Role of structure and morphology in organic electronics · ¾Paracrystallinity: Warren – Averbach...
Transcript of Role of structure and morphology in organic electronics · ¾Paracrystallinity: Warren – Averbach...
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Michael ToneySynchrotron Materials Sciences Division
Stanford Synchrotron Radiation Lightsource (SSRL)SLAC National Accelerator Laboratory
http://www-ssrl.slac.stanford.edu/toneygroup
Role of structure and morphologyin organic electronics
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Ed Kramer
28/5/1939 - 12/27/2014
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Outline
3
1. Organic Electronics Thin FilmsWide range of length scales
2. Quantitative Molecular PackingSmall molecules – Pentacene (&TIPS-Pentacene)Polymers – PBTTT
3. Nanoscale (dis)order - lattice variations, “grains”Paracrystallinity
4. Organic Photovoltaics (OPV) Blendsnm-scale blend morphology
5. Summary
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SLAC National Accelerator Laboratory
• ~1,700 employees + 3,400 users, visitingscientists per year; 300 postdocs andstudents; 75 PhD theses
• Major DOE-BES scientific user facilities:o Linac Coherent Light Source (LCLS)o Stanford Synchrotron Radiation
Lightsource (SSRL)• Science Programs:o Particle Physics & Astrophysicso Accelerator Researcho Photon Sciences
Chemical and Materials SciencesSustainable Energy Materials
4
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SLAC National Accelerator Laboratory
5
SSRL
LCLS-offices
Few other labs in the world currently hosts such a unique andcomprehensive suite of x-ray sources and instrumentation
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Organic Semiconductors
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PolyICSony
OLEDsDisplaysLighting
GE
OFETsDisplay Backplanes
RFID TagsMemory
Logic
OPVPlastic Solar Cells
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Organic Semiconductors
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Ease of processing:• semiconducting inks• printing - i.e. newsprint• low temperature deposition• ambient pressure
Conjugated bonding structure allowsfor semiconducting properties
Unique Opportunities:• Flexible substrates• Large area/High throughput• Chemically tailor properties• Sensing capabilities• Biocompatible
Organic Semiconductor Materials
Small Molecules:Pentacene,TIPS-Pentacene
Polymers:P3HTPBTTT
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Organic Semiconductors
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Transistors (OFET)• 10-5 cm2/Vs (1980s) -> 20-30 cm2/Vs (2014) & poly-Si
Photovoltaics (OPV)
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Organic Semiconductors
9
Chemistry &Processing
PhysicalMicrostructure
Performance• transistors• photovoltaics
Design Rules for New Functional Organic Electronics
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How does structure affect performance?
10Rivnay, Mannsfeld, Miller, Salleo, Toney, Chem. Rev. 112, 5488 (2012).
OPV
OFET
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Outline
11
1. Organic Electronics Thin FilmsWide range of length scales
2. Quantitative Molecular PackingSmall molecules – Pentacene (& TIPS-Pentacene)Polymers – PBTTT
3. Nanoscale (dis)order - lattice variations, “grains”Paracrystallinity: Warren – Averbach
4. Organic Photovoltaics (OPV) Blendsblend morphology
5. Summary PBTTT:Chad MillerRoman GyselNicky Cates MillerAlex MayerMike McGeheeEK ChoChad RiskoJean Luc Brédas
PentaceneStefan MannsfeldZhenan BaoAjay VirkarColin Reese
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Pentacene Films
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Single crystal transistors on SiO2:
= 0.1 - 0.5 cm2/Vs
Why do pentacene TFTs perform as good or better thanpentacene single crystal transistors?
Pentacene:
Butko et al., Appl. Phys. Lett. 83, 4773 (2003).
Knipp et al, J.Appl. Phys. 93, 347 (2003).
= 0.3 cm2/VsTakeya et al., J. Appl. Phys. 94, 5800 (2003).
