2012-small molecule semiconductors for high-efficiency organic photovoltaics

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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http://dx.doi.org/10.1039/c2cs15313k

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http://pubs.rsc.org/en/journals/journal/CS?issueid=CS041011

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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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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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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Fig. 2 Chemical structure of dye-based donors.

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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


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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Fig. 4 Chemical structure of oligothiophene donors.

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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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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


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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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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