Novel conjugated alternating copolymer based on 2,7-carbazole and 2,1,3-benzoselenadiazole

Article abstract of DOI:10.1016/j.polymer.2010.04.057

A novel conjugated alternating copolymer (PCzDBSe) based on N-9'-heptadecanyl-2,7-carbazole and 5,5-(4',7'-di-2-thienyl-2',1',3'-benzoselenadiazole) was synthesized by Suzuki polycondensation. The polymer reveals excellent thermal stabilities with the decomposition temperature (5% weight loss) of 390°C and the glass-transition temperature of 140°C. The absorption peaks of the polymer are located at 412 and 626nm, respectively, while the absorption onset is extended to 716nm, which is 56nm red-shifted as compared with its analogue, poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT). The HOMO and LUMO levels of the polymer were estimated to be -5.28 and -3.55eV, respectively, with an optical bandgap of 1.73eV. The hole mobility of PCzDBSe as deduced from a solution-processed organic field effect transistor (OFET) was found to be 3.9×10^-4cm²V^-1s^-1. Polymer solar cells (PSCs) based on the blends of PCzDBSe and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) with a weight ratio of 1:4 were fabricated. Under AM 1.5 (AM, air mass), 100mWcm^-2 illumination, the devices were found to have an open-circuit (V_oc) of 0.75V, a short-circuit current density (J_sc) of 7.23mAcm^-2, a fill factor (FF) of 45% and a power conversion efficiency (PCE) of 2.58%. The primary results indicate that 5,5-(4',7'-di-2-thienyl-2',1',3'-benzoselenadiazole) is a promising unit for low bandgap polymer for polymer solar cells.

Full text of DOI:10.1016/j.polymer.2010.04.057

Polymer 51 (2010) 3196e3202
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Novel conjugated alternating copolymer based on 2,7-carbazole
and 2,1,3-benzoselenadiazole
*
*
Wei Zhao, Wanzhu Cai, Ruixia Xu, Wei Yang , Xiong Gong, Hongbin Wu , Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Specially Functional Materials, Ministry of Education,
South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
A novel conjugated alternating copolymer (PCzDBSe) based on N-90-heptadecanyl-2,7-carbazole and 5,5-
(40,70-di-2-thienyl-20,10,30-benzoselenadiazole) was synthesized by Suzuki polycondensation. The poly-
mer reveals excellent thermal stabilities with the decomposition temperature (5% weight loss) of 390 ꢀC
and the glass-transition temperature of 140 ꢀC. The absorption peaks of the polymer are located at 412
and 626 nm, respectively, while the absorption onset is extended to 716 nm, which is 56 nm red-shifted
as compared with its analogue, poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-
benzothiadiazole)] (PCDTBT). The HOMO and LUMO levels of the polymer were estimated to be ꢁ5.28
and ꢁ3.55 eV, respectively, with an optical bandgap of 1.73 eV. The hole mobility of PCzDBSe as deduced
Article history:
Received 14 January 2010
Received in revised form
18 April 2010
Accepted 26 April 2010
Available online 20 May 2010
Keywords:
2,7-Carbazole
Benzoselenadiazole
Alternating copolymer
from a solution-processed organic field effect transistor (OFET) was found to be 3.9 ꢂ 10ꢁ4 cm2
V .
ꢁ1 sꢁ1
Polymer solar cells (PSCs) based on the blends of PCzDBSe and [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM) with a weight ratio of 1:4 were fabricated. Under AM 1.5 (AM, air mass), 100 mW cmꢁ2 illu-
mination, the devices were found to have an open-circuit (Voc) of 0.75 V, a short-circuit current density
(Jsc) of 7.23 mA cmꢁ2, a fill factor (FF) of 45% and a power conversion efficiency (PCE) of 2.58%. The
primary results indicate that 5,5-(40,70-di-2-thienyl-20,10,30-benzoselenadiazole) is a promising unit for
low bandgap polymer for polymer solar cells.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
bandgap to enable photocurrent generation from lower energy
photons. It had been demonstrated that the incorporation of electron
Since the discovery of the ultrafast photo-induced charge
transfer between conjugated polymers and fullerene, polymer solar
cells (PSCs) have attracted much attention as potential large-area,
flexible, and low-cost solar cells [1]. Many conjugated polymers
such as poly(p-phenylenevinylene)s (PPV) [2], poly(fluorene)s
(PFO) [3], poly(3-alkylthiophene) [4], and polythiophene deriva-
tives [5] were extensively studied. As a result of the development of
new materials and the optimization of processing techniques, so
far, the power conversion efficiency of the device based on these
conjugated polymers and fullerene derivatives had exceeded 6.0%
[4e7].
