An A-D-A small molecule based on the 3,6-dithienylcarbazole electron donor (D) unit and nitrophenyl acrylonitrile electron acceptor (A) units for solution processed organic solar cells

Article abstract of DOI:10.1039/c2ta00749e

A new A-D-A small molecule (SM) based on a 3,6-dithienylcarbazole donor unit at the center and nitrophenyl acrylonitrile acceptor units as the terminal ends was synthesized by the Pd catalyzed Stille coupling reaction for application as a donor material in the organic bulk heterojunction solar cells. The SM possesses a low lying HOMO energy level at -5.34 eV and optical band gap of 1.74 eV. The photovoltaic performance of the SM was investigated with PC ₆₀BM and PC₇₀BM as acceptors cast from THF with 3% CN additive and without additive. The power conversion efficiency (PCE) of the organic solar cells based on SM:PC₆₀BM cast from THF solvent reached 1.94% with a high V_oc of 0.94 V, a J_sc of 6.08 mA cm ^-2 and FF of 0.34, where as the PCE of SM:PC₇₀BM cast from THF solvent was 2.65% with a V_oc of 0.96 V, a J_sc of 7.26 mA cm^-2 and FF of 0.38%, under the illumination intensity of 100 mW cm^-2. The higher PCE of the organic solar cells based on PC ₇₀BM as an electron acceptor has been attributed to its strong absorption in the visible region than PC₆₀BM. The PCE of the organic solar cells has been further improved to 3.76% and 4.96% for SM:PC ₆₀BM and SM:PC₇₀BM, respectively, cast with CN-THF solvent. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ta00749e

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Materials Chemistry A
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An A–D–A small molecule based on the 3,6-
dithienylcarbazole electron donor (D) unit and
nitrophenyl acrylonitrile electron acceptor (A) units for
solution processed organic solar cells
Cite this: DOI: 10.1039/c2ta00749e
a
a
b
c
Manjeet Singh, Rajnish Kurchania,* J. A. Mikroyannidis, S. S. Sharma
d
and G. D. Sharma*
A new A–D–A small molecule (SM) based on a 3,6-dithienylcarbazole donor unit at the center and
nitrophenyl acrylonitrile acceptor units as the terminal ends was synthesized by the Pd catalyzed Stille
coupling reaction for application as a donor material in the organic bulk heterojunction solar cells. The
SM possesses a low lying HOMO energy level at ꢀ5.34 eV and optical band gap of 1.74 eV. The
photovoltaic performance of the SM was investigated with PC60BM and PC70BM as acceptors cast from
THF with 3% CN additive and without additive. The power conversion efficiency (PCE) of the organic
solar cells based on SM:PC60BM cast from THF solvent reached 1.94% with a high Voc of 0.94 V, a Jsc of
ꢀ
2
6
.08 mA cm and FF of 0.34, where as the PCE of SM:PC70BM cast from THF solvent was 2.65% with a
ꢀ2
ꢀ2
V
oc of 0.96 V, a Jsc of 7.26 mA cm and FF of 0.38%, under the illumination intensity of 100 mW cm
.
Received 18th October 2012
Accepted 10th December 2012
The higher PCE of the organic solar cells based on PC70BM as an electron acceptor has been attributed
to its strong absorption in the visible region than PC60BM. The PCE of the organic solar cells has been
further improved to 3.76% and 4.96% for SM:PC60BM and SM:PC70BM, respectively, cast with CN–THF
solvent.
DOI: 10.1039/c2ta00749e
www.rsc.org/MaterialsA
further increasing the PCE over 15% is needed for OSCs to
compete with silicon solar cells and other thin lm solar cell
Introduction
Solution processed organic solar cells (OSCs) are a promising
cost effective alternative for utility of solar energy converters,
technologies for commercialization. The thermodynamic effi-
ciency limit of OSC is 22–27%. Most of these efficiencies have
4
and possess the advantages of being light weight, low cost and been reported in OSCs fabricated with BHJ active layers
1

exible. In recent years, efforts have been intensively focused composed of low band gap conjugated polymers and [6,6]-
5
on improving device performance towards achievement of the phenyl-C61 (or 71)-butyric acid methyl ester (C (or 71)-PCBM).