= 0.62 cm2/Vs
= 1.0 - 5.5 cm2/Vs on other substrates
Polycrystalline thin film transistors on SiO2:Klauk et al., J.Appl. Phys. 92, 5259 (2002).= 0.4 cm2/Vs
Film packing bulk packing: Fritz et al., JACS 126, 4084 (2004).
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X-ray Diffraction and Scattering
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Q = (4 ) sin
Baker et al., Langmuir 2010, 26, 9146ACS Nano 6, 5465 (2012), JACS 134.,6337 (2012);Advanced Materials 23, 127 (2011); Chem Rev 112, (2012)
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Pentacene Films
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Qxy
b*
a*
Pentacene (small molecule) films:• highly textured 2D powder
• aligned out-of plane (001)• in-plane powder: randomorientation in substrate
(1 1)
(1-1)
(-1 1)
(-1 -1)
(1 -2)(-1 -2) (0 -2)
(2 0)
(0 2) (1 2)(-1 2)
(-2 0)
monolayer
(00Qz) (10Qz) (20Qz)
Qz
Qxy
(11 L)&
(1-1 L) c*
(0 -2 L)
(12 L)&
(1-2 L)
thin film
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Pentacene Films
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Q
20 nm film
Qxy
Qz
(±1 ±1 L)
(0 ±2L)(±1 ±2 L)
(±2 0L)
a = 5.920 Å, b = 7.556 Å, c = 15.54 Å= 81.6 deg, = 87.2 deg, = 89.84 deg
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Pentacene Films – structure refinement
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1. Diffraction peaks& intensities 3. Calculation of integrated intensities from theory
Bragg peak
Bragg rod
Monolayer films
Multilayer films
4. Crystallographic refinement
2. Self-consistent indices and extract unit cell
( ) exp( )i iF q f iqr
2( ) ( ) | ( ) |hkl ABCD hkl hklI q KLPA D q F qK- scaling factorL-P-A-D-
Lorentz factorpolarization correctioncrossed-beam correctionDebye-Waller factor
f -r -q -
i
i
atomic scattering factoratom positionmomentum vector
2 2 2( ) ( ) ( ) | ( ) |hk z ABCD hk z z xy zI q KLPA D q T2 q F q qFormula for intensity along Bragg rods:
Formula for Bragg peaks:
T- Fresnel transm. coeff.
Atoms
(1)
(2)
Q. Yuan, et al, JACS. 130, 3502 (2008); Chem Matls. 20, 2763 (2008).
Crystallographic refinement of diffraction intensities
Necessary simplification:• assume rigid molecules. Reduces degrees of
freedom from 72 to 9 -> makes feasible• justified for fused-ring aromatic molecules.
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Pentacene Films – Structure
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0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.40.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
q z[Å
]
qxy [Å]
ObservedCalculated
55°
View down ontosubstrate plane
substrate plane
20 nm film
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.40.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
q z[Å
]
qxy [Å]
ObservedCalculated
substrate planeView down ontosubstrate plane
60 nm film
a
b
18.5°
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Pentacene Films
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Centered - Rectangular cell:• molecules vertical• a= 5.905 Å, b= 7.562 Å
Mannsfeld, Virkar, Reese, Bao, Toney,Adv. Mater. 21, 2294 (2009).
substrate plane
52°
View down ontosubstrate plane
0.0 0.1 0.2 0.3 0.40
1
2
3
4
5
6
7
8
9 meas. I01(qZ) calc. I01(qZ) meas. I10(qZ) calc. I10(qZ) meas. I11(qZ) calc. I11(qZ) meas. I02(qZ) calc. I02(qZ) meas. I12(qZ) calc. I12(qZ)
I(qZ)a
.U.
qZ [Å-1]
Pentacene sub-monolayer (nominal 1.5 nm, Tsub=60°C) on SiO2.
