Organic bulk heterojunction solar cells based on solution processable small molecules (A-π-A) featuring 2-(4-nitrophenyl) acrylonitrile acceptors and phthalimide-based π-linkers

Article abstract of DOI:10.1039/c2jm16915k

A novel small molecule-based organic donor SM [(2Z,2′Z)-3,3′- (((1E,1′E)-(2-cyclohexyl-1,3-dioxoisoindoline-4,7-diyl)bis(ethene-2, 1-diyl))bis(4,1-phenylene))bis(2-(4-nitrophenyl)-acrylonitrile)] featuring 2-(4-nitrophenyl)acrylonitrile as the acceptor and a π-conjugation bridge composed of phthalimide and styryl units, with an A-π-A type structure, has been synthesized. It showed a long wavelength absorption band having an absorption maximum around 635 nm and the optical bandgap was 1.63 eV, which is lower than most reported conjugated polymers, including poly(3-hexylthiophene) (P3HT). The photovoltaic properties were investigated by constructing bulk heterojunction organic solar cell devices using SM as the electron donor and fullerene derivatives, i.e. PC₆₀BM and PC₇₀BM as the electron acceptors with the device architecture ITO/PEDOT:PSS/SM:PC ₆₀BM or PC₇₀BM/Al. The effect of the SM/fullerene derivative weight ratio and the processing solvent were carefully investigated to improve the performance of the organic solar cells. The optimized organic solar cell with SM:PC₆₀BM and SM:PC₇₀BM cast from THF solvent, at a weight ratio of 1:3 showed power conversion efficiencies (PCEs) of about 1.70% and 2.56%, respectively. The enhanced value of PCE for the BHJ photovoltaic device based on PC₇₀BM is related to the better absorption of PC₇₀BM in the visible region compared to that of PC₆₀BM. The SM:PC₇₀BM blends cast from a DIO-THF mixture and subsequent thermal annealing exhibited improved PCEs of 3.68% and 4.14%, respectively.

Full text of DOI:10.1039/c2jm16915k

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PAPER
Organic bulk heterojunction solar cells based on solution processable small
molecules (A–p–A) featuring 2-(4-nitrophenyl) acrylonitrile acceptors and
phthalimide-based p-linkers
a
G. D. Sharma, J. A. Mikroyannidis, Rajnish Kurchania and K. R. Justin Thomas
b
c
d
*
*
Received 30th December 2011, Accepted 17th May 2012
DOI: 10.1039/c2jm16915k
A novel small molecule-based organic donor SM [(2Z,20Z)-3,30-(((1E,10E)-(2-cyclohexyl-1,3-
dioxoisoindoline-4,7-diyl)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(2-(4-nitrophenyl)-acrylonitrile)]
featuring 2-(4-nitrophenyl)acrylonitrile as the acceptor and a p-conjugation bridge composed of
phthalimide and styryl units, with an A–p–A type structure, has been synthesized. It showed a long
wavelength absorption band having an absorption maximum around 635 nm and the optical
bandgap was 1.63 eV, which is lower than most reported conjugated polymers, including poly(3-
hexylthiophene) (P3HT). The photovoltaic properties were investigated by constructing bulk
heterojunction organic solar cell devices using SM as the electron donor and fullerene derivatives, i.e.
PC60BM and PC70BM as the electron acceptors with the device architecture ITO/PEDOT:PSS/
SM:PC60BM or PC70BM/Al. The effect of the SM/fullerene derivative weight ratio and the
processing solvent were carefully investigated to improve the performance of the organic solar cells.
The optimized organic solar cell with SM:PC60BM and SM:PC70BM cast from THF solvent, at
a weight ratio of 1 : 3 showed power conversion efficiencies (PCEs) of about 1.70% and 2.56%,
respectively. The enhanced value of PCE for the BHJ photovoltaic device based on PC70BM is
related to the better absorption of PC70BM in the visible region compared to that of PC60BM. The
SM:PC70BM blends cast from a DIO–THF mixture and subsequent thermal annealing exhibited
improved PCEs of 3.68% and 4.14%, respectively.
A combination of polymer design, morphology control, struc-
tural insight and device engineering has led to power conversion
efficiencies (PCEs) reaching 6–9% for conjugated polymer/
fullerene blends.3,4a Theoretical considerations estimate the PCE
limit to be between 10%4b and 15%4c for single BHJ and tandem
BHJ configurations respectively.
1. Introduction
Organic photovoltaic (OPV) cells are promising alternatives for
the silicon photovoltaics currently dominating the markets as the
new generation solar cell technologies due to the fact that a large
area can be fabricated with high homogeneity through low cost
solution coating.1 Most of the attention has been focused on
solution processed polymer bulk heterojunction (BHJ) solar
cells.2 The active layer of BHJ OPVs is the admixture of organic
donor and acceptor. Fullerene and its derivatives are the best
acceptors for OPVs in terms of photovoltaic performance,
whereas various strategies have been reported in search of better
organic and polymeric donor materials. The organic donor and
acceptor strongly affect the photovoltaic performance of OPVs.
