Enhanced performance of bulk heterojunction solar cells using novel alternating phenylenevinylene copolymers of low band gap with cyanovinylene 4-nitrophenyls

Article abstract of DOI:10.1021/ma100943t

Two novel low band gap soluble copolymers, P1 and P2, were synthesized and characterized. P1 consisted of alternating dihexyloxyphenylene and α-[[4-(diphenylamino) phenyl]methylene]-4-nitro-ben-zeneacetonitrile. P2 consisted of alternating dihexyloxyphenylene and α,α'-[(1,4- phenylene)dimethy-lidyne]bis(αZ,α'Z)-4-nitrobenzeneacetonitrile. These copolymers showed broad absorption curves with long-wavelength absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for P1 and P2, respectively. Both P1 and P2 were blended with PCBM to study the photovoltaic response of bulk heterojunction (BHJ) solar cells. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of both P1 and P2 are well aligned with those of PCBM acceptor. This allows efficient photoinduced charge transfer and high open circuit voltage, leading to an overall power conversion efficiency (PCE) of 3.15% and 2.60% for the as cast P1:PCBM-and P2:PCBM-based devices, respectively. The PCE of the devices has been further improved up to 4.06% and 3.35% for the devices based on thermally annealed P1:PCBM and P2:PCBM blends, respectively.

Full text of DOI:10.1021/ma100943t

5544 Macromolecules 2010, 43, 5544–5553
DOI: 10.1021/ma100943t
Enhanced Performance of Bulk Heterojunction Solar Cells Using Novel
Alternating Phenylenevinylene Copolymers of Low Band Gap with
Cyanovinylene 4-Nitrophenyls
J. A. Mikroyannidis,*,† A. N. Kabanakis,† P. Balraju,‡,^ and G. D. Sharma*,‡,§
†Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece,
‡Physics Department, Molecular Electronic and Optoelectronic Device Laboratory JNV University, Jodhpur
(Raj.) 342005, India, §Jaipur Engineering College, Kukas, Jaipur (Raj.), India, and ^Molecular Electronics
Laboratory JNCASR, Bangalore, India
Received April 28, 2010; Revised Manuscript Received May 27, 2010
ABSTRACT: Two novel low band gap soluble copolymers, P1 and P2, were synthesized and characterized.
P1 consisted of alternating dihexyloxyphenylene and R-[[4-(diphenylamino)phenyl]methylene]-4-nitro-ben-
zeneacetonitrile. P2 consisted of alternating dihexyloxyphenylene and R,R0-[(1,4-phenylene)dimethy-
lidyne]bis(RZ,R0Z)-4-nitrobenzeneacetonitrile. These copolymers showed broad absorption curves with
long-wavelength absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for P1 and
P2, respectively. Both P1 and P2 were blended with PCBM to study the photovoltaic response of bulk
heterojunction (BHJ) solar cells. The highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) levels of both P1 and P2 are well aligned with those of PCBM acceptor. This
allows efficient photoinduced charge transfer and high open circuit voltage, leading to an overall power
conversion efficiency (PCE) of 3.15% and 2.60% for the as cast P1:PCBM- and P2:PCBM-based devices,
respectively. The PCE of the devices has been further improved up to 4.06% and 3.35% for the devices based
on thermally annealed P1:PCBM and P2:PCBM blends, respectively.
Introduction
polymer synthesized from a copolymerization of alkylated carba-
zole and 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT) recently
Organic solar cells using conjugated molecular materials have
become increasingly feasible through the development of BHJ
structures based on intimate blends of polymeric donors and
soluble fullerene acceptors,1-4 where efficient exciton separation
is enabled by a large-area donor-acceptor interface.5,6 The
attractive features of this type of solar cell include the ease of
solution-processing, low costs, light weight, and the potential
application in flexible large-area devices.7,8 So far, such devices
based on polymer/fullerene blends have achieved high PCEs of
more than 6%.9-12
In order to further improve the efficiency of BHJ polymer solar
cells, the research is focused on the synthesis of new polymers.
From the materials perspective, these donor polymers should not
only have a low band gap (to increase the short circuit current,
Jsc) but also bear a low energy level of the highest occupied
molecular orbital (HOMO) (to improve the open circuit voltage,
Voc).13 Typically, a low band gap polymer is designed via a
“donor-acceptor” (D-A) approach, which is to incorporate
electron-rich and electron-deficient moieties in the polymer back-
bone. The low band gap is mainly caused by the intramolecular
charge transfer between donor and acceptor units.14 In D-A
copolymer, the electron unit provides a deeper HOMO level,
whereas electron acceptor is used to tune the electronic bang gap.10
In fact, several low band gap polymers have been demonstrated
with decent efficiencies (3-6%) when they were blended with
PCBM or [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) in
typical BHJ devices.15-35 For example, PCDTBT, a low band gap
demonstrated a record high efficiency of 6.1% when it was blended
in a BHJ composite with PC70BM.24
Among conjugated polymers, poly(p-phenylenevinylene) (PPV)
and its derivatives have been at the center of focus. For the
applications in polymer solar cells (PSCs), PPVs36 and polythio-
phenes (PTs)37 have attracted much attention due to their good
film-forming and optical properties. Up to now, the PSCs based on
a blend of PPVs or PTs and PCBM have exhibited PCE over 3%38
and 6%,39,34 respectively. Recently, a PCE about 7.4% has been
achieved from PTB7/PC71BM solar cell devices.40 However, the
PCE of the PSCs is still low, which is mainly suffered from the
narrow absorption band and the low charge carrier mobility of
the π-conjugated polymers. To fabricate high-efficiency solar cells,
the promising conjugated polymers should possess a broader ab-
sorption to enlarge absorption spectrum range and higher charge
carrier mobility to reduce charge recombination and increase the
photocurrent.
