ISSN 0306-0012
CRITICAL REVIEWYuze Lin, Yongfang Li and Xiaowei ZhanSmall molecule semiconductors for high-effi ciency organic photovoltaics
www.rsc.org/chemsocrev Volume 41 | Number 11 | 7 June 2012 | Pages 4089–4380
Chemical Society Reviews
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This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4245–4272 4245
Cite this: Chem. Soc. Rev., 2012, 41, 4245–4272
Small molecule semiconductors for high-efficiency organic photovoltaics
Yuze Lin,ab
Yongfang Liaand Xiaowei Zhan*
a
Received 18th November 2011
DOI: 10.1039/c2cs15313k
Organic photovoltaic cells (OPVs) are a promising cost-effective alternative to silicon-based solar
cells, and possess light-weight, low-cost, and flexibility advantages. Significant progress has been
achieved in the development of novel photovoltaic materials and device structures in the last
decade. Nowadays small molecular semiconductors for OPVs have attracted considerable
attention, due to their advantages over their polymer counterparts, including well-defined
molecular structure, definite molecular weight, and high purity without batch to batch variations.
The highest power conversion efficiencies of OPVs based on small molecular donor/fullerene
acceptors or polymeric donor/fullerene acceptors are up to 6.7% and 8.3%, respectively, and
meanwhile nonfullerene acceptors have also exhibited some promising results. In this review we
summarize the developments in small molecular donors, acceptors (fullerene derivatives and
nonfullerene molecules), and donor–acceptor dyad systems for high-performance multilayer,
bulk heterojunction, and single-component OPVs. We focus on correlations of molecular chemical
structures with properties, such as absorption, energy levels, charge mobilities, and photovoltaic
performances. This structure–property relationship analysis may guide rational structural design
and evaluation of photovoltaic materials (253 references).
Introduction
Nowadays, fossil fuel (such as coal, oil, and gas) production and
use gives rise to a mass of environmental problems, and also their
stocks are diminishing. The need to develop renewable energy
sources has become urgent. The development of photovoltaic
cells (PVs), which transform inexhaustible solar energy into
electricity, is therefore one of the most promising long-term
solutions for clean, renewable energy. Currently, the main
barrier that prevents PV technology from providing a large
fraction of energy is the high cost of silicon-based PVs.
Organic photovoltaic cells (OPVs) are a promising cost-effective
alternative to silicon-based solar cells, and possess low-cost, light-
weight, and flexibility advantages. Contemporary OPVs are based
a Beijing National Laboratory for Molecular Sciences and KeyLaboratory of Organic Solids, Institute of Chemistry,Chinese Academy of Sciences, Beijing 100190, China.E-mail: [email protected]
bGraduate University of Chinese Academy of Sciences,Beijing 100049, China
Yuze Lin
Yuze Lin received a BS degreein chemistry from BeijingInstitute of Technology in2009. Now he is a PhD studentat the Institute of Chemistry,Chinese Academy of Sciences.His research interests includesynthesis of conjugated smallmolecules and polymers andtheir application in solar cells.
Xiaowei Zhan
Xiaowei Zhan obtained a PhDdegree in chemistry fromZhejiang University in 1998.He was then a postdoctoralresearcher at the Institute ofChemistry, Chinese Academyof Sciences (ICCAS) from1998 to 2000, and in 2000 hewas promoted to AssociateProfessor at ICCAS. Dr Zhanworked in the University ofArizona and Georgia Instituteof Technology from 2002 to2006 as Research Associateand Research Scientist. He hasbeen a full professor at ICCAS
since 2006. His research interests are in the development of organicand polymeric materials for organic electronics and photonics.
Chem Soc Rev Dynamic Article Links
www.rsc.org/csr CRITICAL REVIEW
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4246 Chem. Soc. Rev., 2012, 41, 4245–4272 This journal is c The Royal Society of Chemistry 2012
on a heterojunction resulting from the contact of an electron
donor (D) and an electron acceptor (A) material. Absorption
of solar photons creates excitons, which diffuse to the D/A
interface, where they are dissociated into free holes and
electrons, and opposite polarity carriers (holes and electrons)
transport through the donor and acceptor channels to anodes
and cathodes respectively, subsequently charges are collected at
the electrodes, resulting in the generation of electrical power.
D/A heterojunctions can be created with two main types of
architectures, bilayer heterojunction1 and bulk heterojunction
(BHJ).2
Before the mid 1980s, in conventional OPVs, a single layer
of single component organic material was sandwiched between
two different electrodes with different work functions.3 In
these single-layer and single-component cells, the built-in
potential is derived from either a Schottky-type potential
barrier at one of the metal/organic contacts or the difference in
work function of the electrodes, and the photovoltaic properties
are strongly dependent on the nature of the electrodes. These
early OPVs showed very poor performance.
In 1986, Tang fabricated a bilayer heterojunction solar cell
with an efficiency approaching 1%, which was a milestone in
the development of OPVs.1 Bilayer heterojunction architecture
has been intensively investigated and still is an invaluable tool
for the evaluation of new active materials, nevertheless,
performance of OPVs based on this structure is limited by the
short exciton diffusion length in organic materials (typically
5–20 nm).4 Since the exciton dissociation process is confined to
the D/A interfacial zone, only excitons produced at a distance
shorter than their diffusion length have a good probability to
reach the interfacial zone and generate free charge carriers. So
the exciton diffusion length limits the maximum thickness of the
active layer and thus the maximum fraction of the incident light
that the cell can absorb and covert into electricity.
In 1991, Hiramoto et al. fabricated a novel type of three-
layered OPV with a codeposited interlayer of mixed pigments
between the respective pigment layers, and the interlayer acted as
an efficient carrier photogeneration layer.5 Actually, this type
OPV device is the predecessor of hybrid planar-mixed molecular
heterojunction (PMHJ) OPVs.6 And the mixed interlayer was
recognized as the first bulk heterojunction layer in small
molecule-based OPVs.
In 1992, Sariciftci et al.7 demonstrated that photoexcitation
of a mixture of a conjugated polymer and fullerene (C60)
resulted in an ultrafast, highly efficient photoinduced electron
transfer. And then Yu et al.2 and Halls et al.8 created the ‘‘bulk
heterojunction’’ (BHJ) concept, which is one of the best OPV
device architectures so far. BHJ is a blend of bicontinuous and
interpenetrating donor and acceptor components in a bulk
volume. Such a nanoscale network exhibits a D/A phase
separation in a 5–20 nm length scale, which is within a distance
less than the exciton diffusion length. Compared to bilayer
heterojunction, BHJ significantly increases the D/A interfacial
area, leading to enhanced efficiency of the OPV devices.9
Two or even more OPV cells can be stacked on top of each
other to form a tandem OPV structure, which enables one to
resolve two limiting factors existing intrinsically among organic
semiconductor molecules: poor charge carrier mobility and a
narrow light absorption range.
The bilayer heterojunction and BHJ OPV device structures
are shown in Fig. 1. In the two devices, the photoactive layers
both sandwiched between a high work function anode, typically
a transparent indium tin oxide (ITO) layer, and a relatively low
work function metal cathode, such as Ca, Al. In the bilayer
heterojunction device, the donor materials stick to the anode
and the acceptor materials stick to the cathode, while the active
layer is blend of donor and acceptor materials in BHJ device.
In principle, there are two processing techniques for the
fabrication of OPV devices, vacuum deposition and solution
processing. Generally, the bilayer heterojunction was fabri-
cated by vacuum deposition since it is difficult to find suitable
solvents for donor layer and acceptor layer without destroying
the D/A interface. And both of the two processing techniques
are suitable for the BHJ devices. Some of small molecules such
as metal phthalocyanine and C60 can be deposited under high
vacuum conditions by thermal evaporation. By coevaporation
of donor and acceptor materials, BHJ layers can be obtained.
On the other hand, soluble materials can be deposited from
solution, by spin coating, inkjet printing, gravure or flexographic
printing.
In OPV devices, principal figures-of-merit include power
conversion efficiency (PCE), short-circuit current density
(JSC), open-circuit voltage (VOC), and fill factor (FF), defining,
respectively, the ratio between the output device electrical
energy versus the input solar energy, the device current density
when no reverse bias is applied, and the device voltage when
no current flows through the cell, and the ratio between
maximum power of the device and JSC � VOC.
Fig. 1 The architecture structure of bilayer heterojunction (a) and
BHJ (b) OPV devices.
Yongfang Li
Yongfang Li has been aprofessor at the Institute ofChemistry, Chinese Academyof Sciences (ICCAS) since1993. He obtained his PhDdegree in physical chemistryin 1986 from Fudan University,then came to ICCAS as a post-doctoral fellow working onconducting polymers with Prof.Renyuan Qian (1986–1988).He did visiting research in Prof.Hiroo Inokuchi’s lab at theInstitute for Molecular Sciencein Japan from 1988 to 1991and in Prof. Alan J. Heeger’s
lab at UCSB from 1997 to 1998. His present research interestsare polymer solar cells and related photovoltaic materialsincluding conjugated polymer donor, solution-processable organicmolecule donor and fullerene derivative acceptor materials.
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This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4245–4272 4247
The photoactive materials, including polymeric and small
molecular semiconductors, play a key role in influencing
physical processes involved in energy conversion, which in
turn determine the electrical characteristics of the solar cell,
such as JSC, VOC, and FF, and ultimately PCE. Recently,
OPVs based on a p-type conjugated polymer as a donor and
a fullerene derivative [6,6]-phenyl-C61 (or C71)-butyric acid
methyl ester (PC61BM or PC71BM) as an acceptor have been
rapidly developing, and so far the highest reported PCEs of
this type OPVs are up to 8.3%,10 but still below 10% that is
often regarded as being a prerequisite for large-scale commercial
applications. On the other hand, small molecular semiconductors
for OPVs have attracted considerable attention, due to their
advantages over their polymer counterparts, which include
well-defined molecular structure, definite molecular weight,
and high purity without batch to batch variations.11 An
increasing number of publications on OPVs based on small
molecules have appeared, the PCEs of devices based on small
molecule donors and fullerene acceptors fabricated by vacuum
deposition or solution processing are both in excess of
6%.12–15 As for small molecule acceptors, compared to the
fullerene derivatives such as PC61BM and PC71BM, the develop-
ment of nonfullerene small molecular acceptors has been lagged
with relatively low performance. Recently, research on non-
fullerene acceptors has become more active due to their
interesting potentials, such as convenient synthesis, low cost,
easy tunability of energy levels, and perhaps most importantly,
better absorption in the visible spectrum.
A number of reviews have summarized the synthesis and
application of conjugated polymeric active materials,16–31
small molecular donors,32–38 fullerene acceptors,39–41 small
molecular nonfullerene acceptors42,43 in OPVs as well as
device physics.44–52 In the present review, we focus on the
representative small molecular donors, acceptors and donor–
acceptor dyads in single OPVs. Progress in the past decade has
been substantial, but continued development of OPV materials
will require a better understanding of the relationships between
molecular structure, electronic structure, materials microstructure,
charge transport and photovoltaic properties than is currently
available. For these reasons, we will survey and analyze what
is currently known concerning structure/property relationships
of photovoltaic small molecules.
Small molecular donors
Small molecular semiconductors can be generally classified as
hole or electron transporting (p-type or n-type) materials
according to the type of orderly transferring charge carriers,
under a given set of conditions, stemming from removal of
electrons from the filled molecular orbitals or from the addition of
electrons to empty orbitals, respectively. Many small molecular
p-type semiconductors have been studied for decades.53
Among these molecules, only a small fraction has been applied
successfully as electron donors in OPV devices due to the
various optical, electrical, and stability requirements demanded
of the chosen materials. The properties of materials, such as
hole mobility (i.e., the distance over which holes are transported
per second under the unit electric field), exciton diffusion
length, thin film morphology, frontier energy level alignment,
band gap, and absorption coefficient, all greatly affect the
performance of OPV device. In this section, some representative
small molecular donors such as dyes, fused acenes, oligothiophenes,
and triphenylamine-based molecules used in the active layer of
OPVs are described and discussed.
Dyes
Of a variety of small molecular donors that have been reported
in the literature, dye-based molecules are first and common.
Several famous classes of dyes are believed to be potential
materials in OPVs, such as phthalocyanine (Pc), subphthalo-
cyanine (SubPc), merocyanine (MC), squaraine (SQ), diketo-
pyrrolopyrroles (DPP), borondipyrromethene (BODIPY),
isoindigo (ID), perylene diimides (PDI), and quinacridone
(QD). As several groups have demonstrated, functionalizing
a dye molecule has been confirmed to be a successful approach
to donor design. Table 1 provides a summary of electronic
properties as well as OPV data for representative dye-based
donors (Fig. 2).
Pc, comprising four isoindole units connected by 1,3-aza
linkages, is a planar and highly aromatic 18-p-electron macro-
cycle. Pc derivatives typically exhibit excellent thermal and
chemical stability,54 and they also offer flexibility in their
optical and electronic properties through synthetic modifica-
tions, such as attaching functional groups to the molecule
perimeter. The optoelectronic properties and stacking in the
solid state of Pc derivatives can be turned by replacing of the
two protons in the molecular cavity with a metal ion.
Although many metal Pc complexes have been used in OPVs,
CuPc (a1) and ZnPc (a2) have been the most common choices
to date for application in Pc-based OPV devices, due to longer
exciton diffusion length of CuPc and ZnPc as compared to the
other Pcs.55
In 1986, CuPc was first used as a donor material in bilayer
heterojunction OPV by Tang, and the device showed a PCE
value of 1%.1 Inspired by this pioneering contribution, Pc has
been commonly applied in vacuum-deposited OPV devices, due to
high absorption coefficient and long exciton diffusion length.56–60
And Pcs are frequently paired with fullerenes in the OPV active
layer. The CuPc/C60 combination is among the most common for
small molecule OPV active layers at present, and devices
deriving from this system exhibited high performance.6 In
2001, Forrest and Peumans demonstrated that bilayer hetero-
junction OPVs based on CuPc and C60 with PCE of 3.6% can be
achieved under 150 mW cm�2 simulated AM1.5G illumination,57
while the BHJ OPVs based on a mixture of vacuum codeposited
CuPc and C60 were fabricated and the best PCE was up to 3.5%
at 100 mW cm�2 simulated AM1.5G illumination.59 Later,
Forrest et al. fabricated an OPV structure with a planar-mixed
heterojunction (PMHJ), i.e., a mixed layer consisting of CuPc
and C60 sandwiched between homogeneous CuPc and C60
layers, and the device afforded a maximum PCE of 5.0%, at
120 mW cm�2 under simulated AM1.5 solar illumination.6
Additionally, stacking two of these cells together in a tandem
cell resulted in a ca. 15% increase in device performance and
a PCE of 5.7% at 100 mW cm�2 AM1.5 simulated solar
illumination.60
Planar Pcs have relatively narrow absorption band and cannot
effectively absorb low-energy photons, while ca. 50% of photons
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4248 Chem. Soc. Rev., 2012, 41, 4245–4272 This journal is c The Royal Society of Chemistry 2012
in the solar spectrum have energies corresponding to wave-
length of 600–1000 nm. Nonplanar Pc molecules, such as
chloroaluminum phthalocyanine (AlClPc, a3), exhibited an
absorption peak in the near-infrared around 755 nm and
harvested a greater percentage of infrared photons.61 The
out-of-plane Cl atom bonded to the central Al atom with
square-pyramidal geometry strongly influences the molecular
packing, inducing an interleaved slip-stack arrangement and
significant red shift in absorption compared to CuPc (maximum
absorption wavelength of ca. 630 nm). Additionally, ultraviolet
photoelectron spectroscopy (UPS) measurements indicate that
the highest occupied molecular orbital (HOMO) energy of a3 is
shifted to �5.4 eV, ca. 0.1 eV farther from vacuum than that of
a1, which is beneficial for an increased VOC. The optimized
a3/C60 bilayer devices exhibit an overall improvement in PCE
from 1.8% of a1/C60 bilayer devices to 2.1%.61
Oxo-titanium phthalocyanine (a4) has also been applied in
the OPV devices as donor materials leading to both a higher
absorbance at long wavelengths and an increased VOC in
a4/C60 bilayer OPV devices as compared to a1.62,63 The studies
of a4 films have determined that vapor deposition results in
the formation of at least two distinct phases with different
absorption spectrum. In 2009, Armstrong et al. reported that
polymorph a4 film has changed to crystalline form that
absorbs light at the longest wavelengths by exposure of the
as-deposited thin film to concentrated solvent vapors (solvent
annealing). The optimized OPVs based on a4 showed a
relatively high PCE of 4.2%, with JSC of 15.1 mA cm�2,
VOC of 0.57 V, and FF of 0.53.63
A metal-free Pc (a5) with aliphatic side chains was one of
the earliest small molecule donors in solution-processed BHJ
OPVs.64 The reported a5-based devices were prepared by spin-
casting a mixed solution of a5 and PDI derivative (f3, Fig. 7)
in chloroform. These very first reported devices exhibited poor
device performance, but recent work has shown that Pc derivatives
are viable donors for application in solution-processed BHJOPVs.