61
power conversion efficiency (PCE) of 10%. Research has mainly Generally, polymers have many advantages such as strong
been concerned with further development of active materials absorption ability, admirable solution processability, good lm
such as conjugated polymers and fullerene derivatives or
forming ability and tunable energy levels. However, the polymer
donor materials always suffer from batch to batch variation,
difficulty of purication and broad molecular weight distribu-
functional layers such as buffering, charge transporting and
2
optical spacing layers. Recently the encouraging PCEs in the
range of 7–10% have been achieved from bulk heterojunction tions. The impurity and relatively high molecular weight dis-
BHJ) OSCs based on conjugated polymer donors and fullerene persity signicantly decreases the charge carrier mobility of
(
acceptors and has rendered OSCs strong candidates for next polymers and further lead to a relatively low ll factor and PCE
6
generation solar cells over the inorganic thin lm solar cells and of the photovoltaic devices.
dye sensitized solar cells. Despite this signicant progress,
3
To overcome these issues, solution processed small mole-
cules (SMs) for BHJ OSCs, combining the advantages of purity,
Department of Physics, Maulana Azad National Institute of Technology (MANIT), well dened structure and molecular weight, no batch to batch
a
Bhopal 462051, MP, India
variations and easily tunable band gap and energy levels have
stimulated more and more attention. Moreover, the intra-
molecular associations, which can assist with charge transport,
are also typically stronger in SMs than in polymeric materials.
b
Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-
7
2
6500 Patras, Greece
c
Department of Engineering Physics, Government Engineering College for Women,
7
Ajmer, Rajasthan, India
d
So far, signicant progress has been made and PCEs of 4–7%
have been achieved for OSCs with SM as the donor. This PCE is
R & D Center for Engineering and Science, JEC Group of colleges, 303101, JEC campus,
8
Kukas, Jaipur, Rajasthan, India. E-mail: sharmagd_in@yahoo.com
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always below that of their polymer counterpart. In most cases, remove the catalyst. It was poured into a mixture of methanol
devices based on solution processed small molecules, always (100 mL) and 2N hydrochloric acid (10 mL) and stirred for 1 h.
seem to have poorer lm quality than their polymer counter- The precipitate was ltered. Compound 2 was puried by silica
8
parts, in the BHJ active layer. Thus it is expected that a better gel column chromatography eluting with toluene and precipi-
PCE can be achieved if the intrinsic poor lm quality and tated in methanol (0.32 g, 73%).
1
morphology in the BHJ active layer could be improved.
3
H NMR (CDCl ) ppm: 8.08–8.03 (m, 3H, carbazole at 4,5,9
However, in order to achieve this, careful molecular design has positions); 7.42 (m, 4H, carbazole at 1,2,7,8 positions); 7.33 (m,
to be carried out to address many factors simultaneously, 2H, thiophene at 5 position); 7.11 (m, 4H, thiophene at 3,4
including the material solar light absorption associated with its positions).
highest occupied molecular orbital (HOMO) and lowest unoc-
Anal. Calcd for C H NS : C, 72.47; H, 3.95; N, 4.23. Found:
20 13 2
cupied molecular orbital (LUMO) energy levels, mobility, lm C, 71.95; H, 3.76; N, 4.32%.
forming quality and morphology compatibility with acceptors.
Push–pull chromophores involving electron donating (D) and Compound 3
electron accepting (A) groups have been widely investigated for
A ask was charged with a solution of 2 (0.20 g, 0.556 mmol) in
formic acid (5 mL). The mixture was stirred and reuxed for
use in OSCs. This type of molecular structure can lower the
material band gap and extend the absorption spectrum towards
longer wavelengths, and the same time the HOMO and LUMO
15 h. Water was subsequently added to the mixture and the
precipitate was ltered off and washed with water. The crude
9
energy levels can be tuned effectively. Continuing our research
product was recrystallized from acetonitrile (0.13 g, 67%).
work on small molecule for BHJ organic solar cells, herein we
have synthesized a small molecule end capped with 2-(4-nitro-
phenyl) acrylonitrile as an electron acceptor and central 3,6-
dithienyl carbozole as an electron donor unit and used it as
electron donor along with different fullerene derivatives as an
electron acceptor for the fabrication of BHJ organic solar cells.
