Introducing Solubility Control for Improved Organic P-Type Dopants

Article abstract of DOI:10.1021/acs.chemmater.5b02340

To overcome the poor solubility of the widely used p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), we have synthesized a series of structure-modified, organic p-type dopants to include alkyl ester groups designed to enable solubility and miscibility control. UV-vis-NIR and cyclic voltammetry measurements show increased solubility of mono- and diester substituted dopants with only modest changes to acceptor strength. Using absorption spectroscopy, photoluminescence, and in-plane conductivity measurements, we demonstrate that the new dopants can successfully p-type dope poly(3-hexylthiophene-2,5-diyl) (P3HT). Monoester substituted dopants are characterized by only slightly reduced electron affinity relative to F4TCNQ, but greater doping effectiveness due to increased miscibility with P3HT. Diester substituted dopants undergo a dimerization reaction before assuming their doped states, which may help anchor dopants into position post deposition, thus decreasing the negative effect of dopant drift and diffusion. We conclude that increased dopant solubility/miscibility increases the overall effectiveness of doping in solution-cast polymer films and that ester modification is a practical approach to achieving solubility/miscibility control in TCNQ-type dopants (Figure Presented).

Full text of DOI:10.1021/acs.chemmater.5b02340

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Introducing solubility control for improved
organic p-type dopants
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Jun Li,†,¶ Guangwu Zhang,‡,¶ Daniella E. Holm,† Ian E. Jacobs,† Bin Yin,‡
Pieter Stroeve,† Mark Mascal,∗,‡ and Adam J. Moul´e∗,†
Department of Chemical Engineering and Materials Science, University of California,
Davis, USA, and Department of Chemistry, University of California, Davis, USA
E-mail: mjmascal@ucdavis.edu; amoule@ucdavis.edu
Abstract
To overcome the poor solubility of the widely used p-type dopant 2,3,5,6-tetrafluoro-
7,7,8,8-tetracyanoquinodimethane (F4TCNQ), a series of structure-modified, organic
p-type dopants have been synthesized to include alkyl ester groups designed to enable
solubility and miscibility control. UV-vis-NIR and cyclic voltammetry measurements
show increased solubility of mono- and di-ester substituted dopants with only mod-
est changes to acceptor strength. Using UV-vis-NIR, photoluminescence, and in-plane
conductivity measurements, we demonstrate that the new dopants can successfully
p-type dope poly(3-hexylthiophene-2,5-diyl) (P3HT). Mono-ester substituted dopants
are characterized by only slightly reduced electron affinity relative to F4TCNQ, but
greater doping effectiveness due to increased miscibility with P3HT. Di-ester substi-
tuted dopants undergo a dimerization reaction before assuming their doped states,
∗To whom correspondence should be addressed
†Department of Chemical Engineering and Materials Science, University of California, Davis, USA
‡Department of Chemistry, University of California, Davis, USA
¶Contributed equally to this work
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which may help anchor dopants into position post deposition, thus decreasing the neg-
ative effect of dopant drift and diffusion. We conclude that increased dopant solubil-
ity/miscibility increases the overall effectiveness of doping in solution cast polymer films
and that ester modification is a practical approach to achieving solubility/miscibility
control in TCNQ-type dopants.
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Introduction
Organic semiconductors have recently drawn much interest due to a number of advan-
tages they have over conventional inorganic materials, which may include low cost, light
weight, compatibility with flexible substrates, biocompatibility, low environmental impact,
and chemical tailorability.1–4 Organic electronic devices, such as organic light emitting diodes
(OLEDs),5,6 organic photovoltaics (OPV)7,8 and organic field effect transistors (OFETs),9,10
show significant improvements in performance with the addition of dopants. Intrinsic or-
ganic semiconductors in general have low free-charge densities and therefore low conductivity
compared to inorganic semiconductors. To increase conductivity, conjugated polymers or
small molecule semiconductors can be doped via the addition (n-type) or removal (p-type)
of electrons. One method of doping a polymer is to form an organic salt using acidic or
basic doping groups for p- and n-type doping, respectively.11 Polyethylenedioxythiophene
polystyrenesulfonate (PEDOT:PSS) is a well known example of an acidically doped p-type
organic conductive material.12,13 An alternative method to dope organic semiconductors is
to add a neutral molecule with an electron affinity (EA) higher than the ionization energy
(IE) of the organic semiconductor.14 In this case, an electron from the semiconductor is
spontaneously donated to the high EA molecule to create a hole state, a classic example
being the use of iodine to dope polythiophenes.15,16
High EA organic molecular dopants have been studied since the 1960’s. Recent stud-
ies on such dopants have focused on the role that ground state and excited state charge
transfer (CT) states play in quenching excited state fluorescence and in recombination pro-
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cesses in OPV devices.17–20 Assuming no CT-state formation, charge transfer occurs when
the lowest unoccupied molecular orbital (LUMO) of the dopant is accessible to the high-
est occupied molecular orbital (HOMO) of the organic semiconductor matrix. The dopant
molecule must also have a stable structure that can be re-oxidized without reaction with
nearby molecules. The quinone structure has these characteristics and early studies of molec-
ular organic dopants focused on tetrachlorobenzoquinone (chloranil; LUMO: -2.76 eV)21–23
and tetracyanoquinodimethane (TCNQ; LUMO: -2.8 eV).24,25 More recently, dopants with
higher EA were prepared by incorporating electron-withdrawing groups into the quinone ring.
Dopants in this category include 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ; LUMO:
-4.6 eV)26–28 and the more widely used 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ; LUMO: -5.24 eV).14,29,30 Following the same concept of molecular design, even
higher EA dopants, such as 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6TN-
AP; LUMO: -5.37 eV)31,32 and 3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodimethane (F2HCNQ;
LUMO: -5.59 eV)33 have been recently synthesized. Other recently reported p-type dopants
with high EA include C60 fullerene (LUMO: -3.6 eV),34 its fluorinated derivative C60F36
(LUMO: -5.38 eV),35,36 and hexaazatriphenylene hexacarbonitrile (HAT-CN6; LUMO: 5.7
eV).37–39 However, most of these dopants have low solubility in common solvents, and are
incorporated into devices structures using either evaporation processes14,40 or premixing in
an inert matrix41 because they cannot be effectively solution processed. This limitation
significantly restricts further development of solution-processed and mass-produced organic
electronics applications.42
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In this paper, we first demonstrate a straightforward synthetic route to soluble versions
of F4TCNQ-type dopants by substituting the cyano groups with either methyl or n-octyl
esters (Figure 1). A comprehensive study of the electrochemical properties of these F4TCNQ
analogues is performed using cyclic voltammetry. With a combination of optical absorption
spectroscopy, photoluminescence spectroscopy, and conductivity measurements, we not only
demonstrate the p-type doping of P3HT using these new dopants, but also show that com-
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5 eV and LUMO 3 eV) was purchased from Sigma-Aldrich. F4TCNQ (>98%) was purchased
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from TCI. All chemicals were used as received unless otherwise indicated. All solvents were
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dried over molecular sieves (3A) before use.
