Improved power conversion efficiency of bulk-heterojunction organic solar cells using a benzothiadiazole-triphenylamine polymer

Article abstract of DOI:10.1039/c2jm14671a

A comparative study of the properties of bulk-heterojunction organic photovoltaic cells (OPVs) using a benzothiadiazole (BTD)-triphenylamine (TPA) small molecule and its polymerized molecule (poly(BTD-TPA)) is presented. OPVs using BTD-TPA or poly(BTD-TPA

Full text of DOI:10.1039/c2jm14671a

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PAPER
Improved power conversion efficiency of bulk-heterojunction organic solar
cells using a benzothiadiazole–triphenylamine polymer
a
Takeshi Yasuda, Yuki Shinohara, Takaaki Matsuda, Liyuan Han and Tsutomu Ishi-i
b
b
a
b
*
*
Received 20th September 2011, Accepted 25th November 2011
DOI: 10.1039/c2jm14671a
A comparative study of the properties of bulk-heterojunction organic photovoltaic cells (OPVs) using
a benzothiadiazole (BTD)–triphenylamine (TPA) small molecule and its polymerized molecule (poly
(BTD-TPA)) is presented. OPVs using BTD-TPA or poly(BTD-TPA):PC60BM at a 1 : 2 mixing weight
ratio were fabricated. The power conversion efficiency (PCE) of the poly(BTD-TPA)-based OPV was
twice that of the BTD–TPA-based OPV. The field-effect hole mobility of poly(BTD-TPA) is two orders
of magnitude higher than that of BTD–TPA and the absorption peak of poly(BTD-TPA) is at a longer
wavelength than that of BTD–TPA. Accordingly, the improved hole mobility and enhanced absorption
of AM1.5 solar-simulated light led to a high short-circuit current (Jsc) and PCE in OPVs based on poly
(BTD-TPA). Using OPVs with poly(BTD-TPA):PC70BM (1 : 4), the device performance exhibited a Jsc
value of 7.45 mA cmꢀ2, an open-circuit voltage of 0.92 V, a PCE of 2.65%, and incident photon to
current conversion efficiencies of around 50% at wavelengths ranging from 360 to 560 nm. The
experimental results for the OPVs with BTD–TPA-based materials indicate that the polymer is effective
for obtaining high-performance OPVs.
small molecules and polymers within the same p-conjugation
skeleton.13
1. Introduction
Solution-processable bulk-heterojunction (BHJ) organic photo-
voltaic cells (OPVs) offer a new source of electrical energy and
capitalize on advantages such as low fabrication cost and easy
processing on flexible substrates.1–3 To date, through the deve-
lopment of new organic semiconductors for BHJ OPVs, power-
conversion efficiencies (PCEs) greater than 7% for OPVs that
incorporate a low bandgap p-conjugated polymer have been
reported.4–8
Along these lines, we report a comparative study for the
properties of BHJ OPVs with benzothiadiazole (BTD)–triphe-
nylamine (TPA) containing either small molecules (BTD–TPA)
or the corresponding polymer (poly(BTD-TPA)). The chemical
structures of these materials are shown in Fig. 1. We chose BTD–
TPA-based materials as the donor in this study because TPA and
its derivatives have attracted attention as OPV materials due to
their excellent thermal and electrochemical stability, electron-
donating ability, and charge-transport properties.14–17 Moreover,
Recent years have seen the emergence of two approaches for
the development of materials for high-performance BHJ OPVs:
one approach uses small molecules (or oligomers)9,10 while the
other uses polymers as the donor materials.11,12 Small molecular
donors feature some specific advantages in terms of structural
definition, synthetic reproducibility, and ease of purification.
However, p-conjugated polymers possess other advantages in
terms of excellent flexibility and workability and long p-conju-
gation. While there remains uncertainty about which small
molecule and polymer materials would be the most appropriate
for BHJ OPVs, very few studies have compared BHJ OPVs using
aOrganic Thin-Film Solar Cells Group, Photovoltaic Materials Unit,
National Institute for Materials Science (NIMS), 1-2-1 Sengen,
Tsukuba, Ibaraki, 305-0047, Japan. E-mail: YASUDA.Takeshi@nims.
go.jp
bDepartment of Biochemistry and Applied Chemistry, Kurume National
College of Technology, 1-1-1 Komorino, Kurume, Fukuoka, 830-8555,
Japan. E-mail: ishi-i@kurume-nct.ac.jp
Fig. 1 Chemical structures of BTD–TPA and poly(BTD-TPA).