a
b
Markus theory of electron transfer:• more overlap in monolayer• explains higher mobility
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Tuning the structure
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Solution Shearing to tune properties
G. Giri, .., M.F. Toney, Z. Bao, Nature 480, 504–508 (2011)
TIPS-pentacene
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52°
Organic Thin Film Microstructure - Polymers
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Semicrystalline polymers: partly crystalline & partly disordered
Brinkmann et al., Adv Mater. (2006)
Small Molecules Semi-crystalline Polymers:• P3HT, PBTTT
transport:• fast: (001) – along chains• pretty fast: (010) – along stacking• slow: (100) – along alkyl chains
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PBTTT – semiconducting polymer
21
PBTTT• poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene)• high performance p-type semiconducting polymer
Semi-crystalline Polymers:• few (broad & overlapping) peaks• combine theory/modeling & experiment
q z(A
-1)
qxy (A-1)
McCulloch et al., Nat Mat. 5, 328 (2006).Brocorens et al., Adv. Mater. 21, 1193 (2009).Cho et al., JACS 134, 6177 (2012)
Approach:• PBTTT – C14• 2D random GIXD -> initial structure via
modeling and GIXD simulation• biaxial textured films -> refine model• molecular mechanics (T = 0K)
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PBTTT structure
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Out-of-plane orientation In-plane orientation
Triclinic:a = 21.5 Å; b = 5.4 Å; c = 13.5 Å
= 137 deg; = 86 deg; = 89 deg Miller et al., Advanced Materials 24, 607 (2012).
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PBTTT – GIXD & modeling
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Approach:• 2D random GIXD -> initial strcuture via modeling and GIXD simulation• biaxial textured films -> refine structural model
• excellent agreement in peak positions Q= 0.68, 1.19, 1.35, 1.41, 1.71 Å-1
• d(001) = 21.3 Å(MM) vs 21.5 Å (GIXD)• agreement with (H00) intensities
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PBTTT – GIXD & modeling
24
Approach:• 2D random GIXD -> initial structure via modeling and GIXD simulation• biaxial textured films -> refine structural model
Q= 1.71 Å -1(h10): Q = 1.71 Å -1 & = 0 deg
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PBTTT – semiconducting polymer
25
Strong hole transport along the b-axis
B3LYP/6-31G** PBTTT-C14(meV)
b-axisth 114.65
te 138.72
a-axisth 0.00007
te 0.00002
Flat energy landscape:• many local minima• prevalence for disorder
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Outline
26
1. Organic Electronics Thin FilmsWide range of length scales
2. Quantitative Molecular PackingSmall molecules - PentacenePolymers – PBTTT
3. Nanoscale (dis)order - lattice variations, “grains”Paracrystallinity
4. Organic Photovoltaics (OPV) Blendsblend morphology
StanfordJonathan RivnayRodrigo NoriegaLeslie JimisonAlberto Salleo
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Organic Film Microstructure
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Semicrystalline polymers: partly crystalline & partly disordered
Brinkmann et al.,Adv Mater. (2006)
Semicrystalline polymers: disorder• crystallinity• pole figure (crystallite orientation distribution)• d-spacing (packing distance) variation• “grain” size• grain boundary structure grain size