Despite the high PCEs reported for polymer BHJ solar cells,
one finds that for a given polymer structure, batch to batch
variations in solubility, molecular weight, polydispersity and
purity can lead to different processing properties and perform-
ance.1b In contrast, solution processed BHJ solar cells based on
small molecules as electron donors have attracted much attention
owing to their advantages over their polymer counterparts. The
small molecules have well defined molecular structure, definite
molecular weight, and high purity which do not vary between
batches.5 There are also examples of small molecules that have
been used as solution processable donor materials in BHJ
architectures.6 However, the PCEs of solution processed small
molecule organic solar cells (OSCs) are below that of their
polymer counterparts. In most cases, small molecule devices
using solution processing display inferior film quality to that of
their polymeric counterparts. It is thus expected that the better
PCE could be achieved if the intrinsic film quality and
aR & D Centre for Science and Technology, JEC Group of Colleges,
Delhi-Jaipur road, Kukas, Jaipur, Raj., India 303101. E-mail:
sharmagd_in@yahoo.com
bChemical Technology Laboratory, Department of Chemistry, University
of Patras, GR-26500 Patra, Greece
cDepartment of Physics, Maulana Azad National institute of Technology
(MANIT), Bhopal, MP, India
dOrganic Materials Laboratory, Department of Chemistry, Indian Institute
of Technology Roorkee, Roorkee-247667, India. E-mail: krjt8fcy@iitr.
ernet.in
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morphology in the BHJ architecture could be improved.
However, in order to achieve this, careful molecule design has to
be carried out to address device specific parameters simulta-
neously, including the materials absorption coefficient, bandgap,
its highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energies, mobility and
film forming quality and morphology compatibility with the
acceptors and so on. The PCE of solution processed BHJ solar
cells based on small molecule as the electron donor have reached
up to 4.3–4.4%7 and 6.7%8 after several processing steps. The
PCEs of the BHJ solar cells based on a thermally evaporated
active layer have been reported as 5.2%9a and 6.4%.9b
stannane in toluene in the presence of Pd(PPh3)4.10 4-Nitrobenzyl
cyanide was synthesized by the nitration of benzyl cyanide with
concentrated nitric acid in the presence of sulfuric acid.11,12 It was
recrystallized from ethanol. N,N-Dimethylformamide (DMF)
was dried by distillation over CaH2. Triethylamine was purified
by distillation over KOH. All other reagents and solvents were
commercially procured and used as supplied.
4,7-Dibromoisobenzofuran-1,3-dione (1). Was synthesized
according to a reported method.13
4,7-Dibromo-2-cyclohexylisoindoline-1,3-dione (2). A flask was
charged with a solution of 1 (0.20 g, 0.654 mmol) and cyclo-
hexylamine (0.10 g, 1.003 mmol) in glacial acetic acid (10 mL).
The solution was stirred and refluxed for 5 h. It was subsequently
concentrated under reduced pressure. The concentrate was
cooled in a refrigerator to obtain the precipitate of compound 2.
It was filtered, washed thoroughly with water, dried and
recrystallized from ethanol (0.12 g, 48%).
Herein, we describe the synthesis of a small molecule (SM)
with an A–p–A structure, phthalimide core and phenyl-
enevinylene segments embedded with electron-withdrawing
groups (cyano- and nitro-) on both sides. The central p-conju-
gation may act as a donor and allow an intramolecular charge
transfer (ICT) that reduces the optical band gap (1.63 eV) of SM.
We have used SM as the electron donor along with the PC60BM
or PC70BM as the electron acceptor for the fabrication BHJ
organic solar cells. The PCEs of the devices based on the
SM:PC60BM and SM:PC70BM blends cast from THF solvent
were 1.70% and 2.56%, respectively. In addition, we have
investigated the effect of additive solvent used for the spin casting
of the SM:PC70BM blend film on the photovoltaic response of
the device and found that the device shows a PCE of about
3.68%, when the DIO–THF mixture was used for the deposition
of the film. The PCE has been further improved up to 4.14%
when the blend film of SM:PC70BM cast from the solvent
mixture was subjected to thermal annealing.
FTIR (KBr, cmꢀ1): 2912, 2848 (C–H stretching of cyclohexane
ring); 1702 (imide carbonyl).
1H NMR (CDCl3) ppm: 7.22 (s, 2H, aromatic); 2.62 and 1.97–
0.72 (m, 11H, cyclohexane ring).