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materials with TPA moieties linked to acceptor moieties such as
BTD, which have a low optical band gap due to intramolecular
charge transfer between electron-donating TPA and electron-
accepting BTD, have seen remarkable progress in their synthesis
for OPVs.18–28
1311, 1282, 1072, 1007, 820; 1H NMR (CDCl3) d 0.89 (t, J ¼ 6.8
Hz, 3H, CH3), 1.22–1.38 (m, 10H, CH2), 1.60 (quint, J ¼ 7.8 Hz,
2H, CH2), 2.56 (t, J ¼ 7.8 Hz, 2H, CH2), 6.91 (d, J ¼ 8.8 Hz, 4H,
ArH), 6.97 (d, J ¼ 8.3 Hz, 2H, ArH), 7.08 (d, J ¼ 8.3 Hz, 2H,
ArH), 7.32 (d, J ¼ 8.8 Hz, 4H, ArH). FAB-MS (positive, NBA,)
m/z 513, 515, 517 (M+). HR-FAB-MS (positive, NBA) m/z
515.0645 (M+, calcd for C26H29Br2N 515.0648).
2. Experimental
2.1. Materials
4-Octyl-N,N-bis[4-(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)-
phenyl]aniline (4). A 1.65 M butyllithium hexane solution (5.75
mL, 9.5 mmol) was added dropwise to a solution of 3 (2.33 g,
4.52 mmol) in dry THF (22 mL) at ꢀ78 ꢁC over 5 min under an
The preparation of BTD–TPA was previously reported.29 4-
Octyl-N,N-diphenylaniline (2) and 4-octyl-N,N-bis(4-bromo-
phenyl)aniline (3) were prepared according to similar reported
syntheses.30 4-Octylaniline (1), 4,7-dibromo[2,1,3]benzothiadia-
zole (5), trimethylborate, neopentyl glycol, and Aliquat 336
(methyltrioctylammonium chloride) were purchased from Tokyo
Chemical Industry. Tri-tert-butylphosphine and butyllithium
hexane solution were purchased from Kanto Chemical. Poly(3,4-
ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS,
CLEVIOS P VP AI 4083) was purchased from HC Starck.
PC60BM and PC70BM (purity 99%) were purchased from
Solenne. Toluene and THF were distilled from sodium and
benzophenone under an argon atmosphere just before use.
Dimethylformamide (DMF) was dried overnight over 4A
molecular sieves before use.
ꢁ
argon atmosphere. After the mixture was stirred at ꢀ78 C for
1 h, trimethylborate (1.1 mL, 9.9 mmol) in dry THF (4 mL) was
ꢁ
added dropwise over 5 min. The mixture was stirred at ꢀ78 C
for 1 h and then warmed to room temperature. After the reaction
was quenched with aqueous 1.2 M hydrochloric acid solution (to
pH ¼ ꢂ5 to 7), the mixture was extracted with ethyl acetate. The
combined organic layers were washed with brine and water, dried
over anhydrous magnesium sulfate, and concentrated in vacuo.
Neopentyl glycol (940 mg, 9.0 mmol) was added to the concen-
trated solution, and the mixture was evaporated to dryness in
vacuo. The residue was purified by silica gel column chroma-
tography (KANTO 60N) with dichloromethane as the eluent to
give 4 in a 74% yield (943 mg, 1.62 mmol) as a white viscous solid:
mp 41–43 ꢁC; IR (KBr, cmꢀ1) 3033, 2958, 2927, 2854, 1594, 1507,
1315 (nB–O), 1306 (nB–O), 1284, 1135, 668, 649; 1H NMR (CDCl3)
d 0.89 (t, J ¼ 6.8 Hz, 3H, CH3), 1.02 (s, 12H, CH3), 1.22–1.37 (m,
10H, CH2), 1.61 (quint, J ¼ 7.8 Hz, 2H, CH2), 2.56 (t, J ¼ 7.8 Hz,
2H, CH2), 3.74 (s, 8H, CH2), 7.00 (d, J ¼ 8.3 Hz, 2H, ArH), 7.04
(d, J ¼ 8.3 Hz, 4H, ArH), 7.06 (d, J ¼ 8.3 Hz, 2H, ArH), 7.64 (d,
J ¼ 8.3 Hz, 4H, ArH); FAB-MS (positive, NBA) m/z 581 [(M +
1)+]. Anal. Calcd for C36H49B2NO4 (581.40): C, 74.37; H, 8.49;
N, 2.41. Found: C, 74.31; H, 8.44; N, 2.46%.
