Class Organic Electronic Materials 150527TS
Transcript of Class Organic Electronic Materials 150527TS
5
Charge Transport in Organic Materials
Table of Contents
188
1 h 4 h 1 h 3 h 3 h 1 h 1 h 1 h 6 h 1 h 2 h 2 h 1 h 6 h 1 h 2 h 2 h 1 h 6 h 2 h 2 h 2 h
1. Introduction and Overview 2. Electron Delocalization in Organic Molecules and Polymers
2.1. Chemical Bonding in Organic Molecules 2.2. Intramolecular Electron Delocalization in π-Conjugated Systems
3. Supramolecular Organization in the Solid State and Intermolecular Electron Delocalization 3.1. The Origin of π-π Interactions 3.2. Supramolecular Organization of π-Conjugated Molecules in the Solid State 3.3. Intermolecular Electron Delocalization
4. Chemical Synthesis of Small Molecules and Polymers for Organic Electronic Materials 4.1. General Strategies for the Synthesis of Conjugated Oligomers and Polymers 4.2. Poly(phenylene)s, Poly(phenylene vinylene)s, and Poly(thiophene)s (and Their Oligomers) 4.3. Acenes, Coronenes, Perylenes, Porphyrins, and Other Selected Examples 4.4. Fullerenes, Carbon Nanotubes, and Graphene
5. Charge Transport in Organic Materials 5.1. Interaction of π-Conjugated Systems with Light 5.2. Charge Carriers in π-Conjugated Molecules 5.3. Charge Carriers in Organic Materials 5.4. Macroscopic Charge Transport in Organic Materials
6. Fabrication and Characterization of Organic Electronic Devices 6.1. Organic Field-Effect Transistors 6.2. Organic Photovoltaic Devices 6.3. Organic Light-Emitting Diodes
5.1
Interaction of π-Conjugated Molecules with Light
• electron excited to higher MO by light absorption if photon energy matches MO energy difference
From Molecular Orbitals to Optoelectronic Properties
190
• HOMO–LUMO gap (free electron–hole pair) ≠ optical transition energy (bound electron–hole pair) • exciton binding energy due to coulomb interaction between an electron and a hole (0.1 to 1 eV )
☀ h∙ν☀
h∙ν
intersystem crossing (ISC)
• excitation to vibrationally excited (“hot”) state of S1 with “inappropriate” bond lengths like those in S0 • subsequent vibrational relaxation results in energy loss as heat • when electron relaxes to ground state (same rules apply), photon with lower energy is emitted
• Franck-Condon principle: fast “vertical” excitation (10–15 s), initially without geometry changes (10–12 s)
Franck-Condon Principle
191
0 1
2 3
4
E
geometry coordinate r
electronic ground state S0
1st excited electronic state S1
0 1
2 3
4
vibrational levels
vibrational levels
+
–
ground state S0 1st excited state S1
• absorption and emission (fluorescence) spectra of a perylene bisimide in solution
Fluorescence and Stokes Shift
192
• emission is usually at higher wavelengths compared to absorption • Stokes shift is the difference in the wavelengths of maximum absorption and emission
0 1
2 3
4
E
geometry coordinate r
electronic ground state S0
1st excited electronic state S1
0 1
2 3
4
vibrational levels
vibrational levels
300 400 500 600 700λ / nm
norm
alize
d abs
orpt
ion / e
missi
on
531 540
461
493
576
• Jablonski diagram illustrates the electronic states of a molecule and the transitions between them
Jablonski Diagram
193
• intersystem crossing (ISC) is a non-radiative transition to a state with a different spin multiplicity • the much larger time scale of phosphorescence is caused by “spin-forbidden" transition
1st singletexcited state S1 1st triplet
excited state S1
electronicground state S0
absorption fluorescence phosphorescence
intersystemcrossing (ISC)
2nd singletexcited state S2
E
Optical Gap and Degree of Delocalization
194
• with increasing conjugation length the main absorption shifts towards longer wavelength
SS
SS
SS
SS
S SS
SS
SS
SS
SS
SS
S SS
SS
SS
SS
S SS
SS
402
324353
372
386
396
400
250 450350 550Wavelength / nm
650
2.5
2
1.5
1
0.5
0
Abso
rptio
n (a.u
.)
SS
SS
SS
SS
S SS
SS
SS
SS
SS
SS
S SS
SS
SS
SS
S SS
SS
402
324353
372
386
396
400
250 450350 550Wavelength / nm
650
2.5
2
1.5
1
0.5
0
Abso
rptio
n (a.u
.)
• linear extrapolation would yield band gap of 2.7 eV for high molecular weight poly(thiophene) • but data deviate starting from 8–10 units and eventually saturate (“effective conjugation length”)
• Kuhn plot of transition energy E = h c / λ (in eV) versus inverse length of the π-conjugated system
Effective Conjugation Length
195
4.0
3.5
3.0
2.50 0.1 0.2 0.3 0.4 0.5
E / eV
1/(n+0.5)
n
S
• blue/red shifted H-type/J-type spectroscopic aggregates from head-head/head-tail transition dipoles • not related to “solid state red shift” due to (limited) intermolecular delocalization by π-stacking
• dipolar coupling of transition dipoles of chromophores with defined orientation in the crystalline state • splitting of the transition state, but only one transition allowed (predominant), depending on geometry
Spectroscopic Aggregates
196
ε / 10
4 L mo
l-1 cm
-1
wavelength / nm250 650450
12
8
4
0
422385H aggregates single
molecules
SRS
SS
SS R
H aggregate J aggregate
S1
S0
E
S1
S0
S2
S1
S2
S0monomer head-to-tail
dimerface-to-facedimer
5.2
Charge Carriers in π-Conjugated Molecules
Selected Examples of Polymer Semiconductors
198
x x
R
poly(acetylene)s
x R
R
x xS
xNH
xS
OO
xS
Rpoly(phenylene)s
poly(phenylene vinylene)s
poly(thiophene)s
poly(pyrrole)
RR
x
poly(fluorene)s
x
RR
R R
“ladder-type poly(phenylene)s
x x
R
R
Z
Z
PEDOT
e.g., P3HTPTPPP
PPV e.g., MEH-PPV PPy
• poly(acetylene) is the only π-conjugated polymer with two equivalent resonance structures
Degenerate Ground States In Poly(acetylene)
199
• poly(acetylene) has two degenerate ground states (energetically and geometrically equivalent) • all other π-conjugated polymers have “benzenoid” ground state and “quinoid” excited state
benzenoid (aromatic)
quinoid (antiaromatic)
E E
ground state A ground state B ground state
excited state
polyene polyene
Peierl’s distorion
• at finite temperature, “lattice defects” associated with domain boundaries result in neutral solitons
“Spontaneous” Formation of Neutral Solitons in Poly(acetylene)
200
• lattice distortion results in additional, localized energy level in band gap, with limited delocalization • in organic chemistry view: solitons are (delocalized) radicals (spin s=½ and charge q=0) • neutral solitons do not carry charge, can not contribute to conduction, but are easily oxidized/reduced
7 π
ground state A ground state Bdomainboundaryq = 0, s = ½
electronic structure lattice defect
• chemically doped poly(acetylene) in the crystalline state becomes semiconducting or even metallic
Chemical Doping ofPoly(acetylene)
201
• different from inorganic semiconductor “impurity doping” (at ppm concentrations) • dopant is strong “single electron transfer” oxidant/reducant, but must not induce follow-up reaction • dopant is applied at high concentrations (0.1–10 mol%), strongly disturbs structure/geometry • conductivity & mobility increased by several orders of magnitude by chemical doping
I2, AsF5, SbF5
poly(acetylene) poor semiconductor
p-type doping good semiconductor
oxidant single electron acceptor
xx x
Na, K, Li
n-type doping good semiconductor
reductant single electron donor
• chemical doping converts neutral solitons into positive or negative soliton charge carriers
Formation of Positive or Negative Soliton Charge Carriers
202
• positive/negative solitons have no spin (s = 0) but carry a charge (q = ±e) • in organic chemistry view: carbocations or carbanions (delocalized over 7–23 carbon atoms) • isolated charge carriers with limited delocalization; doped poly(acetylene) becomes a semiconductor
Naq = 0 s = ½
q = –e s = 0
I2q = +e s = 0
positive soliton negative solitonneutral soliton
+ –
• formation of soliton defects would be energetically costly, “soliton pairs” spontaneously recombine • charge separation/transport additionally impeded by energy for geometry rearrangement • typical pristine π-conjugated polymers are insulators, become semiconductors only upon doping (etc.)
• energy difference “aromatic“ and “quinoid” state for typical π-conjugated polymers
Degenerate Ground States In Poly(acetylene) Different from Other π-Conjugated Polymers
203
benzenoid
quinoid
E
ground state
excited state
n
n
E
• exciton formation by excitation of electron from ground state S0 to first singlet excited state S1
Exciton Formation Upon Irradiation
204
• singlet excitons are neutral (q = 0) and spin-less (s = 0) species associated with a lattice defect • in organic chemistry view: “tightly bound” radical cation/anion pairs with limited delocalization • excitons diffuse slowly under geometry rearrangement, transport excitation energy
q = 0 s = 0
ground state excitonn
☀ h∙ν☀
h∙νfundamental
band gapoptical
band gap +
–
• chemically doped poly(acetylene) in the crystalline state becomes semiconducting or even metallic
Chemical Doping ofPoly(acetylene)
205
• different from inorganic semiconductor “impurity doping” (at ppm concentrations) • dopant is strong “single electron transfer” oxidant/reducant, but must not induce follow-up reaction • dopant is applied at high concentrations (0.1–10 mol%), strongly disturbs structure/geometry • conductivity & mobility increased by several orders of magnitude by chemical doping
I2, AsF5, SbF5
oxidant single electron acceptor
Na, K, Lireductant
single electron donor
typical π-conjugated polymer poor semiconductor
p-type doping good semiconductor
n-type doping good semiconductor
x x x
• chemical oxidation results in formation of positive polaron and/or bipolaron charge carriers
Formation of Positive Polarons or Bipolarons
206
• positive polarons have charge (q = +e), spin (s = ½); delocalized radical cations plus lattice defect • positive bipolarons have charge (q = +2e), no spin (s = 0); delocalized dications plus lattice defect • polarons/bipolarons are one single species, correlation length = effective conjugation length
ox.q = +e s = ½
q = +2e s = 0
+
+
+
n npositive polaron positive bipolaron
ox.