donating and accepting units along the polymer backbone can lead
to intramolecular charge transfer thus resulting in lower bandgap
polymer [8]. As compared with many other conjugated polymer
donors, poly(2,7-carbazole) derivatives exhibit some unique merits
such as high hole mobility, low energy lying highest occupied
molecular orbital (HOMO), and therefore can result in high air
stability and high Voc [9e12]. Furthermore, the absorption spectra
could be easily tuned for optimal solar spectral match. In the past
several years, some of the 2,7-carbazole based polymers were
synthesized as donor materials for highly efficient polymer solar
cells [13], especially a low bandgap polymer, poly[N-90-heptade-
canyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadia-
zole)] (PCDTBT) with an absorption spectrum onset extended to
660 nm was reported [6,11,12,14]. Inspired by the purpose of har-
vesting the lower energy photons in the solar spectra, there is a need
to further lower the bandgap of the 2,7-carbazole based polymer. As
compared to 2,1,3-benzothiadiazole (BT), an electron deficient unit,
which was widely used as the backbone in low bandgap polymers
[2,3,5,15], its analogue 2,1,3-benzoselenadiazole (BSe) is more
effective in extending the absorption spectrum towards the infrared
In order to further enhance the PCE of the polymer based solar
cells, an effective method is to develop donor polymers with low
* Corresponding authors. Institute of Polymer Optoelectronic Materials and
Devices, South China University of Technology, Guangzhou 510640, PR China.
Tel.: þ86 20 87114346; fax: þ86 20 87110606.
E-mail addresses: pswyang@scut.edu.cn (W. Yang), hbwu@scut.edu.cn (H. Wu).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.04.057

W. Zhao et al. / Polymer 51 (2010) 3196e3202
3197
region. For example, Hou et al. demonstrated that as a result of
incorporating the BSe unit in benzo[1,2-b:4,5-b]dithiophene (BDT)
based polymers, the bandgaps can be lowered from 1.70 eV (BDT-BT)
to 1.52 eV (BDT-BSe) [16].
In this paper, 2,7-carbazole-alt-DBSe copolymer (PCzDBSe) with
an extended absorption spectrum was synthesized. The photo-
physical properties of the DBSe based polymer and its photovoltaic
properties to act as an electron donor to PCBM were investigated
and discussed.
dry toluene at room temperature for 24 h. After the OTS SAM
formation, the substrates were rinsed with toluene, and dried in
a vacuum oven. Films of PCzDBSe were deposited on pre-treated
SiO2/Si substrates by spin-coating a 10 mg mLꢁ1 solution of the
polymer in chlorobenzene, followed by annealing under nitrogen
to remove the residual solvent. The source and drain electrodes
were defined by thermally evaporating gold (70 nm) through
a shadow mask on top of the organic thin film forming top contact
geometry transistors. Channel length and width of the obtained
OFET were 0.1 and 10 mm, respectively. The OFET was character-
ized under ambient conditions by a semiconductor parameter
analyzer (model: 4155B).
2. Experimental section
2.1. Materials
2.4. Fabrication and characterization of polymer solar cells
All the starting materials were purchased from Aldrich or Alfa
Aesar and used without further purification. All the reactions were
carried out under argon at 1 atmosphere unless mentioned other-
wise. THF and CH3CN were distilled from sodium and CaH2 before
use, respectively.
Patterned indium tin oxide (ITO) coated glass with a sheet
resistance of 15e20 ohm/square was cleaned by a surfactant scrub,
then underwent a wet-cleaning process inside an ultrasonic bath,
beginning with de-ionized water, followed by acetone and iso-
propanol. After oxygen plasma cleaning for 5 min, a 40 nm-thick
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS) (Bayer Baytron 4083) anode buffer layer was spin-cast on the
ITO substrate and then dried by baking in a vacuum oven at 80 ꢀC
overnight. The active layer, with thickness in the 60e80 nm range,
was then deposited on top of the PEDOT:PSS layer, by casting from
a chlorobenzene solution (20 mg mLꢁ1) containing PCzDBSe:PCBM
(1:x, w/w). The thickness of the PEDOT:PSS and active layer was
verified by a surface profilometer (Tencor, Alpha-500). Finally,
100 nm aluminum layer was thermally evaporated with a shadow
mask at a base pressure of 3 ꢂ 10ꢁ4 Pa. The overlapping area
between the cathode and anode defined a pixel size of 0.15 cm2.