2.2. Synthesis
4-Octyl-N,N-diphenyl-aniline (2). A stock solution of tris
(dibenzylideneacetone)dipalladium (1.29 g, 1.25 mmol) in dry
toluene (20 mL) was added to a mixture of 4-octylaniline (1) (22.9
mL, 100 mmol), bromobenzene (23.2 mL, 220 mmol), and
sodium tert-butoxide (23.1 g, 240 mmol) in dry toluene (130 mL)
at room temperature under an argon atmosphere. The mixture
was then refluxed for 16 h. After being cooled to room temper-
ature, the mixture was washed with water and brine. The organic
layer was dried over anhydrous magnesium sulfate and evapo-
rated to dryness in vacuo. The residue was purified by silica gel
column chromatography (WAKO C300) using hexane as the
eluent to give 2 in a 95% yield (33.9 g, 94. 9 mmol) as a colourless
oil: IR (NaCl, cmꢀ1) 3061, 3026, 2954, 2925, 2854, 1589, 1508,
1494, 1326, 1312, 1278, 752, 695, 622; 1H NMR (CDCl3) d 0.89 (t,
J ¼ 7.3 Hz, 3H, CH3), 1.22–1.37 (m, 10H, CH2), 1.59 (quint, J ¼
7.8 Hz, 2H, CH2), 2.55 (t, J ¼ 7.8 Hz, 2H, ArCH2), 6.97 (t, J ¼
7.8 Hz, 2H, ArH), 7.00 (d, J ¼ 7.8 Hz, 4H, ArH), 7.06 (d, J ¼ 7.8
Hz, 6H, ArH), 7.22 (t, J ¼ 7.8 Hz, 4H, ArH); FAB-MS (positive,
NBA,) m/z 357 (M+). HR-FAB-MS (positive, NBA) m/z
357.2452 (M+, calcd for C26H31N 357.2457).
Preparation of poly(BTD-TPA). 4 (581 mg, 1.0 mmol),
potassium carbonate (2.21 g, 16.0 mmol), and Aliquat 336
(40 mg, 0.1 mmol) water solution (5 mL) were added to a mixture
of 5 (294 mg, 1.0 mmol) and tetrakis(triphenylphosphine)palla-
dium(0) (23 mg, 0.02 mmol) in deaerated toluene (10 mL) under
an argon atmosphere; the mixture was heated at 90 ꢁC for 17 h.
After reaction completion, bromobenzene (11 mL, 0.10 mmol)
was added to the reaction mixture. After one hour, phenyl-
boronic acid (12 mg, 0.10 mmol) was added and the reaction
mixture was heated at 90 ꢁC for 11 h to complete the end-capping
reaction. The reaction mixture was then poured into water and
extracted with dichloromethane. The organic layer was washed
with brine and water, dried over anhydrous magnesium sulfate,
and evaporated to dryness in vacuo. The residue was purified by
silica gel column chromatography (WAKO C300) with
dichloromethane as the eluent and GPC (JAIGEL-2H/3H) with
chloroform as the eluent to give poly(BTD-TPA) in a 53% yield
(259 mg, 0.529 mmol) as a dark red film: IR (KBr, cmꢀ1) 3031,
4-Octyl-N,N-bis(4-bromophenyl)aniline (3). NBS (8.90 g,
50.0 mmol) in dry DMF (25 mL) was added dropwise to a solu-
tion of 2 (8.94 g, 25.0 mmol) in dry DMF (100 mL) at 0 ꢁC over
8 min under an argon atmosphere. The mixture was stirred at
room temperature for 3 h then poured into water and extracted
with ethyl acetate. The organic layer was washed with water,
dried over anhydrous magnesium sulfate, and evaporated to
dryness in vacuo. The residue was purified by silica gel column
chromatography (WAKO C300) with hexane as the eluent to
give 3 in a 90% yield (14.31 g, 27.8 mmol) as a pale yellow oil: IR
(NaCl, cmꢀ1) 3060, 3029, 2953, 2925, 2853, 1578, 1508, 1485,
1
2952, 2921, 2850, 1597, 1507, 1474, 1318, 1280, 886, 822; H
NMR (CDCl3) d 0.90 (t, J ¼ 6.8 Hz, 3H, CH3), 1.21–1.42 (m,
10H, CH2), 1.64 (quint, J ¼ 7.6 Hz, 2H, CH2), 2.62 (t, J ¼ 7.6 Hz,
2H, CH2), 7.10–7.36 (m, 8H, ArH), 7.67–7.70 (m, capping-PhH),
7.73–7.81 (m, 2H, ArH), 7.85–7.90 (m, capping-PhH), 7.90–7.99
(m, 4H, ArH); 13C NMR (CDCl3) d 14.1, 22.7, 29.3, 29.4, 29.5,
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31.6, 31.9, 35.5, 122.4, 123.2, 123.3, 124.5, 125.3, 125.9, 126.9,
127.5, 129.3, 129.3, 129.5, 129.8, 129.8, 129.9, 130.0, 131.0, 131.2,
131.3, 132.2, 139.0, 144.6, 147.7, 147.8, 147.9, 154.2, 155.7; GPC
(CHCl3): Mn ¼ 4200 g molꢀ1, Mw ¼ 6200 g molꢀ1, Mw/Mn ¼ 1.5.
