IMPROVING EFFICIENCY OF ORGANIC PHOTOVOLTAICS 11/08/12 Nanjia Zhou Northwestern University.

IMPROVING EFFICIENCY OF ORGANIC PHOTOVOLTAICS

11/08/12

Nanjia ZhouNorthwestern University

http://econews.com.au/news-to-sustain-our-world/un-takes-hard-look-at-20-years-since-rio-earth-summit/http://planetark.org/enviro-news/item/53111http://thecityfix.com/blog/choking-on-smog/

Nanjia ZhouChang/Marks Group

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International Energy Outlook 2011Nanjia ZhouChang/Marks Group

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International Energy Outlook 201103/23/2012

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Department of Materials Science03/23/2012

Photo courtesy: NREL

Clean, renewable

Reduced dependence on Fossil Fuels

Distributed generation

Proven, reliable

Modularity and Scalability

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http://www.heliatek.com/technologie/organische-photovoltaik/?lang=en



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National Renewable Energy Laboratory (NREL)



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Mitsubishi Chemical announced a certified 10.0% cell

National Renewable Energy Laboratory (NREL) Nanjia ZhouChang/Marks Group

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Department of Materials Science03/23/2012


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http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2000/press.html


Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa "for the discovery and development of conductive polymers".

The Nobel Prize in Chemistry 2000


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Common conducting polymers

Soliton

Polaron

Bipolaron


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Gate

Insulating layer

Organic layer

Source Drain

+ ++ +++ + + ++ + + + + + + + + +

-30V

- - - - - - - - - - - - - - - - - - -

++++++++++++++

1. Organic Field Effect Transistor (OFET)


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1. OFET


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1. OFET


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2. Organic Light Emitting Diode (OLED)07 08 09 10

2.0， 2.2， 2.4， 2.6’’ QVGA2.5’’ QVGA(Landscape)2.6， 2.8， 3.0’’ LQVGA3.1’’ WVGA

4.1’’WQVGA

3.5’’ WQVGA*3D4.8’’ WVGA7.0’’ WSVGA

14-15.4’’WXGA

21-23’’UXGA 40’’/42’’ Full HD


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2. OLED

1. Brighter, thinner, lighter, faster 2. Bright from all viewing angles 3. Need less power to run 4. A lot cheaper to produce

5. Expanding memory capability - coating new layer on top of existing one

6. Wider temperature range

OLED is a display device that sandwiches carbon based films between the two electrodes and when voltage is applied creates light.


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2. OLED


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3. OPV


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3. OPV


Holger Spanggaard, Frederik C. Krebs, Solar Energy Materials and Solar Cells, Volume 83, Issues 2–3, 15 June 2004, Pages 125-146

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Harvard Science in the News: https://sitn.hms.harvard.edu/sitnflash_wp/2012/03/issue113/


OPV vs. Silicon Cell


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Narrower absorption range than silicon cellLower current output

Donor: Electron rich materialAcceptor: Electron poor material

Bulk Heterojunction: Blend of D/A materials forming a heterogeneous mixture

Domain size

OPV vs. Silicon Cell


Li et al. Nature Photonics 6, 153–161, (2012)

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Kietzke, Advances in OptoElectronicsVolume 2007 (2007), Article ID 40285

a. Short circuitb. Open circuitc. Forward biasd. Reverse bias

Characterization


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• Class A Solar Cell Analyzer– AM 1.5G simulated light– Scan area: 5 x 5 cm– Light and dark evaluation– Series resistance evaluation– Xenon lamp with an adjustable

intensity of 850 - 1150 W/m2

– Calibrated with KG3-filtered Si standard from NREL. Spectral mismatch = 1.00

• Spectral Response– Examines wavelengths from 400 - 1100

nm– Includes 15 filters in this range

• Circulating Bath– Cell test temperature variable from -

15C to +75C

NU Characterization


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ORGANIC PHOTOVOLTAICSCathode – Conventional cellLow work function metal: Ca/Al(Anode – Inverted CellHigh work function metal: Ag/Au)Cathode IFL – LiF, n-type semiconductors: ZnO, TiO2Active Layer: blend of donor and acceptor materials

Transparent conductive substrate – ITO, Carbon based material

Anode IFL: PEDOT:PSSP-type semiconductor: NiO, WO3, MoO3, V2O5

graphene oxide


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ORGANIC PHOTOVOLTAICSμED μCT

μCC

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ORGANIC PHOTOVOLTAICS

Band Gap Tuning:• Lower band gap to raise Jsc.• Higher IP polymers and lower EA

acceptors to reduce energy loss during charge separation.

