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
Ed Kramer
28/5/1939 - 12/27/2014
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
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
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
Organic Semiconductors
6
PolyICSony
OLEDsDisplaysLighting
GE
OFETsDisplay Backplanes
RFID TagsMemory
Logic
OPVPlastic Solar Cells
Organic Semiconductors
7
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
Organic Semiconductors
8
Transistors (OFET)• 10-5 cm2/Vs (1980s) -> 20-30 cm2/Vs (2014) & poly-Si
Photovoltaics (OPV)
Organic Semiconductors
9
Chemistry &Processing
PhysicalMicrostructure
Performance• transistors• photovoltaics
Design Rules for New Functional Organic Electronics
How does structure affect performance?
10Rivnay, Mannsfeld, Miller, Salleo, Toney, Chem. Rev. 112, 5488 (2012).
OPV
OFET
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
Pentacene Films
12
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).
X-ray Diffraction and Scattering
13
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)
Pentacene Films
14
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
Pentacene Films
15
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
Pentacene Films – structure refinement
16
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.
Pentacene Films – Structure
17
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°
Pentacene Films
18
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
Tuning the structure
19
Solution Shearing to tune properties
G. Giri, .., M.F. Toney, Z. Bao, Nature 480, 504–508 (2011)
TIPS-pentacene
52°
Organic Thin Film Microstructure - Polymers
20
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
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)
PBTTT structure
22
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).
PBTTT – GIXD & modeling
23
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
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
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
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
Organic Film Microstructure
27
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
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
Diffraction Peaks & Disorder
29
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)
Diffraction Peaks & Disorder
30
analysis approach:• Fourier transform isolated diffraction peaks• A(L) Fourier coefficients product of finitecrystallite size & disorder terms
Diffraction Peaks: Warren-Averbach
31
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]
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)
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)()(
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
PBTTT: directional dependence
35
D. DeLongchamp, Adv. Mater 2011Joe Kline & DeanDeLongchamp (NIST)
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)
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
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)
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%
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
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
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
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
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
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
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
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
Blend Structure of C16-PDPP2FT
48
0.9%
5.6%
P = 3.5
P = 2.7
PDPP2FT: PC71BM 1:3
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
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
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
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
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
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
Backup
Michael Toney
Probing Solid State Film and Casting Solution
Solution SAXS
Solid State SAXS
q = (4 / ) sin
56
• 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:
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
GIXS
59
(h00):slow
(0k0): fast(00l): fast
P3HT structure:
Qxy or Q
Qz
bad - OPV
good - OPV
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
Organic Semiconductors
61
Something on performance
Small Molecules Polymers