Anal. Calcd for C268H258N24S8 (n ¼ 8) (4067.87): C, 79.06; H,
6.39; N, 8.26. Found: C, 78.52; H, 6.59; N, 8.26%.
110 ꢁC for 10 min on a hot plate. Source drain electrodes of Au
(40 nm) for the BTD–TPA-based materials and Ag (40 nm) for
PC60BM were thermally evaporated through shadow masks. The
channel length and width were fixed at 75 mm and 5 mm,
respectively. The OFET measurement was conducted under
vacuum conditions using a Keithley 2636A System Source
Meter.
2.3. General measurements and characterization
2.5. Fabrication and characterization of OPVs
All melting points are uncorrected. The IR spectra were recorded
on a JASCO FT/IR-470 plus Fourier-transform infrared spec-
trometer and measured either as KBr pellets or on NaCl plates.
The OPVs were fabricated in the following configuration: ITO/
PEDOT:PSS/active layer/LiF/Al. The patterned ITO (conduc-
tivity: 10 U per square) glass was pre-cleaned in an ultrasonic
bath of acetone and ethanol and then treated in an ultraviolet-
ozone chamber. A thin layer (40 nm) of PEDOT:PSS was spin-
coated onto the ITO at 3000 rpm and dried at 110 ꢁC for 10 min
on a hot plate in air. The substrate was then transferred to an N2
glove box and dried again at 110 ꢁC for 10 min on a hot plate. An
o-dichlorobenzene solution of BTD–TPA or poly(BTD-TPA):
PCBM blend, which varied from 1 : 1 to 1 : 4 (weight ratio), was
subsequently spin-coated onto the PEDOT:PSS surface to form
1
The H NMR spectra were obtained in CDCl3 using a JEOL
JNM-LA 400 spectrometer. Residual solvent protons were used
as the internal standard and chemical shifts (d) are given relative
to tetramethylsilane (TMS). The coupling constants (J) are
reported in hertz (Hz). Elemental analysis was performed at the
Elemental Analytical Center, Kyushu University. Fast atom
bombardment mass spectra (FAB-MS) were recorded using
a JEOL JMS-70 mass spectrometer with m-nitrobenzyl alcohol
(NBA) as the matrix. Analytical TLC was carried out on silica
gel-coated aluminium foil (Merck 60 F254). Column chroma-
tography was carried out on silica gel (WAKO C300 or KANTO
60N). Gel permeation chromatography (GPC) was performed
using a Japan Analytical Industry Co., Ltd LC-918 with
a combination of JAIGEL-2H and JAIGEL-3H (20 ꢃ 600 mm)
ꢁ
the active layer. The resultant substrate was dried at 110 C for
10 min in an N2 glove box. Finally, LiF (1 nm) and Al (80 nm)
were deposited onto the active layer by conventional thermal
evaporation at a chamber pressure lower than 5 ꢃ 10ꢀ4 Pa, which
provided the devices with an active area of 2 ꢃ 2 mm2. The
thicknesses of the blend films and PEDOT:PSS layers were
measured using an automatic microfigure measuring instrument
(Surfcorder ET200, Kosaka Laboratory Ltd.). The current
density–voltage (J–V) curves were measured using an ADCMT
6244 DC Voltage Current Source/Monitor under an AM 1.5
solar-simulated light irradiation of 100 mW cmꢀ2 (Wacom
Electric Co., Ltd.). The incident photon to current conversion
efficiency (IPCE) was measured using a CEP-2000 system
(Bunkoh-Keiki Co., Ltd.).
columns and chloroform as the eluent at a rate of 3.0 mL minꢀ1
.