• chemical reduction results in formation of negative polaron and/or bipolaron charge carriers
Formation of Positive Polarons or Bipolarons
207
• negative polarons have charge (q = –e), spin (s = ½); delocalized radical anions plus lattice defect • negative bipolarons have charge (q = –2e), no spin (s = 0); delocalized dianions plus lattice defect • polarons/bipolarons are one single species, correlation length = effective conjugation length
red.q = –e s = ½
q = –2e s = 0
negative polaron negative bipolaron
red.
n n
– –2
• polarons typically have two optical absorption bands P1 and P2 in the red or near infrared range
Characterization of Positive Polarons by Near Infrared Spectroscopy
208
• wavelengths λ(P1) > λ(P2); other bands forbidden due to orbital symmetry considerations
P1
++P2
geradeungerade
geradeungerade
ε / 10
3 L mo
l-1 cm
-1
wavelength / nm200015001000500
1
2
0
0
1766
×20
400
1
2
3
×20
853
705 1171
369
T6
T44S RR 6S RR
T4 T6
Energy Gap and Poltroons of Oligothiophenes
209
• the optical gap decreased (slightly) by increasing the oligomer length, and further to polaron gap P2
• polaron transition P1 becomes smaller, lower polaron level closer to valence band • but due to effective conjugation length, Eg, P1 , and P2 reach saturation values
+
++ + + +
T3 T4 T5 T6 T7 T8
R. Rathore, J. Am. Chem. Soc. 2009, 1780-1786
• chemical oxidation can be used for quantitative polaron formation
Crystal Structure of the Quaterphenyl and its Radical Cation
210
• quinoidal structure of polaron requires planarization of molecular structure • polaron structure is better delocalized than ground state, bond lengths become similar
NO SbF6
– NOSbF6
Räder et al., Macromolecules 1999, 32, 1073.
• effective conjugation length is the “correlation length” of the exciton/polaron/bipolaron wave package
Effective Conjugation Length (2)
211
• correlation length decreased by chain twisting due to steric repulsion and/or structural dynamics • correlation length increased by packing in the solid state, reinforced by favorable crystallinity • correlation length drastically increased by enforced molecular rigidity and planarity
delocalized polaron
H H
H H
≈ 10–15° in solution
≈ 2–5° in crystal
SS
H
H
SS
R
R
poly(thiophene) (amorphous)
P3HT (highly crystalline)
RR
poly(phenylene)
poly(fluorene)s
poly(phenylene) (solution)
poly(phenylene) (solid state)
H H
n
• correlation length decreased by chain twisting due to steric repulsion and/or structural dynamics
Delocalization and Dynamic Disorder on the Molecular Level
212
• dynamic disorder increases, charge carriers become more localized with increasing temperature • band gap and also distance between valence/conduction band and polaron levels increase
+
temperature
++
SS
SS
SS
SS
SS
SS
SS
SS
• correlation length decrease by chain twisting disfavored by increasing molecular rigidity
Delocalization and Molecular Rigidity
213
• dynamic disorder decreases, charge carriers become more delocalized with increasing molecular rigidity • band gap and also distance between valence/conduction band and polaron levels decrease
+
molecular rigidity
++
R R
R R
R R
R R
• correlation length decrease by chain twisting disfavored by favorable crystalline packing
Delocalization and Packing Effects on Dynamic Disorder on the Molecular Level
214
• dynamic disorder decreases, charge carriers become more delocalized with improved crystalline packing • band gap and also distance between valence/conduction band and polaron levels decrease
+
supramolecular rigidification
++
SS
SS
SS
SS
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13PT (amorphous)P3HT (crystalline)
SS
SS
SS
SS
5.3
Charge Carriers in Organic Materials and Devices
• π-π stacking between polarons and neutral molecules has charge-transfer character
Intermolecular Delocalization of a Polaron Defect
216
• π-π stacking interaction energetically more favorable and stronger, higher correlation • π-π stacking results in intermolecular delocalization of the charge (typically over several molecules)
two neutral benzene molecules
++ – –
benzene radical cation and a neutral benzene molecule
benzene radical anion and a neutral benzene molecule
E
• intermolecular delocalization decreased by molecular motions in the solid state
Delocalization and Dynamic Disorder on the Molecular Level
217
• dynamic disorder increases, charge carriers become more localized with increasing temperature • band gap and also distance between valence/conduction band and polaron levels increase
+
temperature (dynamic disorder)
++
• intermolecular delocalization increases with crystallinity and crystalline order in the solid state
Delocalization and Static Disorder oin the Solid State
218
• static disorder decreases, charge carriers become more delocalized with improved crystalline packing • static disorder are packing defects, lattice disorder, domain boundaries, impurities, …
+
static disorder
++
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
SS
SS
SS
SS
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
C6H 13
• at high doping levels (≥0.1 mol%) the polaron defect states start to interact with one another
Band Formation in Highly Delocalized Systems at High Charge Carrier Densities
219
• at doping level >1/1000 molecules, polaron defect every 1/10 molecules along any lattice direction • interaction of defect states results in MO splitting and formation of a narrow polaron band
++ +n
number of charge carriers
+
• for more disordered systems, the threshold concentration of polaron defects increases
Band Formation in Less Delocalized Systems at High Charge Carrier Densities
220
• increase above threshold will initially locally create more extended states • the bandwidth of the final polaron band is lower than for highly ordered systems
number of charge carriers
+ + + +n+++ +++
• for highly disordered (amorphous) systems, no polaron bands are formed even at high doping levels
Formation of Extended States in Highly Disordered Systems at High Charge Carrier Densities
221
• even in amorphous solids, however, polaron defects can locally start to interact with one another • formation of more extended polaron defect states with limited delocalization
++ ++ +++ ++ +n
number of charge carriers
SS
SS
SS
SS
S
S S
SS
SS
S
SS
SSSS
SS
S
SS
SS
S
• from single-crystalline to polycrystalline to amorphous materials, static and dynamic disorder increase
Effect of Combined Static and Dynamic Disorder
222
• not only the correlation length and the extended nature of the polaron levels change • also the local structure of both neutral states and polaron defects and, hence, their energy levels change
static & dynamic disorder
++++ ++++++ ++++n ++++ ++++++ +++
SS
SS
SS
S
S
S S
SS
SS
S
SS
SSSS
SS
S
SS
SS
S
Active Regions in p-Type and n-Type Organic Semiconductors
223
+n ++++ ++++++ +++
p-type transport active region
n-type transport active region
–––– –––––– ––––n
• Fermi-Dirac distribution for thermal distribution of electron energies
Fermi Distribution and Fermi Level
224
• for E–EF ≫ kBT, the Fermi-Dirac distribution becomes the Boltzmann distribution • Fermi energy EF is defined (!) as the energy level where probability to find an electron is f(EF)= 0.5
Fermi energy EFenergy
occu
patio
n prob
abilit
y f(E,
T)T = 0 K
T
T
f (E) = 1
e(E°EF )/kB T +1
• electronic structure of a material is described by the density of occupied states (DOS)
Density of Occupied States (DOS)
225
• density of states g(E) represents all available allowed states of a material (e.g., the 3s band in Na) • these available states filled according to Fermi-Dirac distribution f(E,T) • density of occupied states is g(E) ∙ f(E,T), represents the states of all electrons at given temperature
Fermi energy EF
dens
ity of
occu
pied s
tates
g(E) f(E
,T)
Fermi-Dirac distribution f(E,T)density of states
g(E)
4 kBT0.1 eV (300 K)3.2 eV for Na
density of occupied states
• for metallic conductors, Fermi level is, by definition, equal to the “work function” (ionization energy) • for semiconductors and insulators, the Fermi level is, by definition, in the middle of the band gap • however, density of states within the band gap g(E) = 0; Fermi level not equal to work function
Density of States in Metals, Semiconductors, Insulators
226
• only a tiny fraction of “intrinsic” charge carriers even in semiconductors, increasing with temperature • for silicon with a band gap of 1.1 eV, 1010 cm–3 (10–12 at%) at 300 K,1014 cm–3 (10–9 at%) at 500 K
E
EF
metal
0 1
semiconductor insulator
f(E,T)
0 1
f(E,T)
0 1
f(E,T)
E
EF
0 1
semiconductor
f(E,T)
0 1
f(E,T)
0 1
f(E,T)
• upon bringing different materials into contact, Fermi level is defined for the whole materials system
p-Type Organic Semiconductors in Contact with a Metal Electrode
227
• due to large excess of free charge carriers in the metal, this adjusts Fermi level to the one of the metal • ideal combination is noble metal (large work function) near the (high) HOMO of the semiconductor
not in contact in contactE/eV
0
–5.1
Au
0 1
f(E,T)EF
EF
Au
++++ ++++++ +++
E/eV0
–5.1
Au
0 1
f(E,T)EF
• upon bringing different materials into contact, Fermi level is defined for the whole materials system
n-Type Organic Semiconductors in Contact with a Metal Electrode
228
• due to large excess of free charge carriers in the metal, this adjusts Fermi level to the one of the metal • ideal combination is non-noble metal (small work function) near the (low) LUMO of the semiconductor
not in contact in contactE/eV
0
–4.2
Al
0 1
f(E,T)
EFEF
E/eV0
–4.2
Al
0 1
f(E,T)EF
Al
–––– –––––– –––
Typical p-Type and n-Type Organic Semiconductors
229
• typical are Au electrodes for p-type, Al or Ca electrodes for n-type semicoductors • Au electrodes are also often used for n-type semicoductors, due to reactivity/passivation of Al and Ca
Ph
Ph
Ph
Ph
SS
S
E/eV
Au
Al
Cugraphene
–2
–3
–4
–5
–6
–7
ORN O
ONR
O
ORN O
ONR
O
CNNC
ONR
O
ORN O
p-type n-type
230
5.4
Charge Transport in Organic Materials
• models have initially been developed for disordered inorganic semiconductors • these do not really apply to molecular organic materials, mostly because of “strong coupling” • different experimental techniques emphasize different aspects of transport mechanisms
Transport Mechanism Models in Organic Semiconductors
232
• for different incoherent transport models, start / end point “look the same” in structure / energy • models aim to describe thermal distribution of involved levels, traps, transport probability • different mechanisms in parallel, which one dominant depends on temperature, material, disorder (...)
band conductivity
band-like conductivity
tunneling & self-exchange
multiple trap-and-release
variable range hopping
nearest neighborhopping
incoherent transport via increasingly localised states
increasing static & dynamic disorder
coherent transport
Band Transport
Kittel, Introduction to Solid State Physics, John Wiley & Sons, Inc., New York, 1996.
• band transport occurs via coherent Bloch waves, extended states in a periodic lattice • intramolecular transport through “lattice” of repeating units • intermolecular transport via lattice of molecules with intermolecular π-interaction
Band Conductivity
234
• injected charge drifts at constant velocity v, acceleration in electric field F versus scattering in the crystal • mobility µ = v / F is materials property (but may be non-linear and field-dependent at high fields) • limited to intramolecular transport or intermolecular transport in single crystals, at low temperatures
+n–n
material material
Bredas et al., Proc. Natl. Acad. Sci. USA 2002, 99, 5804.