The thickness of the evaporated cathodes was monitored by
a quartz crystal thickness/ratio monitor (model: STM-100/MF,
Sycon). Except for the deposition of the PEDOT:PSS layers, all the
fabrication processes were carried out inside a controlled atmo-
sphere of nitrogen dry-box (Vacuum Atmosphere Co.) containing
less than 10 ppm oxygen and moisture. The power conversion
efficiency of the resulting polymer solar cells was measured under
1 sun AM 1.5G (air mass 1.5 global) spectrum from a solar simulator
(Oriel model 91192) (1000 W mꢁ2). The current densityevoltage
(JeV) characteristics were recorded with a Keithley 2410 source
unit. The spectral response of the devices was measured with
a commercial EQE/incident photon to charge carrier efficiency
(IPCE) measurement setup (Oriel). A calibrated silicon detector was
used to determine the absolute photosensitivity at different
wavelengths.
2.2. Characterization
1H and 13C NMR spectra were recorded by Bruker DRX 300 and
Varian INOVA 500NB spectrometers operating at 300 MHz and
125 MHz respectively, in CDCl3 at 298 K. Chemical shifts were
reported as
d values (ppm) relative to an internal tetramethylsilane
(TMS) standard. The FTIR spectrum was recorded with a Bruker
Vector 33 FTIR spectrometer. Elemental analysis was performed on
a ELEMENTAR Vario ELIII Elemental Analyzer. Number-average (Mn)
and weight-average (Mw) molecular weights were determined by
GPC, using polystyrene as standards. Thermogravimetric analysis
(TGA) was performed on Netzsch TG 209 in nitrogen, with a heating
rate of 20 ꢀC minꢁ1. Differential scanning calorimetry (DSC) analysis
was performꢁe1d on Netzsch DSC 204 in nitrogen, with a heating rate
of 20 ꢀC min
.
Cyclic voltammograms (CV) were recorded on a CHI 660A
electrochemical workstation. conventional three-electrode
A
system consisting of a platinum electrode as working electrode, an
Ag/AgCl reference electrode and a platinum wire as counter elec-
trode was used. The polymer film was deposited on the working
electrode of platinum electrode by solvent casting. The electrode
was polished by using 0.3 mm Al2O3 slurry followed by rinsing with
de-ionized water and acetonitrile, dried over 105 ꢀC before use. A
0.1 M solution of tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in acetonitrile was used as the supporting electrolyte.
Argon was bubbled through the solution for 10 min to remove
the dissolved oxygen and also over the solution throughout the
experiment.
2.5. Synthetic procedures
Photoluminescence spectra (PL) of the polymer were recorded
with a CCD spectrophotometer (Instaspec IV, Oriel Co.) under
excitation of a 325 nm HeCd laser.
2,7-Bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolan-20-yl)-N-900-
heptadecanylcarbazole [14,17] and 4,7-bis(5-bromo-2-thienyl)-
2,1,3-benzoselenadiazole [18] were prepared according to the
literature procedure.
2.3. Fabrication and characterization of organic field effect
transistor
Synthesis of poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-
(40,70-di-2-thienyl-20,10,30-benzoselenadiazole)](PCzDBSe).
In a 50 mL flask, 328.5 mg (0.5 mmol) of 2,7-bis(40,40,50,50-tet-
ramethyl-10,30,20-dioxaborolan-20-yl)-N-900-heptadecanylcarbazole,
253.6 mg (0.5 mmol) of 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzo-
selenadiazole, 3 mg (0.003 mmol) of Pd(PPh3)4, were dissolved in
15 mL of toluene and 0.6 mL of 20% aqueous tetraethylammonium
hydroxide. The reaction mixture was refluxed with vigorous stir-
Heavily p-doped Si wafers with 600 nm thermal SiO2 grown,
were used both as the substrate and the gate electrode. The
substrates were cleaned by subsequent soaking in acetone, iso-
propanol and de-ionized water in ultrasonic bath for 10 min, and
dried with nitrogen. Then, the substrates were treated with
oxygen plasma for 5 min to remove any residual organic materials
and to create a high density of silanol groups at the surface. To
improve the chemical and electrical properties of the gate
dielectric, the SiO2/Si substrates were modified with a self-
assembled monolayer (SAM) octyltrichlorosilane (OTS) solution in
ring for 48 h under argon atmosphere. Bromobenzene (5 mL,
0.05 mmol) was added to the reaction mixture, 4 h later, phenyl-
boronic acid (6.1 mg, 0.05 mmol) was added to the reaction
mixture, and then the reaction was continued for 4 h. After cooling

3198
W. Zhao et al. / Polymer 51 (2010) 3196e3202
to room temperature, the mixture was poured into 500 mL meth-
anol, and filtered. The precipitate was washed in a Soxhlet appa-
ratus with hexane, acetone, toluene, dichloromethane and
chloroform for 24 h, respectively. The chloroform fraction was
condensed to 10 mL, and precipitated in methanol and finally dried
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes of monomers and copolymer are outlined in
Scheme 1. 2,7-Dibromocarbazole (4) was synthesized according to
the literature [14,17] under PPh3-mediated reductive cyclization.
