efficiencies.

vi

ACKNOWLEDGMENTS

It is a pleasure to thank my advisor Dr. Andre J. Gesquiere who made this thesis and research

endeavors possible for me through research assistant funding at the Nanoscience Technology

Center in the Laboratory of Nanoscale Imaging and Spectroscopy (LNIS) at University of

Central Florida. I learned so much not only about science, research and academia but also life in

general.

I would like to thank my committee members Drs. Andres D. Campiglia, Florencio Eloy

Hernández, J. Manuel Perez, Swadeshmukul Santra, Christina Fernandez-Valle and Jingdong Ye

for their support and invaluable time and, for their enlightening discussions and suggestions

during my doctorial studies and research pursuits. I would like to show my gratitude to Dr. Diego

J. Diaz and Jordan Anderson for assistance in cyclic voltammetry experiments. I owe my deepest

gratefulness to Drs. Mircea Cotlet and Zhihua Xu at Brookhaven National Laboratory for their

assistance with the time-correlated single photon counting experiments and length discussions

concerning the data analysis.

I would specially like to thank the past and present research colleagues of LNIS for helping

make the arduous experiments bearable, while providing laugher and a place to talk during the

mundane days at the research center. May your families be blessed and the future brings you

success and happiness.

I am deeply indebted to my family and friends for their undo voted love and support.

Throughout my research studies they constantly reminded me that life did not completely

revolve around research, however, “I had a job… research… and the job must be done….” To

vii

the youngest part of the Bonner family, James, thank you for always putting a smile on my face

regardless of the time of day. Finally, I owe my deepest gratitude to my wife, Brandi, for her

love, support, understanding, appreciation and encouragement, for being my better part and for

being with me during the thick and the thin.

Cheers!

Max

If we knew what we were doing, it wouldn't be called research, would it?

-- Albert Einstein

viii

TABLE OF CONTENTS

LIST OF FIGURES ...................................................................................................................................... x

LIST OF TABLES .................................................................................................................................... xvii

SUMMARY .................................................................................................................................................. 1

CHAPTER 1: SIGNIFICANCE, MOTIVATION AND SPECIFIC AIMS OF DISSERTATION

RESEARCH .................................................................................................................................................. 3

1.1 Significance of organic photovoltaic devices ..................................................................................... 3

1.2 Motivation of dissertation research .................................................................................................... 4

1.3 Specific aims of dissertation research ................................................................................................ 6

CHAPTER 2: BACKGROUND OF DISSERTATION RESEARCH .......................................................... 9

2.1 Principles of bulk heterojunction solar cells ....................................................................................... 9

2.2 Single molecule effect: removing the ensemble-averaging effect of bulk measurement by single

molecule spectroscopy ............................................................................................................................ 11

2.3 Issues of broadband absorption and morphology in bulk heterojunction solar cells ........................ 15

CHAPTER 3: EXPERIMENTAL METHODS .......................................................................................... 20

3.1 Nanoparticle Preparation .................................................................................................................. 20

3.2 Nanoparticle Characterization .......................................................................................................... 23

3.2.1 Material characterization of pure (undoped) and composite (doped) nanoparticles .................... 23

3.2.2 Uv-vis and Fluorescence Spectroscopy ....................................................................................... 25

3.2.3 Single Particle Spectroscopy (SPS) ............................................................................................. 26

3.2.4 Time-Correlated Single Photon Counting (TC-SPC) .................................................................. 28

CHAPTER 4: CHARACTERIZATION OF BROADBAND ABSORBING CONJUGATED POLYMER

NANOPARTICLES BY SINGLE PARTICLE SPECTROSCOPY (SPS) ................................................ 34

4.1 Introduction ...................................................................................................................................... 34

4.2 Results and Discussion ..................................................................................................................... 38

4.2.1 Steady-state spectroscopy of pure (undoped) molecular solutions and nanoparticle suspensions

.............................................................................................................................................................. 38

4.2.2 Steady-state spectroscopy of composite (doped) molecular solutions and nanoparticle

suspensions ........................................................................................................................................... 41

4.2.3 Single Particle Spectroscopy of pure (undoped) and composite (doped) nanoparticles .............. 52

4.3 Conclusion ........................................................................................................................................ 61

CHAPTER 5: EXCITED-STATE DYNAMICS OF BROADBAND ABSORBING ENERGY

TRANSFER CONJUGATED POLYMER NANOPARTICLES ............................................................... 62

5.1 Introduction ...................................................................................................................................... 62

ix

5.2 Results and Discussion ..................................................................................................................... 62

5.2.1 Steady-state spectroscopy for pure (undoped) and molecular solutions and nanoparticle

suspensions ........................................................................................................................................... 67

5.2.2 Time-resolved spectroscopy of pure (undoped) molecular solutions .......................................... 71

5.2.3 Time-resolved spectroscopy of pure (undoped) nanoparticle suspensions .................................. 71

5.2.4 Steady-state spectroscopy of composite (doped) molecular solutions and nanoparticle

suspensions ........................................................................................................................................... 79

5.2.5 Time-resolved spectroscopy of composite (doped) nanoparticle suspensions. ........................... 86

5.3 Conclusion ........................................................................................................................................ 98

CHAPTER 6: INFLUENCES OF FULLERENE DOPING IN BROADBAND ABSORBING ENERGY

TRANSFER CONJUGATED POLYMER NANOPARTICLES ............................................................. 100

6.1 Introduction .................................................................................................................................... 100

6.2 Results and Discussion ................................................................................................................... 103

6.2.1 Electrochemical Properties ........................................................................................................ 103

6.2.2 Bulk Spectroscopy ..................................................................................................................... 105

6.2.3 Single Particle Spectroscopy ..................................................................................................... 117

6.3 Conclusion ...................................................................................................................................... 132

REFERENCES ......................................................................................................................................... 134

x

LIST OF FIGURES

Figure 1: Illustration depicting a conjugated polymer nanoparticle model system for simplifying the bulk

film, where single nanoparticles are representative of the nanodomains found in the active layer

of a bulk heterojunction OPV. ....................................................................................................... 6

Figure 2: Schematic illustrating the basic four-step principle of OPVs: 1) charge generation, 2) exciton

migration, 3) charge dissociation and 4) charge transport and collection. Reprinted from

Account Chemical Research 2009, 42, 11, 1740-1479. ................................................................ 9

Figure 3: Schematic illustrating a blended donor-acceptor active layer typically found in bulk

heterojunction OPVs. The enlarged depictions of the active layer illustrates conjugated polymer

and fullerene domains and grain boundaries, where the bicontinuous percolated network provide

enhanced interfacial areas for exciton dissociation and charge transport. .................................. 11

Figure 4 a) Normalized absorption and fluorescence spectra of MEH-PPV solutions and thin films in

different solvents: chlorobenzene (blue), chloroform (green) and toluene (blue). Reprinted from

J. Phys. Chem. Lett. 2011, 2, 1520-1525. b) Ensemble distribution of single MEH-PPV

molecules fluorescence emission maxima displaying a bimodal distribution, indicating that

MEH-PPV molecules exhibit two different types of polymer confirmations. This figure was

adopted from Science 2000, 289, 5483, 1327-1330. c) Single molecules fluorescence transients

of MEH-PPV. Left) Fluorescence intermittency (blinking) of a collapsed chain confirmation

that results from intramolecular energy transfer to a quenching site. Right) Extended chain

confirmation that displays continuous photobleaching that results from limited interactions with

between chromophore segments. Reprinted from J. Phys. Chem. B 2008, 112, 12575. ............. 14

Figure 5: Depicts the standard AM 1.5 solar spectrum (red) and several organic thin film absorption

spectrum (black). The absorption spectra are expressed in term of absorption coefficient. This

figure was adopted from Chemical Review 2007, 107, 1324-1338. ........................................... 16

Figure 6: Illustrates the competition between energy transfer and charge transfer in P3HT:PCBM bulk

heterojunction OPVs that are doped with small molecules and low band gap polymers. a) bulk

heterojunction illustration that depicts percolated pathways of P3HT and PCBM where dye

molecules are located at the donor/acceptor interface and b-c) energy level align between the

ternary components that depicts energetically favorable processes. These figures are reprinted

from Journal of Physical Chemistry C 2011, 115, 11306-11317, Acs Applied Materials &

Interfaces 2009, 1, 804-810 and Advanced Functional Materials 2010, 20, 338-346. ............... 19

Figure 7: Chemical structures of BPPV, MEH-PPV, PTB7 and PCBM .................................................... 20

Figure 8: Schematic illustrating the modified reprecipitation method that involves rapid injection of an

organic material in a “good” solvent into a miscible solvent that acts as a “poor” solvent system

for the organic materials. The bottom left and right images illustrate the emission color of

molecular solutions and nanoparticle suspension from an external ultraviolent light source. .... 22

Figure 9: Material characterization of undoped and doped nanoparticle suspensions by SEM and TEM

(insert) depicting quasi-spherical shapes with diameters of ~ 37 nm. Scale bar represents 100

nm. ............................................................................................................................................... 25

Figure 10: Schematic illustrating the optical setup for the home-built single particle spectroscopy imaging

system described in Section 3.2.3. .............................................................................................. 27

Figure 11: Schematics illustrating the method of constructing TCSPC histograms. Top) depicts the start-

stop timing events of single photons. Middle) illustrates the output from a constant fraction

discriminator. Bottom) a histogram illustration of the photon distribution per unit time for

photoluminescent lifetime decay profile. Reprinted from Principles of Fluorescence, 3rd

ed. .... 30

Figure 12: Assessment of the fitting quality for a nanoparticle suspension with single and multi-

exponential functions. a-b) single and double exponential fitting functions display poor fitting

quality with χ2 value larger than 1.2 and autocorrelation functions with large random

distributions around zero. c) triple exponential fitting function showing an autocorrelation

function of a good fit with a χ2 value < 1.2. ............................................................................... 32

xi

Figure 13: Chemical structures of Poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-

phenylenevinylene)] (BPPV) and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV) a-c)Material characterization of undoped and doped BPPV/MEH-PPV

nanoparticles. a) SEM and TEM (insert) image of 5wt% nanoparticle suspension depicting

quasi-spherical shapes. SEM, TEM and DLS (data not shown) characterization was carried out

on all undoped and doped nanoparticles. Analysis in excess of 100 nanoparticles per

composition yields an average SEM, TEM and DLS diameter of 38.2 ± 6.6, 36.7 ± 8.9 and 37.1

± 1.2nm, respectively. Scale bar represents 100nm. b-c) undoped and doped molecular solutions

and nanoparticle suspensions excited with an external 365nm lamp, respectively. A slight

variation of emission colors occurs in the molecular solution with increased doping, while the

nanoparticle suspensions yield emission colors from blue to red with respect to increasing

doping levels. .............................................................................................................................. 37

Figure 14: Steady-state spectroscopy of undoped BPPV (blue) and MEH-PPV (orange) depicting spectral

overlap between the emission of BPPV and absorption of MEH-PPV a) Normalized absorption

spectra (dash line) and emission spectra (solid line) of BPPV and MEH-PPV molecular

solutions dissolved in THF. The absorption and emission maxima of BPPV are located at 353

and 466 nm, respectively. MEH-PPV depicts absorption and emission maxima at 500 and 552

nm, respectively. b) Normalized absorption spectra (dash line) and emission spectra (solid line)

of undoped BPPV and MEH-PPV nanoparticle suspensions in deionized water. The vibronic

structure of both nanoparticle suspensions are nearly unchanged while obtaining absorption

maxima similar to the molecular solutions in THF. The BPPV and MEH-PPV emission maxima

are further red shifted to 504 and 593 nm, respectively. In addition, the blue vibronic shoulder

of BPPV nanoparticles is significantly reduced, while the MEH-PPV nanoparticles exhibit

increased aggregate emission shoulder at 630nm with respected to the molecular solution. ...... 40

Figure 15: Steady-state bulk spectroscopy of undoped and doped molecular solutions and nanoparticle

suspensions. a-b) Uv-vis spectra of molecular solutions dissolved in THF and nanoparticles

suspended in deionized water, respectively. The molecular solutions absorption maxima of

BPPV are located at 353nm, while MEH-PPV absorption maxima reside at 500nm. The

absorption maximum of BPPV in the nanoparticle suspension resembles the molecular

solutions, while the nanoparticle absorption maxima of MEH-PPV are red shifted to 503nm

with respect to increased doping levels. c-d) Fluorescent spectra of molecular solutions

dissolved in THF and nanoparticles suspended in deionized water, respectively. The BPPV

emission maximum in the molecular solutions resides at 466nm, and decreases in fluorescent

intensity with respect to a reduced doping level. Compared to molecular solutions, the

nanoparticle suspensions display red shifted emission maxima. The emission maxima of

undoped BPPV and MEH-PPV nanoparticles are located 504nm and 593nm, respectively.

BPPV emission intensity is quenched and blue shifts to 498nm with respect to increased doping

levels of MEH-PPV, while the emission intensity of MEH-PPV monotonically increases and

blue shifts to 585nm. In addition, the MEH-PPV emission maximum of 5wt% is located at

576nm and deviates for the monotonic increase in emission intensity. ...................................... 42

Figure 16: Illustration of the evolution of polymer chain interactions and morphologies within the

nanoparticle as the doping level of MEH-PPV varies from 0 to 100 wt%. The blue and red

segments are representative of BPPV and MEH-PPV, respectively. .......................................... 44

Figure 17: Nanoparticle quenching model fitting results for bulk spectroscopy (red) and SPS (black)

which are in good agreement. Both data sets depict a quenching efficiency (q) of >90%, while

the number of donor (N) is in excess of 2000 molecules. ........................................................... 49

Figure 18: Fluorescent spectra illustrating direct excitation of MEH-PPV in undoped and doped molecular

solutions and nanoparticle suspensions. Both systems depict a decrease in emission intensity

with respect to reduced doping level of MEH-PPV, which is directly correlated to the decrease

in MEH-PPV absorption presented in Figures 3a-b. a) the doped molecular solutions depict a

constant 552nm emission maxima that resemble the undoped molecular solution b) undoped

xii

MEH-PPV nanoparticle emission maximum is located at 595nm, while the doped emission

maxima blue shift to 586nm with respect to decreasing doping levels. These emission maxima

are in good agreement with those presented in Figure 3d. .......................................................... 50

Figure 19: Single particle ensemble spectra with deconvoluted donor and acceptor spectrum and peak

wavelength histograms of undoped and doped nanoparticles. a-f) Ensembles (black) were

deconvoluted by fitting the onset of undoped (0wt%) nanoparticle and performing a least square

analysis, yielding the corresponding donor (blue) and acceptor (orange) spectrum. These data

were used to determine the donor and acceptor emission maxima since the spectral overlap of

BPPV and MEH-PPV can in effect alter the spectral progression of the unmixed spectrum.

Undoped nanoparticle emission maximum is located at 518nm, while the donor emission in

composite nanoparticles blue shifts to 509nm. In turn, the deconvoluted MEH-PPV emission

maximum varies slight (596nm ± 2nm) compared to the unmixed composite ensembles. g-l)

Peak wavelength histogram of individual undoped and doped nanoparticles. 0wt% nanoparticles

depict a single modal distribution at 518nm, while the bimodal distribution in composite

nanoparticles less than 15wt% are centered around 509nm and no donor emission is observed

thereafter. In addition, the distribution of MEH-PPV peak wavelength narrows when increasing

the doping level of MEH-PPV. At doping levels less than 15wt% the MEH-PPV peak

wavelength is centered at 594nm, while majority of the peak wavelengths in 50wt% are red

shifted to 603nm. ......................................................................................................................... 55

Figure 20Figure 7: Corrected FRET efficiencies for individual nanoparticle of a given composition. 5wt%

(a) and 10wt% (b) display a large distribution of energy transfer efficiency, while majority of

the efficiencies of 15wt%, 25wt% and 50wt% (c-d) resides between 80 to 100%. .................... 57

Figure 21: Correct rates per unit time of energy transfer for individual nanoparticles of a given

composition. The rates are displayed on a logarithmic scale for convince. As illustrated, the rate

of energy transfer decreases when the MEH-PPV doping level is reduced. a-b) kcFRET_SPS ≈ 108

to 109 s

-1 and c-e) kcFRET_SPS ≈ 10

9 to 10

10 s

-1. ............................................................................... 58

Figure 22: Exemplifies the complex heterogeneity within the composite nanoparticle systems. Spectrum

from averaged single particle spectra corresponds to the ensemble(s) in Figure 6, and

normalized at 590nm for comparison. a) 50wt% acceptor maxima remain constant with small

intensity differences in the donor maxima. b) The subtle variation in 25wt% acceptor maxima

occurs with different donor intensities, while providing nanoparticle spectrum that resemble that

of 50 wt%. c) 15wt% nanoparticle obtains blue shifted spectra and spectra that are similar to

50wt% and 25wt%. However, nanoparticles with equal or higher donor-to-acceptor intensities

are revealed. d-e) 10wt% and 5wt% nanoparticle obtain spectra that resemble Figures 8a-c.

However, the presences of undoped BPPV nanoparticles were observed, and are depicted by the

elevated normalized intensity (~2). Furthermore these averaged spectra explicitly illustrate the

heterogeneous arrangement of BPPV and MEH-PPV molecules residing within a given

nanoparticle and/or differences in BPPV/MEH-PPV composition per nanoparticle. ................. 60

Figure 23: Undoped absorption and fluorescence spectra and fluorescence decay profiles. a) Normalized

absorption (dash line) and fluorescence (solid line) spectra for BPPV (blue) and MEH-PPV

(orange) dissolved in THF. BPPV and MEH-PPV obtains peak wavelength absorption

maximum at 353 and 500 nm, respectively. BPPV emission maximum is located at 466 nm with

a blue shoulder emission at ~425 nm. The emission maximum for MEH-PPV is located at 552

nm with a red shoulder emission at ~610 nm. b) Normalized absorption (dash line) and

fluorescence (solid line) spectra for BPPV (red) and MEH-PPV (green) nanoparticles suspended

in deionized water. Compared to the molecular solutions, BPPV and MEH-PPV absorption

maximum remain constant, while the emission maximum are red shifted to 504 and 596 nm for

BPPV and MEH-PPV, respectively. MEH-PPV displays an enhanced emission from the

aggregate emitter (630 nm), while BPPV blue shoulder emission is significantly reduced

compared to the molecular solutions. c). BPPV exhibits a bi-exponential decay behavior with an

averaged lifetime of 1.5 ns, while MEH-PPV in THF display a single excited specie with a

xiii

lifetime of 0.25 ns. d) Fluorescent decay profiles of BPPV at 504 nm (red) and MEH-PPV at

590 nm (green) and 630 nm (cyan) nanoparticles suspensions. BPPV nanoparticle obtain

averaged lifetimes of 100 ps. MEH-PPV display emission dependencies with averaged lifetimes

of 93 and 136 ps for 590 and 630 nm, respectively. Individual lifetime components and

contributions are summarized in Tables 1-2. .............................................................................. 70

Figure 24: Bulk spectroscopy of undoped and doped molecular solutions and nanoparticle suspensions as

a function of MEH-PPV doping level. a) Absorption spectra of molecular solutions in THF,

resulting in a constant absorption maxima for BPPV (353 nm) and MEH-PPV (500 nm). b)

Absorption spectra of nanoparticle suspensions displaying at constant BPPPV absorption

maximum at 353 nm, while the maxima of MEH-PPV red shifts of 525 nm with respect to a

decreased doping level. c) Fluorescence spectra of molecular solutions in THF, depicting a

constant BPPV emission maximum at 466 nm with an appearance of MEH-PPV (552 nm)

emission at high doping levels. Solutions were excited at 350 nm with constant excitation and

emission slit widths of 0.5 and 0.5 nm, respectively. d) Fluorescence spectra of nanoparticle

suspensions excited at 350 nm with constant slit widths of 2 and 2 nm, respectively. The

emission intensity of BPPV is shown to decrease and blue shift as the doping of MEH-PPV

increases, while the emission intensity of MEH-PPV monotonically decreases and red shifts to

600 nm. Exceptions to the observed MEH-PPV emission trend occur at 5, 30 and 40 wt%. In the

case of 5 wt%, MEH-PPV emission maximum is located at 587 nm while the emission intensity

is reduced. For 30 and 40 wt%, enhanced emission intensity is observed with respect to the

other nanoparticle compositions. ................................................................................................. 82

Figure 25: Changes of MEH-PPV and BPPV emission intensity for undoped and doped nanoparticle

suspension as a function of the doping weight percent of MEH-PPV. s The quenching with

elevated dopant levels, while the . With elevated dopant levels of MEH-PPV, the BPPV (blue

line) emission intensity is shown to quench hyperefficiently. In the case of MEH-PPV (black

line), the emission intensity gradually decreases and deviates from the monotonic trend at a

dopant level of 5, 30 and 40 wt%. ............................................................................................... 86

Figure 26: Photoluminescent decay profiles and exponential fits of undoped and doped nanoparticle

suspension as a function of the doping percent of MEH-PPV. Nanoparticle suspensions were

excited with a 375nm (a-c) and 490nm (d-e) pulse excitation source, while the PL decays were

collected were collected at the emission maximum of BPPV (504±5nm; 3a) and MEH-PPV

(590±5nm; 3b, 3d), and the red shoulder emission of MEH-PPV (630±5nm; 3c, 3e). ............... 89

Figure 27: Averaged lifetime of undoped and doped nanoparticle suspensions excited at 375nm (solid

line) and 490nm (dash line). The decay profiles were detected at 504 (black circle), 590 (orange

circle) and 630nm (blue circles). These averaged lifetime display excitation and emission

dependencies with lifetimes that increase at 30 and 40wt% as the doping level of MEH-PPV is

increased. The average lifetimes at 504nm also display a general decrease with decreasing

doping levels. .............................................................................................................................. 91

Figure 28: Extrapolated intensity projection from the raw decay profiles as a function of doping weight

percent of MEH-PPV. Nanoparticle suspensions were excited with a 375nm (a-c) and 490nm

(d-e) pulse excitation source, while the PL decays were collected were collected at the emission

maximum of BPPV (504±5nm; 4a) and MEH-PPV (590±5nm; 4b, 4d), and the red shoulder

emission of MEH-PPV (630±5nm; 4c, 4e). ................................................................................ 94

Figure 29: Individual lifetime components and amplitude weighted contribution as a function of MEH-

PPV doping levels. These data are excited with a 375 nm pulse excitation sources, while the PL

decays were collected at 504 (black), 590 (orange) and 630 nm (blue). Left) the individual life

components illustrate a gradual reduction in lifetime with increasing doping levels of MEH-

PPV, while the lifetimes of 30 and 40wt% are increased. Inserted graph depicts the fast τ3

lifetimes. Right) amplitude weighted contributions of the respective individual life components,

which display decreased A1and A2 and increased A3 contributions with respect to increased

xiv

dopant levels of MEH-PPV. Additionally, the relative weighted percentages τ1 and τ2 amplitudes

are the opposite of τ3 amplitudes. ................................................................................................ 96

Figure 30: Individual lifetime components and amplitude weighted contribution as a function of MEH-

PPV doping levels. These data are excited with a 490 nm pulse excitation sources, while the PL

decays were collected at 590 (orange) and 630 nm (blue). Left) the individual life components

illustrate a gradual reduction in lifetime with increasing doping levels of MEH-PPV, while the

lifetimes of 30 and 40wt% are increased. Inserted graph depicts the fast τ3 lifetimes. Right)

amplitude weighted contributions of the respective individual life components, which display

decreased A1and A2 and increased A3 contributions with respect to increased dopant levels of

MEH-PPV. Additionally, the relative weighted percentages τ1 and τ2 amplitudes are the

opposite of τ3 amplitudes. ............................................................................................................ 97

Figure 31: Energy level diagram displaying the HOMO and LUMO energies of the semiconducting

polymers (BPPV and MEH-PPV) and fullerene derivative (PCBM) used in the monofunctional

and difunctional doped nanoparticles. Here, kET and kCT are the rates of energy and charge

transfer, respectively. ................................................................................................................ 105

Figure 32: The absorbance spectra of molecular solutions dissolved in THF (dash dot line) and aqueous

nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue) and 50 (red) total

weight percent (wt%) of PCBM. The presence of PCBM is confirmed by the absorbance peak

located at 330 nm, which increases in absorbance with higher doping levels of PCBM. a-c) The

absorption spectra of the molecular solutions exhibit a constant BPPV and MEH-PPV

maximum at 353 and 500 nm, respectively. d-f) The absorption spectra of the aqueous

nanoparticle suspensions exhibit a BPPV absorption maximum that resemble the molecular

solution, while MEH-PPV absorption maximum red shifts to 509 and 512 nm for 15 and 50

wt% nanoparticle suspensions, respectively.............................................................................. 106

Figure 33: Fluorescence spectra of BPPV molecular solutions dissolved in THF (dash dot line) and

aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue) and 50

(red) wt% of PCBM. a) Molecular solutions of BPPV depicting a constant emission maximum

located at 466 nm, displaying slightly quenched fluorescence intensity with respect to higher

PCBM doping levels. b) Aqueous nanoparticle suspensions depicting an undoped BPPV

emission maximum at 503 nm. As the doping level of PCBM increased from 5 to 50 wt%, the

fluorescence intensity becomes increasingly quenched and the emission maximum blue shifts to

452 nm. The inserted graph in Figure 2b depicts the normalized fluorescence intensity of the

nanoparticle suspensions. A new vibronic structure is observed for 25 and 50 wt% at

approximately 680 nm. This new feature may arise from PCBM emission when directly exciting

the fullerene molecules with an ultraviolent excitation source (375 nm). ................................. 114

Figure 34: Fluorescence spectra of 50 wt% molecular solutions dissolved in THF (dash dot line) and

aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue) and 50

(red) total wt% of PCBM. a) Molecular solutions in THF excited at 375 nm, displaying a BPPV

emission maximum at 466 nm with a 552 nm shoulder that results from direct excitation of

MEH-PPV. b) Molecular solutions in THF excited at 488 nm, which confirms the presence of

MEH-PPV at an emission maximum at 552 nm. c) Aqueous nanoparticle suspension excited at

375 nm, displaying quenched emission intensity and blue shifting emission maxima for both

BPPV and MEH-PPV with higher PCBM doping levels. The BPPV emission peaks at 480 nm

and shifts to 453 nm for 50 wt% PCBM, while the undoped MEH-PPV emission maximum is

located at 593 nm and shifts to 586 nm for 50 wt% PCBM. The 680 nm vibronic structures

observed in the 25 and 50 wt% PCBM nanoparticles suggest that vibronic feature results from

PCBM emission. The inset in Figure 3c illustrates the fluorescence spectra normalized to the

emission maximum of MEH-PPV, which display variations in the peak intensity ratios of BPPV

to MEH-PPV. d) Aqueous nanoparticle suspensions excited at 488 nm, depicting quenched

emission intensity and blue shifting emission maximum for MEH-PPV with respect to higher

doping levels of PCBM. The undoped MEH-PPV emission maximum is located at 593 nm and

xv

shifts to 588 nm for 50 wt% PCBM. The insert in Figure 3d illustrates the normalized

fluorescence spectra for the MEH-PPV emission, which display variations in the 0-0 to 0-1

vibronic transitions. ................................................................................................................... 115

Figure 35: Fluorescence spectra of 15 wt% molecular solutions dissolved in THF (dash dot line) and

aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue) and 50

(red) total wt% of PCBM. a) Molecular solutions in THF excited at 375 nm, displaying a BPPV

emission maximum at 466 nm. b) Molecular solutions in THF excited at 488 nm, which

confirms the presence of MEH-PPV at an emission maximum at 552 nm. c) Aqueous

nanoparticle suspension excited at 375 nm, displaying quenched emission intensity and blue

shifting emission maxima for both BPPV and MEH-PPV with higher PCBM doping levels. The

BPPV emission peaks at 498 nm and shifts to 453 nm for 50 wt% PCBM, while the undoped

MEH-PPV emission maximum is located at 586 nm and shifts to 579 nm for 50 wt% PCBM.

The 680 nm vibronic structures observed in the 25 and 50 wt% PCBM nanoparticles suggest

that vibronic feature results from PCBM emission. The inset in Figure 3c illustrates the

fluorescence spectra normalized to the emission maximum of MEH-PPV, which display

variations in the peak intensity ratios of BPPV to MEH-PPV. d) Aqueous nanoparticle

suspensions excited at 488 nm, depicting quenched emission intensity and blue shifting

emission maximum for MEH-PPV relative to higher PCBM doping levels. The undoped MEH-

PPV emission maximum is located at 587 nm and shifts to 581 nm for 50 wt% PCBM. The

insert in Figure 3d illustrates the normalized fluorescence spectra for the MEH-PPV emission,

which display variations in the 0-0 to 0-1 vibronic transitions. ................................................ 116

Figure 36: Line scan images (10 x 10 μm) of individual undoped BPPV and monofunctional doped BPPV

nanoparticles that are dispersed in a non-fluorescence polyvinyl alcohol film. a) Single particle

image of undoped BPPV nanoparticles using a laser power of 10 nW. b) Single particle image

of monofunctional doped BPPV nanoparticles (0 wt% - 5 wtT% PCBM using a laser power ~

90 nW to achieve similar fluorescence count with respect to the undoped BPPV image. ........ 118

Figure 37: Single particle ensemble spectra and peak wavelength distribution of BPPV nanoparticles

doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. a-d) Single particle

ensemble spectra that display an undoped BPPV emission maximum at 525 nm, which blue

shift to 480 nm with respect to higher PCBM doping levels. New vibronic features are observed

at 640 and 730 nm for the PCBM doped nanoparticles, suggesting that fluorescence emission

may result from excited state complexes and singlet emission from PCBM. e-h) Peak

wavelength histograms illustrating blue shifted emission maxima with broadening distributions

as the doping level of PCBM rises. In the case of 50 wtT% PCBM nanoparticles, the peak

wavelength frequency at 740 nm is higher than the peak wavelength frequency of BPPV,

indicating that the primary emitting specie of these composite nanoparticles results from

fullerene molecules. .................................................................................................................. 122

Figure 38: Schematic illustrating the changes in BPPV (blue) and MEH-PPV (red) inter- and intrachain

interactions, resulting from increasing PCBM (black) doping levels. Scheme 1, 2, and 3 are

representative of 0, 15 and 50 wt% nanoparticles, respectively. ............................................... 123

Figure 39: Single particle ensemble spectra and peak wavelength distributions of 15 wt% nanoparticles

doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. a-d) Normalized

fluorescence ensemble spectra depicting an undoped emission maximum located at 596 nm.

With increasing PCBM doping levels, the MEH-PPV emission maximum blue shifts to 580 nm,

while the peak wavelength intensity of BPPV decreases relative to MEH-PPV. Additional

vibronic structure is observed at 730 nm at high PCBM doping levels. e-h) Peak wavelength

histograms illustrating the emission maximum blue shift with increasing PCBM doping levels.

................................................................................................................................................... 128

Figure 40:Figure 7: Single particle ensemble spectra and peak wavelength distribution of 50 wt%

nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. Left

column) Normalized fluorescence ensemble spectra depicting an undoped emission maximum

xvi

located at 600 nm, while the MEH-PPV emission maximum blue shifts to 589 nm with

enhanced vibronic structure at 730 nm as the doping level of PCBM increases. Right column)

Peak wavelength histograms illustrating that the MEH-PPV emission maxima distribution

exhibits blue shifting with higher PCBM doping levels. In the case of 50 % PCBM

nanoparticles, peak intensity ratio of BPPV to MEH-PPV increases and the peak wavelength

distribution becomes trimodal. .................................................................................................. 129

Figure 41: Single particle FRET efficiencies (EFRET_SPS) and rates (kFRET_SPS) of 15 wt%

nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. a-d) The

EFRET_SPS distribution maximum for the undoped 15 wt% nanoparticles is approximately 50

%, while the efficiencies increase with higher PCBM doping levels. e-h) The kFRET_SPS of 0

to 50 % PCBM are shown to reside between 109 to 1011 s-1. Similar to the EFRET_SPS, the

energy transfer rates display a slight increase above 1010 s-1 with respect to higher PCBM

doping levels, indicating that the energy transfer process is enhanced at excess PCBM levels.

................................................................................................................................................... 130

Figure 42: Single particle FRET efficiencies (EFRET_SPS) and rates (kFRET_SPS) 50 wt%

nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) total wt% of PCBM. a-d)

The EFRET_SPS of undoped 50 wt% nanoparticles is approximately 95 %. The efficiencies of

nanoparticle with up to 25 % PCBM resemble the undoped nanoparticle efficiencies, while the

EFRET_SPS for 50 % PCBM display a bimodal distribution that is approximately equal in

abundance at less than 5 and 85 %. e-h) The kFRET_SPS for 0 to 25 % PCBM displays a broad

distribution of rates ranging from 1010 to 1012 s-1, while the kFRET_SPS distribution for 50 %

PCBM become bimodal with rates ranging from 107 to 1012 s-1. These data suggest that excess

PCBM disrupts the energy transfer process thereby decreasing the energy transfer efficiency

and rate for 50 wt% nanoparticles. ............................................................................................ 131

Figure 43: Single particle ensemble spectra normalized to the emission maximum of MEH-PPV. a) 15

wt% nanoparticles doped with 0, 5, 25 and 50 total wt% of PCBM. These spectra display a

decrease in BPPV emission intensity with respect to higher PCBM doping levels, suggesting

that the energy transfer process is enhanced at higher doping levels. b) 50 wt% nanoparticles

doped with 0, 5, 25 and 50 total wt% of PCBM. These spectra display an increased BPPV

emission for the 50 % PCBM nanoparticles, while BPPV emission intensity of 5 and 25 %

PCBM resemble the undoped 50 wt% nanoparticles. These data suggest that the energy transfer

process occurring at doping levels greater than 25 % are disrupted. ........................................ 132

xvii

LIST OF TABLES

Table 1: Material characterization of undoped and doped nanoparticles .................................................... 24

Table 2: Individual lifetime components, contributions and averaged lifetimes using 375 nm pulse

excitation. .................................................................................................................................... 90

Table 3: Individual lifetime components, contributions and averaged lifetimes using 490 nm pulse

excitation. .................................................................................................................................... 90

Table 4: Monofunctional and Difunctional Doped Nanoparticle Suspensions Quenching Factors ( and

Rate Constants for Energy Transfer and Charge Transfer . ............ 112

1

SUMMARY

The dissertation presented herein will consist of seven chapters which includes the

introduction and background as the first two chapters and the fundamental research of broadband

absorbing conjugated polymer nanoparticle as the last five chapters. Chapter 1 introduces the

field organic electronic and addresses the existing limitations, in particular polymer based bulk

heterojunction organic photovoltaics (BHJ-OPVs), while introducing the areas currently

researched for improving the efficiency of OPVs. In turn, the motivation and specific aims of

this dissertation will present the significance of investigating the steady-state dynamics, excited-

state decay dynamics, morphological and photophysical properties at the nanometer scale.