• negative temperature factor, i.e., mobility µ decreases with temperature T (not a sufficient criterion)
Criteria for Band Conductivity
235
• only in this way “mean free path” of electron much larger than intermolecular distance • function of temperature because W = f(S), typically only at very low temperatures • crystal defects result in traps, detrimental for band transport
W = ~øvib
∏ 0.1°0.2eV
dµdT
< 0
µ/ T °3/2
• for semiconductor in which band transport is limited by scattering from acoustical phonons:
• band width W must be sufficient to enable transport before geometric relaxation (vibration time τvib)
Karl, Charge-Carrier Mobility in Organic Crystals, Springer, Germany, 2001.
• aromatic molecules exhibit metallic conductivity in single crystals at low temperatures
Example of Band Transport at Low Temperature
236
• band transport at T < 100 K, in all three lattice directions, with different mobilities • at room temperature, incoherent transport in two lattice directions becomes equal
3.2 Well-Ordered Systems: Organic Single Crystals 71
30
0.3
1
3
10
b = −1.48
b = −1.48
µ1−
µ3−
µ2−
b = −1.47
100 300
µ− [cm
2 V
−1 s
−1]
T [K]
Figure 3.2 Three principal components of the mobility tensor in a naphthalene crys-tal measured by ToF, and fitted to a Tb dependence. A clear T−3/2 dependence is ob-served only below ∼100 K. (Reprinted with permission from Ref. [9]. Copyright (2001) bySpringer-Verlag.)
In ToF measurements, charge is generated by using radiation that is wellabove the bandgap and as a consequence is in an excited state. This situation is notnecessarily representative of what happens in an electronic device in which electrons(holes) are injected at the bottom (top) of the conduction (valence) band. Recentexperiments pioneered by Podzorov et al. using vacuum-gap field-effect transistor(FET) structures fabricated with elastomers showed that a negative temperaturedependence of the mobility can be observed in single-crystal electronic devices[15–18]. Rubrene FETs displayed an increasing mobility from room temperature(∼10 cm2 V−1 s−1) down to 150 K (>20 cm2 V−1 s−1). The mobility was also foundto depend on the crystallographic direction [17–19]. At T < 150 K, the mobilitywas thermally activated with an activation energy of ∼ 70 meV. A gated Hall-effectmeasurement showed that the activated behavior was because of shallow traps nearthe valence-band edge (the FETs were hole-only devices): the mobility of the freecarriers keeps increasing as the temperature is decreased, even at temperatures atwhich the apparent mobility shows the opposite trend (Figure 3.3).
Owing to constraints in the experimental set-up, the measurements could notbe carried out at temperatures low enough to observe the T−3/2 dependence ofµ, and therefore, a weaker temperature dependence was observed. The use ofFET structures also somewhat relaxes the purity and quality requirements of thecrystal. Indeed, in FETs, the areal charge density is much higher than in ToFmeasurements and the gate voltage can be used to populate traps and push theFermi level close to the valence-band edge. It should be noted, however, that
Phys. Rev. Lett. 1988, 60, 1418.
Charge Transfer Salt of TTF and TCNQ
237
Tetrathiafulvalene (TTF) Tetracyanoquinodimethane (TCNQ)NC
NC
CN
CNS
S
S
S
σ ≈ 10–5 S cm–1
µ = 2 cm2/V⋅sσ ≈ 10–5 S cm–1
TTF ⋅TCNQ Single Crystal
σ = 500 S cm–1, metallic at T < 54 K
Charge Transfer Complexes between Electron Donor and Acceptor Materials (1)
238
• if electron donors and acceptors crystallized in pairs, electron transfer would just yield localized charges • ‘charge transfer complex’ with poor conductivity
NCNC CN
CN
SS
SS
NCNC CN
CN
NCNC CN
CN
SS
SS NC
NC CNCN
SS
SS
NCNC CN
CN
SS
SS
SS
SS
e
NCNC CN
CN
SS
SS
NCNC CN
CN
NCNC CN
CN
SS
SS NC
NC CNCN
SS
SS
NCNC CN
CN
SS
SS
SS
SS
Charge Transfer Complexes between Electron Donor and Acceptor Materials (2)
239
• electron donors and acceptors crystallize in segregated stacks, charge transfer between stacks • electron transfer yields strongly delocalized charge carriers • ‘charge transfer complex’ with high (even metallic) conductivity
NCNC CN
CN
NCNC CN
CN
NCNC CN
CN
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
NCNC CN
CN
NCNHC CN
CNe
NCNC CN
CN
NCNC CN
CN
SS
SS
SS
SS
SS
SS
SS
SS
NCNC CN
CN
NCHNHC CN
CNSS
SS
NCNC CN
CN
240
Band-Like Transport
• often negative temperature factor observed, but mean free path smaller than intermolecular distance • localization due to dynamic disorder, but transport associated with coupled molecular motions
Band-Like Transport in Highly Ordered Crystalline Solids
242
• transport “looks like” coherent transport, although mediated by incoherent (localized) charge carriers • expected µ ~ T–2.1 but typically weaker temperature dependence due to disorder and trap states • typically, inversion to positive temperature factor at about T = 150 K
+ + + e+ + –e – + + +
material material material
Incoherent Charge Transport Mechanisms
• fluctuations of transfer integral S on the order of S itself due to dynamic disorder at room temperature • static disorder is slow on the time scale of charge transport, results in energetic heterogeneity
Incoherent Transport in Disordered Organic Solids
244
• premises for band or band-like transport usually not fulfilled in partially ordered organic solids • even in (partially) crystalline solids, molecular motions at r.t. destroy translational symmetry
++++ ++++++ +++++++ ++++++ +++
polycrystalline solid amorphous solid
• charge carriers with limited delocalization move in electric field independently and statistically • “random walk” with a directional bias by electric field
Incoherent Charge Transport Mechanisms
245
+ + + + + + + + + + + +
e –e –
material material
• charged state (positive polaron) interacts with “nearby” neutral state (HOMO) • if lower polaron level and neutral HOMO level are close in energy, increases probability of exchange • hence, large π-conjugated systems with good intramolecular delocalization and good overlap preferred
• charge carriers with limited delocalization move in electric field independently and statistically • “random walk” with a directional bias by electric field
Incoherent Charge Transport Mechanisms
246
• charged state (negative polaron) interacts with “nearby” neutral state (LUMO) • if upper polaron level and neutral LUMO level are close in energy, increases probability of exchange • hence, large π-conjugated systems with good intramolecular delocalization and good overlap preferred
– – – – – – – – – – – –
e –e –
material material
• electronic frequency unchanged, but probability decreases exponentially with distance • process itself has no temperature dependence, but since disorder does, one observes µ ~ T–n
• finite potential well results in certain probability to find electron outside the well • tunneling to nearby molecules if energy levels identical (required for “resonant exchange”)
Charge Transport by Tunneling
247
ktun / e°B ·d
M1 M2 M1+ +– M2
–
E
ΔG = 0
M1 + M2
–initial state final stateM1 M2+–
• transfer integral J large (large π systems), λ (energetic/geometric difference) small • weak temperature dependence, typical activation energies 50–200 meV (5–20 kJ/mol)
• hopping is a thermally promoted process at higher temperatures (“thermally assisted tunneling”)
Charge Transport by Hopping
248
M1 M2 M1+ +– M2
–
kCT = |J |2~
rº
∏kB T·e° ∏
4kB T
ΔG = 0
λ
EA
geometry
EM1 + M2
–initial state final stateM1 M2+–
• with increasing disorder, λ increases, J decreases, activation energy EA increases for certain steps • with increasing disorder lower energy states increasingly become “traps” for charge carriers
• hopping also allows for transport between different states in disordered systems
Charge Transport by Hopping in Disordered Systems
249
M1 M2 M1+ +– M2
–
λ
EA
geometry
EM1 + M2
–initial state final stateM1 M2+–
• “band edge disorder” model assumes disorder, distribution of states near the band edge • states more located within the bands more extended, states more at the band edge more localized traps
Multiple Trap and Release (MTR) Model
250
• charge carriers temporarily trapped in localized trap states, but no “deep traps” if disorder not too large • promotion of charge “above mobility edge” into more extended state allows for drift in electric field • subsequent “retrapping” into more localized resting state
material material material
trapped above mobility edge trapped
– – –
• “band edge disorder” model assumes disorder, distribution of states near the band edge • states more located within the bands more extended, states more at the band edge more localized traps
Multiple Trap and Release (MTR) Model
251
• charge carriers temporarily trapped in localized trap states, but no “deep traps” if disorder not too large • promotion of charge “above mobility edge” into more extended state allows for drift in electric field • subsequent “retrapping” into more localized resting state
+ + +
material material material
trapped above mobility edge trapped
• in highly disordered (amorphous) systems, only hopping through states with limited delocalization • nearest neighbor hopping (NNH) for highly disordered systems and /or at low temperatures (ΔE ≫ kB T)
Nearest Neighbor Hopping and Variable Range Hopping
252
• variable range hopping (VRH) for less disordered systems and /or at higher temperatures (ΔE ≈ kB T) • VRH and Mott’s law abundant in the literature, but lack physical basis, superseded by other models
variable range hopping (VRH)
– –
kVRH =C VRH ·e°(T0/T )1/4Mott’s law
nearest neighbor hopping (NNH)
Charge Transport in Amorphous Polymers
253
• amorphous conjugated polymers inherently low mobility; but mechanical toughness, strength
S
S
S
SS
S S
S
S
S S SS S S
S
S
S
SS
S SS S
S SS S
SS
S
S
S
S
S
SS
S S
S
S
S S SS S S
S
S
S
SS
S
S SS S
SS
S
S
S
S
S
S
S
S
S SS
SS
S S
S
S
S S SS S S
S
S
S
SS
S SS S
S SS S
SS
S
S
S
S
S
S
S
S
A B
C D
S
S
S
SS
S S
S
S
S S SS S S
S
S
S
SS
S SS S
S SS S
SS
S
S
S
S
S
positive polaron crystalline domain
E E
E E
Charge Transport in Single-Crystalline Solids
254
• single-crystalline organic solids comparably high mobility; but mechanically brittle
A B
C D
positive polaron grain boundary
E E
E E
Summary
255
• π-conjugated molecules: polarizable electrons, low HOMO-LUMO gap • extended π-conjugated systems: electron delocalization for low band gap • doping for charge carrier generation (polarons, bipolarons) • high doping levels for band conductivity
• amorphous π-conjugated polymers: • band conductivity within molecules, as long as coplanarity not interrupted by conformation • intramolecular hopping as molecular level static and dynamic disorder increase • intermolecular hopping/tunneling, depending on intermolecular order
• single crystalline solids of π-conjugated molecules: • band(-like) conductivity within domains (grains), depending on packing and constructive π-overlap • intermolecular hopping within domains as static and dynamic disorder increase • hopping at grain boundaries and defects, depending on size, orientation
256
5.5
The Interplay of Short-Range Order and Microstructure for Macroscopic Charge Transport
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Length Scales in Organic Electronic Materials and Devices
258
carrier dynamics, and advanced computational and simulationtechniques, organic electronics has progressed toward completeand general quantitative descriptions of relevant systems, whichwill continue to facilitate rational synthetic molecular engineer-ing and processing parameter selection.To this end, we describe the contributions made by X-ray
science to elucidate the film morphology and microstructureover 5 orders of magnitude in length scales. Access to such abroad range of length-scales is possible through theextraordinary versatility of X-ray techniques. Recent reviewshave focused on a select range of structural features anddescribed a broad range of techniques that can be used to probethem.4,5 In this review, we discuss the broad range of physicalphenomena and their importance and focus solely on the use ofX-ray based techniques, and specifically quantitative X-raytechniques, for the accurate determination of structuralproperties ranging from molecular packing and defects todevice-scale phase segregation.First, it is important to understand the complexity of
molecular arrangement and film composition in the solid state,as well as what optoelectronic properties are at play in differentstructural regimes. We start at the smallest length scales withstudies of atomic bonding and molecular orbital interactionsand zoom out to describe device-scale phase arrangements anddomain alignment. We choose to highlight organic TFTmaterials and those for organic photovoltaics (OPVs) asexamples of pristine and blend films with a number of distinctmaterials requirements. We also focus on OTFT and OPVmaterials because they are the most frequent subjects of organicelectronic structural characterization by X-ray techniques.