ThePCE of the devicesbased onSM:PC60BMand SM:PC70BMcast
from THF solvent were 1.94% and 2.65%, respectively. In addi-
tion, we have investigated the effect of the solvent additive used
for spin casting of SM:PC BM and SM:PC BM blend lm and
1
3
H NMR (CDCl ) ppm: 9.65 (s, 1H, formyl); 8.08 (m, 2H,
carbazole at 4,5 positions); 7.42 (m, 4H, carbazole at 1,2,7,8
positions); 7.33 (m, 2H, thiophene at 5 position); 7.11 (m, 4H,
thiophene at 3,4 positions).
Anal. Calcd for C H NOS : C, 70.17; H, 3.65; N, 3.90.
2
1
13
2
Found: C, 69.85; H, 3.88; N, 3.64%.
Compound 4
A ask was charged with a solution of 3 (0.47 g, 1.32 mmol) in
6
0
70
1,2-dichloroethane (20 mL). DMF (0.32 g, 4.36 mmol) and POCl
3
found that the device based on solvent additive has increased the
PCE signicantly and yielded a PCE of 3.76% and 4.96%,
respectively for the CN-additive processed BHJ active layers.
(0.67 g, 4.36 mmol) were added dropwise and the mixture was
reuxed for 5 h. Aer cooling to room temperature and addition
of dichloromethane (15 mL) and a saturated aqueous solution
3
of CH COONa (30 mL), the mixture was stirred for 2 h at room
Experimental
Reagents and solvents
temperature. The organic phase was then washed with water
and dried over Na SO . Solvent removal and column chroma-
2
4
tography (silica gel/dichloromethane) gave 4 (0.34 g, 62%).
2
-Tributylstannylthiophene was prepared by the reaction of
1
H NMR (CDCl
thiophene); 9.65 (s, 1H, formyl at position 9 of carbazole); 8.08
m, 2H, carbazole at 4,5 positions); 7.42 (m, 4H, carbazole at
1,2,7,8 positions); 7.11 (m, 4H, thiophene at 3,4 positions).
Anal. Calcd for C23 : C, 66.49; H, 3.15; N, 3.37.
3
) ppm: 10.02 (s, 2H, formyl at position 5 of
thiophene with n-BuLi in hexane and subsequently with tribu-
tylchlorostannane according to the literature. 4-Nitrobenzyl
cyanide was synthesized by the nitration of benzyl cyanide with
concentrated nitric and sulfuric acid. It was recrystallized
from ethanol. N,N-Dimethylformamide (DMF) and tetrahydro-
10
(
11
H
3 2
13NO S
Found: C, 66.05; H, 3.26; N, 3.21%.
2
furan (THF) were dried by distillation over CaH .
Small molecule SM
Compound 1
A ask was charged with a solution of 4 (0.55 g, 1.32 mmol) and
This compound was synthesized by bromination of carbazole by
4-nitrobenzyl cyanide (0.64 g, 3.96 mmol) in DMF (15 mL).
means of N-bromosuccinimide (NBS) in N,N-dimethylforma-
NaOH (0.20 g, 5.00 mmol) dissolved in ethanol (5 mL) was
added to this solution. The reaction mixture was stirred for 1 h
12
mide (DMF) according to a reported method.
at room temperature under N
under reduced pressure. Water was added to the concentrate
2
and then was concentrated
Compound 2
A ask was charged with a solution of 1 (0.43 g, 1.323 mmol) and and SM precipitated as a green solid. It was puried by column
-tributylstannylthiophene (1.1850 g, 3.175 mmol) in freshly chromatography (silica gel/THF) (0.65 g, 58%).
2
1
dried THF (30 mL). The ask was degassed and then nitrogen
3
H NMR (CDCl ) ppm: 8.22 (m, 6H, phenylene ortho to nitro);
was bubbled through the ask for 15 min. The palladium 8.08 (m, 2H, carbazole at 4,5 positions); 7.72 (s, 3H, cyanovi-
catalyst PdCl (PPh ) (0.0557 g, 0.07936 mmol) was added to the nylene); 7.47 (m, 6H, phenylene meta to nitro); 7.42 (m, 4H,
2
3 2
mixture and the reaction was maintained at reux for 48 h carbazole at 1,2,7,8 positions); 7.11 (m, 4H, thiophene at 3,4
under nitrogen. The mixture was subsequently ltered to positions).
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Anal. Calcd for C47
Journal of Materials Chemistry A
H
25
N
7
O
6
S
2
: C, 66.58; H, 2.97; N, 11.56. measured with a computer controlled Keithley electrometer
Found: C, 65.83; H, 3.18; N, 11.12%.
under short circuit conditions. The intensity of each wavelength
was kept constant.