Chapter 2 introduces previous single molecule spectroscopy (SMS) research of conjugated

polymers that provides molecular level information. These molecular level properties provide

additional motivation for simplifying the complexity of the solid state material by fundamentally

studying a limited number of conjugated polymer chains that reside between the single molecule

and bulk film levels (i.e. nanoparticles that resemble the nanodomains found with the active layer

of OPVs). This chapter then describes the various methods for fabricating conjugated polymer

nanoparticles, and their integration into biological and organic electronic applications. In

Chapter 3 the experimental methods (i.e. preparation and instrumentation) used throughout the

dissertation research are described. The first section includes nanoparticle preparation by means

of a self-assembled reprecipitation method where a fullerene derivative and a low bandgap

conjugate polymer were incorporated into the nanoparticle composition to address the need for

efficient electron transfer and increased light harvesting capabilities. The following nanoparticle

characterization section includes material characterization of individual nanoparticle

compositions and, the spectroscopy instrumentation used to evaluate nanoparticles at the single

2

particle level (Single Particle Spectroscopy, SPS) and the bulk photoluminescence decay

dynamics (Time-Correlated Single Photon Counting, TC-SPC) for a given nanoparticle

composition. Chapter 4 focuses on the SPS analysis of broadband absorbing energy transfer

conjugated polymer nanoparticles that are composite blends of poly[(o-phenylenevinylene)-alt-

(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV) and poly[2-methoxy-5-(2-

ethylhexyl-oxy)-p-phenylenevinylene] (MEH-PPV). A SPS computer algorithm was

implemented to correct for direct excitation of MEH-PPV. These SPS data clearly reveal spectral

variations for a given nanoparticle composition, alluding to changes in the morphology, chemical

and structure properties of individual nanoparticles that ultimately effects the spectral and

photophysical properties of a given broadband absorbing energy transfer nanoparticle. Moreover,

Chapter 5 uncovers the excited-state dynamics of the broadband absorbing nanoparticles in bulk

solution. The TC-SPC data depicts a self-assembled nanoparticle composition that consist of

multiple emitting species, where the steady-state fluorescence and time-resolved spectroscopy

results both illustrate an enhanced energy transfer process for two nanoparticle compositions.

These results suggest of a composite nanoparticle with a mixed morphology of molecular-like

and aggregate emitters. To explore the relationship between electron transfer and energy transfer

in the composite nanoparticle and the affects that polymer chain compartmentalization has on the

photophysical efficiency of energy transfer, the effects of [6,6]-phenyl-C61-butyric acid methyl

ester (PCBM) doping are discussed in Chapter 6. A key finding of the polymer-polymer-

fullerene blended nanoparticles demonstrates that the efficiency and rate of energy transfer for

individual nanoparticles at the single particle level can be improved or reduced with increased

doping levels of PCBM.

3

CHAPTER 1: SIGNIFICANCE, MOTIVATION AND SPECIFIC AIMS OF

DISSERTATION RESEARCH

1.1 Significance of organic photovoltaic devices

In 1954, after several years of material and device optimization, Bell Laboratories developed

the first silicon based solar cell that achieved conversion efficiency of approximately 6%.1

Today’s consumer modules composed of amorphous and polycrystalline silicon exhibit

efficiencies between 7-15%, while Shire recently reported a conversion efficiency of 42.3%

using a triple junction device based on gallium arsenide.2,3

However, inorganic devices that

incorporate materials such as amorphous and polycrystalline silicon, cadmium telluride, gallium

arsenide or copper indium diselenide present two major hurdles in providing affordable

consumer products. First, handling and recycling toxic metals such as cadmium have health and

environmental risks. Second, on a dollar-per-watt basis, the inorganic materials and production

costs of these devices are currently too expensive to compete with natural resources such as

fossil fuel.

Since the discovery and development of polyacetylene in 19774,5

(for which Alan Heeger,

Alan MacDiarmid and Hideki Shirakawa shared the Nobel Prize in chemistry in 2000), great

progress has been made in new technologies based on conjugated polymers. These advancements

partially revolve around the ability to synthetically tailor the electronic, magnetic and optical

properties of semiconducting materials6 for specific technologies, such as organic photovoltaics

(OPVs), organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs),7-9

which are currently being integrated into commercially available products that include flat panel

displays, electronic skin, solar modules and complex surface shapes for consumer electronics.10-

13 The prototypical organic electronic devices consist primarily of low molecular weight

4

materials and/or conjugated polymers that are deposited into thin film active layer(s). The low

molecular weight materials are efficiently deposited by techniques such as organic vapor-phase

deposition or supersonic molecular-beam deposition,14,15

while conjugated polymer thin films are

routinely solution processed at room temperature by a number of methods including spin-

coating, roll-to-roll and inkjet printing.16-18

In the specific case of photovoltaic modules, these

deposition techniques offer the possibility of reducing manufacturing costs and producing large

area devices on flexible and lightweight substrates in comparison to inorganic photovoltaic

devices, while still addressing international issues such as climate control, energy independence

and environmental regulations.7-9,19

1.2 Motivation of dissertation research

The goal of developing new alternative energy sources has prompted research institutions to

extensively study the fundamental physics associated with OPVs. These applied studies have

generated numerous publications devoted to improving and optimizing the device performance

(See references within Angewandte Chemie-International Edition 2008, 47, 58-77). With respect

to the first reported conjugated polymer based photovoltaic cell in 1982,20

the current “gold

standard” conversion efficiency for organic photovoltaics is 8.3%.10

Although these devices are

becoming commercially available, several barriers still challenge the performance of OPVs

(e. g., light harvesting, charge generation, dissociation, transport and recombination) in

comparison to inorganic devices.21

These limitations are attributed to mixing of electron donor

and acceptor molecules that result in a thin film active layer that exhibits complex interfacial

energetics, interfacial interactions and nanostructured morphology due to a network of individual

polymer chains that interact as a single entity. Subtle changes in the structural or chemical

composition of the organic material or changes in the composition of the active layer can lead to

5

various degrees of inhomogeneous morphologies, and thus, different photophysical properties for

the bulk thin film. Consequently, a fundamental understanding of the structure and function

relationship of the active layer plays a significant role in the device performance, which can then

be used to understand the functionality and properties of organic electronic devices while

advancing and forecasting the synthetic and material design of next generation organic electronic

devices. In other words, taking a bottom-up approach in understanding the relationship between

morphology and photophysical properties from individual molecules to bulk films is extremely

necessary due to the ensemble averaging of bulk measure that may produce misleading

information.22,23

Unfortunately, methodologies for ascertaining these properties in thin films at

sub-nanometer resolution (< 100 nm) are extremely limited.24-26

Consequently, the structural and

optoelectronic properties at the nanometer scale or molecular-level are still complex when

addressing the solid-state material. These issues highlight the importance of fundamentally

understanding the morphological, photophysical and photochemical properties of individual

molecules. For instance, single conjugated polymer molecules are characterized as alternating

single and double bonds between adjacent carbon atoms that result in a multichromophoric

system with a large number of quasi-localized chromophores per polymer chain, connected by

chemical and structural defects.22,23

The variations in chain length and chain confirmation

naturally result in a disordered polymer system with photophysical properties such as

intramolecular charge and energy transfer, and electronic properties that include aggregate

formation in the ground and excited states.22,23,27

On the other hand, interaction between

neighboring polymer chains can alter the polymer chain morphology and photophysical

properties. While these phenomena, as well as others, are extensively reviewed and provide the

fundamental stepping stones for understanding polymer chain morphologies and chain

6

interactions, it is important to note that individual chains are not completely representative of the

photophysical and optoelectronic properties associated with solid-state thin films.

Accordingly, further work is needed to fundamentally understand the photophysical and

optoelectronic properties of polymer-polymer and polymer-fullerene blended materials that are

closely related to the domain sizes found in thin film active layers (i.e., domain sizes are

nanoscale representations of the exciton diffusion length in two dimensions from a central

location). Thus, the presented dissertation research in this thesis aims to simplify the bulk film by

developing a novel conjugated polymer nanoparticle model system that studies a limited number

of polymer chains and retains bulk thin film properties (Figure 1.1), while providing a

fundamental insight into the morphology, optoelectronic and photophysical properties at the

molecular and nanoscale level.

Figure 1: Illustration depicting a conjugated polymer nanoparticle model system for

simplifying the bulk film, where single nanoparticles are representative of the nanodomains

found in the active layer of a bulk heterojunction OPV.

1.3 Specific aims of dissertation research

To achieve the research goals of this thesis, the fabricated conjugated polymer nanoparticles

will be extensively studied by steady-state and time-resolved spectroscopy to gain a detailed and

7

fundamental understanding of morphology, optoelectronic properties, light harvesting,

intermolecular energy transfer mechanism, interfacial charge transfer processes and competition

between energy and charge transfer mechanisms. Additionally, the fluorescence spectra of

individual nanoparticles will be studied for each composition to gain insight into the electronic,

morphology and kinetic properties per nanoparticle per composition.

To fundamentally address these issues, the following specific aims are proposed:

Specific Aim 1: Fabrication and characterization of composite nanoparticles as a novel model

system for the study of intermolecular energy transfer and interfacial charge transfer: with

respect to bulk films, a simplified nanoparticle model system will be fabricated with different

doping ratios of conjugated polymers and electron accepting molecules (PCBM), potentially

creating a composite broad band absorbing nano material.

Specific Aim 2: Fundamentally study the optoelectronic and morphological properties of pure

and composite nanoparticles in bulk solution, and how it relates to light harvesting, energy

transfer and interfacial charge transfer: steady-state spectroscopy will be used to gain insight

into the optoelectronic and photophysical properties of the composite nanoparticles as a function

of composition, electronic structure and morphology due to changes in the UV-vis and

fluorescence spectra. Quantitatively, the measured fluorescence spectra will yield rates of energy

transfer and interfacial charge transfer.

Specific Aim 3: To gain insight into excited-state dynamics of pure and composite nanoparticles

due to structural and photophysical processes such as energy traps, recombination, exciton

dissociation and energy transfer: the excited-state dynamics of the nanoparticles will be studied

by time-correlated single photon counting (TC-SPC) as a function of excitation and emission

8

wavelengths. Variations in the portion(s) of emitting specie(s), their molecular environment, or

both will be indicative by changes in the fluorescence decay profiles as a function of

nanoparticle composition. These data will be directly correlated to Specific Aim 2.

Specific Aim 4: To gain insight into the optoelectronic and morphological properties of

individual nanoparticles as a function of doping composition, and how these properties affect the

energy transfer efficiencies, rates of energy transfer and rates of interfacial charge transfer:

fluorescence spectra of individual nanoparticles will be measured with the optical resolution of

single particle spectroscopy (SPS) in a device environment similar to OPVs to uncover any

distributions of optoelectronic properties, morphologies or photophysical processes as a function

of composition that remain masked in the bulk measurements. These results will be directly

correlated to Specific Aim 2.

9

CHAPTER 2: BACKGROUND OF DISSERTATION RESEARCH

2.1 Principles of bulk heterojunction solar cells

To address the global need for renewable green energy resources, organic photovoltaic

devices (OPVs) are one of the most promising candidates for mobile solar power generation as

they are capable of providing large area substrates with flexible and lightweight material

characteristics. Regardless of the material design or device architecture, the basic mechanism for

converting photogenerated excitons into an energy conversion process involves a four-step

process. As illustrated in Figure 2, donor-acceptor materials are sandwiched between a respective

anode and cathode exhibiting different work function to promote the charge transport of free

charge carriers.

Figure 2: Schematic illustrating the basic four-step principle of OPVs: 1) charge generation,

2) exciton migration, 3) charge dissociation and 4) charge transport and collection. Reprinted

from Account Chemical Research 2009, 42, 11, 1740-1479.

During light absorption (primarily in the organic layer consisting of conjugated polymers),

bound electron-hole pairs or excitons are formed by promoting an electron from the highest

occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) level

10

(Step 1). To obtain charge dissociation at the donor-acceptor interface (Step 3), the exciton must

migrate to the interface where the two materials differ in electron affinity and ionization potential

(Step 2). Once an energetically favored charge separated state materializes, the different work

functions of the electrodes create an internal electric field that leads to charge transport of

electrons and holes to their respective electrodes (Step 4). However, to achieve these

fundamental physical processes, appropriate material design and morphology optimization is

required for an ideal active layer.

Unlike free charge carrier generation of inorganic materials, the low dielectric constant of

conjugated polymers results in a coulombic bound electron-hole pair or exciton.28

Therefore, to

achieve efficient exciton dissociation in OPVs, material with high electron affinity29

such as, but

not limited to, other conjugated polymers and small molecules are needed. To date, the most

efficient electron accepting materials for OPVs are fullerene derivatives,11

while recent reports

have shown increase conversion efficiency with next generation conjugated polymers.30,31

Consequently, these donor-acceptor organic layers must be integrated into the device

architecture. Different architectures (bilayer and bulk heterojunction) have been adopted to

maximize the interfacial area for achieving the most efficient exciton dissociation while

minimizing the distance required for the exciton to reach such an interface. The major advantage

of developing blended bulk heterojunction over bilayer devices is the ability to harvest more

solar energy at appropriate film thickness (> 100 nm),21

while promoting the maximum charge

dissociation at interfaces that will contribute to the OPV’s photocurrent. The illustration in

Figure 3 displays a typically bulk heterojunction architecture of donor and acceptor molecules

randomly mixed in a thin film active layer, which leads to bicontinuous percolated network for

better charge dissociation and charge mobility/transport.

11

Figure 3: Schematic illustrating a blended donor-acceptor active layer typically found in bulk

heterojunction OPVs. The enlarged depictions of the active layer illustrates conjugated polymer

and fullerene domains and grain boundaries, where the bicontinuous percolated network provide

enhanced interfacial areas for exciton dissociation and charge transport.

2.2 Single molecule effect: removing the ensemble-averaging effect of bulk

measurement by single molecule spectroscopy

Conjugated polymers are characterized as ridged polymeric chains with extensive π-electron

conjugation (i.e., alternating single and double bonds). In dilute solution, polymers in the PPV

family can be considered as an ensemble of quasi-localized chromophores of various lengths,

resulting from physical defects (i.e., bending and kinking) along the polymer backbone that does

not allow complete delocalization of π-electrons.32-34

On average, an individual chromophore

consists of seven repeat units.35

This results in various chromophoric lengths per polymer chain

that is indicative of the broad structureless absorption spectrum of conjugated polymers (Figure

12

3a). Since the chromophores are electronically coupled though the strong σ-bond network of the

polymer backbone,31

subtle changes in the polymer confirmation can alter the optical and

photophysical properties of conjugated polymers.36,37

These interacting chromophores lead to

interchain or intrachain energy transfer38-40

along the polymer chain to limited number of

emissive low energy chromophores that results in a narrow and vibronically structured

fluorescence spectrum (Figure 3a). This unique photophysical process arise primary through an

intermolecular Fӧrster mechanism, which has been supported by quantum mechanics, time-

resolved spectroscopy and single molecule spectroscopy.22,31,39-43

While solid-state films are

recognized to retain some degree of the polymer confirmation that is observed in bulk solution

during film casting,44

the difference in the vibronic structure and spectral shift of the solid-state

films displayed in Figure 3a indicate that the confirmation structure changes during solvent

evaporation. Consequently, aggregation in solid-state films remains controversial because of the

broad distribution of morphologies and heterogeneity at the nanoscale, which remains virtually

impossible to study on the macro scale due to an ensemble-averaging effect of traditional

spectroscopy methods.

With the advent of single molecule spectroscopy (SMS), the nature of aggregation,

morphology, specific roles of interchain and intrachain interactions and photophysical dynamics

of isolated molecules could be studied. In 1997, Vanden Bout et al. published a unique SMS

study on a PPV copolymer derivative isolated at room temperature in an inert polystyrene

matrix. These investigators observed two key phenomena. First, polymer molecules displayed

single-step photobleaching that is commonly observed in small molecules. Second, polymer

molecules exhibited fluorescence intermittency (i.e., blinking) to different and discrete energy

levels. While the former was postulated to arise from defect sites along the polymer chain that

13

ultimately quench excitations, the authors postulated that intramolecular energy migration to

photogenerated defect sites temporarily quench the fluorescence of the latter phenomena.45

Gesquiere et al. later demonstrated that the quenching process is reversible using Fluorescence-

Voltage/Single Molecule Spectroscopy (F-V/SMS).46

Hues et al. successfully demonstrated that

the chain confirmation of MEH-PPV molecules could be solvent controlled, while exhibiting

distinct single molecule fluorescence spectra when prepared in different solvent qualities.47,48

In

a good solvent such as chloroform, the polymer conformation is extended with minimal

interaction between chromophore segments and emit independently. This extended confirmation

leads to higher emission energies (Figure 4b, blue histogram) and continuous bleaching of the

fluorescence intensity projections (Figure 4c, right transient), while exhibiting a low degree of

polarization. In a poor solvent such as toluene, the polymer confirmation collapses and exhibits

efficient exciton funneling to low energy sites (Figure 4b, red histogram) that results in

fluorescence blinking (Figure 4c, left transient) and broad distribution of polarization ratios.

14

Figure 4 a) Normalized absorption and fluorescence spectra of MEH-PPV solutions and thin

films in different solvents: chlorobenzene (blue), chloroform (green) and toluene (blue).

Reprinted from J. Phys. Chem. Lett. 2011, 2, 1520-1525. b) Ensemble distribution of single

MEH-PPV molecules fluorescence emission maxima displaying a bimodal distribution,

indicating that MEH-PPV molecules exhibit two different types of polymer confirmations. This

figure was adopted from Science 2000, 289, 5483, 1327-1330. c) Single molecules fluorescence

transients of MEH-PPV. Left) Fluorescence intermittency (blinking) of a collapsed chain

confirmation that results from intramolecular energy transfer to a quenching site. Right)

Extended chain confirmation that displays continuous photobleaching that results from limited

interactions with between chromophore segments. Reprinted from J. Phys. Chem. B 2008, 112,

12575.

15

By studying individual MEH-PPV molecules at low temperatures (20K), Yu et al. obtained a

better understanding of the emission spectra due to the high resolution of the vibronic bands.49

These studies revealed a bimodal distribution of low energy red sites and high energy blue sites.

Compared to SMS at room temperature, these data imply that the SMS at low temperature result

from either single chromophores (blue) or multiple chromophores (red). The red site emission is

thought to occur through efficient energy transfer from blue to red chromophores, whereas the

low energy red sites act as energy acceptors due to intramolecular contact along the polymer

chain or from long chromophore segments.22,50,51

In fact, the excited state dynamics of

conjugated polymers has been investigated by SMS. Specifically, through the studies conducted

by Barbara et al. the following phenomena are better understood: i) intermolecular energy

transfer by means of singlet excitons, ii) singlet excitons quenching by triplet excitons through

Fӧrster energy transfer and iii) the decay of triplet exciton through triplet-triplet annihilation.52-54

In addition, Monte Carlo simulation and polarization data was further used to support the

structural confirmations of single polymer chains55

that exhibit slight variations in the SMS

emission maxima (Figure 4b).38,49

These simulation studies predicted two distinct confirmations

i) collapsed and ordered structure called a defect cylinder (Figure 4c, left cartoon) and ii) an

extended structure connected through defects called a defect coil (Figure 4c, right cartoon).

2.3 Issues of broadband absorption and morphology in bulk heterojunction solar

cells

Despite recent improvements in OPV power conversion efficiency, the current limitation for

global marketing still remains the limited lifetime of the device and the low power conversion

efficiency. One of the primary bottlenecks of OPVs surrounds the limited light harvesting ability

of the organic materials used in the solid-state films. Figure 5 illustrates the standard AM 1.5

16

solar spectrum and the absorption spectrum of several organic materials that are commonly used

in OPVs,56

depicting that the maximum spectral flux of the solar spectrum resides between 700 -

900 nm. Due to the large band gap (>2 eV or 620 nm) of these organic materials as well as

others, only a small portion of the terrestrial solar irradiation can be absorbed (~ 30%)57

whereas

materials with band gaps of 1.1 eV (1,100 nm) can absorb 77% of the solar irradiation.57

Therefore, the light harvesting ability of OPVs must be enhanced by synthetically tailoring

organic materials and/or incorporating energy transfer cascades into the solid-state films.

Figure 5: Depicts the standard AM 1.5 solar spectrum (red) and several organic thin film

absorption spectrum (black). The absorption spectra are expressed in term of absorption

coefficient. This figure was adopted from Chemical Review 2007, 107, 1324-1338.

The development of low band gap polymers is one approach for improving the spectral

coverage of OPVs,58-60

but there are several considerations that must be taken into account when

designing the appropriate polymer. First, for exciton separation and charge generation, a

minimum energy different (0.3 eV) between the LUMO of the donor and acceptor is required.

17

Previous reports have demonstrated that lowering the band gap of the donor can lead to a

decrease in open circuit voltage (i.e., the difference between the HOMO of the polymer and

LUMO of the acceptor) with respect to a constant acceptor LUMO level (e.g., PCBM; LUMO =

-4.2 eV), which makes the polymer inefficient for power conversion.21,56,61

Second, despite the

paradox of synthetically engineering low band gap polymers, the active layer’s morphology must

be optimized with bicontinuous pathways that include large interfacial regions, suitable domain

sizes and domain ordering for exciton generation, charge separation and diffusion and fast charge

transport, respectively. For example, the low charge mobility of PCPDTBT (poly[2,6-(4,4-bis-

(2-ethylhexyl)-4H- cyclopenta[2,1-b;3,4-b’]dithiophene-alt-4,7-(2,1,3-benzothiadiazole) only

produces a power conversion efficiency of ~ 3.0% even with the large photocurrent response.62

However, there are recent reports of low band gap polymer such as PTB-7 (poly[[4,8-bis[(2-

ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-

ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) and PCDTBT (poly [[9-(1-octylnonyl)-9H-

carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]) that

display great enhancement in the polymer conversion efficiency of OPVs.63-65

Another approach to increasing the spectral coverage of a binary bulk heterojunction system

is to dope the active layer with a third organic component that provides a complementary

absorption range (e.g., near infrared). Several groups have incorporated dyes such as porphyries

and phthalocyanines into the polymer-fullerene system.66-72

In these ternary systems, the choice

of organic dye can improve or reduce the overall performance of the photovoltaic device. A

behavior attributed to morphological changes in the solid-state film due to dye aggregation. As

expected, the photocurrent spectrum extends to longer wavelengths for dye doped OPV devices

with improved performance. Transient absorption spectroscopy reveals that the dye undergoes

18

electron transfer to a PCBM molecule, followed by hole transfer to a P3HT molecule and the

charges are then transported though the percolated pathways of P3HT and PCBM. This implies

that the dye molecules are located at the donor/acceptor interface, as illustrated in Figure 6a.

Perhaps more interesting is a recent P3HT:PCBM:dye bulk heterojunction study by Ohkita et al.,

which observed similar photophysical mechanism (i.e. energetically favorable energy level align

between the ternary components, Figure 6b) and interfacial dye coverage, but describes the

enhanced photocurrent of P3HT and dye to efficient light harvesting due to energy transfer from

P3HT to dye molecules.70,71

In addition, a similar approach was taken by incorporating the low

band gap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-b;3,4-b’]dithiophene-alt-

4,7-(2,1,3-benzothiadiazole) (PCPDTBT) into a P3HT:PCBM bulk heterojunction system.73

Koppe et al. outlines the critical importance of reducing bimolecular recombination and aligning

the energy level of the low band gap polymer for cascade of different energetically favorable

electron and hole transfer pathways,73

as shown in Figure 6c.

19

Figure 6: Illustrates the competition between energy transfer and charge transfer in

P3HT:PCBM bulk heterojunction OPVs that are doped with small molecules and low band gap

polymers. a) bulk heterojunction illustration that depicts percolated pathways of P3HT and

PCBM where dye molecules are located at the donor/acceptor interface and b-c) energy level

align between the ternary components that depicts energetically favorable processes. These

figures are reprinted from Journal of Physical Chemistry C 2011, 115, 11306-11317, Acs

Applied Materials & Interfaces 2009, 1, 804-810 and Advanced Functional Materials

2010, 20, 338-346.

20

CHAPTER 3: EXPERIMENTAL METHODS

3.1 Nanoparticle Preparation

Poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)]

(BPPV), poly[2-methoxy-5-(2-ethylhexyl-oxy)-p-phenylenevinylene] (MEH-PPV, Mn =

~ 200,000; PDI = ~ 5) and 1-(3-methoxycarbonylpropyl)-1-phenyl-[6.6]C61 (PCBM) were

purchased from Sigma Aldrich. PTB7 was purchased from 1-Materials. Chemical structures are

provided in Figure 7. BPPV and PTB7 were used as received, while MEH-PPV was purified in a

hot acetone wash before use. The number average molecular weight and polydispersity index

were measured as 1,575 and 1.4, and 377,000 and 2.45 for BPPV and MEH-PPV, respectively,

by gel permeation chromatography (GPC) equipped with in-line refractive index and light

scattering detectors and a tetrahydrofuran (THF) mobile phase. All molecular solutions were

dissolved in anhydrous THF (EMD, Inhibitor free, DriSolve©

) purchased from VWR.

Figure 7: Chemical structures of BPPV, MEH-PPV, PTB7 and PCBM

21

A modified reprecipitation method, initially developed by Nakanishi and co-workers74

, was

used to fabricate doped and undoped nanoparticles (Figure 8). In this solution processing

technique, conjugated polymers or dyes were dissolved in a “good” organic solvent (i.e., THF),

and rapidly injected into a miscible solvent system (i.e., water or cyclohexane) that attacked as a

“poor” solvent to the solute. As a result, the solute underwent supersaturation, nucleation and

growth to form nanoparticles.75

The repulsive electrostatic forces between nanoparticles of

similar surface charges yielded dispersed nanoparticle suspensions in aqueous solution (which

are stable for several months)76

The empirical surface charge on the nanoparticle resulted from

two dielectric materials in close contact that undergo electrostatic charge separation. According

to Coehn’s rule,77

the substance with the lowest dielectric constant will receive a negative charge

while the other receives a positive charge. In the case of MEH-PPV nanoparticles dispersed in

water, negative charges should have formed around the nanoparticle since the relative dielectric

constant of MEH-PPV (3.0) is much less than that of water (80). Zeta potential measurements

verified the repulsive electrostatic interaction of the dispersed nanoparticles in aqueous solution

due to the negative surface charge. BPPV and MEH-PPV exhibited zeta potentials of -11 mV

and -15 mV, respectively.

The fabrication method entailed a 100 mg/L molecular solution of BPPV or MEH-PPV

dissolved in THF, where 1 mL was rapidly injected into 4 mL of deionized water while stirring

at 1,000 rpm. For doped composite nanoparticles, the weight average molecular weights of

BPPV and MEH-PPV were used to dope BPPV according to weight percent (wt%). To

qualitatively verify that nanoparticles would be formed after flash injection, an external

ultraviolent lamp (365 nm) was used to excite the molecular solutions and nanoparticle

suspension. The molecular solutions depicted before injection (Figure 8, bottom left) display a

22

subtle color variation, while the nanoparticle suspension (Figure 8, bottom right) displays

emission colors from blue to red. This observation is indicative of intermolecular Fӧrster

Resonant Energy Transfer (FRET) process. When the composite molecular solutions were doped

with PCBM, the doping wt% was expressed in terms of total mass (mTotal (g) = mBPPV + mMEH-

PPV). Compared to the two component composite systems, the three component systems (BPPV-

MEHPPV-PTB7) were doped by the composite absorption spectrum mimicking the solar

spectrum.

Figure 8: Schematic illustrating the modified reprecipitation method that involves rapid

injection of an organic material in a “good” solvent into a miscible solvent that acts as a “poor”

solvent system for the organic materials. The bottom left and right images illustrate the emission

color of molecular solutions and nanoparticle suspension from an external ultraviolent light

source.

For single particle studies, glass coverslips (No.1) substrates were pre-cleaned by two

methods before sealing the samples from oxygen by metal deposition. First, the coverslips were

23

cleaned with acetone (EMD, OmniSolve©

), 10% w/v sodium hydroxide (NaOH, Sigma Aldrich,

99.9%) and deionized water (Millipore, 18MΩ) using ultrasonication. The coverslips were then

dried with nitrogen and UV-ozone cleaned for 15 minutes. Nanoparticle suspensions were

diluted in 4% w/v poly(vinyl alcohol) (PVA, Mn = 30,000 - 70,000), and spin-coated onto the

pre-cleaned coverslips at 2,000 rpm for 1 minute. An additional layer of PVA was spin-coated on

top of the nanoparticle/PVA film to protect the integrity of the nanoparticles during thermal

evaporation. The single particle samples were placed inside an MBraun (< 0.1 ppm O2;

< 0.1 ppm H2O) glove box containing a thermal evaporator, and 100 nm gold film (Cerac Inc.,

99.9%) was deposited on the surface using R.D. Mathis tungsten boat sources at a rate of 1 Å/s.

3.2 Nanoparticle Characterization

3.2.1 Material characterization of pure (undoped) and composite (doped) nanoparticles

Material characterization was performed on all nanoparticle suspensions by dynamic light

scattering (DLS, data not shown), low resolution transmission electron microscopy (TEM, Insert

in Figure 9) and scanning electron microscopy (SEM, Figure 9). A Precision Detector 2000

Dynamic Light Scattering apparatus collected the average diameter over three trials acquiring

thirty data points per trial maintaining a 20 °C working temperature. TEM experiments were

performed using a JOEL JEM-1011. The TEM samples were prepared initially by casting a poly-

lysine (PDL) film on a 400 mesh copper grid (Electron Microscopy Sciences, FCF400) and

vacuum drying for 15 minutes, followed by drop casting the net nanoparticle suspensions onto

the PDL coated grid and vacuum drying for 15 minutes. SEM experiments were performed using

a Zeiss Ultra-55 microscope. SEM samples were prepared by drop casting the net nanoparticle

suspension onto a clean silicon wafer and vacuum drying for 15 minutes. SEM and TEM

analysis was performed on more than 100 nanoparticles per composition where the well contrast

24

image result from the electron rich nature of several hundred conjugated polymer molecules

densely packed into a quasi-spherical shape. The nanoparticle diameters of individual

compositions are summarized in Table 1, which resulted in average DLS, TEM and SEM

diameters of 37.1 ± 1.2, 36.7 ± 8.9 and 38.2 ± 6.6 nm, respectively.

Table 1: Material characterization of undoped and doped nanoparticles

25

Figure 9: Material characterization of undoped and doped nanoparticle suspensions by SEM

and TEM (insert) depicting quasi-spherical shapes with diameters of ~ 37 nm. Scale bar

represents 100 nm.

3.2.2 Uv-vis and Fluorescence Spectroscopy

Bulk spectroscopy was performed in a 1 cm path length quartz cuvette. Absorption

spectra were obtained with a Varian Cary 300 Bio UV–vis scanning spectrometer or Agilent

8543 photoarray spectrometer. Emission spectra were then obtained with a NanologTM

Horiba

Jobin Yvon fluorometer using excitation wavelengths of 350 and 488 nm. Excitation and

emission slit widths for molecular solutions were held constant at 0.25 nm, while the slit widths

were increased to 3 nm for nanoparticle suspensions.

26

3.2.3 Single Particle Spectroscopy (SPS)

Optically, a home-built scanning confocal microscope (Figure 10) was constructed

around an inverted Zeiss Axiovert 200. Collimated laser light from a 375 nm diode laser

(PicoQuant, LDH-P-C-375) or 488 nm laser line from an argon-ion laser (Melles Griot, 43

series) was cleaned by its respective interference filter (Semrock, LD01-375 or Chroma, 488IF),

sent through the back port, reflected by a appropriate dichroic mirror (Chroma, z375dcr or

495dclp) and focused to the sample surface with a diffraction limited spot size of ~ 300nm using

a Zeiss 100x Fluar objective lens (NA 1.3, WD 0.17 mm). The samples were placed on a

NanoLP1000 piezoelectric stage (Mad City Labs), and raster-scanned utilizing home-

programmed LabView software. Before being diverted to an avalanche photodiode (PerkinElmer

SPCM-AQR-14) for fluorescent imaging, the emitted photons were collected by the objective

lens, transmitted through the dichroic mirror and filtered through the respective Raman edge

filter (Chroma, 458REF or 488REF). Single particle fluorescent spectra were obtained by using a

PI Acton SP-2156 spectrograph with the grating centered at 600 nm (150 g/mm blaze: 500 nm)

which was coupled to a thermoelectrically cooled Andor iXon electron multiplying charge

coupled device (EMCCD, EM+ DU-897 BI). After subtracting the background, the single

particle spectra were averaged over three frames with an acquisition time of 10 s per frame

(unless otherwise noted) to reduce the single-to-noise ratio. For each composition, more than

>100 nanoparticles were studied.

27

Figure 10: Schematic illustrating the optical setup for the home-built single particle

spectroscopy imaging system described in Section 3.2.3.

Data analysis was performed on a MatLab interface with a home-programmed computer

algorithm previously modeled by Chen and co-workers,78,79

unless otherwise noted. The

algorithm assessed the acceptor spectral bleedthrough (ASBT) which resembled the uncorrected

FRET spectrum (sFRET). This phenomenon occurred from direct excitation of the acceptor

molecules at the donor excitation wavelength. The assumption was made that the correction data

donor-acceptor pair under donor excitation (DA@D), donor-acceptor pair under acceptor

excitation (DA@A), acceptor only under donor excitation (A@D), and acceptor only under

acceptor excitation (A@A) are organized in sequential order according to the excitation

wavelength data array, and the donor-acceptor pairs at the respective excitation wavelength for a

given composition display similar fluorescent intensities (i.e., similar radiative rates).

Additionally, a constant acceptor-only ratio was applied to the unmixed FRET spectra to obtain

the ASBT spectrum for a given doping level. This constant ratio was justified by the constant

28

quantum efficiency of the Andor iXon EMCCD over the visible spectrum;79

as expected, the

ratio decreased when the doping level of MEH-PPV was reduced.

Experimentally, a sequence of confocal images and single particle spectrum were required to

remove ASBT spectrum from the composite nanoparticles sFRET. DA@D and A@D were

excited at 55 nW, while DA@A and A@A varied from 3.5 to 16.5 nW with respect to a

decreasing in the doping concentration. For example, the 50 wt% nanoparticles line scan image

depicted in Figure 1b was scanned with 375 nm excitation source at 55 nW (DA@D). Spectra of

individual nanoparticles were acquired, providing the uncorrected FRET ensemble spectrum.

The same area was then scanned with a 488 nm laser source at 3.5 nW (DA@A). Spectra of

individual nanoparticles were acquired, resulting in a DA@A ensemble spectrum with similar

fluorescent counts as DA@D. The undoped MEH-PPV nanoparticles were then excited with the

identical laser source and power as the donor-acceptor pair to obtain A@D and A@A,

respectively. This systematic protocol was performed on each nanoparticle composition before

administering computer algorithm.

3.2.4 Time-Correlated Single Photon Counting (TC-SPC)

Time-domain and frequency-domain (phase-shift) spectroscopy are widely used strategies for

measuring excited state processes such as fluorescent lifetimes, radiative and nonradiative rates,

and rates of energy transfer. These spectroscopy methods have been applied to a broad spectrum

of biological and materials based research, and have been extensively reviewed.80-83

In this work,

a time-domain technique, time-correlated single photon counting (TCSPC), was selected to

resolve photoluminescent decay profiles in the nanosecond to picoseconds time range. TCSPC

offers high time resolution for analysis of complex excited state dynamics by exciting a sample

using high repetition rate mode-locked picoseconds or femtosecond lasers that deliver a pulse

29

width preferably shorter than the sample decay time, such as pulse diode laser or titanium

sapphire laser. Photomultipliers or avalanche photodiodes were used to record the time-

dependent intensity, , distribution of emitted photons following pulse excitation, resulting in

start-stop timing events. With periodic repetitive rates of the pulse excitation source, single

photon counting events were constructed into histograms over several cycles. Construction of the

PL decay profile, for example, is illustrated in Figure 11. TCSPC electronics measure the time

difference between the excitation pulse and fluorescent photon (Figure 11, top), and store single

photon counting events for a corresponding time channel in a histogram (Figure 11, bottom).