2. RELEVANT LENGTH SCALES IN ORGANICSEMICONDUCTORS
An outstanding challenge for the development of high-performing organic semiconductor devices is to arrange theindividual phases (and molecules within those phases) in amanner that optimizes all relevant optoelectronic processesrequired for efficient operation. For OTFTs, these processesinclude charge injection and charge transport on both themolecular and device-size scales. In organic photovoltaics,efficient solar radiation absorption and exciton diffusion anddissociation rely on features found at the molecular andnanoscopic-size scale. Charge transport for carrier collectionrelies on details of molecular and mesoscale microstructure andmorphology. Unfortunately, however, it is challenging tocontrol how molecules arrange at different length scales.Organic solids are held together by weak intermolecularattractive forces that make it difficult to engineer stable anddefect-free microstructures and morphologies. One can finelycontrol the absorption and local intermolecular charge transferamong neighboring molecules by finely tuning the molecularstructure and chemistry,4,6,7 but these chemical changes oftenaffect the film-forming properties and, therefore, the larger-scalemorphology in a way that may be detrimental to otherelectronic processes. Often it is convenient to focus on onespecific length scale due to access to a particular set ofexperimental techniques or tools, but it has become clear thatthe most enlightening work comes from combining techniquesthat deliver insight into the microstructure and morphologyover a wide range of size scales. These size scales, and thestructural order or physical arrangements that are commonlyaddressed, are outlined in Figure 2; additionally, Table 1
Figure 2. Size scales and relevant morphological features in organic electronic devices. Within each schematic, the square denotes the enlargedregion preceding it. Each type of physical feature shown is assigned a range of length scales on the scale at center, described in the text. Althoughmany materials systems and device types/architectures are often investigated, two examples are chosen to display the various morphologies andmicrostructures possible. Top row: a two-component blend film typical of a bulk heterojunction used in organic photovoltaic devices. Themorphology of phase separation is invariably more complex than the schematic shown here, with impure/mixed phases coexisting with purecrystalline phases. Bottom row: single-component small-molecule semiconductor film used in OTFTs. Domains refer to regions composed ofsimilarly oriented grains.
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Structural and Energetic Disorder
259
summarizes examples of the optoelectronic processes that arerelevant for the physical structures at play at these size scales, aswell as the X-ray techniques suited to study them.For the purposes of this discussion of X-ray-based analyses
for the quantitative determination of microstructure in organicsemiconductors, these rough groupings of length scale areuseful because they are closely linked to specific experimentaltechniques, which are described later. The following sectionswill focus on each of the microstructural size categories, startingwith the smallest length scales.2.1. 1 Å−10 nm: Chemistry, Local Molecular Packing, andDefects
The chemistry of a molecule influences the energetics of asystem at the most basic level. The π-orbital delocalization orconjugation length8,9 and induction or hybridization effects10
are common methods used to control the bandgap and theenergy of the highest occupied and lowest unoccupiedmolecular orbitals (HOMOs and LUMOs) (Figure 3). Thelocation of energy levels with respect to each other and/orvacuum determines the onset of absorption, charge carrierinjection, band alignment, and environmental stability. Tounderstand these properties, it is therefore important to focuson the molecular scale, where the chemical structure bothdirectly and indirectly influences other microstructural features.Molecule size, rigidity, or the presence of solubilizing sidegroups (as is necessary for solution-processable materials)dictate the local molecular packing,4,6 the interaction with thesubstrate surface,11−13 and the solubility, which in turninfluence domain size and phase segregation. The structuralfeatures in question are those related to the moleculararchitecture and its associated interactions with neighboringmolecules, which spans the Ångstrom to nanometer scale.Ultimately, packing order at the molecular scale affects chargeand exciton localization. Localized cahrges are incapable ofdiffusing in a concentration gradient or drifting in a weakelectric field.Although the molecular structure is a starting point, the
specific solid-state arrangement of (macro)molecules ultimatelyinfluences the local optoelectronic properties of the sys-tem.15−17 Molecular packing profoundly affects the interactionwith neighboring molecular orbitals, which dictates intermo-lecular charge transfer, charge delocalization, and opticalabsorption. In the solid state, high-performing materials oftenhave a tendency to partially arrange in a periodic lattice or inaggregates. The specific symmetry and packing structure has
been found to sensitively depend on the aspect ratio, planarity,size, and density of side-chains or solubilizing groups.4,18 Forexample, the packing motif of pentacene is edge-to-face (Figure4). When a solubilizing side-group is added at the 6 and 13positions, packing adopts a face-to-face brickwork-like motif. Ifthis side-group is shifted over by one position (5, 14) on theacene core, a mixed-motif is observed, called a sandwichherringbone, where cofacial dimers of the molecules form edge-to-face arrangements with each other.6 Ultimately thecrystalline lattice arrangement will control the ease (efficiency)of transport (drift and diffusion) for both free charges andbound electron−hole pairs (excitons). These structural featuresare characteristically 1−10 nm in size and are important for thecharge generation, exciton diffusion, exciton recombination,and free charge carrier collection steps often considered inOPVs, as well as charge injection and local charge transport inOTFTs.
Table 1. Important Basic Morphological Features Commonly Discussed in Organic Semiconductors, Their Relevant Size Scale,X-Ray-Based Technique Used to Probe That Feature, And the Most Important Optoelectronic Properties That Are at Play forEach Feature
morphological or physical feature size scaleappropriate experimental X-ray
technique relevant optoelectronic processes
molecular packing/chemistry <1 Å−3 nm NEXAFS, WAXS, XRR (monolayers) energy levels (absorption, emission), charge generation,charge transfer
crystalline order and defects 0.5−10 nm WAXS (peak shape analysis) charge trapping, exciton recombinationinterfaces, interface roughness,grain boundaries
1−10 nm WAXS, SAXS, r-SoXR exciton dissociation/charge separation, charge carrierrecombination, charge trapping
grain size (and orientation),coherence length
7−200 nm WAXS (peak shape and pole f igureanalysis), STXM
charge and exciton transport
phase segregation, domain size(and connectivity)
20 nm−1 μm(polymers)
r-SoXR/S, SAXS, STXM charge pathways and barriers, exciton dissociation,charge injection
70 nm−5 μm(small molecules)
STXM, XRR
device size (area/thickness) 100 nm−100 μm charge transport, charge extraction, optical in- and out-coupling
Figure 3. Energy levels in organic semiconductors. (A) Highestoccupied molecular orbital (HOMO) and lowest unoccupiedmolecular orbital (LUMO), with respect to the vacuum level. Theoptical band gap (Eg), ionization potential (IP), and electron affinity(EA) are labeled, as well as the presence of intragap trap states. (B) Amore realistic description is that of the density of states. The mobilityor transport edge delineates the energy at which transport occurs (seeref 14), with trapping occurring in shallow traps in a broad tail ofstates, or deep trap states. (C) Schematic representation of transportbarriers due to traps, and scattering centers, which depend on therelative energy with respect to the transport level. Hole conduction isassumed in B and C.
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Structure Determination
260
Unfortunately, the data-collection process is serial, where eachscattering direction/angle is measured separately, making dataacquisition times long; because small diffracting volumes areprobed, damage to the film is a constant consideration.Conversely, an area detector setup (Figure 5C) allows for
rapid data collection over a large range of scattering angles butsuffers from lower accuracy and resolution. The detector in thiscase essentially takes a snapshot of the scattered X-rays, withthe exposure time depending on the detector, source intensity,diffracting volume, and crystalline qualityvarying from a fewseconds to tens of minutes.82 Shorter acquisition timesminimize beam damage and allow in situ and time-resolvedmeasurements. The scattering angle resolution of the recordedprofile depends on the pixel size of the detector, the beam size,the incidence angle, and the sample-to-detector distance. Alarge illumination area can produce broadening artifacts in thescattering peaks, limiting the measurement resolution.88
Another constraint of area detectors in 2D GIXD is the fixedincidence angle and sample orientation. Because of thescattering geometry, nominal cuts along qz (qxy = 0) on thearea detector are not true specular scans but instead providescattering data from planes that are misoriented from thesubstrate normal by a few degrees with larger distortions athigher scattering angles. This effect also occurs for the in-planescattering for single crystals and films that are highly orientedin-plane. These distortions have been mentioned before89−91
and addressed by completely removing the near-specular regionfrom the 2D pattern88 or by labeling the scattering vectors asapproximate (i.e., ∼qz) and using slices or cuts to make
qualitative or relative comparisons. This distortion however, isoften ignored in the literature.