Significantly improved performance was realized by using
a combination of three ZnPc-based donors and a fullerene
acceptor to give PCE of 0.12%, with a JSC of 1.24 mA cm�2,
VOC of 0.41 V, and FF of 0.24.65
The OPVs based on Pcs showed relatively low VOC (generally
less than 0.6 V), which was one of factors limiting the PCEs of
OPVs. TheVOC value generally depends on the energy difference
between the lowest unoccupied molecular orbital energy
(LUMO) of the acceptor and HOMO energy of the donor.66
Thus, one way to increase the VOC is to lower the HOMO level
Table 1 Optical and electronic properties, mobilities, and OPV performance of dye-based donors
lmaxa/nm Eg
opt/eV mhb/cm2 V�1 s�1 HOMOc/LUMO/eV Active layerd JSC /mA cm�2 VOC/V FF PCEe (%) Ref.
a1 a1/f1 2.3 0.45 0.65 0.95f 1a1/C60 18.8 0.58 0.52 3.6g 57a1:C60 (1 : 1 vac) 15.4 0.50 0.46 3.5 59a1/a1:C60 (1 : 1 vac)/C60 15.0 0.54 0.61 5.0h 6
a3 755 �5.4(U)/— a3/C60 — 0.68 0.50 2.1i 61a4 850 �5.2(U)/— a4/C60 15.1 0.57 0.53 4.2 63a6 590 �5.6/�3.6 a6/C60 3.36 0.97 0.57 2.1 67a7 688 2 � 10�5 (S, N) �5.4/�3.6 a7(sol)/C60 5.6 0.55 0.49 1.5 69
1.7 a7(vac)/C60 6.1 0.79 0.49 2.5 70a8 607 1 � 10�5 (O, N) �5.80/�3.76 a8:PC61BM (1 : 3) 5.3 0.90 0.32 1.54 72a9 649 �5.59/�3.68 a9:PC61BM (3 : 7) 6.3 0.76 0.36 1.74 72a10 616 5 � 10�5 (O, N) �5.75/�3.59 a10:PC61BM (9 : 11) 8.24 0.94 0.34 2.59 73
a10:C60 (1 : 1 vac) 11.5 0.80 0.48 4.9j 74a10:C60 (9 : 11 vac) 12.6 0.96 0.47 6.1 12
a11 595 7 � 10�6 (O, N) �5.69/�3.54 a11:PC71BM (9 : 11) 10.2 1.0 0.44 4.5 75a12 760 10�5–10�4 (O) �5.0/�3.3 a12:PC61BM (1 : 3) 5.70 0.62 0.35 1.24 79a13 770 1.2 � 10�4 (O, N) �5.0/�3.3 a13:PC71BM (1 : 3) 9.32 0.57 0.37 1.99 80a14 820 1.3 � 10�3 (O, N) �5.14/�3.37 a14:PC71BM (3 : 2) 12.6 0.31 0.47 1.79 81a15 700 �5.3/�3.4 a15/C60 6.89 0.83 0.55 3.2 82
a15(sol)/C60 9.71 0.78 0.54 4.1 83a15:PC71BM (1 : 6) 12.0 0.92 0.5 5.5 84
a16 710 �5.3/�3.7 a16(sol)/C60 10.0 0.90 0.64 5.7 86a17 588 1.95 �5.69/�3.66 a17:PC61BM (1 : 2) 4.43 0.80 0.34 1.17 88a18 675 1.70 5.1 � 10�5 (S, N) �5.56/�3.75 a18:PC61BM (1 : 2) 4.14 0.75 0.44 1.34 88
a17:a18:PC61BM (1 : 1 : 2) 4.70 0.87 0.42 1.70 89a19 672 1.70 9.7 � 10�5 (S, N) a19:PC61BM (1 : 2) 7.00 0.75 — 2.17k 90a20 742 2.01 5 � 10�7 (S, B) �5.03/�3.0 a20:PC61BM (7 : 3) 8.42 0.67 0.45 2.33 91a21 720 1.0 � 10�4 (S, N) �5.2(U)/�3.7 a21:PC71BM (1 : 1) 9.2 0.75 0.44 3.0 92a22 660 1.7 3 � 10�5 (S, B) �5.2(U)/�3.4 a22:PC71BM (3 : 2) 10 0.92 0.48 4.4 93a23 695 1.73 4 � 10�5 (O, N) �5.46/�3.46 a23:PC61BM (1 : 1) 4.9 0.77 0.41 1.53 94a24 2.5 � 10�3 (S, N) a24:PC71BM (2 : 1) 8.3 0.76 0.58 3.7 95a25 676 1.72 7.18 � 10�3 (O, N) �5.40/�3.68 a25:PC61BM (3 : 2) 11.27 0.84 0.42 4.06 96a26 660 1.67 �5.5/�3.9 a26:PC61BM (1 : 1) 6.3 0.76 0.38 1.76 97a27 3.64 � 10�5 (S, B) �5.27/�3.54 a27:PC61BM (1 : 2) 5.94 0.78 0.31 1.42 99a28 550 1.94 5.25 � 10�5 (S, N) �5.55/�3.42 a28:PC71BM (1 : 2) 8.87 0.72 0.35 2.22 100
a In film. b O and S: measured by OFET or SCLC method, N and B: in neat or blend film. c From electrochemistry unless stated otherwise,
U: from UPS. d Donor/acceptor: bilayer by vacuum deposition unless stated otherwise; donor:acceptor: blend by solution process unless stated
otherwise; vac: vacuum deposition; sol: solution process. e AM1.5, 100 mW cm�2 unless stated otherwise. f AM2, 75 mW cm�2. g 150 mW cm�2.h 120 mW cm�2. i 119 mW cm�2. j 88 mW cm�2. k 90 mW cm�2.
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This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4245–4272 4249
Fig. 2 Chemical structure of dye-based donors.
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4250 Chem. Soc. Rev., 2012, 41, 4245–4272 This journal is c The Royal Society of Chemistry 2012
of donor materials. In 2006, Thompson and coworkers
reported a subphthalocyanine (a6)/C60-based bilayer hetero-
junction OPV.67 Due to the deep HOMO level of a6, the
devices exhibited much higher VOC (0.97 V) as compared to
conventional devices based on CuPc/C60 bilayer heterojunction
(0.42 V), without a concomitant reduction in JSC, resulting in
enhancement in PCE from 0.9% to 2.1%. Gommans et al.
reported a higher JSC (5.4 mA cm�2) and higher PCE (3.0%)
from this device architecture.68
In 2009, solution processing of subnaphthalocyanine (SubNc, a7)
was carried out for the first time to form a donor layer in
efficient bilayer heterojunction OPVs.69 Due to its unique
properties, such as good solubility, low tendency to aggregate,
and strong light absorption in the visible region, amorphous
a7 films with good charge transporting and light-harvesting
properties can be prepared via simple solution casting. The
a7/C60 bilayer device based on solution processed a7 donor
layer demonstrated a PCE of 1.5%, with a JSC of 5.6 mA cm�2,
VOC of 0.55 V, and FF of 0.49 after thermal annealing at 120 1C
for 40 min. At the same time, Verreet et al. reported that the
a7/C60 bilayer OPVs based on vacuum deposited a7 donor layer
produced a higher PCE of 2.5%, with JSC of 6.1 mA cm�2, VOC
of 0.79 V, and FF of 0.49.70 The decreased VOC as compared
to that of a6-based devices results from greater conjugation
imparted by the additional benzene rings in a7, which raises
the HOMO energy level.71 However, the benzene rings are also
responsible for the shift in absorption to longer wavelengths and
a resultant improvement of JSC, compared to a6-based OPVs.
Merocyanine (MC) dye-based molecules offer high absorption
coefficients (usually over 1� 105M�1 cm�1) and sufficiently large
variability in the position of the HOMO and LUMO levels.72,73
In 2008, Wurthner, Meerholz and coworkers have successfully
applied MC dyes in solution-processed BHJ OPVs for the first
time. The optimized PCEs were up to 1.54% or 1.74% for blends
of MC dyes (a8 or a9):PC61BM.72 Later, they modified the push–
pull dye a8 with a flexible alkyl chain to achieve more efficient
photovoltaic molecules by bridging the electron donating unit
with a propylene group in a10 to diminish the flexibility of the
structure and to ensure a more planar geometry.73 a10 exhibited
relatively high hole mobility of 5 � 10�5 cm2 V�1 s�1, five times
of that for a8. Solution-processed devices based on a a10:PC61BM
blend showed a JSC of 8.24 mA cm�2, larger than that of
a8:PC61BM (5.30 mA cm�2), resulting in a remarkably improved
PCE of 2.59%. Moreover, vacuum processed BHJ OPVs based
on a10 and C60 were also fabricated, and the best vacuum
processed device showed a promising PCE of 4.9%.74 More
recently, a PCE of 6.1% was achieved by introducing MoO3
instead of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)
(PEDOT:PSS) as hole-collecting contact sandwiched between
the active layer and anode after solvent annealing in CH2Cl2for 10 min.12 They developed a new dye a11 for application in
solution-processed OPVs, the optimized device showed a PCE
of 4.5% with a VOC of 1.0 V.75 They reported a parallel-BHJ
cell that forms in situ driven by dipolar intermolecular forces
when mixing two merocyanine dyes with complementary
absorption as double-donors with C60 acceptor. By optimizing
the ratio of the two donor components and the thickness of the
active layer, the PCE (3.2%) of the blend donor cells was
higher than either (2.6–2.7%) of the reference devices based on
the individual dyes only. This synergetic effect is attributed to
a more efficient photon harvesting efficiency of the mixed
donor cells compared to either of the single donor devices.76
MC dyes show a great potential for the application in highly
efficient tandem solar cells. Very recently, a dye a8 and C60-based
novel tandem-cell device architecture, combining bilayer and
bulk heterojunctions, was fabricated by fully vacuum depositing,
consisting of only four organic layers.77 The optimized PCE was
up to 4.8%, with a strikingly high VOC of 2.1 V, which is the
highest VOC value reported for small molecule tandem solar
cells. Wurthner et al. also investigated the charge dissociation at
the D/A heterointerface of thermally evaporated bilayer hetero-
junctionMC dye (an analog with propyl replacing butyl in a8)/C60
OPVs, where they found that the FF value can be improved by
evaporating the dye film on a heated substrate or post-
annealing the completed devices above the glass transition
temperature (Tg) of this MC donor. The optimized bilayer
device showed PCE up to 3.9% with very high FF of 0.70.78
Squaraine (SQ) has been effectively applied to small mole-
cule OPV devices.79–86 The SQ dye-based molecules showed
broad absorption from 500 to 900 nm in the film, high
absorption coefficients (over 1 � 105 M�1 cm�1), good photo-
chemical and thermal stability.87 In 2008, Marks and coworkers
reported that a series of SQ-based molecules exhibited promising
performance in solution processed BHJ OPV, and the optimized
a12-based device processed in air exhibited a PCE of 1.24%.79
Subsequently, the structure modification by using hexenyl
groups (a13) instead of 2-ethylhexyl side chains improved
the device PCE up to 1.99% after thermal annealing at 50 1C
for 30 min.80 Wurthner, Meerholz and coworkers replaced the
ketone on the SQmoiety with dicyanovinyl group (a14), leading
to increased crystallinity and relatively high hole mobility
(1.3 � 10�3 cm2 V�1 s�1 after annealing).81 The BHJ device
based on a14 and PC61BM (annealing at 110 1C for 5 min)
showed a PCE of 1.79%, with an unusually high JSC of up to
12.6 mA cm�2, a high FF of 0.47, but a rather low VOC of
0.31 V. Thompson, Forrest and coworkers also recently
reported a SQ-based molecule (a15) as donor in OPVs.82–84
Initially, a15was used to fabricate bilayer heterojunction OPVs by
vacuum deposition, which exhibited a PCE of 3.2% under 1 sun,
AM1.5G simulated solar irradiation.82 Later, bilayer devices
using solution processed a15 layers with evaporated C60 layers
were found to have a higher PCE of 4.1%, while the BHJ
device achieved a PCE of 2.9%.83 Recently, they found that
post-annealing through additional extended exposure of the
blend to dichloromethane can lead to control of the nanoscale
phase separation of a15:PC71BM (1 : 6) blend films and an
optimized morphology reduces series resistance.84 By optimizing
morphology and molecular ordering of the a15:PC71BM BHJ
OPVs, a peak PCE of 5.5% has been achieved, with a maximum
cell performance achieved when the exciton diffusion length is
approximately equal to the mean a15 crystallite size. This
result suggests that the high BHJ OPV performance could be
achieved by the precise structural control of phase separation.
Furthermore, they developed a series of new SQ-based dyes;85,86
the optimized a16/C60 bilayer device with using solution processed
a16 layer with evaporated C60 layer showed the best OPV
performance after thermal annealing at 90 1C: PCE of 5.7%,
JSC of 10.0 mA cm�2, VOC of 0.90 V, and FF of 0.64.86
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Crystallographic data suggest that the intermolecular stacking
of a16 molecules is closer than that of a15, thereby reducing
the device series resistance and increasing its fill factor.