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Characterization
1H-NMR (300 MHz), 13C-NMR (75 MHz) and 19F-NMR (282 MHz) spectra were recorded on
a Varian Mercury 300 NMR spectrometer. Spectral data were processed using MestReNova
(version 6.2.0) desktop NMR data processing software.
Cyclic voltammograms (CVs) were recorded on a BASi Epsilon MF-9092 Electrochemi-
cal Workstation. The redox potentials of all of substrates were measured in anhydrous ace-
tonitrile (MeCN) solution containing tetramethylammonium tetrafluoroborate (Me4NBF4,
0.05M) using a platinum disk (Φ =1.6 mm) as the working electrode and Ag/AgCl as the
reference electrode. The concentration of substrates in the working solution was 0.5 mM,
and the electrochemical potential sweep rate was fixed at 100 mV/s.
High resolution mass spectra (HRMS) were obtained on a Thermo Fisher Hybrid LTQ-
Orbitrap XL mass spectrometer equipped with electrospray. Liquid chromatography-mass
spectra (LC-MS) were recorded on Qtrap LC/MS instruments.
UV-vis-NIR and photoluminescence spectra were recorded on a Perkin Elmer Lambda 750
spectrophotometer and a Varian Eclipse photoluminescence spectrophotometer, respectively.
For the solubility measurements, 5 mg of F4TCNQ and 100 mg of the four new dopants were
◦
introduced into 1.0 mL of chloroform. The mixtures were stirred on a hotplate at 60 C for
24 h, and then rested at room temperature for another 24 h. Saturated solutions were diluted
accordingly in order to be measured in a useful absorbance range. Calibration curves were
also measured for each dopant. For characterization of the blended films, glass substrates
were cleaned in ultrasonic baths of acetone, Mucasol detergent (5%), and deionized water,
followed by drying with nitrogen. The substrates were then exposed to UV/ozone for 30
min before use. Solutions of 5 mg/mL P3HT and p-type dopants (0.5 mg/mL for F4TCNQ
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and 5 mg/mL for the ester derivatives) in chloroform were mixed in appropriate ratios to
achieve the desired mole fraction of dopants to P3HT. Except neat F4TCNQ, all other films
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were spin coated from blend solutions at 60 C inside a nitrogen glovebox. Neat F4TCNQ
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films were deposited using an MBruan thermal evaporator at deposit rate ∼0.2 A/s. Film
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thicknesses were measured with a Veeco Dektak 150 Surface profilometer. All UV-vis-NIR
and photoluminescence spectra were measured under ambient conditions in air.
Conductivity measurements were performed with a four-point probe setup using a Kei-
thely 2420 source sourcemeter unit. Four electrodes (5 nm Cr / 95 nm Au, 1 × 5 mm2, 1
mm spacing) were deposited through a shadow mask by thermal evaporation. The same
procedure was used for substrate cleaning and blended film deposition described above. All
conductivity measurements were performed in the dark under a nitrogen atmosphere in a
glovebox.
Results and Discussion
Synthesis of P-type Dopants
Synthesis of the symmetrical di(alkoxycarbonyl)dicyanotetrafluoroquinodimethanes F4DM-
CDCNQ and F4DOCDCNQ starts from commercial 1,4-bis(chloromethyl)-2,3,5,6-tetrafl-
uorobenzene 1 (Scheme 1). Conversion to the required bis(cyanomethyl) derivative 3 does
not take place by direct substitution with cyanide, so the intermediate bis(bromomethyl)tetra-
fluorobenzene 2 was prepared. Remarkably, direct substitution of 1 with bromide also proves
challenging, but 2 could be made via the corresponding (bis)benzyl alcohol. Substitution of
2 with cyanide is facile, and methoxycarbonylation of 3 by deprotonation and reaction with
dimethyl carbonate gives diester 4. Oxidation of 4 to the dimethyl ester F4TCNQ analogue
F4DMCDCNQ with bromine proceeds in high yield. Alternatively, transesterification of
4 with octanol gave the longer-chain diester 5 which could likewise be readily oxidized to
F4DOCDCNQ.
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Scheme 1: Synthesis of F4DMCDCNQ and F4DOCDCNQ. Reagents and conditions:
◦
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a) NaOAc, Ac2O, AcOH, 105 C, 36 h; b) NaOH, EtOH/THF/H2O, 75 C, 24 h; c) PBr3,
DCM, 10 h, 76% over 3 steps; d) NaCN, CF3COOH, DMSO, 5 h, 51%; e) (MeO)2CO, NaH,
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THF, 75 C, 18 h, 83%; f) Br2, H2O, 2 h, 94%; g) octanol, TsOH·H2O, toluene, 110 C, 6 h,
72%; h) Br2, H2O, 3 h, 93%.
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The synthesis of the mono(alkoxycarbonyl)tetrafluorotricyanoquinodimethanes F4MCT-
CNQ and F4OCTCNQ starts from commercial hexafluorobenzene (Scheme 2). Reaction
with methyl cyanoacetate in the presence of a base gives the intermediate methyl 2-cyano-2-
(pentafluorophenyl)acetate, which is decarboxylated in situ to give 2-(pentafluorophenyl)eth-
anenitrile 6. SN Ar substitution with the sodium salt of malononitrile provides 7, which is
methoxycarbonylated by reaction with dimethyl carbonate and base to give 8. Oxidation of
8 with bromine gives F4MCTCNQ, the monomethyl ester analogue of F4TCNQ. Transes-
terification of 8 with octanol followed again by oxidation gives the mono-octyl ester dopant
F4OCTCNQ. Detailed synthetic procedures are provided in the Supporting Information.