Additionally, TCSPC methodology is conditioned so the detection efficiency of a single photon

is less than one per laser pulse. This is depicted in the middle panel of the electronic output in

Figure 11.

30

Figure 11: Schematics illustrating the method of constructing TCSPC histograms. Top)

depicts the start-stop timing events of single photons. Middle) illustrates the output from a

constant fraction discriminator. Bottom) a histogram illustration of the photon distribution per

unit time for photoluminescent lifetime decay profile. Reprinted from Principles of Fluorescence,

3rd

ed.

These constructed histograms are representative of a single decay profile, where the

photoluminescent lifetime is calculated by the slope of the decay curve using the exponential

decay function expressed in Equation 1,

31

where represents the number of exponential decay functions used in the fitting procedure,

represents the amplitudes of the individual exponential decay components used in the fitting

procedure at = 0, and is the decay time of individual exponential decay component . For

multi-exponential PL decays, the weighted amplitude contributions ( ) of the individual lifetime

components and the amplitude-averaged decay times were calculated by Equations 2 and 3,

respectively,

where is the area under the decay curve for each component.

PL decay profiles for undoped MEH-PPV nanoparticles, obtained from TCSPC method

described below, are shown in Figure 6 with single and multi-exponential fitting functions. The

intensity decays were measured through a polarizer orientated at 54.7° from the z-axis (i.e.,

magic angle) to avoid effects of rotational diffusion and/or anisotropy. Assessing the fitting

quality for single and double exponential functions in Figures 12 a-b, the χ2 values were

significantly larger than 1.2 with the autocorrelation function displaying large random

distribution around zero. These fits should be rejected. Moreover, the use of three exponential

fitting functions display a χ2 value of 0.95 with the values of the autocorrelation function were

small and distributed randomly around zero. This fitting quality is indicative of a good fit. It

32

should be noted that the minimal fluorescent lifetime detected by TCSPC is not limited by the

IRF. The photoluminescent decays are modeled with an exponential decay function by iterative

reconvolution of the IRF, where lifetimes of approximately 20% of the IRF width can be

obtained. With a 45ps IRF, our data is limited to resolving a lifetime decay of ~ 10 ps.

Figure 12: Assessment of the fitting quality for a nanoparticle suspension with single and

multi-exponential functions. a-b) single and double exponential fitting functions display poor

fitting quality with χ2 value larger than 1.2 and autocorrelation functions with large random

distributions around zero. c) triple exponential fitting function showing an autocorrelation

function of a good fit with a χ2 value < 1.2.

33

Optically, TCSPC was performed with a FluoroTime 200 spectrometer (Picoquant) equipped

with a monochromator, microchannel-plate photomultiplier (Hamamatsu) and a PicoHarp 300

TCSPC analyzer, in combination with a 375 or 490 nm output of a frequency-doubled

Ti:Sapphire laser system (NewPort Spectra Physics, Tsunami, 85 fs pulse width, 80 MHz

repetition rate) to collect photoluminescent decays. FluoroFit Professional Software (Picoquant

GmbH, Germany) was used to estimate lifetimes by iterative reconvolution of an average

instrument response function of 45 ps with a three exponential model function. Lifetime fits were

judged according to literature methodologies,84

and weighted amplitudes were calculated to

describe the lifetime contribution of the photoluminescent lifetime components.

34

CHAPTER 4: CHARACTERIZATION OF BROADBAND ABSORBING

CONJUGATED POLYMER NANOPARTICLES BY SINGLE PARTICLE

SPECTROSCOPY (SPS)

4.1 Introduction

Semiconducting polymers have evolved into a vital component of the active layer found

within organic electronic devices such as organic field-effect transistors (OFETs), polymer light

emitting diodes (PLEDs), and organic photovoltaic (OPVs).14,56,85

Organic semiconducting

polymer materials exhibit remarkable electrical conductivity resulting from the electronic

delocalization within the polymer, and allow for flexible synthetic methodologies in material

design. In the case of PLEDs, controlling the electrical properties and emission color has been

achieved by solution processing of blended binary conjugated polymer composites through

various energy level alignment schemes and small molecule doping. Blending fractions of low-

band gap dopant material with a high-band gap material, for example, has lead to single emitting

LEDs and white-LEDs.30,86-94

In the field of OPVs, to date power conversion efficiencies over 8-

9% have been achieved in single layer bulk heterojunctions,64

while polymer blends have

achieved power conversion efficiencies of ~2%.95,96

For these applications, it has been well-

established that conjugated polymer chain morphology and interchain interactions greatly

influence the optoelectronic properties of the resulting active materials, and consequently, the

performance of these organic devices.21,92,97-99

Single molecule spectroscopy (SMS) has proven too been a vital tool in understanding the

optoelectronic properties of conjugated polymers by uncovering phenomena such as fluorescence

intensity intermittency and effects of solvent on polymer chain morphology, which led to the

discovery of rapid and efficient energy funneling to low energy chromophores.47,49,100

38,45

Although the use of SMS has proven to be hugely successful in uncovering the fundamental

35

optoelectronic properties of single conjugated polymers and has shown that even single chains

exhibit some degree of bulk-like properties, the complex and heterogeneous nature of a bulk thin

film in terms of nanoscale optical and electronic interactions, and phase separation into domains,

is insufficiently represented. Unfortunately, direct measurements of these properties at the

nanoscale in bulk films has proven challenging in the past.101

While alternative approaches such

as atomic force and electron microscopy were previously used to characterize and study the

evolution of nanoscale morphology (i.e. a few domains) in polymer-fullerene bulk films on a

length scale less than 100 nm,98,99,102,103

techniques such as scanning transmission x-ray

microscopy (STXM) and femtosecond transient absorption spectroscopy were recently used to

map the nanoscale morphology of polymer-polymer bulk films on the length scale of ~30 to sub-

10 nm, respectively.104,105

Alternatively, our group106-111

and others100,112-115

have recently

proposed and explored the use of conjugated polymer nanoparticles as model systems for the

study of polymer chain interactions, polymer chain morphology and photophysical processes at

the nanoscale by single particle spectroscopy (SPS). With this approach, conjugated polymer

nanoparticles have been shown to be representative of nanoscale cross-sections of bulk materials,

consisting of a limited number of molecules while maintaining the functionality of the bulk

material. The limited number of molecules in the nanoparticles still allows for analysis of the

morphology of polymer chains, resulting in a simplified system that can relate the material

functionality to the molecular level.

While recent SPS studies have determined the morphology and polymer chain interactions in

composite polymer-fullerene systems, 106-111

blended nanoparticle systems with broad absorption

regions and tunable emission colors are primarily characterized by the quenching efficiency of

36

the acceptor molecules in bulk solution.116-118

For example, conjugated polymers nanoparticles

doped with photochromic dyes and metalloporphyrins have recently been explored. Harbron et

al. demonstrated energy transfer quenching of MEH-PPV nanoparticles doped under ultraviolent

irradiation with a photochromic spiroxamzine dye.119

Wu et al. incorporated metalloporphyrins

into polyfluorene nanoparticles that resulted in oxygen sensing by phosphorescence

quenching.120

In addition, conjugated polymers nanoparticles provide additional strategies for

developing biological probes. In comparison to conventional dyes, fluorescent proteins and

semiconducting quantum dots, these nanoparticles offer size tunability with high fluorescence

emission and rates, photostability and negligible fluorescence intermittency, one and two photon

absorption capabilities and cellular uptake.120-122

However, the effects polymer-polymer interaction have on the optoelectronic and

photophysical properties in blended conjugated polymer nanoparticles, has not, to the best of

our knowledge, been considered or investigated on a single particle basis.. As a result, a new

insight into the nanoscale morphology, optoelectronic, and photophysical processes of composite

broad band absorbing conjugated polymer nanoparticles is validated by the results presented

within this paper. By blending the broad and featureless absorption spectra of two

semiconducting polymers, poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-

phenylenevinylene)] (BPPV, Top Left in Figure 13) and poly[2-methoxy-5-(2-ethylhexyloxy)-

1,4-phenylenevinylene] (MEH-PPV, Top Right in Figure 13), the spectral coverage of the

composite nanoparticles was extended from the ultraviolent region to 600nm. Material

characterization indicates that the nanoparticles are quasi-spherical in shape, which exhibit

remarkable emission tunability through an intermolecular Förster Resonance Energy Transfer

(FRET) mechanism by simply varying the molecular fraction of quencher. While the steady-state

37

spectroscopy data indicates that the broad band absorbing nanoparticles consists of band gap

changes and physical changes (e.g., polymer chain folding and chain-chain stacking), it conceals

a distributions of photophysical processes and spectral shapes for a given nanoparticle

composition at the single particle level. Specifically, the SPS data displays decreasing energy

transfer properties (i.e. efficiencies and rates), spectral shifting and a bimodal distribution in the

emission maximum of the single particle spectra with respect to lower dopant concentrations.

These data imply that individual nanoparticles of a given composition consist of different

BPPV:MEH-PPV doping ratios and/or heterogeneous arrangement of BPPV and MEH-PPV

molecules per nanoparticle.

Figure 13: Chemical structures of Poly[(o-phenylenevinylene)-alt-(2-methoxy-5-(2-

ethylhexyloxy)-p-phenylenevinylene)] (BPPV) and Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-

phenylenevinylene] (MEH-PPV) a-c)Material characterization of undoped and doped

BPPV/MEH-PPV nanoparticles. a) SEM and TEM (insert) image of 5wt% nanoparticle

38

suspension depicting quasi-spherical shapes. SEM, TEM and DLS (data not shown)

characterization was carried out on all undoped and doped nanoparticles. Analysis in excess of

100 nanoparticles per composition yields an average SEM, TEM and DLS diameter of 38.2 ±

6.6, 36.7 ± 8.9 and 37.1 ± 1.2nm, respectively. Scale bar represents 100nm. b-c) undoped and

doped molecular solutions and nanoparticle suspensions excited with an external 365nm lamp,

respectively. A slight variation of emission colors occurs in the molecular solution with

increased doping, while the nanoparticle suspensions yield emission colors from blue to red with

respect to increasing doping levels.

4.2 Results and Discussion

4.2.1 Steady-state spectroscopy of pure (undoped) molecular solutions and

nanoparticle suspensions

The steady-state spectroscopy of BPPV (black, donor) and MEH-PPV (orange, acceptor)

dissolved in THF and undoped nanoparticles suspended in deionized water are shown in Figure

14. As depicted in the absorption spectra of Figure 14a and 14b (dash line), these materials show

broad and featureless absorption spectra, owing to the broad distribution of chromophore

energies.44

The absorption maximum of molecular solution of BPPV, an alternating copolymer

of MEH-PPV, is located at significantly higher energy with respect to the absorption maximum

of MEH-PPV. The high energy BPPV absorption maximum suggests that the delocalization of π-

electrons in the BPPV polymer backbone is interrupted by the ortho linkage of the unsubstituted

phenylene unit, yielding an overall decrease in conjugation length. The absorption spectra of

undoped BPPV and MEH-PPV nanoparticles are nearly unchanged with respect to the molecular

solutions suggesting that during nanoparticle assembly the intra- and interchain electronic

interactions remain unaltered in the ground state.

39

The corresponding fluorescence spectra are depicted in Figures 14a and 14b (solid line).

The molecular solutions of BPPV and MEH-PPV have emission maxima located at 466 and 552

nm, respectively, indicative of red site emission by means of intrachain or interchain energy

transfer.31,34,38-40

The latter being described as energy funneling among chromophore segments

because the polymer chain folds back onto itself or because the polymer chains are in close

proximity of each other. Interchain interactions of this nature predominantly occur in solvent

systems such as THF, where the polymer chains adopt tightly coiled confirmations that avoid

interactions between the aromatic backbone of the polymer with the solvent. 123 Compared to the

molecular solutions, the emission maxima of BPPV and MEH-PPV nanoparticle suspensions are

red shifted to 503 and 593 nm, respectively. Similar to thin films, the red shifted fluorescence

spectra of the nanoparticles is attributed to energy transfer to low energy chromophores and the

formation of weakly emissive aggregates due to enhanced interchain and intrachain pi-stacking

interactions during nanoparticle assembly. 38,44,47,124,125

Interestingly, the BPPV nanoparticles

display a reduced blue vibronic shoulder, while an enhanced red shoulder emission is observed

for the MEH-PPV nanoparticles. The reduced blue shoulder of BPPV nanoparticles may suggest

that exciton efficiently migration to low energy π-stacked region in the nanoparticles or changes

in the molecular confirmation in the solid state material during stacking interaction of the

polymer chains. In the case of MEH-PPV nanoparticles, the enhanced red shoulder emission

implies that exciton efficiently migrate within the nanoparticle to regions of aggregated emitting

species.111

40

Figure 14: Steady-state spectroscopy of undoped BPPV (blue) and MEH-PPV (orange)

depicting spectral overlap between the emission of BPPV and absorption of MEH-PPV a)

Normalized absorption spectra (dash line) and emission spectra (solid line) of BPPV and MEH-

PPV molecular solutions dissolved in THF. The absorption and emission maxima of BPPV are

located at 353 and 466 nm, respectively. MEH-PPV depicts absorption and emission maxima at

500 and 552 nm, respectively. b) Normalized absorption spectra (dash line) and emission spectra

(solid line) of undoped BPPV and MEH-PPV nanoparticle suspensions in deionized water. The

vibronic structure of both nanoparticle suspensions are nearly unchanged while obtaining

absorption maxima similar to the molecular solutions in THF. The BPPV and MEH-PPV

emission maxima are further red shifted to 504 and 593 nm, respectively. In addition, the blue

vibronic shoulder of BPPV nanoparticles is significantly reduced, while the MEH-PPV

41

nanoparticles exhibit increased aggregate emission shoulder at 630nm with respected to the

molecular solution.

4.2.2 Steady-state spectroscopy of composite (doped) molecular solutions and

nanoparticle suspensions

The steady-state spectroscopy of undoped BPPV and MEH-PPV and composite

BPPV/MEH-PPV molecular solutions in THF and corresponding nanoparticles suspended in

deionized water are shown in Figure 15. The composite systems are expressed in dopant weight

percent (wt%) of MEH-PPV (100wt%, orange; 50wt%, green; 25wt%, magenta; 15wt%, black;

10wt%, cyan; 5wt%, red; 0wt% blue). The absorption spectra in Figure 15a and 15b depict two

key observations that demonstrate a high degree of experimental control when preparing the

molecular solutions and nanoparticle suspensions. First, the isosbestic point suggests that BPPV

and MEH-PPV are non-interacting components in the ground state but have the same total

concentration at a single point. The non-interacting components further support the observed

vibronic progression of the composite absorption spectra that depict nearly unchanged

superposition spectra of the undoped BPPV and MEH-PPV spectra. This implies that there are

no new detectable ground state interactions that occur when blending both polymers such as

previously observed charged transfer absorption bands.126,127

Second, MEH-PPV absorbance

increases with doping, a result that is consistent with respect to increased concentrations of the

acceptor and the increases in MEH-PPV emission intensity when directly exciting the acceptor

(Figure 5).

42

Figure 15: Steady-state bulk spectroscopy of undoped and doped molecular solutions and

nanoparticle suspensions. a-b) Uv-vis spectra of molecular solutions dissolved in THF and nanoparticles

suspended in deionized water, respectively. The molecular solutions absorption maxima of BPPV are

located at 353nm, while MEH-PPV absorption maxima reside at 500nm. The absorption maximum of

BPPV in the nanoparticle suspension resembles the molecular solutions, while the nanoparticle absorption

maxima of MEH-PPV are red shifted to 503nm with respect to increased doping levels. c-d) Fluorescent

spectra of molecular solutions dissolved in THF and nanoparticles suspended in deionized water,

respectively. The BPPV emission maximum in the molecular solutions resides at 466nm, and decreases in

fluorescent intensity with respect to a reduced doping level. Compared to molecular solutions, the

nanoparticle suspensions display red shifted emission maxima. The emission maxima of undoped BPPV

and MEH-PPV nanoparticles are located 504nm and 593nm, respectively. BPPV emission intensity is

43

quenched and blue shifts to 498nm with respect to increased doping levels of MEH-PPV, while the

emission intensity of MEH-PPV monotonically increases and blue shifts to 585nm. In addition, the MEH-

PPV emission maximum of 5wt% is located at 576nm and deviates for the monotonic increase in

emission intensity.

Interestingly, the BPPV contribution in the absorption spectra of the composite

nanoparticles depicts similar structure and peak wavelength maxima as the undoped BPPV

nanoparticle suspension, BPPV absorption maximum blue shifts from 525 to 503 nm as the

doping level is increased. This observation indicates that the interchain and intrachain electronic

interaction of BPPV in the ground state remain unaffected during doping, while the interchain

and intrachain π-stacking interaction of MEH-PPV become less hindered as the doping level

increases. In the case of 5wt% nanoparticles, the MEH-PPV absorption maximum red shifted by

22nm with respect to the 503 nm absorption maximum of undoped MEH-PPV nanoparticles,

suggesting that the polymer chains adopt a defective cylinder confirmation with excess

interchain and intrachain π-stacking. Polymer chain confirmations of this nature were previously

shown to occur in aggregates and concentrated solutions.44,128

In nanoparticles with low MEH-

PPV doping levels, the MEH-PPV polymer chains may arrange within the nanoparticle as tightly

packed “isolated” molecules with excess pi-stacking interactions or as a small aggregated region

of molecules that overall exhibit enhanced pi-stacking interactions. As the doping level of MEH-

PPV increases, the defective cylinder confirmation of MEH-PPV the pi-stacking interaction of a

MEH-PPV defective cylinder confirmation may become less hindered. This may result from the

increased concentration of high molecular weight MEH-PPV polymer chains, where steric or

repulsive effects may lead to reduced pi-stacking interactions. These polymer chain packing

geometries are illustrated in Figure 16.

44

Figure 16: Illustration of the evolution of polymer chain interactions and morphologies

within the nanoparticle as the doping level of MEH-PPV varies from 0 to 100 wt%. The blue and

red segments are representative of BPPV and MEH-PPV, respectively.

Another possible scenario to consider is a MEH-PPV packing geometry where polymer

chains stack in a disordered face-to-face configuration (i.e. H-aggregation). Davydov and

Kasha129

exciton coupling model of describing the photophysical nature of H- and J-aggregates

has previous been applied to oligo- and poly(phenylene-vinylene) derivatives.130,131

In this model

the absorption of strongly interacting monomers will split into two bands, one in-phase and one

out-of-phase that couples with the transition moments of the monomer. The allowed in-phase

transition of H-aggregation absorbs higher energies than the respective monomer, resulting in

blue shift absorption maximum and weakly emissive aggregates (vide infra) with respect to the

monomer. Although blue shifted absorption spectra have been observed in undoped MEH-PPV

45

nanoparticle and MEH-PPV nanoparticles doped with fullerenes with respect to the molecular

solution,106,110,111

the latter does not fit well with the occurrence of blue shifted absorption

spectra in a H-aggregates packing geometry.

The fluorescence spectra in Figures 15c and 15d are representative of the corresponding

undoped and doped molecular solutions and nanoparticle suspensions at donor excitation (350

nm), respectively. These doped fluorescence spectra depict similar vibronic structures compared

to the undoped constituents, suggesting that the formation of an excited state complex (i.e.

exciplex or excimer) does not exist, however, a nonemissive excited state complex cannot be

ruled out. As previously observed in Figure 14, the doped nanoparticle suspensions are red

shifted with respect to the molecular solutions, indicative of an enhancement of interchain and

intrachain π-stacking interactions.38,44,47

In addition, Figures 14 depict the spectral overlap ( )

between the emission spectra of BPPV and the absorption spectra of MEH-PPV. According to

Equation 4, the extent of spectral overlap is directly proportional to the rate of energy transfer:82

(4)

(5)

where QD is the quantum yield of the donor; κ2 is the orientation factor; τD is the lifetime

of the donor; r is the distance between donor and acceptor; N is Avogadro’s number; n is the

refractive index of the medium; FD corresponds to the normalized fluorescent spectrum of the

donor; εA is the molar absorption coefficient of the acceptor at λ and λ is the wavelength. The

overlap integrals for molecular solutions and nanoparticle suspensions were calculated by

Equation 5, resulting in values of 2.9x1018

M-1

cm-1

nm4 and 3.7x10

18 M

-1 cm

-1 nm

4,

46

respectively. These values imply the potential for efficient intermolecular energy transfer within

a BPPV/MEH-PPV composite nanoparticle.

The BPPV emission of the doped nanoparticles is shown to blue shift from 504 to 489 nm

as the doping level of MEH-PPV is increased. Similar observations were previously reported for

composite P3HT:PCBM and MEH-PPV:PCBM nanoparticles suspension with respect to

undoped nanoparticle suspensions.106-111

These authors contribute this spectral feature to the

reduction in the conjugation length of polymer chains, a result caused by PCBM steric effect

during doping. The vibronic progression of BPPV, however, remains unaltered during doping

indicating minimal reduction in intermolecular π-stacking interactions.

Moreover, the emission intensity of BPPV is sequentially reduced with increased doping, and

can be assigned to the energy transfer mechanism (i.e. Trivial, Förster or Dexter) that occurs

between polymer chains of BPPV and MEH-PPV. In the case of a Trivial mechanism, donor and

acceptor interactions occur without affecting the emissive ability of donor. X. Li et al. recently

excluded energy transfer through a trivial mechanism by observing a decreasing in the donor

lifetime, where decay of the donor ( ) was proportional to the change of fluorescent intensity

( ) with respect to the quenchers molecular fraction.116

These measurements further illustrate

positive nonlinear deviation in the Stern-Volmer relationship that has been demonstrated in gold

and blended organic nanoparticles,116-118,132

which was indicative of a dynamic Förster process.

In the latter, quenching of the donor emission intensity via Förster and Dexter mechanisms occur

over short distances through nonradiative dipole-dipole interaction (<10nm) and electron

exchange (<2nm), respectively. Conjugated polymers furthermore provide an additional energy

migration mechanism, where excitons have been shown to migrate along polymer chains at

lengths of 5-20 nm,133,134

a process characterized by the exciton diffusion length.To properly

47

assign the energy transfer mechanism, we recently studied the excited-state dynamics of these

undoped and doped nanoparticle suspensions by time-correlated single photon counting

spectroscopy.135

These data illustrate that the donor lifetime decreases as the doping level of

MEH-PPV increases; therefore, rules out the possibility of a Trivial energy transfer mechanism.

In addition, the dependence of BPPV fluorescence intensity with respect to the MEH-PPV

doping concentration was modeled using a modified Stern-Volmer relationship (Figure 17),

where F0 and F are the fluorescence intensity in the absence and presence of the acceptor,

respectively, and q is the MEH-PPV concentration expressed as a molecule fraction. It was

observed that the modified Stern-Volmer relationship deviates (positive) from linearity,

especially at high doping concentrations. This phenomenon is indicative of a hyperefficient

quenching system, 41-43,63

suggesting that the nanoparticles consist of densely packed polymer

chains that efficiently transfer energy from BPPV to MEH-PPV. Consequently, these steady-

state and time-resolved studies demonstrate that our blended nanoparticle system primarily

consist of a dynamic Fӧrster process, but a combination of energy migration and energy transfer

cannot be ruled out. Moreover, we cannot rule out a Dexter quenching mechanism involving

electron exchange at short distances (i.e. over an order of one to two molecules.

To gain further insight into the random distribution of donor and acceptor molecules

residing within a composite nanoparticle, the effect of multiple quenchers and their statistical

distribution were evaluated by the following nanoparticle quenching model117

given in Equation

6 to the Stern-Volmer relationship depicted in Figure 17,

48

where the Poisson probability distribution function describes the quenchers per nanoparticle,

number of donor molecules ( ), molecular fraction of the quencher ( ), and the quenching

efficiency ( ) defined by . The curve fittings depicted in Figure 4

shows that the quenching model fits our experimental data and are, therefore, applicable in

describing the composite nanoparticles. Previous studies have demonstrated quenching

efficiencies that exceed 90% for hyperefficient blended nanoparticle systems.116-118

Interestingly,

the number of BPPV molecules (N > 2000) exceeds previous N calculation by more than an

order of magnitude, and likely results from low molecular weight of BPPV. These fitting

variables further suggest that a Dexter type mechanism would be negligible in blended

nanoparticles obtaining a diameter of ~36nm. In a scenario where the number of BPPV

molecules per nanoparticle exceeds the number of MEH-PPV quenches, a single MEH-PPV

should efficiently quench thousands of BPPV molecules over a larger Förster distance verse the

molecular scale of a Dexter mechanism.

49

Figure 17: Nanoparticle quenching model fitting results for bulk spectroscopy (red) and SPS

(black) which are in good agreement. Both data sets depict a quenching efficiency (q) of >90%,

while the number of donor (N) is in excess of 2000 molecules.

In addition to the BPPV emission component of the composite nanoparticle suspensions, the

MEH-PPV emission component in Figure 14d displays several key features of the composite

nanoparticle suspensions with respect to decreasing doping levels of MEH-PPV. First, from 50

wt% to 10 wt% the MEH-PPV emission increases while the emission spectra blue shifts to 587

nm. Second, the MEH-PPV emission intensity of the 5 wt% nanoparticles does not follow the

monotonic intensity progression from 50 wt% to 10 wt% (i.e. the emission intensity of the MEH-

PPV component gradually increase as the doping level decreases, but decreases at doping level

of 5 wt%). To provide further insight of the MEH-PPV emission intensity, the molecular

solutions (Figure 18a) and nanoparticle suspensions (Figure 18b) were studied under direct

50

excitation of the acceptor.. While the doped molecular solutions emission maxima coincides with

undoped molecular solution in Figure 13b, the nanoparticle suspension emission maxima are

within good agreement to the composite nanoparticle suspension depict under BPPV excitation

in Figure 14d. This data implies that the fluorescence spectrum of the 5 wt% nanoparticle

suspension in not artificially influenced by the fluorescence spectrum of the BPPV component.

In fact, these data are directly correlated to the absorption spectra (Figure 14a-b) of the

composite molecular solutions and nanoparticle suspensions, which display an increase in the

absorbance of MEH-PPV with respect to increased doping levels.

Figure 18: Fluorescent spectra illustrating direct excitation of MEH-PPV in undoped and

doped molecular solutions and nanoparticle suspensions. Both systems depict a decrease in

emission intensity with respect to reduced doping level of MEH-PPV, which is directly

correlated to the decrease in MEH-PPV absorption presented in Figures 3a-b. a) the doped

molecular solutions depict a constant 552nm emission maxima that resemble the undoped

51

molecular solution b) undoped MEH-PPV nanoparticle emission maximum is located at 595nm,

while the doped emission maxima blue shift to 586nm with respect to decreasing doping levels.

These emission maxima are in good agreement with those presented in Figure 3d.

On the bases of these data, we have outlined several plausible mechanisms that could

effectively influence the peak wavelength maximum and emission intensity of MEH-PPV: i)

fewer absorbing BPPV molecules ii) phase separation iii) reduction in quantum yield iv) the

formation of H-aggregates. In the case of a lower concentration of donor molecules per

nanoparticle, the rate of energy transfer would be limited by BPPV ability to absorb photons and

emit non-radiatively. Although concentration of donor molecules would coincide with the

relatively low molar extinction coefficient (ε = 64,000 M-1

cm-1

), we have ruled out this

possibility since the energy transfer nanoparticle model predicts a large number BPPV molecules

(N>2000) residing within a nanoparticle. We further believe that fewer absorbing BPPV

molecules and phase separation are limiting factors since the rate of energy transfer per

nanoparticle (kET_SPS) are orders of magnitude larger at higher doping levels (Figure 8l). The

contribution of a reduced MEH-PPV quantum yield (ΦSOLN = 0.21; ΦNPs = 0.02) in composite

nanoparticles could also influence the fluorescent emission intensity of MEH-PPV. In this case,

the presence of aggregate quenching increases with concentration and, consequently, provides a

larger number of excited state species that funnel to weakly emissive interchain states. These

interchain states act as energy traps and effectively reduce the photoluminescence of the solid

state material.136,137

Lastly, it is important to specifically consider the assignment of H-

aggregates to the reduced photoluminescence and red shifted MEH-PPV emission maximum

while increasing the dopant level. In oligo- and poly(phenylene-vinylene) derivatives130,131

, the

delocalized exciton coupling of the interacting chromophores split the excited state in the face-

52

to-face stacking geometry of H-aggregates, while forbidding the transition to the lowest excited

state.129

This architecture leads to blue shifted Uv-vis spectrum, which is corroborated by the

bulk Uv-vis spectroscopy, forms of excited state excimers. These excited state complexes

subsequently reduce the photoluminescence efficiency and exhibit red shifted fluorescent

spectra.131

We note, however, that the formation of interchain excited state complexes (i.e.

excimer) are typically nonemissive at ambient temperature and rapid internal conversion to the

lower energy excited state can contribute to the reduction in emission intensity 138-140

and that the

increased aggregated state could result in quenching of the excited state by excimers.

In the case of 5wt%, we have estimated that approximately 1-6 MEH-PPV molecules

resided within nanoparticle consisting of a 36nm diameter. We propose that the reduction in

MEH-PPV emission intensity at 5wt% may result from dilution effects that form isolated MEH-

PPV molecules which are phase separated from regions of aggregated BPPV molecules. These

aggregated regions effectively enhance nonradiative deactivated pathways through excess

interchain energy traps (i.e. aggregate quenching), leading to a partial intermolecular energy

transfer process that is limited by the inability of locating isolated MEH-PPV molecules over the

donor diffusion length during energy migration. It should be noted, that this proposed

mechanism would not explain this phenomenon if the ultrafast energy transfer observed at 5wt%,

resulted in complete quenching of the donor emission.

4.2.3 Single Particle Spectroscopy of pure (undoped) and composite (doped)

nanoparticles

Single particle spectroscopy was performed on undoped and doped nanoparticles by spin

coating a dilute solution of nanoparticles suspended in a nonfluorescent inert polymer matrix (i.e.

PVA). The resulting film provided an imaging substrate where the nanoparticles were separated

53

by several micrometers. Fluorescent spectra of ~100 individual nanoparticles per composition

were compiled into corrected FRET ensemble spectra (Figure 19a-f, black). The BPPV and

MEH-PPV emission maxima appear to shift in the ensemble spectra during doping, however, the

broad spectral overlap of BPPV provides inaccurate assignment of these values. To minimize the

error when determining the individual emission maxima, the ensemble spectra were

deconvoluted into donor-only (blue) and acceptor-only (orange) spectra by fitting the onset of

the ensemble with the undoped BPPV ensemble spectrum (Figure 19a). These spectra were fitted

with three Gaussian functions to obtain the respective emission maximum. In addition, the

resulting data was used to construct peak wavelength histogram for each composition (Figure

19a-f, grey). These data depict two key observations of the optoelectronic and photophysical

properties, and discussed hereafter: i) the individual donor and acceptor spectra shift with respect

to the MEH-PPV doping level and ii) the peak wavelength histogram become broad and bimodal

when the doping level of MEH-PPV is reduced. Compared to the steady-state spectroscopy,

undoped (Figure 19a, black) and doped (Figure 19b-f, blue) BPPV emission spectra are red

shifted to 518nm and 509nm, respectively, indicative of enhance pi-stacking interactions at the

single particle level. MEH-PPV emission spectra of 50wt% and 25wt% are red shifted to

approximately 600nm, while the emission spectra blue shifts to 590nm as the MEH-PPV doping

level is reduce below 25wt%. The red shifted MEH-PPV spectra are in agreement with other

undoped MEH-PPV nanoparticle and thin film studies,38,44,47

while the blue shifted spectra of

BPPV and MEH-PPV are indicative of kinking and bending along the polymer backbone. The

latter implies that BPPV and MEH-PPV sterically affect the interchain interaction of the

counterpart during doping and effectively reduces the polymer conjugation length, while the red

shifted spectra suggest that pi-stacking interaction are slightly enhanced. Moreover, the enhanced

54

π-stacking interactions and steric effects coincide with bulk spectroscopy observed in the

nanoparticle suspensions (Figure 14d), while the unaltered vibronic progress of the deconvoluted

BPPV and MEH-PPV spectra suggest that intrachain interaction remains unaffected during

doping. Interestingly, the spectra for doped nanoparticles display a broad distribution of

maximum wavelength ranging from 500nm to 612nm, which is inaccessible at the bulk level. In

addition, the peak wavelength distribution becomes bimodal and increasingly broader (Figures

19b-c), while those of 15wt%, 25wt% and 50wt% display similar single modal peak wavelength

distributions of previously studied MEH-PPV nanoparticles.106,111

. These observations validate

the distribution of photophysical properties displayed in Figures 20 and 21 for a given

nanoparticle composition.

55

Figure 19: Single particle ensemble spectra with deconvoluted donor and acceptor spectrum

and peak wavelength histograms of undoped and doped nanoparticles. a-f) Ensembles (black)

were deconvoluted by fitting the onset of undoped (0wt%) nanoparticle and performing a least

square analysis, yielding the corresponding donor (blue) and acceptor (orange) spectrum. These

data were used to determine the donor and acceptor emission maxima since the spectral overlap

of BPPV and MEH-PPV can in effect alter the spectral progression of the unmixed spectrum.

Undoped nanoparticle emission maximum is located at 518nm, while the donor emission in

composite nanoparticles blue shifts to 509nm. In turn, the deconvoluted MEH-PPV emission

maximum varies slight (596nm ± 2nm) compared to the unmixed composite ensembles. g-l)

Peak wavelength histogram of individual undoped and doped nanoparticles. 0wt% nanoparticles

depict a single modal distribution at 518nm, while the bimodal distribution in composite

56

nanoparticles less than 15wt% are centered around 509nm and no donor emission is observed

thereafter. In addition, the distribution of MEH-PPV peak wavelength narrows when increasing

the doping level of MEH-PPV. At doping levels less than 15wt% the MEH-PPV peak

wavelength is centered at 594nm, while majority of the peak wavelengths in 50wt% are red

shifted to 603nm.

Corrected energy transfer efficiencies (EcFRET_SPS, Figures 20a-e) and rates (kcFRET_SPS,

Figures 21a-e) were computed from a least squares deconvolution of individual correct

nanoparticle spectra with the undoped BPPV ensemble spectrum. Individual nanoparticle

EcFRET_SPS and kcFRET_SPS were calculated by Equation 7 and 8, respectively,

where FDA and FD represent the fluorescent intensity of the donor in the presence and absence

of the acceptor, respectively. is the lifetime of the donor, and represent the

quantum yield and detector efficiency of the donor and acceptor, respectively. The excited-state

lifetime of BPPV nanoparticles was determined by time-correlating single photon counting,

yielding an averaged lifetime of 100ps. The quantum efficiency of the EMCCD is nearly

constant over the spectral range of SPS analysis and resulted in a detector efficiency ratio of 1.