Roncali and coworkers firstly reported borondipyrromethene
(BODIPY) derivatives as photovoltaic materials.88–90 This type
of dyes can be used as a platform for the design of donor
materials owing to a unique combination of facile synthesis,
good stability, and high absorption coefficients. Additionally,
based on the presence of a tetrahedral boron atom in the
structure, BODIPY appears as an interesting potential of
isotropic active materials for OPVs. In a first exploration of
BODIPY-based donors, the dyes a17 and a18 were synthesized
and applied in BHJ OPVs blending with PC61BM acceptor. The
absorption spectra of a17 and a18 show a maximum at 572 and
646 nm, respectively, with high molecular extinction coefficients
(1.0–1.3 � 105 M�1 cm�1). The OPV cells based on a17 or
a18:PC61BM (1 : 2) delivered PCE of 1.17% or 1.34%.88
Combining these two dyes with complementary absorption
characteristics and blending with PC61BM, the devices were
fabricated showing broad IPCE (the incident photon-to-current
conversion efficiency, i.e., the current obtained outside the
photovoltaic device per incoming photon) spectrum from 350
to 700 nm, yielding a PCE of 1.7%, with a JSC of 4.7 mA cm�2,
VOC of 0.87 V, and FF of 0.42.89 Bithiophene was incorporated
into a18 to build the new molecule a19, leading to improved hole
mobility of 9.7 � 10�5 cm2 V�1 s�1 from 5 � 10�5 cm2 V�1 s�1
for a18, while retaining the broad and strong absorption of
BODIPY. The BHJ OPV based on a19 and PC61BM showed an
improved JSC of 7.0 mA cm�2 and PCE of 2.2%.90
Due to several attractive properties of diketopyrrolopyrrole
(DPP) dyes for photovoltaic applications, such as strong light
absorption, good photochemical stability, and facile synthetic
modification, Nguyen et al. have applied a series of DPP-based
materials in solution processable BHJ OPVs.91–93 In 2008, they
explored the first soluble DPP-based molecule (a20) with
terthiophene arms as a donor blending with PC61BM; the
BHJ device exhibited a JSC of 8.42 mA cm�2, VOC of 0.67 V,
FF of 0.45, and an overall PCE of 2.3%.91 However, inherent
aspects of this material limit the device performance, including
high HOMO energy level, potential morphological instability
from the thermally labile alkyl group, and imbalance of carrier
mobilities (hole mobility of 5 � 10�7 cm2 V�1 s�1 and electron
mobility (i.e., the distance over which electrons are transported
per second under the unit electric field) of 3 � 10�4 cm2 V�1 s�1
as measured using single-carrier diodes). By using 2-ethylhexyl
group instead of t-Boc substituent in a20, the new DPP-
based molecule (a21) showed deeper HOMO level (�5.2 eV),
improved morphological and thermal stability, and balanced
carrier mobilities when blending with PC71BM (hole mobility
of 1.0 � 10�4 cm2 V�1 s�1 and electron mobility of 4.8 �10�4 cm2 V�1 s�1). Owing to these improvements, the OPVs
based on a21:PC71BM (1 : 1) exhibited a PCE of 3.0%, with
JSC of 9.2 mA cm�2, VOC of 0.75 V, and FF of 0.44.92
The OPV performance of DPP-based materials was further
improved by replacing hexylbithiophene end groups with
benzofuran (a22). Blending a22 with PC71BM, very little phase
separation was apparent in the as-cast film. However, thermal
annealing leads to suitable phase separation so that effective
BHJ morphologies are obtained. The degree of phase separation
can be controlled by adjusting annealing temperature; 110 1C
yielded optimum device properties: JSC of 10 mA cm�2, VOC
of 0.9 V, FF of 0.48, and PCE of 4.4%.93 In 2010, Luscombe
et al. incorporated selenophene into DPP-based molecules as
the donor material in BHJ OPVs;94 the optimized devices
based on an a23:PC61BM (1 : 1) blend showed a PCE of
1.53%, with a JSC of 4.9 mA cm�2, VOC of 0.77 V, and FF
of 0.41. Recently Frechet and coworkers reported a series of
DPP-based donors with different end groups, and demon-
strated that efficient OPV materials can be constructed by
attaching planar, symmetric end groups to electroactive small
molecules.95 p–p interaction of molecule a24 dictated tight,
aligned crystal packing, favorable morphology, and promoted
intermolecular connectivity, so OPV devices based on blend of
a24:PC71BM (2 : 1) exhibited a maximum PCE of 4.1% with a
FF approaching 0.6. Marks and coworkers first implemented
naphtho[2,3-b : 6,7-b0]dithiophene (NDT) in donor materials
for BHJ OPVs.96 The molecule (a25) with NDT as core and
DPP as arms was synthesized and showed high absorption
coefficient of 1.1 � 105 M�1 cm�1 at maximum absorption
of 624 nm, appropriate HOMO energy level (�5.4 eV), and
relatively high hole mobility of up to 7.18 � 10�3 cm2 V�1 s�1.
Combined with the electron acceptor PC61BM, a high
PCE of 4.06% was achieved, with a JSC of 11.27 mA cm�2,
VOC of 0.84 V, and FF of 0.42, by annealing at 110 1C for
10 min.
Similar to DPP structure, isoindigo (ID) and perylene
diimide (PDI) both have symmetrical lactam structure with
strong electron withdrawing property. Reynolds et al. reported
synthesis of a26 based on ID as core and bithiophene as arms
and application in solution processed BHJ OPVs as donor
material.97 After annealing at 110 1C, the device based on
a26:PC61BM (1 : 1) gave a PCE of 1.76%; the PCE was
increased to 2.15% by adding a polydimethylsiloxane additive,
due to more favorable morphology.98 Ko and coworkers
reported annulated thiophene PDI linked with triphenylamine
through a bithiophene bridge (a27),99 and BHJ OPV based
on a27:PC61BM (1 : 2) showed JSC of 5.94 mA cm�2, VOC of
0.776 V, FF of 0.308 and PCE of 1.42%.
Frechet and coworkers reported a series of multifunctional
linear quinacridone (QD)-based molecules which showed intense
absorption in the visible spectral region, and the absorption
range and intensity were well-tuned by the interaction between
the QD core and the incorporated thiophene arms.100 A
solution processed thin film of a28:PC71BM (1 : 2) showed
bicontinuous nanophase separated morphology, which was
feasible for exciton dissociation and charge transport. a28-based
device gave a PCE of 2.22%, under an AM1.5 simulated solar
illumination.
In dyes, Pc, MC, and DPP-based molecules have exhibited
very high performance in OPVs with PCE values over 6%,
which can be partially attributed to the strong absorption of
these materials. Interestingly, some dyes (e.g., BODIPY and
MC) with very similar structures can show very different and
complementary absorption spectra while retain compatibility
of other properties. Thus, multicomponent devices can be
fabricated by mixing two dyes as double-donors with fullerene
acceptor, and better device performance can be achieved, which
may deserve further attention.
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Fused acenes
In organic field-effect transistors (OFETs), fused acenes, such
as pentacene and tetracene, have been extensively studied as a
p-type semiconductor, and especially, pentacene is well known
to exhibit extremely high hole mobility (over 1 cm2 V�1 s�1).101,102
The high mobility is a key factor to enhance the PCEs of OPV
devices. This type of materials has been successfully applied in
vacuum deposited bilayer or solution processed BHJ OPVs,
due to their high hole mobility, broad absorption and good
thermal stability. Table 2 provides a summary of electronic
properties as well as OPV data for representative fused acene
donors (Fig. 3).
In 2004, Kippelen et al. first fabricated an efficient OPV
based on a bilayer heterojunction of polycrystalline pentacene (b1)
and C60.103 Under illumination of broadband light 100 mW cm�2,
the device exhibited a JSC of 15 mA cm�2, VOC of 0.36 V, FF
of 0.50, and PCE of 2.7%. Obviously, in this device the most
limiting factor is the low VOC. JSC and PCE were projected to be
around 8.2 mA cm�2 and 1.5% under AM1.5, 100 mW cm�2.
In 2005, Yang et al. fabricated bilayer heterojunction OPVs
with tetracene (b2)/C60 as the photoactive layer.104 The PCEs
of the devices were 2.3% under AM1.5 solar illumination at
100 mW cm�2.
Researchers have tried to use the fused acenes for applica-
tions in solution processable BHJ OPVs by properly functio-
nalizing with solubilizing groups such as triisopropylsilylethynyl
(b3).105 Unsubstituted pentacene adopts a herringbone motif
in solid state, while 6,13-bis(triisopropylsilylethynyl)pentacene
exhibits a structural improvement in solid state, promoting
face-to-face instead of face-to-edge (herringbone) packing,
leading to close cofacial p-stacking.36,106 Such modification offers
tunability of the p-orbital overlap and therefore control of intrinsic
charge carrier mobility, an important parameter for OPV applica-
tions. b3 can be crystallized in film and absorb farther into the red
than pentacene, theoretically leading to higher photocurrent and
PCE. Unfortunately, b3 rapidly undergoes a Diels–Alder reac-
tion with fullerene derivatives in solution, and b3-fullerene
adduct ineffectively supports photoinduced charge transfer.
Hereby, Anthony, Malliaras and coworkers fabricated a bilayer
device (spin-coated b3 layer and vacuum-deposited C60 layer),
where adduct formation can be minimized. After optimization
of photoactive layer thickness, incorporation of exciton-blocking
layer, and thermal annealing, PCE reached a peak value of
0.5%.105 Triethylsilylethynyl-substituted anthradithiophene
(b4) exhibited an absorption cutoff of 575 nm in film and a
high hole mobility of 0.11 cm2 V�1 s�1.107 As-cast b4:PC61BM
(7 : 3) film was fairly amorphous and did not generate large
photocurrent. Solvent (dichloromethane) vapor annealing of
these blends led to the formation of spherulites, which is
consisted of a network of b4 crystallites dispersed in an amorphous
matrix composed primarily of PC61BM. The spherulite covered
fraction of the film appeared rough with submicrometer,
needle-like crystals, which led to a marked improvement in
photocurrent generating capacity. The generated photocurrent
was proportional to the area covered by spherulites. Devices
with 82% spherulite coverage gave the best PCE of 1.0%, with
a JSC of 2.96 mA cm�2, VOC of 0.84 V and FF of 0.40.107
Thompson et al. synthesized triethylsilylethynyl-substituted
tetracene monomer (b5) and dimer (b6).108 The PCE of bilayer
OPV device based on b5 and C60 was 0.5%, withVOC of 1.06 V,
JSC of 1.48 mA cm�2, and FF of 0.34. The VOC (0.31 V) for the
dimer (b6) based device was substantially lower than that for
the monomer based device, leading to lower PCE (0.2%), and
the main reason might be that b6 has higher HOMO energy
level (�5.16 eV) than that (�5.36 eV) of b5.
In 2008, Marrocchi and coworkers reported BHJ OPV
devices based on an anthracene derivative (b7):PC61BM
(1 : 1.17) blend with a PCE of 1.12%.109 They found that an
acetylenic spacer yielded significantly better OPV performance
than an olefinic spacer in this system.110 The optimized OPV
device based on b7:PC61BM (2 : 1) showed a JSC of 3.1 mA cm�2,
VOC of 0.89 V, FF of 0.45, and overall improved PCE of 1.27%
upon thermal annealing at 60 1C.111 In 2010, Chung et al.
reported triisopropylsilylethynyl anthracene derivative substi-
tuted with bithiophene (b8).112 The BHJ OPVs were fabricated
by blending b8 with PC61BM (1 : 1–4). As varying the blending
ratio, the blend film morphology changed from obvious phase-
segregated crystalline domains at 1 : 1 ratio to homogeneous,
nearly amorphous phases at 1 : 4 ratio. At the ratio of 1 : 4,
BHJ OPVs yielded a PCE of 1.4%, higher than that for
other ratios, revealing that well-mixed homogeneous phases,
Table 2 Optical and electronic properties, mobilities, and OPV performance of fused-acene-based donors
lmaxa/nm Eg
opt/eV mhb/cm2 V�1 s�1 HOMOc/LUMO/eV Active layerd JSC/mA cm�2 VOC/V FF PCEe (%) Ref.
b1 1.77 b1/C60 15 0.363 0.50 2.7f 103b2 520 2.14 b2/C60 7.0 0.58 0.57 2.3 104b3 1.65 b3(sol)/C60 1.9 0.47 0.52 0.5 105b4 2.16 0.11 (O, N) �5.15/�2.98 b4:PC61BM (7 : 3) 2.96 0.84 0.40 1.0 107b5 535 �5.36/�2.76 b5/C60 1.48 1.06 0.34 0.5 108b6 612 �5.16/�3.00 b6/C60 1.73 0.31 0.37 0.2 108b7 503 2.25 0.07 (O, N) �5.51/�3.00 b7:PC61BM (1 : 1.17) 1.37 0.762 0.44 1.12g 109b8 537 2.19 10�4–10�5 (S, B) �5.3/�2.8 b8:PC61BM (1 : 4) 4.55 0.78 0.40 1.40 112b9 620 2.0 �5.1/�3.0 b9:PC61BM (1 : 1) 6.55 0.83 0.41 2.25 113b10 1.90 �5.46/�3.60 b10:PC71BM (1 : 1) 5.41 0.80 0.45 1.95 114b11 �5.25/�2.64 b11:f3 (2 : 3) 3.35 � 10�2 0.69 0.4 1.95h 115b12 368 2.87 2.3�10�3 (O, N) �5.03/�2.6 b12:PC61BM (1 : 2) 2.68 0.90 0.61 1.46 116b13 375 2.51 �5.28/�2.77 b13:PC71BM (1 : 2) 6.37 1.0 0.38 2.5 117
a In film. b O and S: measured by OFET or SCLC method, N and B: in neat or blend film. c From electrochemistry. d Donor/acceptor: bilayer by
vacuum deposition unless stated otherwise; donor:acceptor: blend by solution process; sol: solution process. e AM1.5, 100 mW cm�2 unless stated
otherwise. f Broad band light 100 mW cm�2. g 41 mW cm�2. h At 490 nm, 0.47 mW cm�2.
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rather than highly crystalline phases, resulted in improved
PCE by allowing for efficient charge separation.
In 2009, Watkins and coworkers reported a new class of
organic semiconductors based on dibenzo[b,def]chrysene (DBC).113
In contrast to well-studied pentacenes, DBC derivatives did
not undergo cycloaddition reactions with fullerenes. In the
BHJ OPVs, one successful result was obtained by using a mixture
of triethylsilylethynyl-substituted DBC (b9) and PC61BM spin-
casted from chloroform solution. Casting solvents played an
important role in determining crystallinity and morphology.
High boiling point solvent such as chlorobenzene led to larger
scale of phase separation, and reduced interfacial area, resulting
in very low device efficiency. Spin-casted BHJ films from low
boiling point solvent such as chloroform showed phase separa-
tion at the nanoscale, which benefited more efficient dissociation
of the photogenerated excitons and collection of separated
charges. At high concentration and high spin speed, the BHJ
films from chloroform have been well modified to enhance the
PCE of OPV devices. Hereby the optimized device showed a
JSC of 6.55 mA cm�2, VOC of 0.83 V, FF of 0.41 and PCE of
2.25%. Very recently, triisopropylsilylethynyl-functionalized
dibenzo[def,mno]chrysene (b10) with similar structure to b9
was synthesized and applied to fabricate OPV devices.114 The
primary BHJ OPVs based on b10:PC71BM exhibited a PCE of
1.95%, lower than that of b9.