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Microstructure: grains, packing disorder
28
grain sizeM non-uniform strain
…within a grain,and/or from onegrain to another
e2 1/2paracrystallinity
deviation frommean d-spacing
g
local packing disorder: variation inspacing between neighboring molecules
disorder
Less
More
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Diffraction Peaks & Disorder
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Disorder/Strain(20nm grain size)
Increasing disorder/strain
Multiple diffraction orders: quantitative analysis of both disorder/strain & grain size
200 nm
20 nm
5 nm
200 nm 20 nm 5 nm
Decreasing grain size
Grain Size(little disorder)
grain size: width independent of orderdisorder/strain: width dependent oforder (g and e different)
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Diffraction Peaks & Disorder
30
analysis approach:• Fourier transform isolated diffraction peaks• A(L) Fourier coefficients product of finitecrystallite size & disorder terms
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Diffraction Peaks: Warren-Averbach
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P(NDI2OD-T2) = poly{[N,N 9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)}• stable, high performance n-type semiconducting polymer
qxy [Å-1]0.0 0.5 1.0
0.0
0.5
1.0
1.5
2.0
qz[Å
-1]
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Diffraction Peaks: Warren-Averbach
32
isolated peaks
analysis approach:• Fourier transform isolated peaks• A(L) - crystallite size & disorder terms
0.2 0.4 0.6 0.8 1 1.2 1.4
10-4
10-3
10-2
10-1
qz (Å-1)
Inte
nsity
(arb
.uni
ts)
-0.1 0 0.1q-qpeak (Å
-1)
Nor
m.I
nten
sity
-0.1 0 0.1q-qpeak (Å
-1)-0.1 0 0.1
q-qpeak (Å-1)
-0.1 0 0.1q-qpeak (Å
-1)-0.1 0 0.1
q-qpeak (Å-1)
SS
N
N
OO
OO
C10H21
H17C8
H21C10
C8H17
n
a)
b)
c)
diffraction alonglamellar stacking
P(NDI2OD-T2)
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Diffraction Peaks: Warren-Averbach
33
normalized FT
synthesized data
result (lamellar direction):M = 27 nm; 22 (14) nm, e = 1.7%, g = 3.6%
0 2 4 6 8 10 12 14 16 180
0.2
0.4
0.6
0.8
1
n
Nor
mal
ized
Am
(n)
0 5 10 15
10-1
100
e)
d)
P(NDI2OD-T2)
-0.1 0 0.1q-qpeak (Å
-1)
Nor
m.I
nten
sity
-0.1 0 0.1q-qpeak (Å
-1)-0.1 0 0.1
q-qpeak (Å-1)
-0.1 0 0.1q-qpeak (Å
-1)-0.1 0 0.1
q-qpeak (Å-1)
ne)
2222222 221)( enmngmhklm ee
MndnA
)()()()( nAnAnAnA gm
em
Sm
dQiQndQInA mm 2exp)()(
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PBTTT: directional dependence
34
PBTTT = poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene• high performance p-type semiconducting polymer
C. Wang, Adv. Mater 2010
D. DeLongchamp, Adv. Mater 2011
q z(A
-1)
qxy (A-1)M.L. Chabinyc, JACS (2007)
Lamellar
Backbone
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PBTTT: directional dependence
35
D. DeLongchamp, Adv. Mater 2011Joe Kline & DeanDeLongchamp (NIST)
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PBTTT: directional dependence
36
0 5 10 15 20 250
0.2
0.4
0.6
0.8
1
n
Nor
mal
ized
An
-0.8 -0.4 0 0.4 0.8qxy-qxy,peak [A
-1]
Nor
mal
ized
Inte
nsity
1 2 3 4 510
-4
10-3
10-2
qxy [A-1]
Inte
nsity
[a.u
.]
SSS
S
H29C14
H29C14 n
result (pi):o M = N/Ao g = 7.3%o e = 0.9%
result (lamella):o M = 25 nm
(large error bars)o g = 2.0%o e = 0.6%
Implications on transport?
(010)
(020)
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PBTTT: directional dependence
37
result (pi):o M = N/Ao g = 7.3%o e = 0.9%
Implications on transport?