Various low band gap small molecules and polymers carrying
cyanovinylene 4-nitrophenyls have been synthesized in our la-
boratory and used for BHJ solar cells41 and dye-sensitized solar
cells42 with enhanced efficiency. In continuation of this research
line, herein we synthesized two novel alternating phenyleneviny-
lene copolymers P1 and P2 with the D-A structure. Particularly,
both copolymers contained the hexyloxyphenylene as one con-
structional unit. The other constructional unit was the R-[[4-
(diphenylamino)phenyl]methylene]-4-nitrobenzeneacetonitrile for
P1 and R,R0-[(1,4-phenylene)dimethylidyne]bis(RZ,R0Z)-4-nitro-
benzeneacetonitrile for P2. The hexyloxy side groups rendered
both copolymers soluble in common organic solvents. The pre-
sence of triphenylamine in P1 is expected to enhance the hole
*Corresponding authors: tel þ30 2610 997115, fax þ30 2610 997118,
e-mail mikroyan@chemistry.upatras.gr (J.A.M.); tel 91-0291-2720857,
fax 91-0291-2720856, e-mail sharmagd_in@yahoo.com (G.D.S.).
pubs.acs.org/Macromolecules
Published on Web 06/10/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 13, 2010 5545
mobility of this copolymer. A literature survey revealed that the
R-[[4-(diphenylamino)phenyl]methylene]-4-nitrobenzeneacetoni-
trile unit has been utilized as nonlinear chromophore.43 In addi-
tion, this unit, but without the nitro group, has been utilized for
electrophotographic photoreceptor and electrochromic display
devices.44 Finally, an alternating phenylenevinylene copolymer
similar to P2, but without the nitro groups, has been reported
in a Chinese patent as electroluminescent photovoltaic (PV)
material.45 The optical, electrochemical, and electrical properties
of copolymers P1 and P2 and their performance for PV devices
are reported. The overall PCE of the devices based on the as cast
P1:PCBM and P2:PCBM is about 3.15% and 2.60%, respec-
tively. The higher value of PCE for P1:PCBM has been attributed
to (i) the higher difference in the LUMO levels between P1 and
PCBM as compared to the difference in LUMO levels of P2 and
PCBM and (ii) the relatively higher value of the hole mobility for
P1. When thermally annealed P1:PCBM and P2:PCBM active
layers are used in the device, the PCE was further increased up to
4.06% and 3.35%, respectively. This enhanced PCE with ther-
mally annealed blends has been related to the increase in both
crystallinity of donor material in blend and hole mobility.
On the other hand, in order to improve organic photovoltaic
cells, our efforts should not be solely focused in enhancing their
power conversion efficiency as it will be proved pointless without
addressing their weaknesses, i.e., poor UV-light stability and
reduced lifetime. These limiting factors must be taken seriously
into account as they are responsible for the loss of the electronic
properties of the material used in the active layer after a very short
time in use conditions. Several recent reports concerning the
study of lifetime of the organic devices, during storage both in
the dark46 and under illumination,47,48 have been published. The
degradation of organic solar cells may be caused by several
factors, and the photodegradation of the organic active layer is
one of them.49,50 It has been well established that the component
has to be protected from the atmosphere;51 however, the behavior
of the active layer materials under illumination and in the absence
of oxygen has to be further investigated. Despite the efforts of
providing an efficient protection to the solar cell against oxygen
and humidity, the photodegradation of the materials cannot be
avoided. The direct absorption of UV-vis light by the chromo-
phores of polymeric matrix can be responsible for photophysical
processes and direct photochemical reactions. It is well-known
that under the impact of light and without involvement of
oxygen,52-55 aromatic polymers for instance can undergo various
reactions, involving rearrangement (photo-Fries, Norrish y),
chain scissions, or cross-linking. Several experiments have been
conducted, involving the artificially accelerated aging of the
materials. The data obtained from these experiments suggest
that, if well protected of oxygen (encapsulation), this active layer
is photochemically stable for several years in real conditions of
aging.56
mmol), DMF (6 mL), and triethylamine (3 mL). The flask was
degassed and purged with N2. The mixture was heated at 90 ꢀC
for 24 h under N2. Then, it was filtered and the filtrate was
poured into methanol. The precipitate was filtered and washed
with methanol. The crude product was purified by dissolving
in THF and precipitating into methanol (0.2673 g, yield 83%).