In 2001 Friend and coworkers used a discotic liquid crystal-
line hexaperihexabenzocoronene (HBC) derivative (b11) in
combination with a PDI acceptor (f3, Fig. 7) to fabricate
solution-processed OPVs with a PCE of 1.95% at 490 nm
(0.47 mW cm�2).115 Recently, Holmes and coworkers reported
Fig. 3 Chemical structure of fused acene donors.
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synthesis of difluorenyl-substituted HBC (b12) and its use in
OPVs. Devices based on a blend of b12:PC61BM (1 : 2) after
thermal annealing at 150 1C generated JSC of 2.68 mA cm�2
and PCE of 1.46%.116 Later, they attached oligothiophene to
b12 to yield b13.117 Relative to b12, b13 has extended con-
jugation and broader absorption. The BHJ OPVs based on
b13:PC71BM (1 : 2) exhibited higher JSC (5.71 mA cm�2) and
higher PCE (2.64%) relative to the devices based on
b12:PC71BM (1 : 2) (JSC of 2.53 mA cm�2 and PCE of 1.0%).
Most fused acenes possess high crystallinity and relatively
high mobility, and these materials show moderate photo-
voltaic performance in vacuum deposited bilayer devices.
For solution-processed BHJ devices, high crystallinity of fused
acenes leads to large phase separation scale when blending with
fullerene acceptor, which causes decreased device performance.
Thus, tuning the crystallinity of fused acenes by modifying their
substituent groups is necessary to achieve high performance
BHJ devices based on fused acenes.
Oligothiophenes
Oligothiophenes, including one-dimensional, two-dimensional
and three-dimensional conjugated systems, are one of the largest
families of organic semiconductors, and have been widely used
in OPVs due to their high charge-carrier mobility and facile
synthesis to tune energy levels.118 Table 3 summarizes electronic
properties as well as OPV data for representative one, two and
three dimensional oligothiophenes (Fig. 4).
BHJ OPVs based on blend of a-sexithiophene (c1) and C70
were fabricated by the vacuum coevaporation method.119
Since c1 was easy to aggregate and crystallize, it was difficult
to mix homogeneously c1 and C70 at a blend ratio of 1 : 1 (w/w),
leading to insufficient formation of carrier transport network
and charge separation. When c1:C70 ratio was 1 : 5 (w/w),
excessive C70 prevented c1 from crystallization, leading to
formation of amorphous structure, so that charge separation
efficiency was improved and desirable carrier transport inter-
penetrating network was formed. After thermal annealing at
140 1C for 20 min, the OPV devices exhibited a PCE of 2.38%.
However, narrow absorption and relatively high HOMO level
of c1 limited JSC and VOC of OPVs, respectively. To achieve
high-efficiency OPVs, conjugated oligothiophenes with low
band gap, broad absorption and appropriate energy levels
are required. One successful approach is to introduce electron
withdrawing units into the conjugated backbone to form D–A
oligothiophenes with highly polarizable p-electron systems,
which can extend the absorption spectrum of the donor toward
longer wavelengths by an intramolecular charge transfer and
thus have a good match with the solar spectrum.
The dicyanovinyl (DCV) group has strong electron-withdrawing
properties leading to efficient intramolecular charge transfer.
Some DCV-substituted oligothiophenes have been synthe-
sized and applied in vacuum-deposited and solution-processed
OPVs.120–126 In 2006, Bauerle, Leo and coworkers fabricated
bilayer heterojunction OPVs based on terminally DCV-substituted
oligothiophenes bearing butyl side chains (c2) as donor and
C60 as acceptor.120 Due to low HOMO level and red-shifted
absorption of c2, these OPVs afforded PCEs of up to 3.4%,
with high VOC of 0.98 V, JSC of 10.6 mA cm�2 at 118 mW
cm�2 simulated sunlight. Later, they used selenophene instead
of thiophene in c2 to synthesize a series of c2 analogs.
Selenophene-containing oligomers showed slightly lower perfor-
mance due to lower degree of donor/acceptor phase separation
compared to c2 in BHJ OPVs, but PCE was still in a good range
of 2.5–3.1%.123 Recently, they synthesized a series of terminally
DCV-substituted oligothiophenes without solubilizing side
chains via a novel convergent approach and used them as
electron donors in vacuum-processed bilayer heterojunction
and BHJ OPVs.124 OPV devices incorporating c3 and C60
showed PCEs of up to 2.8% for bilayer heterojunction
Table 3 Optical and electronic properties, mobilities, and OPV performance of oligothiophene-based donors
lmaxa/nm Eg
opt/eV mhb/cm2 V�1 s�1 HOMOc/LUMO/eV Active layerd JSC/mA cm�2 VOC/V FF PCEe (%) Ref.
c1 �5.3(U)/�3.1 c1:C70 (1 : 5) (vac.) 9.2 0.58 0.45 2.38 119c2 573 1.77 �5.6(U)/� c2/C60 10.6 0.98 0.49 3.4f 120c3 579 1.68 �5.43/�3.87 c3:C60 (2 : 1) (vac.) 11.1 0.97 0.49 5.2 124c4 614 1.68 1.5 � 10�4 (S, N) �5.13/�3.42 c4:PC61BM (1 : 1.4) 12.4 0.88 0.34 3.7 126c5 580 1.74 3.3 � 10�4 (S, N) �5.13/�3.29 c5:PC61BM (2 : 1) 10.74 0.86 0.55 5.08 127c6 618 1.69 1.5� 10�4 (S, N) �5.21/�3.68 c6:PC61BM (2 : 1) 13.98 0.92 0.47 6.10 14c7 563 1.83 4.5� 10�4 (S, N) �5.11/�3.54 c7:PC61BM (2 : 1) 9.77 0.93 0.60 5.44 128c8 650 1.73 1.8� 10�4 (S, N) �4.95/�3.26 c8:PC61BM (1 : 0.8) 11.51 0.80 0.64 5.84 129c9 503 2.00 �5.50/�3.34 c9/C60 3.1 0.98 0.57 1.73 133c10 548 1.87 �5.65/�3.64 c10/C60 4.7 1.00 0.67 3.15 133c11 415 1.9 2.5 � 10�4 (S, N) �5.28/�3.38 c11:PC71BM (1 : 4) 8.45 0.82 0.43 3.0 134c12 720 1.51 �5.16/�3.60 c12:PC71BM (3 : 2) 10.9 0.7 0.42 3.2 135c13 720 1.5 0.12 (O, N) �5.2/�3.6 c13:PC71BM (7 : 3) 14.4 0.78 0.59 6.7 15c14 390 �5.26/�2.66 c14:PC61BM (1 : 1.2) 3.65 0.85 0.26 0.80 136c15 426 2.13 1.1 � 10�4 (O, N) �5.29/�3.16 c15:PC71BM (1 : 2) 4.61 0.94 0.36 1.54 137c16 450 2.1 c16:PC61BM (1 : 4) 3.35 0.94 0.40 1.3 138c17 535 2.06 �5.4/�3.3 c17:PC61BM (1 : 4) 2.5 0.93 0.47 1.12 140c18 495 1.74 �5.1/�3.2 c18:PC71BM (1 : 4) 4.79 0.93 0.37 1.64 145c19 400 2.28 �5.28/�3.07 c19:PC61BM (1 : 2) 4.19 0.97 0.42 1.72 147c20 390 2.65 c20:PC61BM (1 : 3) 1.13 0.85 0.24 0.29g 148c21 399 c21:PC61BM (1 : 3) 1.33 0.51 0.27 0.20h 149
a In film. b O and S: measured by OFET or SCLC method, N: in neat film. c From electrochemistry unless stated otherwise, U: from UPS.d Donor/acceptor: bilayer by vacuum deposition; donor:acceptor: blend by solution process unless stated otherwise; vac: vacuum deposition.e AM1.5, 100 mW cm�2 unless stated otherwise. f 118 mW cm�2. g 80 mW cm�2. h 99 mW cm�2.
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This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4245–4272 4255
Fig. 4 Chemical structure of oligothiophene donors.
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4256 Chem. Soc. Rev., 2012, 41, 4245–4272 This journal is c The Royal Society of Chemistry 2012
and 5.2% for BHJs under simulated AM1.5 100 mW cm�2
illumination.
In 2010, Chen and coworkers reported the synthesis of a
DCV-substituted oligothiophene with six solubilizing side chains
(c4) and application in solution-processed BHJ OPVs.125,126
Absorption spectra of c4 films showed good solar spectral
coverage. Hole mobility of pristine c4was 1.5� 10�4 cm2 V�1 s�1
measured by the space charge limited current (SCLC) model.125
Under illumination of 100 mW cm�2, the devices based on
c4:PC61BM blend (1 : 1.4) displayed a PCE of 3.7%, with JSCof 12.4 mA cm�2, VOC of 0.88 V, and FF of 0.34.126 Carefully
designing and controlling the position and density of different
alkyl side or end chains can not only increase solubility of
materials in organic solvents for spin-casting devices but also
improve their packing structure and solid-state miscibility
with fullerenes. Recently, Chen and coworkers designed and
synthesized a series of oligothiophenes end-capped with electron-
withdrawing alkyl cyanoacetate groups instead of DCV and
investigated the correlation between these different end groups
and their BHJ device performance.127 OPVs based on
c5:PC61BM (2 : 1) exhibited a PCE of 5.08%, with a JSC of
10.74 mA cm�2 VOC of 0.86 V and FF of 0.55. Compared to c4
with DCV end group, c5 with alkyl cyanoacetate end group
gave higher PCEs. The better PCEs benefited from improved
FF (over 50%), which came from better film quality and
morphology. Later, 3-ethylrhodanine was introduced as an
end acceptor group to synthesize oligothiophene c6.14 c6
showed stronger solar absorption than c5, which is beneficial
to improving the JSC. A very high PCE of 6.10% was obtained
by using a blend of c6:PC61BM as the active layer, with a
remarkable JSC of 13.98 mA cm�2 and VOC of 0.92 V. Mean-
while, they replaced the central thiophene unit in c5 with a
more electron-rich and planar structure such as benzodithio-
phene and dithienosilole to synthesize c7 and c8. Improved
mobility and absorption of c7 and c8 led to enhanced PCEs
(5.44% for c7128 and 5.84% for c8129). The high PCE values
5.08–6.10% of c5–c8 based devices suggest that the oligothio-
phenes with electron-withdrawing end groups are very promis-
ing donor materials for solution processed BHJ OPVs. In
addition, Roncali et al. synthesized a series of symmetrical and
unsymmetrical septithiophenes end-capped with DCV and
thiobarbituric (TB) electron-withdrawing groups, and investi-
gated their photovoltaic properties in bilayer OPVs based on
spin-coated donor layer and vacuum-deposited C60 acceptor
layer.130 They found an interesting phenomenon that breaking
the symmetry of the donor structure can lead to a significant
increase of VOC, which may be attributed to the intermolecular
interactions and molecular orientation.
Other electron withdrawing units such as benzothiadiazole
(BT), thiadiazolopyridine (TP) and trifluoroacetyl (TFA) were
also introduced into oligothiophene systems.131–135 Bauerle and
coworkers reported two linear oligothiophenes c9 and c10 end-
capped with BT and TP acceptor units, respectively.133 Bilayer
heterojunction OPVs based on c10 and C60 showed a higher PCE
(3.15%) with a very high FF (0.67) compared to that of c9-based
devices (1.73%). The excellent FF, which is among the highest
values reported for small molecule-based OPVs, might be due to
better stacking in thin film caused by intermolecular hydrogen-
bonding interactions by the nitrogen atom of the pyridine ring.
In 2010, Frechet and coworkers reported a series of platinum-
acetylide linear oligothiophenes containing a thienyl-BT-thienyl
core, and various oligothiophenes were connected to control the
molecular packing by changing the number of thiophene units
from two to four.134 The best device based on the oligomer with
terthiophene (c11) blending with PC71BM (1 : 4) after annealing
at 70 1C for 30 min exhibited a JSC of 8.45 mA cm�2, VOC of
0.82 V, FF of 0.43, and PCE of 3.0%. Recently, Bazan and
coworkers introduced dithienosilole (DTS) in oligothiophenes.135
The DTS-containing oligothiophenes showed broad absorption
extending beyond 700 nm, due to intramolecular charge transfer.
After annealing at 110 1C for 2 min, BHJ OPVs based on
c12:PC71BM (3 : 2) showed a JSC of 10.9 mA cm�2 and PCE of
3.2%. Later, Heeger, Bazan and coworkers synthesized one
novel isomeric compound (c13) of c12, and used it to fabricate
solution processed BHJ OPVs.15 Compound c13 exhibited
strong optical absorption, especially from 600 to 800 nm,
and a high hole mobility of ca. 0.1 cm2 V�1 s�1 measured by
organic field-effect transistor (OFET). Under AM1.5 irradia-
tion (100 mW cm�2), a record PCE of 6.7% was achieved for
small-molecule donor based BHJ devices from c13:PC71BM
(7 : 3, w/w). This high efficiency was obtained by adding
remarkably low percentage of solvent additive (0.25% v/v of
1,8-diiodooctane) during the film-forming process, which led
to decreased domain sizes in the BHJ layer. Additionally, the
low fraction of fullerene in BHJ layer is surprising and
interesting. The high performance at such a low fullerene
concentration indicated that the crystalline donor may render
the fullerene acceptor to form percolated pathways to the
electrode. Moreover, it seems that lower fullerene concen-
tration has some connection with the small amount of additive
required to achieve optimized BHJ morphologies.
Aside from linear oligothiophenes, two-dimensional oligo-
thiophene systems have also been reported. Early in 2006, Liu,
Tian and coworkers reported X-shaped conjugated systems
with four linear oligothiophene arms connected to a central
thiophene core.136 c14 with the longest arms had the lowest
bandgap with an absorption onset of 520 nm; the blend of c14
and PC61BM exhibited the smallest feature size in AFM
images. BHJ OPVs based on c14:PC61BM gave a PCE of
0.80% under simulated solar illumination. Recently, Zhan and
coworkers reported a new X-shaped oligothiophene (c15) with
four longer arms than c14.137 Owing to the longer conjugation,
c15 exhibited red shifted absorption with the maximum at
426 nm and the onset of 582 nm in film relative to c14.
BHJ OPVs based on the blend of c15:PC61BM (1 : 2) gave a
VOC of 0.93 V, JSC of 2.71 mA cm�2, FF of 0.40, and PCE of
1.02%. Replacing PC61BM with PC71BM led to an improved
JSC of 4.61 mA cm�2 and a higher PCE of 1.54%.
In 2006, Kopidakis et al. investigated 1,3,5 and 1,2,4,5-
substituted phenyl-cored thiophene dendrimers as donor materials
in photovoltaic application.138 Once again, the oligomers with
the longest oligothiophene arms exhibited the best performance.