Figures 6 and 7 illustrate a broad distribution of energy transfer efficiencies and rates per

unit over several orders of magnitude, resulting from the degree of spectral variation depicted in

the SPS ensemble data. In the case of 15wt%, 25wt% and 50wt%, the majority of the energy

57

transfer efficiencies reside around 80 to 100%. This suggest that BPPV and MEH-PPV polymer

are dispersed in the nanoparticle where direct energy transfer could occur through close

proximity of polymer chains or the excited state can effectively transfer energy within the

exciton diffusion length. The data also display rates of energy transfer on the time scale between

109 and 10

10 s

-1. Additionally, 10wt% and 5wt% display a largest distribution of correct FRET

efficiencies and rates of energy transfer between 108 and 10

9 s

-1. These photophysical

parameters, once more, directly correlated to the broad distribution of spectral variation depicted

in ensemble spectra of Figure 19, where the chemical, structural and morphological properties

consequently effect the electronic and photophysical properties on the nanoscale for a given

nanoparticle composition, which remain masked at the bulk level.

Figure 20Figure 7: Corrected FRET efficiencies for individual nanoparticle of a given

composition. 5wt% (a) and 10wt% (b) display a large distribution of energy transfer efficiency,

while majority of the efficiencies of 15wt%, 25wt% and 50wt% (c-d) resides between 80 to

100%.

58

Figure 21: Correct rates per unit time of energy transfer for individual nanoparticles of a

given composition. The rates are displayed on a logarithmic scale for convince. As illustrated,

the rate of energy transfer decreases when the MEH-PPV doping level is reduced. a-b) kcFRET_SPS

≈ 108 to 10

9 s

-1 and c-e) kcFRET_SPS ≈ 10

9 to 10

10 s

-1.

Lastly, we will specifically address the degree of spectral variation when assembling a

broad absorbing composite nanoparticle consisting of blended BPPV and MEH-PPV, as

illustrated in Figure 9. These data are normalized at 590nm to display the donor-to-acceptor

intensity ratio. A slight fluctuation in the shape of the fluorescence spectra is observed in 50wt%

nanoparticles (Figure 22a) with a small variation in donor intensity and constant MEH-PPV

emission maximum, while 25wt% nanoparticles (Figure 22b) resembled the latter but were found

to have increased donor emission with a slight blue shift in the MEH-PPV peak maximum. As

the doping level was reduced below 25wt% (Figure 22c and 22d), single nanoparticle spectra

were found with similar vibronic progressions of 50wt% and 25 wt%, while obtained spectra

59

with higher and equal donor-to-acceptor intensity ratios. Additionally, 10wt% (Figure 22d, green

line) and 5 wt% (Figure 22e, red line) nanoparticle contained undoped BPPV nanoparticle

emission as depicted by the normalized intensity of ~2, and demonstrates a broad distribution of

peak wavelength maxima. To our knowledge, spectral variation at the single particle level has

never been demonstrated for a blended polymer system. We therefore proposed two plausible

circumstances that could account for the degree of spectral variations of a given composition: i)

heterogeneous arrangement of donor and acceptor molecules with a given nanoparticle and/or ii)

nanoparticle suspension which obtains different BPPV and MEH-PPV concentrations per

nanoparticle. In the former, the arrangement of BPPV and MEH-PPV may result in a nanoscale

morphology that becomes increasingly heterogeneous and multifaceted when lowering the

dopant level of MEH-PPV suggesting that inter- and intrachain interaction are severely

interrupted during doping. In turn, the relationship between energy transfer and photophysical

properties with respect to the dopant concentration would be affected by exciton migration and

the broad distribution of aggregate morphologies found within the blended energy transfer

nanoparticles, which can be attributed to variation in the polymer folding and stacking geometry.

In the latter, variation in the donor and acceptor concentration per nanoparticle would lead to a

broad distribution of optical and photophysical properties since the FRET models described

above are directly related to the quenching of the donor emission intensity. We do acknowledge

that neither scenario can fully explain the variation in spectral shapes, since both scenarios may

occur simultaneously.

60

Figure 22: Exemplifies the complex heterogeneity within the composite nanoparticle

systems. Spectrum from averaged single particle spectra corresponds to the ensemble(s) in

Figure 6, and normalized at 590nm for comparison. a) 50wt% acceptor maxima remain constant

with small intensity differences in the donor maxima. b) The subtle variation in 25wt% acceptor

maxima occurs with different donor intensities, while providing nanoparticle spectrum that

resemble that of 50 wt%. c) 15wt% nanoparticle obtains blue shifted spectra and spectra that are

similar to 50wt% and 25wt%. However, nanoparticles with equal or higher donor-to-acceptor

intensities are revealed. d-e) 10wt% and 5wt% nanoparticle obtain spectra that resemble Figures

8a-c. However, the presences of undoped BPPV nanoparticles were observed, and are depicted

by the elevated normalized intensity (~2). Furthermore these averaged spectra explicitly illustrate

61

the heterogeneous arrangement of BPPV and MEH-PPV molecules residing within a given

nanoparticle and/or differences in BPPV/MEH-PPV composition per nanoparticle.

4.3 Conclusion

In this work, a modified reprecipitation method was utilized to fabricate self-assembled

composite broadband absorption BPPV/MEH-PPV nanoparticles. These nanomaterials reside

between single molecules and bulk materials; retain the bulk materials functionality, while

providing molecular level properties at the nanoscale. The undoped and doped nanoparticles

were characterized by size, morphology, bulk and single particle photophysical properties. These

broadband absorbing nanoparticles display a range of electronic and morphological interaction,

including Fӧrster Resonance Energy Transfer, band gap changes and physical changes in chain

folding and chain-chain stacking. SPS revealed a broad distribution of spectral shapes and

photophysical properties which signifies a heterogeneous arrangement of donor and acceptor

molecules within a given nanoparticle. These findings provide an understanding of

morphological, optoelectronic and photophysical properties of polymer-polymer active layers in

organic electronic devices, while suggesting that the doping level of energy transfer

nanoparticles for biological probes should be carefully considered.

62

CHAPTER 5: EXCITED-STATE DYNAMICS OF BROADBAND

ABSORBING ENERGY TRANSFER CONJUGATED POLYMER

NANOPARTICLES

5.1 Introduction

The conversion of solar energy into a viable energy source through artificial light

harvesting systems continues to attract much interest in the global market of renewable energy.

Semiconducting polymers processed from solution have emerged into a vital class of materials

for organic photovoltaic (OPV) active layers due to their low cost and easy

processability.85,141,142

While the most efficient devices are blends of conducting polymers and

fullerene derivatives,64

blending two or more semiconducting polymers (electron donor and

acceptor polymers) has also been explored.30

More recently, research efforts have been focused

on enhancing the light harvesting ability of OPV devices by blending conducting polymers with

complementary light absorbing materials such as inorganic semiconductors,143

small

molecules144

and low bandgap polymers.73

While these blended systems already exhibit

complex morphological and photophysical properties,21,99,101

fullerene derivatives are typically

still required for efficient charge separation. Under ideal conditions, charge transfer from

polymer to fullerene is thought to occur on the femtosecond time scale while the kinetically

hindered back transfer process is orders of magnitude slower.145

However, Janssen et al.

demonstrated that the degree of solvent polarity can control the dominating electron or energy

transfer process that occurs in fullerene-oligothiophene-fullerene triads, a result consistent with

quantitative calculation rationalized by the Weller relationship.146,147

On the other hand, near

equal contributions of electron and energy transfer processes in perylene derivative-C60 dyads

were reported by Martini et al., implying that simultaneous photophysical process may occur

between two closely packed chromophores.148

More recently, Ohkita and co-workers

63

incorporated a third light harvesting component into P3HT/PCBM active layer that absorb longer

wavelengths and displays an efficient cascade of photocurrent.70,71

The ternary blends indicate

that the light-harvesting dyes are distributed at phase separated polymer/fullerene interface

without forming dye aggregates, while the dyes efficiently serving as a photosensitizer and

extending the exciton diffusion length through long-range Fӧrster energy transfer mechanism.

These studies underline the importance of fundamentally understanding competing

photophysical processes in blended organic systems regardless of the original donor and acceptor

materials. In polymer-polymer-fullerene ternary blends, for example, energy transfer between the

different polymers will be probable, while charge transfer between the constituent conducting

polymers and fullerene will occur as well. Moreover, photophysical process such as energy

transfer between constituent conducting polymers and fullerene and charge transfer between the

different polymers must be considered. Thus, the relative quantum yields of these competitive

processes will mainly depend on the spatial distribution of the components and the

optoelectronic properties of the polymers. The latter being more strongly influenced by the

morphology of individual polymer chains, morphology of the blended material (which

determines interchain interactions and exciton migration), and blending or phase separation

between polymers and fullerene (which in part determines charge transfer kinetics).

Additionally, research has extensively demonstrated that the degree of aggregation (i.e.

morphological properties that result from interchain and intrachain interactions) directly affects

the optical, electronic and photophysical properties of semiconducting materials,22,23,44,149-152

which consequently can be controlled by solution processing conditions.123,125,134,136,138,153-158

Specifically, understanding the excited-state dynamics of conjugated polymers is vital for

optimizing the active layer in organic electronics. For example, the excited-state dynamics of

64

classical conjugated polymers such as PPV159-161

and MEH-PPV37,124,162-164

have been

extensively studied, while reports of new generation conjugated polymers such as APFO3,165

PBTTT166

and PCDTBT167

have become more available. These studies general focus on the

nature and evolution of the excited-state dynamics in non-aggregated and aggregated forms of

the conjugated polymer by using time-resolved fluorescence spectroscopy techniques such as

time-correlated single photon counting (TC-SPC). In “good” solvent systems, dilute conjugated

polymer solutions typically exhibit single excited-state dynamics owing to isolated polymer

chains with minimal interchain interactions.123,124,140,168-170

For conjugated polymer films casted

from solution, the degree of aggregation in the molecular solution to a large extent is retained

after spin-coating procedures. However, conjugated polymer films obtain a significantly larger

degree of interchain interaction in comparison to dilute molecular solutions, leading to more

complex excited-state dynamics that often display nonexponential decays and bimolecular

quenching. While the nonexponential behavior of conjugated polymer films can be ascribed to

polymer chain aggregation and the local environment of the exciton,171-173

high excitation power

densities on average reduce the exciton density directly after photoexcitation by means of rapid

exciton-exciton annihilation quenching process of neighboring excitons.124,174-178

Moreover,

conjugated polymer films display detection wavelength dependencies that give rise to spectral

diffusion most likely resulting from rapid Fӧrster energy transfer migration from high energy

chromophores with faster initial decays to longer lived low energy chromophores.160,161,179-181

Although the phenomena described above provide substantial understanding of the effects of

interchain and intrachain interaction on the optoelectronic and photophysical properties for

complex conjugated systems, these traditional steady-state studies fail to spectrally resolve single

65

conjugated polymer chains and correlated the photophysical and optoelectronic properties of

single polymer chains with their respective morphologies.

With the advent of single molecule spectroscopy (SMS), Barbara et al. undoubtedly

revealed a rich molecular level picture of a multichromophoric conjugated polymer with single-

step photobleaching and fluorescence intermittence, which is attributed to efficient

intramolecular energy transfer to photogenerated quenching sites.45

These quenching sites were

later revealed as a product of photochemical reactions between exciton traps and oxygen;38

thereafter, Fluorescence-Voltage/Single Molecule Spectroscopy (F-V/SMS) demonstrated that

the fluorescence quenching reaction is reversible.46

Additionally, the relationship between chain

confirmation (i.e. morphology adopted by single polymer chains) and spectroscopic properties

were extensively studied by several groups.22,47,49,51,55,100,182

For example, Hu et al. utilized

Monte Carlo simulations of MEH-PPV polarization distributions to predict two districted

polymer confirmations (collapsed defect cylinder and extended defect coil),55

while Huser et al.

demonstrated polymer conformational changes in single MEH-PPV with respect to the solvent

preparation conditions.47

Together, these single molecule studies demonstrate that the degree to

intrachain π-stacking greatly affects the extent of intramolecular energy transfer (i.e. energy

funneling) and, as a consequence, the polymer chain confirmations, photophysical and

optoelectronic properties of multichromophoric conjugated polymers.

Recently, pure (undoped) and composite (doped) organic aggregates/nanoparticles that

consist of a limited number of conjugated oligomer or polymer molecules have been fabricated

and spectroscopically characterized for material and biological based purposes.46,100,106-

111,115,120,121,183-186 These nanoparticle systems provide researches the unique opportunity to study

polymer chain interaction of the bulk film (i.e. interchain interactions), which are inaccessible at

66

the single molecule level. Our group has recently shown how the morphology and photophysical

properties at the molecular level are affected by polymer chain folding and the interactions

between polymer-fullerenes107-111

and polymer-polymer183

at the nanoscale by using single

particle spectroscopy (SPS). Specifically, we demonstrated that broadband absorbing energy

transfer nanoparticles display a degree of spectral variations and broad distribution of energy

transfer efficiencies and rates for a given nanoparticle composition, which remain masked in

steady-state measurements due to ensemble averaging. These data are indicative of a nanoscale

morphology that becomes heterogeneous in the solid state due to polymer-polymer aggregation,

changing polymer chain morphologies, and strong variations in polymer distribution at the

nanoscale. Furthermore, Peteanu et al. demonstrated that long-chain (13 rings) alkoxy-

substituted oligomeric PPVs (OPPVs) aggregates exhibit similar spectral and time-resolved

properties as aggregated MEH-PPV polymers.184

Specifically, the fluorescence spectra display

an anomalous Franck-Condon structure with emission dependent fluorescence lifetimes,

implying that oligomeric aggregates self-assemble into a “shell” of loosely packed chains

(monomer-like emitters, ~0.6 ns) that surrounds tightly packed chains (aggregate-like emitters,

~0.3 ns) located in the “core” of the aggregate. More recently, Peteanu’s group provided addition

evidence of oligomeric aggregate “core-shell” model by studying the spatial variations in the

excited-state dynamic of individual OPPV aggregates with fluorescence lifetime imaging

microscopy (FLIM).185

Herein, we extend our research endeavors in fundamentally understanding the optical,

electronic and morphological properties of broadband absorption energy transfer nanoparticles

by examining the excited-state dynamics of pure (undoped) and composite (doped) nanoparticle

suspensions consisting of BPPV and MEH-PPV. By combining steady-state absorption, steady-

67

state fluorescence spectroscopy and time-resolved fluorescence spectroscopy, changes in

polymer chain morphologies and polymer-polymer aggregation as a function of doping

concentrations of MEH-PPV, from 0 wt% to 100 wt%, were studied. Similar to the OPPV

aggregates,184,185

these nanoparticles display multiple fluorescence lifetime components with

respect to the molecular solutions, indicating that the pure and composite nanoparticles consist of

different types of emitting chains that differ in their interchain and intrachain interactions.

Moreover, the broadband absorbing composite nanoparticles depict an enhanced Fӧrster energy

transfer mechanism with higher doping concentrations of MEH-PPV, implying a higher

probability of intermolecular interactions between BPPV and MEH-PPV with respect to the

BPPV to MEH-PPV doping ratio. Perhaps most interesting is the observed photophysical and

optoelectronic properties of the 30 and 40 wt% composite nanoparticles, which deviate from the

monotonic trends observed in the remaining nanoparticle compositions. We hypothesize that

these enhanced photophysical and optoelectronic properties result from nanoscale morphology of

well mixed molecular-like and aggregate-like polymer chains with increased interfacial

interactions between donor and acceptor molecules, which consequently leads to a more efficient

non-radiative energy transfer process from BPPV to MEH-PPV.

5.2 Results and Discussion

5.2.1 Steady-state spectroscopy for pure (undoped) and molecular solutions and

nanoparticle suspensions

Figure 23 displays the absorption and photoluminescence spectra, and time-resolved

fluorescence decays of neat BPPV and MEH-PPV molecular solutions in THF and the

corresponding aqueous nanoparticle suspensions. The absorption spectra of BPPV (blue, dash

68

line) and MEH-PPV (orange, dash line) molecular solution in THF are shown in Figure 23a with

peak absorption wavelengths of 353 nm and 500 nm, respectively. As expected the absorption

spectra display broad and featureless vibronic structure, indicating that chemical and physical

defects along the polymer backbone causes individual chromophores to adopt various

conjugation lengths which absorb different energies.32-34

The fluorescence spectra (Figure 23a,

solid lines) of the molecular solutions in THF display red shifted emission maxima with respect

to the absorption spectra. . The emission maximum of BPPV (blue, solid line) is located at 466

nm with a blue shoulder at ~415 nm, while the emission maximum of MEH-PPV (orange, solid

line) is located at 552 nm with a red shoulder at ~590 nm. In the case of BPPV, the broad and

blue shoulder emission indicates that intramolecular energy transfer is significantly reduced

which likely results from poorly coupled intrachain interactions between portions of short chain

chromophores and low energy chromophores. This hypothesis is consistent with previously

report tetrahedral defected MEH-PPV studies, demonstrating that the magnitude of

conformational disordered leads to multiple emitting chromophoric species.187,188

On the other

hand, MEH-PPV displays a vibronic structured fluorescence spectrum which clearly indicates

that excitation energy from high energy chromophores efficiently funnels to lower energy

chromophores during the exciton lifetime, a result indicative of an intramolecular Fӧrster energy

transfer process.38,44

The absorption (dash line) and fluorescence (solid line) spectra of undoped BPPV (red) and

MEH-PPV (green) nanoparticle suspensions in aqueous solution are shown in Figure 23b. The

absorption maxima of BPPV and MEH-PPV nanoparticles, each in their pure form, are located at

353 nm and 502 nm, respectively. While the nearly unchanged absorption spectra with respect to

solution for BPPV nanoparticles suggest that intrachain and interchain electronic interactions

69

remain unaltered in the ground state during nanoparticle assembly, the MEH-PPV nanoparticles

absorption spectrum displays a red tail with respect to the molecular solutions. The broadening

absorbance spectrum at the red spectral side indicates that interactions between chromophore

segments on the polymer chain alter the ground state during nanoparticle assembly, a result

indicative of aggregate states.123,189

Additionally, the corresponding fluorescence spectra for

both BPPV and MEH-PPV nanoparticles are red shifted by ~40 nm with respect to molecular

solutions in THF, an observation indicative of enhanced inter- and intrachain pi-stacking

interactions.38,44,47,174

For the undoped BPPV nanoparticles, the emission maximum is located at

504 nm compared to the molecular solution in THF. Interestingly, the intramolecular blue

shoulder emission of the molecular solution, ascribed to short chain chromophores with weak

coupling interactions to low energy chromophores, is absent in the undoped BPPV nanoparticles

suspension. This may allude to changes in molecular conformation upon stacking of BPPV

chains in the solid state material, or efficient exciton migration to lower energy π-stacked regions

in the nanoparticle (i.e. intramolecular and intermolecular aggregates). In the case of undoped

MEH-PPV nanoparticles, the emission maximum is located at 596 nm, and the emission spectra

exhibit a red shoulder at ~640 nm that is enhanced with respect to molecular solution. The

enhanced red shoulder of MEH-PPV implies efficient exciton migration to emitting aggregate

species in the pure (undoped) nanoparticles, which is in agreement with previous studies of semi-

conducting polymer nanoparticles109-111

and films.130,190-192

70

Figure 23: Undoped absorption and fluorescence spectra and fluorescence decay profiles. a)

Normalized absorption (dash line) and fluorescence (solid line) spectra for BPPV (blue) and

MEH-PPV (orange) dissolved in THF. BPPV and MEH-PPV obtains peak wavelength

absorption maximum at 353 and 500 nm, respectively. BPPV emission maximum is located at

466 nm with a blue shoulder emission at ~425 nm. The emission maximum for MEH-PPV is

located at 552 nm with a red shoulder emission at ~610 nm. b) Normalized absorption (dash line)

and fluorescence (solid line) spectra for BPPV (red) and MEH-PPV (green) nanoparticles

suspended in deionized water. Compared to the molecular solutions, BPPV and MEH-PPV

absorption maximum remain constant, while the emission maximum are red shifted to 504 and

596 nm for BPPV and MEH-PPV, respectively. MEH-PPV displays an enhanced emission from

71

the aggregate emitter (630 nm), while BPPV blue shoulder emission is significantly reduced

compared to the molecular solutions. c). BPPV exhibits a bi-exponential decay behavior with an

averaged lifetime of 1.5 ns, while MEH-PPV in THF display a single excited specie with a

lifetime of 0.25 ns. d) Fluorescent decay profiles of BPPV at 504 nm (red) and MEH-PPV at 590

nm (green) and 630 nm (cyan) nanoparticles suspensions. BPPV nanoparticle obtain averaged

lifetimes of 100 ps. MEH-PPV display emission dependencies with averaged lifetimes of 93 and

136 ps for 590 and 630 nm, respectively. Individual lifetime components and contributions are

summarized in Tables 1-2.

5.2.2 Time-resolved spectroscopy of pure (undoped) molecular solutions

In Figure 23c, the PL decay profile of BPPV dissolved in THF (blue line) is depicted. The

photophysical properties of BPPV exhibit bi-exponential excited-state behavior with individual

lifetimes components (and corresponding contributions, weighted amplitudes) of 1.7 ns (83%)

and 0.35 ns (17%). These individual lifetime components correspond to an average lifetime of

1.5 ns. It has been well documented that the PL lifetime increases with respect to decreasing

conjugation length.193-195

More recently, Lim et al. demonstrated that MEH-PPV of short

conjugation length exhibits excited-state lifetimes ranging from 1.2-1.4 ns.196

By taking the

molecular weight (Mw = ~2200) and the molecular weight per monomer (Mw = 362) of BPPV

into account, these data suggest that the BPPV polymer chains obtain relatively short conjugation

length which is in agreement with the long averaged lifetime. However, the bi-exponential

behavior of BPPV implies that the dilute molecular solution consists of multiple emitting

species. Wang et al. recently reported bi-exponential behavior in ethylene glycol substituted

poly(phenylene vinylene) derivatives.197

and ascribed the long (>1.1 ns) and short (<0.5 ns) PL

lifetimes to unquenched and partially quenched chromophore segments, respectively. With the

72

latter arising from exciton trap sites (such as H-aggregation138,154

or photooxidized chromophore

segments)22,137,198

that form on the polymer chain and partially or completely quench the

excitations.171

Therefore, the assumption is made that a small portion of the short conjugated

segments in the molecular solution of BPPV acting as energy traps are partially quenched (0.35

ns, 17%). While the majority of the short conjugated segments in BPPV account for unquenched

chromophores that exhibit long lifetimes (1.7 ns, 83%).

In the case of MEH-PPV dissolved in THF, a single exponential function was fitted to the PL

decay indicative of the presence of a single excited species. The observed PL lifetime is 240 ps,

which is in good agreement with previous literature reports of single emitting specie in dilute

molecular solutions of MEH-PPV.136,171,199

The radiative lifetimes (τr) of BPPV and MEH-PPV in THF solution were estimated by

means of the measured PL lifetimes and the quantum efficiencies of BPPV and MEH-PPV

determined in solution. The MEH-PPV radiative lifetime was determined to be 1.2 ns by

Equation 7,

Φ

where kr and knr are the radiative and nonradiative rate constants, and τ and τr are the measured

lifetime and radiative lifetime, respectively. For BPPV, the radiative lifetime was estimated to be

3.4 ns by Equation 8,

Φ

73

where α1and α2 represent the weighted amplitude contributions of τ1 and τ2, respectively.199

These

estimated radiative lifetime calculations are used in the following sections to discuss the

presence/absence of excited-state complexes in undoped and doped nanoparticle suspensions.

5.2.3 Time-resolved spectroscopy of pure (undoped) nanoparticle suspensions

Figure 23d shows the PL decay profiles for the undoped BPPV and MEH-PPV nanoparticle

suspension in aqueous solution. From these data it can be clearly seen that the PL decays faster

for the undoped nanoparticles compared to the corresponding dilute molecular solutions. In both

nanoparticle suspensions, the PL decay profiles were modeled with multiexponential functions,

indicating the presence of multiple emitting species that may be due to differences in polymer

chain aggregation and local environment of the exciton.171

. For BPPV, the fitting procedure

yields lifetimes (contributions) of 0.91 ns (1%), 0.24 ns (18%) and 0.06 ns (81%). In the case of

MEH-PPV, the fitting procedure yields lifetimes (contributions) of 0.80 ns (2%), 0.22 ns (16%)

and 0.04 ns (82%) for 590 nm emission, and 0.92 ns (5%), 0.29 ns (18%) and 52 ns (77%) for

630 nm emission. The lifetime components of the undoped MEH-PPV nanoparticles exhibit

similar fluorescence decay components previously reported by Collison et al. for MEH-PPV in

reduced solvent quality medium.201

Additionally, the undoped MEH-PPV nanoparticles display

detection wavelength dependencies in comparison to the dilute molecular solution of MEH-PPV.

This observation is indicative of enhanced interchain interaction and aggregate emission, as

supported by the enhancement of the aggregate shoulder emission of the undoped MEH-PPV

nanoparticles with respect to the molecular solution.111

Compared to molecular solution the undoped BPPV and MEH-PPV nanoparticle suspensions

exhibit reduced amplitude-weighted averaged lifetimes. In the case of BPPV, the nanoparticle

suspension obtains an averaged lifetime of 100 ps. For the MEH-PPV nanoparticle suspension,

74

the averaged lifetimes range from 85-93 ps and 107-137 ps at the detection wavelengths of 590

and 630 nm, respectively. These reduced averaged lifetimes imply that new nonradiative

pathways are created or preexisting nonradiative pathways are enhanced during nanoparticle

assembly and polymer chain aggregation. In fact this observation correlates directly with the

reduced quantum efficiency of BPPV (0.01) and MEH-PPV (0.02) nanoparticle suspensions

resulting from weakly emissive interchain trap sites with respect to the molecular solution.135-137

These findings are discussed in detail in the following section based on the individual lifetime

components found for the nanoparticles, and will be correlated with molecular solution and solid

state properties.

Turning our attention to individual lifetime components (contributions) of the undoped

BPPV and MEH-PPV nanoparticle suspensions, the excited state dynamics are governed by fast

(τ3), intermediate (τ2) and slow residual (τ1) lifetime components.

The long PL lifetime decay components found for the BPPV and MEH-PPV nanoparticles

are 0.91 ns and 0.80 ns, respectively, with very small (<2%) weighted amplitude contributions.

With respect to the molecular solution experiments completed in our studies for these

compounds, this is a new and longer lifetime component in the case of the undoped MEH-PPV

nanoparticles. The τ1 lifetime may be attributed to emission from MEH-PPV chains that

compartmentalize in the nanoparticles and act as isolated polymer chains or shorter conjugation

length polymer segments that contain weak interchain interactions. As such, these chains are

compartmentalized from the bulk of the MEH-PPV nanoparticles, suggesting that this emission

component may arise from chains on the nanoparticle surface. A similar core-shell model has

been recently reported for aggregates fabricated from PPV oligomers.184,202

Scheblykin et al.

recently studied the excited-state dynamics of MEH-PPV dispersed in PMMA as a function of

75

MEH-PPV concentration in order to elucidate effects of interchain aggregation. The findings of

these investigators demonstrate that concentrated film-like samples of MEH-PPV embedded in

PMMA matrix exhibit non-exponential decay dynamics with the major lifetime component at 0.3

ns, which are assigned to aggregation of MEH-PPV chains, while isolated MEH-PPV chains

show lifetimes in the range of 0.4 to 1.2 ns.203,204

Moreover, Sherwood et al. 184,185

recently

proposed a core-shell model for PPV oligomer aggregates where the two emitting species likely

occur from differences in the strength of interchain interactions as a result of variations in

polymer chain packing. For this model the PPV oligomer aggregates consist of a core with

“aggregate-like” polymer chains (short-lived dynamics) obtaining strong interchain interactions,

while a shell of “monomer-like” polymer chains (long-lived dynamics) exhibiting weak

interchain interaction surrounds the aggregate core. Smith et al.205

more recently reported long-

lived PL decay components in MEH-PPV thin films cast from various solvents that are similar to

τ1 decay component of the undoped MEH-PPV nanoparticles and, thus, suggesting emission

from short conjugated length chromophores due to reduced interchain energy migration or

differences in the conjugation length of individual polymer chains. When comparing the

fluorescence spectra associated with the long-lived decay component in these studies to the

undoped MEH-PPV nanoparticles. It is clearly observed that the undoped nanoparticles do not

exhibit blue site emission, a result consistent with single particle spectroscopy studies.106,111

Consequently, the spectral behavior of the undoped MEH-PPV nanoparticles indicates that the τ1

lifetime likely results from shorter conjugation length segments rather than variation in the

strength of interchain interactions (i.e. blue emitting chromophores due to reduced energy

migration from high energy chromophores to low energy chromophores). When considering the

substantial polydispersity (~5) of the samples received as-is, the long τ1 lifetime agrees well with

76

chromophores of shorter conjugation length, as reported in PPV oligomers and short MEH-PPV

chains. 193,194,196

Because our purification method of MEH-PPV in combination with the high

Mw are expected to virtually eliminate short MEH-PPV chains, we hypothesize that the long-

lived τ1 lifetime component of the undoped MEH-PPV nanoparticles may arise from polymer

chains possibly on/near the nanoparticle surface that contain weak interchain interactions, i.e.

small regions coexisting within the nanoparticle where polymer chains are only loosely

aggregated and exhibit limited electronic coupling with neighboring chains. This interpretation is

supported by (i) the core-shell model proposed by Sherwood et al.184,185

and (ii) the spectral and

photophysical properties of “highly” aggregated regions within MEH-PPV thin films, as reported

by Smith et al.205

In the case of the undoped BPPV nanoparticles, the t1 lifetime component decreases to 0.85

ns with respect to the 1.7 ns t1 lifetime component measured in dilute BPPV molecular solution.

The reduced t1 lifetime may be contributed to short chain excitations that efficiently migrate to

defect sites or undergo nonradiative quenching. While intrachain exciton migration is known to

be a less efficient process than interchain migration (i.e., ~100 times slower),39

enhanced

interchain pi-stacking regions are expected upon nanoparticle aggregation. Thus, excitations

generated in the BPPV nanoparticles may obtain a significantly increased probability of finding

nonradiative chains, which may manifest into a reduced t1 lifetime component. On the other

hand, conformational changes have previously shown changes in fluorescence lifetimes because

of changes in the electronic coupling between the ground-state and excited-state manifold.175

An

observation previously associated to regions of enhanced nonradiative channels in dye-doped

and polymer-doped polymer systems.206,207

Although, the lifetime measurements cannot solely

distinguish which proposed mechanism dominates the reduced t1 lifetime component of the

77

undoped BPPV nanoparticle suspensions, these interpretations coincide closely with the

mechanisms proposed for the reduced blue shoulder emission of the undoped BPPV nanoparticle

suspension (i.e., changes in molecular confirmation upon stacking or efficient exciton migration

to low energy pi-stacked regions).

Alternative interpretations of these data have also been considered. Rothberg et al. previously

reported long-lived PL decay components for PPV and MEH-PPV.137,201

The nanosecond decay

component of PPV was attributed to relaxed intrachain exciton formed through interchain

excitons, while the singlet state of MEH-PPV was attributed to back transfer from nonradiative

interchain excited states to intrachain excited states. In addition, time-resolved PL spectroscopy

studies previously uncovered long lifetime components in conjugated oligomers, polymers and

cofacially aggregated molecules that correspond well with the τ1 component found for the BPPV

and MEH-PPV nanoparticle suspensions.34,123,153,174,208-210

These long lifetime components are

indicative of an intermolecular excited-state complex (i.e. excimer), which show distinct

unstructured red shifted emission and PL lifetimes that are longer than the molecular solution

radiative lifetime of the respective monomer. For example, films and oligomer aggregates

containing electronegative functional groups such as cyano- display excimer emission with

lifetimes approximately three times longer than the radiative lifetime.153,186,209,210

While oligo-

PPV aggregates have displayed long lifetimes of 2-4 ns, these components contribute to ≤20% of

the total weighted amplitude.184

The data presented within this paper provide little evidence for

assigning the τ1 lifetime in the undoped and doped nanoparticles to an excited-state complex,

since excited-state complex spectral properties where not observed in the steady-state

spectroscopy, and the τ1 lifetime is substantially shorter than the radiative lifetime of BPPV (3.4

ns) and MEH-PPV (1.2 ns). However, nonradiative excimer emission should not be ruled out.

78

This process would compete with overall photophysical properties of the nanoparticle by

reducing fluorescent lifetime, and was shown to decay through a nonradiative pathway at room

temperature.138,211,212

Second, the long lifetime may result from chromophores with shorter

conjugation lengths, as reported in PPV oligomers and short MEH-PPV chains, when

considering the substantial polydispersity of the samples received as-is.193,194,196

Our purification

method of MEH-PPV in combination with the high Mw is expected to virtually eliminate this

issue. However, in the case of BPPV this does not preclude that the polymer chains intrinsically

consist of short conjugation length chromophore segments. Finally, the polymer nanoparticles

may consist of polymer chains that stack as weakly emissive H-aggregates. . While the strongly

reduced quantum yields of the nanoparticles and longer lifetime component with respect to

molecular solution suggest the presence of weakly emissive H-aggregates, the absorption spectra

of the conjugate polymers remain unperturbed. The latter does not fit well with the typical

observations made for H-aggregates, specifically the occurrence of blue shifted absorption

spectra129,213

and long excited-state dynamics with respect to molecular solution that were

previously reported for aggregates and thin films of organic materials such as PPV, thiophene

oligomers and P3HT.149,156,214-216

.

The intermediate τ2 lifetimes of MEH-PPV (0.22ns) and BPPV (0.24ns) nanoparticles

are consistent with the observations made for MEH-PPV and BPPV molecular solutions, as

reported above and in literature.136,171,199

These lifetimes can be attributed to emission from

partially quenched chromophores, in part due to the intrinsic non-radiative decay of these

polymers as well as aggregation of the chains that leads to efficient exciton migration on

individual chains and in the bulk of the nanoparticles. This is rationalized by a Franck-Condon

model of aggregated conjugated polymers, where the polymer aggregate consist to dual emission

79

from molecular-like (resembling the singlet state emitter) and aggregate

emitters.109,110,125,184,190,217,218

The BPPV and MEH-PPV nanoparticles exhibit a fast decay component (τ3) of 40ps and

60ps, respectively, with >80% of the overall amplitude weighted contribution. This is the major

decay component, which is assigned to emission from chromophores that are quenched by intra-

and intermolecular energy transfer to chemical or structural defects along the polymer backbone

and in the bulk of the nanoparticles, as well as aggregates.31,34,38-40,219

While intramolecular

energy transfer primarily occurs in polymer chains with extended confirmations (i.e. good

solvents), thin films and nanoparticles, for example, are dominated by more efficient

intermolecular energy transfer.44,106,111,201

This was previously demonstrated in quantum

mechanical simulations.31

In addition, it has also been shown that aggregate emission exhibits a

slow PL rise time upon pulsed excitation, which again suggests that aggregates are excited for

the most part through an energy transfer process originating from surrounding polymer

chains.168,181,220-222

While future experiments are planned to investigate the aggregate species by

pulse excitation , these literature reports give aggregate lifetimes similar to the short lifetime

component observed for our undoped and doped nanoparticle suspensions.