Anal. calcd for C14H13Br2NO2: C, 43.44; H, 3.39; N, 3.62.
Found: C, 43.17; H, 3.43; N, 3.55%.
4,40-((1E,10E)-(2-Cyclohexyl-1,3-dioxoisoindoline-4,7-diyl)bis-
(ethene-2,1-diyl))dibenzaldehyde (3). A flask was charged with
a mixture of 2 (0.235 g, 0.608 mmol), 4-vinylbenzaldehyde (0.177
g, 1.337 mmol), Pd(OAc)2 (0.007 g, 0.025 mmol), P(o-tolyl)3
(0.043 g), DMF (5 mL) and triethylamine (3 mL). It was degassed
ꢁ
by purging with N2. The mixture was heated at 90 C for 24 h
2. Experimental
under N2. Then, it was filtered and n-hexane was added to the
filtrate. The mixture was stirred and the supernatant solvents
were decanted. The crude product was purified by column
chromatography, eluting with a mixture of dichloromethane and
n-hexane (1 : 1) (0.17 g, 56%).
2.1 Characterization methods
FTIR spectra were recorded on a Perkin-Elmer 16PC FT-IR
spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were
obtained with a Bruker spectrometer. The electrochemical
properties, i.e. onset oxidation and reduction potentials of SM,
were examined by cyclic voltammetry (CH instruments). We
used an electrochemical cell with a conventional three electrode
assembly and tetrabutylammonium hexafluorophosphate
(Bu4NPF6) (0.1 M) acting as the electrolyte in anhydrous
acetonitrile solution. The voltammetric measurements were
performed at a scan rate of 100 mV sꢀ1 at room temperature. SM
was deposited on the glassy carbon electrode which was used as
the working electrode, while a Pt wire and an Ag/AgCl electrode
were used as the counter and reference electrodes, respectively.
The optical absorption spectra of the pure SM (in solution and
thin film) and blended films were measured using a Perkin Elmer
UV-Vis spectrophotometer.
FTIR (KBr, cmꢀ1): 2912, 2848 (C–H stretching of cyclohexane
ring); 1703 (imide and formyl carbonyl).
1H NMR (CDCl3) ppm: 10.02 (s, 2H, formyl); 7.87 (m, 4H,
aromatic ortho to formyl); 7.56 (m, 4H, aromatic meta to
formyl); 7.22 (s, 2H, aromatic of phthalimide ring); 7.15 (s, 4H,
vinyl); 2.61 and 1.97–0.72 (m, 11H, cyclohexane ring).
Anal. calcd for C32H27NO4: C, 78.51; H, 5.56; N, 2.86. Found:
C, 78.15; H, 5.24; N, 2.51%.
(2Z,20Z)-3,30-(((1E,10E)-(2-Cyclohexyl-1,3-dioxoisoindoline-4,7-
diyl)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(2-(4-nitrophenyl)
acrylonitrile) (SM). A flask was charged with a solution of 3
(0.298 g, 0.608 mmol) and 4-nitrobenzyl cyanide (0.197 g, 1.216
mmol) in a mixture of ethanol (15 mL) and DMF (5 mL).
Sodium hydroxide (0.18 g, 4.50 mmol) dissolved in ethanol
(5 mL) was added portion wise to the stirred solution. The
mixture was stirred for 1 h at room temperature under N2 and
then concentrated under reduced pressure. Water was added to
the concentrate and SM precipitated as a dark green solid. It was
recrystallized from DMF–water (0.30 g, 63%).
The crystallinity of the thin films was studied using the X-ray
diffraction (XRD) technique (panalytical make, USA) having
ꢀ
CuKa, as radiation source of wavelength l ¼ 1.5405 A with the
film coated on quartz substrate.
2.2 Synthesis of SM
Reagents and solvents. 4-Vinylbenzaldehyde was synthesized
FTIR (KBr, cmꢀ1): 2930, 2852 (C–H stretching of cyclohexane
a Stille reaction of 4-bromobenzaldehyde with tributyl(vinyl)
ring); 1704 (imide carbonyl); 1526, 1346 (nitro); 2172 (cyano).
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1H NMR (CDCl3) ppm: 8.22 (m, 4H, aromatic ortho to nitro);
7.72 (s, 2H, cyanovinylene); 7.50–7.10 (m, 14H, other aromatic
and 4H vinylene); 2.64 and 1.97–0.72 (m, 11H, cyclohexane ring).
Anal. calcd for C48H35N5O6: C, 74.12; H, 4.54; N, 9.00.
Found: C, 73.89; H, 4.58; N, 9.12%.
The incident photon to current efficiency (IPCE) of the devices
was measured using a monochromator and 100 W halogen lamp
as light source. The resulting current was measured with
computer controlled Keithley electrometer (model 6514), under
short circuit conditions. The intensity for each wavelength was
kept constant.