To measure the electron mobility in the BHJ active layer, this
layer was sandwiched between the top and bottom Al electrodes
acting as the injecting electrode and the electron collecting
electrode, respectively. For hole mobility measurement, the BHJ
layer was sandwiched between PEDOT:PSS coated ITO as the
hole injecting electrode and Au as the hole collecting electrode.
The mobilities were extracted by tting the J–V characteristics in
space charge limited region.
Characterization methods
1
H NMR (400 MHz) spectra were obtained using a Bruker
spectrometer. Chemical shis (d values) are given in parts per
million with tetramethylsilane as an internal standard.
The electrochemical properties i.e. onset oxidation potential
and reduction potentials of SM, were examined by cyclic vol-
tammetry. We used an electrochemical cell with a conventional
three electrode assembly and tetrabutylammonium hexa-

4 6
uorophosphate (Bu NPF ) (0.1 M) acting as the electrolyte in
anhydrous acetonitrile solution. SM was deposited on the glassy Results and discussion
carbon electrode which was used as the working electrode,
while Pt wire and an Ag/AgCl electrode were used as the counter
Synthesis of SM
Scheme 1 outlines the ve-step synthesis of SM. In particular,
electrode, respectively. The cyclic voltammetry measurement at
ꢀ
1
carbazole was brominated by N-bromosuccinimide (NBS) in
DMF to afford 3,6-dibromocarbazole (1). The Stille coupling of 1
with 2-tributylstannylthiophene in the presence of a palladium
catalyst gave 2. The latter reacted with formic acid to afford
aldehyde 3. Compound 3 was further formylated by means of
room temperature was performed at a 100 mV s scan rate. The
optical absorption spectra of SM in solution and thin lms, and
blend in thin lms were measured using a Perkin Elmer UV-
visible spectrophotometer.
3
DMF and POCl to give trialdehyde 4. The condensation of 4
with 4-nitrobenzyl cyanide in the presence of NaOH afforded
Device fabrication and characterization
The organic solar cells were prepared on a commercial indium
tin oxide (ITO) coated glass substrates with a sandwiched
structure: glass/ITO/PEDOT:PSS/SM:PC60BM or PC70BM/Al.
Prior to use, the patterned ITO coated glass substrates were
cleaned with deionised water, acetone and isopropyl alcohol,
the target SM. This small molecule and all the intermediate
compounds were characterized by H NMR spectroscopy.
1
Optical and electrochemical properties of SM
using ultrasonication and then dried in an ambient atmo- Fig. 1 shows the optical absorption spectra of the SM in THF
sphere. thin layer (about 60 nm) of poly(3,4-ethyl- solution and thin lm form. The absorption spectra of SM
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) showed two absorption bands in the wavelength region 400–
Bayon, Germany) was spin coated onto the ITO glass and baked 800 nm. The absorption band observed in the longer wave-
A
(
ꢁ
at 100 C for 10 min. Blends of SM and fullerene derivates in length region results from the intramolecular charge transfer
different weight ratios from 1 : 05 to 1 : 4 w/w) were dissolved (ICT) between the central di-thienyl carbazole donor unit and
ꢀ
1
for 6 h in THF (concentration 10 mg mL ) solution and the terminal acceptor groups present in the SM, originating
subsequently spin coated at 1500 rpm for 60 s on to the from HOMO / LUMO monoexcitations. The absorption band
PEDOT:PSS layer of ITO coated glass. The deposition of all observed at the 400–500 nm could be assigned to p–p*
organic layers was carried out in ambient atmosphere. The transitions.
thickness of the photoactive layer is about 80–85 nm.
The absorption spectrum of SM in the lm state (also shown
Aluminium (Al) was then evaporated onto the surface of the in Fig. 1) shows an obvious broadening bathochromic shi of 42
ꢀ
5
photoactive layer under a vacuum of 10 Torr. The active area nm in the longer wavelength band compared to the solution
2
of the devices is about 5 mm .
spectra. Such a spectral change is typically attributed to plana-
To prepare the lm from the additive solvent, a blend solu- rization of the conjugated backbone and stronger p–p interac-
13
tion was prepared by dissolving SM and the fullerene derivative tions in the solid state. The solid state absorption of conjugated
in a host solvent, i.e. THF, and then small amounts of pro- materials is primarily determined by conformation and molec-
14
cessing additives were incorporated and the solution was stirred ular packing. The thin lm absorption spectra of SM also
ꢁ
on a hot plate at 60 C for 4 h. The blend solution was cooled to showed a well dened vibronic shoulder around 656 nm, indi-
room temperature prior to spin coating deposition.