On the basis of the above considerations, the undoped nanoparticle suspensions reported

herein are representative of a core-shell assembly of interacting polymer chains that are governed

by the following excited-state dynamics: isolated-like molecules (τ1), molecular-like (τ2) and

lifetime component that resemble aggregate emission or energy transfer process from polymer

chains to aggregates (τ3). ). Moreover, it is important to note that a three exponential decay

profile is observed at emission wavelength of 640 nm for the undoped MEH-PPV nanoparticle

suspension, while similar emitting species are observed at the emission wavelengths of 504, 590

80

and 630 nm for the doped nanoparticle suspensions. These data are discussed in greater detail in

the proceeding sections.5.2.4 Steady-state spectroscopy of composite (doped) molecular

solutions and nanoparticle suspensions

In Figure 24, the absorption and fluorescence spectra of undoped and doped molecular

solutions in THF and the corresponding nanoparticles suspended in deionized water are shown.

Steady-state and single particle spectroscopy of these composite broadband absorbing molecular

solutions and nanoparticle suspensions were recently studied in detail.183

The absorption spectra

obtained for molecular solutions and shown in Figure 24a depict a peak wavelength maximum of

353 nm and 500 nm for BPPV and MEH-PPV, respectively, with absence of vibronic structure.

For the nanoparticle suspensions shown in Figure 24b, the vibronic structure of the absorption

spectra remains nearly unchanged with respect to the molecular solutions in THF and pure

(undoped) nanoparticle suspensions. As the dopant level of MEH-PPV increases per

nanoparticles composition, the absorption spectra peak wavelengths for BPPV and MEH-PPV

appear to blue shift with respect to the molecular solutions and undoped nanoparticle

suspensions. For example, the peak wavelength for BPPV in the 5 wt% absorption spectrum blue

shifts to 340 nm, while the peak wavelength for MEH-PPV in the 75 wt% absorption spectrum

blue shifts to 510 nm. At first glance these data suggest that during nanoparticle assembly

excesses bending and kinking along the polymer backbone, leads to polymer chains with reduced

conjugation length that collapse into coiled structure.44,100,106,111,128

While coiled MEH-PPV

chain confirmations were previously reported for undoped and doped nanoparticles,76,77

there are

several reasons to believe that the polymer chains of BPPV and MEH-PPV adopt different

confirmations during nanoparticle assembly. In the case of BPPV, proper deconvolution of these

doped absorption spectra may lead to a doped BPPV absorption maximum that resembles the

81

undoped nanoparticle suspension, resulting from the additional ultraviolet vibronic progress of

MEH-PPV that may distort the actual absorption maximum. This data suggest that doping does

not affect the interchain and intrachain electronic interactions of BPPV in the ground state during

nanoparticle assembly. On the other hand, when comparing MEH-PPV absorption maximum of

the doped nanoparticles to undoped MEH-PPV nanoparticle absorption maximum, it is clearly

illustrated that the absorption maxima red shift by greater than 10 nm as the dopant levels of

MEH-PPV decreases. The fact that MEH-PPV may adopt a defected cylinder confirmation with

excess interchain and intrachain π-stacking interactions is inconsistent with previous MEH-PPV

studies of aggregates and concentrated solutions that demonstrate red shifts in the absorption

spectra.44,128

However, a nanoparticle morphology that consist of MEH-PPV molecules located

in segregated regions that are tightly packed into collapsed defected cylinder confirmations or a

“highly” aggregated regions that share an overall enhancement in the π-stacking interactions may

account for the red shifted MEH-PPV absorption maximum in the doped nanoparticle

suspensions.183

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Figure 24: Bulk spectroscopy of undoped and doped molecular solutions and nanoparticle

suspensions as a function of MEH-PPV doping level. a) Absorption spectra of molecular

solutions in THF, resulting in a constant absorption maxima for BPPV (353 nm) and MEH-PPV

(500 nm). b) Absorption spectra of nanoparticle suspensions displaying at constant BPPPV

absorption maximum at 353 nm, while the maxima of MEH-PPV red shifts of 525 nm with

respect to a decreased doping level. c) Fluorescence spectra of molecular solutions in THF,

depicting a constant BPPV emission maximum at 466 nm with an appearance of MEH-PPV (552

nm) emission at high doping levels. Solutions were excited at 350 nm with constant excitation

and emission slit widths of 0.5 and 0.5 nm, respectively. d) Fluorescence spectra of nanoparticle

suspensions excited at 350 nm with constant slit widths of 2 and 2 nm, respectively. The

emission intensity of BPPV is shown to decrease and blue shift as the doping of MEH-PPV

83

increases, while the emission intensity of MEH-PPV monotonically decreases and red shifts to

600 nm. Exceptions to the observed MEH-PPV emission trend occur at 5, 30 and 40 wt%. In the

case of 5 wt%, MEH-PPV emission maximum is located at 587 nm while the emission intensity

is reduced. For 30 and 40 wt%, enhanced emission intensity is observed with respect to the other

nanoparticle compositions.

The fluorescence spectra of the doped molecular solution dissolved in THF are depicted in

Figure 24c. These data display similar fluorescence spectra to the corresponding undoped

molecular solution of BPPV. The spectral feature at 552 nm is due to superposition of MEH-PPV

emission resulting from direct excitation of MEH-PPV that becomes apparent at high doping

levels. For the nanoparticle suspensions shown in Figure 24d, the fluorescence spectra display

red shifted emission maximum with respect to the molecular solutions, a result consistent with

enhanced inter- and intrachain pi-stacking interactions.38,44,47,174

In addition, the vibronic

structure of the emission spectra obtain for doped nanoparticle suspensions are the superposition

of the undoped nanoparticle suspensions of BPP and MEH-PPV, suggesting that excited-state

complexes do not occur. When comparing the emission maxima of the undoped BPPV (500 nm)

and MEH-PPV (596 nm) nanoparticles to the peak wavelengths of the doped nanoparticle

suspensions, it is clearly illustrated that the peak wavelengths in the doped nanoparticle

suspensions blue shifts for both BPPV and MEH-PPV. For example, the peak wavelength of

BPPV blue shifts to 480 nm for the 75 wt% nanoparticle suspension, while the peak wavelength

of MEH-PPV blue shifts to 576 nm for the 5 wt% nanoparticle suspension. These data imply that

the conjugation length of BPPV and MEH-PPV reduces as a result of steric effects that are

imposed by the opposite component. Perhaps more interestingly is the change in the relative

intensity of both BPPV and MEH-PPV per composition. In Figure 25 (blue line), the Stern-

84

Volmer relationship displays hyperefficient quenching of BPPV emission intensity with respect

to increased doping levels of MEH-PPV, a results consistent with other blended organic

nanoparticles and representative of a dynamic Fӧrster energy transfer process.117,118,183

On the

other hand, the emission intensity of MEH-PPV (Figure 25, black line) gradually increases with

respect to decreasing the dopant level of MEH-PPV, a finding indicative of reduced aggregate

quenching (i.e. reduced intrachain π-stacking interaction that form trap sites). However,

deviation from the monotonic increase of MEH-PPV emission intensity is observed at the doping

levels of 5, 30 and 40 wt%. For the 5wt% nanoparticle suspension, the emission intensity of

MEH-PPV is slightly reduced in comparison to the 10 wt% nanoparticle suspension, suggesting

of an incomplete energy transfer process to a limited number of segregated polymer chains (i.e.

the excitons of BPPV recombine to the ground state before effectively locating MEH-PPV

molecules during exciton diffusion over the exciton lifetime). Based on the absorption spectra of

the doped nanoparticle suspension, we have approximated the total number of MEH-PPV

molecules present in a 5 wt% nanoparticle by assume that MEH-PPV molecules adopt a defected

cylinder confirmation (r = 2.5 nm; h = 10 nm) previously reported by Barbara et al. This packing

arrangement indicates that a undoped 36 nm diameter MEH-PPV nanoparticle (i.e. 100 wt%)

consist of approximately 124 MEH-PPV molecules and, thus, a composite nanoparticle of the

same diameter but doped with 5 wt% of MEH-PPV accounts for approximately 6 MEH-PPV

molecules present in a composite nanoparticle. Such calculations support the hypothesis of an

incomplete energy transfer process segregated MEH-PPV molecules. Additionally, these

calculations support the hypothesis of incomplete energy transfer to a region of highly

aggregated MEH-PPV polymer chains, where the reduced emission intensity may result from a

combination of (i) incomplete energy transfer described above and (ii) enhanced intrachain π-

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stacking interaction, in other words, enhanced aggregate quenching by a limited number of

“highly” aggregated polymer chains. Wu et al. also observed similar spectral characteristics in

composite nanoparticle with low dopant concentrations. In contrast to the 5 wt% nanoparticle

suspension, the concentration range of 30 and 40 wt% exhibit an enhanced MEH-PPV emission

intensity which is clearly more intense than the remaining composite nanoparticle suspensions.

Several studies have previously demonstrated that the FRET emission intensity and spectral

shape of the acceptor in composite thin films and nanoparticles is dependent on the dopant

concentration, especially for dye doped systems. The investigators contribute these

photophysical phenomena to the extent of dopant segregation or aggregation. Consequently, the

fluorescence spectra of the 30 and 40 wt% nanoparticle suspensions signifies a mixed molecular-

like and aggregate-like morphology that efficiently transfers energy nonradiatively from BPPV

to MEH-PPV whereby photons are emitted from MEH-PPV before decaying through a

nonradiative channel, i.e. MEH-PPV molecules are “uniformly” distributed throughout the

nanoparticle matrix which exhibits limited polymer-polymer segregation or aggregation

86

Figure 25: Changes of MEH-PPV and BPPV emission intensity for undoped and doped

nanoparticle suspension as a function of the doping weight percent of MEH-PPV. s The

quenching with elevated dopant levels, while the . With elevated dopant levels of MEH-PPV, the

BPPV (blue line) emission intensity is shown to quench hyperefficiently. In the case of MEH-

PPV (black line), the emission intensity gradually decreases and deviates from the monotonic

trend at a dopant level of 5, 30 and 40 wt%.

5.2.5 Time-resolved spectroscopy of composite (doped) nanoparticle suspensions.

In Figure 26, the raw PL decay profiles and exponential fits for the doped nanoparticle

suspensions are shown and numerically provided in Tables 2-3 using the same experimental

parameters as the undoped nanoparticle suspensions (i.e. donor (375 nm) and acceptor (490 nm)

excitation wavelengths while detecting emission maxima of the BPPV (504 nm) and MEH-PPV

(590 nm) nanoparticles and the 630nm red shoulder of MEH-PPV). When probing the

87

nanoparticle suspension at the donor excitation wavelength, in a concentration range from 5 to

75 wt%, the fitted decay profiles in Figure 26a-c yield individual lifetimes (contributions)

ranging from 0.71-0.95 ns (0.2-1.3%), 0.09-0.23 ns (3-18%) and 0.02-0.06 ns (81-97%) for

BPPV. For the same concentration range, the individual lifetimes (contributions) of MEH-PPV

that range from 0.85-1.4 ns (2.4-4.4%), 0.25-0.37 ns (14-31%) and 0.05-0.10 ns (65-83%) at a

detection wavelength of 590 nm, and 0.93-1.4 ns (4.0-6.2%), 0.29-0.37 ns (17-37%) and 0.05-

0.09 ns (57-79%) at a detection wavelength of 630 nm. When probing the nanoparticle

suspension at the acceptor excitation wavelength, in a concentration range from 5 to 75 wt%, the

fitted decay profiles in Figure 26d-e yield individual lifetimes (contributions) of MEH-PPV that

range from0.84-1.4 ns (4.0-9.0%), 0.24-0.36 ns (17-26%) and 0.05-0.07 ns (65-81%) at a

detection wavelength of 590 nm, and 0.96-1.4 ns (6.0-14%), 0.29-0.39 ns (20-26%) and 0.05-

0.07 ns (60-74%) at a detection wavelength of 630 nm. In addition, the individual lifetime

components reported were used to calculate the amplitude-weighted averaged lifetimes of BPPV

and MEH-PPV per composition. While the averaged lifetimes for BPPV monotonically decrease

from 96 ps to 23 ps as the doping level of MEH-PPV increases from 5 to 75 wt%, the averaged

lifetime of MEH-PPV resides between 95 and 342 ps regardless of the excitation or emission

wavelengths. The resulting data were used to construct individual lifetimes (components) as a

function of MEH-PPV dopant level for each of the excitation and detection wavelengths. Several

key observations can be made from these data, which will be discussed in further detail in the

proceeding sections: (i) similar to the case of the undoped nanoparticle suspensions, the local

environment of the exciton and/or polymer chain aggregation may directly contribute to the

presence of these multiple emitting species,171

(ii) the averaged lifetimes of the doped

nanoparticle suspension are clearly shorter with respect to the molecular solutions, again, imply

88

that during nanoparticle assembly and polymer chain aggregation, preexisting nonradiative

pathways are enhanced or new nonradiative pathways are created and (iii) the averaged MEH-

PPV lifetimes of the doped nanoparticles are longer than the average lifetime of the undoped

MEH-PPV nanoparticle suspensions, suggest dissimilar but close morphologies and chain

stacking as the undoped nanoparticle suspensions.

89

Figure 26: Photoluminescent decay profiles and exponential fits of undoped and doped

nanoparticle suspension as a function of the doping percent of MEH-PPV. Nanoparticle

suspensions were excited with a 375nm (a-c) and 490nm (d-e) pulse excitation source, while the

PL decays were collected were collected at the emission maximum of BPPV (504±5nm; 3a) and

MEH-PPV (590±5nm; 3b, 3d), and the red shoulder emission of MEH-PPV (630±5nm; 3c, 3e).

90

Table 2: Individual lifetime components, contributions and averaged lifetimes using 375 nm

pulse excitation.

Table 3: Individual lifetime components, contributions and averaged lifetimes using 490 nm

pulse excitation.

Doping

Percent

(wt%)

Excitation

Wavelength

(nm)

Emission

Wavelength

(nm) t1 (pS) A1 (%) t2 (pS) A2 (%) t3 (pS) A3 (%) <t> (pS)

0 375 504 906±87.1 1.3±0.28 235±14.5 18±0.9 57±2.9 81±1.1 100±2.5

5 375 504 894±47.1 1.3±0.37 233±4.4 18±2.2 58±1.0 81±2.5 96±11.0

10 375 504 805±52.0 1.2±0.17 215±15.9 16±2.0 54±4.4 82±2.2 89±11.2

15 375 504 804±17.7 0.7±0.0 201±1.4 13±0.1 49±0.0 87±0.1 73±0.0

25 375 504 830±60.2 0.6±0.22 179±5.1 12±2.3 44±1.2 88±2.5 64±5.8

30 375 504 706±91.5 0.6±0.22 153±14.7 11±3.2 34±6.1 88±3.4 51±2.3

40 375 504 701±5.6 0.6±0.4 142±18.8 10±4.4 32±0.6 89±4.8 48±9.8

50 375 504 906±82.3 0.2±0.04 132±12.7 6±1.3 26±4.6 94±1.3 35±4.0

75 375 504 951±77.9 0.5±0.05 85±8.5 3±1.0 16±2.9 97±0.9 23±2.1

5 375 590 1374±24.8 4.4±1.17 374±21.3 31±6.1 103±7.2 65±5.1 242±2.6

10 375 590 1300±29.9 3.1±0.47 350±9.0 27±1.1 93±7.4 70±1.0 200±1.0

15 375 590 1226±85.5 3.1±1.44 337±54.3 27±3.1 91±11.4 70±4.6 193±52.0

25 375 590 991±47.1 2.3±0.45 270±15.3 22±1.1 66±4.0 76±1.5 132±11.4

30 375 590 1082±66.5 2.9±0.15 297±14.4 19±2.3 67±3.2 78±2.4 141±1.0

40 375 590 1056±14.7 3.2±0.51 298±15.9 19±6.1 63±9.1 78±6.6 139±26.3

50 375 590 1008±62.1 3.4±0.9 284±2.9 17±1.2 57±8.7 80±0.2 127±1.7

75 375 590 849±35.2 2.4±0.10 246±12.7 14±1.8 47±6.1 83±1.8 95±10.4

100 375 590 803±10.2 1.7±0.26 236±4.5 13±0.5 46±1.7 85±0.6 85±0.6

5 375 630 1396±25.6 6.2±1.80 366±11.3 37±3.8 92±13.4 57±2.6 270±10.8

10 375 630 1323±35.9 4.4±0.51 360±12.7 30±0.7 94±12.1 66±1.1 228±2.6

15 375 630 1257±61.2 4.3±2.63 357±36.9 27±6.4 90±4.0 68±9.0 216±65.4

25 375 630 1087±24.2 3.2±0.35 308±19.6 23±0.3 73±3.5 74±0.6 159±12.1

30 375 630 1138±61.7 4.4±0.79 335±22.1 21±2.0 72±3.5 75±1.5 174±12.5

40 375 630 1154±34.8 5.7±0.79 355±21.7 22±4.7 71±5.0 72±4.0 194±1.5

50 375 630 1105±73.5 5.7±1.9 352±29.3 19±0.7 66±3.5 75±1.6 182±26.0

75 375 630 929±8.1 4.0±0.46 294±15.0 17±1.6 53±4.0 79±1.1 130±0.6

100 375 630 869±17.4 2.6±0.38 273±3.2 16±0.4 50±1.5 82±0.7 107±3.8

Doping Percent

(wt%)

Excitation

Wavelength

(nm)

Emission

Wavelength

(nm) t1 (pS) A1 (%) t2 (pS) A2 (%) t3 (pS) A3 (%) <t> (pS)

5 490 590 1359±42.9 9±1.4 357±22.5 26±8.1 68±1.5 65±7.2 256±7.4

10 490 590 1289±59.3 7±0.8 330±17.6 23±2.2 58±1.0 70±1.4 207±11.1

15 490 590 1233±58.5 5±1.6 328±23.5 23±2.9 59±6.0 71±3.3 197±25.1

25 490 590 1057±46.4 5±1.1 274±13.9 21±2.6 52±4.0 74±3.4 150±21.7

30 490 590 1135±40.4 6±0.7 304±24.8 20±2.7 55±8.9 74±3.3 171±23.6

40 490 590 1073±59.3 5±1.2 291±27.1 22±5.2 56±11.0 73±5.8 159±36.4

50 490 590 980±134.2 4±1.6 271±53.2 20±4.7 54±15.0 76±5.9 139±50.7

75 490 590 843±92.0 4±0.7 239±30.8 17±2.9 47±8.2 81±4.2 119±9.2

100 490 590 803±57.5 3±0.7 224±17.4 16±1.6 42±2.3 82±2.3 93±10.3

5 490 630 1429±28.3 14±2.7 391±16.7 26±6.5 71±3.2 60±3.8 342±24.6

10 490 630 1347±26.4 13±2.6 376±12.3 24±1.3 65±2.5 64±1.7 297±25.2

15 490 630 1303±42.5 11±2.0 373±16.9 24±1.3 66±6.6 65±2.8 280±35.9

25 490 630 1171±55.5 8±1.7 337±5.0 23±1.5 60±6.2 69±3.1 209±22.6

30 490 630 1234±7.9 9±1.3 361±8.6 24±3.3 62±6.5 67±2.3 239±14.2

40 490 630 1139±23.6 8±0.7 342±5.1 23±2.9 63±9.6 68±3.7 219±25.4

50 490 630 1108±68.1 6±2.4 331±23.5 23±3.1 60±12.5 70±5.3 191±51.3

75 490 630 959±40.9 6±1.2 291±7.8 20±1.7 54±7.2 74±2.9 153±22.0

100 490 630 925±61.0 5±1.1 289±15.7 18±1.2 52±4.5 77±2.3 136±22.1

91

Figure 27: Averaged lifetime of undoped and doped nanoparticle suspensions excited at

375nm (solid line) and 490nm (dash line). The decay profiles were detected at 504 (black circle),

590 (orange circle) and 630nm (blue circles). These averaged lifetime display excitation and

emission dependencies with lifetimes that increase at 30 and 40wt% as the doping level of MEH-

PPV is increased. The average lifetimes at 504nm also display a general decrease with

decreasing doping levels.

In Figure 27, the averaged lifetime of the undoped and doped nanoparticle suspensions are

shown as a function doping weight percent of MEH-PPV and emission wavelength. The average

lifetimes of BPPV (black circles) display a monotonic decrease from 100 ps (0 wt%) to 23 ps (75

wt%),indicative of a dynamic nonradiative dipole-dipole energy transfer process (i.e. Fӧrster)

92

from BPPV to MEH-PPV. 71

Other donor-acceptor systems containing multiple emitting

components (i.e. nanoparticles and solid materials) previously observed this phenomenon. The

averaged lifetime components of BPPV (black circles) are depicted at donor (black line, solid) in

Figure 27 with respect to the MEH-PPV doping level. The average lifetime monotonically

decreases from 100 ps (0 wt%) to 23 ps (75 wt%); coincides with the BPPV decay profiles

(Figure 26a) and the extrapolated intensity projection (Figure 28a) that become shorter and

subside after 4ns. Previous studies have observed this phenomenon in other donor-acceptor

systems containing multiple emitting components (i.e. nanoparticles and solid materials), for

which, the observed decrease in excited state lifetime of the donor was described by an energy

transfer model.116,223

In addition, the hyperefficient quenching of BPPV and in other doped

conjugated polymer nanoparticle systems further supports the proposed dynamic nonradiative

dipole-dipole energy transfer process (i.e. Fӧrster) from BPPV to MEH-PPV.117,118,183

In the case of MEH-PPV, the averaged lifetimes plotted in Figure 27 show the excited state

dynamics at the MEH-PPV emission maximum (blue circles) and red shoulder emission (red

circles) at donor (black line, solid) and acceptor (black line, dash) excitation wavelengths with

respect to the doping level of MEH-PPV. Several key observations are made as the doping level

of MEH-PPV increases. First, when comparing the blue emitter (λems = 590nm, blue circles) of

MEH-PPV to the corresponding red emitter (λems = 630nm, orange circles), the average lifetimes

decrease with the exception of the 30 and 40 wt% nanoparticle suspension s. In addition, the

excited-state dynamics of the blue or red emitter display excitation wavelength dependencies, as

demonstrated by the reduction of the average lifetimes at 375nm pulse excitation with respect to

490nm pulse excitation. These general trends are further depicted in the extrapolated intensity

projections (Figures 28b-e) where the white dotted lines represent the doping weight percent of

93

MEH-PPV for a given nanoparticle composition. The data presented are indicative of excitation

and emission dependencies of the excited state dynamics, which were attributed to enhanced

interchain interaction from aggregate emission111

and primary excitation of high energy

chromophores followed by rapid energy transfer to low energy chromophores through interchain

and intrachain interactions.224

Second, ss the doping level of MEH-PPV increases, the averaged

lifetime of MEH-PPV decreases from 300 ps (5 wt%) to 100 ps (75 wt%), suggesting a transition

in morphology from isolated MEH-PPV molecules or a single region of densely packed MEH-

PPV molecules to aggregated MEH-PPV domains. In other words, at low MEH-PPV dopant

levels the excited-state dynamics exhibit similar fluorescence lifetimes as the molecular

solutions, while increasing the dopant level results in fluorescence lifetimes that are comparable

to enhanced interchain and intrachain pi-stacking interactions of the undoped MEH-PPV

nanoparticle suspension.. In the case of 5wt%, we have calculated that approximately 1-6 MEH-

PPV molecules reside within a 36nm diameter nanoparticle. This suggests that the nanoscale

morphology may result in spatially separated domains larger than the Forster distance (i.e. 2xR0)

or BPPV inability to locate MEH-PPV molecules over the polymer exciton diffusion length. For

either scenario, this would lead to inefficient Forster energy transfer process, while reducing the

5wt% nanoparticle emission intensity of MEH-PPV as shown in Figure 24b (red line). On the

other hand, increasing the relative concentration of MEH-PPV may lead to increased interchain

quenching effects (e.g., aggregate quenching) and, thus, a decreases in the emission intensity of

MEH-PPV as shown in Figure 24b (blue line). Finally, the 30 and 40 wt% nanoparticle

suspensions display enhanced excited-state dynamics with respect to the decreasing trend of the

averaged lifetimes, which directly correlates to the enhanced MEH-PPV emission intensity of 30

and 40 wt% in Figure 24b. This indicates, in both cases, that during nanoparticle assembly and

94

polymer chain packing events new radiative decay pathways are created or preexisting pathway

are enhance by constructing a mixed morphology of organized molecular-like and aggregate

emitting domains that efficiently transfer energy within the exciton diffusion length and

respective lifetime.

Figure 28: Extrapolated intensity projection from the raw decay profiles as a function of

doping weight percent of MEH-PPV. Nanoparticle suspensions were excited with a 375nm (a-c)

and 490nm (d-e) pulse excitation source, while the PL decays were collected were collected at

the emission maximum of BPPV (504±5nm; 4a) and MEH-PPV (590±5nm; 4b, 4d), and the red

shoulder emission of MEH-PPV (630±5nm; 4c, 4e).

95

Apart for the averaged lifetimes, the individual lifetime components (τ1,2,3) and

corresponding weighted amplitude contributions (A1,2,3) provide substantial insight into the

changing polymer-polymer and polymer-aggregate interactions which ultimately affects the

optoelectronic properties of the composite nanoparticle suspensions. The data plotted in Figures

29 and 30 again, displays excitation and emission dependencies of the excited state decays,

which are in agreement with the averaged lifetime data discussed above. Interestingly, the τ1,2

components for 30 and 40 wt% are reduced at the emission maximum of BPPV (Figure 24a,

black circles), while the τ1,2 components simultaneously increase at MEH-PPV emission

maximum (Figure 29a, orange circles) and red shoulder of MEH-PPV (Figure 29a, blue circles).

In fact the same trend is observed in the weighted amplitude contributions (Figure 29b),

implying that the unquenched and partial quenched chromophores of BPPV effectively transfer

energy more efficiently to MEH-PPV in the morphology of 30 and 40 wt% nanoparticles. This

data once more supports the hindrance of exciton migration or energy transfer to nonradiative

states which are manifest into increased emission intensities and fluorescence lifetimes for the 30

and 40 wt% nanoparticle suspensions.

96

Figure 29: Individual lifetime components and amplitude weighted contribution as a function

of MEH-PPV doping levels. These data are excited with a 375 nm pulse excitation sources,

while the PL decays were collected at 504 (black), 590 (orange) and 630 nm (blue). Left) the

individual life components illustrate a gradual reduction in lifetime with increasing doping levels

of MEH-PPV, while the lifetimes of 30 and 40wt% are increased. Inserted graph depicts the fast

τ3 lifetimes. Right) amplitude weighted contributions of the respective individual life

components, which display decreased A1and A2 and increased A3 contributions with respect to

increased dopant levels of MEH-PPV. Additionally, the relative weighted percentages τ1 and τ2

amplitudes are the opposite of τ3 amplitudes.

97

Figure 30: Individual lifetime components and amplitude weighted contribution as a function

of MEH-PPV doping levels. These data are excited with a 490 nm pulse excitation sources,

while the PL decays were collected at 590 (orange) and 630 nm (blue). Left) the individual life

components illustrate a gradual reduction in lifetime with increasing doping levels of MEH-PPV,

while the lifetimes of 30 and 40wt% are increased. Inserted graph depicts the fast τ3 lifetimes.

Right) amplitude weighted contributions of the respective individual life components, which

display decreased A1and A2 and increased A3 contributions with respect to increased dopant

levels of MEH-PPV. Additionally, the relative weighted percentages τ1 and τ2 amplitudes are the

opposite of τ3 amplitudes.

Additionally, the insert in Figures 29 and 30 depict a reducing τ3 component and rising

A3 contribution with respect to increased doping levels. In the case of BPPV, we would expect

the opposite trend if the τ3 component was governed by aggregate emission (i.e. the contribution

of aggregate emissive should be higher for undoped BPPV nanoparticles). The data, therefore,

suggest that the resulting τ3 component of BPPV consist of energy transfer from polymer chain

98

to aggregate species, as previously discussed in the undoped nanoparticle sections. It should be

noted that the observation is purely speculative since the dynamic energy transfer mechanism

would ultimately result in quench of the donor emission with respect to increased doping levels.

As for MEH-PPV, reducing and rising trend of τ3 and A3 alludes to the presence of aggregate

quenching and/or energy transfer from molecular emitters to aggregate emitters. Assessment of

the relative A3 contribution at 590nm and 630nm it is clear that the excited-state dynamics at

590nm display a higher percent contribution, which point to more a likely scenario of facilitate

energy transfer from molecular to aggregate emitters. On the basis of specifically assigning the

excited-state dynamic mechanism responsible for the τ3 contribution in MEH-PPV, additional

experiments are planned to probe the aggregate specie at longer wavelength. These studies

should provide insight into the initial energy transfer process from polymer chains to trap sites,

or intermediate decay times which transfer from polymer chain to aggregates.

5.3 Conclusion

Broadband absorbing nanoparticles consisting of Poly[(o-phenylenevinylene)-alt-(2-

methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV) and Poly[2-methoxy-5-(2-

ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were fabricated by the reprecipitation

method to address the issues of morphology, intermolecular interactions and efficient light

absorption in polymer-polymer organic photovoltaic devices (OPVs). These nanomaterials

provide a model system that maintains the functionality of the bulk material, while relating the

material function to molecular level properties. Time-correlated single photon counting (TC-

SPC) was applied to fundamentally excited-state properties of the resulting broadband absorbing

composite nanoparticles. TC-SPC demonstrates that the composite nanoparticle suspensions

consist of a dynamic fluorescent resonance energy transfer mechanism involving molecular and

99

aggregate emitters. Surprisingly, with nanoparticle suspensions with 30 and 40 weight

percentage (wt%) of MEH-PPV display enhanced fluorescence emission and lifetimes,

suggesting a mixed molecular-like and aggregate morphology. These data clearly illustrate that

the nanoscale morphology of densely packed (i.e. solid state) broadband absorbing composite

nanoparticles is extremely complex and heterogeneous due to changing polymer chain

morphologies and changing polymer-polymer aggregation, which ultimately affect the optical

and electronic properties of the bulk materials found within OPVs.

100

CHAPTER 6: INFLUENCES OF FULLERENE DOPING IN BROADBAND

ABSORBING ENERGY TRANSFER CONJUGATED POLYMER

NANOPARTICLES

6.1 Introduction

Organic photovoltaic (OPV) devices have attracted increasing attention because of the

potential advantages that include large area, light weight and flexible devices through low cost

solution processing.225

Over the past decade, considerable progress has been made in the power

conversion efficiency (PCE) of OPVs, as evidenced by the increase PCE from 1% to 9%.226

In

most cases, OPVs adopt a bulk heterojunction (BHJ) structure, where the light harvesting active

layer consist of a phase separated blend of donor and acceptor materials21,56

(e.g., the most

prominent BHJ mixture of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl C61-butyric acid

methylester (PCBM)227

). Because conjugated polymers harvest a limited amount of the solar

energy, extensive research efforts have been devoted to improving the light harvesting efficiency

with the use of low band gap polymers and dye sensitization. In the case of low band gap

polymers, it is well recognized that high PCE are achieved when simultaneously optimizing the

polymer band gap and energy levels.228

While the latter is a simple and versatile method for

adding a third light absorbing component, dye aggregation typically results in reduced charge

mobility or absorption efficiency of the active layer.66,71,229-231

On the other hand, blending two

or more conjugated polymers offers a unique opportunity to enhance the light harvesting ability

and open circuit voltage of OPV devices (See reference within Advanced Materials 2009, 21,

3840-3850); however, efficient charge separation typically requires the addition of fullerene

derivatives. More importantly, the fundamental understanding of the principle phenomena and

mechanisms of the solid-state films such as charge transfer/transport, chain-chain interactions

and morphological aggregation, must be unraveled in order to increase device efficiency.21,44,232-

101

237 For example, aggregation in solid-state films provides good charge transport through strong

interacting pi-electron systems, but evidently induces emissive quenching traps and

heterogeneous morphologies which can greatly affect the device performance.138,155,174

238

Consequently, the photophysical and optoelectronic properties of conjugated polymer

materials have attracted a lot of scientific interest with respect to the bulk material morphology.

Single molecule spectroscopy (SMS) has been proven to be very successful for unraveling the

complex excited-state dynamics of conjugated polymers such as correlating spectroscopic

properties of the solid-state material to different morphology that can be adopted by single

polymer chains.45,47,55,237

Additionally, it was discovered that extended chromophore segments

undergo ultrafast energy transfer processes from high energy chromophores to low energy

chromophores (i.e., energy funneling), which results in photophysical phenomena that includes

fluorescence blinking, single-step photobleaching and variations in the fluorescence spectral

properties with respect to the preparation conditions.38,49,215

However, these SMS studies are not

completely representative of the function and properties of the bulk material.

More recently, conjugated polymer nanoparticles have been fabricated and

spectroscopically characterized.100,106-111,115,120,121,183

These nanoparticle studies are motivated by

the ability to study a limited number of molecules and interchain interactions at the molecular

level on a length scale between single molecule and bulk material. We recently reported the

fabrication of composite nanoparticles of the conjugated polymers poly[(o-phenylenevinylene)-

alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV), poly[2-methoxy-5-(2-

ethylhexyl-oxy)-p-phenylenevinylene] (MEH-PPV). By using single particle spectroscopy

(SPS), we were able to show how variations in polymer chain folding, polymer-polymer

interaction and polymer aggregation affect the nanoparticles morphology and photophysical

102

properties of individual nanoparticle at the molecular scale. In particular, the shape of the

fluorescence spectra and energy transfer properties for individual nanoparticles was found to

change with the MEH-PPV dopant level, implying that the heterogeneous arrangement of donor

and acceptor molecules results in band gap changes and physical changes in chain folding and

chain-chain stacks.

Herein, we extend our research endeavors to fundamentally understand the

optoelectronic, morphological and photophysical properties of ternary broadband absorption

nanoparticles that consist of BPPV, MEH-PPV and 1-(3-methoxycarbonylpropyl)-1-phenyl-

[6.6]C61 (PCBM). By combining stead-state absorption and fluorescence spectroscopy and

single particle spectroscopy, we were able to report a comprehensive study on the extent of

which polymer chain folding and the interactions between different conjugated polymers and

fullerenes affect the nanoparticle morphology and photophysical processes (e.g., the competition

between energy transfer and charge transfer) as a function of different MEH-PPV (0, 15 and 50

wt%) and PCBM (0, 5, 25 and 50 wt%) blending ratios. While the fluorescence spectroscopy

measurements reveal quantitative estimates of the charge transfer rates in bulk solution that are

orders of magnitude larger than the rates of energy transfer, the SPS measurement unmask

several key features that are unobtainable in the bulk measurements. Specifically, SPS

demonstrates that the extent of polymer chain folding is greatly affected by increased PCBM

dopant concentrations and, the enhancement or reduction of energy transfer efficiencies and rates

for a given PCBM dopant concentration is dependent on the BPPV:MEH-PPV blending ratio

(i.e. the energy transfer process enhances in the 15 wt% nanoparticles with respect to increased

PCBM doping levels, while the opposite is shown to occur in the 50 wt% nanoparticles).