Hole or electron only devices were fabricated using the
configurations: ITO/PEDOT:PSS/SM:PC70BM/Au for holes
and Al/SM:PC70BM/Al for electrons. Mobilities are extracted by
fitting the current–voltage curves using the Mott–Gurney rela-
tionship (space charge limited current).19
2.3 Computational methods
We have performed theoretical calculations to understand the
electronic structure of the functional materials by using the
Gaussian 09W (Revision-A.02) program suite.13 The geometries
of the dyes in vacuum were optimized using density functional
theory (DFT)14 with the hybrid B3LYP15 density functional and
the 6-31G* basis set. All optimized geometries were subjected to
vibrational analysis, and characterized as a minimum (no
imaginary frequencies). The time-dependent DFT (TD-DFT)14
at the level of B3LYP/6-31G* was further carried out to deter-
mine at least 10 vertical excitations to the excited state of the
molecules. The computation in THF was performed by applying
the polarizable continuum model (PCM) using the integral
equation formalism variant (IEFPCM) to describe the electro-
static solute–solvent interactions by the creation of solute cavity
via a set of overlapping spheres.16 The absorption spectra were
simulated using the 10 lowest spin-allowed singlet transitions,
mixed Lorentzian–Gaussian lineshape (0.5) and an average full-
width at half maximum (3000 cmꢀ1) for all peaks.17,18
3. Results and discussion
3.1 Synthesis of SM
The synthesis of SM is shown in Scheme 1 and characterized by
FTIR and NMR spectra. The terminal groups at both sides of
the SM (left and right) behave as electron acceptor units, since it
contains the nitro and cyano groups. The SM was characterized
by FTIR and NMR spectra. The central phthalimide unit is a p-
conjugated segment and the cyclohexyl ring is attached to the
central unit to enhance the solubility. It is expected that the
conjugated phthalimide unit will behave as an electron donating
group. Therefore, this SM can be represented as A–p–A or A–
D–A small molecule.
2.4 Device fabrication and characterization
Photovoltaic devices were fabricated with ITO coated glass as the
positive electrode and Al as the negative electrode. The patterned
indium tin oxide (ITO) glass substrate was pre-cleaned in an
ultrasonic bath of acetone and isopropanol and then dried at
room temperature. A thin layer (60 nm) of poly(3,4-ethyl-
3.2 Theoretical computations
The computed vertical excitation and frontier orbital energies,
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 significantly to the prominent
electronic excitations in the dyes are presented in Fig. 1. The
simulated absorption spectra in vacuum and THF phase are
shown in Fig. 2. For the dye SM in tetrahydrofuran, two
prominent absorptions with the peak positions at 550 and
ꢂ410 nm are predicted from the computations. The lower
energy absorption at 550 nm corresponds to the HOMO to
LUMO transition while the shorter wavelength peak originates
due to HOMO to LUMO+2 and HOMOꢀ1 to LUMO+1
excitations. The HOMO, HOMOꢀ1 and LUMO are p-type
orbitals and delocalized over the entire molecule, however the
nitro groups do not participate in the construction of HOMO
and HOMOꢀ1. The LUMO+2 is mainly contributed by the
phthalimide and nitro units and can be described as an
acceptor orbital. In view of the observed electronic distribu-
tions of the participating orbitals, the longer wavelength
absorption may be described as a p–p* transition while the
shorter wavelength absorption may originate from the elec-
tronic migration from the delocalized p-segment to the nitro/
phthalimide units (cf. HOMO to LUMO+2 contribution to
this transition). The high oscillator strengths observed for the
p–p* transition appearing at a longer wavelength is consistent
with the extended p-delocalization present in the molecular
orbitals.
enedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS,
Bayon, Germany) was spin coated onto the ITO glass and baked
at 100 ꢁC for 20 min. A tetrahydrofuran (THF) solution of blend
SM : PC60BM or PC70BM (1 : 2, 1 : 3, 1 : 4, w/w) was subse-
quently spin coated on the PEDOT:PSS layer to form a photo-
active layer. The thickness of the photoactive layer is about
80–85 nm. The deposition of all organic layers was carried out in
ambient atmosphere. An aluminum (Al) was then evaporated
onto the surface of the photoactive layer under vacuum of 10ꢀ5
Torr. The active area of the device is about 5 mm2. The current–
voltage characteristics of the device was measured with
a computer controlled Keithley 238 source meter unit. A halogen
lamp coupled with AM1.5 solar spectrum filters was used as the
light source and the optical power at the surface of device was
100 mW cmꢀ2. All the measurements were also carried out in
ambient conditions.