FT-IR (KBr, cm-1): 3028, 1592, 1506 (aromatic); 1690 (formyl);
2952, 2924 (C-H stretching of hexyloxy chains); 1284, 1210
(ether bond); 1314 (C-N stretching of triphenylamine); 964
1
(trans vinylene bond). H NMR (CDCl3) ppm: 10.01 (s, 1H,
formyl); 7.87 (m, 2H, triphenylamine ortho to formyl); 7.35 (m,
4H, triphenylamine ortho to vinylene); 7.11 (m, 10H, vinylene
and triphenylamine ortho to nitrogen); 6.75 (s, 2H, phenylene
ortho to oxygen); 3.96 (m, 4H, OCH2(CH2)4CH3); 1.81 (m, 4H,
OCH2CH2(CH2)3CH3); 1.37 (m, 12H, O(CH2)2(CH2)3CH3);
0.91 (t, 6H, O(CH2)5CH3). Anal. Calcd for (C41H45NO3)n: C,
82.10; H, 7.56; N, 2.34. Found: C, 81.94; H, 7.48; N, 2.27.
Copolymer P1. A flask was charged with a solution of 3
(0.3214 g, 0.537 mmol of the repeating unit) and 4-nitrobenzyl
cyanide (0.869 g, 0.537 mmol) in a mixture of ethanol (15 mL)
and chloroform ((10 mL). Sodium hydroxide (0.18 g, 4.50 mmol)
dissolved in ethanol (5 mL) was added portionwise to the stirred
solution. The mixture was stirred for 1 h at room temperature
under N2 and then was concentrated under reduced pressure. The
concentrate was cooled in a refrigerator to precipitate a dark-
green solid. It was filtered, washed thoroughly with water, and
dried to affordP1. The crude product was purified by dissolving in
THF and precipitating into methanol (0.2950 g, 74%). FT-IR
(KBr, cm-1): 3030, 1590, 1502, 1452 (aromatic); 2954, 2922 (C-H
stretching of hexyloxy chains); 2186 (cyano); 1530, 1346 (nitro);
1286, 1212 (ether bond) and 962 cm-1 (trans vinylene bond). 1H
NMR (CDCl3) ppm: 8.22 (m, 2H, phenylene ortho to nitro); 7.72
(s, 1H, cyanovinylene);7.47 (m, 2H, phenylene metatonitro);7.34
(m, 6H, triphenylamine meta to nitrogen); 7.10 (m, 10H, vinylene
and triphenylamine ortho to nitrogen); 6.74 (s, 2H, phenylene
ortho to oxygen); 3.96 (m, 4H, OCH2(CH2)4CH3); 1.81 (m, 4H,
OCH2CH2(CH2)3CH3); 1.36 (m, 12H, O(CH2)2(CH2)3CH3); 0.92
(t, 6H, O(CH2)5CH3). Anal. Calcd for (C49H49N3O4)n: C, 79.11;
H, 6.64; N, 5.65. Found: C, 78.83; H, 6.75; N, 5.54.
Compound 7. A flaskwas charged with a solution of6 (0.3300 g,
1.130 mmol) and 4-nitrobenzyl cyanide (0.3666 g, 2.260 mmol)
in ethanol (20 mL). Sodium hydroxide (0.60 g, 16.70 mmol)
dissolved in ethanol (10 mL) was added portionwise to the
stirred solution. The mixture was stirred for 1 h at room
temperature under N2 and then was concentrated under reduced
pressure. The concentrate was cooled in a refrigerator to pre-
cipitate a dark-green solid. It was filtered, washed thoroughly
with water, and dried to afford 7. It was purified by column
chromatography, eluting with a mixture of dichloromethane
and hexane (1:1) (0.54 g, 83%). FT-IR (KBr, cm-1): 1596, 1490
(aromatic); 2162 (cyano); 1522, 1346 (nitro). 1H NMR (CDCl3)
ppm: 8.19 (m, 4H, phenylene ortho to nitro); 7.70 (s, 2H, cyano-
vinylene); 7.50 (m, 4H, phenylene meta to nitro); 7.22 (s, 2H,
phenylene ortho to bromine). Anal. Calcd for C24H12Br2N4O4:
C, 49.68; H, 2.08; N, 9.66. Found: C, 49.34; H, 2.13; N, 9.75.
Copolymer P2. This copolymer was synthesized in 78% yield
by reacting 7 with 1,4-divinyl-2,5-bis(hexyloxy)benzene in DMF
in the presence of Pd(OAc)2, P(o-tolyl)3 and triethylamine
according to the procedure described for copolymer 3. FT-IR
(KBr, cm-1): 3060, 1594, 1492, 1453 (aromatic); 2952, 2926
(C-H stretching of hexyloxy chains); 2162 (cyano); 1522, 1346
(nitro); 1290, 1206 (ether bond); 963 (trans vinylene bond). 1H
NMR (CDCl3) ppm: 8.20 (m, 4H, phenylene ortho to nitro);
7.70 (s, 2H, cyanovinylene); 7.50 (m, 4H, phenylene meta to
nitro); 7.25 (s, 2H, phenylene ortho to cyanovinylene); 7.12 (s,
4H, vinylene); 6.80 (s, 2H, phenylene ortho to oxygen); 3.96 (m,
4H, OCH2(CH2)4CH3); 1.82 (m, 4H, OCH2CH2(CH2)3CH3);
1.35 (m, 12H, O(CH2)2(CH2)3CH3); 0.93 (t, 6H, O(CH2)5CH3).