103

6.2 Results and Discussion

6.2.1 Electrochemical Properties

To rationalize the differentiation between energy transfer and charge transfer in the

monofunctional and difunctional PCBM doped nanoparticles, the energy levels of BPPV and

MEH-PPV were determined by cyclic voltammetry. The cyclic voltammograms were measured

in tetrahydrofuran (THF) at room temperature in a potential window ranging from -1.8 to +2.5 V

(vs Ag/AgCl). Figure 31 illustrates the molecular energy level diagram and chemical structures

of the materials used in the monofunctional and difunctional doped nanoparticle suspensions and

presents the photophysical processes involved in the BPPV/MEH-PPV/PCBM system.

The ground state energy levels ( ) were estimated by using a ferrocene/ferrocenium

(Fc/Fc+) internal standard and calculated by the following equation:

, (9)

where is the onset of the oxidation potential. The excited state energy levels of BPPV

and MEH-PPV were estimated from the polymer band gap obtained in the UV-vis spectroscopy

data according to the following equations:239

(10)

(11)

where is the excited state energy, is the band gap of the conjugated polymer, and

is the absorption wavelength onset of the conjugated polymer. The ground state energy levels

104

for BPPV and MEH-PPV were estimated at -5.6 and -5.4 eV, respectively. The band gap energy

for BPPV (2.6 eV) and MEH-PPV (2.2 eV) then correspond to excited state energy levels of -3.0

and -3.2 eV for BPPV and MEH-PPV, respectively. The results show that the LUMO level of

both BPPV and MEH-PPV lie above the LUMO level of PCBM (-3.7 eV) with molecular orbital

level offsets of > 0.3 eV and indicate that photo-induced charge transfer from conjugated

polymer to PCBM is likely to be thermodynamically favorable due to the required driving force

needed to overcome the Coulomb binding energy of the singlet exciton.61

However, the LUMO

level offset of 0.2 eV between BPPV and MEH-PPV indicates that photo-induced electron

transfer is an unfavorable thermodynamic process, suggesting that intermolecular energy transfer

primarily accounts for the observed exciton quenching of BPPV. These electrochemical

properties are discussed hereafter to support the photophysical processes observed in the bulk

and single particle spectroscopy data of the monofunctional and difunctional doped

nanoparticles.

105

Figure 31: Energy level diagram displaying the HOMO and LUMO energies of the

semiconducting polymers (BPPV and MEH-PPV) and fullerene derivative (PCBM) used in the

monofunctional and difunctional doped nanoparticles. Here, kET and kCT are the rates of energy

and charge transfer, respectively.

6.2.2 Bulk Spectroscopy

The absorption and fluorescence spectra of undoped, monofunctional doped and difunctional

doped molecular solutions dissolved in THF and aqueous nanoparticle suspensions were

collected. These spectra are shown in Figures 32-35, where the molecular solutions and

nanoparticle suspensions are presented in the left (dash dot lines) and right columns (solid lines),

respectively. The absorption spectra for the molecular solutions (Figures 32a-c) depict BPPV and

MEH-PPV absorption maxima located at 353 and 500 nm, respectively. The nanoparticle

suspensions absorption spectra (Figure 32d-e) are nearly unchanged with respect to the

molecular solutions and suggest that during nanoparticle assembly, interchain and intrachain

106

interactions remain unaltered in the ground state. In addition, the absorption spectra illustrate the

presence of PCBM by the characteristic absorption peak located at 330 nm that increases in

absorbance in response to increased doping levels of PCBM. The absorption spectra are the sum

of pure PCBM and the corresponding BPPV or BPPV/MEH-PPV constituents, indicating that

there are no detectable ground state interactions occurring in the molecular solutions or

nanoparticle suspensions.240,241

These spectral characteristics are similar to previous reports of

composite nanoparticle suspensions.111,135

Figure 32: The absorbance spectra of molecular solutions dissolved in THF (dash dot line)

and aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue) and

50 (red) total weight percent (wt%) of PCBM. The presence of PCBM is confirmed by the

absorbance peak located at 330 nm, which increases in absorbance with higher doping levels of

107

PCBM. a-c) The absorption spectra of the molecular solutions exhibit a constant BPPV and

MEH-PPV maximum at 353 and 500 nm, respectively. d-f) The absorption spectra of the

aqueous nanoparticle suspensions exhibit a BPPV absorption maximum that resemble the

molecular solution, while MEH-PPV absorption maximum red shifts to 509 and 512 nm for 15

and 50 wt% nanoparticle suspensions, respectively.

In Figures 33-35, the fluorescence spectra of the molecular solutions dissolved in THF and

aqueous nanoparticle suspensions are depicted and are comparable to previous bulk film and

conjugated polymer nanoparticle fluorescence studies.106,111,135,174

The emission maxima for

BPPV (Figure 33a, black line) and MEH-PPV (Figures 34b-35b, black line) dissolved in THF

are located at 504 and 552 nm, respectively. While the undoped BPPV emission maximum is

located at 504 nm (Figure 33b, black line), the BPPV emission intensity and peak wavelength for

the 15 and 50 wt% monofunctional doped BPPV/MEH-PPV nanoparticles (Figures 34c-35c,

black line) decrease and blue shift to 496 and 485 nm, respectively. In addition, the

monofunctional doped nanoparticle suspensions obtain MEH-PPV emission maxima at 581 and

594 nm for the 15 and 50 wt% monofunctional doped BPPV/MEH-PPV nanoparticles,

respectively. These red shifted nanoparticle fluorescence spectra with respect to the molecular

solutions are ascribed to enhanced interchain and intrachain π-stacking interactions,38,44,47

while

the reduced BPPV emission of the monofunctional doped BPPV/MEHPPV nanoparticles is

indicative of an intermolecular Fӧrster energy transfer process through non-radiative dipole-

dipole interactions.135

Consequently, the blue shifted fluorescence spectra are indicative of

disrupted intrachain and interchain interaction which lead to disordered polymer chain

morphologies.

108

For the monofunctional doped BPPV nanoparticles and difunctional doped BPPV/MEH-PPV

nanoparticles, the fluorescence spectra display emission maxima that gradually blue shift to

higher energies compared to the representative undoped nanoparticle suspension (Figures 33b,

34c-d, 35c-d). Similar to the monofunctional doped BPPV/MEH-PPV nanoparticles, higher

PCBM doping levels severely disrupt interchain and intrachain interactions of BPPV, resulting in

more disordered chain morphologies and blue shifted fluorescence spectra. In addition, the 0-0

and 0-1 peak intensities of MEH-PPV vary in the bulk material under direct excitation (Inserts in

Figures 34d and 35d), suggesting that exciton migration to low energy sites is limited through

compartmentalization of polymer chain segments by reducing intermolecular π-stacking

interactions with respect to higher PCBM doping levels. These observations (i.e., more disorder

chain morphologies and limited exciton migration with increased PCBM doping) are in

agreement with previous studies of composite MEH-PPV/PCBM and P3HT/PCBM

nanoparticles.106-111

Compared to the undoped nanoparticle suspensions, the fluorescence intensity of the PCBM

doped nanoparticle suspensions is quenched by greater than 75%. Fluorescence quenching of this

nature was previously observed in blended conjugated polymer solutions,242

nanoparticles106-

108,110,111 and thin films.

240 Several electron spin resonance studies have deduced that the

fluorescence quenching mechanism is dominated by a charge transfer process.240,243

Heeger and

colleagues, for example, demonstrated two resolvable photo-induced spin signals in a composite

MEH-PPV/C60 film that were associated with the charge separated state. Additionally, it has

been widely reported that ultrafast interfacial charge transfer occurs in intercalated networks of

conjugated polymer-fullerene films on the time scale of ~ 40 fs,145,240

which is faster than the

energy transfer reaction. A quantitative estimate of the composite nanoparticle interfacial charge

109

transfer process (Table 4) can be determined by the extent of fluorescence quenching and the

excited state lifetime of the donor.146,244

For the monofunctional doped BPPV nanoparticles, the

photoinduced rate constant is given by Equation 12,

, (12)

where is the rate of charge transfer from BPPV to PCBM, is the quenching emission

intensity ratio of the BPPV/PCBM doped nanoparticles with respect to the undoped BPPV

nanoparticles, and is the excited state lifetime of undoped BPPV nanoparticles (100 ps).135

The values were estimated on the time scales of 2.0 x 10

10, 1.9 x 10

11 and 4.6 x 10

11 s

-1 for 5,

25 and 50 wtT% PCBM, respectively. These calculations indicate that the interfacial charge

transfer process is enhanced with respect to increasing doping concentrations of PCBM that is

likely to result from greater intermolecular interactions between BPPV and PCBM. In the case of

the difunctional doped BPPV/MEH-PPV nanoparticles, the photoinduced charge transfer process

can take place by: i) an indirect two-step process where energy is initial transfer from BPPV to

MEH-PPV followed by intermolecular electron transfer from MEH-PPV to PCBM, or ii) direct

electron transfer from BPPV to PCBM. These photoinduced charge transfer processes are shown

in Figure 31, where the difunctional doped nanoparticle rate for indirect charge transfer is

quantitatively estimated by Equation 13.

, (13)

Here, is the rate of indirect charge transfer from MEH-PPV to PCBM, is the

MEH-PPV quenching emission intensity ratio of the difunctional doped nanoparticles with

respect to the monofunctional undoped BPPV/MEH-PPV nanoparticles, and is the

110

excited state lifetime of undoped MEH-PPV nanoparticles (140 ps).135

The estimated values

were on the time scale of 2.7 x 1010

, 3.6 x 1011

and 7.3 x 1011

s-1

for the difunctional doped 15

wt% nanoparticle suspensions with 5, 25 and 50 wtT% PCBM, respectively, and 5.2 x 1010

,

4.8 x 1011

and 5.1 x 1011

s-1

for the difunctional doped 50 wt% nanoparticle suspensions with

5, 25 and 50 wtT% PCBM, respectively. These estimated rates for both difunctional doped

nanoparticle suspensions are comparable to the monofunctional doped BPPV nanoparticles and

suggest that the charge transfer process of these broadband absorbing nanoparticles are only

affected by the addition of PCBM. Alternatively, if direct electron transfer takes place in the

difunctional doped nanoparticles, additional fluorescence quenching is expected because the rate

of electron transfer would transpire at a faster rate with respect to the energy transfer process.

Compared to other OPVn-C60 dyads,244,245

the fluorescence spectra of the difunctional doped

BPPV/MEH-PPV nanoparticles shown in Figures 5b and 6b clearly depict significant

fluorescence quenching in comparison to the undoped 15 and 50 wt% nanoparticle suspensions.

In this case, the rate for direct charge transfer is quantitatively estimated by a modified model

according to Equation 14,

(14)

where is the rate of direct charge transfer from BPPV to PCBM,

is the BPPV

emission intensity quenching ratio of the difunctional doped nanoparticles with respect to the

monofunctional undoped BPPV/MEH-PPV nanoparticles,

is the MEH-PPV emission

intensity quenching ratio of the difunctional doped nanoparticles with respect to the

monofunctional undoped BPPV/MEH-PPV nanoparticles, and are the quantum yields of

the donor and acceptor, respectively, and is the excited state lifetime of undoped BPPV

111

nanoparticles (100 ps).135

The estimated values are on the time scale of 7.2 x 10

10, 2.9 x 10

13

and 7.7 x 1013

s-1

for the difunctional doped 15 wt% nanoparticle suspensions with 5, 25 and 50

wtT% PCBM, respectively, and 1.3 x 1011

, 2.8 x 1012

and 5.3 x 1012

s-1

for the difunctional doped

50 wt% nanoparticle suspensions with 5, 25 and 50 wtT% PCBM, respectively. Compared to the

indirect charge transfer calculations, the rates of direct charge transfer in both difunctional doped

compositions are faster by approximately one order of magnitude. These rates indicate that the

bulk of the initial BPPV excited state charge transfers to PCBM before energy transferring to

MEH-PPV and coincides with the significant reduction in the difunctional doped nanoparticle

fluorescence intensity. In addition, the direct charge transfer rates are faster by approximately

one order of magnitude for the 15 wt% difunctional doped nanoparticles compared to the 50 wt%

difunctional doped nanoparticles. This possibly results from a higher doping wtT% of PCBM in

the case of the 15 wt% difunctional doped nanoparticles, where enhanced intermolecular

interactions between BPPV and PCBM over short distances may increase the probability of

direct charge transfer. Nonetheless, the estimated calculations for both photoinduced charge

transfer processes quantitatively support the quenched photoluminescent intensity of the PCBM

doped nanoparticle suspensions with respect to the undoped nanoparticle constituents, and the

interfacial charge transfer process that dominates the rate of intermolecular energy transfer.

112

Table 4: Monofunctional and Difunctional Doped Nanoparticle Suspensions Quenching

Factors ( and Rate Constants for Energy Transfer and Charge Transfer

.

0wt%-5wtT% PCBM 3

2.0 x 10

10

0wt%-25wtT% PCBM 19 1.9 x 1011

0wt%-50wtT% PCBM 46 4.6 x 1011

15wt%-0wtT% PCBM 1.6 3.1 x 109

15wt%-5wtT% PCBM 5 5 2.7 x 1010

7.2 x 1010

15wt%-25wtT% PCBM 110 53 3.6 x 1011

2.9 x 1013

15wt%-50wtT% PCBM 147 107 7.3 x 1011

7.7 x 1013

50wt%-0wtT% PCBM 7.9 3.4 x 1010

50wt%-5wtT% PCBM 5 9 5.2 x 1010

1.3 x 1011

50wt%-25wtT% PCBM 9 71 4.8 x 1011

2.8 x 1012

50wt%-50wtT% PCBM 14 85 5.1 x 1011

5.3 x 1012

Additional quenching mechanisms (i.e., Fӧrster or Dexter) may account for the quenched

fluorescence intensity, where the excited state is quenching through a non-radiative deactivation

pathway. In the case of Fӧrster mechanism, the fluorescence quenching may result from the

spectral overlap between the conjugated polymer emission spectrum and the absorption spectrum

of PCBM. The sharp vibronic transition (λmax = 680 nm) observed at high PCBM doping levels

(i.e., 25 and 50 wtT% PCBM) may allude to PCBM emission by means of an Fӧrster mechanism,

which therefore reduces the overall emission intensity of the difunctional doped nanoparticle

suspensions. The overlap integral between BPPV or MEH-PPV and PCBM is small compared to

the spectral overlap integral of BPPV and MEH-PPV,246

suggesting negligible energy transfer

from BPPV and/or MEH-PPV to PCBM through an Fӧrster mechanism. However, Cook and co-

workers previously observed similar fluorescence spectra in pristine PCBM thin films using an

ultraviolet excitation source.247

This data suggests that the 680 nm vibronic transition may result

from direct excitation interfacial regions of aggregated PCBM molecules. On the other hand, the

fluorescence quenching resulting from a Dexter mechanism cannot be ruled out due to

113

intermolecular interactions between polymer chains and fullerenes. In this case, energy may be

transferred over short distances.

Interestingly, bulk spectroscopy displayed variations in the ratio between the BPPV peak

intensity relative to the peak intensity of MEH-PPV (IBPPV/IMEH-PPV). As depicted in the

normalized intensity inserts in Figures 34c and 35c, the 15 and 50 wt% monofunctional doped

nanoparticle suspensions (black lines) peak intensity ratios were calculated as 60 and 7 %,

respectively. As the doping level of PCBM increases, the peak intensity ratio increased to 200

and 37 % for the 15 and 50 wt% difunctional doped nanoparticle suspensions, respectively,

indicating that PCBM doping reduces the efficiency of the energy transfer process. Several

possible mechanisms may contribute to the observed reduction in FRET efficiency. First, the

initial excited state of BPPV may be dominated by the charge transfer process to PCBM at short

distances240

and in doing so, limiting the intermolecular energy transfer process between densely

packed BPPV and MEH-PPV polymer chains of a given nanoparticle composition. This

interpretation is supported by the estimated charge transfer rates, where direct charge transfer

was shown to exceed indirect charge transfer by an order of magnitude. In addition, the emission

intensity of MEH-PPV in both difunctional doped BPPV/MEH-PPV nanoparticle suspensions is

enhanced under direct excitation (Figures 34d and 35d) with respect to the fluorescence intensity

of MEH-PPV when directly exciting BPPV (Figures 34c and 35c), which further supports a

dominant charge transfer process of the initial excited state of BPPV. Second, nanoparticle

morphology may consist of phase separated polymer and fullerene interfacial regions, thus

preventing intermolecular energy transfer by separating BPPV and MEH-PPV molecules on a

size scale greater than the Fӧrster distance. Phase separation of this nature has been extensively

reviewed in blended polymer films (i.e., polymer-fullerene and polymer-

114

polymer).21,92,98,103,174,248-250

Finally, the changes in MEH-PPV vibronic structure during PCBM

loading may allude to a nanoscale morphology where the overall disruption of intermolecular π-

stacking interactions between dissimilar polymers (i.e., BPPV and MEH-PPV) occurs throughout

the nanoparticle, thereby reducing intermolecular energy transfer by limiting exciton migration.

Figure 33: Fluorescence spectra of BPPV molecular solutions dissolved in THF (dash dot

line) and aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue)

and 50 (red) wt% of PCBM. a) Molecular solutions of BPPV depicting a constant emission

maximum located at 466 nm, displaying slightly quenched fluorescence intensity with respect to

higher PCBM doping levels. b) Aqueous nanoparticle suspensions depicting an undoped BPPV

emission maximum at 503 nm. As the doping level of PCBM increased from 5 to 50 wt%, the

fluorescence intensity becomes increasingly quenched and the emission maximum blue shifts to

452 nm. The inserted graph in Figure 2b depicts the normalized fluorescence intensity of the

nanoparticle suspensions. A new vibronic structure is observed for 25 and 50 wt% at

approximately 680 nm. This new feature may arise from PCBM emission when directly exciting

the fullerene molecules with an ultraviolent excitation source (375 nm).

115

Figure 34: Fluorescence spectra of 50 wt% molecular solutions dissolved in THF (dash dot

line) and aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue)

and 50 (red) total wt% of PCBM. a) Molecular solutions in THF excited at 375 nm, displaying a

BPPV emission maximum at 466 nm with a 552 nm shoulder that results from direct excitation

of MEH-PPV. b) Molecular solutions in THF excited at 488 nm, which confirms the presence of

MEH-PPV at an emission maximum at 552 nm. c) Aqueous nanoparticle suspension excited at

375 nm, displaying quenched emission intensity and blue shifting emission maxima for both

BPPV and MEH-PPV with higher PCBM doping levels. The BPPV emission peaks at 480 nm

and shifts to 453 nm for 50 wt% PCBM, while the undoped MEH-PPV emission maximum is

located at 593 nm and shifts to 586 nm for 50 wt% PCBM. The 680 nm vibronic structures

observed in the 25 and 50 wt% PCBM nanoparticles suggest that vibronic feature results from

116

PCBM emission. The inset in Figure 3c illustrates the fluorescence spectra normalized to the

emission maximum of MEH-PPV, which display variations in the peak intensity ratios of BPPV

to MEH-PPV. d) Aqueous nanoparticle suspensions excited at 488 nm, depicting quenched

emission intensity and blue shifting emission maximum for MEH-PPV with respect to higher

doping levels of PCBM. The undoped MEH-PPV emission maximum is located at 593 nm and

shifts to 588 nm for 50 wt% PCBM. The insert in Figure 3d illustrates the normalized

fluorescence spectra for the MEH-PPV emission, which display variations in the 0-0 to 0-1

vibronic transitions.

Figure 35: Fluorescence spectra of 15 wt% molecular solutions dissolved in THF (dash dot

line) and aqueous nanoparticle suspensions (solid line) doped with 0 (black), 5 (green), 25 (blue)

117

and 50 (red) total wt% of PCBM. a) Molecular solutions in THF excited at 375 nm, displaying a

BPPV emission maximum at 466 nm. b) Molecular solutions in THF excited at 488 nm, which

confirms the presence of MEH-PPV at an emission maximum at 552 nm. c) Aqueous

nanoparticle suspension excited at 375 nm, displaying quenched emission intensity and blue

shifting emission maxima for both BPPV and MEH-PPV with higher PCBM doping levels. The

BPPV emission peaks at 498 nm and shifts to 453 nm for 50 wt% PCBM, while the undoped

MEH-PPV emission maximum is located at 586 nm and shifts to 579 nm for 50 wt% PCBM.

The 680 nm vibronic structures observed in the 25 and 50 wt% PCBM nanoparticles suggest that

vibronic feature results from PCBM emission. The inset in Figure 3c illustrates the fluorescence

spectra normalized to the emission maximum of MEH-PPV, which display variations in the peak

intensity ratios of BPPV to MEH-PPV. d) Aqueous nanoparticle suspensions excited at 488 nm,

depicting quenched emission intensity and blue shifting emission maximum for MEH-PPV

relative to higher PCBM doping levels. The undoped MEH-PPV emission maximum is located at

587 nm and shifts to 581 nm for 50 wt% PCBM. The insert in Figure 3d illustrates the

normalized fluorescence spectra for the MEH-PPV emission, which display variations in the 0-0

to 0-1 vibronic transitions.

6.2.3 Single Particle Spectroscopy

Fluorescence imaging of individual BPPV, BPPV/MEHPPV and BPPV/MEH-PPV/PCBM

nanoparticles were investigated by raster scanning laser confocal microscopy. As shown in

Figure 36, the bright spots in the fluorescence images are representative of individual undoped

BPPV and monofunctional doped BPPV nanoparticles. Compared to the undoped BPPV

nanoparticles fluorescence intensity, the excitation power was increased by 9 to 12 times to

achieve similar fluorescence counts for the PCBM doped nanoparticles. However, these images

118

confirm the presence of ultra fast charge transfer from conjugated polymer to fullerene that leads

to a dynamic quenching process, as previously described by Heeger and co-workers.240,242

The

co-existence of fluorescence emission during the dynamic quenching process implies that instead

of complete charge transfer to fullerene molecules every time the polymer molecule is cycled to

the excited state, the fluorescence signal results from the probability of the excited state returning

to the ground state through a radiative pathway.

Figure 36: Line scan images (10 x 10 μm) of individual undoped BPPV and monofunctional

doped BPPV nanoparticles that are dispersed in a non-fluorescence polyvinyl alcohol film. a)

Single particle image of undoped BPPV nanoparticles using a laser power of 10 nW. b) Single

particle image of monofunctional doped BPPV nanoparticles (0 wt% - 5 wtT% PCBM using a

laser power ~ 90 nW to achieve similar fluorescence count with respect to the undoped BPPV

image.

Single particle fluorescence spectra were collected from ~100 nanoparticles per composition.

The individual fluorescence spectra for 0, 15 and 50 wt% nanoparticles doped with 0 (black), 5

(green), 25 (blue) and 50 (red) total weight percent (wtT%) of PCBM were averaged into a single

119

particle ensemble spectra (Figures 37,39 and 40, left column) and peak wavelength distributions

(Figures 37, 39 and 40, right column). In addition, the energy transfer efficiencies (EFRET_SPS)

and rates of energy transfer (kFRET_SPS) in the left and right columns of Figures 41-42,

respectively, were calculated for the 15 and 50 wt% monofunctional and difunctional doped

nanoparticles by least square deconvolution of individual nanoparticle spectra with the

corresponding undoped BPPV or BPPV/PCBM ensemble spectrum via Equations 15 and 16.

(15)

(16)

Here, FDA and FD represent the fluorescent intensity of the donor in the presence and absence

of the acceptor, respectively. is the lifetime of the donor, and represent the

quantum yield and detector efficiency of the donor and acceptor, respectively. The excited-state

lifetime of undoped BPPV nanoparticles was determined by time-correlating single photon

counting, yielding an averaged lifetime of 100 ps, while the quantum efficiency of the EMCCD

over the spectral range of the SPS analysis resulted in a detector efficiency of 1. Consequently,

these data clearly indicate that the optoelectronic and photophysical properties per nanoparticle

per composition are greatly affected by conformational changes that the polymer chains adopt

through intermolecular and intramolecular interactions, and the nanoscale morphology that

evolves during nanoparticle assembly. Nonetheless, by assuming that the nanoparticles are

viewed as a composite of interacting polymer chains and fullerenes in a closely packed nanoscale

structure, the SPS data can provide further insight into the polymer chain confirmations,

nanoscale morphology and photophysical processes that remain masked at the bulk level.

120

In Figure 37, the undoped BPPV and monofunctional doped BPPV nanoparticle ensemble

spectra and peak wavelength distributions are depicted, and correspond well with steady-state

fluorescence measurements. The undoped BPPV nanoparticles emission maximum red shifted to

510 nm with respect to the molecular solution dissolved in THF, while emission maximum of the

monofunctional doped BPPV nanoparticle ensemble spectra continuously blue shifted to 480 nm

with increasing doping concentration of PCBM. In Scheme 1 of Figure 38, the polymer chain

confirmations of undoped BPPV and monofunctional doped BPPV nanoparticles are presented.

These illustrations depict collapsed polymer chains with excess intrachain and interchain π-

stacking interactions that yield polymer segments that fold back onto themselves and form

emissive π-stacked red sites. With increasing doping concentrations of PCBM, however,

interchain and intrachain interactions are severely altered due to PCBM steric effects, resulting in

blue shifted fluorescence spectra due to reduced exciton migration to low energy sites and

disordered BPPV chain morphologies. Moreover, broadening of the peak wavelength

distributions with respect to higher PCBM concentrations further support changes in chemical,

structural and morphological properties of BPPV polymer chains; consequently, the electronic

properties of individual nanoparticles are altered.

Interestingly, two new broad and structureless vibronic features were observed in Figure 37

with PCBM addition. Compared to the 5 wtT% PCBM doped nanoparticles, the red shifted and

structureless emission with a peak wavelength of ~ 650 nm is more pronounced in the 25 wtT%

PCBM doped nanoparticles. Broad, structureless and red shifted fluorescence features have been

widely reported in bulk films with respect to molecular solutions,138,153,251-254

indicative of an

interchain excited-state (i.e., exciplex or excimer) that forms between the excited-state of a

polymer chain with the ground-state of a fullerene neighbor or vice versa. The intermolecular

121

excited-state complex, however, is not clearly observed for the 50 wtT% PCBM doped

nanoparticles, which may result from a new non-radiative interchain state or reduced interfacial

interactions. A new non-radiative pathway does not fit well with emissive excited-state

complexes; specifically, the red shifted emission intensity grows with respect to higher PCBM

doping concentrations. If, however, interfacial contact between BPPV and PCBM is primarily

limited to grain boundary lines along phase separated regions of BPPV and PCBM, interaction

between the excited-state chain and the ground-state neighbor may reduce the probability of an

emissive excited-state complex. In fact, the broad red shifted vibronic transition that resembles

the S1 singlet excited state emission of PCBM (λmax = 740 nm) may spectrally mask the

interchain excited-state complex. With higher PCBM doping levels, the PCBM emission

increased while becoming the primary emitting specie in the 50 wtT% PCBM doped

nanoparticles, which may result from direct excitation of PCBM regions or by means of non-

radiative energy transfer process from BPPV to PCBM. The latter seems less likely since the

overlap integral of BPPV and PCBM is negligible, and several orders of magnitude smaller with

respect to the overlap integral of BPPV and MEH-PPV.246

The former, however, fits well with

the photophysical properties of pristine PCBM films which display S1 S0 singlet emission

with an emission maximum around 730 nm.247

In fact, direct excitation of PCBM regions further

support the interpretation of limited interfacial interaction due to phase separated BPPV and

PCBM regions, and the primary emitting specie of PCBM for the highly doped PCBM

nanoparticles.

122

Figure 37: Single particle ensemble spectra and peak wavelength distribution of BPPV

nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. a-d) Single

particle ensemble spectra that display an undoped BPPV emission maximum at 525 nm, which

blue shift to 480 nm with respect to higher PCBM doping levels. New vibronic features are

observed at 640 and 730 nm for the PCBM doped nanoparticles, suggesting that fluorescence

emission may result from excited state complexes and singlet emission from PCBM. e-h) Peak

wavelength histograms illustrating blue shifted emission maxima with broadening distributions

as the doping level of PCBM rises. In the case of 50 wtT% PCBM nanoparticles, the peak

wavelength frequency at 740 nm is higher than the peak wavelength frequency of BPPV,

indicating that the primary emitting specie of these composite nanoparticles results from

fullerene molecules.

123

Figure 38: Schematic illustrating the changes in BPPV (blue) and MEH-PPV (red) inter- and

intrachain interactions, resulting from increasing PCBM (black) doping levels. Scheme 1, 2, and

3 are representative of 0, 15 and 50 wt% nanoparticles, respectively.

The normalized ensemble spectra, peak wavelength distributions and photophysical

properties for the 15 and 50 wt% monofunctional and difunctional doped nanoparticles are

124

depicted in Figures 39-42. In the case of the monofunctional doped nanoparticles, the spectral

and photophysical properties are within good agreement with previous SPS studies of undoped

MEH-PPV nanoparticles and composite BPPV/MEH-PPV nanoparticles106,111

(add BPPV/MEH-

PPV citation). For the 15 wt% monofunctional doped nanoparticles, the BPPV peak wavelength

and MEH-PPV emission maximum were located at 510 and 596 nm, respectively. The EFRET_SPS

values depicted in Figure 41 displayed a broad distribution ranging from 10 to 90 % with the

distribution maximum at ~ 50 %, while the kFRET_SPS values ranged between 109 - 10

10 s

-1 (Figure

41). Compared to the 15 wt% monofunctional doped nanoparticles, the 50 wt% monofunctional

doped nanoparticles displayed a BPPV peak wavelength and MEH-PPV emission maximum

positioned at 500 and 600 nm, respectively, while the EFRET_SPS values displayed a narrower

distribution ranging from 80 – 100 % with the distribution maximum at ~ 95 % and kFRET_SPS

values residing between 1010

- 1012

s-1

(Figure 42).

Additionally, the interchain and intrachain interactions of both BPPV and MEH-PPV are

illustrated for the monofunctional doped nanoparticles in Schemes 2 and 3 of Figure 38.

Compared to the undoped BPPV nanoparticles, the monofunctional doped BPPV polymer chain

morphologies are more disordered and extended, while a higher degree of disordered BPPV

chain morphologies are depicted for the 15 wt% monofunctional doped nanoparticles than the 50

wt% monofunctional doped nanoparticles. These disordered polymer chain morphologies are

indicative of disrupted interchain and intrachain π-stacking interactions of BPPV due to MEH-

PPV steric effects, which leads to blue shifted fluorescence spectra due to limited exciton

migration to low energy red sites. Moreover, the spectral characteristics of MEH-PPV suggest

that the polymer chains co-exist as defect cylinder confirmations whereas the long conjugated

segments form π-stacked red sites during self-assembly by folding back onto themselves.

125

However, the defected cylinder confirmations of the 15 wt% monofunctional doped

nanoparticles are slightly disordered relative to the 50 wt% monofunctional doped nanoparticles.

Again, the difference in chain morphologies is most likely to result from the higher BPPV

concentration (BPPV concentration for 15 wt% monofunctional doped nanoparticles was ~ 85%

higher than the 50 wt% monofunctional doped nanoparticles) that induces steric effects where

interchain and intrachain π-stacking interactions of MEH-PPV are slightly interrupted, leading to

more disordered chain morphologies and blue shifted fluorescence spectra due to reduced

exciton migration to low energy sites.

Compared to the monofunctional doped BPPV nanoparticles, similar spectral characteristics

(blue shifting peak wavelengths, broadening peak wavelength distribution and pronounced 740

nm emissions) were observed in the 15 and 50 wt% difunctional doped nanoparticles when

increasing the PCBM doping concentration. Moreover, we cannot exclude an interchain excited-

state complex since the emission spectra of MEH-PPV may spectrally mask the excited-state

complex. With increased PCBM doping levels, the interchain and intrachain π-stacking

interactions for both BPPV and MEH-PPV are severely disrupted, resulting in more disordered

chain morphologies and blue shifted spectral characteristics due to reduced exciton migration to

low energy red sites. These chain morphologies with respect to excess PCBM doping are

illustrated in Schemes 2-3. Interestingly, the overlaid normalized ensemble spectra of the

monofunctional and difunctional doped nanoparticles (Figure 43) depict an intermolecular

Fӧrster energy transfer process that amplifies or reduces with excess PCBM doping for a given

difunctional doped nanoparticle composition. In accordance, the calculated EFRET_SPS and

kFRET_SPS values for individual nanoparticles per composition exhibit similar photophysical

properties. For example, the 15 wt% difunctional doped nanoparticles, from 0 to 50 wtT%

126

PCBM, displayed EFRET_SPS distribution that became narrower and shifted to higher efficiencies

while the kFRET_SPS values ranged from 1010

- 1011

s-1

increase in relative abundance. On the other

hand, the 50 wt% - 5 wtT% and 50 wt% - 25 wtT% difunctional doped nanoparticles displayed

similar EFRET_SPS and kFRET_SPS distributions ranging from 70 - 100% and 109 - 10

12 s

-1,

respectively, while the distribution maxima shifted slightly to lower values. As the PCBM

doping level reached 50wtT%, the 50 wt% difunctional doped nanoparticles displayed a

broadening bimodal EFRET_SPS and kFRET_SPS distribution where the distribution maxima was

reduced considerably. In fact, the bimodal kFRET_SPS distribution agreed well with the trimodal

peak wavelength distribution for the 50 wt% - 50 wtT% PCBM nanoparticles (Figure 39, red

histogram), suggesting the PCBM doping effectively reduced energy transfer by separating

intermolecular interactions at distances larger than the quenching radius or individual

nanoparticles that obtain different doping ratios for a given composition. While these SPS data

clearly signifies that the interchain and intrachain interactions between polymer chains and

fullerenes, to a great extent, affect the optoelectronic and photophysical properties of both

monofunctional and difunctional doped nanoparticles compositions, the variations of the energy

transfer process per nanoparticle per composition cannot solely be explained through interchain

and intrachain interactions.