The thin film of the blend was cast from mixed solvent and is
described as follows: the blend of the solution of SM : PC70BM
(1 : 3 w/w) was first prepared using THF solvent and stirred for
4 h. A 0.5 mL of 1,8-diiodooctane (DIO) was added to the
SM:PCBM solution prepared from the THF solvent and stirred
for another 2 h. The pre-thermal annealing of the active layer was
ꢁ
carried out at 110 C for 2 min on a hot plate before the depo-
sition of final Al electrode. The performances presented here are
the average of the four pixels of each device.
13988 | J. Mater. Chem., 2012, 22, 13986–13995
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Scheme 1 Synthesis of SM.
3.3 Optical and electrochemical properties
Fig. 3 presents the UV-visible absorption spectra of SM in both
dilute (10ꢀ5 M) THF solution and a thin film. These spectra were
normalized by their long wavelength absorption peak. In the
solution, the short wavelength maximum (lmax) appeared at
approximately 495 nm, corresponding to the n–p* transition.
The absorption band in the longer wavelength region has been
assigned to the intra-molecular charge transfer (ICT) between
the acceptor unit and central donor unit in the SM. The thin film
onset of the absorption SM was found at 760 nm, which corre-
sponds to an optical band gap about 1.63 eV. It has been well
established in our earlier communication that the presence of
cyanovinylene 4-nitrophenyl terminal moieties affects the
absorption spectra of the compounds by significant redshift and
broadening.20
Fig. 1 Electronic distributions observed (B3LYP/6-31G*) for SM in
In order to evaluate the electrochemical properties of the SM,
cyclic voltammetry was performed in a 0.1 M solution as
described in the experimental part. The CV curve was recorded in
reference to the Ag/AgCl reference electrode, which is calibrated
using the ferrocene/ferrocenium (Fc/Fc+) redox couple (4.7 eV
selected molecular orbitals contributing to the electronic excitations.
(versus Ag/AgCl), respectively. The highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
below the vacuum level) as an external standard. The SM
ox
(LUMO) energy levels were determined to be ꢀ5.3 eV
ox
onset
exhibited a oxidation peak with onset potential (E
red
onset
) and
onset
and ꢀ3.35 eV, respectively, from the corresponding E
and
red
onset
E
according to following equations21
reduction peak with onset potential (E
) of 0.6 V and ꢀ1.35 V
Table 1 Computed vertical transition details and orbital energies for the SM
Phase
Gas
labs/nm
Oscillator strength
Assignment
HOMO / LUMO (98%)
Dipole moment/Debye
518.8
392.2
550.3
417.0
401.7
2.986
0.662
3.028
0.496
0.353
3.96
4.79
HOMO / LUMO+3 (53%), HOMOꢀ1 / LUMO+1 (41%)
THF
HOMO / LUMO (97%)
HOMO / LUMO+2 (89%), HOMOꢀ1 / LUMO+1 (7%)
HOMO / LUMO+1 (87%), HOMO / LUMO+2 (8%)
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It is important to note that the HOMO level of the SM is
greater than that of P3HT,23 which indicates that the present SM
has better oxidative stability as compared to P3HT. In addition,
the LUMO energy level of SM is well aligned with the LUMO
levels of PCBMs24 (Fig. 4) with energy difference >0.3 eV, indi-
cating that the generated excitons can effectively be dissociated at
the interface between the SM and PCBMs.25 Since open circuit of
BHJ organic solar cell is determined by the difference between
the HOMO level of the donor and LUMO level of the acceptor,
the HOMO energy level of the related donor molecules in the
bulk heterojunction photovoltaic cell is very important for a high
efficiency device.26 Obviously it is expected that with a relatively
low HOMO level (ꢀ5.3 eV) compared with P3HT, this SM may
favor for the improvement of the Voc on construction of a BHJ
photovoltaic device with fullerene derivatives as the acceptor.
Fig. 2 Absorption spectra simulated for SM using the theoretically
3.4 Photovoltaic properties
computed excitation wavelengths, their oscillator strengths and with
a fwhm of 3000 cmꢀ1
.
Fig. 5 shows the thin film absorption spectra of SM:PC60BM and
SM:PC70BM blends. The film absorption of the blends shows
two bands i.e. one in the shorter wavelength region corre-
sponding to fullerene derivatives and another in the longer
wavelength region corresponding to SM. A broader absorption
through the whole solar spectrum is observed for the
SM:PC70BM blend as compared to the SM:PC60BM blend and
this has been attributed to the absorption of PC70BM.27a The
broader absorption spectrum of SM:PC70BM indicates higher
light harvesting efficiency and exciton generation for the
photovoltaic device.