STRATEGIES FOR EFFICIENCY ENHANCEMENT

Decrease LUMO-LUMO offset

Morphology Control:• Domain sizes comparable to exciton diffusion

length• Efficient carrier transport• Experimentally: solvent, additive, processing

conditions, and self organizing properties of materials

Photon Management:• Alter device architecture to focus photo

absorption in active layer• Potentially reduce active layer thickness for

higher IQE

Ordered Heterojunction:• Donor/acceptor form interdigitated

morphology within nanometer scale• Direct pathways of donor/acceptor to

electrodes

Nelson, J. Materials Today 2011, 14, 462–470.Li, G.; Zhu, R.; Yang, Y. 2012, 1–9.Kang, M.-G.; Xu, T.; Park, H. J.; Luo, X.; Guo, L. J. Adv. Mater. 2010, 22, 4378.

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RESEARCH OUTLINE

Novel high efficiency donor-type polymers, study of device characteristics

Materials Development

Nanoscale patterning of polymer active layer surface.Photonic and plasmonic enhancement of photocurrent

Plasmonic Structure

Tandem device to enhance light harvesting;Optimize electron/hole transport

Nanowire, Tandem, Hybrid, etc.


Band Gap Tuning

Photon Management

Novel Device Architecture

Processing conditions;Solvent choice;Polymer stacking;Characterization

Morphology-Carrier Transport Relationship

Morphology Control

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Control of nanoscale morphology: Domain size variation

Morphology Carrier Transport Relationship


BILAYER VS. BHJ28


EFFECT OF PROCESSING CONDITIONS29

Annealing

Treat, N. D., Brady, M. A., Smith, G., Toney, M. F., Kramer, E. J., Hawker, C. J. and Chabinyc, M. L. (2011), Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater., 1: 82–89. doi: 10.1002/aenm.201000023


EFFECT OF PROCESSING CONDITIONS30

Solvent choice

Lee et al. J. Am. Chem. Soc. 130, 3619 (2008)


CRYSTALLINITY AND PACKING31

Huang et al. J. Phys. Chem. C, 2012, 116 (18), pp 10238–10244


CHARACTERIZATION32

Lu et al. Nature Communications 3, Article number: 795

Atomic Force Microscope (AFM),Transmission Electron Microscope (TEM)

Grazing Incidence Wide Angle X-ray Scattering (GIWAXS)

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Increase DIO concentration


PC71BM BTIBDT

High sensitivity of DIO

0 0.5% 1.0% 3.0% 10.0%

N. Zhou, A. Guerrero, H. Heitzer, S. Lou et. al.

PRECISE MANIPULATION OF DOMAIN SIZES

S

N

S

O OR1

S

S

OR2

OR2

n

High Performance Donor Materials



Li et al. Nature Photonics 6, 153–161, (2012)

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Zhou et al. Adv. Mater., 24: 2242–2248

1) R1=2-hexyldecyl, R2=2-ethylhexyl; PCE=3.4%2) R1=2-butyloctyl, R2=2-ethylhexyl; PCE=4.8%3) R1=2-hexyldecyl, R2=n-dodecyl; PCE=5.5%4) R1=2-butyloctyl, R2=n-decyl; PCE=3.3%5) R1=n-octyl, R2=2-butyloctyl; PCE=1.6%

BTIBDT BASED COPOLYMERS35

BTIBDT BASED COPOLYMERSBithiophene Imide (BTI) and Benzodithiophene (BDT) Copolymers Inverted OPVs

S

S

OR2

OR2

Sn Sn

S

N

S

O OR1

Br BrS

N

S

O OR1

S

SOR2

OR2

+

n

P1: R1 = 2-butyloctyl, R2 = 2-ethylhexylP2: R1 = 2-hexyldodecyl, R2 = n-dodecyl

i

The electron deficiency of the BTI units leads to polymers with a low-lying HOMOs (~-5.6 eV). Inverted solar cells are fabricated to investigate the OPV performance of the BTI-based polymers and achieve power conversion efficiencies up to 5.5%, with substantial Vocs above 0.9 V which are among the highest Vocs reported to date for polymer/PCBM solar cells.