Anal. Calcd for (C46H44N4O6)n: C, 73.78; H, 5.92; N, 7.48.
Found: C, 73.51; H, 6.07; N, 7.39.
Experimental Section
Reagents and Solvents. 4-Nitrobenzyl cyanide was synthe-
sized from the nitration of benzyl cyanide with concentrated
nitric and sulfuric acid.57 It was recrystallized from ethanol. 1,4-
Divinyl-2,5-bis(hexyloxy)benzene was prepared by Stille cou-
pling reaction58 of 1,4-dibromo-2,5-bis(hexyloxy)benzene with
tributylvinyltin.59 N,N-Dimethylformamide (DMF) and tetra-
hydrofuran (THF) were dried by distillation over CaH2.
Triethylamine was purified by distillation over KOH. All other
reagents and solvents were commercially purchased and were
used as supplied.
Preparation of Compounds and Copolymers. Copolymer 3. A
flask was charged with a mixture of 2 (0.2315 g, 0.537 mmol),
1,4-divinyl-2,5-bis(hexyloxy)benzene (0.1775 g, 0.537 mmol),
Pd(OAc)2 (0.0050 g, 0.022 mmol), P(o-tolyl)3 (0.0376 g, 0.124

5546 Macromolecules, Vol. 43, No. 13, 2010
Mikroyannidis et al.
Scheme 1. Synthesis of Copolymer P1
Characterization Methods. IR spectra were recorded on a
Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. H
(equivalent to AM1.5 irradiation) at the surface of the device.
The photoaction spectrum of the devices was measured using a
monochromator (Spex 500 M), and the resulting photocurrent
was measured with a Keithley electrometer (model 6514), which
is interfaced to the computer by LABVIEW software. We have
prepared separate hole and electron only devices having struc-
ture ITO/PEDOT:PSS/copolymer:PCBM/Au and Al/copoly-
mer:PCBM/Al, respectively, to measure the hole and electron
mobility in the BHJ active layer.
1
NMR (400 MHz) spectra were obtained using a Bruker spectro-
meter. Chemical shifts (δ values) are given in parts per million
with tetramethylsilane as an internal standard. UV-vis spectra
were recorded on a Beckman DU-640 spectrometer with spec-
trograde THF. Elemental analyses were carried out with a Carlo
Erba model EA1108 analyzer.
Cyclic voltammetry experiment was carried out on a poten-
tiostat/galvanostat in an electrolyte solution of 0.1 M tetrabu-
tylammonium hexafluorophosphate (Bu4NPF6) in dichlorome-
thane, using a three-electrode cell with a gold working electrode,
a platinum wire counter electrode, and Ag/Agþ (0.01 M) refe-
rence electrode. Copolymers to be measured were coated by
drop-casting on the gold working electrode from its THF
solution.
Results and Discussion
Synthesis and Characterization. Copolymer P1 was syn-
thesized by a four-step reaction sequence which is outlined in
Scheme 1. In particular, compounds 160 and 260 were synthe-
sized according to the literature. The Heck reaction of 2 with
1,4-divinyl-2,5-bis(hexyloxy)benzene gave copolymer 3. This
reaction took place in DMF utilizing Et3N as acid scavenger
and Pd(OAc)2 as catalyst. Finally, 3 reacted with 4-nitrobenzyl
cyanide in EtOH in the presence of NaOH to afford the target
copolymer P1.
X-ray diffraction experiments were performed on a Bruker
D8 advanced model diffractometer with Cu KR radiation (λ =
1.542 A) at a generator voltage 40 kV and current of 40 mA.
˚
Atomic force microscopy (AFM) studies were performed using
digital instrument nanoscope in trapping mode.
Device Fabrication and Characterization. The copolymer
photovoltaic (PV) devices were prepared in a glovebox on
indium tin oxide (ITO)-coated glass substrates. The substrates
were cleaned in an ultrasonic bath with water and acetone.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) was deposited by spin-coating at 3000 rpm for
60 s and baked for 30 min at 80 ꢀC, as hole transport layer. The
thickness of PEDOT:PSS layer is about 50 nm. In the glovebox,
a mixture of copolymer:PCBM (1:1 ratio) was dissolved in
THF and spin-coated on the top of ITO/PEDOT:PSS film at
2000 rpm for 60 s. The thickness of the active layer was 70-
80 nm. Finally, aluminum (Al) electrode was thermally evapo-
rated through a shadow mask under high vacuum. The active
area of the device is 5 mm2. The thermal annealing of the active
blend layers was carried out at 100 ꢀC for 2 min on the hot plate
before the deposition of the Al electrode.
Scheme 2 outlines the five-step reaction synthesis of
copolymer P2. Specifically, compounds 4,61 5,61 and 661 were
synthesized according to reported methods. The reaction of 6
with 4-nitrobenzyl cyanide gave 7. Finally, the latter reacted
with 1,4-divinyl-2,5-bis(hexyloxy)benzene to afford P2.