The current–voltage characteristics of the devices was in the solid state. This is similar to what is seen in regioregular
measured with a computer controlled Keithley 238 source meter semiconducting polymers such as P3HT and some conjugated
cating an effective p–p packing between the molecule backbones
15
16
15
unit. A halogen lamp coupled with AM1.5 solar spectrum lter small molecules, and is indicate of effective p-stacking and
17
was used as the light source and the optical power at the surface strong intermolecular interaction in the lm. The optical band
ꢀ
2
of the device was 100 mW cm . All the measurements were gap of SM estimated from the onset of the absorption spectra in
carried out under ambient conditions.
the thin lm was found to be 1.74 eV.
The incident photon to current efficiency (IPCE) of the
The electrochemical properties of SM and its HOMO and
devices was measured using a monochromator and 100 W LUMO levels were estimated from the cyclic voltammetry (CV)
onset
halogen lamp as the light source and the resulting current was and we have observed that the onset reduction potential (Ered
)
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Scheme 1 Synthesis of small molecule SM.
exciton i.e. 0.3–0.4 eV) to guarantee photoinduced electron
transfer between them. The difference between the HOMO of
donor and LUMO of the acceptor is as large as 1.44 eV, which
could lead to high open circuit voltage for BHJ solar cells using
SM as the donor and fullerene as the acceptor, since the Voc is
mainly determined by this difference.
DFT computation
We have performed theoretical calculations to understand the
electronic structure of SM and to elucidate the origin of possible
21
vertical transitions, by using the Gaussian 09W (Revision-A.02)
program suite. The geometries of the dye in a vacuum were
22
optimized using density functional theory (DFT) with the
23
hybrid B3LYP density functional and the 6-31G* basis set.
Optimized geometry was subjected to vibrational analysis, and
characterized as a minimum due to the absence of imaginary
frequencies. The time-dependent DFT (TD-DFT) calculations
were carried out at the same theory level to determine at least 10
vertical excitations to the excited state of SM.
Fig. 1 Absorption spectra of SM in THF solution and in thin film form cast from
THF solution.
24
onset
and the onset oxidation potential (Eoxd ) were ꢀ1.49 V and 0.63
The computed vertical excitation and frontier orbital ener-
gies, oscillator strengths for electronic excitations and
compositions of vertical transitions in terms of molecular
orbitals are listed in Table 1. The electronic distributions
observed for selected molecular orbitals contributing signi-
cantly to the prominent electronic excitations in the SM are
presented in Fig. 2. They mainly originate due to the electronic
excitations HOMO to LUMO, HOMO ꢀ 1 to LUMO and HOMO
of the SM are ꢀ 1 to LUMO + 1. Since in the HOMO and HOMO ꢀ 1 of the
V (vs. NHE). The HOMO (EHOMO) and LUMO (ELUMO) energy
elec
g
levels and electrochemical bandgap (E
from the following expressions
) of SM were estimated
18
onset
oxd
onset
red
E
HOMO ¼ ꢀqðE
ꢀ 4:7Þ eV
ꢀ 4:7Þ eV
ELUMO ¼ ꢀqðE
elec
Eg ¼ EHOMO ꢀ ELUMO
elec
g
The values of EHOMO and ELUMO as well as E
ꢀ
5.34 eV, ꢀ3.2 eV, and 2.14 eV, respectively, calculated from dye the electronic distribution occurs over the electron
above expressions. The bandgap estimated CV is higher than donating 3,6-dithienylcarbazole unit and the electron delocal-
that obtained from the absorption spectrum in thin lm, which ization in LUMO and LUMO + 1 is spread over the electron-
is commonly observed in most of the conjugated polymers and accepting nitrophenyl acrylonitrile units. The HOMO and
19
small molecules. The LUMO level of the SM is higher than that LUMO values estimated are also shown in Table 1 and these
20
of PC60BM or PC70BM (ꢀ3.9 eV); the LUMO gap between the values are in good agreement with the experimentally esti-
donor and acceptor is enough larger than the binding energy of mated from CV measurement.