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Scheme 2: Synthesis of F4MCTCNQ and F4OCTCNQ. Reagents and conditions: a)
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MeOCOCH2CN, K2CO3, DMF, 115 C, 6 h, 72%; b) 50% aq HOAc, conc. H2SO4, reflux,
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15 h, 75%; c) CH2(CN)2, NaH, THF, 70 C, 18 h, 66%; d) (MeO)2CO, NaH, THF, 80 C,
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48 h, 89%; e) Br2, H2O, 1 h, 82%; f) octanol, TsOH·H2O, toluene, 85 C, 9 h, 71%; g) Br2,
H2O, 1.5 h, 81%.
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Characterization of P-type Dopants
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Our dopant design goal was to improve the solubility of the dopant molecules without com-
promising doping efficiency. To demonstrate the improved solubility of the new F4TCNQ
analogues, saturated solutions were prepared in chloroform (Figure 2a) and their absorbance
spectra were recorded by UV-vis-NIR at room temperature. The solubility limits (Figure
2b) were then determined based on calibration curves of each compound. The details of the
calibration curves and measurements can be found in the Supporting Information (Figure
S47). As can be seen from Figure 2b, the solubility of the ester-substituted dopants is signif-
icantly greater than F4TCNQ itself, by a factor of at least 55 for F4MCTCNQ and 100 for
F4OCTCNQ. Interestingly, although the solubility of the diesters are still ∼30 times higher
than that of F4TCNQ, they are less soluble than the monoesters, which is attributed to the
higher symmetry of the diesters.43
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5 mg/mL
100 mg/mL
a)
b)
F4OCTCNQ
F4MCTCNQ
F4DOCDCNQ
F4DMCDCNQ
F4TCNQ
Figure 2: (a). Photos of saturated solutions (from left to right: F4TCNQ, F4MCTCNQ,
F4OCTCNQ, F4DMCDCNQ, and F4DOCDCNQ) in chloroform. (b). Measured solubility
limits of the dopants in chloroform at room temperature.
Since the EA of the dopant is an important predictor of doping efficiency, the next
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thing we needed to show is what effect substitution of an ester group for a cyano group
on the F4TCNQ structure would have on the reduction potential. The electrochemical
states of the dopants were investigated using cyclic voltammetry (CV). Solutions of dopants
in acetonitrile were measured under reducing potentials and the CV data are shown in
Figure 3a. As expected, F4TCNQ itself undergoes two reversible one-electron reduction
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steps corresponding to F4TCNQ and F4TCNQ2−. The monoester-substituted dopants
•−
F4MCTCNQ and F4OCTCNQ also exhibit two reduction peaks, but interestingly, they
are located between the first and second reductions of F4TCNQ, and the first reduction
peak Ered1 of the monoester-substituted dopants is greater than the second reduction peak
Ered2. Unlike F4TCNQ, only one reduction peak is seen in the CV of diester-substituted
dopants. Closer inspection of the CV data reveals that both the mono and diesters undergo
redox processes that are either irreversible or quasi-reversible. Therefore, instead of using
the mean values of anodic and corresponding cathodic peak potentials (E1/2), reduction
potentials were used for comparison. The reduction potentials for all dopants vs. Ag/AgCl
are summarized in Table 1.
Table 1: Summary of reduction potentials from CV and determined LUMO levels of
F4TCNQ and its ester derivatives
Compound
F4TCNQ
F4MCTCNQ
F4OCTCNQ
F4DMCDCNQ
F4DOCDCNQ
Ered1(V) ELUMO1(eV) Ered2(V) ELUMO2(eV)
0.51
0.42
0.40
0.25
0.21
-5.23
-5.14
-5.12
-4.97
-4.93
0
0.12
0.11
-4.72
-4.84
-4.83
As can be seen from Table 1, Ered1 shows a decreasing trend from 0.51 V to ∼0.4 V and
∼0.2 V as cyano groups are substituted by one or two ester groups, respectively, showing
that F4TCNQ is a stronger p-type dopant than its ester analogues. This is expected since
the cyano group is more electron withdrawing than the ester group, which results in a more
electron deficient quinoid ring in the F4TCNQ than in the ester-substituted derivatives.
Comparing the octyl and methyl ester substituted dopants, subtle differences in Ered1 are
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ultraviolet photoelectron spectroscopy (UPS) (5.24 eV),46 which verifies the accuracy of our
assignments.
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To enable a visual comparison, the LUMO levels of all investigated p-type dopant molecules
are compiled into one energy diagram (Figure 3b). The HOMO and LUMO levels of P3HT
and P3HT positive polaron (P3HT+) reported from literature15,47–49 are also shown to scale.
Further discussion of the P3HT matrix/dopants system is deferred to the next section, as we
focus here on the comparison of LUMO levels of the dopants. Similar to F4TCNQ, Ered1 and
Ered2 for the ester analogues can be assigned to the formation of anion radicals from the neu-
tral molecules and the formation of dianions from the anion radicals, respectively. Therefore,
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we assign singly and doubly reduced dopants as compound and compound2−, respectively.
•−
•−
Among the compounds considered, F4TCNQ shows the deepest LUMO level, F4MCTCNQ
•−
•−
•−
and F4OCTCNQ show the next deepest levels, and F4DMCDCNQ and F4DOCDCNQ
show relatively shallow levels. In addition, doubly reduced dopants show weaker reduction
strength (smaller EA) and thereby shallower LUMO levels than singly reduced dopants,
as expected. Another highlight of the CV measurements is that there is only one reduc-
tion peak for diester-substituted dopants. Thus, the LUMO levels of F4DMCDCNQ2− and
F4DOCDCNQ2− are not shown because these species do not form based on the CV data.
This suggests that the diester-substituted dopants undergo a different chemical process dur-
ing reduction than the F4TCNQ and monoester-substituted dopants, the details of which
will be discussed below. Finally, only subtle differences in LUMO levels are found between
the methyl ester and octyl ester analogues, merely 0.01–0.04 eV, indicating that the –R
group on the ester does not strongly influence the electronic nature of the TCNQ dopants.