Both copolymers were soluble in common organic sol-
vents such as dichloromethane, chloroform, and THF due to
the hexyloxy side groups. P1 displayed higher solubility in
these solvents than P2. The noncoplanar configuration of
triphenylamine of P1 enhanced its solubility. In contrast, the
two cyanovinylene 4-nitrophenyls of P2 suppressed the
solubility of this copolymer. The crude products were pur-
ified by dissolution in THF and precipitation into methanol.
The copolymers were obtained in 74% and 78% yield and
had number-average molecular weight (Mn) of 13 500 and
11 200, by GPC, with a polydispersity of 2.3 and 2.1 for P1
and P2, respectively.
The J-V characteristics of the devices in the dark and under
illumination were measured by a semiconductor parameter
analyzer (Keithley 4200-SCS). A xenon light source (Oriel)
was used to give a simulated irradiance of 100 mW/cm2
The FT-IR spectra of copolymers showed common ab-
sorption bands which for P1 appeared at 3030, 1590, 1502,

Article
Macromolecules, Vol. 43, No. 13, 2010 5547
Scheme 2. Synthesis of Copolymer P2
1452 (aromatic); 2954, 2922 (C-H stretching of hexyloxy
chains); 2186 (cyano); 1530, 1346 (nitro); 1286, 1212 (ether
bond), and 962 cm-1 (trans vinylene bond). Figure 1 presents
following expressions:62
EHOMO ¼ -qðEox þ 4:71Þ eV
ELUMO ¼ -qðEred þ 4:71Þ eV
1
typical H NMR spectrum of P1. It displayed an upfield
signal at 8.22 ppm assigned to the phenylene “j” ortho to
nitro which is deshielded by the nitro group. The cyanovi-
nylene “h” resonated at 7.72 ppm, while the unsubstituted
vinylene “f” near 7.10 ppm. The phenylene “i” meta to nitro
gave peak at 7.47 ppm, the triphenylamine “f” and “g” at
7.10-7.34 ppm, and the phenylene “e” ortho to oxygen
at 6.74 ppm. Finally, the aliphatic “a”-“d” resonated at
the region of 0.92-3.96 ppm. The small signals between
5 and 6 ppm are assigned to the terminal vinyl groups
-CHdCH2.
The electrochemical data of both copolymers were obtained
from the oxidation and reduction voltammograms as shown
in Figure 3 and summarized in Table 1. The electrochemical
band gap (Eeglec) of both copolymers has also been estimated
from the difference between the HOMO and LUMO energy
levels and listed in Table 1. The Eeglec values of copolymers are
slightly higher than the corresponding optical band gaps,
which is probably due to the exciton binding energy of
copolymers.63 It has been generally accepted that an energy
difference of 0.3-0.4 eV between the LUMO levels of
acceptor and donor materials (i.e., LUMO-LUMO offset)
used in BHJ layer is necessary for efficient photoinduced
electron transfer from polymer to PCBM.64 Since the LUMO
energy level of PCBM has a value ranging between -4.0 and
-4.3 eV,64 the LUMO-LUMO offset between P1 or P2 and
PCBM is larger than 0.3 eV, and therefore it is expected that
excitons can be easily dissociated at the interface between P1 or
P2 and PCBM. Furthermore, the HOMO energy levels of P1
(-5.2 eV) and P2 (-5.3 eV) are well aligned with the HOMO
level of the PEDOT:PSS (-5.1 eV), indicating that holes can
be easily transported from the HOMO level of P1 and P2 to
PEDOT:PSS. Therefore, both copolymers can be used as
electron donors with PCBM as electron acceptor for BHJ
PV devices.
Electrical and Photovoltaic Properties of Pristine P1 and
P2. We have investigated the electrical and photovoltaic
properties of single-layer devices based on P1 or P2, before
using these materials as donor component for BHJ photo-
voltaic devices. Figure 4a,b shows the J-V characteristics of
the devices based on pristine P1 and P2 thin films sand-
wiched between ITO/PEDOT:PSS and Al electrodes, in dark
as well as under illumination intensity of 100 mW/cm2 at
room temperature. The J-V characteristics of both devices
in dark show a rectification effect when positive potential is
applied to ITO/PEDOT:PSS electrode with respect to the Al
electrode. Since the HOMO level of PEDOT:PSS is very
close to HOMO levels of both P1 and P2, this electrode
behaves as Ohmic contact for hole injection from the PED-
OT:PSS-coated ITO electrode into the HOMO of copoly-
mer. However, the LUMO level of both copolymers is very
Photophysical and Electrochemical Properties. Figure 2
presents the UV-vis absorption spectra of copolymers in
both dilute (10-5 M) THF solution and thin film. Table 1
summarizes the photophysical and electrochemical charac-
teristics of copolymers. The absorption curves were broad
and extended from 300 up to ∼750 nm in solution and
∼800 nm in thin film. P2 exhibited broader absorption than
P1 in both solution and thin film. The copolymers displayed
a short-wavelength absorption maximum (λa,max) at the
range of 350-400 nm. Besides, they displayed a long-wave-
length λa,max at 610-623 nm which is attributable to the
intramolecular charge transfer (ICT) between the electron-
donating dialkoxyphenylene and/or triphenylamine and the
electron-withdrawing cyanovinylene 4-nitrophenyl. The
three-dimensional configuration of triphenylamine of P1
may restrict the π-conjugation along the polymer backbone
as well as the ICT. In contrast, P2 is a fully conjugated
copolymer with two electron-withdrawing moieties per re-
peating unit which favor both π-conjugation and ICT. For
this reason, P2 showed lower optical band gap (Eogpt) than
P1. Specifically, the thin film absorption onset is located at
739 and 758 nm corresponding to Eogpt of 1.68 and 1.64 eV for
P1 and P2, respectively. These Eogpt values are in agreement
with those of other related materials containing cyanoviny-
lene 4-nitrophenyls which have been synthesized in our
laboratory.41,42
Electrochemical cyclic voltammetry is often used to mea-
sure the redox properties of the copolymers from which their
energy levels of HOMO and LUMO are estimated. The
HOMO and LUMO levels are estimated from the onset
) potentials from the
red
onset
oxidation (Eoonxset) and reduction (E

5548 Macromolecules, Vol. 43, No. 13, 2010
Mikroyannidis et al.