V. Coropceanu, et al, Chem. Rev., (2007)
Mobility:• strong dependence on overlap& molecular packing
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Packing Disorder - Transport
38Rivnay, et al., Phys Rev B RC, (2011)
Increase in paracrystalline disorder produceslocalized tail states in the bandgap
first principle simulation:• 2D system – DOS• 20 sites along the backbone• 50 -stacked molecules with varying g• disorder creates tail states
backbone (20 monomers)
50 -stackedmolecules
delocalized(µ0)
localized (Nt & E0)
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Packing Disorder – small molecules
390 20 40 60 80
0
0.2
0.4
0.6
0.8
1
n
Nor
mal
ized
Am
(n)
-.02 0 .02qxy-qxy,peak (Å
-1)
Nor
mal
ized
Inte
nsity
1 1.5 2 2.510
-4
10-2
100
qxy (Å-1)
Inte
nsity
(arb
.uni
ts)
In plane[100] direction
TIPS-Pentacene
FET 0.5-5 cm2/Vs
result [100]:o M = 41 +/- 7 nmo g = 0.9 +/- 0.6 %o e = 0.1 +/- 0.1 %
PBTTT (pi):o M = N/Ao g = 7.3%o e = 0.9%
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Organic Solar Cells: Morphology
40
Order in semicrystalline polymers:• packing disorder -paracrystallinity (g)• semicrystalline (P3HT, PBTTT)• weak order (PCDTBT)• poor order -> amorphous(rRA-P3HT)
Noriega et al., Nature Materials, doi:10.1038/nmat3722
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Organic Solar Cells: Morphology
41
Order in semicrystalline polymers:• packing disorder - paracrystallinity• semicrystalline (P3HT, PBTTT)• weak order (PCDTBT)• poor order/amorphous (rRA-P3HT)
-2 -1 0 1 2
01
2
qxy (Å-1)
~qz
(Å-1)
sem
icrys
tallin
e3D
amor
phou
s
P3HT PBTTT
PDPPBT P(NDI2OD-T2)
IDT-BT PCDTBT
PTAAr-Ra P3HT
Noriega et al., Nature Materials, doi:10.1038/nmat3722
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Summary + Outline
42
general observations:polymers
g (sometimes e) is largeidea of grains may not be relevant
small moleculesg and e are small, grains can be large
Organic thin film microstructure - local packing disorder:distribution of packing (neighbor) distances - paracrystallinitysignificant impact on charge transport
1. Organic Electronics Thin Films2. Quantitative Molecular Packing3. Nanoscale (dis)order - lattice variations, “grains”
Paracrystallinity4. Organic Photovoltaics (OPV) Blends
blend morphology
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Organic Solar Cells: Morphology
43
Gomez, et al. Chem Comm, 47, 436 (2011)Treat, et al., Adv. Energy Mater. 1, 82 (2011).Chen et al., NanoLetts. (2011).
Three separate regions:• pure donor – “semicrystalline”• some ( 20%) fullerene in amorphous donor• pure fullerene – amorphous
Some issues:• Molecular packing in donor
polymer: carrier & excitontransport
• BHJ morphology (nm lengths);close to exciton diffusion length
• Intermixing of donor & acceptor• Interface structure
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BHJs: nanoscale phase segregation
44
Need to combine several methods:• Imaging – EF-TEM• Scattering (SAXS + R-SoXS)
Sizes of three separate regions:• pure donor• mixed fullerene-donor• pure fullerene
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Probe Morphology with Scattering
45
Transmission Scattering:• hard x-rays (films, solutions)probe structures up to 50 nm• soft x-rays (films) probestructures up to 1 µm• solution SAXS
• Guinier - domain size (D)• Porod (P) exponent –
interface roughness
q = (4 / ) sin
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Understanding the Porod Exponent
`
Porod (P) exponent:• shape of scatterers (particles)• interface roughness between domains
46
diffuseness of interface (fractal)P= 4->3, more mixed, jaggedP= 3->2, more loose, mixed
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Improvements Using Additives - PDPP2FT:PC71BM
47
Five fold Efficiency Enhancement in PDPP2FT:PC71BM Additives:DIOODTClN
0% ClN: PCE = 0.9%,Jsc = -1.9, Voc = 0.69, FF = 0.65
5% ClN: PCE = 5.7%,Jsc = -12.6, Voc = 0.65, FF = 0.69
Yiu et al., JACS 2012, 134, 2180
PDPP2FT:PC71BM 1:3
C16
KAUST & UC-BerkeleyAlan YiuJeremy NiskalaOlivia LeePierre BeaujugeJean Fréchet
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Blend Structure of C16-PDPP2FT
48
0.9%
5.6%
P = 3.5
P = 2.7
PDPP2FT: PC71BM 1:3
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Influence on Blend Microstructure
49
Additive leads to:• decrease in phase segregated domainsize: 100s nm -> 80 nm• more intermixed interfaces (smaller P)
Additive:smaller domains & more intermixed domain interfaceresults in better exciton splitting and charge separation
What’s the mechanism behind these changes?