It has been widely reported that controlling the polymer blend morphology (i.e., the nature of

phase separation) is highly dependent upon solutions processing variables such as solvent

additives,248

annealing,92,174

blending ratios,103,249

concentration98,174

and molecular weight.21,250

In the case of MEH-PPV and other related conjugated polymers, increasing the fullerene doping

level above the optimal polymer-fullerene blending ratio inherently results in phase separation

and poor optoelectronic device performance.58,98

Consequently, the nanoparticle morphology

127

likely consists of compartmentalized regions (polymer-only, fullerene-only and polymer-

fullerene) that are interfacial separated on the nanometer scale, where the extent of phase

separated interfacial regions directly affects the intermolecular energy transfer process for a

given nanoparticle composition. In other words, PCBM addition could lead to the following: i)

nanoscale morphology that enhances energy transfer through improved BPPV and MEH-PPV

interactions while simultaneously allowing both charge and energy transfer processes to occur as

efficient mechanisms (Figure 38, Scheme 2), and ii) nanoscale morphology that in many cases

limits, severely reduces or forbids intermolecular energy transfer process by reducing BPPV and

MEH-PPV interactions during the photoinduced charge transfer mechanism (Figure 38, Scheme

3). The enhanced photophysical properties of the 15 wt% difunctional doped nanoparticles

suggest that the nanoparticle morphology consists of well mixed polymer-polymer and polymer-

fullerene regions, and, as a result, interfacial boundary lines co-exist between these regions

where energy and charge transfer can effectively take place. On the other hand, the

photophysical properties of the 15 wt% difunctional doped nanoparticles suggest that excess

PCBM doping creates an undesirable nanoparticle morphology for efficient energy transfer

because the interactions between polymer-polymer regions are effectively limited by phase

separation. Phase separation of this nature seems likely when considering the extent of

morphologically induced steric effects for a given nanoparticle composition. Several

monofunctional doped conjugated polymer nanoparticle studies have reported induced PCBM

steric effects, which leads to limited exciton migration to low energy sites and reduced π-

stacking interactions (interchain and intrachain). In the case of the difunctional doped

nanoparticles, the high molecular weight of MEH-PPV may contribute additional steric effects;

in doing so, it creates nanoscale phase separated compartments of polymer-only and fullerene-

128

only regions with limited polymer-polymer regions. This interpretation of the nanoscale

morphology is quantitatively supported by larger abundance of high molecular weight MEH-

PPV molecules (~ 62) for the 50 wt% monofunctional doped nanoparticle with respect to the 15

wt% monofunctional doped nanoparticle (~ 19).255

Figure 39: Single particle ensemble spectra and peak wavelength distributions of 15 wt%

nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. a-d)

Normalized fluorescence ensemble spectra depicting an undoped emission maximum located at

596 nm. With increasing PCBM doping levels, the MEH-PPV emission maximum blue shifts to

580 nm, while the peak wavelength intensity of BPPV decreases relative to MEH-PPV.

Additional vibronic structure is observed at 730 nm at high PCBM doping levels. e-h) Peak

129

wavelength histograms illustrating the emission maximum blue shift with increasing PCBM

doping levels.

Figure 40:Figure 7: Single particle ensemble spectra and peak wavelength distribution of 50

wt% nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. Left

column) Normalized fluorescence ensemble spectra depicting an undoped emission maximum

located at 600 nm, while the MEH-PPV emission maximum blue shifts to 589 nm with enhanced

vibronic structure at 730 nm as the doping level of PCBM increases. Right column) Peak

wavelength histograms illustrating that the MEH-PPV emission maxima distribution exhibits

blue shifting with higher PCBM doping levels. In the case of 50 % PCBM nanoparticles, peak

intensity ratio of BPPV to MEH-PPV increases and the peak wavelength distribution becomes

trimodal.

130

Figure 41: Single particle FRET efficiencies (EFRET_SPS) and rates (kFRET_SPS) of 15

wt% nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) wt% of PCBM. a-d)

The EFRET_SPS distribution maximum for the undoped 15 wt% nanoparticles is approximately

50 %, while the efficiencies increase with higher PCBM doping levels. e-h) The kFRET_SPS of

0 to 50 % PCBM are shown to reside between 109 to 1011 s-1. Similar to the EFRET_SPS, the

energy transfer rates display a slight increase above 1010 s-1 with respect to higher PCBM

doping levels, indicating that the energy transfer process is enhanced at excess PCBM levels.

131

Figure 42: Single particle FRET efficiencies (EFRET_SPS) and rates (kFRET_SPS) 50 wt%

nanoparticles doped with 0 (black), 5 (green), 25 (blue) and 50 (red) total wt% of PCBM. a-d)

The EFRET_SPS of undoped 50 wt% nanoparticles is approximately 95 %. The efficiencies of

nanoparticle with up to 25 % PCBM resemble the undoped nanoparticle efficiencies, while the

EFRET_SPS for 50 % PCBM display a bimodal distribution that is approximately equal in

abundance at less than 5 and 85 %. e-h) The kFRET_SPS for 0 to 25 % PCBM displays a broad

distribution of rates ranging from 1010 to 1012 s-1, while the kFRET_SPS distribution for 50 %

PCBM become bimodal with rates ranging from 107 to 1012 s-1. These data suggest that excess

PCBM disrupts the energy transfer process thereby decreasing the energy transfer efficiency and

rate for 50 wt% nanoparticles.

132

Figure 43: Single particle ensemble spectra normalized to the emission maximum of MEH-

PPV. a) 15 wt% nanoparticles doped with 0, 5, 25 and 50 total wt% of PCBM. These spectra

display a decrease in BPPV emission intensity with respect to higher PCBM doping levels,

suggesting that the energy transfer process is enhanced at higher doping levels. b) 50 wt%

nanoparticles doped with 0, 5, 25 and 50 total wt% of PCBM. These spectra display an increased

BPPV emission for the 50 % PCBM nanoparticles, while BPPV emission intensity of 5 and 25 %

PCBM resemble the undoped 50 wt% nanoparticles. These data suggest that the energy transfer

process occurring at doping levels greater than 25 % are disrupted.

6.3 Conclusion

Broadband absorbing nanoparticles of the conjugated polymers poly[(o-phenylenevinylene)-

alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)] (BPPV) and poly[2-methoxy-5-(2-

ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were fabricated with different 1-(3-

methoxycarbonylpropyl)-1-phenyl-[6.6]C61 (PCBM) and MEH-PPV doping ratio by a modified

reprecipitation method to address the issue of morphology, intermolecular chain interactions,

light harvesting and the competition between charge and energy transfer in ternary polymer-

polymer-fullerene OPVs. These model nanoparticle systems related the material function to

133

molecular level properties, while maintaining the functionality of the bulk material. The

monofunctional and difunctional doped nanoparticles display a broad distribution of electronic

and morphological properties, including Fӧrster energy transfer, change transfer, band gap

changes and physical changes in polymer chain stacking and folding. In particular, SPS revealed

that the energy transfer process was not only dependent of the PCBM doping concentration, but

also dependent on the BPPV:MEH-PPV blending ratio. These findings provide a fundamental

understanding of the morphological, optical, electronic and photophysical properties that may

occur in the active layer of ternary doped OPVs, while suggesting that energy transfer and

interfacial charge transfer can simultaneously occur at the nanoscale between closely packed

conjugated polymer constituents and fullerenes.

134

REFERENCES

(1) Perlin, J.: From space to earth : the story of solar electricity / John Perlin;

1st Havard University Press ed. ed.; Harvard University Press: Cambridge, Mass, 2002.

(2) Corporation, S.; Vol. 2011.

(3) Ranjan, S.; Balaji, S.; Panella, R. A.; Ydstie, B. E.: Silicon solar cell

production. Computers & Chemical Engineering 2011, 35, 1439-1453.

(4) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.;

Louis, E. J.; Gau, S. C.; Macdiarmid, A. G.: ELECTRICAL-CONDUCTIVITY IN

DOPED POLYACETYLENE. Physical Review Letters 1977, 39, 1098-1101.

(5) Shirakawa, H.; Louis, E. J.; Macdiarmid, A. G.; Chiang, C. K.; Heeger, A.

J.: SYNTHESIS OF ELECTRICALLY CONDUCTING ORGANIC POLYMERS -

HALOGEN DERIVATIVES OF POLYACETYLENE, (CH)X. Journal of the Chemical

Society-Chemical Communications 1977, 578-580.

(6) Handbook of conducting polymers; 3rd ed / edited by Terje A. Skotheim

and John R. Reynolds. ed.; CRC: Boca Raton, FLA, 2007.

(7) Organic photovoltaics : mechanism, materials, and devices / [edited by]

Sam-Shajing Sun, Niyazi Serdar Sariciftci; Taylor & Francis: Boca Raton, FL, 2005.

(8) Organic light-emitting devices : Synthesis, properties, and applications /

edited by Klaus Müllen, Ullrich Scherf; Wiley-VCH ; John Wiley [distributor: Weinheim

: Chichester, 2006.

(9) Kymissis, I.: Organic field effect transistors : theory, fabrication and

characterization / Ioannis Kymissis; Springer: New York, NY, 2009.

(10) GmbH, H.; Vol. 2011.

(11) Konarka Technologies, I.; Vol. 2011.

(12) Plextronics; Vol. 2011.

(13) Solarmer Energy, I.; Vol. 2011.

(14) Dimitrakopoulos, C. D.; Malenfant, P. R. L.: Organic thin film transistors

for large area electronics. Advanced Materials 2002, 14, 99-+.

(15) Iannotta, S.; Toccoli, T.: Supersonic molecular beam growth of thin films

of organic materials: A novel approach to controlling the structure, morphology, and

functional properties. Journal of Polymer Science Part B-Polymer Physics 2003, 41,

2501-2521.

(16) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.;

Wu, W.; Woo, E. P.: High-resolution inkjet printing of all-polymer transistor circuits.

Science 2000, 290, 2123-2126.

(17) Krebs, F. C.: All solution roll-to-roll processed polymer solar cells free

from indium-tin-oxide and vacuum coating steps. Organic Electronics 2009, 10, 761-768.

(18) Muller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn, V.;

Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K.: Multi-colour organic light-

emitting displays by solution processing. Nature 2003, 421, 829-833.

135

(19) Nielsen, T. D.; Cruickshank, C.; Foged, S.; Thorsen, J.; Krebs, F. C.:

Business, market and intellectual property analysis of polymer solar cells. Solar Energy

Materials and Solar Cells 2010, 94, 1553-1571.

(20) Weinberger, B. R.; Akhtar, M.; Gau, S. C.: POLYACETYLENE PHOTO-

VOLTAIC DEVICES. Synthetic Metals 1982, 4, 187-197.

(21) Thompson, B. C.; Frechet, J. M. J.: Organic photovoltaics - Polymer-

fullerene composite solar cells. Angewandte Chemie-International Edition 2008, 47, 58-

77.

(22) Barbara, P. F.; Gesquiere, A. J.; Park, S. J.; Lee, Y. J.: Single-molecule

spectroscopy of conjugated polymers. Accounts of Chemical Research 2005, 38, 602-610.

(23) Lupton, J. M.: Single-Molecule Spectroscopy for Plastic Electronics:

Materials Analysis from the Bottom-Up. Advanced Materials 2010, 22, 1689-1721.

(24) McNeill, C. R.; Watts, B.; Swaraj, S.; Ade, H.; Thomsen, L.; Belcher, W.;

Dastoor, P. C.: Evolution of the nanomorphology of photovoltaic polyfluorene blends:

sub-100 nm resolution with x-ray spectromicroscopy. Nanotechnology 2008, 19, 7.

(25) Westenhoff, S.; Howard, I. A.; Friend, R. H.: Probing the morphology and

energy landscape of blends of conjugated polymers with sub-10 nm resolution. Phys.

Rev. Lett. 2008, 101, 4.

(26) Ma, W. L.; Yang, C. Y.; Heeger, A. J.: Spatial Fourier-transform analysis

of the morphology of bulk heterojunction materials used in "plastic" solar cells.

Advanced Materials 2007, 19, 1387-+.

(27) Rossi, G.; Chance, R. R.; Silbey, R.: CONFORMATIONAL DISORDER

IN CONJUGATED POLYMERS. Journal of Chemical Physics 1989, 90, 7594-7601.

(28) Clarke, T. M.; Durrant, J. R.: Charge Photogeneration in Organic Solar

Cells. Chemical Reviews 2010, 110, 6736-6767.

(29) Tang, C. W.: 2-LAYER ORGANIC PHOTOVOLTAIC CELL. Applied

Physics Letters 1986, 48, 183-185.

(30) McNeill, C. R.; Greenham, N. C.: Conjugated-Polymer Blends for

Optoelectronics. Advanced Materials 2009, 21, 3840-3850.

(31) Beljonne, D.; Pourtois, G.; Silva, C.; Hennebicq, E.; Herz, L. M.; Friend, R.

H.; Scholes, G. D.; Setayesh, S.; Mullen, K.; Bredas, J. L.: Interchain vs. intrachain

energy transfer in acceptor-capped conjugated polymers. Proceedings of the National

Academy of Sciences of the United States of America 2002, 99, 10982-10987.

(32) Kohler, B. E.; Samuel, I. D. W.: EXPERIMENTAL-DETERMINATION

OF CONJUGATION LENGTHS IN LONG POLYENE CHAINS. Journal of Chemical

Physics 1995, 103, 6248-6252.

(33) Kohler, B. E.; Woehl, J. C.: A SIMPLE-MODEL FOR CONJUGATION

LENGTHS IN LONG POLYENE CHAINS. Journal of Chemical Physics 1995, 103,

6253-6256.

(34) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L.:

Influence of interchain interactions on the absorption and luminescence of conjugated

oligomers and polymers: A quantum-chemical characterization. Journal of the American

Chemical Society 1998, 120, 1289-1299.

136

(35) Dykstra, T. E.; Hennebicq, E.; Beljonne, D.; Gierschner, J.; Claudio, G.;

Bittner, E. R.; Knoester, J.; Scholes, G. D.: Conformational Disorder and Ultrafast

Exciton Relaxation in PPV-family Conjugated Polymers. Journal of Physical Chemistry

B 2009, 113, 656-667.

(36) Hennebicq, E.; Pourtois, G.; Scholes, G. D.; Herz, L. M.; Russell, D. M.;

Silva, C.; Setayesh, S.; Grimsdale, A. C.; Mullen, K.; Bredas, J. L.; Beljonne, D.: Exciton

migration in rigid-rod conjugated polymers: An improved Forster model. Journal of the

American Chemical Society 2005, 127, 4744-4762.

(37) Dykstra, T. E.; Kovalevskij, V.; Yang, X. J.; Scholes, G. D.: Excited state

dynamics of a conformationally disordered conjugated polymer: A comparison of

solutions and film. Chemical Physics 2005, 318, 21-32.

(38) Yu, J.; Hu, D. H.; Barbara, P. F.: Unmasking electronic energy transfer of

conjugated polymers by suppression of O-2 quenching. Science 2000, 289, 1327-1330.

(39) Nguyen, T. Q.; Wu, J. J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H.: Control

of energy transfer in oriented conjugated polymer-mesoporous silica composites. Science

2000, 288, 652-656.

(40) Nguyen, T. Q.; Wu, J.; Tolbert, S. H.; Schwartz, B. J.: Control of energy

transport in conjugated polymers using an ordered mesoporous silica matrix. Advanced

Materials 2001, 13, 609-+.

(41) Meskers, S. C. J.; Hubner, J.; Oestreich, M.; Bassler, H.: Time-resolved

fluorescence studies and Monte Carlo simulations of relaxation dynamics of

photoexcitations in a polyfluorene film. Chemical Physics Letters 2001, 339, 223-228.

(42) Graupner, W.; Leising, G.; Lanzani, G.; Nisoli, M.; DeSilvestri, S.; Scherf,

U.: Femtosecond relaxation of photoexcitations in a poly(para-phenylene)-type ladder

polymer. Physical Review Letters 1996, 76, 847-850.

(43) Muller, J. G.; Lupton, J. M.; Feldmann, J.; Lemmer, U.; Scharber, M. C.;

Sariciftci, N. S.; Brabec, C. J.; Scherf, U.: Ultrafast dynamics of charge carrier

photogeneration and geminate recombination in conjugated polymer : fullerene solar

cells. Physical Review B 2005, 72, 10.

(44) Schwartz, B. J.: Conjugated polymers as molecular materials : How chain

conformation and film morphology influence energy transfer and interchain interactions.

Annual Review of Physical Chemistry 2003, 54, 141-172.

(45) VandenBout, D. A.; Yip, W. T.; Hu, D. H.; Fu, D. K.; Swager, T. M.;

Barbara, P. F.: Discrete intensity jumps and intramolecular electronic energy transfer in

the spectroscopy of single conjugated polymer molecules. Science 1997, 277, 1074-1077.

(46) Gesquiere, A. J.; Park, S. J.; Barbara, P. F.: F-V/SMS: A new technique for

studying the structure and dynamics of single molecules and nanoparticles. Journal of

Physical Chemistry B 2004, 108, 10301-10308.

(47) Huser, T.; Yan, M.; Rothberg, L. J.: Single chain spectroscopy of

conformational dependence of conjugated polymer photophysics. Proceedings of the

National Academy of Sciences of the United States of America 2000, 97, 11187-11191.

(48) Huser, T.; Yan, M.: Solvent-related conformational changes and

aggregation of conjugated polymers studied by single molecule fluorescence

137

spectroscopy. Journal of Photochemistry and Photobiology a-Chemistry 2001, 144, 43-

51.

(49) Yu, Z. H.; Barbara, P. F.: Low-temperature single-molecule spectroscopy

of MEH-PPV conjugated polymer molecules. Journal of Physical Chemistry B 2004,

108, 11321-11326.

(50) De Leener, C.; Hennebicq, E.; Sancho-Garcia, J. C.; Beljonne, D.:

Modeling the Dynamics of Chromophores in Conjugated Polymers: The Case of Poly (2-

methoxy-5-(2 '-ethylhexyl)oxy 1,4-phenylene vinylene) (MEH-PPV). Journal of Physical

Chemistry B 2009, 113, 1311-1322.

(51) Mirzov, O.; Scheblykin, I. G.: Photoluminescence spectra of a conjugated

polymer: from films and solutions to single molecules. Physical Chemistry Chemical

Physics 2006, 8, 5569-5576.

(52) Gesquiere, A. J.; Lee, Y. J.; Yu, J.; Barbara, P. F.: Single molecule

modulation spectroscopy of conjugated polymers. Journal of Physical Chemistry B 2005,

109, 12366-12371.

(53) Lee, Y. J.; Kim, D. Y.; Grey, J. K.; Barbara, P. F.: Variable temperature

single-molecule dynamics of MEH-PPV. Chemphyschem 2005, 6, 2404-2409.

(54) Yu, J.; Lammi, R.; Gesquiere, A. J.; Barbara, P. F.: Singlet-triplet and

triplet-triplet interactions in conjugated polymer single molecules. Journal of Physical

Chemistry B 2005, 109, 10025-10034.

(55) Hu, D. H.; Yu, J.; Wong, K.; Bagchi, B.; Rossky, P. J.; Barbara, P. F.:

Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical

conformations. Nature 2000, 405, 1030-1033.

(56) Gunes, S.; Neugebauer, H.; Sariciftci, N. S.: Conjugated polymer-based

organic solar cells. Chem. Rev. 2007, 107, 1324-1338.

(57) Nunzi, J. M.: Organic photovoltaic materials and devices. C. R. Phys. 2002,

3, 523-542.

(58) Winder, C.; Sariciftci, N. S.: Low bandgap polymers for photon harvesting

in bulk heterojunction solar cells. Journal of Materials Chemistry 2004, 14, 1077-1086.

(59) Bundgaard, E.; Krebs, F. C.: Low band gap polymers for organic

photovoltaics. Solar Energy Materials and Solar Cells 2007, 91, 954-985.

(60) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; De Boer, B.:

Small bandgap polymers for organic solar cells (polymer material development in the last

5 years). Polymer Reviews 2008, 48, 531-582.

(61) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.;

Heeger, A. J.; Brabec, C. L.: Design rules for donors in bulk-heterojunction solar cells -

Towards 10 % energy-conversion efficiency. Advanced Materials 2006, 18, 789-+.

(62) Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z. G.; Waller, D.;

Gaudiana, R.; Brabec, C.: High photovoltaic performance of a low-bandgap polymer (vol

18, pg 2884, 2006). Advanced Materials 2006, 18, 2931-2931.

(63) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses,

D.; Leclerc, M.; Lee, K.; Heeger, A. J.: Bulk heterojunction solar cells with internal

quantum efficiency approaching 100%. Nature Photonics 2009, 3, 297-U5.

138

(64) Liang, Y. Y.; Xu, Z.; Xia, J. B.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L.

P.: For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power

Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135-+.

(65) Soci, C.; Hwang, I. W.; Moses, D.; Zhu, Z.; Waller, D.; Gaudiana, R.;

Brabec, C. J.; Heeger, A. J.: Photoconductivity of a low-bandgap conjugated polymer.

Advanced Functional Materials 2007, 17, 632-636.

(66) Dastoor, P. C.; McNeill, C. R.; Frohne, H.; Foster, C. J.; Dean, B.; Fell, C.

J.; Belcher, W. J.; Canipbell, W. M.; Officer, D. L.; Blake, I. M.; Thordarson, P.;

Crossley, M. J.; Hush, N. S.; Reimers, J. R.: Understanding and improving solid-state

Polymer/C-60-fullerene bulk-heterojunction solar cells using ternary porphyrin blends.

Journal of Physical Chemistry C 2007, 111, 15415-15426.

(67) Cooling, N.; Burke, K. B.; Zhou, X.; Lind, S. J.; Gordon, K. C.; Jones, T.

W.; Dastoor, P. C.; Belcher, W. J.: A study of the factors influencing the performance of

ternary MEH-PPV:porphyrin:PCBM heterojunction devices: A steric approach to

controlling charge recombination. Solar Energy Materials and Solar Cells 2011, 95,

1767-1774.

(68) Burke, K. B.; Belcher, W. J.; Thomsen, L.; Watts, B.; McNeill, C. R.; Ade,

H.; Dastoor, P. C.: Role of Solvent Trapping Effects in Determining the Structure and

Morphology of Ternary Blend Organic Devices. Macromolecules 2009, 42, 3098-3103.

(69) Belcher, W. J.; Wagner, K. I.; Dastoor, P. C.: The effect of porphyrin

inclusion on the spectral response of ternary P3HT : porphyrin : PCBM bulk

heterojunction solar cells. Solar Energy Materials and Solar Cells 2007, 91, 447-452.

(70) Honda, S.; Yokoya, S.; Ohkita, H.; Benten, H.; Ito, S.: Light-Harvesting

Mechanism in Polymer/Fullerene/Dye Ternary Blends Studied by Transient Absorption

Spectroscopy. Journal of Physical Chemistry C 2011, 115, 11306-11317.

(71) Honda, S.; Nogami, T.; Ohkita, H.; Benten, H.; Ito, S.: Improvement of the

Light-Harvesting Efficiency in Polymer/Fullerene Bulk Heterojunction Solar Cells by

Interfacial Dye Modification. Acs Applied Materials & Interfaces 2009, 1, 804-810.

(72) Johansson, E. M. J.; Yartsev, A.; Rensmo, H.; Sundstrom, V.: Photocurrent

Spectra and Fast Kinetic Studies of P3HT/PCBM Mixed with a Dye for Photoconversion

in the Near-IR Region. Journal of Physical Chemistry C 2009, 113, 3014-3020.

(73) Koppe, M.; Egelhaaf, H. J.; Dennler, G.; Scharber, M. C.; Brabec, C. J.;

Schilinsky, P.; Hoth, C. N.: Near IR Sensitization of Organic Bulk Heterojunction Solar

Cells: Towards Optimization of the Spectral Response of Organic Solar Cells. Advanced

Functional Materials 2010, 20, 338-346.

(74) Kasai, H.; Nalwa, H. S.; Oikawa, H.; Okada, S.; Matsuda, H.; Minami, N.;

Kakuta, A.; Ono, K.; Mukoh, A.; Nakanishi, H.: A NOVEL PREPARATION METHOD

OF ORGANIC MICROCRYSTALS. Japanese Journal of Applied Physics Part 2-Letters

1992, 31, L1132-L1134.

(75) Horn, D.; Rieger, J.: Organic nanoparticles in the aqueous phase - theory,

experiment, and use. Angew. Chem.-Int. Edit. 2001, 40, 4331-4361.

139

(76) Zhang, X. R.; Wang, Y. F.; Ma, Y.; Ye, Y. C.; Wang, Y.; Wu, K.: Solvent-

stabilized oxovanadium phthalocyanine nanoparticles and their application in xerographic

photoreceptors. Langmuir 2006, 22, 344-348.

(77) Gement, A.: Liquid Dielectrics; Chapman and Hall, Ltd.: London, 1933.

(78) Chen, Y.; Mauldin, J. P.; Day, R. N.; Periasamy, A.: Characterization of

spectral FRET imaging microscopy for monitoring nuclear protein interactions. Journal

of Microscopy-Oxford 2007, 228, 139-152.

(79) Chen, Y.; Periasamy, A.: Intensity range based quantitative FRET data

analysis to localize protein molecules in live cell nuclei. Journal of Fluorescence 2006,

16, 95-104.

(80) Becker, W.: Advanced Time-Correlated Single Photon Counting

Techniques; Springer: New York, 1995.

(81) Becker, W.: The bh TCSPC Handbook; 3rd ed.; Becker & Hickl GmbH:

Berlin, Germany, 2008.

(82) Lakowicz, J. R.: Principles of Flourescence Spectroscopy; 3rd ed., 2006.

(83) O'Connor, D. V.; Phillips, D.: Time-correlated single-photon counting;

Academic Press: New York, 1983.

(84) Maus, M.; Rousseau, E.; Cotlet, M.; Schweitzer, G.; Hofkens, J.; Van der

Auweraer, M.; De Schryver, F. C.; Krueger, A.: New picosecond laser system for easy

tunability over the whole ultraviolet/visible/near infrared wavelength range based on

flexible harmonic generation and optical parametric oscillation. Review of Scientific

Instruments 2001, 72, 36-40.

(85) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R.

N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.;

Salaneck, W. R.: Electroluminescence in conjugated polymers. Nature 1999, 397, 121-

128.

(86) Granstrom, M.; Inganas, O.: White light emission from a polymer blend

light emitting diode. Appl. Phys. Lett. 1996, 68, 147-149.

(87) Yu, G.; Nishino, H.; Heeger, A. J.; Chen, T. A.; Rieke, R. D.: ENHANCED

ELECTROLUMINESCENCE FROM SEMICONDUCTING POLYMER BLENDS.

Synthetic Metals 1995, 72, 249-252.

(88) Berggren, M.; Inganas, O.; Gustafsson, G.; Rasmusson, J.; Andersson, M.

R.; Hjertberg, T.; Wennerstrom, O.: LIGHT-EMITTING-DIODES WITH VARIABLE

COLORS FROM POLYMER BLENDS. Nature 1994, 372, 444-446.

(89) Zhang, C.; Vonseggern, H.; Pakbaz, K.; Kraabel, B.; Schmidt, H. W.;

Heeger, A. J.: BLUE ELECTROLUMINESCENT DIODES UTILIZING BLENDS OF

POLY(P-PHENYLPHENYLENE VINYLENE) IN POLY(9-VINYLCARBAZOLE).

Synthetic Metals 1994, 62, 35-40.

(90) D'Andrade, B. W.; Forrest, S. R.: White organic light-emitting devices for

solid-state lighting. Advanced Materials 2004, 16, 1585-1595.

(91) Gather, M. C.; Kohnen, A.; Meerholz, K.: White Organic Light-Emitting

Diodes. Advanced Materials 2011, 23, 233-248.

140

(92) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J.: Thermally

stable, efficient polymer solar cells with nanoscale control of the interpenetrating network

morphology. Advanced Functional Materials 2005, 15, 1617-1622.

(93) Moons, E.: Conjugated polymer blends: linking film morphology to

performance of light emitting diodes and photodiodes. Journal of Physics-Condensed

Matter 2002, 14, 12235-12260.

(94) Zhong, C. M.; Duan, C. H.; Huang, F.; Wu, H. B.; Cao, Y.: Materials and

Devices toward Fully Solution Processable Organic Light-Emitting Diodes. Chemistry of

Materials 2011, 23, 326-340.

(95) He, X. M.; Gao, F.; Tu, G. L.; Hasko, D.; Huttner, S.; Steiner, U.;

Greenham, N. C.; Friend, R. H.; Huck, W. T. S.: Formation of Nanopatterned Polymer

Blends in Photovoltaic Devices. Nano Letters 2010, 10, 1302-1307.

(96) Mori, D.; Benten, H.; Kosaka, J.; Ohkita, H.; Ito, S.; Miyake, K.:

Polymer/Polymer Blend Solar Cells with 2.0% Efficiency Developed by Thermal

Purification of Nanoscale-Phase-Separated Morphology. Acs Applied Materials &

Interfaces 2011, 3, 2924-2927.

(97) Yang, X.; Loos, J.: Toward high-performance polymer solar cells: The

importance of morphology control. Macromolecules 2007, 40, 1353-1362.

(98) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.;

Meissner, D.; Sariciftci, N. S.: Nanoscale morphology of conjugated polymer/fullerene-

based bulk-heterojunction solar cells. Advanced Functional Materials 2004, 14, 1005-

1011.

(99) Hoppe, H.; Sariciftci, N. S.: Morphology of polymer/fullerene bulk

heterojunction solar cells. Journal of Materials Chemistry 2006, 16, 45-61.

(100) Grey, J. K.; Kim, D. Y.; Norris, B. C.; Miller, W. L.; Barbara, P. F.: Size-

dependent spectroscopic properties of conjugated polymer nanoparticles. Journal of

Physical Chemistry B 2006, 110, 25568-25572.

(101) Nguyen, T. Q.; Schwartz, B. J.; Schaller, R. D.; Johnson, J. C.; Lee, L. F.;

Haber, L. H.; Saykally, R. J.: Near-field scanning optical microscopy (NSOM) studies of

the relationship between interchain interactions, morphology, photodamage, and energy

transport in conjugated polymer films. Journal of Physical Chemistry B 2001, 105, 5153-

5160.

(102) Martens, T.; D'Haen, J.; Munters, T.; Beelen, Z.; Goris, L.; Manca, J.;

D'Olieslaeger, M.; Vanderzande, D.; De Schepper, L.; Andriessen, R.: Disclosure of the

nanostructure of MDMO-PPV : PCBM bulk hetero-junction organic solar cells by a

combination of SPM and TEM. Synthetic Metals 2003, 138, 243-247.

(103) van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma, C. W. T.;

Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J.: Relating the morphology of poly(p-

phenylene vinylene)/methanofullerene blends to solar-cell performance. Advanced

Functional Materials 2004, 14, 425-434.

(104) McNeill, C. R.; Watts, B.; Swaraj, S.; Ade, H.; Thomsen, L.; Belcher, W.;

Dastoor, P. C.: Evolution of the nanomorphology of photovoltaic polyfluorene blends:

sub-100 nm resolution with x-ray spectromicroscopy. Nanotechnology 2008, 19.

141

(105) Westenhoff, S.; Howard, I. A.; Friend, R. H.: Probing the morphology and

energy landscape of blends of conjugated polymers with sub-10 nm resolution. Physical

Review Letters 2008, 101.

(106) Gesquiere, A. J.; Tenery, D.; Hu, Z. J.: Single-particle spectroscopy on

conducting polymer-fullerene composite materials for application in organic photovoltaic

devices. Spectroscopy 2008, 23, 32-44.

(107) Hu, Z. J.; Gesquiere, A. J.: PCBM concentration dependent morphology of

P3HT in composite P3HT/PCBM nanoparticles. Chemical Physics Letters 2009, 476, 51-

55.

(108) Hu, Z. J.; Tenery, D.; Bonner, M. S.; Gesquiere, A. J.: Correlation between

spectroscopic and mophological properties of composite P3HT/PCBM nanoparticles

studied by single particles spectroscopy. Journal of Luminescence 2010, 130, 771-780.

(109) Tenery, D.; Gesquiere, A. J.: Effect of PCBM Concentration on

Photoluminescence Properties of Composite MEH-PPV/PCBM Nanoparticles

Investigated by a Franck-Condon Analysis of Single-Particle Emission Spectra.

Chemphyschem 2009, 10, 2449-2457.

(110) Tenery, D.; Gesquiere, A. J.: Interplay between fluorescence and

morphology in composite MEH-PPV/PCBM nanoparticles studied at the single particle

level. Chemical Physics 2009, 365, 138-143.

(111) Tenery, D.; Worden, J. G.; Hu, Z. J.; Gesquiere, A. J.: Single particle

spectroscopy on composite MEH-PPV/PCBM nanoparticles. Journal of Luminescence

2009, 129, 423-429.

(112) Gesquiere, A. J.; Park, S. J.; Barbara, P. F.: Photochemistry and kinetics of

single organic nanoparticles in the presence of charge carriers. European Polymer

Journal 2004, 40, 1013-1018.

(113) Gesquiere, A. J.; Uwada, T.; Asahi, T.; Masuhara, H.; Barbara, P. F.: Single

molecule spectroscopy of organic dye nanoparticles. Nano Letters 2005, 5, 1321-1325.

(114) Palacios, R. E.; Fan, F. R. F.; Grey, J. K.; Suk, J.; Bard, A. J.; Barbara, P.

F.: Charging and discharging of single conjugated-polymer nanoparticles. Nature

Materials 2007, 6, 680-685.

(115) Palacios, R. E.; Lee, K. J.; Rival, A.; Adachi, T.; Bolinger, J. C.; Fradkin,

L.; Barbara, P. F.: Single conjugated polymer nanoparticle capacitors. Chemical Physics

2009, 357, 21-27.

(116) Li, X. P.; Qian, Y.; Wang, S. Q.; Li, S. Y.; Yang, G. Q.: Tunable

Fluorescence Emission and Efficient Energy Transfer in Doped Organic Nanoparticles.

Journal of Physical Chemistry C 2009, 113, 3862-3868.

(117) Wu, C. F.; Peng, H. S.; Jiang, Y. F.; McNeill, J.: Energy transfer mediated

fluorescence from blended conjugated polymer nanoparticles. Journal of Physical

Chemistry B 2006, 110, 14148-14154.

(118) Wu, C. F.; Zheng, Y. L.; Szymanski, C.; McNeill, J.: Energy transfer in a

nanoscale multichromophoric system: Fluorescent dye-doped conjugated polymer

nanoparticles. Journal of Physical Chemistry C 2008, 112, 1772-1781.

142

(119) Harbron, E. J.; Davis, C. M.; Campbell, J. K.; Allred, R. M.; Kovary, M.

T.; Economou, N. J.: Photochromic Dye-Doped Conjugated Polymer Nanoparticles:

Photomodulated Emission and Nanoenvironmental Characterization. Journal of Physical

Chemistry C 2009, 113, 13707-13714.

(120) Wu, C. F.; Bull, B.; Christensen, K.; McNeill, J.: Ratiometric Single-

Nanoparticle Oxygen Sensors for Biological Imaging. Angewandte Chemie-International

Edition 2009, 48, 2741-2745.

(121) Wu, C.; Bull, B.; Szymanski, C.; Christensen, K.; McNeill, J.: Multicolor

Conjugated Polymer Dots for Biological Fluorescence Imaging. Acs Nano 2008, 2, 2415-

2423.

(122) Wu, C. F.; Szymanski, C.; Cain, Z.; McNeill, J.: Conjugated polymer dots

for multiphoton fluorescence imaging. Journal of the American Chemical Society 2007,

129, 12904-+.

(123) Nguyen, T. Q.; Doan, V.; Schwartz, B. J.: Conjugated polymer aggregates

in solution: Control of interchain interactions. Journal of Chemical Physics 1999, 110,

4068-4078.

(124) Martini, I. B.; Smith, A. D.; Schwartz, B. J.: Exciton-exciton annihilation

and the production of interchain species in conjugated polymer films: Comparing the

ultrafast stimulated emission and photoluminescence dynamics of MEH-PPV. Physical

Review B 2004, 69, 12.

(125) Padmanaban, G.; Ramakrishnan, S.: Fluorescence spectroscopic studies of

solvent- and temperature-induced conformational transition in segmented poly[2-

methoxy-5-(2 '-ethylhexyl)oxy-1,4-phenylenevinylene] (MEHPPV). Journal of Physical

Chemistry B 2004, 108, 14933-14941.

(126) Panda, P.; Veldman, D.; Sweelssen, J.; Bastiaansen, J.; Langeveld-Voss, B.