A mechanism is proposed to explain the charge transfer processes of F4TCNQ and ester-
substituted dopants, as illustrated schematically in Figure 4. Assuming only one electron is
involved at each step, radical anions are formed from neutral molecules in the first reduc-
tion. Resonance structures are also shown. Due to the molecular symmetry of the F4TCNQ
and the diester molecules, the two resonance structures of their anion radicals are identical,
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that diester dopants form dimers after reduction while monoester dopants and F4TCNQ do
not. The structure of the dimer shown in Figure 4 is a proposal based on the most likely site
for spin coupling, involving no interference with the aromaticity of the benzene rings and
with maximum possible separation of the charges. However, for the purposes of this work,
the exact structure of the dimer is of no real consequence.
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Characterization of Doped Films
It has been shown in the above sections that F4TCNQ can be synthetically altered to in-
corporate esters in place of nitrile groups, where the ester can support different alkyl chains
that can be used to affect the solubility of the molecule. In this section we demonstrate
successful p-type doping of the well-characterized conjugated polymer P3HT by the above
described ester-substituted dopants. In particular, we investigate the effect of altering the
solubility of the dopant on the charge density and conductivity of the polymer.
Measuring Charge Density
Absorption spectroscopy (UV-vis-NIR) is commonly used to observe changes in the crys-
talline to amorphous content of P3HT samples.50,51 The doping level can also be monitored
using UV-vis-NIR.30 Grazing incidence x-ray spectra and concentration-dependent conduc-
tivity were recently measured for the P3HT/F4TCNQ system.52 For doping levels below 4
mol%, F4TCNQ occupies locations within the amorphous part of the P3HT52 but dopes ex-
clusively in the crystalline domains.20 Between 4 mol% and 17 mol% doping ratios, F4TCNQ
intercalates between the P3HT chains within crystalline domains, increasing the (010) crys-
52
˚
˚
tal spacing (along the π-stacking direction) from 3.83 A to 7.18 A. The 17 mol% blend
ratio is the point when this blend system reaches the upper limit on both conductivity and
low energy absorptivity (1 ∼ 1.7 eV) according to literature.52
The normalized UV-vis-NIR absorption spectra for P3HT/dopant at selected doping ra-
tios are shown in Figure 5. The full doping concentration series of UV-vis-NIR spectra can
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F4MCTCNQ
F4OCTCNQ
F4DMCDCNQ
Neat P3HT
3 mol%
17 mol%
Neat dopant
F4DOCDCNQ
0.5
1
1.5
2
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3
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Energy (eV)
Figure 5: Normalized UV-vis-NIR absorbance spectra of neat P3HT (black), dopants (ma-
genta dashed line), and P3HT/dopant blend films at 3 mol% (red) and 17 mol% (blue) doping
ratios. Arrows show the dedope peak for P3HT/dopant system.
be found in the Supporting Information (Figure S48). These data exactly reproduce the
literature results for P3HT/F4TCNQ.52 Undoped P3HT has a band gap of ∼1.9 eV. Upon
oxidation by F4TCNQ, two broad sub-bandgap absorbance peaks centered around 0.4 eV
and 1.5 eV are observed, which can be assigned to the polarons of P3HT.53,54 In addition,
two sharper peaks at 1.43 and 1.62 eV can be detected that correspond to absorbance by
•−
F4TCNQ . As shown in Figure 5, sub-gap absorptions similar to those of F4TCNQ
•−
•−
•−
are seen in F4MCTCNQ and F4OCTCNQ . The double peaks at 1.43 and 1.62 eV are
replaced by a single absorbance at the same energy that increases with increased dopant con-
centration. In fact, the NIR absorbance is stronger at both 0.4 eV and 1.5 eV in films doped
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with the monoester dopants than with F4TCNQ at the same mol% doping concentration.
For instance, at 17 mol% the ratios between polaron absorbance at 1.5 eV to ground-state
absorbance at 2.3 eV are 1:1, 1.04:1 and 1.12:1 for F4TCNQ, F4MCTCNQ and F4OCTCNQ,
respectively. The increased polaron/ground-state ratio is an indication that higher P3HT
polaron density is achieved for monoester derivatives. Conversely, only a slight increase in
P3HT polaron peaks is observed in films containing F4DMCDCNQ and F4DOCDCNQ, even
as the doping concentration reaches 17 mol%. This result indicates that there is less charge
transfer between P3HT and the diester dopants compared with the F4TCNQ and monoester
dopants at the same doping concentration.
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Changes in the short-range ordered regions (aggregates)55,56 of P3HT can be probed by
monitoring the absorbance ratio of the 0–0 to 0–1 vibronic peaks at 2.03 eV (610 nm) and 2.25
eV (550 nm) because these peak are associated with interchain-delocalized excitation47,57 and
the degree of P3HT ordering.55,58,59 The concentration-dependent spectra in the Supporting
Information (Figure S48) show that the relative intensity of the peak at 610 nm compared
to the other P3HT vibronic peaks increases with increasing doping concentration up to 10
mol%. This result implies one of two things. It could indicate that crystalline domains of
P3HT are becoming more ordered (more planar) upon doping. Alternatively, the relative
increase in the peak at 2.03 eV could indicate an increased probability of doping in less
crystalline domains. The ground-state signal is bleached upon doping and only the most
crystalline sections of the sample are isolated from contact with the dopants. A relative
increase in the peak at 610 nm occurs in all P3HT/acceptor systems, although the diester
dopants induce less pronounced shifts in the vibronic spectra.
Besides the characteristics of the doping effects mentioned above, UV-vis-NIR spectra
for all neat dopants are also presented. The absorbance spectra of these five dopants do not
show many differences other than the broader and red-shifted absorption peak for F4TCNQ.
The blueshift of the absorption peak from F4TCNQ (∼2.8 eV) to the monoester derivatives
(∼3.3 eV) and diester derivatives (∼3.4 eV) is expected due to less delocalization of charge
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resulting from replacing cyano with ester groups. The appearance of an additional peak
in the high energy range of blend systems is also noted, as indicated by arrows (Figure 5).
These peaks are assigned to the dopant anion, although in monoester systems an overlapping
of the monoester anion absorbance and neutral dopant molecule absorbance is observed.
A recent study from our group showed that this optical transition can be used to dedope
P3HT/F4TCNQ films with complete recovery of optical characteristics and mobilities.60 The
related transitions for ester-substituted dopants are blue-shifted with respect to F4TCNQ
by ∼0.5 eV. The UV-vis-NIR spectra of the dopant anions can be found in the Supporting
Information (Figure S49).