Although three-armed dendrimers had superior mobility,
four-armed dendrimers enabled solution-cast film formation
and yielded smaller optical bandgap. The best material (c16)
consisted of a 1,2,4,5-substituted phenyl core with each arm
containing six thiophenes. Solution processed BHJ devices
based on c16:PC61BM (1 : 4) exhibited PCE of 1.3%,
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with a JSC of 3.35 mA cm�2, VOC of 0.94 V and FF of 0.40.
However, due to high bandgaps (over 2.3 eV) and poor
morphology, 1,3,5-substituted phenyl-cored thiophene dendri-
mers did not show promising photovoltaic performance,
which was similar to the work from Roncali et al.139 In an
effort to improve overlap with the solar spectrum, they
reported a thiophene dendrimer that was modified with an
electron-withdrawing group in order to lower the bandgap.140
The dendrimer c17 had an electron-withdrawing tricyanoben-
zene core and three oligothiophene arms to create a push–pull
structure that lowers LUMO level. Compared to its counter-
part without cyano groups and other 1,3,5-substituted phenyl-
cored thiophene dendrimers, the optical absorption onset of
c17 was down to 2.1 eV, attributed to lowering of the LUMO
level. The density functional theory calculations showed that
the optimized geometry of c17 is totally planar and therefore
p-stacking should be favored in films.141 Planarized structure
led to improved morphology both in neat films and in blends
with PC61BM. The BHJ OPVs based on c17:PC61BM showed
an improved FF, from 0.28 for that without cyano groups to
0.47, leading to an overall PCE enhancement from 0.40%
to 1.12%.140 The authors attributed the higher FF to the
improved morphology and reduced carrier recombination of
the c17:PC61BM blend.
Extended push–pull oligothiophenes with DCV groups
exhibited narrow optical energy gap, strong absorption and
favorable intermolecular p–p interactions in the solid state
and were promising donors for high-efficiency photovoltaic
applications.142–144 Wong and coworkers synthesized a series
of dendritic oligothiophenes with carbazole and DCV, the
optical bandgap of these molecules in thin films greatly
reduced to 1.74 eV with strong spectral broadening and a
high ionization potential at ca. 5.5 eV as determined by UPS.
The BHJ OPVs fabricated from dendrimer c18 blended with
PC71BM (1 : 4) showed a PCE of 1.64% with a Voc of 0.93 V
after annealing at 100 1C for 10 min.145
In 2008, Bauerle and coworkers reported promising OPV
results with large, highly branched oligothiophene dendrimers
blended with PC61BM.146,147 The largest dendrimer showed
absorption cutoff up to 600 nm. Oligothiophene c19 showed the
best result in BHJ OPV blending with PC61BM, and the optimized
device produced a PCE of 1.72%, with JSC of 4.19 mA cm�2,
VOC of 0.97 V, and FF of 0.42.147
One issue with many OPV materials, such as fused acenes
and low dimensional oligomers, is molecular and crystal
orientation. High anisotropy in light absorption and charge
transport is an important concern for the future of OPV
materials, especially the solution processable BHJ materials.
In 2006, Roncali and coworkers sought to overcome the low
dimensionality of planar small molecules via synthesis of three
dimensional oligothiophenes with a tetrahedral silicon core.148
Absorption spectra of these compounds are identical in thin
film and solution state, indicating an absence of aggregation or
intermolecular p-interactions in the solid state. Within a BHJ,
these molecules are likely to form amorphous hole-transporting
networks that may hinder charge collection. The PCE of device
based on c20:PC61BM (1 : 3) under white light irradiation at
80 mW cm�2 is 0.3%, a relatively high value considering the
charge transport limitation and the 440 nm light absorption onset.
However, the insufficient robustness of the silicon-thiophene
bond was identified as a problem, and in some cases degradation
of the molecules was observed during vacuum sublimation. In
order to solve this problem, they have developed a different
approach based on the use of a bithiophene twisted by steric
effect as a core for building three dimensional oligothiophenes.
The steric interactions associated with the fixation of bulky
groups at the 3,30-positions of bithiophene produced a dihedral
angle between the thiophene rings, thus generating a tetrahedral
starting unit for the construction of three dimensional systems.149
BHJ OPVs based on c21 and PC61BM in a 1 : 3 (w/w) ratio
under white light irradiation at 99 mW cm�2 gave a JSC of
1.33 mA cm�2, VOC of 0.51 V, and PCE of 0.20%. The low
PCE was considered in relation with the poor absorption of
c21, the maximum peak was only 399 nm.
One-dimensional oligothiophenes with pull–push structure
exhibited promising OPV performance. Especially, oligothio-
phenes with electron-withdrawing end groups yielded very high
PCEs up to 6.10% when blending with PC61BM. Meanwhile,
oligothiophenes with fused rings (such as benzodithiophene and
dithienosilole) showed better performance in BHJ devices. On
the other hand, the issue mentioned above in low dimensional
oligomers is molecular and crystal orientation. High anisotropy
in light absorption and charge transport is critical concern for
OPV materials, especially for the solution processable BHJ
materials. Thus, introducing the electron-withdrawing units
and fused rings into three-dimensional oligothiophenes may
produce surprising and superb results in BHJ OPVs.
Triphenylamine derivatives
Triphenylamine (TPA) has been regarded as a promising unit
for organic semiconductor materials due to its good hole-
transporting and electron-donating capabilities.53,150 TPA-
based small molecules, including push–pull molecules with
TPA as the terminal group and star-shaped molecules with
TPA as core have been widely investigated for application in
OPVs, and they have exhibited good photovoltaic perfor-
mance. Table 4 provides a summary of electronic properties
as well as OPV data for representative linear and star-shaped
TPA-based donors (Fig. 5).
Li and coworkers reported a series of TPA-based linear
push–pull chromophores with benzothiadiazole (BT) pull
moieties.151,152 Early in 2006, they synthesized a D–A–D
molecule d1 with TPA as the donor, BT as the acceptor and
vinylthiophene as the bridge, and the solution-processed BHJ
OPVs based on d1:PC61BM (1 : 1) blend showed a PCE of
0.26%.151 Replacing thiophene in d1 with 4-hexylthiophene
(d2) or 4-hexylthieno[3,2-b]thiophene (d3) led to higher PCEs
(1.44%) due to better film formation caused by alkyl chains.152
Zhan and coworkers synthesized d4, similar to d2 but without
a vinyl group.153 BHJ OPVs based on d4:PC71BM (1 : 3) gave a
higher Voc (0.93 V) and higher PCE (2.21%) relative to
d2:PC71BM (1 : 3).154 They used 1% 1,8-octanedithiol (ODT)
as additive to further improve PCE to 2.86%. This 30% PCE
enhancement was attributed to aggregated domain formation,
enhanced absorption, improved hole mobility, and more
balanced charge transport. Later, they replaced benzothiadiazole
in d4 with thiazolothiazole to synthesize d5; BHJ OPVs
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4258 Chem. Soc. Rev., 2012, 41, 4245–4272 This journal is c The Royal Society of Chemistry 2012
based on d5:PC71BM (1 : 4) after thermal annealing at 110 1C for
10 min afforded JSC of 9.39 mA cm�2, VOC of 0.91 V, FF of
0.44, and PCE of 3.73%.155 Bo and coworkers reported a
series of X-shaped TPA-based molecules with BT as core.156
Interestingly, the X-shaped molecules exhibited deeper HOMO
levels and wider bandgaps than corresponding linear mole-
cules. The best X-shaped molecule (d6) blending with PC71BM
showed a JSC of 4.9 mA cm�2,VOC of 0.92 V, FF of 0.41, and
PCE of 1.8%, similar to that for its linear counterpart.
Cyano-containing electron withdrawing units, such as dicyano-
methylenepyran (DCP),157–159 DCV13,160–162 and cyanoacetic
acid,163 were also combined with TPA to build push–pull
molecules. In 2007, Li and coworkers designed and synthesized
a D–A–D molecule (d7) with TPA as the donor, DCP as the
acceptor, and divinylbenzene as the bridge.157 The VOC, JSC,
FF, and PCE of the optimized device based on d7:PC61BM
(1 : 3) reached 0.9 V, 2.14 mA cm�2, 0.41, and 0.79%, respec-
tively. Replacing benzene bridge in d7 with thiophene (d8) led
to broader absorption, beneficial to generating excitons and
improving the JSC of OPVs.159 The OPV devices based on
d8:PC71BM (1 : 3) blend showed a JSC of 5.94 mA cm�2
and PCE of 2.06%, higher than that (1.40%) of devices based
on d7.
In 2011, Roncali and coworkers reported a series of molecules
with TPA as the end group, a DCV dimer as the core and
thiophene as the bridge.160 These compounds showed interesting
light-harvesting properties (absorption onset at ca. 700 nm)
and low-lying HOMO levels. The symmetrical molecule d9
with one thiophene as bridge showed promising performance
in d9/C60 bilayer heterojunction OPVs: JSC of 3.06 mA cm�2,
VOC of 0.97 V, FF of 0.33, and PCE of 1.08%. Lin, Wong
and coworkers reported a molecule (d10) adopting coplanar
diphenylsubstituted dithienosilole as a central p-bridge betweenTPA and DCV, with a cutoff absorption wavelength of 650 nm;
vacuum deposited planar-mixed heterojunction (PMHJ) OPVs
incorporating C60 or C70 as an acceptor showed an appreciable
PCE of 2.69 or 3.82%, respectively.161 More recently, they
reported a novel D–A–A molecule (d11) with ditolylamino-
thienyl as the donor and BT and DCV as acceptors for
vacuum-deposited OPV devices;162 PMHJ OPVs using C70
acceptor delivered very high Jsc of 14.68 mA cm�2 and PCE of
5.81%, which was attributed to the solar spectral response
extending to the near-IR region and the ultracompact absorption
dipole stacking of the thin film. Later, they further developed
D–A–A molecule (d12) where the pyrimidine acceptor was
employed to replace the BT block in d11.13 The PMHJ
device based on d12 and C70 exhibited a markedly high
spectra-mismatch-corrected PCE of 6.4% with a JSC of
12.1 mA cm�2, VOC of 0.95 V, and FF of 0.56. The high
VOC value for d12 is attributed to its low-lying HOMO level
(�5.46 eV) acquired by using UPS, which is deeper than that
of d11 (�5.30 eV measured by UPS). Compared to d11, the
JSC is slightly lower possibly due to the blue-shifted absorption
of d12. The PCE value of 6.4% is among the highest ever
obtained for organic vacuum-deposited single cells. Pei and
coworkers reported a D–A–A molecule (d13) with TPA as
donor and BT and cyanoacetic acid as acceptors, which
exhibited broad absorption (300–800 nm) in thin film.164
Solution-processed BHJ OPVs based on d13:PC61BM (1 : 2)
blend gave a moderate PCE of 1.23%.163
Table 4 Optical and electronic properties, mobilities, and OPV performance of TPA-based donors
lmaxa/nm Eg
opt/eV mhb/cm2 V�1 s�1 HOMOc/LUMO/eV Active layerd JSC/mA cm�2 VOC/V FF PCEe (%) Ref.
d1 560 1.8 �5.1/�3.3 d1:PC61BM (1 : 1) 0.867 0.76 0.33 0.26f 151d2 569 1.73 1.04 � 10�4 (S, N) �5.14/�3.37 d2:PC71BM (1 : 3) 4.84 0.79 0.375 1.44 152d3 581 1.64 1.48 � 10�4 (S, N) �5.10/�3.42 d3:PC71BM (1 : 3) 5.71 0.74 0.34 1.44 152d4 534 2.03 3.5 � 10�7 (S, B) �5.16/�2.99 d4:PC71BM (1 : 3) 7.49 0.93 0.41 2.86 154d5 436 2.31 1.3 � 10�6 (S, B) �5.39/�2.91 d5:PC71BM (1 : 4) 9.39 0.91 0.437 3.73 155d6 508 2.09 �5.4(U)/� d6:PC71BM (1 : 3) 4.9 0.92 0.41 1.8 156d7 498 1.88 1.19 � 10�6 (S, N) �5.14/�2.76 d7:PC71BM (1 : 3) 5.07 0.71 0.38 1.40 159d8 536 1.79 �5.16/�3.37 d8:PC71BM (1 : 3) 5.94 0.79 0.44 2.06 159d9 592 1.7 d9/C60 3.06 0.97 0.33 1.08g 160d10 542 1.91 �5.4(U)/� d10:C70 (1 : 1) (vac) 9.53 0.83 0.48 3.82 161d11 684 �5.15/�3.71 d11/d11:C70 (1 : 1)/C70 14.68 0.79 0.50 5.81 162d12 550 �5.46(U)/� d12/d12:C70 (1 : 1)/C70 12.1 0.95 0.56 6.4 13d13 587 1.86 �5.43/�3.57 d13:PC61BM (1 : 2) 6.32 0.67 0.29 1.23 163d14 435 2.38 d14/C60 2.33 0.48 0.41 0.46 165d15 d15:PC61BM (1 : 3) 4.10 0.66 0.30 0.81 165d16 544 1.91 d16/C60 1.97 0.72 0.34 0.49 165d17 540 1.84 d17/C60 3.65 0.89 0.36 1.17 165d18 538 1.78 2.9 � 10�5 (S, N) �6.02/� d18/C60 4.59 1.15 0.28 1.85h 167d19 0.011 (O, N) �5.50/� d19/C60 1.7 0.67 0.3 0.32 168d20 1.78 3.9 � 10�5 (S, N) �5.72/� d20:PC61BM (1 : 2) 5.30 0.87 0.39 1.80i 169d21 1.78 5.6 � 10�5 (S, N) �5.78/� d21:PC61BM (1 : 2) 5.83 1.07 0.31 2.02i 169d22 538 1.9 4.9 � 10�4 (O, N) �5.28/�3.11 d22:PC71BM (1 : 3) 9.51 0.87 0.52 4.3 170d23 563 1.83 �5.22/�3.34 d23:PC71BM (1 : 2) 5.21 0.84 0.308 1.4 171d24 585 1.65 �5.03/�3.42 d24:PC71BM (1 : 2) 7.66 0.88 0.439 3.0 171d25 486 2.14 �5.41/�3.37 d25:PC61BM (1 : 3) 1.66 0.89 0.41 0.61 172d26 541 1.86 4.7 � 10�5 (S, N) �5.3/�3.27 d26:PC61BM (1 : 3) 4.18 0.81 0.39 1.33 173d27 529 1.96 �5.19/�3.08 d27:PC71BM (1 : 3) 8.58 0.85 0.327 2.39 174
a In film. b O and S: measured by OFET or SCLC method, N and B: in neat or blend film. c From electrochemistry unless stated otherwise,
U: from UPS. d Donor/acceptor: bilayer by vacuum deposition; donor:acceptor: blend by solution process unless stated otherwise; vac: vacuum
deposition. e AM1.5, 100 mW cm�2 unless stated otherwise. f 85 mW cm�2. g 90 mW cm�2. h 80 mW cm�2. i 95 mW cm�2.
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Fig. 5 Chemical structure of triphenylamine-based donors.
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Benefiting from its special propeller starburst molecular
structure, three-dimensional, amorphous materials with isotropic
optical and charge-transporting properties could be expected
when combining TPA core with linear p-conjugated arms.