M. W.; Meskers, S. C. J.: Charge transfer absorption for pi-conjugated polymers and

oligomers mixed with electron acceptors. Journal of Physical Chemistry B 2007, 111,

5076-5081.

(127) Bakulin, A. A.; Martyanov, D. S.; Paraschuk, D. Y.; Pshenichnikov, M. S.;

van Loosdrecht, P. H. M.: Ultrafast Charge Photogeneration Dynamics in Ground-State

Charge-Transfer Complexes Based on Conjugated Polymers. Journal of Physical

Chemistry B 2008, 112, 13730-13737.

(128) Traiphol, R.; Sanguansat, P.; Srikhirin, T.; Kerdcharoen, T.; Osotchan, T.:

Spectroscopic study of photophysical change in collapsed coils of conjugated polymers:

Effects of solvent and temperature. Macromolecules 2006, 39, 1165-1172.

(129) Kasha, M. R., H. R.; El-Bayoumi, M. A: THE EXCITON MODEL IN

MOLECULAR SPECTROSCOPY. Pure and Applied Chemistry 1965, 11, 371-392.

(130) Meskers, S. C. J.; Janssen, R. A. J.; Haverkort, J. E. M.; Wolter, J. H.:

Relaxation of photo-excitations in films of oligo- and poly(para-phenylene vinylene)

derivatives. Chemical Physics 2000, 260, 415-439.

(131) Xie, Z. Q.; Xie, W. J.; Li, F.; Liu, L. L.; Wang, H.; Ma, Y. G.: Controlling

supramolecular microstructure to realize highly efficient nondoped deep blue organic

143

light-emitting devices: The role of diphenyl substituents in distyrylbenzene derivatives.

Journal of Physical Chemistry C 2008, 112, 9066-9071.

(132) Fan, C. H.; Wang, S.; Hong, J. W.; Bazan, G. C.; Plaxco, K. W.; Heeger, A.

J.: Beyond superquenching: Hyper-efficient energy transfer from conjugated polymers to

gold nanoparticles. Proceedings of the National Academy of Sciences of the United States

of America 2003, 100, 6297-6301.

(133) Markov, D. E.; Amsterdam, E.; Blom, P. W. M.; Sieval, A. B.; Hummelen,

J. C.: Accurate measurement of the exciton diffusion length in a conjugated polymer

using a heterostructure with a side-chain cross-linked fullerene layer. Journal of Physical

Chemistry A 2005, 109, 5266-5274.

(134) Ruseckas, A.; Namdas, E. B.; Ganguly, T.; Theander, M.; Svensson, M.;

Andersson, M. R.; Inganas, O.; Sundstrom, V.: Intra- and interchain luminescence in

amorphous and semicrystalline films of phenyl-substituted polythiophene. Journal of

Physical Chemistry B 2001, 105, 7624-7631.

(135) Bonner, M. S.; Gesquiere, A. J.: Excited-state dynamics of broadband

absorbing conjugated polymer nanoparticles. Expected publishing data 05-2011.

(136) Yan, M.; Rothberg, L. J.; Kwock, E. W.; Miller, T. M.: INTERCHAIN

EXCITATIONS IN CONJUGATED POLYMERS. Physical Review Letters 1995, 75,

1992-1995.

(137) Yan, M.; Rothberg, L. J.; Papadimitrakopoulos, F.; Galvin, M. E.; Miller,

T. M.: DEFECT QUENCHING OF CONJUGATED POLYMER LUMINESCENCE.

Physical Review Letters 1994, 73, 744-747.

(138) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B. R.:

Aggregation quenching of luminescence in electroluminescent conjugated polymers.

Journal of Physical Chemistry A 1999, 103, 2394-2398.

(139) Jakubiak, R.; Rothberg, L. J.; Wan, W.; Hsieh, B. R.: Reduction of

photoluminescence quantum yield by interchain interactions in conjugated polymer films.

Synthetic Metals 1999, 101, 230-233.

(140) Wang, P.; Collison, C. J.; Rothberg, L. J.: Origins of aggregation quenching

in luminescent phenylenevinylene polymers. Journal of Photochemistry and

Photobiology a-Chemistry 2001, 144, 63-68.

(141) Heeger, A. J.: Semiconducting and metallic polymers: The fourth

generation of polymeric materials (Nobel lecture). Angewandte Chemie-International

Edition 2001, 40, 2591-2611.

(142) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay,

K.; Friend, R. H.; Burns, P. L.; Holmes, A. B.: LIGHT-EMITTING-DIODES BASED

ON CONJUGATED POLYMERS. Nature 1990, 347, 539-541.

(143) Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G.:

Photovoltaic Devices with a Low Band Gap Polymer and CdSe Nanostructures

Exceeding 3% Efficiency. Nano Letters 2010, 10, 239-242.

(144) Sharma, S. S.; Sharma, G. D.; Mikroyannidis, J. A.: Improved power

conversion efficiency of bulk heterojunction poly(3-hexylthiophene):PCBM photovoltaic

144

devices using small molecule additive. Solar Energy Materials and Solar Cells 2011, 95,

1219-1223.

(145) Brabec, C. J.; Zerza, G.; Cerullo, G.; De Silvestri, S.; Luzzati, S.;

Hummelen, J. C.; Sariciftci, S.: Tracing photoinduced electron transfer process in

conjugated polymer/fullerene bulk heterojunctions in real time. Chemical Physics Letters

2001, 340, 232-236.

(146) van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.;

Hummelen, J. C.; Janssen, R. A. J.: Photoinduced energy and electron transfer in

fullerene-oligothiophene-fullerene triads. Journal of Physical Chemistry A 2000, 104,

5974-5988.

(147) van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.;

Hummelen, J. C.; Janssen, R. A. J.: Photoinduced singlet and triplet energy transfer in

fullerene-oligothiophene-fullerene triads. Synthetic Metals 2001, 116, 123-127.

(148) Martini, I. B.; Ma, B.; Da Ros, T.; Helgeson, R.; Wudl, F.; Schwartz, B. J.:

Ultrafast competition between energy and charge transfer in a functionalized electron

donor/fullerene derivative. Chemical Physics Letters 2000, 327, 253-262.

(149) Spano, F. C.: Excitons in conjugated oligomer aggregates, films, and

crystals. Annual Review of Physical Chemistry 2006, 57, 217-243.

(150) Barbara, P. F.; Chang, W. S.; Link, S.; Scholes, G. D.; Yethiraj, A.:

Structure and dynamics of conjugated polymers in liquid crystalline solvents. In Annual

Review of Physical Chemistry; Annual Reviews: Palo Alto, 2007; Vol. 58; pp 565-584.

(151) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A.: About

supramolecular assemblies of pi-conjugated systems. Chemical Reviews 2005, 105, 1491-

1546.

(152) Bredas, J. L.; Beljonne, D.; Coropceanu, V.; Cornil, J.: Charge-transfer and

energy-transfer processes in pi-conjugated oligomers and polymers: A molecular picture.

Chemical Reviews 2004, 104, 4971-5003.

(153) Samuel, I. D. W.; Rumbles, G.; Collison, C. J.: EFFICIENT INTERCHAIN

PHOTOLUMINESCENCE IN A HIGH-ELECTRON-AFFINITY CONJUGATED

POLYMER. Physical Review B 1995, 52, 11573-11576.

(154) Hsu, J. H.; Fann, W. S.; Tsao, P. H.; Chuang, K. R.; Chen, S. A.:

Fluorescence from conjugated polymer aggregates in dilute poor solution. Journal of

Physical Chemistry A 1999, 103, 2375-2380.

(155) Nguyen, T. Q.; Kwong, R. C.; Thompson, M. E.; Schwartz, B. J.:

Improving the performance of conjugated polymer-based devices by control of interchain

interactions and polymer film morphology. Applied Physics Letters 2000, 76, 2454-2456.

(156) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C.: Role of intermolecular

coupling in the photophysics of disordered organic semiconductors: Aggregate emission

in regioregular polythiophene. Physical Review Letters 2007, 98, 4.

(157) Traiphol, R.; Charoenthai, N.: Effects of conformational change and

segmental aggregation on photoemission of illuminophores in conjugated polymer MEH-

PPV: Blue shift versus red shift. Synthetic Metals 2008, 158, 135-142.

145

(158) Arnrutha, S. R.; Jayakannan, M.: Probing the pi-stacking induced molecular

aggregation in pi-conjugated polymers, oligomers, and their blends of p-

phenylenevinylenes. Journal of Physical Chemistry B 2008, 112, 1119-1129.

(159) Kersting, R.; Lemmer, U.; Mahrt, R. F.; Leo, K.; Kurz, H.; Bassler, H.;

Gobel, E. O.: FEMTOSECOND ENERGY RELAXATION IN PI-CONJUGATED

POLYMERS. Physical Review Letters 1993, 70, 3820-3823.

(160) Kersting, R.; Mollay, B.; Rusch, M.; Wenisch, J.; Leising, G.; Kauffmann,

H. F.: Femtosecond site-selective probing of energy relaxing excitons in

poly(phenylenevinylene): Luminescence dynamics and lifetime spectra. Journal of

Chemical Physics 1997, 106, 2850-2864.

(161) Warmuth, C.; Tortschanoff, A.; Brunner, K.; Mollay, B.; Kauffmann, H. F.:

Excitonic fs-luminescence rise in poly(phenylenevinylene), PPV. Journal of

Luminescence 1998, 76-7, 498-501.

(162) Ruseckas, A.; Wood, P.; Samuel, I. D. W.; Webster, G. R.; Mitchell, W. J.;

Burn, P. L.; Sundstrom, V.: Ultrafast depolarization of the fluorescence in a conjugated

polymer. Physical Review B 2005, 72, 5.

(163) Moses, D.; Dogariu, A.; Heeger, A. J.: Ultrafast detection of charged

photocarriers in conjugated polymers. Physical Review B 2000, 61, 9373-9379.

(164) Scheblykin, I. G.; Yartsev, A.; Pullerits, T.; Gulbinas, V.; Sundstrom, V.:

Excited state and charge photogeneration dynamics in conjugated polymers. Journal of

Physical Chemistry B 2007, 111, 6303-6321.

(165) Zhang, F. L.; Jespersen, K. G.; Bjorstrom, C.; Svensson, M.; Andersson, M.

R.; Sundstrom, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Inganas, O.: Influence of

solvent mixing on the morphology and performance of solar cells based on polyfluorene

copolymer/fullerene blends. Advanced Functional Materials 2006, 16, 667-674.

(166) Hwang, I. W.; Soci, C.; Moses, D.; Zhu, Z. G.; Waller, D.; Gaudiana, R.;

Brabec, C. J.; Heeger, A. J.: Ultrafast electron transfer and decay dynamics in a small

band gap bulk heterojunction material. Advanced Materials 2007, 19, 2307-+.

(167) Banerji, N.; Cowan, S.; Leclerc, M.; Vauthey, E.; Heeger, A. J.: Exciton

Formation, Relaxation, and Decay in PCDTBT. Journal of the American Chemical

Society 2010, 132, 17459-17470.

(168) Blatchford, J. W.; Jessen, S. W.; Lin, L. B.; Gustafson, T. L.; Fu, D. K.;

Wang, H. L.; Swager, T. M.; MacDiarmid, A. G.; Epstein, A. J.: Photoluminescence in

pyridine-based polymers: Role of aggregates. Physical Review B 1996, 54, 9180-9189.

(169) Samuel, I. D. W.; Rumbles, G.; Collison, C. J.; Friend, R. H.; Moratti, S.

C.; Holmes, A. B.: Picosecond time-resolved photoluminescence of PPV derivatives.

Synthetic Metals 1997, 84, 497-500.

(170) Dogariu, A.; Vacar, D.; Heeger, A. J.: Picosecond time-resolved

spectroscopy of the excited state in a soluble derivative of poly(phenylene vinylene):

Origin of the bimolecular decay. Physical Review B 1998, 58, 10218-10224.

(171) Smilowitz, L.; Hays, A.; Heeger, A. J.; Wang, G.; Bowers, J. E.: TIME-

RESOLVED PHOTOLUMINESCENCE FROM POLY[2-METHOXY, 5-(2'-ETHYL-

146

HEXYLOXY)-P-PHENYLENE-VINYLENE] - SOLUTIONS, GELS, FILMS, AND

BLENDS. Journal of Chemical Physics 1993, 98, 6504-6509.

(172) Markov, D. E.; Hummelen, J. C.; Blom, P. W. M.; Sieval, A. B.: Dynamics

of exciton diffusion in poly(p-phenylene vinylene)/fullerene heterostructures. Physical

Review B 2005, 72, 5.

(173) Rothberg, L.: Photophysics of Conjugated Polymers. In Semiconducting

polymers : Chemistry, physics and engineering; Malliaras, G. H. a. G. G., Ed.; Wiley-

VCH, 2007; Vol. 1; pp 179-204.

(174) Nguyen, T. Q.; Martini, I. B.; Liu, J.; Schwartz, B. J.: Controlling

interchain interactions in conjugated polymers: The effects of chain morphology on

exciton-exciton annihilation and aggregation in MEH-PPV films. Journal of Physical

Chemistry B 2000, 104, 237-255.

(175) Nguyen, T. Q.; Yee, R. Y.; Schwartz, B. J.: Solution processing of

conjugated polymers: the effects of polymer solubility on the morphology and electronic

properties of semiconducting polymer films. Journal of Photochemistry and

Photobiology a-Chemistry 2001, 144, 21-30.

(176) Lim, S. H.; Bjorklund, T. G.; Bardeen, C. J.: The role of long-lived dark

states in the photoluminescence dynamics of poly(phenylene vinylene) conjugated

polymers. II. Excited-state quenching versus ground-state depletion. Journal of Chemical

Physics 2003, 118, 4297-4305.

(177) Stevens, M. A.; Silva, C.; Russell, D. M.; Friend, R. H.: Exciton

dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and

poly(9, 9-dioctylfluorene-co-benzothiadiazole). Physical Review B 2001, 63, 18.

(178) Maniloff, E. S.; Klimov, V. I.; McBranch, D. W.: Intensity-dependent

relaxation dynamics and the nature of the excited-state species in solid-state conducting

polymers. Physical Review B 1997, 56, 1876-1881.

(179) Hayes, G. R.; Samuel, I. D. W.; Phillips, R. T.: EXCITON DYNAMICS IU

ELECTROLUMINESCENT POLYMERS STUDIED BY FEMTOSECOND TIME-

RESOLVED PHOTOLUMINESCENCE SPECTROSCOPY. Physical Review B 1995,

52, 11569-11572.

(180) Hayes, G. R.; Samuel, I. D. W.; Phillips, R. T.: Ultrafast dynamics of

photoexcitations in conjugated polymers. Synthetic Metals 1997, 84, 889-890.

(181) Mahrt, R. F.; Pauck, T.; Lemmer, U.; Siegner, U.; Hopmeier, M.; Hennig,

R.; Bassler, H.; Gobel, E. O.; Bolivar, P. H.; Wegmann, G.; Kurz, H.; Scherf, U.; Mullen,

K.: Dynamics of optical excitations in a ladder-type pi-conjugated polymer containing

aggregate states. Physical Review B 1996, 54, 1759-1765.

(182) Schindler, F.; Lupton, J. M.; Feldmann, J.; Scherf, U.: A universal picture

of chromophores in pi-conjugated polymers derived from single-molecule spectroscopy.

Proceedings of the National Academy of Sciences of the United States of America 2004,

101, 14695-14700.

(183) Bonner, M. S.; Gesquiere, A. J.: Broad Band Energy Transfer

Nanoparticles: A Single Particle Spectroscopy Study. J. Phys. Chem. C 2011,

(submitted).

147

(184) Sherwood, G. A.; Cheng, R.; Smith, T. M.; Werner, J. H.; Shreve, A. P.;

Peteanu, L. A.; Wildeman, J.: Aggregation Effects on the Emission Spectra and

Dynamics of Model Oligomers of MEH-PPV. Journal of Physical Chemistry C 2009,

113, 18851-18862.

(185) Peteanu, L. A.; Sherwood, G. A.; Werner, J. H.; Shreve, A. P.; Smith, T.

M.; Wildeman, J.: Visualizing Core–Shell Structure in Substituted PPV Oligomer

Aggregates Using Fluorescence Lifetime Imaging Microscopy (FLIM). The Journal of

Physical Chemistry C 2011, null-null.

(186) Sherwood, G. A.; Cheng, R.; Chacon-Madrid, K.; Smith, T. M.; Peteanu, L.

A.; Wildeman, J.: Chain Length and Substituent Effects on the Formation of Excimer-

Like States in Nanoaggregates of CN-PPV Model Oligomers. Journal of Physical

Chemistry C 2010, 114, 12078-12089.

(187) Hu, D. H.; Yu, J.; Padmanaban, G.; Ramakrishnan, S.; Barbara, P. F.:

Spatial confinement of exciton transfer and the role of conformational order in organic

nanoparticles. Nano Letters 2002, 2, 1121-1124.

(188) Padmanaban, G.; Ramakrishnan, S.: Conjugation length control in soluble

poly 2-methoxy-5-((2 '-ethylhexyl)oxy)-1,4-phenylenevinylene (MEHPPV): Synthesis,

optical properties, and energy transfer. Journal of the American Chemical Society 2000,

122, 2244-2251.

(189) Traiphol, R.; Charoenthai, N.; Srikhirin, T.; Kerdeharoen, T.; Osotchan, T.;

Maturos, T.: Chain organization and photophysics of conjugated polymer in poor

solvents: Aggregates, agglomerates and collapsed coils. Polymer 2007, 48, 813-826.

(190) Wang, P.; Cuppoletti, C. M.; Rothberg, L. J.: Bimodal inhomogeneity in

conjugated polymer spectroscopy: Experimental tests of a two conformation model.

Synthetic Metals 2003, 137, 1461-1463.

(191) Liu, J.; Shi, Y. J.; Ma, L. P.; Yang, Y.: Device performance and polymer

morphology in polymer light emitting diodes: The control of device electrical properties

and metal/polymer contact. Journal of Applied Physics 2000, 88, 605-609.

(192) Quan, S.; Teng, F.; Xu, Z.; Quan, L.; Zhang, T.; Liu, D.; Hou, Y.; Wang,

Y.; Xu, X.: Temperature dependence of photoluminescence in MEH-PPV blend films.

Journal of Luminescence 2007, 124, 81-84.

(193) Gierschner, J.; Cornil, J.; Egelhaaf, H. J.: Optical bandgaps of pi-

conjugated organic materials at the polymer limit: Experiment and theory. Advanced

Materials 2007, 19, 173-191.

(194) Peeters, E.; Ramos, A. M.; Meskers, S. C. J.; Janssen, R. A. J.: Singlet and

triplet excitations of chiral dialkoxy-p-phenylene vinylene oligomers. Journal of

Chemical Physics 2000, 112, 9445-9454.

(195) Chang, R.; Hsu, J. H.; Fann, W. S.; Liang, K. K.; Chiang, C. H.; Hayashi,

M.; Yu, J.; Lin, S. H.; Chang, E. C.; Chuang, K. R.; Chen, S. A.: Experimental and

theoretical investigations of absorption and emission spectra of the light-emitting

polymer MEH-PPV in solution. Chemical Physics Letters 2000, 317, 142-152.

148

(196) Lim, T. S.; Hsiang, J. C.; White, J. D.; Hsu, J. H.; Fan, Y. L.; Lin, K. F.;

Fann, W. S.: Single short-chain conjugated polymer studied with optical spectroscopy: A

donor-accepter system. Physical Review B 2007, 75, 9.

(197) Wang, C. C.; Tsai, H. H.; Shih, H. H.; Jeon, S. H.; Xu, Z. H.; Williams, D.;

Iyer, S.; Sanchez, T. C.; Wang, L.; Cotlet, M.; Wang, H. L.: Synthesis and

Characterization of Ethylene Glycol Substituted Poly(phenylene Vinylene) Derivatives.

Acs Applied Materials & Interfaces 2010, 2, 738-747.

(198) Park, S. J.; Gesquiere, A. J.; Yu, J.; Barbara, P. F.: Charge injection and

photooxidation of single conjugated polymer molecules. Journal of the American

Chemical Society 2004, 126, 4116-4117.

(199) Samuel, I. D. W.; Crystall, B.; Rumbles, G.; Burn, P. L.; Holmes, A. B.;

Friend, R. H.: THE EFFICIENCY AND TIME-DEPENDENCE OF LUMINESCENCE

FROM POLY(P-PHENYLENE VINYLENE) AND DERIVATIVES. Chemical Physics

Letters 1993, 213, 472-478.

(200) Footnote, Q. Y.: The quantum yields of BPPV and MEH-PPV in THF

were measured by using standard solutions of anthracene in cyclohexane and

rhodamine 101 in ethanol, respectively. Solutions exhibiting absorbances less than 0.1

were used at a single excitation wavelength with constant excitation and emission slit

widths. The refractive indices of the solvent systems were taken into account during the

calculation.

(201) Collison, C. J.; Rothberg, L. J.; Treemaneekarn, V.; Li, Y.: Conformational

effects on the photophysics of conjugated polymers: A two species model for MEH-PPV

spectroscopy and dynamics. Macromolecules 2001, 34, 2346-2352.

(202) Sumpter, B. G.; Kumar, P.; Mehta, A.; Barnes, M. D.; Shelton, W. A.;

Harrison, R. J.: Computational study of the structure, dynamics, and photophysical

properties of conjugated polymers and oligomers under nanoscale confinement. Journal

of Physical Chemistry B 2005, 109, 7671-7685.

(203) Lin, H. Z.; Hania, R. P.; Bloem, R.; Mirzov, O.; Thomsson, D.; Scheblykin,

I. G.: Single chain versus single aggregate spectroscopy of conjugated polymers. Where

is the border? Physical Chemistry Chemical Physics 2010, 12, 11770-11777.

(204) Lin, H. Z.; Tabaei, S. R.; Thomsson, D.; Mirzov, O.; Larsson, P. O.;

Scheblykin, I. G.: Fluorescence blinking, exciton dynamics, and energy transfer domains

in single conjugated polymer chains. Journal of the American Chemical Society 2008,

130, 7042-7051.

(205) Hao, X. T.; McKimmie, L. J.; Smith, T. A.: Spatial Fluorescence

Inhomogeneities in Light-Emitting Conjugated Polymer Films. Journal of Physical

Chemistry Letters 2011, 2, 1520-1525.

(206) Kwak, E. S.; Vanden Bout, D. A.: Fully time-resolved near-field scanning

optical microscopy fluorescence imaging. Analytica Chimica Acta 2003, 496, 259-266.

(207) Cadby, A.; Dean, R.; Fox, A. M.; Jones, R. A. L.; Lidzey, D. G.: Mapping

the fluorescence decay lifetime of a conjugated polymer in a phase-separated blend using

a scanning near-field optical microscope. Nano Letters 2005, 5, 2232-2237.

149

(208) Conwell, E. M.; Perlstein, J.; Shaik, S.: Interchain photoluminescence in

poly(phenylene vinylene) derivatives. Physical Review B 1996, 54, R2308-R2310.

(209) Gierschner, J.; Ehni, M.; Egelhaaf, H. J.; Milian Medina, B.; Beljonne, D.;

Benmansour, H.; Bazan, G. C.: Solid-state optical properties of linear polyconjugated

molecules: pi-stack contra herringbone. Journal of Chemical Physics 2005, 123, 9.

(210) Lowe, C.; Weder, C.: Oligo(p-phenylene vinylene) excimers as molecular

probes: Deformation-induced color changes in photoluminescent polymer blends.

Advanced Materials 2002, 14, 1625-1629.

(211) Bjorklund, T. G.; Lim, S. H.; Bardeen, C. J.: Use of picosecond

fluorescence dynamics as an indicator of exciton motion in conjugated polymers:

Dependence on chemical structure and temperature. Journal of Physical Chemistry B

2001, 105, 11970-11977.

(212) Jakubiak, R.; Yan, M.; Wan, W. C.; Hsieh, B. R.; Rothberg, L. J.:

Description and importance of interchain excited states in conjugated polymer

photophysics. Israel Journal of Chemistry 2000, 40, 153-157.

(213) Davydov, A. S.: Theory of Molecular Excitons; McGraw-Hill: New York,

1962.

(214) Clark, J.; Chang, J. F.; Spano, F. C.; Friend, R. H.; Silva, C.: Determining

exciton bandwidth and film microstructure in polythiophene films using linear absorption

spectroscopy. Applied Physics Letters 2009, 94, 3.

(215) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H. J.; Hanack, M.;

Hohloch, M.; Steinhuber, E.: Tuning of fluorescence in films and nanoparticles of

oligophenylenevinylenes. Journal of Physical Chemistry B 1998, 102, 1902-1907.

(216) Brown, P. J.; Thomas, D. S.; Kohler, A.; Wilson, J. S.; Kim, J. S.;

Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H.: Effect of interchain interactions on the

absorption and emission of poly(3-hexylthiophene). Physical Review B 2003, 67, 16.

(217) Kim, D. Y.; Grey, J. K.; Barbara, P. F.: A detailed single molecule

spectroscopy study of the vibronic states and energy transfer pathways of the conjugated

polymer MEH-PPV. Synthetic Metals 2006, 156, 336-345.

(218) Menon, A.; Galvin, M.; Walz, K. A.; Rothberg, L.: Structural basis for the

spectroscopy and photophysics of solution-aggregated conjugated polymers. Synthetic

Metals 2004, 141, 197-202.

(219) Collini, E.; Scholes, G. D.: Electronic and Vibrational Coherences in

Resonance Energy Transfer along MEH-PPV Chains at Room Temperature. Journal of

Physical Chemistry A 2009, 113, 4223-4241.

(220) Fakis, M.; Anestopoulos, D.; Giannetas, V.; Persephonis, P.;

Mikroyannidis, J.: Femtosecond time resolved fluorescence dynamics of a cationic water-

soluble poly(fluorenevinylene-co-phenylenevinylene). Journal of Physical Chemistry B

2006, 110, 12926-12931.

(221) Hardison, L. M.; Zhao, X. Y.; Jiang, H.; Schanze, K. S.; Kleiman, V. A.:

Energy Transfer Dynamics in a Series of Conjugated Polyelectrolytes with Varying

Chain Length. Journal of Physical Chemistry C 2008, 112, 16140-16147.

150

(222) Herz, L. M.; Silva, C.; Phillips, R. T.; Setayesh, S.; Mullen, K.: Exciton

migration to chain aggregates in conjugated polymers: influence of side-chain

substitution. Chemical Physics Letters 2001, 347, 318-324.

(223) Ajayaghosh, A.; Vijayakumar, C.; Praveen, V. K.; Babu, S. S.; Varghese,

R.: Self-location of acceptors as "isolated" or "stacked" energy traps in a supramolecular

donor self-assembly: A strategy to wavelength tunable FRET emission. Journal of the

American Chemical Society 2006, 128, 7174-7175.

(224) Kim, Y. H.; Kim, D.; Jeoung, S. C.; Han, J. Y.; Jang, M. S.; Shim, H. K.:

Ultrafast energy-transfer dynamics between block copolymer and pi-conjugated polymer

chains in blended polymeric systems. Chemistry of Materials 2001, 13, 2666-2674.

(225) Brabec, C. J.: Organic photovoltaics: technology and market. Solar Energy

Materials and Solar Cells 2004, 83, 273-292.

(226) Service, R. F.: Outlook Brightens for Plastic Solar Cells. Science 2011,

332, 293-293.

(227) Dang, M. T.; Hirsch, L.; Wantz, G.: P3HT:PCBM, Best Seller in Polymer

Photovoltaic Research. Advanced Materials 2011, 23, 3597-3602.

(228) Kitamura, C.; Tanaka, S.; Yamashita, Y.: Design of narrow-bandgap

polymers. Syntheses and properties of monomers and polymers containing aromatic-

donor and o-quinoid-acceptor units. Chemistry of Materials 1996, 8, 570-578.

(229) Kaulach, I.; Muzikante, I.; Gerca, L.; Plotniece, M.; Roze, M.; Kalnachs, J.;

Shlihta, G.; Shipkovs, P.; Kampars, V.; Tokmakov, A.: PV and magnetic field effects in

poly(3-hexylthiophene)-fullerene cells doped with phthalocyanine soluble derivative.

Eur. Phys. J.-Appl. Phys 2007, 40, 169-173.

(230) Ltaief, A.; Ben Chadbane, R.; Bouazizi, A.; Davenas, J.: Photovoltaic

properties of bulk heterojunction solar cells with improved spectral coverage. Mater. Sci.

Eng. C-Biomimetic Supramol. Syst. 2006, 26, 344-347.

(231) Xu, Z. X.; Roy, V. A. L.; Low, K. H.; Che, C. M.: Bulk heterojunction

photovoltaic cells based on tetra-methyl substituted copper(II)

phthalocyanine:P3HT:PCBM composite. Chem. Commun. 2011, 47, 9654-9656.

(232) Chabinyc, M. L.: Characterization of semiconducting polymers for thin

film transistors. Journal of Vacuum Science & Technology B 2008, 26, 445-457.

(233) Chabinyc, M. L.: X-ray scattering from films of semiconducting polymers.

Polymer Reviews 2008, 48, 463-492.

(234) Hao, X. T.; Hosokai, T.; Mitsuo, N.; Kera, S.; Okudaira, K. K.; Mase, K.;

Ueno, N.: Control of the interchain pi-pi interaction and electron density distribution at

the surface of conjugated poly(3-hexylthiophene) thin films. Journal of Physical

Chemistry B 2007, 111, 10365-10372.

(235) Hao, X.-T.; Chan, N. Y.; Dunstan, D. E.; Smith, T. A.: Conformational

Changes and Photophysical Behavior in Poly 2-methoxy-5-(2 '-ethyl-hexyloxy)-1,4-

phenylene vinylene Thin Films Cast under an Electric Field. Journal of Physical

Chemistry C 2009, 113, 11657-11661.

151

(236) Lee, S. S.; Loo, Y.-L.: Structural Complexities in the Active Layers of

Organic Electronics. In Annual Review of Chemical and Biomolecular Engineering, Vol

1; Prausnitz, J. M. D. M. F. S. M. A., Ed., 2010; Vol. 1; pp 59-78.

(237) Ravindranath, R.; Ajikumar, P. K.; Bahulayan, S.; Hanafiah, N. B. M.;

Baba, A.; Advincula, R. C.; Knoll, W.; Valiyaveettil, S.: Ultrathin conjugated polymer

network films of carbazole functionalized poly(p-phenylenes) via electropolymerization.

Journal of Physical Chemistry B 2007, 111, 6336-6343.

(238) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock,

E. W.; Miller, T. M.: Photophysics of phenylenevinylene polymers. Synthetic Metals

1996, 80, 41-58.

(239) Hagfeldt, A.; Gratzel, M.: LIGHT-INDUCED REDOX REACTIONS IN

NANOCRYSTALLINE SYSTEMS. Chemical Reviews 1995, 95, 49-68.

(240) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F.: PHOTOINDUCED

ELECTRON-TRANSFER FROM A CONDUCTING POLYMER TO

BUCKMINSTERFULLERENE. Science 1992, 258, 1474-1476.

(241) Kim, H.; Lee, K.; Park, Y. G.; Suh, H.: Role of interpenetrating networks in

the device performance of polymer-fullerene photovoltaic cells. Journal of the Korean

Physical Society 2003, 42, 183-186.

(242) Wang, J.; Wang, D. L.; Moses, D.; Heeger, A. J.: Dynamic quenching of 5-

(2 '-ethyl-hexyloxy)-p-phenylene vinylene (MEH-PPV) by charge transfer to a C-60

derivative in solution. Journal of Applied Polymer Science 2001, 82, 2553-2557.

(243) Dyakonov, V.; Zoriniants, G.; Scharber, M.; Brabec, C. J.; Janssen, R. A.

J.; Hummelen, J. C.; Sariciftci, N. S.: Photoinduced charge carriers in conjugated

polymer-fullerene composites studied with light-induced electron-spin resonance.

Physical Review B 1999, 59, 8019-8025.

(244) Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.;

Hummelen, J. C.; Janssen, R. A. J.: Synthesis, photophysical properties, and photovoltaic

devices of oligo(p-phenylene vinylene)-fullerene dyads. Journal of Physical Chemistry B

2000, 104, 10174-10190.

(245) Segura, J. L.; Martin, N.; Guldi, D. M.: Materials for organic solar cells: the

C-60/pi-conjugated oligomer approach. Chemical Society Reviews 2005, 34, 31-47.

(246) Footnote: PhotochemCAD software was using to calculated the individual

spectral overlap integral between the ternary component nanoparticles: MEH-

PPV/PCBM (5.7 x 1014

M-1

cm-1

nm4) < BPPV/PCBM (3.7 x 10

14 M

-1cm

-1nm

4) <

BPPV/MEH-PPV (1.4 x 1018

M-1

cm-1

nm4).

(247) Cook, S.; Ohkita, H.; Kim, Y.; Benson-Smith, J. J.; Bradley, D. D. C.;

Durrant, J. R.: A photophysical study of PCBM thin films. Chemical Physics Letters

2007, 445, 276-280.

(248) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.;

Bazan, G. C.: Efficiency enhancement in low-bandgap polymer solar cells by processing

with alkane dithiols. Nature Materials 2007, 6, 497-500.

152

(249) Snaith, H. J.; Arias, A. C.; Morteani, A. C.; Silva, C.; Friend, R. H.: Charge

generation kinetics and transport mechanisms in blended polyfluorene photovoltaic

devices. Nano Letters 2002, 2, 1353-1357.

(250) Snaith, H. J.; Friend, R. H.: Morphological dependence of charge

generation and transport in blended polyfluorene photovoltaic devices. Thin Solid Films

2004, 451, 567-571.

(251) Jenekhe, S. A.; Osaheni, J. A.: EXCIMERS AND EXCIPLEXES OF

CONJUGATED POLYMERS. Science 1994, 265, 765-768.

(252) Osaheni, J. A.; Jenekhe, S. A.: EFFICIENT BLUE LUMINESCENCE OF

A CONJUGATED POLYMER EXCIPLEX. Macromolecules 1994, 27, 739-742.

(253) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.;

Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R.: Charge carrier

formation in polythiophene/fullerene blend films studied by transient absorption

spectroscopy. Journal of the American Chemical Society 2008, 130, 3030-3042.

(254) Morteani, A. C.; Dhoot, A. S.; Kim, J. S.; Silva, C.; Greenham, N. C.;

Murphy, C.; Moons, E.; Cina, S.; Burroughes, J. H.; Friend, R. H.: Barrier-free electron-

hole capture in polymer blend heterojunction light-emitting diodes. Advanced Materials

2003, 15, 1708-+.

(255) Footnote3: The number of MEH-PPV molecules per nanoparticle

composition were estimated from the doping weight percent of ME-PPV by assuming i)

an average nanoparticle diameter of 36 nm and ii) MEH-PPV collapsed cylinder

confirmation (L = 10 nm and R = 2.5 nm). By calculating the volume of a sphere and

cylinder, approximately 124 molecules of MEH-PPV could reside within a single

undoped MEH-PPV nanoparticle. The 50 wt% monofunctional doped nanoparticles, for

example, calculate as: 124 * 0.5 = 62 MEH-PPV molecules per nanoparticle.