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Table 1 Computed oscillator strength, assignment and orbital energies for SM
f
Assignment
HOMO, eV
LUMO, eV
Band gap, eV
2.24
0
0
0
1
.967
.483
.189
.207
HOMO / LUMO (98%)
ꢀ5.48
ꢀ3.24
HOMO / LUMO + 1 (97%)
HOMO ꢀ 1 / LUMO (96%)
HOMO ꢀ 1 / LUMO + 1 (76%)
HOMO / LUMO + 2 (19%)
HOMO ꢀ 1 / LUMO + s2 (96%)
0
.124
Photovoltaic properties
based on SM:PCBM with a weight ratio 1 : 1 are shown in Fig. 4
and the corresponding photovoltaic parameters are summa-
rized in Table 2. The device with SM:PC BM yielded a V of
To probe the photoinduced charge transfer from the donor SM
to the acceptor, the photoluminescence (PL) spectra of the SM
and its blends with fullerene derivatives were measured and
shown in Fig. 3. The pristine SM lm exhibited an emission
band with an emission maximum at 830 nm. For the SM:ful-
lerene blend lms, emission intensity exhibited signicant
quenching (about 90%). This indicates a photo induced charge
transfer from the SM donor to the fullerene acceptors, which is
highly favorable for competing with radiative recombination of
the excitons. The emission intensity for the SM:PC70BM is
slightly more quenched, indicating that the photo induced
charge transfer in this BHJ active layer is more efficient.
6
0
oc
ꢀ2
0.94 V, a Jsc of 6.08 mA cm and a FF of 0.34, resulting in a PCE
of 1.94%. In contrast, the device with SM:PC70BM with same
weight ratio shows a higher photovoltaic performance with a Voc
ꢀ
2
of 0.96 V, a Jsc of 7.26 mA cm and a FF of 0.38, yielding an
overall PCE of 2.65%. The increase in the PCE for the device
based on PC70BM as the electron acceptor is mainly due to the
enhancement of the Jsc and FF, attributed the stronger
absorption of PC BM than PC BM, below 500 nm. The
7
0
60
broader absorption band of SM:PC BM as compared to
7
0
SM:PC60BM leads to the absorption of more photons by the
former active layer, generating more excitons in the active layer
in the device based on SM:PC70BM, resulting in higher Jsc. The
almost same value of Voc is related to the same value of LUMO
level of both the fullerene derivatives, since the Voc is related to
the energy difference between the HOMO level of donor and
LUMO level of acceptor employed in BHJ active layer.
The low values of PCEs of these BHJ organic solar cells may
be due to the poor nanoscale interpenetrating morphology of
the BHJ active layer. In organic BHJ solar cells based on
To demonstrate the potential application of the SM in OSCs,
we used SM as an electron donor and PC60BM and PC70BM as an
electron acceptor and fabricated the BHJ OSCs with a structure
ITO/PEDOT:PSS/SM:PC BM or PC BM/Al, consisting of
6
0
70
different donor–acceptor weight ratios. It is found that the Voc is
almost insensitive to the weight ratio, while the Jsc is strongly
dependent on the donor to acceptor weight ratio in the active
layer. The donor : acceptor ratio of 1 : 1 gave the best perfor-
mance. The current–voltage characteristics of the BHJ OSC
Fig. 2 Electronic distributions in the selected molecular orbitals for SM.
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consisting of interpenetrating and interconnected D and A
domains that are of the order of 10–30 nm is ideal for efficient
26
solar cell. It is well known that the solvent additive (SA) i.e.
addition of relatively small amount of high boiling point
solvents, can have tremendous impact on the morphology of the
BHJ active layer and hence affecting the photovoltaic perfor-
27
mance of the device. The use of SA has also been employed for
28
BHJ solar cells based on small molecules to improve the PCE.
Fig. 5 shows the J–V characteristics of SM:PC60BM and
SM:PC BM based devices fabricated with solvent additive (SA),
7
0
CN (3% by volume) and photovoltaic parameters are complied
in table. Both the devices showed increase in photovoltaic
performance. The device with SM:PC60BM showed a PCE of
ꢀ
2
3
.76% with a Jsc of 9.73 mA cm , Voc of 0.92 V and FF of 0.42.
Fig. 3 Photoluminescence spectra of SM, SM:PC60BM and SM:PC70BM.