Early in 2006, a series of three-dimensional, star-shaped
molecules with TPA as core for OPVs were firstly reported
by Roncali and coworkers.165–168 They synthesized a series of
star-shaped molecules based on a triphenylamine core modified
by various combinations of thienylenevinylene conjugated
branches and electron-withdrawing indanedione or dicyanovinyl
groups (d14–d18).165 Compared to d14, d15–d18 with electron-
withdrawing end groups showed broadening and red shift
of absorption and IPCE spectra as well as down-shifting of
HOMO levels. Under the same conditions, the PCE of vacuum-
deposited bilayer OPVs increased from 0.46% (d14) to 1.17%
(d17). Optimized devices based on d18 exhibited a PCE of
1.85% with a very high VOC of 1.15 V.167 In 2006, they
synthesized a star-shaped molecule with TPA as core and
terthiophene as arms (d19).168 d19 exhibited high mobility
(0.011 cm2 V�1 s�1) but narrow absorption (lmax = 429 nm),
leading to a low PCE (0.32%). Replacing terminal bithiophene
in d19 with one (d20) or two (d21) dicyanovinyl groups extended
absorption bands and lowered HOMO levels as a result of
intramolecular charge transfer,169 resulting in a large improve-
ment of VOC and JSC. The BHJ OPV devices based on d20
or d21:PC61BM (1 : 2) showed PCEs of 1.80 and 2.02%,
respectively.
Recently, Zhan and coworkers reported a new three-
dimensional, star-shaped, D–A–D small molecule (d22) with
TPA as the core, BT as the bridge and terthiophene as the
arms.170 Relative to its counterpart d19 without BT, d22
exhibited broader absorption as a result of intramolecular
charge transfer. The d22:PC71BM (1 : 2) blend film exhibited
nanoscale aggregated domains, which are beneficial to charge
separation and enhanced efficiency of the OPVs. Without any
post-treatment, the BHJ OPV devices exhibited a JSC of
9.51 mA cm�2, VOC of 0.87 V, FF of 0.52 and PCE of
4.3%. The PCE value of 4.3% is among the highest reported
for solution processed BHJ OPVs based on TPA-containing
small molecules, indicating that d22 is a promising three
dimensional donor molecule for OPVs.
To improve absorption and solution processability of d18,
Li and coworkers replaced thiophene with 4,40-dihexyl-2,20-
bithiophene to give d23.171 Solution-processed BHJ OPVs
based on d23:PC71BM (1 : 2) showed a PCE of 1.4%. To
further extend absorption, they incorporated vinylene bridge
between TPA and bithiophene units in d23 to give d24. The
absorption spectrum of the d24 film covered a broad wavelength
range in the visible region from 380 to 750 nm, which red-shifted
by ca. 40 nm relative to that of the d23 film. BHJ OPVs based
on d24:PC71BM (1 : 2) showed a JSC of 7.76 mA cm�2, VOC of
0.88 V, FF of 0.439 and PCE of 3.0%, better than that of d23.
In 2008, Li and coworkers synthesized a star-shaped, D–A
molecule (d25) with TPA as core, BT as arm and vinylene as
bridge.172 The optimized BHJ device based on d25:PC61BM
(1 : 3) produced a PCE of 0.61%. Attaching vinyl-TPA to d25
as end groups (d26) extended the absorption and the absorp-
tion maximum red shifted from 486 to 541 nm.173 The PCE of
BHJ devices based on d26:PC61BM (1 : 3) reached 1.33%,
higher than that of the devices based on d25 or the corre-
sponding linear molecule. The result indicates that the three
dimensional, star-shaped structure may be beneficial to improving
the photovoltaic performance compared to its linear counterpart.
Subsequently, PCE of the devices was further improved to
2.39% by using PC71BM as an electron acceptor and replacing
the terminal group of vinyl-TPA in d26 with 4-hexylthiophene
(d27).174
In materials based on TPA and its analogs, the D–A–A
structure (e.g., d11 and d12) showed amazing results in PMHJ
OPVs, with PCEs up to 6.4%. Meanwhile, the three-dimensional
pull–push materials based on TPA as core (e.g., d22) also gave
high performance in solution-processed BHJ OPVs with PCEs
up to 4.3%. These results suggest that combining multi
electron-withdrawing units with TPA core to form three-
dimensional D–A–A structure may be one good direction
for developing high performance OPV materials.
Small molecular acceptors
As the rapid development of donor materials, including
polymers and small molecules, PCEs of the OPVs have reached
over 8%. The acceptors are of the same importance as the
donors for high performance OPVs. However, research efforts
devoted to the acceptors are much less than those on the
donors. So far, fullerenes and their derivatives still dominate
the acceptors although nonfullerene-based acceptors have
attracted increasing attention in recent years.
Fullerenes and their derivatives
Fullerenes and their derivatives have been widely used in
bilayer heterojunction and BHJ OPVs largely due to their
strong tendency to accept electrons from donor semiconducting
materials and high electron mobilities in the films even in
composite form.39,40 Additionally, fullerene derivatives readily
form favorable nanoscale morphological network with donors,
which could improve BHJ OPV performance. Table 5 provides
a summary of electronic properties as well as OPV data for
representative fullerene derivatives (Fig. 6).
Generally, C60 (e1) and C70 (e2) were mainly used in vacuum
deposited OPVs as acceptors, while their soluble derivatives
PC61BM (e3) and PC71BM (e4) were mainly used in solution
processed OPVs. C60 was discovered by Kroto et al. in 1985.175
The spherical shape of C60 renders it a good acceptor in any
direction, and this isotropy toward electron transfer is advan-
tageous versus planar molecular structures because it greatly
increases the chance for a beneficial alignment with the donor
p-system. Leo and coworkers obtained an over 25% improve-
ment in device performance in ZnPc:C70 BHJ devices over that
using ZnPc:C60.176 The PCE enhancement from 2.27 to 2.87%
was attributed to an increase in photocurrent (from 7.52 to
9.88 mA cm�2), due to stronger long-wavelength absorption of
C70 as compared to C60, caused by a relaxation of symmetry-
forbidden transitions in C70.
PC61BM was firstly synthesized by Hummelen et al. for the
application in photophysical studies toward improvement of
photoinduced electron transfer efficiencies in the fabrication of
photodetectors and photodiodes.177 PC61BM has much better
solubility in organic solvents than its parent compound C60.
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In 1995, Yu et al. invented the BHJ structure for the OPVs
using soluble PC61BM as acceptor blended with conjugated
polymer donor.2 A deficiency of PC61BM as a photovoltaic
material is its very weak absorption in the visible region, due
to the high degree of symmetry of C60 which causes the lowest-
energy transition formally dipole forbidden. To improve the
visible absorption of PC61BM, Wienk et al. synthesized the
corresponding C70 derivative PC71BM possessing stronger
absorption from 400 to 700 nm than PC61BM.178 Since then,
PC61BM and PC71BM have been widely used in the fabrica-
tion of BHJ OPVs. So far the BHJ OPVs based on polymer
donors and PC61BM or PC71BM acceptors have PCE values
of over 7%.10,179–185 In recent years, some other promising
fullerene derivatives have also been reported.39 PC61BM
analogues with the phenyl ring replaced by thiophene,186 fluorene,
or triphenylamine187 units have been synthesized successfully
Table 5 Optical and electronic properties, mobilities, and OPV performance of fullerene-based acceptors
mea/cm2 V�1 s�1 LUMOb/eV Active layerc JSC/mA cm�2 VOC/V FF PCEd (%) Ref.
e1 �3.9 ZnPc:e1 (vac, 1 : 2) 7.52 0.56 0.543 2.27 176e2 1.3 � 10�3 (O, N) �3.9 ZnPc:e2 (vac, 1 : 2) 9.88 0.56 0.522 2.87 176e5 7 � 10�4 (S, N) �3.7 P3HT:e5 (1 : 1.2) 9.14 0.724 0.68 4.5 188e6 �3.74 P3HT:e6 (1 : 1) 10.61 0.84 0.727 6.48 190e7 �3.72 P3HT:e7 (1 : 1) 10.79 0.86 0.721 6.69 192e8 9.0 � 10�5 (S, B) �3.85 P3HT:e8 (1 : 1.2) 9.05 0.87 0.655 5.2 193e9 2.0� 10�4 (S, B) �3.66 P3HT:e9 (1 : 0.6) 10.3 0.83 0.62 5.3 195e10 4.0 � 10�4 (S, N) �3.63 P3HT:e10 (1 : 1) 8.64 0.81 0.61 4.2 199e11 �3.74 BP/BP:e11/e11 10.5 0.75 0.65 5.2 200e12 1.1 � 10�3 (S, B) �3.75 P3HT:e12 (1 : 1) 10.3 0.81 0.63 5.25 201e13 P3HT:e13 (55 : 45) 11.3 0.65 0.57 4.2 202
a O and S: measured by OFET or SCLC method, N and B: in neat or blend film. b From electrochemistry. c donor:acceptor: blend by solution
process unless stated otherwise; vac: vacuum deposition. d AM1.5, 100 mW cm�2.
Fig. 6 Chemical structure of fullerene derivative acceptors.
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and exhibited PCEs of 4% in solution processed BHJ OPV
devices in combination with regioregular poly(3-hexylthiophene)
(P3HT) donor.
Relatively deep LUMO levels of the PC61BM analogues
lead to VOC loss in OPV devices, and further limit increase in
PCEs. In 2008, Blom and coworkers introduced bisPC61BM (e5),
the bisadduct analogue of PC61BM, as a new fullerene-based
acceptor material in OPVs.188 The cyclic voltammetry measure-
ments showed the LUMO energy level of bisPC61BM was
0.1 V higher than that of PC61BM, so that the device based on
P3HT:bisPC61BM (1 : 1.2) showed a higher VOC of 0.73 V
(0.58 V for PC61BM), ultimately a higher PCE of up to 4.5%
(3.8% for PC61BM).
Li and coworkers developed indene–C60 bisadduct (IC60BA,
e6)189,190 and indene-C70 bisadduct (IC70BA, e7)191,192 which
exhibited 0.17 and 0.19 eV LUMO up-shift, respectively,
relative to PC61BM and PC71BM. Meanwhile, IC60BA and
IC70BA were easier to synthesize and more soluble in common
solvents than PC61BM and PC71BM. The photovoltaic prop-
erties of the fullerene derivatives were studied by fabricating
BHJ OPVs with P3HT as donor and the fullerene derivatives
as acceptor. Thanks to the LUMO up-shift, the VOC of the
devices with IC60BA and IC70BA reached 0.84 V, which was
0.26 V higher than that (0.58 V) of the devices with PC61BM as
acceptor, and the PCE values of the devices were up to 5.44
and 5.64%, respectively, which were over 40% enhancement in
comparison with that (3.88%) of the BHJ OPVs based on
P3HT and PC61BM.189,191 After thermal annealing at 150 1C
for 10 min, BHJ OPVs based on P3HT and IC60BA provided a
PCE of 6.48%.190 By additive (3-hexylthiophene) processing,
BHJ OPVs based on P3HT and IC70BA showed a PCE of up
to 6.69%,192 which is the highest value in the OPVs based on
P3HT reported in the literature so far.
Cheng and coworkers reported a new class of diphenyl-
methano-based C60 bisadduct.193 The plane of the phenyl
groups lying parallel to the fullerene surface sterically protects
and shields the core C60 structure from severe intermolecular
aggregation, rendering it intrinsically soluble, morphologically
amorphous, and thermally stable. The double functionaliza-
tion raises the LUMO energy level (�3.85 eV) by ca. 0.1 eV,
compared to that of PC61BM (�3.95 eV). The BHJ devices
based on P3HT:e8 (1 : 1.2) blend after thermal annealing at
140 1C for 10 min exhibited a JSC of 9.05 mA cm�2, VOC of
0.87 V, FF of 0.655, and a high PCE of 5.2%. Voroshazi
et al.,194 Kim et al.195 and Wang et al.196 independently
reported novel o-xylenyl C60 bisadduct e9 with a higher
LUMO level (�3.66 eV) relative to PC61BM. e9 was success-
fully used as an electron acceptor with P3HT in BHJ OPVs,
showing a high PCE of 5.31% with a high VOC of 0.83 V.195
Endohedral fullerenes were also confirmed to have much higher
LUMO levels than their corresponding empty-cage fullerenes in
theory197 and experiment.198 Ross et al. synthesized a series of
soluble PCBM-like Lu3N@C80 derivatives, and the LUMO
energy level of Lu3N@C80-PCBH (e10) is 0.28 eV higher than
that of PC61BM.199 The BHJ devices based on P3HT:e10 (1 : 1)
after thermal annealing at 110 1C for 10 min displayed high
VOC of 0.89 V and PCE of 4.2%. This result indicates that this
series of endohedral fullerene derivatives might be another
type of high VOC acceptor for application in OPVs.
Matsuo et al. synthesized a new fullerene derivative with
Si-containing side chains (e11), which has good thermal stability
and LUMO up-shift relative to PC61BM.200 They fabricated
OPVs with a three-layer structure: an interdigitated BHJ layer
of tetrabenzoporphyin (BP):e11 sandwiched by BP donor and
e11 acceptor layers. The JSC, VOC, FF and PCE of the best
device reached 10.5 mA cm�2, 0.75 V, 0.65 and 5.2%, respec-
tively. Recently, Sharma and coworkers synthesized a new
fullerene derivative (e12) from PC61BM; e12 displayed better
solubility in common solvents and stronger absorption in the
film than PC61BM.201 After optimizing mixed solvents and
subsequent thermal annealing, the best device with P3HT and
e12 showed a JSC of 10.3 mA cm�2, VOC of 0.81 V, FF of 0.63,
and PCE of 5.25%. Frechet and coworkers synthesized a
dihydronaphthyl fullerene benzyl alcohol benzoic acid ester
(e13);202 the best BHJ OPV based on P3HT:e13 (55 : 45) after
thermal annealing at 150 1C for 30 min exhibited a JSC of
11.3 mA cm�2, VOC of 0.65 V, FF of 0.57, and PCE of 4.2%.
Without question, fullerene derivatives are the most successful
acceptors in the OPVs so far. During the last decade, significant
advances have been made in fullerene-based acceptors, appro-
priate chemical modifications have up-shifted their inherent
LUMO energy levels, and the cost of fullerene production has
also decreased along with an increase in synthetic yields and
material purity. Nevertheless, there remain incentives to develop
nonfullerene acceptors that will not only retain the favorable
electron accepting and transporting properties of fullerenes, but
also overcome their insufficiencies such as the limited spectral
breadth and bandgap variability.
Rylene diimides
On the basis of recent developments in high-performance
electron transporting materials for OFETs, some research
groups have begun investigating nonfullerene acceptors for
use in OPVs. Rylene diimides have attracted interest as alter-
native acceptor materials since they exhibit excellent photo-
stability, easy alteration of HOMO and LUMO energies, large
absorption coefficients, high electron mobilities, electron affi-
nities similar to those of fullerenes and each of these properties
can be readily tailored through either variation of substituents
on the imide nitrogen atoms or on the rylene core.203–206
Table 6 provides a summary of electronic properties as well
as OPV data for representative rylene diimide related acceptors
(Fig. 7).