To explore the photovoltaic performance of the SM, bulk
heterojunction OSCs were fabricated using SM as the electron
donor and fullerene derivatives PC60BM or PC70BM as the
electron acceptor with a device architecture ITO/PEDOT:PSS/
SM:PC60BM or PC70BM/Al. We have fabricated devices with
Fig. 3 Optical absorption spectra of SM in THF solution and thin film
form.
ox
onset
EHUMO ¼ ꢀq(E
EHUMO ¼ ꢀq(E
+ 4.7) eV
+ 4.7) eV
red
onset
The results of energy levels of the SM are also listed in Table 2.
The electrochemical band gap of SM is estimated to be 1.95 eV,
which very close to the value estimated from the DFT calcula-
tions, which is also listed in Table 2. The electrochemical band
gap is higher than that estimated from the optical absorption
spectra (1.63 eV) which are due to the exciton binding energy of
the organic material. This is the common phenomena observed in
most of the organic semiconductors as reported in the
literature.22
Table 2 Experimental and computed values of the HOMO, LUMO and
band gap of SM
Method
HOMO/eV
LUMO/eV
Band gap/eV
Cyclic voltammetry
Computed
ꢀ5.30
ꢀ5.69
ꢀ3.35
ꢀ3.10
1.95
2.59
Fig. 4 HOMO and LUMO levels of SM, PC60BM1,24 and PC70BM.1
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The incident photon to current efficiency (IPCE) spectra of the
BHJ devices has been investigated to get information on the
contributions of the SM and PC60BM or PC70BM to photocur-
rent generation. The values IPCE have been estimated from
following expression
IPCE(l) ¼ 1240Jsc/lPin
where Jsc is the photocurrent in the short circuit condition, Pin
and l are the illumination intensity and wavelength of the
monochromatic incident light, respectively. The IPCE spectra of
the devices are shown in Fig. 7. It can be seen from this figure that
the IPCE spectra of the devices closely resemble the absorption
spectra of the respective BHJ active layer indicating that both
donor and acceptor components employed in the BHJ active
layer contribute in photocurrent generation. It is noted that the
IPCE values for the device based on PC70BM as the electron
acceptor, in the wavelength region below 560 nm, are higher than
that for device based on PC60BM as the electron acceptor. This
could be ascribed to higher photon absorption by PC70BM as
compared to PC60BM, in the wavelength region below 560 nm.
PC70BM has more absorption in the visible region than
PC60BM. Therefore, under identical device conditions, PC70BM
also contributes to the generation of excitons and may result
more photocurrent. Since the LUMO levels of both PC60BM and
PC70BM lie at same energy level, the Voc of the devices remain
same for both devices. Indeed, after changing the electron
acceptor from PC60BM to PC70BM, a higher Jsc value was
obtained due to the higher absorption coefficient of PC70BM in
the visible region.27 The Jsc strongly depends on the number of
excitons generated in the photoactive layer of the device and their
dissociation into free charge carriers at the interfaces, which is
directly related to the optical absorption spectra of the photo-
active layer. The stronger absorption of PC70BM in the visible
region than PC60BM, which may contribute to more excitons
and charge generation in the SM:PC70BM based device than in
the SM:PC60BM device. The large number of generated excitons
is likely one of the main reasons for the enhanced Jsc in the
SM:PC70BM device.
Fig. 5 Optical absorption spectra of SM:PC60BM and SM:PC70BM
blends in thin film form.
different SM/fullerene weight ratios and found that the optimum
blend ratio is 1 : 3 (w/w) for both SM:PC60BM and
SM:PC70BM. The current–voltage (J–V) characteristics of the
devices under illumination of 100 mW cmꢀ2, based on SM as the
donor and PC60BM or PC70BM as the electron acceptor are
shown in Fig. 6 and the corresponding photovoltaic parameters
i.e. short circuit current (Jsc), open circuit voltage (Voc), fill factor
(FF) and power conversion efficiency (PCE) are summarized in
Table 3. The device with SM:PC60BM provided a Voc of 0.86 V,
a Jsc of 4.5 mA cmꢀ2 and a FF of 0.44, resulting in a PCE of
1.70%. In contrast, the device based on SM:PC70BM with the
same weight ratio, shows higher photovoltaic performance with
a Voc of 0.86 V, a Jsc of 6.2 mA cmꢀ2 and a FF of 0.48, yielding
a PCE of 2.56%. The increase in the PCE is mainly due to the
enhancement of the Jsc and Voc for both the devices. The same
value of Voc is related to the same LUMO level for the both
PC60BM and PC70BM.