The PCE of the device based on SM:PC70BM reaches 4.96% with
ꢀ
2
J
J
sc of 11.23 mA cm , Voc of 0.92 V and FF of 0.48. The value of
sc has been drastically enhanced with the SA treatment without
signicant loss in V . Moreover the FF has also increased upon
oc
the SA treatment. Therefore, the enhancement in the PCE, upon
SA treatment is mainly as a result of increased Jsc and FF. We
have observed from the J–V characteristics of the devices in dark
that the devices fabricated with CN additives have a higher value
s
for the rectication ratio and smaller value for R , estimated
from the J–V characteristics of the devices measured in dark
compared to those made without SA and may be attributed to
the enhancement in the Jsc and FF. We have observed that the J–
V dark currents in the reverse bias and low forward voltages is
lower for device with SA as compared to the device without SA,
indicates that the recombination has been reduced with the SA
processing of the BHJ active layer.
The X-ray diffraction method has been widely used to
determine the structural ordering of the donor–acceptor blend
29
Fig.
4
Current–voltage characteristics of BHJ photovoltaic devices based thin lms.
To investigate the structural ordering of
SM:PC60BM and SM:PC70BM blends processed from THF solvent.
SM:PC BM in the solid state, we performed X-ray diffraction
7
0
(XRD) analysis on thin lm spin coated on to glass substrates
from THF and CN–THF solutions and the XRD patterns are
Table 2 Photovoltaic parameters of BHJ organic solar cells
shown in Fig. 6. As shown in Fig. 6, SM:PC70BM display Bragg
Processing
solvent
ꢀ2
Blend
Jsc (mA cm
)
Voc (V) FF
PCE (%)
SM:PC60BM THF
SM:PC70BM THF
SM:PC60BM THF (with CN)
SM:PC70BM THF (with CN) 11.23
6.08
7.26
9.73
0.94
0.96
0.92
0.92
0.34 1.94
0.38 2.65
0.42 3.76
0.48 4.96
conjugated polymer and small molecules, the morphology of
the D–A BHJ active layer is of critical importance to their high
25
PCE. For an efficient solar cell, the excitons generated on the D
and A material used in the active layer must be able to diffuse to
the D–A interface where they can be dissociate into free elec-
trons and free holes. Therefore, an interpenetrating network of
D and A phases is then required for the transport of the electron
and hole to the respective collecting electrode. Moreover, the
hole and electron mobilities must be sufficiently high and there
ratio must be unity for balance and efficient charge transport.
Fig. 5 Current–voltage characteristics of BHJ photovoltaic devices based on
For these requirements,
a
phase separated morphology SM:PC60BM and SM:PC70BM blends processed from CN–THF solvent.
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is the permittivity of the free space (8.85 ꢂ 10ꢀ
12
F
3
m
.5), 3
0
ꢀ
1
), b is the eld activation factor and V is the applied
voltage. The current–voltage characteristics of the devices are
3
2
1/2
plotted in the form of ln(Jd /V ) vs. (V/d)
and shown in
Fig. 7a and b for the hole only device and electron only device,
respectively. From the analysis of these plots and the equation
we have estimated the charge carrier mobilities in the BHJ
active layer processed from THF and with a solvent additive.
The hole and electron mobilities for the BHJ active layer
ꢀ
7
2
ꢀ1
processed from THF solvent are 5.87 ꢂ 10 cm V s and
ꢀ4
2
ꢀ1
2
.05 ꢂ 10 cm V s , respectively. However, with the addi-
tion of a solvent additive (CN), the hole mobility has been
ꢀ5
2
ꢀ1
increased to 2.38 ꢂ 10 cm V s but the electron mobility
ꢀ5
2
ꢀ1
has been decreased to 9.8 ꢂ 10 cm V s . It can be seen
from the absorption spectra of the SM:PCBM processed with
additives, the vibronic peak is enhanced indicating that aer
the SA the more ordered lm could be the result of enhanced
p–p stacking. Thus SA leads to hole mobility enhanced by a
factor of 40. However, the increase in the order to the donor
phase probably causes decrease in the order of the acceptor
phase, which leads to electron mobility reduction by a factor
of 3. Similar results have been observed for the device with
PC60BM as electron acceptor. The simultaneous increase and
Fig. 6 The XRD pattern of SM:PC70BM blend thin films cast from THF and CN–
THF.