Perylene diimides (PDIs) are among the earliest and most
common nonfullerene acceptors investigated in OPVs. Many
early studies of OPVs incorporating PDIs consisted of layered
structures fabricated by vacuum deposition. The first bilayer
heterojunction OPV was reported by Tang where a PDI-based
small molecule (f1) was used as the acceptor along with CuPc
as the donor.1 When ZnPc was employed as a donor in place
of CuPc in a simple bilayer device, the PCE was improved
from 0.95% to 1.3%.207 Later, the cis-isomer f2 was prepared,
and a bilayer heterojunction device based on CuPc/f2 showed
a PCE of 0.93%, slightly lower than that of f1 (1.1%).208 The
lower efficiency was attributed to less efficient packing in
acceptor f2, leading to shorter exciton diffusion lengths in
the system.
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BHJ OPVs based on solution processed blends of PDI-based
materials with appropriate donor materials are attracting increasing
attention. Solution processable PDI-based acceptors can be
prepared by introducing solubilizing groups, such as alkyl, on
the imide nitrogen atoms. 3-Pentyl substituted PDI (f3) was
prepared and blended with a variety of donors such as P3HT209
and polycarbazole210 to fabricate OPVs by spin coating. By
careful control over the solvent, substrate temperature during
deposition, and annealing temperature and time, the devices
based on P3HT:f3 (1 : 4) film showed a JSC of 1.65 mA cm�2,
VOC of 0.45 V, FF of 0.34, and PCE of 0.25%.209 Nonetheless
the tendency for f3 to form crystalline domains within polymer
matrices limited the efficiencies of BHJ devices since the crystal-
line domains acted as electron traps to decrease photocurrents.
PDIs with longer alkyl substituents on the imide nitrogen
atoms have also been prepared, and when blended with P3HT
gave low efficiencies. For instance, a maximum PCE of 0.18%
was achieved with a P3HT:f4 weight ratio of 1 : 4 after
annealing at 80 1C for 1 h.211 Along with varying substituents
on the imide nitrogen atoms, recent studies have also examined
the impact of substituents in the bay regions of the PDIs on thin-
film morphology and device performance.211,212 Although the
PDIs with electron donating or withdrawing substituents have
tuned the LUMO energy level over a range of nearly 0.7 eV and
impacted on device VOC, none of these PDI bay-substituted
derivatives yielded higher performance than simple PDIs
without bay-region substituents.211 Very recently, Laquai
et al. demonstrated that the photovoltaic characteristics of
blend films of P3HT and PDIs are improved upon using a
core-alkylated PDI derivative (f5) instead of the often used
N-alkylated.212 The alkyl-substitution pattern affected the
packing of the PDI and improved the blend aggregation.
Table 6 Optical and electronic properties, mobilities, and OPV performance of rylene diimide-based acceptors
lmaxa/nm Eg
opt/eV meb/cm2 V�1 s�1 HOMOc/LUMO/eV Active layerd JSC/mA cm�2 VOC/V FF PCEe(%) Ref.
f1 ZnPc/f1 1.3 207f2 CuPc/f2 3.66 0.93f 208f3 530 �5.8/�3.8 P3HT:f3 (1 : 4) 1.65 0.45 0.34 0.25 209f4 542 2.13 �5.82/�3.69 P3HT:f4 (1 : 4) 1.32 0.36 0.38 0.182 211f5 P3HT:f5 (1 : 1) 1.74 0.75 0.38 0.50 212f6 8.8 � 10�4 �6.0/�3.85 X:f6 (1 : 1) 6.8 0.88 0.47 2.85 213f7 5.6 � 10�4 �5.9/�3.8 Y:f7(1 : 3.5) 6.3 0.95 0.53 3.17 214f8 455 2.13 4.6 � 10�4 �5.90/�3.95 Z:f8 (1 : 1) 8.30 0.90 0.52 3.88 215f9 �5.78/�3.87 ZnPc/f9 2.11 0.50 0.51 0.54 216f10 �5.5/�4.1 P3HT/P3HT:f10 (1 : 1)/f10 3.51 0.82 0.52 1.50 217
a In film. b Measured by SCLC method in blend film. c From electrochemistry. d Donor/acceptor: bilayer by vacuum deposition; donor:acceptor:
blend by solution process. e AM1.5, 100 mW cm�2. f 94 mW cm�2.
Fig. 7 Chemical structure of rylene diimide acceptors.
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The optimized device based on P3HT:f5 exhibited a PCE of
0.5%, with JSC of 1.74 mA cm�2, VOC of 0.75 V, and FF
of 0.38.
PDI-based small molecular acceptors had no promising
results in OPVs, until Sharma and coworkers developed a series
of high-performance acceptors by attaching tert-butylphenoxy
groups to the bay-region of PDIs and varying the imide
substituent.213–215 The acceptors f6–f8 with different fused-
ring substituents on the imide exhibited electron mobilities
(Measured by SCLC) of 4–9� 10�4 cm2 V�1 s�1 when blended
with small molecular donors X, Y, or Z (Fig. 7). The devices
based on X:f6, Y:f7, and Z:f8 blends gave PCEs of 2.85%,213
3.17%,214 and 3.88%,215 respectively, after annealing and/or
inserting a ZnO layer between the active layer and the cathode.
The PCE of 3.88% is the highest ever reported for nonfullerene-
based BHJ OPVs.
Compared to the PDI-based molecules, the smaller fused-
ring unit naphthalene diimide (NDI) derivatives were less
successful as acceptors in OPVs, because they possess a larger
bandgap and thus absorb poorly in the visible spectrum
(generally at onset less than 400 nm). For example, bilayer
OPVs based on ZnPc/f9 exhibited a PCE of 0.54%, lower
than that (1.3%) of its analog, f1.216 Recently, Jenekhe and
coworkers reported a new nonfullerene acceptor (f10) with
NDI as core and oligothiophene as arms used in BHJ OPVs.
After annealing at 100 1C for 10 min and adding 0.2%
diiodooctane additive, the optimized P3HT:f10 based device
showed a PCE of 1.5%.217,218
Although rylene diimides have strong absorption and high
electron mobility, the performance of OPVs using rylene
diimides as acceptors do not yet rival those of fullerene-based
systems. The planar shapes of rylene diimides may result in
enhanced p-stacking and more quasi one-dimensional electron
transport, compared to the spherical/ellipsoidal molecular
shapes of fullerenes. So when rylene diimide-rich phases do
not have high degrees of long-range order, there can be
multiple orientations of p-stacked phases, which may decrease
long-range mobilities and therefore act as recombination
centers. In this system, introducing bulky side chains can
decrease the molecular planarity and may be helpful to
improve the photovoltaic performance.
Other nonfullerene acceptors
Apart from the rylene-based acceptors, some other n-type
molecules have been reported as the nonfullerene acceptors
for OPVs. Table 7 provides a summary of electronic properties
as well as OPV data for representative other nonfullerene
small molecular acceptors (Fig. 8). One method of increasing
the n-type character of molecules is to introduce electron-
withdrawing units, such as fluorine and cyano, to the periphery
of the aromatic rings.219 For example, hexadecafluorinated CuPc
g1 showed electron mobilities of up to 5 � 10�3 cm2 V�1 s�1 in
OFETs.220,221 When incorporating g1 in bilayer heterojunction
OPVs as an acceptor and sexiphenyl (p-6P) as a donor, a PCE
of 0.18% was obtained.222 Using subphthalocyanine (SubPc)
instead of p-6P led to improved PCE of 0.56%.223 Torres and
coworkers synthesized a series of fluorinated SubPc as acceptors
in OPVs.224 Bilayer OPV devices of the acceptors with a variety
of donors, such as pentacene, CuPc, AlClPc, SubPc and SubNc,
were prepared by vacuum evaporation. In these devices, the
SubPc/g2-based device showed the highest PCE of 0.96%, with
JSC of 2.1 mA cm�2, VOC of 0.94 V, and FF of 0.49. However,
the PCE value of 0.96% was obviously lower than that (3.0%)
of SubPc/C60-based control device. The moderate efficiencies
could be limited by the low electron mobility of the amor-
phous fluorinated SubPc as well as series resistance effects in
the active layer. Jones and coworkers found that replacing
fluorinated SubPc (g2) with chlorinated SubPc (g3) led to
Table 7 Optical and electronic properties, mobilities, and OPV performance of other nonfullerene-based acceptors
lmaxa/nm Eg
opt/eV meb/cm2 V�1 s�1 HOMOc/LUMO/eV Active layerd JSC/mA cm�2 VOC/V FF PCEe (%) Ref.
g1 780 1.5 p-6P/g1 0.96 0.42 0.18 222�6.4/�4.9 SubPc/g1 2.54 0.40 0.55 0.56 223
g2 SubPc/g2 2.1 0.94 0.49 0.96 224g3 SubPc/g3 3.53 1.31 0.58 2.68 225g4 710 �5.7/�3.95 SubPc/g4 7.8 0.95 0.54 4.0 226g5 1.82 �5.29/�3.50 P3HT:g5 (1 : 1) 3.72 0.84 0.41 1.29 228g6 700 �5.47/�3.64 P3HT:g6 (1 : 1) 1.93 0.54 0.41 0.43 227g7 P3HT:g7 (1 : 1) 2.44 0.95 0.43 1.00 228g8 P3HT:g8 (1 : 1) 3.17 0.80 0.50 1.26 228g9 620 �5.34/�3.21 P3HT:g9 (5 : 3) 1.93 1.05 0.39 0.80 229g10 P3HT:g10 (1 : 1) 0.24 0.93 0.21 0.06f 232g11 �6.27/�3.92 P3HT:g11 (1 : 1) 0.3–0.4f 233g12 490 1.15 � 10�5 (S) �5.47/�3.42 P3HT:g12 (1 : 1) 0.66 0.78 0.27 0.14 235g13 640 1.8 1.14 � 10�4 (S) �5.9/�4.1 P3HT:g13 (1 : 1) 5.72 0.48 0.57 1.57 236g14 475 �6.2/�3.6 P3HT:g14 (1 : 2) 2.36 0.62 0.50 0.73 237g15 2.40 �6.02/�3.50 P3HT:g15 (1 : 1) 1.79 0.67 0.37 0.45 240g16 �6.0/�3.6 P3HT:g16 (1 : 1) 3.00 0.76 0.48 1.10 241
POPT:g16 (1 : 1) 5.50 0.62 0.40 1.40 241g17 2.34 �5.77/�3.35 P3HT:g17 (1 : 1.4) 4.7 0.96 0.56 2.54 244g18 1.81 �5.26/�3.52 P3HT:g18 (1 : 2) 2.36 0.71 0.52 1.00 245g19 580 1.81 3 � 10�3 (O) �5.9/�4.09 P3HT:g19 (1 : 1) 1.93 0.52 0.31 0.31 246g20 410 �6.1/�3.5 P3HT:g20 (1 : 2) 4.40 0.76 0.56 1.86 247g21 517 2.18 �5.17/�3.24 P3HT:g21 (1.5 : 1) 3.9 1.1 0.4 1.7 248
a In film. b O and S: measured by OFET or SCLCmethod in neat film. c From electrochemistry. d Donor/acceptor: bilayer by vacuum deposition;
donor:acceptor: blend by solution process. e AM1.5, 100 mW cm�2. f 80 mW cm�2.
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improved performance; SubPc/g3-based device showed pro-
mising PCEs of 2.68% with very high VOC of 1.31 V.225 After
selective halogenation to tune the energy levels of SubPc, g3
provided sufficient interfacial HOMO and LUMO offsets for
efficient exciton dissociation, whilst maximizing the interface
gap. Recently, Verreet et al. reported fluorinated fused SubPc
dimer (g4, mixture of two isomers) with strong and comple-
mentary absorption to the donor material SubPc.226 The
optimized SubPc/g4-based bilayer device exhibited a high
PCE of 4%, with JSC of 7.8 mA cm�2, VOC of 0.95 V, and
FF of 0.54. The PCE of 4% is the highest ever reported for
nonfullerene-based bilayer OPVs.
Fig. 8 Chemical structure of other nonfullerene acceptors.
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4266 Chem. Soc. Rev., 2012, 41, 4245–4272 This journal is c The Royal Society of Chemistry 2012
Although pentacene and its derivatives belong to a promising
class of donors in OPVs, Anthony and coworkers demonstrated
that it is possible to switch them from donors to acceptors
in OPVs by lowering their LUMO energy levels through
cyanation.227 Cyanopentacenes g5 and g6 with tricyclopentylsilyl
groups have been synthesized;227,228 the energy levels can be
tuned by varying the number of cyano groups, while the
trialkylsilyl groups control crystal packing and filmmorphology.
They found that the HOMO and LUMO energy levels down-
shifted by ca. 0.14 eV for every cyano group that was introduced
to the pentacene core, and a particularly strongly one-dimensional
‘‘sandwich herringbone’’ crystal packing motif yielded devices
with the highest photocurrent. In the initial devices, blended
with P3HT donor, dicyanopentacene g6 exhibited a best PCE
of 0.43%, with JSC of 1.93 mA cm�2, VOC of 0.54 V, and FF of
0.41. Reducing the number of cyano from 2 (g6) to 1 (g5) led
to a higher performance: JSC of 3.72 mA cm�2, VOC of 0.84 V,
FF of 0.41, and PCE of up to 1.29%.228 The 0.3 V improve-
ment in VOC was attributed to an up-shift of LUMO energy
level caused by a decrease in cyano number. Subsequently,
several other pentacene derivatives with a single electron
withdrawing group were investigated, and the better acceptors
were found to be chloropentacene g7 and trifluoromethylpentacene
g8 with PCE of 1.00% and 1.26%, respectively, when blending
with P3HT.228 Recently, anthradithiophene was also converted
into acceptor for OPVs, through the introduction of amide
groups.229 In most literature work, anthradithiophene deriva-
tives were studied as a mixture of syn and anti isomers due to
the great challenge in the preparation of isomerically pure
materials. However, amide groups caused different self-assembly
of the syn- and anti-isomers that allowed for the first time
separation and property evaluation of isomerically pure
anthradithiophenes. Anthony and coworkers evaluated the
acceptor properties of two pure isomers and their mixture in
BHJ OPVs with P3HT as donor; syn-isomer g9 yielded much
better PCE (0.8%) than that of anti-isomer (0.008%) or their
mixture (0.09%) after annealing at 120 1C for 1 min. For
anti-isomer, larger scale aggregation was observed in the blend
film, while syn-isomer:P3HT blend showed a more uniform
coverage textured with small grains of the acceptor. The
difference in blend film morphology is responsible for the
difference in OPV performance.