In addition to two different fullerene derivatives, the variation
of solvent was also considered for further improvement of the
photovoltaic performance of the device. The choice of the solvent
can greatly affect the morphology of the active layer and thus
influence the overall PCE of the photovoltaic device.28 An
additive solvent approach has been shown to be effective in
several polymers.29 This approach has also been successfully
applied to solution processable BHJ solar cells based on small
molecules: fullerene derivatives30 and planar heterojunction solar
cells.31 Since the device based on PC70BM as electron acceptor
shows higher PCE, we have investigated the effect of mixed
solvent and subsequent thermal annealing on the photovoltaic
performance of device based on SM:PC70BM blend. The J–V
characteristics of BHJ organic photovoltaic device cast from
additive solvent (DIO–THF) is shown in Fig. 8 and photovoltaic
parameters are compiled in Table 3. The Jsc and FF of the
photovoltaic device cast from DIO–THF additive solvent has
been increased to 8.13 mA cmꢀ2 and 0.54, respectively, resulting
in an overall PCE of 3.68%. The PCE has been further increased
up to 4.14%, when the SM:PC70BM blend cast from mixed
Fig. 6 Current–voltage (J–V) characteristics of (a) SM:PC60BM (1 : 3
w/w ratio) and (b) SM:PC70BM (1 : 3 w/w ratio) BHJ solar cells, blends
cast from THF solvent under an illumination of 100 mW cmꢀ2
.
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Table 3 Photovoltaic parameters of the BHJ photovoltaic devices based on SM:PC60BM and SM:PC70BM blends
Blend
Jsc/mA cmꢀ2
Voc/V
FF
PCE (%)
SM:PC60BM (THF cast)
SM:PC70BM (THF cast)
SM:PC70BM (DIO–THF cast)
SM:PC70BM (thermally annealed DIO–THF cast)
4.5
6.2
8.13
8.8
0.86
0.86
0.84
0.84
0.44
0.48
0.54
0.56
1.70
2.56
3.68
4.14
SM. The absorption spectrum of the SM:PC70BM blend cast
from additive solvent, the absorption peak corresponding to SM
shifts towards the higher wavelength region and also the
absorption coefficient increases, indicating the increase in degree
of SM crystallinity due to the enhanced interchain p–p* stack-
ing. Fig. 8 clearly shows that the further redshift in the absorp-
tion band corresponding to SM. The additional application of
thermal annealing associated with mixed solvent cast film
allowed the film to crystallize more extensively, as indicated by
the increased absorption coefficient and prominent redshift. The
improved crystallinity of SM, facilitated through self-organized
chain stacking, can enhance the light harvesting capabilities,
improving the Jsc.
The IPCE spectra of the devices based on as the cast and
thermally annealed active layer cast from mixed solvent display
broader IPCE spectral band that cast from THF solvent
(Fig. 10). The values of IPCE in the longer wavelength region
further increased when the active layer cast from mixed solvent
was thermally annealed. The red shift in the IPCE spectra which
agreed with the absorption spectra of the blend films was inter-
preted as arising from the enhanced crystallinity of the SM,
resulting higher hole mobility. The enhanced values of IPCE and
PCE of the devices based on mixed solvent and additional
thermal annealing is mainly due the increase in photocurrent.
To gain a better understanding of the crystalline blend, we
have recorded the X-ray diffraction (XRD) on the spin cast thin
of pristine SM and SM:PC70BM cast from THF and DIO–THF
additive solvent. Fig. 11 shows the XRD patterns of SM cast
from THF and DIO–THF additive solvent. The pristine SM cast
from THF and DIO–THF additive solvent show same XRD
band with a peak at 2q ¼ 7.2ꢁ, but the intensity of the peak
significantly increased, when the film cast from the mixed
Fig. 7 IPCE spectra of (a) SM:PC60BM (1 : 3 w/w ratio) and (b)
SM:PC70BM (1 : 3 w/w ratio) BHJ solar cells, the blends cast from THF
solvent.
Fig. 8 Current–voltage (J–V) characteristics of as cast and thermally
annealed SM:PC70BM (1 : 3 w/w ratio) BHJ solar cells in mixed solvent
(DIO–THF) under an illumination of 100 mW cmꢀ2
.
solvent is thermally annealed before the deposition of Al elec-
trode. The increase in the PCE has been attributed to the increase
in the crystallinity of the blend cast from the DIO–THF additive
solvent, and more balanced charge transport in the device.
The optical absorption spectra of the as cast and thermally
annealed SM:PC70BM blend cast from DIO–THF additive
solvent are shown in Fig. 9. As shown in Fig. 5, the blend
SM:PC70BM, cast from THF only shows an absorption band in
longer wavelength region centered at 635 nm, corresponding to
Fig. 9 Optical absorption spectra of SM:PC70BM blend cast of DIO–
THF solvent and thermally annealed film cast from DIO–THF.
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Another important factor that is responsible for the increased
Jsc is the charge transport in the active layer. To obtain efficient
charge transport of carriers in the photoactive layer of the device,
it is necessary to enhance both te electron and hole mobilities.