Table 1. Optical and Electrochemical Properties of Copolymers
copolymer
P1
P2
λa,maxa in solution (nm)
623
610
739
1.68
623
623
758
1.64
λa,maxa in thin film (nm)
thin film absorption onset (nm)
E ogpt b (eV)
ox
onset
red
onset
E
E
(V)
(V)
0.49
0.56
-1.26
-5.20
-3.45
1.75
-1.11
-5.30
-3.60
1.70
HOMO (eV)
LUMO (eV)
Egel c (eV)
a λa,max: the absorption maxima from the UV-vis spectra in THF
solution or in thin film. b E gopt: optical band gap determined from the
absorption onset in thin film. c E egl: electrochemical band gap determined
from cyclic voltammetry.
effect is due to the formation of the Schottky barrier at the
Al-copolymer layer interface.
The charge mobility of the conjugated materials used as
photoactive layer in organic PV device is also an important
factor, which influences the short circuit current (Jsc). The
hole mobilities of both copolymers were measured using the
space charge limited current (SCLC) method65 with the
device structure ITO/PEDOT:PSS/P1 or P2/Au. The J-V
Figure 1. 1H NMR spectrum of copolymer P1 in CDCl3 solution. The
solvent peak is denoted by an asterisk.
characteristics of the devices were plotted as ln[Jd3/(Vapp
-
Vbi)] vs [(Vapp -Vbi)/d]1/2 (where Vappl and Vbi is the applied
voltage and built-in potential, respectively) and are shown in
Figure 5. The hole mobilities of P1 and P2 calculated from
the intercepts of corresponding lines are 4.5 ꢀ 10-5 and 2.1 ꢀ
10-5 cm2/(V s) for P1 and P2, respectively. These results indi-
cate that both P1 and P2 behave as p-type organic semi-
conductor with high hole mobility and can be used as
electron donor for BHJ devices.
The PV parameters of the devices fabricated with P1 and
P2 were estimated from the J-V characteristics under illu-
mination, as shown in Figure 4b, and the PV parameters are
summarized in Table 2. It can be seen from this table that
both Jsc and PCE are higher for the device based on P1 as
compared to P2 under the same conditions. Since the absorp-
tion spectra and the band gap of both copolymers are almost
the same, the amount of light absorbed by both copolymers
will also be almost the same. This implies that the number of
excitons generated in the photoactive layer is the same.
Therefore, the higher hole mobility of P1 as compared to
P2, as described earlier, may be responsible for the higher Jsc
and PCE. The HOMO level of P1 is more aligned with the
HOMO level of PEDOT:PSS, indicating that holes can be
easily transported from P1 to PEDOT:PSS. However, holes
can be trapped at the interface between the P2 and PEDOT:
PSS due to the difference in the HOMO levels of P2 and
PEDOT:PSS, which causes the slight band bending at the
interface. This band bending is also responsible for the lower
values of Voc for the devices based on P2 due to the voltage
loss across the interface.
Bulk Heterojunction Devices Based on P1 and P2 with
PCBM. Before testing the P1 and P2 copolymers as electron
donor, with PCBM as acceptor for BHJ devices, photolu-
minescence (PL) measurements were performed to examine
the charge transfer efficiency from the copolymer to
PCBM. Figure 6 compares the PL spectra of P1, P1:PCBM,
and P2:PCBM composite in solid state. P1 shows strong PL
with emission maximum at 815 nm. The intensity of both
P1:PCBM and P2:PCBM is quenched by addition of
PCBM, indicating that a photoinduced charge transfer
takes place effectively from copolymer to PCBM. The
quenching in P1:PCBM is higher compared to P2:PCBM
which indicates that a more efficient charge transfer occurs
in former.
Figure 2. UV-vis absorption spectra of copolymers in THF solution
(top) and thin film (bottom).
far from the work function of Al (-4.2 eV) and forms the
Schottky barrier for electron injection from Al into the
LUMO level of copolymers. Consequently, the rectification

Article
Macromolecules, Vol. 43, No. 13, 2010 5549
Figure 3. Cyclic voltammogram of P1 and P2.
Figure 4. Current-voltage characteristics of the devices based on P1 and P2 in dark (a) and under illumination intensity of 100 mW/cm2 (b).