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Bithiophene Imide (BTI) and Benzodithiophene (BDT) Copolymers Inverted OPVs


BTIBDT BASED COPOLYMERS• Inverted architectures:

ITO/ZnO/polymer:PC71BM/MoOx/Ag

• Inverted structures avoid oxidation of low work-function cathodes, such as Al or Ca, and acidic PEDOT:PSS etching of ITO, both of which limit conventional devices.

• ZnO is an excellent cathode interfacial layer due to its high electron mobility, excellent thermal stability, and hole-blocking properties.

• MoOx conduction band (-2.60 eV) lies sufficiently above the donor LUMOs (-3.7 eV, estimated from the HOMOs and optical band gaps), to block electrons, while the low work function (-5.6 eV) forms a good anode contact with the donor polymers


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Polymer:PC71BM Solvent Thickness (nm) Voc (V) Jsc (mA/cm2) FF (%) PCE (%)

P1 1:1.5 DCB 95 0.992 4.16 47.9 1.98

P2 1:1 DCB 100 0.940 4.76 56.3 2.52

P1 1:1.5 DCB+DIOa 95 0.876 9.69 51.8 4.39

P2 1:1 DCB+DIOa 100 0.922 9.62 62.0 5.50

J-V Curve EQE and UV-vis

Stability

Device Performance


BTIBDT BASED COPOLYMERS

Stability

• Best device achieved without thermal annealing

• Initial drop due to microstructural reorganization


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BTIBDT BASED COPOLYMERSConventional Device and SCLC measurement

Polymer:PC71BM Voc [V] Jsc [mA/cm2] FF (%) PCE (%)

P1 1:1.5 0.872 7.87 51.1 3.51

P2 1:1 0.905 8.06 55.1 4.02

Space charged limited current (SCLC) measurement

ITO/PEDOT:PSS/polymer/Au

Hole mobilities OTFT: 1.2 × 10-4 and 2.8 × 10-4 cm2V-1s-1 SCLC: 5.5 × 10-5 and 1.9 × 10-4 cm2V-1s-1

for P1 and P2-based OTFTs and hole only devices, respectively

Conventional Device Performance


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• Without DIO, predominantly phase-segregated morphologies for the P1/PC71BM and P2/PC71BM blends.

• The dark regions in the TEM images confirm large PC71BM domain sizes -- far larger than typical exciton diffusion lengths (~10 nm). Consequently, poor exciton dissociation and low current density are expected.

• Significantly more homogeneous morphologies are found for P1/PC71BM and P2/PC71BM films processed with DIO.

• nanosacle phase separation and bicontinuous interpenetrating networks result in more efficient charge separation and transport, leading to more than doubled Jsc.


BTIBDT BASED COPOLYMERSMorphology Optimization

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BTI and Dithienosilole (DTS) Copolymers Inverted OPVs


BTIDTS BASED COPOLYMERS

S

N

S

O O

Br Br

+

P1: R = 2-ethylhexyl, R' = 2-ethylhexylP2: R = n-dodecyl, R' = 2-ethylhexyl

i

S SMe3Sn SnMe3

Si

S

N

S

O O

SS

SiR'R'

R' R'R

R

BTIR-Br2 DTSR'-Sn2

n

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Polymer DCB:DIO

(%:%)

Voc

(V)

Jsc

(mA/cm2)

FF

(%)

PCE (%)

P1 100:0 0.838 6.06 52.6 2.67

P1 98:2 0.834 12.51 53.1 5.54

P2 100:0 0.847 6.46 48.4 2.65

P2 98:2 0.803 12.81 62.3 6.41

X. Guo, N. Zhou, et al. JACS (2012)


BTI BASED COPOLYMERS


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BTIDTG COPOLYMERS

Polymer % of DIO additives Voc (V) Jsc (mA/cm2) FF (%) PCE (%)

PBTISi-C6 2.0 0.824 12.59 60.6 6.29PBTISi-C8 2.0 0.803 12.81 62.3 6.41PBTISi-EH 2.0 0.791 12.49 56.4 5.57PBTIGe-C6 3.0 0.770 11.76 47.2 4.27