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The differential solubility of the mono- and di-ester dopants with methyl and octyl chains
enables us to evaluate the effect of dopant solubility on doping efficiency, with the long chain
esters showing higher solubility. Comparison of the P3HT+/dopant CT-state absorbance
−
features to the ground-state absorbance of bulk P3HT gives a direct measure of the charge
density. The results in this section clearly indicate that a dopant which is more miscible in
the polymer has a higher probability of doping the polymer.
Measuring Dopant Miscibility
Photoluminescence spectroscopy (PL) is another technique that has been extensively used
to evaluate doping interactions in organic materials because photoluminescence is quenched
by the presence of dopants.53,61 Dopants increase the free charge density in the polymer
and free charges react with and quench excitons through a dark process with ∼1/r3 dis-
tance dependence.62 PL experiments were performed using 2.4 eV excitation to optically
excite P3HT and the PL emission for undoped P3HT (1.4 ∼ 2.0 eV) was observed. Figure
6 shows the PL intensity as a function of mol fraction for all dopants. The inset shows
typical PL spectra for neat and doped P3HT at 1 mol% doping ratio. As can be seen,
F4DMCDCNQ and F4DOCDCNQ show only partial PL quenching of ∼20% and ∼40%,
respectively. F4TCNQ is a stronger exciton quencher (∼80%), while monoester dopants
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In this section, we describe concentration-dependent conductivity measurements that were
performed to determine whether the increase in dopant miscibility translates to enhanced
electronic properties. Figure 7a shows a log-log plot of conductivity versus mole fraction
of dopant for P3HT doped with F4TCNQ and each of the new dopants. Blend films at
different doping ratios were spin-cast on pre-patterned substrates and then measured by the
four-point probe method, the details of which have been described in earlier publications.63,64
It is clear that the in-plane conductivity of P3HT can be tuned by at least three orders of
magnitude by all dopants. All dopants also show a similar trend of increased conductivity
with increasing dopant concentration. After reaching a peak, the addition of more dopant
leads to an increase in neutral dopant domains and conductivity decreases with increased
concentration. The upper limit of conductivity in P3HT when doped with F4TCNQ or
monoester dopants is ∼100 S/cm while for diester dopants the maximum conductivity is
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Over most of the doping range, the conductivity of P3HT doped with the monoester
dopants is higher at the same mole fraction than P3HT doped with F4TCNQ, which is
consistent with UV-vis-NIR and PL measurement results. For comparison to the earlier
work of Duong et al.,52 the conductivity data are also plotted on a log linear scale (Figure
7b). Just as was seen for P3HT doped with F4TCNQ, there is a clear change in the slope of
conductivity vs. concentration for all dopants at around 4 mol% dopant loading. We assume
here that the assignments made by Duong et al. are correct in that the initial high slope
comes from dopants mixing into the amorphous P3HT domains.20,52 At higher doping ratios,
the dopants intercalate into the crystalline P3HT domains, increasing the crystal spacing
between P3HT chains. The initial slopes for loading of dopant up to 4 mol% are identical for
the monoester dopants and F4TCNQ. However, when the doping ratio is increased beyond
4 mol%, F4OCTCNQ in particular shows a larger increase in conductivity with increased
doping than F4TCNQ, confirming that the modified dopant is more efficient at intercalating
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Molar fraction of dopants
Figure 7: (a). In-plane conductivity measurements of P3HT/dopant blend films as a function
of doping concentration. The neat P3HT film conductivity is also shown (orange). (b). Fits
to the conductivity data in the weak (dashed lines) and strong (solid lines) doping regimes
for F4TCNQ and the monoester derivatives (top) and diester derivatives (bottom).
between P3HT chains. This result demonstrates that increased miscibility of the dopant has
a clear advantage for doping efficiency. For F4TCNQ, the peak in conductivity occurs at 17
mol% doping followed by a rapid drop in conductivity with increased dopant concentration,
presumably because increased dopant content is neutral and pure F4TCNQ crystals are
formed in the polymer.52 By comparison, both monoester dopants show a much broader peak
in conductivity with increased dopant concentration, which also argues for better miscibility
between the dopant and P3HT.
Another interesting observation is that the conductivity of P3HT samples doped with
diester dopants increases exponentially up to 40 mol% doping ratio, while the F4TCNQ
and monoester dopants show an exponential increase in conductivity until 10 mol% for
F4OCTCNQ and 17 mol% for F4MCTCNQ and F4TCNQ, respectively. This is consistent
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with the UV-vis-NIR results for the diesters (Figure S48 in the Supporting Information)
which show that P3HT polaron density continues to increase with up to 64 mol% dopant
loading. This trend cannot be explained solely by the relatively shallow LUMO level of
diesters. Instead, the increase in conductivity with such high doping levels implies that
the dimerization reaction stabilizes the doped states. The conductivity does not increase
until there is high enough dopant concentration that they are able to dimerize and form
stable dianions. This dimerization reaction shifts equilibrium towards the dianion, which is
trapped by the irreversibility of the process and then cannot give the electron back into the
polymer. While it may be considered disadvantageous that large doping ratios are necessary
to achieve higher conductivity with the diesters, there is a possible processing advantage
to the dimerization reaction. This advantage is that the large dianion is unlikely to diffuse
with thermal stress or drift with electric field stress. The smaller F4TCNQ is known to
diffuse against a concentration gradient and thereby to dope regions of the sample outside of
the originally intended area, thus reducing device lifetimes.35 In addition, since the diester-
substituted dopants do not dimerize until after deposition, they can be efficiently mixed
into the polymer and then doping can occur post film formation through the dimerization
reaction. The dopants, once dimerized, will provide a much higher barrier to diffusion.
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Conclusions
In conclusion, we have developed a method to introduce solubility control into p-type organic
dopants based on the F4TCNQ structure. Solubility control is achieved by replacing cyano
groups on the F4TCNQ framework with ester groups. The results show that the solubility
of the new ester dopants is improved by a factor of at least 30 and up to 100 compared with
F4TCNQ at room temperature. In addition, these new dopants possess a range of interesting
properties. First, mono-substituted ester dopants show only slightly lower (∼0.2 eV) elec-
tron affinity than F4TCNQ. However, in spite of the lower EA, monoester dopants lead to
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increased polaron density and more efficient doping in P3HT than F4TCNQ. F4OCTCNQ,
for instance, has the same conductivity at 10 mol% doping as F4TCNQ at 17 mol% doping.