Table 2. Photovoltaic Parameters of the Devices Based on Pristine P1
and P2 Sandwiched between ITO/PEDOT:PSS and Al Electrodes
short-circuit open-circuit
current (Jsc)
voltage
(Voc) (V)
fill factor power conversion
(FF)
copolymer (mA/cm2)
efficiency (η) (%)
P1
P2
0.52
0.44
0.83
0.78
0.49
0.45
0.21
0.15
Figure 5. Current-voltage characteristics of ITO/PEDOT:PSS/P1 or
P2/Au devices, plotted in the form of ln[Jd3/ (Vapp - Vbi)2] vs [(Vapp
Vbi)/d]0.5
-
.
In order to check the PV properties of copolymers P1 and
P2, we fabricated polymer solar cells with the architecture
ITO/PEDOT:PSS/P1 or P2:PCBM/Al, where both P1 and
P2 were used as electron donor and PCBM as electron
acceptor. The active layer constituted of a copolymer:PCBM
blend at various weight ratios, and it was deposited by spin-
coating on an ITO glass substrate covered by a PEDOT:PSS
layer. We found that the optimum copolymer:PCBM weight
ratio was around 1:1.
Figure 6. Photoluminescence spectra of P1, P1:PCBM, and P2:PCBM
thin films.
The J-V characteristics of the PV devices in dark and
under illumination of AM 1.5 (100 mW/cm2) are shown in
Figure 7. Table 3 lists the PV parameters obtained from the
J-V characteristics for the best performing devices. The
devices fabricated from both copolymers clearly show a

5550 Macromolecules, Vol. 43, No. 13, 2010
Mikroyannidis et al.
Figure 7. Current-voltage characteristics of the devices based on P1:PCBM (a) and P2:PCBM (b) in dark and under illumination intensity of
100 mW/cm2 of the as-cast and thermally annealed layer.
Table 3. Photovoltaic Parameters of the Devices Based on P1:PCBM and P2:PCBM Blend Sandwiched between ITO/PEDOT:PSS and
Al Electrodes
short-circuit current
(Jsc) (mA/cm2)
open-circuit voltage
(Voc) (V)
power conversion
efficiency (η) (%)
copolymer
fill factor (FF)
P1:PCBM (as-cast)
P2:PCBM (as-cast)
P1:PCBM (annealed)
P2:PCBM (annealed)
6.78
5.70
8.35
7.00
0.91
0.95
0.87
0.92
0.51
0.49
0.56
0.53
3.15
2.60
4.06
3.35
Figure 8. IPCE spectra of the bulk heterojunction photovoltaic devices based on P1:PCBM (a) and P2:PCBM (b) blends.
rectifying behavior in dark and exhibit significant PV effects
under illumination. Our measurements (Table 3) indicate
that these devices have a high Voc comparable to that
obtained from P3HT:PCBM device (Voc = 0.55-0.60).
The slight higher value of Voc for device with P2:PCBM as
compared to P1:PCBM is in agreement with the value of the
HOMO levels of P2 and P1 (Table 1) since the Voc is related
to the energy difference between the LUMO level of acceptor
(PCBM) and the HOMO level of the donor (copolymer).66 It
can also be seen that the Jsc and PCE of the device based on
P1 is higher than that to P2. Furthermore, the device based
on P1 shows an enhanced fill factor of 0.52. The improve-
ment in Jsc and PCE could be ascribed to a more efficient
charge separation and transport between P1 and PCBM.
Upon comparing, this result could be attributed to the higher
energy difference in the LUMO levels of P1 and PCBM as
compared to that between P2 and PCBM (Table 1).
where Jsc (mA/cm2) is the short-circuit photocurrent; Pin
(W/m2) and λ (nm) are the illumination intensity and wave-
length of the monochromatic light, respectively. The IPCE
spectra of the devices (Figure 8) show a peak which corre-
sponds to the absorption band of the respective polymers. The
similarity of the absorption spectrum with the IPCE response
demonstrates that the excitons produced by the absorption of
photonsin copolymersare dissociated into free chargecarriers
in the interfaces between P1 or P2 and PCBM in the active
layer and subsequently collected by the electrodes. The maxi-
mum values of IPCE for P1:PCBM and P2:PCBM based
devices are about 54% and 43%, respectively. The IPCE
values are consistent with the relatively higher values of Jsc
and PCE of the P1:PCBM-based BHJ PV device.
The photocurrent in BHJ PV device is determined by the
product of the absorbed photons within the solar spectrum
and incident quantum efficiency (IQE) of the device.67 As it
can be seen from Table 1 and Figure 2, the optical absorption
spectra and band gap of both copolymers are almost similar,
indicating that the number of the photons absorbed by
the active layer used in both devices is also almost the same.