Alkylation of (4) with 9-heptadecane-p-toluenesulfonate afforded
N-90-heptadecanyl-2,7-dibromocarbazole (5) [14]. The Y-shaped
long alkyl chain in the carbazole unit makes the polymer with high
solubility and relative high molecular weight. 2,7-Bis(40,40,50,50-tet-
ramethyl-10,30,20-dioxaborolan-20-yl)-N-900-heptadecanylcarba-
zole (6) was synthesized through a double lithiation of (5) with LiBu
at ꢁ78 ꢀC, and subsequent quenching with isopropoxy boronic
ester. 4,7-Dibromo-2,1,3-benzothiadiazole (8) was reduced by
NaBH4, and subsequently oxidated by SeO2 to give 4,7-dibromo-
overnight (150 mg, yield: 40%). 1H NMR (300 MHz, CDCl3)
d (ppm):
8.10 (br, 4H); 7.88 (br, 4H); 7.60 (br, 2H); 7.51 (br, 2H); 4.70 (br, 1H);
2.38 (br, 2H); 2.00 (br, 2H); 1.20 (m, 24H); 0.80 (t, 6H). 13C NMR
(125 MHz, CDCl3)
d (ppm): 158.26, 157.93, 147.26, 143.11, 141.04,
139.88, 138.93, 131.93, 130.27, 128.77, 137.33, 126.62, 126.28, 125.52,
125.35, 123.93, 123.25, 122.16, 120.80, 119.50, 117.63, 117.22, 115.36,
115.03, 108.75, 106.06, 56.61, 33.89, 31.77, 29.70, 29.48, 29.35, 29.22,
26.87, 24.96, 22.59, 14.04; FTIR (KBr disk cmꢁ1): 3028, 1430, 761,
722, 686, 2919, 2850. Anal. calcd. (%) for C43H47N3S2Se: C, 68.96; H,
6.33; N, 5.61; S, 8.56; Se,10.54. Found: C, 68.69; H, 6.38; N, 5.64; S,
8.64.
Scheme 1. Synthetic route of monomers and the polymer.

W. Zhao et al. / Polymer 51 (2010) 3196e3202
3199
Fig. 1. TGA and DSC plots of PCzDBSe in nitrogen.
Fig. 2. UVevis absorption spectra of PCzDBSe in chloroform solution (a) and in thin
film (b).
2,1,3-benzoselenadiazole (10), which was converted to 4,7-bis(2-
thienyl)-2,1,3-benzoselenadiazole (11) by Stille coupling reaction
with tri-N-butylstannyl-2-thiophene (7) in the presence of
PdCl2(PPh3)2, and subsequently brominated by N-bromosuccini-
mide (NBS) to give 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzoselena-
diazole (12). The alternating copolymer was synthesized via
a Suzuki polycondensation of (6) and (12).
The weight-average molecular weight (Mw) and number-
average molecular weight (Mn) are 17,000 and 14,000 respectively,
with a polydispersity (PDI) of 1.21. The copolymer is soluble in
common organic solvents, such as chloroform, THF and
chlorobenzene.
The thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) plots are shown in Fig. 1 and the thermal
analytical data are summarized in Table 1. The polymer reveals
excellent thermal stability with a high decomposed temperature
(corresponding to 5% weight loss) at 390 ꢀC, while the glass-tran-
sition temperature is determined at 140 ꢀC.
To get an insight into the charge transfer process in the
donoreacceptor blend films, the photoluminescence (PL) of blends
along with that of the pure polymer has been investigated. As
shown in Fig. 3, the PL of the polymer in film locates in the near-
infrared region with a maximum emission at 724 nm, and is thor-
oughly quenched by the addition of 6,6-phenyl-C61-butyric acid
methyl ester (PC61BM) or 6,6-phenyl-C71-butyric acid methyl ester
(PC71BM), indicating a highly efficient exciton dissociation at the
interface between the polymer and PCBM [1]. Therefore efficient
PSCs can be constructed based on the PCzDBSe:PCBM blend.