The increased doping efficiency is proposed to be a result of the ester-substituted dopants
being both more soluble in good solvents for P3HT and more miscible in P3HT itself. This
study not only shows that molecular tailoring of organic dopants for increased miscibility
with the desired polymer is a sound approach to improving doping efficiency, but also that
single ester replacements are a viable method to modify common dopant structures with
cyano attachments. Additionally, we also introduce diester-substituted dopants, which have
distinctly different electrochemical properties than F4TCNQ and the monoester dopants
due to a dimerization reaction that occurs between their radical anions. Despite the shallow
LUMO level of the diesters, this irreversible dimerization also results in effective doping.
However, much higher doping levels are needed for these diester dopants to maximize con-
ductivity in P3HT, as shown by increased polaron density in optical spectra. Viewed broadly,
these results provide a pathway for future dopant molecule design with controlled solubil-
ity/miscibility as a synthetic design parameter. Future studies will include a variety of ester
–R groups that could tailor the placement of molecular dopants with respect to particular
conjugated molecules/polymers.
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Supporting Information
Detailed synthetic procedures, NMR spectra, Mass spectra, UV-Vis spectra, and Photo-
luminescence spectra. The Supporting Information is available free of charge on the ACS
Publications website at DOI:
Acknowledgements
This research project was supported by the U.S. Department of Energy, Office of Basic
Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-
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References
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(1) Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on
plastic. Nature 2004, 428, 911–918.
(2) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar
Cells. Chem. Rev. 2007, 107, 1324–1338.
(3) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of Conjugated Polymers for Organic
Solar Cell Applications. Chem. Rev. 2009, 109, 5868–5923.
(4) Lussem, B.; Riede, M.; Leo, K. Doping of organic semiconductors. Phys. Status Solidi
A 2013, 210, 9–43.
(5) Zhou, X.; Blochwitz, J.; Pfeiffer, M.; Nollau, A.; Fritz, T.; Leo, K. Enhanced Hole
Injection into Amorphous Hole-Transport Layers of Organic Light-Emitting Diodes
Using Controlled p-Type Doping. Adv. Funct. Mater. 2001, 11, 310–314.
(6) Reineke, S.; Thomschke, M.; Lu¨ssem, B.; Leo, K. White organic light-emitting diodes:
Status and perspective. Rev. Mod. Phys. 2013, 85, 1245–1293.
(7) Taima, T.; Sakai, J.; Yamanari, T.; Saito, K. Doping effects for organic photovoltaic
cells based on small-molecular-weight semiconductors. Sol. Energy Mater. Sol. Cells
2009, 93, 742–745.
(8) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Molecular Semiconductors
in Organic Photovoltaic Cells. Chem. Rev. 2010, 110, 6689–6735.
(9) Braga, D.; Horowitz, G. High-Performance Organic Field-Effect Transistors. Adv.
Mater. 2009, 21, 1473–1486.
23
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5
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7
8
9
(10) Lu, G.; Blakesley, J.; Himmelberger, S.; Pingel, P.; Frisch, J.; Lieberwirth, I.; Salz-
mann, I.; Oehzelt, M.; Di Pietro, R.; Salleo, A.; Koch, N.; Neher, D. Moderate doping
leads to high performance of semiconductor/insulator polymer blend transistors. Nat.
Commun. 2013, 4, 1588.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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30
31
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34
35
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37
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39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(11) Heeger, A. J.; Sariciftci, N. S.; Namdas, E. B. Semiconducting and metallic polymers;
Oxford University Press, 2010.
(12) Jonsson, S.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; van der
Gon, A. D.; Salaneck, W.; Fahlman, M. The effects of solvents on the morphol-
ogy and sheet resistance in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid
(PEDOT–PSS) films. Synth. Met. 2003, 139, 1–10.
(13) Elschner, A.; Kirchmeyer, S. PEDOT-Type Materials in Organic Solar Cells; WILEY-
VCH, 2008.
(14) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Highly Efficient Organic Devices Based
on Electrically Doped Transport Layers. Chem. Rev. 2007, 107, 1233–1271.
(15) Br´edas, J.; Wudl, F.; Heeger, A. Polarons and bipolarons in doped polythiophene: A
theoretical investigation. Solid State Commun. 1987, 63, 577–580.
(16) Colaneri, N.; Nowak, M.; Spiegel, D.; Hotta, S.; Heeger, A. J. Bipolarons in poly(3-
methylthiophene): Spectroscopic, magnetic, and electrochemical measurements. Phys.
Rev. B 1987, 36, 7964–7968.
(17) Aziz, E.; Vollmer, A.; Eisebitt, S.; Eberhardt, W.; Pingel, P.; Neher, D.; Koch, N.
Localized Charge Transfer in a Molecularly Doped Conducting Polymer. Adv. Mater.
2007, 19, 3257–3260.
(18) Panda, P.; Veldman, D.; Sweelssen, J.; Bastiaansen, J. J. A. M.; Langeveld-Voss, B.
24
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Chemistry of Materials
1
2
3
4
M. W.; Meskers, S. C. J. Charge Transfer Absorption for -Conjugated Polymers and
5
6
Oligomers Mixed with Electron Acceptors. J. Phys. Chem. B 2007, 111, 5076–5081.
7
8
9
(19) Zhu, L.; Kim, E.-G.; Yi, Y.; Br´edas, J.-L. Charge Transfer in Molecular Com-
plexes with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ): A Den-
sity Functional Theory Study. Chem. Mater. 2011, 23, 5149–5159.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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41
42
43
44
45
46
47
48
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50
51
52
53
54
55
56
57
58
59
60
(20) Gao, J.; Roehling, J. D.; Li, Y.; Guo, H.; Moule, A. J.; Grey, J. K. The effect of 2,3,5,6-
tetrafluoro-7,7,8,8-tetracyanoquinodimethane charge transfer dopants on the confor-
mation and aggregation of poly(3-hexylthiophene). J. Mater. Chem. C 2013, 1, 5638–
5646.