The molecular weight of poly(BTD-TPA) was determined via
gel permeation chromatography (GPC) on a TOSOH HLC-8020
system with two TSK-gel GMHxl-L (7.8 ꢃ 300 mm) columns
using an RI detector and chloroform (HPLC grade) as the eluent
at a rate of 1.0 mL minꢀ1. The number average molecular weight
(Mn), weight average molecular weight (Mw), and polydispersity
(Mw/Mn) were determined using a TOSOH Multi Station GPC-
8020 calibrated with a series of polystyrene standards. The
thermal stability was analyzed using a Mettler Toledo TGA 851
at a heating rate of 10 K minꢀ1 under a nitrogen atmosphere. The
UV-vis spectra in dichloromethane were measured on a JASCO
V-570 spectrophotometer in a 1.0 cm wide quartz cell at 1 ꢃ 10ꢀ5
M. The atomic force microscopy (AFM) measurement of the
surface morphology of samples was conducted on a Nanocute
(SII NanoTechnology Inc.) in tapping mode. The highest occu-
pied molecular orbital (HOMO) energy levels of the organic
semiconductors were estimated by photoelectron yield spec-
troscopy (PYS) using an AC-3 spectrometer (Riken Keiki).31
3. Results and discussion
3.1 Synthesis and characterization of poly(BTD-TPA)
4-Octyl-N,N-bis(4-bromophenyl)aniline (3) was prepared by the
bromination of 4-octyl-N,N-diphenylaniline (2), which was
obtained by a palladium(0)-catalyzed amination reaction of 4-
octylaniline (1) with aniline as displayed in Scheme 1.30 Dibro-
mide 3 was converted to the corresponding bisboronic acid by
lithiation followed by treatment with trimethylborate (Scheme
1). The resulting boronic acid was protected with neopentyl
glycol to generate the bisboronate (4) which was easily isolated
and purified. 4 was then polymerized via a palladium(0)-cata-
lyzed Suzuki coupling reaction with dibromo[2,1,3]benzothia-
diazole (5). After polymerization, an end-capping reaction was
carried out by treatment with bromobenzene and phenylboronic
acid. The obtained polymer, poly(BTD-TPA), is soluble in
common organic solvents including dichloromethane, chloro-
form, THF, and o-dichlorobenzene. TGA analysis of the poly-
mer indicates high thermal stability with a decomposition
temperature around 455 ꢁC (5% weight loss). The number
average molecular weight (Mn), weight average molecular weight
(Mw), and polydispersity (Mw/Mn) were estimated to be 4,200,
6,200, and 1.5, respectively. Although the Mn and Mw of poly
2.4. Fabrication and characterization of organic field-effect
transistors (OFETs)
To estimate the hole mobilities of BTD–TPA-based materials
and the electron mobility of PC60BM, we fabricated and char-
acterized OFETs with a top-contact geometry. A glass/Au gate
electrode/Parylene-C insulator substrate was prepared according
to previously reported methods.32 Parylene-C was deposited with
a thickness ranging from 1201 to 1261 nm. The dielectric
capacitance of the insulating layer per unit area ranged from 2.36
to 2.25 nF cmꢀ2. Organic semiconductors were then spin-coated
from an o-dichlorobenzene solution onto the Parylene-C layer.
The substrate was transferred to an N2 glovebox and dried at
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Scheme 1 Synthetic route to poly(BTD-TPA).