ꢁ
reection (100) peak at 2q ¼ 9.28 (stronger peak) and 2q ¼
ꢁ
1
9.82 (relatively low intensity), that indicate the presence of
crystalline domains. When the SM:PC BM blend is cast from
7
0
the THF solvent low degree of ordering as evident from the
ꢁ
appearance of weak broad reection at 2q ¼ 9.28 . The lack of
sufficient crystallinity and phase separation within the BHJ
30
layer may be attributed to the low value of PCE. However,
when the blend lm is cast from CN–THF mixed solvent, the
ꢁ
intensity of diffraction peak corresponds 2q ¼ 9.28 indicating
the more organized assembly of the SM with the addition of
CN additive. Since THF is not a good solvent for fullerene
derivatives and the CN is good solvent for both PC BM and
6
0
PC70BM, therefore the addition of CN as SA help the crystal-
linity of SM due to the solubility of fullerene derivatives in CN.
This indicates that the addition of CN improves the crystal-
linity and phase separation between the donor and acceptor
materials in the BHJ layer, which leads to an enhancement in
the PCE of the device.
To understand the inuence of charge carrier transport on
the photovoltaic performance, we have used the space charge
limited current (SCLC) technique to measure the hole and
electron mobility in the BHJ layer processed from THF and CN–
THF solvent. In order to satisfy the SCLC conditions, hole only
devices and electron only devices with structure ITO/
PEDOT:PSS/SM:PC70BM/Au and Al/SM:PC70BM/Al, respectively
were fabricated. These devices form Ohmic contacts at low
voltage, as conrmed by the linear current–voltage relationship
with slope of unity and quadratic behavior at SCLC regime
(slope two). The zero eld mobility of charge carrier mobilities
in the BHJ active layer have been measured by the space charge
limited current (SCLC) technique, given by modied Mott–
Gurney's Law:
"
rffiffiffiffi #
ꢀ
ꢁ
2
V
V
J ¼ ð9=8Þ3
0
3
r
m
exp 0:89b
d3
d
where J is the current density at the SCL regime, d is the
thickness of the active layer, m is the zero eld mobility, 3 is
Fig. 7 Current–voltage characteristics in dark for (a) hole only and (b) electron
only devices using the SM:PC70BM blends processed from THF and CN–THF
r
the relative dielectric constant of the blend (assuming that solvent.
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Fig. 8 AFM phase images (3 mm ꢂ 3 mm scale) of the SM:PC70BM cast from THF and CN–THF solvents.
decrease in the hole mobility and electron mobility, respec-
tively, lead to a more balanced hole and electron transport
Conclusion
aer the addition of SA is benecial to enhancement in both In summary, an A–D–A small molecule with a 3,6-dithie-
sc and FF, and consequently the overall PCE of the organic nylcarbazole D unit and nitrophenyl acrylonitrile acceptor (A)
solar cells.
units has been designed, synthesized and employed as a donor
J
In the organic solar cells based on the BHJ active layer, the along with fullerene derivatives as electron acceptors for the
morphology of the active layer plays an important role in the fabrication of solution processed BHJ solar cells. A PCE of
PCE of the device. Fig. 8 shows the surface morphology of the 1.65% and 2.65% was obtained from the SM:PC60BM and
SM:PC70BM BHJ lms cast from THF and CN–THF solvents SM:PC70BM, respectively, processed from THF solvent. The PCE
obtained from atomic force microscopy (AFM). The rms values has been further improved up to 3.76% and 4.96%, for
of the surface roughness are 5.23 nm and 3.96 nm for SM:PC60BM and SM:PC70BM, respectively, when CN as the
SM:PC70BM blend lm cast from the THF and CN–THF, solvent additive was added to the blend solution.
respectively. Similar results were also observed for SM:PC60BM
blend lms. The large value of surface roughness and phase
separation for the THF cast lm is not sufficient for efficient
Acknowledgements
dissociation of the excitons into free charge carrier, leading to We thank Prof. Y. K. Vijay, Department of Physics, Rajasthan
low J . Since CN is a good solvent for PCBM and therefore University, Jaipur, India, for providing the facilities for device
sc
evidence for the large scale phase separation is provided by fabrication and their characterization. We thank Dr K. R. Justin
the smaller surface roughness observed in AFM images for Thomas, Department of Chemistry, Indian Institute of Tech-
SM:PC70BM cast from CN–THF solvent. The smaller roughness nology Roorkee, India for his assistance with computational
value and improved value phase separation for the BHJ active studies.
layer contributes to the increase in the interfacial D–A area
leads to more efficient charge separation, resulting higher
PCE.
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