As same as CuPc and pentacene, oligothiophenes have also
been widely employed as donors in OPVs. However, thienyl-
S,S-dioxide caused a significant increase in electron affinity of
the oligothiophene system.230–234 Barbarella et al. developed a
series of linear and branched thiophene-S,S-dioxide-containing
oligothiophene acceptors for OPVs.232,233 In these studies, due to
the propensity for linear oligothiophene acceptors to crystallize
and form aggregates leading to reduced D/A interfacial area, BHJ
OPVs based on all linear acceptors blending with P3HT showed
very low PCEs, and the asymmetric molecule g10 gave a PCE of
0.06%, higher than the others.232 To suppress crystallization,
V-shaped acceptors were synthesized. Owing to the branched
structure lacking any symmetry elements, these V-shaped
acceptors displayed a low tendency to crystallization and better
film forming properties than their linear counterparts.233 Thus
for the acceptor g11, devices fabricated from a blend of P3HT
yielded estimated PCE of 0.3–0.4% by thermal annealing.233
Cyano is an excellent electron-withdrawing unit in design of
donor and acceptor materials for OPVs. As with the systems
described above, carbazole has also been modified as an
acceptor through cyanation.235 The molecule g12 substituted
with two DCV groups showed broad absorption with the onset of
about 600 nm. By controlling blend ratio, solvent and annealing
conditions, the optimized devices with P3HT:g12 showed a PCE
of 0.14%. The relatively low PCE of OPVs based on g12 is due to
its low electron mobility of 1.15 � 10�5 cm2 V�1 s�1 estimated
by the SCLC method.
Wang and coworkers developed DCV substituted quinacridone
derivatives as acceptors for BHJ OPVs.236 These molecules
exhibited remarkable absorption in the region from 650 to
700 nm. In particular, molecule g13 exhibited a LUMO energy
level of �4.1 eV, small bandgap (1.8 eV) and moderate
electron mobility (1.14 � 10�2 cm2 V� s�1 by SCLC). A device
based on P3HT:g13 exhibited a PCE of 1.57%, with JSC of
5.72 mA cm�2, VOC of 0.48 V, and FF of 0.57. However, the
relatively deep LUMO energy level of g13 led to a significant
loss of photovoltage.
Meredith et al. combined fluorene, BT and DCV together to
synthesize a new nonfullerene acceptor (g14), which can be
processed by vacuum deposition or solution processing to give
amorphous thin films and can be annealed at a modest
temperature to give films with better order and enhanced
charge transport properties.237 The P3HT:g14 BHJ device
spun cast from dichlorobenzene showed a low PCE of
0.04%, and after annealing at 65 1C for 20 min, the PCE
was improved to 0.73%, which can be attributed to an increase
in electron mobility in the acceptor phase and improvement in
the charge percolation network to the contacts.
Recently, a series of substituted dicyanoimidazole deriva-
tives known as Vinazene have received focused attention as
acceptors in BHJ OPVs.238–243 The series of Vinazene-based
molecules displayed a wide range of electronic properties, with
LUMO energy levels ranging from �2.76 to �3.60 eV.238,239
Of these molecules, g15 and g16 containing BT core gave the
most efficient devices when blended with P3HT in BHJ
OPVs.238–241 After thermal annealing, g15 and g16 based
devices showed a PCE of 0.45%240 and 1.1%,241 respectively.
For g16, a further improvement in efficiency can be achieved
by employing an octylphenyl-substituted polythiophene
(POPT) as the donor. The optimized device at a 1 : 1 blend
weight ratio of POPT:g16 yielded a JSC of 5.5 mA cm�2, VOC
of 0.62 V, FF of 0.4, and PCE of 1.4%.241 The higher
performance of POPT could be explained by the ability of
the conjugated phenyl substituent to twist out of planarity,
resulting in a high dissociation efficiency. Very recently,
Sellinger et al. synthesized a series of electron-deficient molecules
based on a central BT moiety flanked with vinylimides and
used for solution processed BHJ OPVs as acceptors.244 Of
these acceptors, phthalimide-BT based molecule (g17) showed
promising photovoltaic performance. BHJ OPVs based on
P3HT:g17 (1 : 1.4) yielded a high VOC of 0.96 V and a maximum
PCE of 2.54%, whereas naphthalimide-BT analog only afforded
0.1% PCE. The high VOC is attributed to the high LUMO level
(�3.3 eV) of g17.
DPP-based materials have been proven promising donors
for OPVs; blended with PC71BM acceptors, they yielded PCEs
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up to 4.4%.93 Interestingly, a variety of DPP derivatives with
electron withdrawing end capping groups (such as aldehyde,
trifluoromethylphenyl and trifluorophenyl) were tested as
acceptors in OPVs.245,246 The inherent electron affinity of
DPP moiety was further improved through the introduction
of electron withdrawing groups. Among these compounds,
g18 provided the highest PCE of 1.0%, when blended with
P3HT donor in a BHJ device,245 while aldehyde substituted
DPP g19 showed a lower PCE of 0.31%.246
Very recently, Pei et al. modified the fluoranthene-fused
imide scaffold to develop a new electron acceptor g20 by
combining with a cyano substituent group.247 The investigation
of the photophysical and electrochemical properties indicates
that LUMO level (ca. �3.5 eV) of g20 both matches the work
function of the LiF/Al cathode and increases VOC of OPV
devices. The preliminary results showed that by thermal
annealing at 100 1C, the solution-processed BHJ OPVs based
on P3HT:g20 (1 : 2) gave a PCE of 1.86%, with relatively high
VOC of 0.76 V, compared to that (0.58 V) of the control device
based on P3HT:PC61BM blend.
Wudl and coworkers reported the use of a 9,90-bifluorenylidene
polycycle for small molecule acceptors.248,249 The 9,90-bifluorenyl-
idene structures are particularly effective at stabilizing a negative
charge, due to both steric and electronic effects. But the steric
effect is relieved when the molecule receives an electron and gains
aromaticity. Meanwhile, the LUMO energy level of this system is
easily tuned by substitution on the aromatic periphery. When
blended with P3HT, the asymmetric molecule g21 showed a
decent PCE of 1.7% and a high VOC of 1.10 V.248
Chemical functionalization allows control over absorption
profile and energy levels of nonfullerene acceptors. However,
compared to fullerenes and their derivatives, nonfullerene
acceptors showed lower performance in OPV devices, especially
solution processed BHJ OPVs. The hurdles that remain arise
from active layer morphology and charge transport issues.
While many of these materials showed good electron mobilities
when measured in neat films, the mobilities in blends with
the donors were generally not as high. The next step in
the development of new nonfullerene acceptors may involve
developing multi-dimensional structures.
Donor–acceptor dyad molecules for
single-component OPVs
For BHJ OPVs, one of the main problems is fine tuning the
complicated physical interactions between donor–donor,
acceptor–acceptor, and donor–acceptor to obtain an ideal and
stable morphology with a well-defined nanostructure. To solve
this problem, acceptors, such as fullerene, PDI, DCV etc. were
attached to donor molecules, such as oligothiophene, oligo-
phenylenevinylene, and triphenylamine, as pendant side chains
or end groups to form donor–acceptor dyad molecules (Fig. 9),
which can be regarded as a molecular heterojunction and used
to fabricate single-component OPVs. Such a structure facilitates
exciton dissociation and homogeneous distribution to prevent
severe phase separation. In recent years, the donor–acceptor
dyad molecules showed some promising performance.250
Hashimoto and coworkers developed a series of fullerene-
dyad molecules for single-component OPVs. In 2009, they
reported synthesis of an oligophenylenevinylene containing
five phenyl rings with C60 connected to the middle phenyl ring
in the conjugated chain by a polyether linker (h1).251 After
thermal annealing at 150 1C for 1 min under inert atmosphere,
the h1-based single-component OPV device gave JSC of
3.30 mA cm�2, VOC of 0.88 V, FF of 0.44 and PCE of
1.28% under white light irradiation at 100 mW cm�2. More
recently, the same group reported an oligothiophene contain-
ing 8 thiophene rings and an electron-withdrawing DPP unit in
the middle with C60 connected to the DPP unit by a polyether
linker (h2).252 h2 showed a low bandgap and the photocurrent
response was extended to 850 nm, so that a h2-based single-
component OPV device gave a high JSC (4.79 mA cm�2). This
high JSC together with VOC of 0.51 V and FF of 0.46 led to a
total PCE of 1.1%.
Fig. 9 Chemical structure of donor–acceptor dyads.
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In 2009, Geng et al. synthesized a series of extended D–A
cooligomers in which substituted fluorene-alt-bithiophene
oligomer was selected as donor block and PDI as acceptor
block.253 They found that solvent vapor annealing significantly
improved the order of the cooligomers with an increase of the
persistent length of the lamellae. After solvent annealing, h3-based
single-component OPV device showed a JSC of 4.49 mA cm�2,
VOC of 0.87 V, FF of 0.38 and PCE of 1.50%, which is the best
value reported for a single-component OPV device so far.
TPA-DCV-based star-shaped D–A molecule (d17, Fig. 5)
reported by Roncali et al. also exhibited promising results in
single-component OPVs: JSC of 1.7 mA cm�2, VOC of 0.70 V,
FF of 0.30, and PCE of 0.40%.167
The fast charge recombination and inefficient charge hopping
and transport in single-component OPVs is probably responsible
for the relatively lower PCEs of single-component OPVs
compared to donor/acceptor blend or bilayer solar cells.17
Conclusions and outlook
We have reviewed some representative and promising small
molecule donors, acceptors, and donor–acceptor dyad systems
for high-performance bilayer, BHJ, and single-component
OPVs. So far a number of small molecular OPV materials
have been discovered, but most of these materials give relatively
low PCEs. With extensive research and accumulated practical
experience that we have outlined in this review, the guidelines
to pursuing and developing high-performance small molecules
for OPVs are becoming legible. The basic requirements of
specific intrinsic properties necessary for an ideal small
molecular donor, acceptor or donor–acceptor dyad materials
include: (a) a low optical bandgap for broad absorption range
matching with solar spectrum and high extinction coefficient
for capturing more solar energy; (b) long exciton diffusion
lengths for effective migration of excitons to D/A interface;
(c) high hole or electron mobility for accelerating charge
transport, which in turn allows a thicker active layer required
for increased light harvesting, as well as reduces charge
recombination and series resistance; (d) suitable HOMO/LUMO
energy levels to ensure a large VOC and a downhill energy
offset for exciton dissociation. Additionally, easy sublimation,
relatively low molecular weight, excellent thermal stability are
also necessary for vacuum deposited materials, while good
film-forming property and sufficient solubility are also neces-
sary for solution processed materials. Meanwhile, since the
optimal OPV material itself is device application and system
specific, for bilayer heterojunction OPVs, high crystallinity
of the active materials may be beneficial to improve the
device performance. On the other hand, for BHJ OPVs, it is
also critical for the active materials that formation of a
bicontinuous interpenetrating network with the optimum
morphology for building two distinct highways for trans-
porting free charge carriers; moderate crystallinity of materials
can improve charge carrier mobility, but high crystallinity
will cause large phase separation scale and low PCE of
devices. Generally, low PCEs in OPV devices may be attrib-
uted to the OPV materials violating one or some of the above
criteria, or the device configuration and fabrication below
optimization.
High-efficiency OPVs have been achieved as a result of
innovations of small molecular materials and device fabrica-
tion technology, boding a bright future for this exciting
research field. For further improving the device performance
by modifying the molecular structure of active materials, it is
necessary that deeply understanding the relationships between
chemical structures and optical, electronic and device properties.
To summarize:
(1) Enhancing and extending the absorption of active materials
to match solar radiation is one of the main ways to improve
JSC and efficiencies of OPV devices. An OPV molecule with
bandgapo 2 eV is necessary but not sufficient for a high PCE.
The oligomers with long conjugation length generally possess
broad and strong absorption. Building the push–pull structure
with electron donating and withdrawing units can create
the intramolecular charge transfer and in turn improve the
molecular absorption. Molecules and oligomers based on
fused-ring blocks such as acenes, dithienocyclopentadiene,
dithienosilole, dithienopyrrole and benzodithiophene could
exhibit relatively low Eg and broad absorption.
(2) Since the VOC of devices is related directly to the
difference between the HOMO level of the donor and the
LUMO level of the acceptor, a lower HOMO of a donor and a
higher LUMO of an acceptor would help to achieve a higher
VOC. In general, a donor with a HOMO below �5.3 eV or an
acceptor with a LUMOabove�3.7 eV tends to giveVOC4 0.8 V.
The energy levels can be tuned by changing the species and
number of substituent group. In general, introducing the
electron-withdrawing units, such as cyano and fluorine etc.,
would down-shift the LUMO levels of materials. On the contrary,
the electron-donating units such as TPA and thiophene help to
raise the HOMO levels.
(3) The high hole mobility of donors and electron mobility
of acceptors (4 10�2 cm2 V�1 s�2) would help to achieve a
high JSC. Molecules based on fused-ring blocks tend to exhibit
relatively high mobilities. Crystal structure and p–p stacking in
the film are the key factors to determining the mobility of
materials. In addition, the mobilities matching of donor and
acceptor benefit also FF and PCE of devices.
(4) For solution processing, the use of long alkyl or alkoxy
side chains has been a common approach. However, the side
chain nature not only affects the solubility, configuration and
intermolecular interaction of the molecules, and also affects
the absorption, energy levels and charge transport properties.
In particular, the side chain affects morphology of blend films
and finally affects the photovoltaic performance of devices.
It should be stressed that increasing the content of insulating
alkyl side chains relative to the conjugated main chain in the
molecule may result in deterioration in charge transport.
Thereby, balanced choice of a suitable solubilizing group at
an appropriate location is of crucial importance for fine-tuning
the structure–properties relationship.
(5) The morphology and phase separation scale in BHJ film
is not only related to the nature of materials, but also can be
optimized by carefully controlling the device fabrication
conditions, such as varying spin-casting solvent and speed or
donor/acceptor radio and concentration, thermal annealing,
exposure to solvent vapor, modifying the surface energy of
the blend component and substrate. With optimal morphology
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on the nanoscale, it becomes easy to translate the microscopic
intrinsic properties of the photovoltaic materials into macro-
scopic OPV device performance.
(6) Merocyanine (MC) dye and TPA-based D–A–A molecule
donors in combination with C60 or C70 acceptors exhibited
very high performance in vacuum deposited OPVs, while one-
dimensional oligothiophenes with push–pull structures in
combination with PC61BM or PC71BM exhibited very high
performance in solution processed BHJ OPVs; the best PCEs
were up to 4 6%. Fused acenes exhibited relatively low
efficiencies in vacuum deposited or solution processed OPVs
(o 3%). Nonfullerene acceptors (PCE o 4%) do not yet rival
fullerene-based systems in vacuum deposited or solution processed
devices, while donor–acceptor dyad molecular heterojunction
(PCE o 1.5%) does not yet rival donor/acceptor bilayer
or bulk heterojunction in vacuum deposited or solution
processed devices.
(7) It should be emphasized that the PCE is more a device
parameter than an intrinsic photovoltaic material parameter.
High efficiency achievement is a systematic combination of
material properties with judicious and careful optimization of
the various device fabrication conditions. So far the best
performances (4 6%) of OPVs were achieved from vacuum
or solution-deposited small molecule donors with fullerene
acceptors. An interdisciplinary approach such as novel photo-
voltaic materials and new advanced device concepts will
probably bring high efficiency over 10% and low cost OPVs
to final commercialization.
Acknowledgements
This work was supported by NSFC (Grants 21025418,
51011130028, 21021091), 973 Project (Grant 2011CB808400),
and the Chinese Academy of Sciences.
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