4.1.2. Molecular Crystal Packing. To determine themolecular packing of the ordered regions of poly- andsemicrystalline films, the size and symmetry of the unit cellmust be determined first, and then the basis or atomicarrangement within that repeating unit cell. This information iscontained in the peak positions and intensities of diffractionpeaks. The general analysis is analogous to structuraldetermination used for biological systems, for example, todetermine protein structures. Similarly to biological entities,one must obtain rather large single crystals to adequatelyperform single-crystal diffraction measurements, such thatthousands of peaks can be observed. Unfortunately, it can bedifficult or impossible to grow large single crystals of organicsemiconductors, and often, the phase of a single crystal isdifferent from what is encountered in a thin film that would beused in a device.92 Therefore, we discuss the quantitativemolecular packing determination as it applies to thin films ofpolycrystalline small molecules and semicrystalline polymers.Diffraction peaks from a polycrystalline thin film or single
crystal only occur at discrete scattering vectors q, specificallythose for which the scattered waves from adjacent lattice planesinterfere constructively. This is Bragg’s law and is expressed bythe Bragg condition (eq 2) that links the allowed q values (qB)to the interplanar lattice spacings in the crystalline sample.87,93
Although the real lattice vectors (a, b, c) describe the distanceand orientation of unit cells in the crystal, the magnitudes andorientations of the corresponding interplanar spacings dhkl for afamily of lattice planes with Miller indices hkl form a lattice oftheir own, called the reciprocal lattice (a*, b*, c*):
π π
π
* = ×· × * = ×
· ×
* = ×· ×
a b ca b c
b c aa b c
c a ba b c
2( )
, 2( )
,
2( ) (3)
It is convenient to discuss diffraction within the context of thereciprocal lattice, because q values that satisfy the Braggcondition (qB) can be constructed from the reciprocal latticevectors:
= * + * + *h k lq a b c( ).hkl (4)
Because of the relationship between real and reciprocal spacelattices, the real space unit cell can be reconstructed fromdiffraction data in a two-step process. First, the q values fromthe crystalline sample are determined from the positions of theexperimentally observed diffraction peaks (typically qxy, qz forsamples that are isotropic in plane). In the second step, thepeaks are indexed by assigning a set of integers h, k, l to eachpeak. The reciprocal lattice can then be reconstructed byapplying eq 4, and the corresponding real space unit cell followsdirectly by application of the inverse relationships from eq 3.94
This analysis has been demonstrated for both vapor-depositedand solution-deposited small-molecule films95 where thenumber of measurable diffraction peaks is typically in therange of 10−40. This analysis of diffraction images only yieldspurely geometrical information (the unit cell size and shape)and does not reveal the arrangement of the molecules withinthat cell.With extended organic semiconductor systems, it is crucial to
know the exact molecular packing in order to evaluatefundamental material parameters such as electrical conductivity
Figure 5. Wide-angle X-ray scattering geometries on thin films. (A)Specular diffraction (also used in X-ray reflectivity (XRR) and powderdiffraction). (B) Grazing incidence wide-angle X-ray diffraction(GIXD) with a point detector. α is the incidence angle, and φ is anin-plane, azimuthal, rotation. (C) Grazing incidence X-ray scattering(GIXS) with a 2D image plate (a similar setup is used for grazingincidence small-angle scattering, with the detector at a larger distance,L; see Figure 16).
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yields more efficient charge transport. The information aboutthe size of crystalline grains is contained in the width of thediffraction peaks, and at the simplest level the Scherrer equationcan be employed to extract a grain size.87,93 The Scherrerequation relates the peak width to the coherence length Lc,
π= ΔL K2
qc
(6)
where K is a shape factor (typically 0.8−1) and Δq is the fullwidth at half-maximum of a diffraction peak. It should be wellunderstood that calling this a grain or crystallite size isrigorously correct when the only factor affecting the measuredpeak breadth is the finite size of the crystalline assemblies. Inorganic semiconductors, especially those that include solubiliz-ing alkyl chains or polymeric species, this is rarely the case, andthe width of a f irst order peak cannot provide a grain size.Many of the recent advances in the analysis of diffraction line
shape for organic semiconductors have focused on decouplingthe effects of finite size from those of cumulative disorder,which is well-established and has been used for inorganicmaterials and bulk polymers over 60 years.118,119 Cumulativedisorder describes variations in lattice parameter within the filmor the loss of predictive ability regarding the position of a unitcell as the displacement along a certain direction of a latticeincreases, and is due to the accumulation of distortions anddefects.86,118 In other words, when cumulative disorder ispresent, the lattice “forgets” its origin (initial location) aftersome distance. This is distinct from noncumulative disorder(e.g., thermal motion) where the average lattice is retainedindependent of distance between lattice sites; cumulativedisorder is often called paracrystalline disorder or latticedisorder (Figure 9D and E).120,121 When analyzing a diffractionpattern, one often observes (or expects to observe) aprogression of higher-order diffraction peaks corresponding toa specific set of crystal planes, i.e., (100), (200), (300). While adiffraction profile from a finite-size-dominated sample has peaksof equal breadth for this progression, one with cumulativedisorder shows successive broadening of the higher-order peaks(Figure 10). This distinction along with the specific peakshapes and functional dependence of the peak broadening ondiffraction order allow for the quantitative decoupling of thecrystallite size and lattice disorder. Such decoupling isimportant from the standpoint of the electronic properties.Indeed, an overall low diffracted intensity can be interpreted asthe presence of a large volume fraction of disorderedcrystallites, through which charges and excitons can percolate.On the other hand, the same signal could be due to a smallvolume fraction of relatively perfect crystallites, in which casecharges and excitons will have to migrate through anamorphous matrix. Hence, quantitative analysis methods areinstrumental in distinguishing these two very differentsituations.Implementation of well-known models that decouple the
effects of size and disorder are limited in organic semi-conductors. Weakly scattering materials, such as polymers,rarely exhibit enough peaks to enable such analysis.Furthermore, the directions that usually limit charge orexcitonic transport are those where intermolecular couplingoccurs. In these directions molecules typically only exhibit vander Waals bonds. As a result, the directions that are mostimportant for transport are also the most disordered ones. Thispoint is interesting considering that the weak scattering (for
higher order peaks) may be due to a low crystalline fraction, butmore likely, it is due to a high degree of paracrystalline disorder,making these analyses (Figure 10) important for both accuratedetermination of crystallite size and quantification of latticedisorder.When both finite size and disorder are present, the functional
form with which higher diffraction order peaks broaden can beattributed to different types of disorder or defects.122 Therefore,by plotting the peak breadth versus diffraction order m (or m2)(Figure 10A, inset), one can extract the coherence length fromthe intercept (m = 0), while the functional form of thebroadening, Δq(m), can reveal the mechanism and magnitudeof the lattice nonidealities.122,123 This integral breadth method(related to the analyses of Hall of Williamson),124,125 is usefulin strongly diffracting materials, but it is not a reliable tool fordiffraction patterns where only two or three diffraction ordersare present. Nevertheless, this approach has been applied with
Figure 10. Peak breadth, shape, and diffraction-order dependentbroadening to decouple size and disorder related terms. (A) Schematicof hypothetical diffraction profile from crystalline lattices affected byfinite size, with (red) and without (blue) the contribution ofcumulative disorder. Inset: Integral peak breadth (Δq) approach fordecoupling coherence length, Lc, from disorder-related terms usingmultiple diffraction order peak widths. Fourier transform-basedtechniques, such as the Warren−Averbach analysis, rely on peakisolation (B and C), followed by Fourier transform (D and E) analysisto decouple the average column length, M, its size distribution, wM,and cumulative disorder terms (g, erms).
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Crstallite Orientation
261lamellar stacking thiophene-based liquid crystalline polymerswhere a number of well-resolved out-of-plane peaks areobserved.91,126 Similar analyses (based on ref 126) have beenperformed by Lilliu et al.127,128 where size and paracrystallinityare decoupled to show the crystal growth and orderingdynamics upon annealing of P3HT/PCBM solar cell blends.They find a growth of crystallites in the alkyl stacking directionof the P3HT and an increase in order on annealing that isdependent on PCBM concentrationindicative of PCBMacting as a plasticizer.127
More accurate techniques involve utilizing the entire peakshape, in the form of a Fourier-transform (FT) analysis ofindividual peaks (Figure 10D and E). The power of suchtechniques, first introduced by Warren and Averbach,119,129 isthat contributions from cumulative disorder and a finitecrystallite size can be decoupled because of their specificfunctional dependences on both the diffraction peak order andthe Fourier frequency. The Fourier components of the peakscan be represented as the product of contributions from a finitecrystallite size, which is independent of peak order, and peak-order-dependent terms related to paracrystalline disorder. Inone variant of this Warren−Averbach (WA) model, eachdiffraction peak is constructed from the superposition of wavesscattered by units whose distortions from the ideal lattice aredescribed by Gaussian statistics and belong to columns of unitcells along the [hkl] direction. The normalized nth coefficientof the Fourier transform of the mth-order diffraction peak isthen given by
π= − +A n A n m ng n e( ) ( ) exp[ 2 ( )]m mS 2 2 2 2
rms2
(7)
where AmS (n) is the size-related broadening contribution that
depends on the column length distribution in the sample, g isthe paracrystalline disorder parameter, and erms is the latticeparameter fluctuation reduced variance.130,131 The general WAmethodology has been applied to the alkyl-stacking (nontran-sport) direction in polyalkylthiophenes, monitoring the changein disorder parameters when heating through a glass
transition.130 Although this routine is normally applied in amultistep manner, which is hampered by iterative fits andaccumulating errors, a full fit to eq 7 can be employed, withthorough error propagation analysis to gain confidence in fitresults.131 The π-stacking analysis of conjugated polymersystems is usually particularly inaccessible, because only thefirst-order peak is measurable. Rivnay, Noriega, and co-workersused alignment methods (see section 4.1.5) to measure asecond-order peak of PBTTT π-stacking, which allowed for fullanalysis; it was found that the π-stacking disorder in PBTTT isg = 7.3%, a value that is close to a 1D amorphous assembly(∼10%).131 This finding also showed that, in π-stackedpolymers, paracrystalline disorder completely dominates thepeak breadth (coherence), opening up the possibility of aScherrer-based single-peak estimation of paracrystallinity fordisordered polymers when specific assumptions are satisfied.Furthermore, using a tight binding model, it was found thatincreasing the paracrystalline disorder in the π-stacking ofPBTTT oligomers introduces a tail of localized states extendinginto the optical band gap that can act as shallow traps, whichlimit charge transport.29 Such a finding provides physicaljustification for transport models that involve interplay oflocalized and delocalized states, such as the mobility edgemodel. Furthermore, it highlights the importance of disen-tangling the effect of paracrystallinity (g) and crystallite sizebecause of the effects of these microstructural parameters onthe physics of charge transport.Although the physical source of paracrystalline disorder is
unknown, it is thought to be an accumulation of intrinsic andextrinsic defects that yield a statistical, static disorder. Intrinsiccontributions may be due to polydispersity, molecular weightand related chain-end effects,132 regioregularity couplingdefects,133 or side-chain disorder. The role and concentrationof typical crystallographic defects (e.g., dislocations) has notbeen studied in depth. There have been numerous accounts ofobservations of screw dislocations (mostly by AFM and TEM)and other local defects (by scanning tunneling microscopy) in
Figure 11. Texture in thin films. (A) Randomly oriented (similar to powder) arrangements of crystallites, with no preference for a specificcrystallographic orientation (100) with respect to the substrate normal produce rings in the diffraction patterns. (B) Textured or oriented films witha distribution of crystallite orientations produce arcs of diffracted intensity. (C) Highly oriented films produce spots or ellipses. The corresponding2D GIXD patterns for PBTTT that are solid state pressed (A), as spun from solution (B) and annealed (C), are used as examples at bottom. Thepressed sample, A, is mostlynot completelyrandomly oriented.