(21) Kearns, D. R.; Tollin, G.; Calvin, M. Electrical properties of organic solids. II. Effects
of added electron acceptor on metalfree phthalocyanine. J. Chem. Phys. 1960, 32,
1020–1025.
(22) Chen, E. C. M.; Wentworth, W. E. A comparison of experimental determinations of
electron affinities of pi charge transfer complex acceptors. J. Chem. Phys. 1975, 63,
3183–3191.
(23) Cooper, C. D.; Frey, W. F.; Compton, R. N. Negative ion properties of fluoranil, chlo-
ranil, and bromanil: Electron affinities. J. Chem. Phys. 1978, 69, 2367–2374.
(24) Ferraris, J.; Cowan, D. O.; Walatka, V.; Perlstein, J. H. Electron transfer in a new
highly conducting donor-acceptor complex. J. Am. Chem. Soc. 1973, 95, 948–949.
(25) Compton, R. N.; Cooper, C. D. Negative ion properties of tetracyanoquinodimethan:
Electron affinity and compound states. J. Chem. Phys. 1977, 66, 4325–4329.
(26) Maitrot, M.; Guillaud, G.; Boudjema, B.; Andr´e, J. J.; Simon, J. Molecular materi-
albased junctions: Formation of a Schottky contact with metallophthalocyanine thin
films doped by the cosublimation method. J. Appl. Phys. 1986, 60, 2396–2400.
25
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(27) Krikor, H.; Nagels, P. Structural characterization and electrical conductivity
of poly(phenylethynyl) copper doped with iodine and 2,3-dichloro-5,6-dicyano-p-
benzoquinone. Synth. Met. 1989, 29, 109–114.
9
10
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33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(28) Tolbert, L.; Edmond, C.; Kowalik, J. Charge-transfer doping of poly(3-alkyl-2,2-
bithiophene). Synth. Met. 1999, 101, 500–501.
(29) Pingel, P.; Schwarzl, R.; Neher, D. Effect of molecular p-doping on hole density and
mobility in poly(3-hexylthiophene). Appl. Phys. Lett. 2012, 100, 143303.
(30) Wang, C.; Duong, D. T.; Vandewal, K.; Rivnay, J.; Salleo, A. Optical measurement
of doping efficiency in poly(3-hexylthiophene) solutions and thin films. Phys. Rev. B
2015, 91, 085205.
(31) Koech, P. K.; Padmaperuma, A. B.; Wang, L.; Swensen, J. S.; Polikarpov, E.;
Darsell, J. T.; Rainbolt, J. E.; Gaspar, D. J. Synthesis and Application of 1,3,4,5,7,8-
Hexafluorotetracyanonaphthoquinodimethane (F6-TNAP): A Conductivity Dopant for
Organic Light-Emitting Devices. Chem. Mater. 2010, 22, 3926–3932.
(32) Tietze, M. L.; Burtone, L.; Riede, M.; Lu¨ssem, B.; Leo, K. Fermi level shift and doping
efficiency in p-doped small molecule organic semiconductors: A photoelectron spec-
troscopy and theoretical study. Phys. Rev. B 2012, 86, 035320.
(33) Gao, Z. Q.; Mi, B. X.; Xu, G. Z.; Wan, Y. Q.; Gong, M. L.; Cheah, K. W.; Chen, C. H.
An organic p-type dopant with high thermal stability for an organic semiconductor.
Chem. Commun. 2008, 117–119.
(34) Schulze, K.; Uhrich, C.; Schu¨ppel, R.; Leo, K.; Pfeiffer, M.; Brier, E.; Reinold, E.;
B¨auerle, P. Efficient Vacuum-Deposited Organic Solar Cells Based on a New Low-
Bandgap Oligothiophene and Fullerene C60. Adv. Mater. 2006, 18, 2872–2875.
26
ACS Paragon Plus Environment

Page 27 of 31
Chemistry of Materials
1
2
3
4
5
6
7
8
(35) Meerheim, R.; Olthof, S.; Hermenau, M.; Scholz, S.; Petrich, A.; Tessler, N.;
Solomeshch, O.; Lu¨ssem, B.; Riede, M.; Leo, K. Investigation of C60F36 as low-
volatility p-dopant in organic optoelectronic devices. J. Appl. Phys. 2011, 109, 103102.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(36) Solomeshch, O.; Yu, Y. J.; Goryunkov, A. A.; Sidorov, L. N.; Tuktarov, R. F.;
Choi, D. H.; Jin, J.-I.; Tessler, N. Ground-State Interaction and Electrical Doping
of Fluorinated C60 in Conjugated Polymers. Adv. Mater. 2009, 21, 4456–4460.
(37) Falkenberg, C.; Olthof, S.; Rieger, R.; Baumgarten, M.; Muellen, K.; Leo, K.; Riede, M.
The role of energy level matching in organic solar cells—Hexaazatriphenylene hexacar-
bonitrile as transparent electron transport material. Sol. Energy Mater. Sol. Cells 2011,
95, 927–932.
(38) Diouf, B. B.; Jeon, W. S.; Park, J. S.; Choi, J. W.; Son, Y. H.; Lim, D. C.; Doh, Y. J.;
Kwon, J. H. High hole mobility through charge recombination interface in organic light-
emitting diodes. Synth. Met. 2011, 161, 2087–2091.
(39) Lin, H.; Lin, W.; Chang, J.; Wu, C. Solution-processed hexaazatriphenylene hexacar-
bonitrile as a universal hole-injection layer for organic light-emitting diodes. Org. Elec-
tron. 2013, 14, 1204–1210.
(40) Tunc, A. V.; Sio, A. D.; Riedel, D.; Deschler, F.; Como, E. D.; Parisi, J.; von Hauff, E.
Molecular doping of low-bandgap-polymer:fullerene solar cells: Effects on transport and
solar cells. Org. Electron. 2012, 13, 290–296.
(41) Rainbolt, J. E.; Koech, P. K.; Polikarpov, E.; Swensen, J. S.; Cosimbescu, L.;
Von Ruden, A.; Wang, L.; Sapochak, L. S.; Padmaperuma, A. B.; Gaspar, D. J. Synthe-
sis and characterization of p-type conductivity dopant 2-(3-(adamantan-1-yl)propyl)-
3,5,6-trifluoro-7,7,8,8-tetracyanoquinodimethane. J. Mater. Chem. C 2013, 1, 1876–
1884.