The space charge limited current (SCLC) method was employed
to estimate the hole and electron mobilities of the devices
prepared from the as cast and thermally annealed films cast from
DIO–THF additive solvent.20,32 The hole and electron mobilities
were estimated from the J–V characteristics of the hole and
electron only devices, respectively and fitting the data in SCLC
region. The current in the SCLC region is described as follow:
JSCLC ¼ (9303rmV2/8d3)exp[0.891g(V/d)0.5
]
where 303r represents the permittivity of the material, m is the
mobility of electron or hole, g is the field activation factor, d is
the thickness of the active layer and V is the effective voltage i.e.
Vappl ꢀ Vbi (Vbi is the built in potential and Vappl is the applied
voltage). The hole mobility of the film cast from THF, DIO–
THF additive solvent and thermally annealed film cast from the
additive solvent are 4.8 ꢃ 10ꢀ6 cm2 Vꢀ1 sꢀ1, 1.2 ꢃ 10ꢀ4 cm2 Vꢀ1
sꢀ1, and 6.5 ꢃ 10ꢀ4 cm2 Vꢀ1 sꢀ1 respectively. The electron mobility
has been slightly increased, when the device is fabricated from the
additive solvent and subsequently thermally annealed. There-
fore, the addition of DIO (additive solvent) and thermal
annealing increases both electron and hole mobilities, but the
degree of increase is more in the hole mobility. For the balance
charge transport, the ratio between the electron and hole
mobilities should be unity. With the increase of hole mobility, the
ratio between the electron and hole mobilities is reduced,
resulting in more balanced charge transport in the device cast
from the mixed solvent as compared to the THF cast film. The
hole mobility in the device fabricated from THF solvent was
insufficient to ensure efficient charge transport as well as the hole
and electron mobilities were less balanced than those measured
for the thin film cast from mixed solvent and thermally annealed
film cast from mixed solvent. The increase in the hole mobility
for the thin film cast from the additive solvent may be due to the
increased crystallinity of the blend, which is further enhanced by
the thermal annealing of the blend as has been evidenced by the
absorption spectrum and XRD patterns for the films.
Fig. 10 IPCE spectra of the BHJ devices based on (a) cast and (b)
thermally annealed SM:PC70BM (1 : 3 w/w ratio) blend cast from mixed
solvent (DIO–THF).
solvent. This indicates the crystallinity of the film has been
increased. However, when the SM is blended with PC70BM, the
diffraction peak corresponds to at 2q ¼ 7.2ꢁ becomes weak,
suggesting an effective mixing of PC70BM with SM. When the
blend is cast from the mixed solvent, the diffraction peak
centered at 2q ¼ 7.2ꢁ reappears. This indicates that the film cast
from the THF solvent was not sufficiently crystallized. More-
over, the blend cast from additive solvent and subsequent
thermal annealing, the intensity of this peak is further increased
(as shown in Fig. 11b). This indicates that the thermally
annealing further increases the film crystallinity. These changes
in film crystallinity after thermal annealing agree with optical
absorption data.
4. Conclusions
In summary, we have synthesized and characterized small
molecule SM based on cyclohexyl ring as the central core donor
unit, with an A–D–A type molecular structure. This small
molecule showed a long wavelength absorption maximum at
635 nm and a low optical band gap of 1.63 eV. HOMO and
LUMO levels estimated from DFT calculations and cyclic vol-
tammetry data closely match and are suitable for an electron
donor along with fullerene derivatives as the electron acceptor,
for the fabrication of BHJ solar cell. The BHJ solar cell based on
SM:PC60BM and SM:PC70BM (cast from THF solution)
showed PCEs of 1.70% and 2.56%, respectively. The higher value
of PCE for the device based on PC70BM as electron acceptor is
attributed to the better absorption capability of PC70BM in the
visible region, resulting in high Jsc. We have also investigated the
effect of additive solvent and subsequent thermal annealing on
Fig. 11 Diffraction patterns of (a) SM cast from THF and mixed DIO–
THF solvent and (b) SM:PC70BM blends cast from THF, DIO–THF and
DIO–THF (thermally annealed).
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L. Yang, Y. Liu, Y. Chen, Q. Qi, F. Zhang and S. Yin, Appl. Phys.
Lett., 2010, 97, 023303.
the performance of the BHJ devices based on SM:PC70BM. The
PCE of the device has been significantly enhanced which is
mainly due to the increase in Jsc and FF. The crystallization of
the SM:PC70BM blend was significantly enhanced by spin
casting from the additive solvent as has been evidenced by the
absorption spectrum and XRD pattern for the films. The
improved crystallinity with the additive solvent cast and ther-
mally annealed blend yields a PCE of 3.68% and 4.14%,
respectively. The improved PCE of the BHJ photovoltaic devices
has been also been attributed to the improved charge transport,
as evidenced from the increase in charge carrier mobility.
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