(BTD-TPA) are slightly low in the polymeric regime, our
previous study confirms that a strong Mw influence on perfor-
mances of OFETs is not observed when using triphenylamine
polymer (TSP-A02) with Mw ranging from 5500 to 9400.14
3.3. Field-effect hole mobilities of BTD–TPA and poly(BTD-
TPA)
The charge carrier mobility of organic semiconductors is an
important factor in the efficiency of OPVs.33–35 Increased carrier
mobility in BHJ OPVs would have a positive effect on carrier
extraction and charge collection efficiency, which is related to the
sum of the hole and electron drift lengths.33 However, high
carrier mobilities will also increase the bimolecular recombina-
tion strength.35 In consideration of the two competing processes,
i.e. carrier extraction and recombination, that are both governed
by carrier mobility, Mandoc et al. demonstrated that the effi-
ciency exhibits a distinct maximum as a function of the carrier
mobilities in BHJ OPVs of P3HT:PC60BM: the maximum PCE
occurs with electron mobilities ranging from 10ꢀ2 to 10ꢀ1 cm2 Vꢀ1
sꢀ1 (hole mobilities ranging from 10ꢀ3 to 10ꢀ2 cm2 Vꢀ1 sꢀ1) and
decreases at both lower and higher mobilities.34 Shieh et al. also
estimated that the highest efficiency in BHJ OPVs can be reached
if one kind of carrier mobility is fixed at 10ꢀ2 cm2 Vꢀ1 sꢀ1 and the
3.2. Thin film properties of BTD–TPA derivatives
The normalized optical absorption spectra of BTD–TPA and
poly(BTD-TPA) in the film state are shown in Fig. 2. The UV-
Vis absorption maxima of BTD–TPA and poly(BTD-TPA)
appeared at 477 and 494 nm in the film state and at 458 and
477 nm in dichloromethane solution, respectively. The absorp-
tion edge in the film state was at 543 and 561 nm, which corre-
sponds to a band gap of 2.28 eV for BTD–TPA and 2.21 eV for
poly(BTD-TPA). A comparison of spectra of the BTD–TPA and
poly(BTD-TPA) films reveals that the latter has a slightly red-
shifted absorption as a result of increased conjugation length.
The HOMO energy levels of BTD–TPA and poly(BTD-TPA)
were determined to be ꢀ5.56 and ꢀ5.47 eV, respectively. The
higher HOMO level of poly(BTD-TPA) implies a lower interface
barrier between PEDOT:PSS (ionization potential of ꢀ5.0 eV)
and poly(BTD-TPA) than between PEDOT:PSS and BTD–
TPA.
other is within a tolerance range of 10ꢀ1 to 10ꢀ3 cm2 Vꢀ1 sꢀ1 35
.
These results indicate that charge mobilities up to around 10ꢀ2
cm2 Vꢀ1 sꢀ1 are beneficial for the performance (especially, the
short-circuit current (Jsc) and fill-factor (FF)) of BHJ OPVs.
To estimate the charge carrier mobility of the materials used in
this study, we fabricated and characterized the OFETs. Although
the OFET measurement provides information on carrier mobili-
ties in the direction parallel to the substrate, the measurement is
helpful for estimating mobilities perpendicular to the substrate
and interpreting the behavior of OPVs, in the case of the amor-
phous films of BTD–TPA and poly(BTD-TPA) in which
isotropic carrier transports are realized. In fact, Veres et al.
reported the field-effect hole mobility of amorphous organic
semiconductors such as polytriarylamines (PTAA) was very
close to the bulk time-of-flight mobility.36
Fig. 3 shows the transfer characteristics (at a drain voltage of
ꢀ100 V) of the OFETs based on BTD–TPA or poly(BTD-TPA).
Above the threshold voltage, the field-effect hole mobility can be
calculated from the slope of the plot of the square root of the
drain current versus gate voltage. From the plots in Fig. 3, the
field-effect hole mobility was calculated to be 5.4 ꢃ 10ꢀ6 cm2 Vꢀ1
Fig. 2 Normalized optical absorption spectra of BTD–TPA and poly
(BTD-TPA) in film state.
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Fig. 3 Transfer characteristics (at a drain voltage of ꢀ100 V) of OFETs
based on BTD–TPA and poly(BTD-TPA).
sꢀ1 with a threshold voltage of ꢀ22 V and an on/off ratio of 1.2 ꢃ
102 for the BTD–TPA-based OFET. For the poly(BTD-TPA)-
based OFET, the field-effect hole mobility was calculated to be
3.1 ꢃ 10ꢀ4 cm2 Vꢀ1 sꢀ1 with a threshold voltage of ꢀ29 V and an
on/off ratio of 1.3 ꢃ 104. Therefore, the hole mobility of poly
(BTD-TPA) is about two orders of magnitude higher than that of
BTD–TPA. This is most likely due to the extended electron
delocalization in the polymer compared to in BTD–TPA.
3.4. Photovoltaic properties of BTD–TPA and poly(BTD-
TPA)
Fig. 4 (a) J–V curves of OPVs prepared with various photoactive layers
and (b) IPCE spectra of OPVs.
