role of structure and morphology in organic electronics · ¾paracrystallinity: warren – averbach...

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