3.3. Electrochemistry
As shown in Fig. 4, the cyclic voltammogram of PCzDBSe pres-
1=2
ents one oxidation process ðE
¼ 1:03 VÞ and one reduction
ox
1=2
re
process ðE
¼ ꢁ1:17 VÞ: Based on the recorded oxidation
potential, PCzDBSe should show good air stability [21]. From the
onset oxidation potential (Eox ¼ 0.88 V) and the onset reduction
potential (Ere ¼ ꢁ0.85 V) of the polymer, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) levels were calculated to be ꢁ5.28 and ꢁ3.55 eV,
respectively (Table 2). The HOMO of PCzDBSe is 0.22 eV higher than
that of PCDTBT. The corresponding electrochemical bandgap is
1.73 eV, which is similar to the optical bandgap within the exper-
imental error. Given these electrochemical data and the observed
PL quenching as shown above in the PCzDBSe:PCBM blend, and
taking into account that the LUMO energy of PCBM is ꢁ4.3 eV, one
can assume that the PCzDBSe can potentially lead to PSCs with PCE
over 6% under ideal conditions [22].
3.2. Photophysical properties
Fig. 2 shows the UVevis absorption spectra of PCzDBSe. The
polymer presents two absorption bands both in chloroform solu-
tion and in thin film, which is in accordance with other D-A
copolymers [13,14]. The absorption peak at long wavelength could
be attributed to the
the absorption peak at short wavelength is possibly the result of the
p* transition of carbazole units [15,19]. We note that the
pe
p* transition of the low bandgap units, while
pe
absorption peak of the film is red-shifted 16 nm as compared with
that in solution, as a result of the intermolecular interaction in solid
state [20]. The optical bandgap of the polymer is 1.73 eV, deduced
from the absorption onset of the film at 716 nm. The optical
properties of PCzDBSe were summarized in Table 2. The absorption
of PCzDBSe has a red-shift of 56 nm compared with that of its BT
analogue (PCDTBT) [6,14].
Table 2
Absorption and electrochemical properties of the copolymer
Table 1
Polymer In
CHCl3
In film
p-Doping
n-Doping
EgEC
(eV)
Molecular weight and thermal properties of PCzDBSe
Polymer
Mn
Mw
PDI
Tg (ꢀC)
Td (ꢀC)a
lmax lmax lonset Egopt Eox,onset HOMO Ere,onset LUMO
(nm) (nm) (nm) (eV) (V) (eV) (V) (eV)
PCzDBSe 610 626 716 1.73 0.88
ꢁ5.28 ꢁ1.17 ꢁ3.55 1.73
PCzDBSe
14,000
17,000
1.21
140
390
a
Temperature of 5% weight loss.

3200
W. Zhao et al. / Polymer 51 (2010) 3196e3202
Fig. 3. PL spectra of PCzDBSe (a), PCzDBSe:PC61BM (1:4, w/w) (b), and PCzDBSe:
PC71BM (1:4, w/w) (c) blends excited at 325 nm by a HeCd laser.
3.4. Field effect transistor performances
Top contact OFETof PCzDBSewasfabricated onpre-treated SiO2/Si
substrates as described in Section 2.3. Fig. 5 shows the output at
different gate voltages and the transfer characteristics for OFET using
spin-coating PCzDBSe on OTS-treated SiO2/Si substrates. The copoly
mer was found to exhibit typical p-type organic semiconductor
characteristics, and the hole mobility of PCzDBSe was calculated in
the saturated regime at VDS ¼ ꢁ30 V to be 3.9 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1
with a current on/off ratio of 1.4 ꢂ102. The results are consistent with
the reported data [11,12], in which the field effect hole mobility of
a
series of polycarbazole derivatives on OTS-treated SiO2/Si
substrates was found to between 2 ꢂ 10ꢁ5 cm2 Vꢁ1 sꢁ1 and
1 ꢂ10ꢁ3 cm2 Vꢁ1 sꢁ1. The high hole mobility of the PCzDBSe is
expected to facilitate charge transport and reduce recombination loss
in the PSCs; therefore it can lead to an improved device performance.
Fig. 5. The output at different gate voltages (VG) (a) and transfer characteristics in the
saturation regime at constant sourceedrain voltage (VDS ¼ ꢁ30 V) (b) for OFETs using
spin-coated PCzDBSe on OTS-treated SiO2/Si substrates.