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Crystal Structure of P3HT
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fiber axis π–π stacking
[010]
SS
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SS
SS
SS
SS
SS
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alkyl packing [100]
chain repeat [001]
Melting of poly(alkylthiophene)s
263
• melting of side chains hard to observe for longer P3HT, kinetically trapped • separation of backbone and side chains persists after melting
T. Thurn-Albrecht et al., Macromolecules 2010, 43, 4646.
DSC heating/cooling trace
Tg
Tm,side
Tm,back
Phase I Phase II III
Morphology of poly(alkylthiophene)s
264
• AFM phase images reveal “hard-soft” contrast, not topology (height)
T. Thurn-Albrecht et al., Macromolecules, 2010, 43, 4646
P3HT (Mn 3000) P3HT (Mn 6000) P3HT (Mn 12000)
Kline et al., Polym. Rev. 2006, 46, 27
Crystallinity of P3HT Thin Films
265
XRD, out-of-plane GIXS, in-plane
Kline et al., Polym. Rev. 2006, 46, 27
Crystallinity of P3HT Thin Films
266
high molecular weightlow molecular weight
High resolution GIXS on P3HT
267
F.P.V. Koch et al. / Progress in Polymer Science 38 (2013) 1978– 1989 1985
Fig. 5. (A) Grazing-incidence X-ray diffraction patterns obtained with a 2D-image plate for solution-processed thin films of selected P3HT of Mn given inthe figure. (B) Evolution of the interplanar spacings d1 0 0 and d0 2 0 as well as the !-stacking disorder parameter g (see Refs. [35–38]) with Mn. At Mn ≥ Mc,d1 0 0, d0 2 0 and g become essentially independent from the materials’ molecular weight.
being interconnected by individual macromolecules. Thissuggests that charge-transport in FETs seems to bestrongly limited in thin-film architectures comprised ofnon-interconnected chain-extended crystals such as thoseobtained with low-molecular weight materials, possiblybecause grain boundaries between the crystalline regions(alluded to above) can act as deep traps or transport bar-riers [43,44]. Hence, in order to obtain efficient deviceperformance in semicrystalline polymer semiconductors itseems to be critical to work with material of sufficientlyhigh Mn that provides for a high crystalline interconnec-tivity through tie molecules, as already postulated, e.g., byKline et al. [12,13]. It is intriguing, however, to also note thatthe dependency of !FET with Mn closely follows the devel-opment of Tm and "Hf (measured in solution-cast films)
with Mn (Fig. 6B and C), while it is inversely correlated withthe !-stacking disorder deduced from g (Fig. 5B/bottompanel). Indeed, materials featuring low g-values (i.e., betterorder along the !-stack) display the worst device perfor-mance. Molecular order in the !-stacking direction may bedecoupled from the order along the chain backbone; how-ever molecular order seems relevant since !FET directlycorrelates with "Hf, which is used in classical polymer sci-ence to compare the molecular order between systems [1].Moreover, higher !FET are measured for architectures com-prising crystalline moieties of an increased lamellar crystalthickness (as deduced from the higher melting tempera-tures), suggesting that processing methods that allow l tobe manipulated might assist in reaching optimum deviceperformance.
Thermomechanical Properties of P3HT
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F.P.V. Koch et al. / Progress in Polymer Science 38 (2013) 1978– 1989 1981
Fig. 2. Differential scanning calorimetry, DSC, heating (A) and cooling (B) thermograms obtained for melt-solidified P3HT samples of a range of molec-ular weights (heating/cooling rate = 10 ◦C/min). Melting- and crystallization temperatures Tm and Tc varied with Mn. In addition, it is indicated how thesupercooling, !T, for P3HT of Mn = 130 kg/mol was deduced from the difference between its Tm and Tc.
charge transport as measured in transistors, among otherthings. For instance, on the basis of a range of P3HTs ofrelatively low Mw (2.4 kg/mol < Mw < 18 kg/mol), Zhanget al. [11] established a correlation between “weight-average contour length”, LW (with LW being a measurefor the lamellar crystal thickness, l, for these short-chainmolecules [7]) and field-effect transistor charge-carriermobilities "FET, such that "FET increases with increasingLW, in accord with findings presented in Refs. [12–14].
A more detailed discussion of the interrelationship ofcharge transport and polymer solid-state microstructurewill be given after highlighting how processing can affectthe molecular assembly process.
2.1. Melt processing
2.1.1. Thermal phase behaviourThermal analysis, viscosity measurements and mechan-
ical testing are useful tools to identify the molecular weightabove which the two-phase morphology of interconnectedlamellar crystals and amorphous regions can be observed.We applied these methodologies to P3HT. For this pur-pose, we selected materials of a wide range of molecularweights; number-average molecular weights, Mn rangedfrom 4 to 128 kg/mol, and Mw from 5.5 to 260 kg/mol(see gel permeation chromatography data displayed inFig. 1B).
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Table 1P3HT of the given number-average molecular weights, Mn, and polydispersity indices (PDI) were investigated in this study. Melting temperatures, Tm andcrystallization temperatures, Tc, for melt-processed materials of these P3HT samples are listed. Larger supercoolings !T = Tm − Tc were required to solidifythe P3HT of higher Mn from the melt due to the entangled nature of these systems.
Mn (kg/mol) 4.0 4.8 5.2 7 8 25 35 80 90 95 110 130Tm (◦C) 189 204 211 221 227 236 237 230 235 225 229 223Tc (◦C) 172 181 186 190 196 203 203 183 193 176 184 180!T (◦C) 17 23 25 31 31 33 34 47 42 49 45 43PDI (−) 1.3 1.4 1.4 1.7 1.6 1.8 1.5 2.5 2.2 3.7 2.9 2.0
Mn and PDI were measured via GPC; Tm and Tc were determined via DSC, as per the procedures described in the Experimental section.
Thermal analysis data is presented in Fig. 2. For melt-processed samples of Mn ≤ 25 kg/mol, we observe anessentially linear increase in melting temperature, Tm(deduced from the end of melting endotherm in differen-tial scanning calorimetry (DSC) thermograms), with Mn.For P3HT of higher molecular weight, the melting pointat first slightly decreased with Mn, levelling off around230 ◦C for materials of Mn ≥ 60 kg/mol (Fig. 2A). Consid-ering that Tm depends on chain length (with Tm beinghigher for longer-chain material) [1–3,26,28] and lamellarcrystal thickness l (with Tm increasing with l) [1–3,26,28],we assign this dependency of Tm with Mn to the factthat the microstructure is evolving from fully extended-chain structures into entangled, interconnected two-phase‘fringed-micelle’ architectures [1,2] at Mn ≈ 25 kg/mol, invery close agreement with the classification for the struc-tural evolution of P3HT with chain length forwarded inRefs. [9–14]. Clearly, higher molecular-weight materials(Mn ≥ 25 kg/mol) entangle in the melt, which is reflectedby the large supercooling !T = Tm − Tc, (with Tc being thecrystallization temperature; see Fig. 2B) that is requiredto crystallize these polymers from the melt. Table 1 sum-marizes the !T values obtained for the systems discussedhere; in addition !T for P3HT of Mn = 130 kg/mol is indi-cated in Fig. 2 with red arrows and lines (for interpretationof the references to colour in text, the reader is referred tothe web version of this article). Note also that the entanglednature of the P3HT samples of higher molecular weight,which can lead to chain folding, results in thinner crys-talline, lamellar moieties [1,2] and, hence, lowers their Tm.
In contrast, P3HT of lower molecular weight(Mn ≤ 25 kg/mol) can readily crystallize because themacromolecular chains are not entangled (hence, theobserved, relatively small !T; cf. Table 1) and can there-fore form chain-extended crystals of a thickness l thatessentially corresponds to the length of the individualmolecules [1–8]. For these structures l is linearly correlatedwith Mn. Since thicker crystal lamellae are more stable[3,26,28], this increase in l results in an increase in Tm.
2.1.2. Mechanical propertiesThe structural classification for P3HT, with materi-
als of Mn ≤ 25 kg/mol forming chain-extended structuresand samples of higher Mn fringed-micelle microstruc-ture, is supported by mechanical analysis of a selectedseries of melt-processed P3HT tapes (Mn = 20, 25, 90 and110 kg/mol; Fig. 3A). The low molecular-weight material(Mn = 20 kg/mol) displayed brittle tensile behaviour imply-ing that this sample is indeed comprised of unconnected,chain-extended crystals that cannot withstand large strain.
For P3HT of Mn = 25 kg/mol, we start to observe, at higherelongations at break, a deviation from the linear relation ofstress, ", and elongation, ε, which signifies the onset of plas-tic (i.e., irreversible) deformation. This is a clear indicationthat elastic percolation is reached, which results from thecrystalline entities being connected through tie molecules.P3HT samples of higher molecular weight (here, 90 and
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Fig. 3. (A) Mechanical properties of four selected P3HT tapes of differentMn that had been solidified from the melt. (B) Top panels: Transmissionwide-angle X-ray diffraction patterns on as-prepared (left) and mechani-cally elongated (right) P3HT samples of Mn = 110 kg/mol. After uniaxiallystretching well-defined diffractions are observed that are characteristicfor anisotropic structures (right panel). Bottom panels: Reflection opticalmicrographs taken with crossed polarizers. The oriented tape (P3HT ofMn = 110 kg/mol) was thereby positioned parallel (left) and at 45◦ (right)with respect to the polarizer/analyser system.
The Role of Entanglement in Crystallization
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Fig. 1. (A) Schematic illustration of the evolution of the molecular arrangement of a (semi-)flexible polymer such as the macromolecular semiconductorpoly(3-hexylthiophene), P3HT (see inset in (B) for the chemical structure of P3HT), in the melt (T > Tm; top panel) and in the solid state (T < Tm; bottompanel). The molecular weight between entanglements Me and the long period L, i.e., the total thickness of the crystalline and amorphous region, are definedas indicated in red. In addition, the crystallographic directions [1 0 0] alkyl stacking, [0 1 0] (!-stacking) and [0 0 1] (direction along polymer backbone)are specified in the schematic. (B) Gel permeation chromatography data obtained for the various P3HT systems, measured at 80 ◦C in chlorobenzeneagainst polystyrene standards. From these plots, their number-average molecular weight, Mn, weight-average molecular weight, Mw, and polydispersity,PDI (PDI = Mw/Mn), were determined.
employed in FETs and PVs. For this material, studies basedon transmission electron microscopy (TEM) have indicatedthat the structural evolution from chain-extended crys-tals to an architecture comprised of interlinked crystallinelamellae and amorphous regions occurs at a weight-average molecular weight, Mw, of around 25–35 kg/mol[9,10]. Tellingly, chain folding, which leads to lamellar crys-talline units found in classical systems such as polyolefinsand poly(ethylene oxides) of sufficiently high molecu-lar weight [1–5,26], was visualized in longer-chain P3HTmolecules by scanning tunnelling microscopy (STM) as
early as 2000 by Mena-Osteriz et al. [27] Note, here, thatthe (1 0 0) distance in such mono-layer architectures devi-ated from the lamellar stacking distance measured forP3HT crystals. This observation by Mena-Osteriz et al. [27],nonetheless, indicates that higher-molecular weight P3HTchains can fold and, hence, similar crystalline moieties asfound in classical polymers may form in P3HT of high Mw.