OPVs based on various blend ratios of BTD–TPA-based mate-
rials and PC60BM or PC70BM with thicknesses of about 200 nm
were prepared and characterized. The photovoltaic parameters
for ITO/PEDOT:PSS (40 nm)/active layer/LiF (1 nm)/Al (80 nm)
under illumination with 100 mW cmꢀ2 of AM 1.5 are
summarized in Table 1. To fairly compare BTD–TPA and poly
(BTD-TPA), we focused on OPVs using BTD–TPA or poly
(BTD-TPA):PC60BM at a 1 : 2 mixing ratio. Fig. 4(a) shows the
J–V curves under an AM 1.5 solar-simulated light irradiation of
100 mW cmꢀ2, and Fig. 4(b) shows the IPCE spectra of the
OPVs. The PCE of the poly(BTD-TPA)-based OPV with a blend
ratio of 1 : 2 was twice as good as that of the BTD–TPA-based
OPV with a blend ratio of 1 : 2. The IPCE was 16% at a wave-
length of 500 nm for the BTD–TPA-based OPV and 27% at
a wavelength of 520 nm for the poly(BTD-TPA)-based OPV. The
relatively high Jsc and PCE of the OPV based on poly(BTD-
TPA) are due to the higher hole mobility and enhanced
absorption of AM1.5 solar-simulated light of poly(BTD-TPA).
As shown in Table 1 and Fig. 4, as the PC60BM concentration
increased from a poly(BTD-TPA):PC60BM ratio of 1 : 2 to 1 : 4,
the PCEs of the OPVs based on poly(BTD-TPA) increased by
1.5. Although the electron mobility of the PC60BM-based
OFETs was calculated to be 6.6 ꢃ 10ꢀ3 cm2 Vꢀ1 sꢀ1 (with
a threshold voltage of 9 V and an on/off ratio of 3.6 ꢃ 103), which
is twenty times higher than the hole mobility of poly(BTD-TPA),
increasing the PC60BM concentration in the OPV films led to
improved Jsc and PCEs. This result suggests the absence of
sufficient percolation paths for electrons to the cathode in the
blend film with a low PC60BM concentration, which is due to the
good miscibility of poly(BTD-TPA) and PC60BM. The AFM
images in Fig. 5 show that all surfaces of the active layers (poly
(BTD-TPA):PC60BM ¼ 1 : 2 and 1 : 4), which consist of the
domains with below 20 nm in diameter, are very similar which
indicates good miscibility in these films.
Table 1 Photovoltaic parameters of ITO/PEDOT:PSS (40 nm)/various
active layers/LiF (1 nm)/Al (80 nm) under illumination by 100 mW cmꢀ2
of AM 1.5
Finally, to obtain the best performance of OPVs using poly
(BTD-TPA) as a donor, PC70BM was chosen as the acceptor
because it has stronger absorption in the visible region from 400
to 600 nm than PC60BM.37 Fig. 4(a) shows the J–V curves of
OPVs using poly(BTD-TPA):PC70BM (1 : 4) with a thickness of
233 nm while Fig. 4(b) shows the IPCE spectra of the OPV. The
device exhibited a Jsc of 7.45 mA cmꢀ2, an open-circuit voltage
(Voc) of 0.92 V, an FF of 0.39, a PCE of 2.65%, and IPCEs of
around 50% at wavelengths ranging from 360 to 560 nm.
Blend
ratio by Thickness/ Jsc/mA Voc
/
PCE
FF (%)
Active layer
weight nm
cmꢀ2
V
BTD–TPA:PC60BM
BTD–TPA:PC60BM
1 : 1
1 : 2
203
205
230
259
249
233
0.43
1.53
2.64
3.46
3.96
7.45
0.88 0.29 0.11
0.87 0.32 0.43
0.93 0.36 0.88
0.90 0.37 1.15
0.90 0.38 1.34
0.92 0.39 2.65
Poly(BTD-TPA):PC60BM 1 : 2
Poly(BTD-TPA):PC60BM 1 : 3
Poly(BTD-TPA):PC60BM 1 : 4
Poly(BTD-TPA):PC70BM 1 : 4
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 2539–2544 | 2543

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4. Conclusion
We investigated the performances of BHJ OPVs incorporating
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hole mobility of poly(BTD-TPA) is two orders of magnitude
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2544 | J. Mater. Chem., 2012, 22, 2539–2544
This journal is ª The Royal Society of Chemistry 2012