3.5. Solar cell performances
PSCs with a device structure of ITO/PEDOT:PSS/PCzDBSe:PCBM/
Al were fabricated following the processes described in Section 2.3
and the performances of a series of devices with different donor/
acceptor blending ratios by weight are summarized in Table 3 for
comparison. On increasing the weight ratio of polymer:PC61BM
from 1:1 to 1:4, the PCE increases from 0.39% to 1.46%. This
phenomenon is quite common in bulk heterojunction PSCs [23]. A
Table 3
Photovoltaic performances of the PSC devices
Polymer:PCBM (wt/wt)
Jsc (mA cmꢁ2
)
Voc (V)
FF (%)
PCE (%)
1:1a
1:2
1:3
1.63
2.17
3.35
4.15
7.23
0.75
0.8
0.8
0.8
0.75
32
38
42
44
45
0.39
0.66
1.13
1.46
2.58
1:4
1:4b
a
Fig. 4. Cyclic voltammogram of PCzDBSe in acetonitrile solution of 0.1 mol Lꢁ1 [Bu4N]
PF6.
PC61BM.
PC71BM.
b

W. Zhao et al. / Polymer 51 (2010) 3196e3202
3201
Fig. 8. Photoresponse of PSCs based on PCzDBSe:PC61BM (1:4, wt/wt).
Fig. 6. IeV curves of PSCs in the dark (a) and under illumination of AM 1.5G,
100 mW cmꢁ2 based on PCzDBSe:PC61BM (b), and PCzDBSe:PC71BM (c) blends.
up to 700 nm is clearly displayed. Note that the EQE curve of the
devices has two features e a peak around 400 nm and another at
600 nm, closely following the trend observed in the absorption
spectra of the PCzDBSe (Fig. 2), indicating that the harvested photons
over the entire absorption spectra contribute to the photocurrent.
The device exhibits a maximal EQE of 58% at 390 nm and an EQE of
40% at 600 nm. Optimization of PSCs based on PCzDBSe is on the way
and it is expected that PCE can be further improved under optimized
fabrication conditions (film morphology, thickness of active layer,
electrode buffer layer etc.).
typical JeV characteristic of the devices under AM 1.5G illumination
(100 mW cmꢁ2) and in the dark is shown in Fig. 6. A Jsc of
4.15 mA cmꢁ2 and a fill factor of 44% are obtained under simulated
solar spectra while the Voc of the device is found to be 0.80 V,
slightly smaller than that of its BT analogue (PCDTBT, 0.89 V) [14].
The slightly lower Voc in our PCzDBSe devices is probably due to its
higher HOMO energy level since it had been suggested that Voc
directly correlates with the difference between the LUMO of the
acceptor and the HOMO of the donor. When PC61BM was replaced
by PC71BM and used as an electron acceptor in the devices,
a significantly improved Jsc of 7.23 mA cmꢁ2 (Fig. 6) was recorded,
due to its unsymmetrical structure with significantly stronger
absorption in the visible region (see Fig. 7) [24]. As a result, the
overall performance of the device is improved, leading to a PCE of
2.58%.
4. Conclusion
In conclusion,
a novel conjugated alternating copolymer
(PCzDBSe) based on N-90-heptadecanyl-2,7-carbazole and 5,5-(40,70-
di-2-thienyl-20,10,30-benzoselenadiazole) was synthesized by Suzuki
polycondensation. Compared with its analogue based on 2,1,3-ben-
zothiadiazole (BT) unit, the absorption onset of the PCzDBSe is red-
shifted by 56 nm, extended to 716 nm. The deduced optical bandgap
of PCzDBSe is 1.73 eV, which is 0.14 eV narrower than that of PCDTBT.
The hole mobility of PCzDBSe as deduced from a solution-processed
OFET was found to be 3.9 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1. Voc, Jsc and FF of the
devices fabricated from the PCzDBSe are 0.75 V, 7.23 mA cmꢁ2, and
45%, respectively, leading to a power conversion efficiency (PCE) of
2.58%.
The external quantum efficiency (EQE) plot of the devices under
short-circuit conditions is shown in Fig. 8, in which photoresponse
Acknowledgements
The authors are deeply grateful to the National Nature Science
Foundation of China (nos. 60906032, 50990065, and U0634003) and
the State Key Basic Research Project of China (nos. 2009CB623602
and 2009CB930604) for the financial support.
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