On the other hand, short-chain P3HT seem to adoptchain-extended microstructures (observed in TEM andatomic-force microscopy (AFM) [9–13]), typical for com-mon oligomers, e.g., paraffins [2,6,7]. This directly affects
Charge Carrier Mobilities in P3HT
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Fig. 6. Comparison of the development with number-average molecularweight, Mn of (A) field-effect transistor mobility, !FET, collected from lit-erature [11–14,41,42], (B) melting temperature, Tm, and (C) enthalpy offusion, "Hf (both obtained for solution-solidified material) of the variousP3HT samples investigated here.
3.2. Bulk charge transport
Bulk hole mobilities seem to be less dependent onan interconnected microstructure than the FET mobility,as already shown by Ballantyne et al. [16]. This can bededuced from time-of-flight photoconduction measure-ments on as-cast P3HT films of selected Mn that werea few micrometres thick and sandwiched between twonon-injecting electrodes. As a consequence, charge trans-port parallel to the film normal are assessed in TOF, while
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Fig. 7. (A) Typical time-of-flight (TOF) photoconductivity hole transientsmeasured at the same electric field for solution-cast films of P3HT ofdifferent molecular weights (casting temperature ≈ 25 ◦C). (B) Molecular-weight-dependence of the bulk hole mobility !TOF for P3HT films castfrom solution at room temperature (squares) or at 115 ◦C (circles). NB.The reported trend is in agreement with the one observed in Ref. [16].
in transistors charge transport parallel to the substratesurface is analyzed. Photo-transients recorded at roomtemperature are displayed in Fig. 7A. Despite the relativelydispersive transport evident in the transients, which is afeature often observed in TOF data of conjugated polymerswith a significant degree of disorder, the indicative transittimes tt (obtained from the intersection of the asymptotesto the long-time and short-time transients when plottedon double-logarithmic axes, and indicated in Fig. 7A forthe data obtained for P3HT of Mn = 4.8 kg/mol, could stillbe distinguished for all materials tested. This allowed usto calculate the time-of-flight hole mobilities !TOF usingthe expression: !TOF = d2/V·tt where d is the sample thick-ness, tt is transit time and V is the applied voltage. Thededuced mobility values are plotted as function of molec-ular weight in Fig. 7B (squares). Highest time-of-flight hole
1986 F.P.V. Koch et al. / Progress in Polymer Science 38 (2013) 1978– 1989
Fig. 6. Comparison of the development with number-average molecularweight, Mn of (A) field-effect transistor mobility, !FET, collected from lit-erature [11–14,41,42], (B) melting temperature, Tm, and (C) enthalpy offusion, "Hf (both obtained for solution-solidified material) of the variousP3HT samples investigated here.
3.2. Bulk charge transport
Bulk hole mobilities seem to be less dependent onan interconnected microstructure than the FET mobility,as already shown by Ballantyne et al. [16]. This can bededuced from time-of-flight photoconduction measure-ments on as-cast P3HT films of selected Mn that werea few micrometres thick and sandwiched between twonon-injecting electrodes. As a consequence, charge trans-port parallel to the film normal are assessed in TOF, while
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Fig. 7. (A) Typical time-of-flight (TOF) photoconductivity hole transientsmeasured at the same electric field for solution-cast films of P3HT ofdifferent molecular weights (casting temperature ≈ 25 ◦C). (B) Molecular-weight-dependence of the bulk hole mobility !TOF for P3HT films castfrom solution at room temperature (squares) or at 115 ◦C (circles). NB.The reported trend is in agreement with the one observed in Ref. [16].
in transistors charge transport parallel to the substratesurface is analyzed. Photo-transients recorded at roomtemperature are displayed in Fig. 7A. Despite the relativelydispersive transport evident in the transients, which is afeature often observed in TOF data of conjugated polymerswith a significant degree of disorder, the indicative transittimes tt (obtained from the intersection of the asymptotesto the long-time and short-time transients when plottedon double-logarithmic axes, and indicated in Fig. 7A forthe data obtained for P3HT of Mn = 4.8 kg/mol, could stillbe distinguished for all materials tested. This allowed usto calculate the time-of-flight hole mobilities !TOF usingthe expression: !TOF = d2/V·tt where d is the sample thick-ness, tt is transit time and V is the applied voltage. Thededuced mobility values are plotted as function of molec-ular weight in Fig. 7B (squares). Highest time-of-flight hole
Salleo, Nature Mater. 2013, 12, 1038.
Long-Range versus Short-Range Order
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a
b
c
Figure 1 |Microstructure of conjugated polymer films. a–c, Schematics of the microstructure of a semicrystalline polymer film, for example P3HT (a),disordered aggregates (b) and a completely amorphous film (c). Note the coexistence of ordered (darker shadowed areas) and spaghetti-like amorphousregions. This microstructure is similar to the concept of fringed micelles. If the molecular weight is high enough and there is a large enough density ofordered material, long polymer chains (highlighted in red) can connect ordered regions without a significant loss of conjugation, greatly improving chargetransport.
relationship relating order at the segmental level to transport at thedevice scale. A survey of over ten years of literature data confirmsthe generality of these concepts.
Charge transport and the importance of aggregatesThe microstructure of high-molecular-weight poly(3-hexyl-thiophene), P3HT—amodel semicrystalline conjugated polymer—exhibits a continuous variation in order parameters17,18 wheresemi-ordered and amorphous ‘spaghetti-like’ regions coexist;long polymer chains are responsible for the connectivity betweenadjacent crystallites (Fig. 1a)19. Semi-ordered regions may becomprised of large domains with three-dimensional long-rangeperiodicity (crystallites) or smaller domains with short-rangeordering of a few molecular units (aggregates, which may also beidentified as fringed micelles; Fig. 1b).
In such a heterogeneous microstructure, the ordered regionsare largely responsible for charge transport because chargesmust overcome an energy barrier to move from ordered toamorphous regions. Indeed, owing to its reduced conjugationlength, the amorphous fraction of regio-regular P3HT (RR-P3HT) has a larger bandgap compared to the aggregates andwe find no evidence of energetic overlap of electronic statesin amorphous and ordered P3HT regions (Supplementary Fig.S2). This energetic offset hinders carrier migration into theamorphous regions, and makes it energetically and statisticallyfavourable for charges at the order/disorder interface to migrateback into the ordered regions. Because, in principle, this offsetcould be reduced by polaronic effects, we test our hypothesisby studying blends of regio-random P3HT (RRa-P3HT) withcontrolled amounts of RR-P3HT nanofibrils (see Methods)20. Evenfor a low concentration of aggregates (⇠10% by volume), atcurrent densities comparable to those encountered in electronicdevices, the electroluminescence spectra resemble that of RR-P3HT (Fig. 2, and Supplementary Figs S3 and S4). Consequently,transported charges recombine within fibrils. Thus, at realisticcurrent densities, charge carriers remain confined in the orderedregions of a heterogeneous microstructure when such orderedregions are spatially close enough to form an interconnectednetwork through tie-molecules.
This conclusion may be challenged by the oft-encounteredassumption, based on calorimetric measurements, that semicrys-talline polymer films have a low degree of crystallinity. However,these measurements systematically underestimate the amount ofordered material (Supplementary Fig. S5). In P3HT, we assessthat such an underestimation is approximately a factor of two,with the aggregate volume fraction in P3HT films near ⇠40–50%(refs 21,22). Hence, in high-mobility polymers the fraction of
the film comprised of ordered material is sufficiently large to beinterconnected by bridging polymer chains, creating a network thatsustains efficient charge transport. Such networks of crystalliteshave been previously observed in conjugated polymers21,23.
The nature of the connections between aggregates is poorlyunderstood and should be the subject of further study.Nevertheless,we are able tomake a few qualitative considerations. Polymer chainsare semiflexible24,25—that is, rigid at length scales comparableto their persistence length and flexible at larger length scales.If the spacing between neighbouring crystallites is only a fewpersistence lengths, a chain that exits one ordered region and enters
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Figure 2 | Photo-physical characterization of pre-aggregated P3HTfibril:amorphous-blend films. a, Reference electroluminescence spectrafrom pure RR and RRa P3HT. Vertical dashed lines are theelectroluminescence peak positions for the amorphous, and 0-0, 0-1aggregate transitions of the pristine films, respectively.b, Photoluminescence spectrum (dotted) showing the signature ofaggregated and amorphous material, along with electroluminescencespectra for increasing values of current in the devices made with films of10% pre-aggregated RR-P3HT. Unlike the photoluminescence spectrum,the low-current (typical of normal device application) electroluminescencespectra show only emission from aggregates, indicating that it is dueexclusively to recombination within the aggregates and not to excitonenergy transfer to the aggregates before recombination. Only at largecurrent densities does the blue-shifted spectral feature from RRa-P3HTappear, indicating emission from the disordered matrix.
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Disorder and Electronic Performance
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dependence, static cumulative disorder is significant in thesematerials. Dynamic disorder will induce further fluctuationsaround the static positions, but these refinements do not qualita-tively alter our conclusions.
The spatial extent of the calculated wavefunctions for thedisordered stack (Fig. 3e–h) shows that disorder-induced states thatlie deeper into the DOS tail are increasingly localized. Even stateswithin the originally ‘delocalized band’ becomemore localized withincreasing paracrystallinity. In the limit of large positional disorder(g ⇠ 10%), the distinction between band and tail disappears: asingle broad distribution of localized states with a monotonicallydecreasingDOS extends into the bandgap, reproducingwell-knownresults of electronic structure theory of amorphous materials34,35.In the intermediate paracrystallinity regime (g ⇠ 3–7%) we observea coexistence of localized and delocalized states, indicating that inparacrystalline aggregates charge is transported by a mechanismwhere mobile charge is temporarily trapped in localized states, akinto multiple trapping and release1,36.