27
ACS Paragon Plus Environment

Chemistry of Materials
Page 28 of 31
1
2
3
4
5
6
7
8
(42) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Materials and
Applications for Large Area Electronics: Solution-Based Approaches. Chem. Rev. 2010,
110, 3–24.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(43) Pinal, R. Effect of molecular symmetry on melting temperature and solubility. Org.
Biomol. Chem. 2004, 2, 2692–2699.
(44) Andersson, M. R.; Berggren, M.; Inganaes, O.; Gustafsson, G.; Gustafsson-
Carlberg, J. C.; Selse, D.; Hjertberg, T.; Wennerstroem, O. Electroluminescence from
Substituted Poly(thiophenes): From Blue to Near-Infrared. Macromolecules 1995, 28,
7525–7529.
(45) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thomp-
son, M. E. Relationship between the ionization and oxidation potentials of molecular
organic semiconductors. Org. Electron. 2005, 6, 11–20.
(46) Gao, W.; Kahn, A. Controlled p-doping of zinc phthalocyanine by coevaporation with
tetrafluorotetracyanoquinodimethane: A direct and inverse photoemission study. Appl.
Phys. Lett. 2001, 79, 4040–4042.
¨
(47) Osterbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. Two-Dimensional Electronic
Excitations in Self-Assembled Conjugated Polymer Nanocrystals. Science 2000, 287,
839–842.
(48) Hwang, I.-W.; Moses, D.; Heeger, A. J. Photoinduced Carrier Generation in
P3HT/PCBM Bulk Heterojunction Materials. J. Phys. Chem. C 2008, 112, 4350–
4354.
(49) Lloyd, M. T.; Prasankumar, R. P.; Sinclair, M. B.; Mayer, A. C.; Olson, D. C.; Hsu, J.
W. P. Impact of interfacial polymer morphology on photoexcitation dynamics and de-
vice performance in P3HT/ZnO heterojunctions. J. Mater. Chem. 2009, 19, 4609–4614.
28
ACS Paragon Plus Environment

Page 29 of 31
Chemistry of Materials
1
2
3
4
5
6
7
8
(50) Bouman, M. M.; Havinga, E. E.; Janssen, R. A. J.; Meijer, E. W. Chiroptical Properties
of Regioregular Chiral Polythiophenes. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A
1994, 256, 439–448.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(51) Moul´e, A. J.; Meerholz, K. Controlling Morpholopy in Polymer-Fullerene Mixtures.
Adv. Mater. 2008, 20, 240–245.
(52) Duong, D. T.; Wang, C.; Antono, E.; Toney, M. F.; Salleo, A. The chemical and
structural origin of efficient p-type doping in P3HT. Org. Electron. 2013, 14, 1330–
1336.
(53) Yim, K. H.; Whiting, G. L.; Murphy, C. E.; Halls, J. J. M.; Burroughes, J. H.;
Friend, R. H.; Kim, J.-S. Controlling Electrical Properties of Conjugated Polymers
via a Solution-Based p-Type Doping. Adv. Mater. 2008, 20, 3319–3324.
(54) Pingel, P.; Neher, D. Comprehensive picture of p-type doping of P3HT with the molec-
ular acceptor F4TCNQ. Phys. Rev. B 2013, 87, 115209.
(55) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.;
Toney, M. F.; Salleo, A. A general relationship between disorder, aggregation and
charge transport in conjugated polymers. Nat. Mater. 2013, 12, 1037–1043.
(56) Koch, F. P. V. et al. The impact of molecular weight on microstructure and charge
transport in semicrystalline polymer semiconductors–poly(3-hexylthiophene), a model
study. Prog. Polym. Sci. 2013, 38, 1978 – 1989, Topical issue on Conductive Polymers.
(57) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Role of Intermolecular Coupling in the
Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioreg-
ular Polythiophene. Phys. Rev. Lett. 2007, 98, 206406.
(58) Chang, L.; Lademann, H. W. A.; Bonekamp, J.-B.; Meerholz, K.; Moul´e, A. J. Effect
29
ACS Paragon Plus Environment

Chemistry of Materials
Page 30 of 31
1
2
3
4
of Trace Solvent on the Morphology of P3HT:PCBM Bulk Heterojunction Solar Cells.
5
6
Adv. Funct. Mater. 2011, 21, 1779–1787.
7
8
9
(59) Roehling, J. D.; Arslan, I.; Moule, A. J. Controlling microstructure in poly(3-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
hexylthiophene) nanofibers. J. Mater. Chem. 2012, 22, 2498–2506.
(60) Jacobs, I. E.; Li, J.; Burg, S. L.; Bilsky, D. J.; Rotondo, B. T.; Augustine, M. P.;
Stroeve, P.; Moul´e, A. J. Reversible Optical Control of Conjugated Polymer Solubility
with Sub-micrometer Resolution. ACS Nano 2015, 9, 1905–1912.
(61) Tsoi, W. C.; Spencer, S. J.; Yang, L.; Ballantyne, A. M.; Nicholson, P. G.; Turnbull, A.;
Shard, A. G.; Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J.-S. Effect of Crys-
tallization on the Electronic Energy Levels and Thin Film Morphology of P3HT:PCBM
Blends. Macromolecules 2011, 44, 2944–2952.
(62) Ferguson, A. J.; Kopidakis, N.; Shaheen, S. E.; Rumbles, G. Quenching of Excitons by
Holes in Poly(3-hexylthiophene) Films. J. Phys. Chem. C 2008, 112, 9865–9871.
(63) Mauger, S. A.; Li, J.; Ozmen, O. T.; Yang, A. Y.; Friedrich, S.; Rail, M. D.;
Berben, L. A.; Moule, A. J. High work-function hole transport layers by self-assembly
using a fluorinated additive. J. Mater. Chem. C 2014, 2, 115–123.
(64) Nardes, A. M.; Kemerink, M.; Janssen, R. A. J.; Bastiaansen, J. A. M.; Kiggen, N.
M. M.; Langeveld, B. M. W.; van Breemen, A.; de Kok, M. M. Microscopic understand-
ing of the anisotropic conductivity of PEDOT:PSS thin films. Adv. Mater. 2007, 19,
1196–1200.
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ACS Paragon Plus Environment