Therefore, the difference in photocurrent in the present
The devices were also tested for their incident photon to
current efficiency (IPCE). The values of IPCE have been
estimated using the following expression:
IPCE ð%Þ ¼ 1240Jsc=λPin

Article
Macromolecules, Vol. 43, No. 13, 2010 5551
Figure 9. AFM images of P1:PCBM blend thin films as cast (a) and thermally annealed (b) films.
devices is attributed to the difference between the IQE of the
devices. The IQE is determined by three processes: (a)
migration and diffusion of photogenerated excitons toward
the D/A interface, (b) exciton dissociation and charge sepa-
ration at the interfaces, and (c) collection of charge carriers
by the collecting electrodes. The process “a” depends upon
the nanoscale phase separation between the donor and
acceptor materials used in the BHJ active layer. The AFM
images of the P1:PCBM and P2:PCBM are similar, which
indicates that both blends have the same nanoscale phase
separation. For process “b”, a sufficient large energy differ-
ence between the LUMO of donor and acceptor material is
required for ultrafast photoinduced charge transfer. As can
be seen from Table 1, this difference is higher for P1:PCBM
as compared to P2:PCBM, resulting in higher IQE for the
device based on the former blend. The process “c” depends
upon the percolated path of electrons and holes in the BHJ
active layer and also on the position of HOMO and LUMO
levels of donor and acceptor materials, respectively, relative
to the work function of anode and the cathode. The HOMO
level of P1 is closer to the HOMO level of PEDOT:PSS,
which results in a more efficient collection of holes. There-
fore, processes “b, c” are responsible for the higher value of
Jsc and PCE of the device fabricated with P1:PCBM blend.
The hole mobility for P1 is higher than that for P2 which
leads to the enhanced PCE for this device.
We have also investigated the effect of thermal annealing
on the PV response of the BHJ devices using both P1 and P2.
The J-V characteristics under illumination and IPCE spec-
tra of the devices are shown in Figures 7 and 8, respectively.
The overall PCE for the devices based on thermally annealed
P1 and P2 are 4.06% and 3.35%, respectively. The increase
in the IPCE and Jsc of the devices based on the thermally
annealed blends is attributed to the increase in the crystal-
linity of the blend and the hole mobility in the copolymer
phase upon thermal annealing. This can be ascribed to an
effective formation of phase-separated structure induced by
thermal annealing, which leads to a better connectivity of
each phase.
in copolymer are crucial for understanding the photogenera-
tion process in BHJ devices. We have also investigated the
effect of thermal annealing on the charge carrier mobility as
reported earlier in the literature for P3HT:PCBM blends. The
electron and hole mobilities in the BHJ thin film layer can be
determined from the J-V measurements using appropriate
electrodes which suppress the injection of either electrons or
holes, resulting in hole and electron only devices, respectively.
We have measured the dark J-V characteristics of P1:PCBM
blends for hole only and electron only devices using both as
cast and thermally annealed blends. We have used devices
havingITO/PEDPOT:PSS/P1:PCBM/Au and Al/P1:PCBM/
Al structure for the estimation of hole mobility and electron
mobility, respectively. The charge carrier mobilities of both P1:
PCBM and P2:PCBM blend films were evaluated by space
charge limited current (SCLC) measurements.65 The hole
mobilities for both the as cast P1:PCBM and P2:PCBM are
quite comparable with the value measured for pristine copo-
lymers. However, the values of hole mobilities are about 1.3 ꢀ
10-4 and 8.4 ꢀ 10-5 cm2/(V s) for the annealed P1:PCBM and
P2:PCBM, respectively. The electron mobility for the as-cast
and annealed blends is almost the same. The enhanced hole
mobility leads to a balanced charge transport in the BHJ PV
device.
We have also investigated the morphologies of P1:PCBM
and P2:PCBM blends to get information about the average
surface roughness for the film, using atomic force micro-
scopy (AFM). The AFM images for the as-cast and ther-
mally annealed P1:PCBM blends are shown in Figure 9.
Similar images have been also obtained for P2:PCBM. The
as-cast P1:PCBM blend forms smooth film with an average
surface roughness 1.2 nm. Upon thermal annealing of the P1:
PCBM film at 100 ꢀC, the average surface roughness in-
creases to ∼1.8 nm. It can be seen from these images that
thermal annealing results in significant changes in the surface
morphology. The surface domain size has also been in-
creased upon thermal annealing. This indicates that both
the increased domain size and average surface roughness
result in an improvement in the crystallinity of the blend,
which is responsible for the enhancement in the PCE due to
the more efficient photoinduced charge at the D/A inter-
faces.
Charge carrier mobilities in donor-acceptor BHJ thin
films play an important role on the performance of the
organic PV devices. After the photoinduced charge transfer
at the D-A interfaces presented in the bulk of the photo-
active layer of the devices, the electrons are localized in the
PCBM phase and holes remain in the copolymer phase, i.e.,
P1 or P2 in the present cases. Subsequently, the free electrons
and holes must be transported via percolated PCBM and
copolymer pathways toward Al and PEDOT:PSS/ITO elec-
trodes, respectively, in order to produce the photocurrent.
Therefore, the electron transport in PCBM and hole transport
Thin film XRD patterns were used to determine the
difference in the crystallinity of the P1:PCBM blended films
(as-cast and thermally annealed) and are shown in Figure 10.
Similar results have also been observed for P2:CBM. It can
be seen in Figure 10 that the as-cast film exhibits a peak
centered at 2θ = 7.8ꢀ, which corresponds to the interplanar
˚
distance of 7.8 A. The thermal annealing leads to higher peak
intensity, indicating a higher degree of crystallinity. Since

5552 Macromolecules, Vol. 43, No. 13, 2010
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