Measuring paracrystallinity in conjugated polymersUsing X-ray diffraction peak-shape analysis (details in Supplemen-tary Text) we quantify paracrystallinity in polymer crystallites todetermine whether this disorder dominates their DOS. In PBTTT,a polymer considered to exhibit an exceptional degree of order37,we find a highly anisotropic lattice disorder. In the lamellar stack-ing direction, glam = 2.6% owing to side chain interdigitation38,39.Long-range lamellar order however does not translate to order inthe⇡-stacks, where g⇡=7.3%, a value close to that of an amorphousstack. Thus, we conclude that the DOS of a PBTTT ⇡-stack isdictated by paracrystalline disorder.
The low amount of paracrystalline disorder measured in a smallmolecule film and its small anisotropy (Fig. 4 and SupplementaryTable S1) shows that organic solids are not inherently disorderedin spite of their weak intermolecular forces. Conversely, thedistinguishing feature of polymers is the pronounced disorder inthe ⇡-stacking direction. To observe this transition, we use thinfilms of P3HT with chain lengths of 8 to >2,000 monomers (Mnfrom 1.33 to >350 kgmol�1), and perform single-peak estimationsof the ⇡-stacking disorder (Supplementary Text and Figs S6, S7).When starting from an oligomer and adding monomers to form ahigh-molecular-weight polymer, paracrystallinity in the ⇡-stackingdirection is initially low (comparable to that in polycrystalline filmsof small molecules) and rises until the incremental effect of addingmonomers vanishes and paracrystallinity becomes independent ofmolecule length (Fig. 4a).
Although P3HT represents a model system, we also exploredparacrystallinity in the ⇡-stacking direction of other high-mobilitypolymers. We collect ⇡-stacking data from over 30 literaturecases (Fig. 4a), where conjugated polymers were processed withcommon solvents and casting techniques. It is apparent thathigh-molecular-weight semicrystalline polymer semiconductorsexhibit a surprisingly common level of minimum lattice disorder(g ⇠ 6–8%). In the direction where intermolecular charge transferoccurs, all high-mobility high-molecular-weight polymers arecloser to being amorphous (g ⇠10–20%) than crystalline (g <1%),irrespective of the degree of order in the lamellar stacking direction.The universal dependence of paracrystalline disorder on molecularweight indicates that chain folding and entanglements are probablesources of defects40. Side-chain conformational disorder may alsodisrupt the backbone packing arrangement, leading to an intrinsiclevel of disorder common to all conjugated polymers.
A simultaneous comparison of the effect of molecular weighton disorder and on mobility (Fig. 4) is a strong indication thatcharge transport is linked to lattice disorder. When the molecularweight of P3HT is high enough such that its lattice disorderbecomes independent of chain length, the carrier mobility is also
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Figure 4 | Chain length effects on paracrystallinity and charge transport.a, The ⇡-stacking paracrystallinity in films of polymeric semiconductors ofdifferent molecular weight (MW), plotted as a function of degree ofpolymerization. Semicrystalline materials (P3HT, PBTTT, PQT) are shown inshades of red. Poorly ordered high performing materials are shown in black.Others are shown in grey. Open symbols are estimates, solid symbols arederived from a full analysis. As a comparison, cumulative disorder inpolycrystalline triisopropylsilyl pentacene (TIPS-Pn) is far smaller than thatof PBTTT and, importantly, shows significantly less anisotropy(Supplementary Table S1). b, Mobility as a function of molecular weight fora variety of semiconducting polymers. P3HT is shown in red with differentsymbols referring to different studies. Other high-performing materials areshown in black. For the complete list of references see Supplementary Text.
remarkably independent of chain length. The link between thesetwo fundamentally different properties is particularly striking, asthe turn-over point is the same for both (chains with 50–100monomers for P3HT) and agrees with recent yield and mobilitymeasurements of photogenerated charges41. Our findings remainvalid, irrespective of whether interface or bulk measurements areconsidered (see Supplementary Text). Similar improvements inmobility with increasing molecular weight have been observedin other polymers11,42.
The effect of molecular weight on charge transport in semicrys-talline polymers is subject to an intrinsic and general trade-off. Longchains provide electrical connectivity between ordered regions,allowing efficient charge transport, but also result in higher struc-tural disorder. In the low-molecular-weight connectivity-limitedregime, mobility rises quickly as average chain length increases43.Eventually, the mobility plateaus as a function of molecular weightwhen enough connectivity is provided for charge to move throughthe paracrystalline regions (see Supplementary Text). Thus, inhigh-molecular-weight polymers charge transport is strongly en-hanced by the connectivity of the ordered regions (crystallites andaggregates) but limited by lattice disorder in these regions oncesufficient connectivity is established.
The spread in mobility values is significant, showing that nosingle microstructural feature is entirely responsible for electronic
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ARTICLES
NATUREMATERIALS DOI: 10.1038/NMAT3722
performance. Differences in morphology arise from variationsin processing conditions and polydispersity, which contribute tothe variability in charge carrier mobility. Despite these factors,the fundamental transport-limiting mechanism and its causalconnection to paracrystallinity is preserved.
Paracrystallinity governs charge transportIn ‘classic’ (that is, designed to exhibit a noticeable degree ofcrystallinity and extended ⇡–⇡ stacking) semicrystalline conjugatedpolymers, paracrystalline disorder in the crystallites limits chargetransport via the introduction of traps. The level of lattice disorderin the ⇡-stacking direction measured in all high-molecular-weightpolymers corresponds to the regimewhere localized and delocalizedstates in the aggregates coexist, giving rise to trapping-limitedtransport, as experimentally observed in field effect transistors(FET) and in localmobilitymeasurements26. In FETs, the differencein energy between the populated traps and the mobile statesmanifests itself in the activation energy for transport and itsdependence on charge density (gate voltage). Hence, the greater thedisorder, the deeper the traps and the larger the activation energy.
In Fig. 5, we gather activation energies derived fromtemperature-dependent FET measurements, and trap depth/tailwidths derived from modelling electrical transport both fromthe literature and from this work. These data were collectedfrom samples spanning a range of mobility, materials purity,device preparation, surface treatments and geometry. In spite ofsuch variety of materials and conditions, a consistent behaviouremerges. Transport in common thiophene-based semicrystallinepolymers—for example, P3HT, poly[5,50-bis(3-alkyl-2-thienyl)-2,20-bithiophene] (PQT), PBTTT—has an average 72 ± 24meVactivation energy. In comparison, completely amorphous ma-terials show a significantly higher average activation energy⇠230±100meV. It is interesting to examine newhigh-performancepolymers, which do not seem to belong in either category. Examplesof these materials are copolymers based onmore complex moieties,such as naphthalene-diimides (PNDI-T2), diketopyrrolopyrroleswith sulphur and selenium (DPP-BT, DSePP-BT), and carbazoles(PCDTBT), but also newer materials such as isoindigo copoly-mers and indacenodithiophene-based polymers. These materialshave mobilities comparable to those of semicrystalline polymers(µFET ⇠ 1 cm2 V�1 s�1), but at first glance, morphologically, theyare closer to the amorphous materials family. Indeed their disorderis higher than that of semicrystalline polymers, as evidenced bylower diffracted intensities, fewer observable diffraction peaks,and/or the large breadth of their ⇡-stacking peak. These newhigh-mobility polymers exhibit activation energies and trapbreadths of 76±33meV: similar to those of ‘classic’ semicrystallinepolymers and significantly lower than the mean value observed foramorphous conjugated polymers. Many of these new reportedly‘amorphous’ materials have at least enough short-range order toresult in detectable diffraction in synchrotron XRD experiments(Supplementary Fig. S8). Though polymers in this new class areseemingly disordered, they exhibit the tell-tale signs of aggregationin their solid-state optical absorption spectra, namely the presenceof a resolvable vibronic progression near the absorption edge22.Short-range order in the form of aggregates is sufficient to supportefficient intermolecular charge transfer. Because charges have ashort scattering mean free path in the ⇡-stacks—on the order of ananometre36—they are mostly affected by such short-range order.Disordered crystallites larger than a scattering length bring hardlyany additional benefit for charge transport. In conjugated polymers,crystallinity—nomatter how low—is a sign that the polymer chainshave a tendency to form stacked aggregates with a paracrystallinitythat can be estimated by the width of the ⇡-stacking XRD peak.This collection of interconnected aggregates enables highmobilitiesin high-molecular-weight polymers despite the consistently large
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Figure 5 |Activation energy for transport in semiconducting polymers.Activation energies obtained from FET data from this work, and from theliterature (dash), as well as trap depth/tail widths derived from devicemodelling (cross) for traditional classic semicrystalline materials (red),new high performers that are found to be poorly ordered (black), andcompletely amorphous materials (blue). Data (84 values for 13 differentmaterials) are binned by material (for the complete list of references seeSupplementary Text). The averages and standard deviations from therespective groups are shown for semicrystalline, poorly ordered, andamorphous materials on the right panel.
paracrystalline disorder in the ⇡-stacking direction and weakdiffracted intensities.
Molecular stacking, even with only short-range order, makesaggregating materials fundamentally different from truly amor-phous polymers. Assuming an exponential dependence of theintermolecular transfer integral t on separation x , t = t0e��x , theprefactor t0 and wavefunction overlap decay length (1/�) governthe extent to which lattice disorder affects charge transport. Thenewer high-mobility polymers can tolerate nanometre-scale disor-der owing to their structure at the ångström-scale. For example,increasing t0 by reducing the ⇡–⇡ stacking distance causes aggre-gates to be electronically less sensitive to large amounts of disorder,increasing the threshold beyond which paracrystallinity-inducedstates severely affect transport. Similarly, reducing � by design-ing materials with orbitals having larger overlaps or favourablesymmetries reduces the effect of positional disorder on chargetransport. Some of these factors are probably at work in recentlyreported high-mobility polymers. The optimization of side-chainsto increase solubility leads to a reduced intermolecular ⇡-stackingdistance in diketopyrrolopyrrole-furan and isoindigo-based poly-mers. Using large and planar fused-ring aromatic cores increasesthe overlapping area between adjacent ⇡-stacked units, makingthe aggregates less sensitive to positional disorder12,13. Therefore,tuning the molecular design to improve packing and orbital shape,orientation and symmetry may contribute to making transport inaggregates—where charge is confined—less sensitive to ⇡-stackingdisorder, thereby preserving efficient device-scale charge transport.
These considerations separate polymeric semiconductors intotwo classes. In one class we have truly amorphous materials,lacking any long-range order and characterized by broad, featurelessamorphous X-ray scattering halos. Charge transport in these trulyamorphous materials is limited by hopping between localized statesin a broad DOS. Such a transport mode is restricted to mobilities<0.1 cm2 V�1 s�1 and is subject to large activation energies44. In thesecond class are materials that can form interconnected aggregatesexhibiting at least short-range order, with transport in field-effectdevices described by a multiple trapping and release mechanism.
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