PDPP2FT: PC71BM
blend blend+DIO
blend+ODT
blend+ClN
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Structure of C16-PDPP2FT in solution
50
w/o additive in CB
• no Guinier regime at low qaggregates > 100 nm
• no Gaussian behavior (P=3)chains aggregate even at lowpolymer concentrations
• broad peak at high q & slopeof -1 appear with increasingconcentrationalkyl chains correlate leadingto longer stiff chain segmentsformation of small nuclei
IncreasedConcentration
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Structure of C16-PDPP2FT in solution
51Schmidt et al. Adv. Mater. 2014, 26, 300.
PDPP2FT in CB
Weakly ordered polymer aggregates act as seed sites for crystallizationpromotes a higher density of seed crystalsFilm has better morphology
SAXS fullerene =>no effect of additives
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Mechanism for C16-PDPP2FT
52Schmidt et al. Adv. Mater. 2014, 26, 300.
Weakly ordered polymer aggregates act as seed sites for crystallizationpromotes a higher density of seed crystals
Film has better morphologyMore jagged interfacesMixed crystallite orientationOptimal length scale phase segregation
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Summary
53
1. Organic Electronics Thin FilmsWide range of length scales
2. Quantitative Molecular PackingSmall molecules – Pentacene (&TIPS-Pentacene)Polymers – PBTTT
3. Nanoscale (dis)order - lattice variations, “grains”Paracrystallinity
4. Organic Photovoltaics (OPV) BlendsBlend Morphology
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54
Thanks
Stanford University• Zhenan Bao• Mike McGehee• Alberto SalleoNIST• Joe Kline, Dean DeLongchamp
GaTech• Jean Luc Brédas
www-ssrl.slac.stanford.edu/toneygroup/ KAUST• Pierre Beaujuge & Jean Fréchet
SSRL (SLAC)• Christopher Tassone• Kristin Schmidt• Chad Miller
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Backup
Michael Toney
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Probing Solid State Film and Casting Solution
Solution SAXS
Solid State SAXS
q = (4 / ) sin
56
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• Additives lower the nucleation barrier for polymer crystallization already in solutionleading to a higher nuclei concentration
• Additives stabilize PCBM aggregates in solutionless substrate effects as crystallization starts in solution leading to a mixedorientation of crystallitesfaster crystallization kinetically traps the system resulting in smaller and moreintermixed domains
• Additives give polymer mobility over a prolonged film drying processincreased coherence length and crystallinity
Michael Toney
Conclusion
Conclusion
w/o additives: w/ additives:
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Microstructure
Porod exponent (fractal interface):diffuseness of interface between
domainsP= 4->3, more mixed, jaggedP= 3->2, more loose, mixedshape of particle
Understanding the Porod Exponent
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GIXS
59
(h00):slow
(0k0): fast(00l): fast
P3HT structure:
Qxy or Q
Qz
bad - OPV
good - OPV
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Michael Toney
Structure of C16-PDPP2FT in SolutionMechanism
additives lower the critical concentrationfor stiff “nuclei” regionsstiff regions 25 nmpossibly - persistence length increases withincreasing polymer concentration &additives
persistence length fromintersection of different slopes
60
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Organic Semiconductors
61
Something on performance
Small Molecules Polymers