BTI/TPD-DTS COPOLYMERS


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S

N

S

O O

SS

SiEHHE

S

N

S

O O

SS

SiEHHE

neat polymerpolymer/PC71BMDCB:DIO=100:0

polymer/PC71BMDCB:DIO=98:2

BTIDTS BASED COPOLYMERSBTI and Dithienosilole (DTS) Copolymers Inverted OPVs

P1

P2π-π stacking distances are 3.6 Å and 3.5 Å for P1 and P2, respectively


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BTIDTS BASED COPOLYMERS


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BTIDTS MORPHOLOGY

TEM images of PBTISi-C8 with different D/A ratio: (a) P:PC71BM (1:1), (b) P:PC71BM (1:1.5), and (c) P:PC71BM (1:2) blend films processed in DCB as solvent with 2% DIO.



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BTIDTG COPOLYMERS



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HiGH FF OPV AND MORPHOLOGY STUDY

X. Guo, N. Zhou, et al. Nature Photonics (2012)


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Nanoscale patterning of polymer active layer surface.Photonic and plasmonic enhancement of photocurrent

Photon Management


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PLASMONIC OPV

Enhancement atFront electrode

M. Heo, H. Cho, J. Jung, J. Jeong, S. Park, J. Kim, Adv. Mater. 2011, 23, 5689M. Kang , T. Xu , H. Park , X. Luo , L.J. GuoAdv. Mater. 2010, 22, 4378J. Yang, J. You, C. Chen, W. Hsu, H. Tan, X. Zhang, Z. Hong, Y. Yang ACS Nano 2011 5 (8), 6210

Device Fabrication

Ag nanowire grating as electrode

blending Au nanoparticle into interfacial layer

metallic nanoparticles at front electrode


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PLASMONIC OPVDevice Fabrication

• Lower transmission at TCO

• Reducing conductivity of TCO

• Disrupting organic/inorganic interface at TCO

• Affecting film quality and possibly polymer packing

• Introducing extra step that cannot be easily incorporated into fabrication, thus affect reproducibility.

Modifying Front Electrode

Modifying Rear Electrode

• No effect on device fabrication prior to metal evaporation

• Easily convert the metallic electrode into a plasmonic grating.

• The grating can increase the amount of light trapped in the active layer through three potential pathways.

• Trap the light in the form of surface plasmon polaritons (SPP).

• Increase the surface area of the interface.

• Increase the diffraction of light.

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PLASMONIC OPV

Collaboration with S. Lubin (Odom group), A. Hryn (Odom group)

Device Fabrication

N. Zhou, S. Lubin, A. Hryn, X. Guo et. al. Manuscript in preparation

ITOZnOActive LayerMoOx

Ag

ITO ZnO solution processing

Active layer Hot embossing

Thermal evaporation of rear electrode

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PLASMONIC OPV

Collaboration with S. Lubin (Odom group), A. Hryn (Odom group)

Device Voc [V] Jsc [mA/cm2] FF (%) PCE (%)

Plasmonic 0.580 11.5 64.0 4.28

No plasmonic 0.582 9.38 68.5 3.74

Device Performance

N. Zhou, S. Lubin, A. Hryn, X. Guo et. al. Manuscript in preparation

Plasmonic 0.565 12.8 56.2 4.07

Sq 600 nm

Sq 400 nm


Tandem device to enhance light harvesting;Optimize electron/hole transport

Novel Device Architectures


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Yang Yang et al. Nature Photonics, 6, 2012, 180

TANDEM SOLAR CELLS


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TANDEM SOLAR CELLS• The maximum efficiency for a two junction tandem under the

AM1.5G spectrum and without concentration is 47 %. At the peak efficiency the top cell has a bandgap of 1.63 eV and the bottom cell has a bandgap of 0.96 eV.


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PCE greater than 40% is the highest efficiency obtained to date for triple junction inorganic photovoltaic cells.

TANDEM SOLAR CELLS


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P3HT PSBTBT

TANDEM ORGANICS SOLAR CELLS

Jin Young Kim, Kwanghee Lee, Nelson E. Coates, Daniel Moses, Thuc-Quyen Nguyen, Mark Dante, and Alan J. HeegerScience 13 July 2007: 317 (5835), 222-225.


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• Dresden, Germany-based OPV thin-film developer Heliatek has achieved a new organic tandem solar cell world record with 10.7% cell efficiency on a 1.1cm2 substrate.

• “Deposition of small organic molecules in a low temperature, roll-to-roll vacuum process”

TANDEM ORGANICS SOLAR CELLS

• Yang Yang group, UCLA• reaches 10.6% efficiency a world’s first for

polymer organic photovoltaic devices• “Their tandem structure consists of a front

cell with a larger (or high) band gap material and a rear cell with a smaller (or low) band gap polymer, connected by a designed interlayer.”

• Sumitomo's low–band gap polymer


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INTERLAYERMetal ohmic contacts with sub-cells

Ag, Au: Yakimov et al. (2002)

Colsmann et al. (2006) used n-doped and p-doped organic layers on either side of a thin Au layer as hole blocking and electron blocking layer, respectively.

first time reaches nearly additive Voc =1 V

p-type layer: high work function metal oxides such as molybdenum oxide (MoO3) or tungsten oxide (WO3), PEDOT:PSS

n-type layer: solution-processed ZnO and TiO2

A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 2002, 80, 1667.A. Colsmann, J. Junge, C. Kayser and U. Lemmer, Appl. Phys. Lett., 2006, 89, 203506.


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PROCESSING• Sustain the solution process of the top polymer layer and protect the bottom

polymer layer• deposited ITO or thermally evaporated semitransparent metal layers

such as Au• An n-type layer using ZnO or TiO2 is coated from an alcohol based solvent• aqueous based PEDOT:PSS

• Difficult to find a solvent for the top polymer layer that does not dissolve the bottom polymer layer. (high boiling point solvents for the top polymer layer may be detrimental to the integrity of underlying layers)

• Wettability of interlayer and top cells• Modified PEDOT:PSS layer (dilute by alcohol, add surfactant)• Thin layer of metal


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Yang Yang et al. Nature Photonics, 6, 2012, 180


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N. Zhou, et al. Manuscript in preparation


ITO NANOWIRE ELECTRODE69

N. Zhou, et al. Manuscript in preparation

SiO OO

ITO

IFL by ALD

Vertically grown ITO nanowire on ITO substrate

Filled with active layer

Vertically grown ITO nanowire on ITO substrate

Surface modification

SAM growth

Active layer fillingElectrode deposition

• Inefficiency of hole transport due to relatively poor hole mobility of polymer semiconductor comparing to the electron mobility of PCBM

• Reduce hole transport distance• Greatly enhance electrode (anode) surface area• Potentially increase hole transport and hole collection efficiencies

Li, S. Q.; Guo, P.; Zhang, L.; Zhou, W.; Odom, T. W.; Seideman, T.; Ketterson, J. B.; Chang, R. P. H. ACS Nano 2011, 5, 9161–9170.


ITO NANOWIRE ELECTRODE70

N. Zhou, P. Guo, S. Li, et. al.

Facilitate hole transport

Current fabricated device structure

Proposed device structure


SUMMARY AND OUTLOOKMorphology control• Manipulation of domain (PCBM cluster) size• Processing conditions• Characterization of domain size • Relate to J-V parameters

Photon management• A simple method through hot embossing• Convert the standard Ag electrode • >20% increase in Jsc

Novel Device Architectures• Tandem OPVs• Facilitate carrier transport/reduce bimolecular recombination probability• Active layer filtration through solution processing/surface functionalization• Further optimize processing conditions to create interdigitated structure

Materials development• BTI-BDT demonstrated one of the highest Voc• BTI-DTS achieved performance of 6.4%• Highest performance in literatures: 8.7%


ACKNOWLEDGEMENTAdvisers: Prof. Tobin Marks Prof. RPH Chang

Prof. Antonio Facchetti

Marks groupDr. Xugang GuoMimi LouStephen Loser

Chang group:Dr. D. B. BuchholzShiqiang LiPeijun GuoMarks B team members

Committee Members: Prof. Lin Chen Prof. Mark Hersam

Collaborators:Dr. Antonio Guerrero, Prof. Juan Bisquert group, SpainSteven Lubin and Alex Hryn, Prof. Teri Odom group

Thank you!

“The Apollo Projects Of Our Time”

- Steven ChuCourtesy of Backwoods Solar Electric Systems & NREL

IMPROVING EFFICIENCY OF ORGANIC PHOTOVOLTAICS 11/08/12 Nanjia Zhou Northwestern University.

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