Showcasing work from the group of Osaka University andNara Institute of Science and Technology (NAIST), Japan.
Impact of substituents on the performance of smallmolecule semiconductors in organic photovoltaic devices
via regulating morphology

As featured in:Volume 10
Number 4
28 January 2022
Pages 1149–1534

Journal of

Materials Chemistry C
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A review on substituent engineering of small-moleculeorganic semiconductors for photovoltaic applications. The
relationship between the structure of electronically inert
peripheral substituents and the resultant photovoltaic
efficiency is highlighted.
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REVIEW

Cite this: J. Mater. Chem. C, 2022,
10, 1162

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Impact of substituents on the performance ofsmall-molecule semiconductors in organic
photovoltaic devices via regulating morphology
Mitsuharu Suzuki,*a Kanta Suzuki,a Taehyun Wonb and Hiroko Yamada*c

Recent years have witnessed a rapid development of organic photovoltaic devices (OPVs). The significant
improvement in power conversion eﬃciency is due to not only the discovery of new p-conjugatedframeworks, but also careful substituent engineering to achieve optimal morphology in bulk-heterojunction
Received 7th September 2021,
Accepted 6th December 2021
DOI: 10.1039/d1tc04237h

active layers. Indeed, all aspects of the light-to-electricity conversion in OPVs, such as light absorption,
exciton diﬀusion, and charge-carrier transport, are influenced by the morphological characteristics of active
layers. The importance of active-layer morphology on the performance of OPVs has made substituent
engineering an increasingly important part of OPV semiconductor design. Herein, we overview recent
prominent examples of substituent engineering, focusing on flexible substituents that regulate morphology,

rsc.li/materials-crather than the molecular electronic structure, of small-molecule organic semiconductors.

1. IntroductionaDepartment of Applied Chemistry, Graduate School of Engineering,Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
E-mail: msuzuki@chem.eng.osaka-u.ac.jpbDivision of Applied Science, School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japanc
Division of Materials Science, Graduate School of Science and Technology,
Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma,
Nara 630-0192, Japan. E-mail: hyamada@ms.naist.jp

Mitsuharu Suzuki received his
Bachelor’s (2000) and Master’s(2002) degrees from Kyoto
University (Japan). After working
in Mitsubishi Gas Chemical
Company, he moved to theUnited States and received his
PhD in chemistry from Universityof California, Los Angeles in 2011.
He then worked as a postdoctoral
fellow at University of California,
Los Angeles then Lawrence
Berkeley National Laboratory,
Mitsuharu Suzuki
before joining Nara Institute of
Science and Technology (Japan) as an assistant professor in 2013.
Since 2018, he has been an associate professor at Osaka University.
His current research focuses on the design, synthesis, and controlledassembly of organic compounds for optoelectronic applications.

1162 | J. Mater. Chem. C, 2022, 10, 1162–1195

Organic photovoltaic technologies have experienced a marked
development in recent years,1 achieving an excellent power
conversion eﬃciency (PCE) of 19% in a single-junction cell.2
This progress was largely enabled by the creation of neworganic semiconductors (SCs) comprising a meticulously
designed p-conjugated backbone, as typified by recent nonfullerene n-type (or acceptor) SCs bearing a highly ring-fusedcore unit flanked by strongly electron-withdrawing end

Kanta Suzuki received hisbachelor’s degree in Science from
University of Hyogo in 2020. Since
2020, he has been working as a
graduate student with Profs. Kenichi Nakayama and Mitsuharu
Suzuki at Osaka University. His
research work focuses on thedesign and synthesis of organic
semiconductors for photovoltaic
applications.
Kanta Suzuki

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Review

Journal of Materials Chemistry Cgroups.3–8 In general, active layers of organic photovoltaic
devices (OPVs) are composed of p-type (or donor) and n-type
SCs to form the p/n interface (i.e., the heterojunction). The
photovoltaic process in OPVs starts with light absorption byeither the p- or n-type material to form excitons, which thendiffuse into the p/n interface and form a charge transfer (CT)state. The CT state is generated by the electron or hole transfer
process between the p- and n-type materials, and in an ideal
sequence these CT states dissociate into free electrons andholes to be transported towards and extracted at the respective
electrodes. Thus, strong light absorption over a wide range of
wavelengths and efficient generation of CT states at the p/ninterface, which can both be achieved by tuning the molecularorbital (MO) energy level, are the main concerns in designing
SCs for OPVs. This MO tuning is mainly linked to the structureof p-backbones.
Another critical concern is related to the morphology ofactive layers.9–13 Herein, the term ‘morphology’ is defined as awide range of structural factors of active layers including
molecular orientation and intermolecular arrangement,
domain size, crystallinity, and surface topology. The current
state-of-the-art OPVs are primarily designed to possess asolution-processed active layer of the bulk-heterojunction(BHJ) type, which is prepared from a blend solution of p- andn-type compounds to form microscale phase separation with alarge heterojunction area.13–15 The morphology of BHJ active
layers affects essentially all aspects of the photovoltaic processfrom light absorption to charge-carrier extraction. For example,
the phase-separation characteristics (e.g., size, connectivity, andpurity of domains) determine the behavior of excitons andcharge carriers. Therefore, optimization of the BHJ morphology
is a prerequisite for achieving the highest possible PCE.

In relation to this, not only the resultant morphology, but also
its evolution is a critical target of study, making the in situ
morphology analysis during deposition of BHJ layers increasingly
common in the development of OPVs.16–18
The intrinsic morphological properties of SCs can be tuned
through modification of substituents on p-conjugated backbones;
accordingly, substituent engineering has become a routine in thedevelopment of new OPV SCs. Note that actual morphologicaloutcomes are also aﬀected by extrinsic factors such as deposition
conditions, annealing conditions, and the nature of the combinedmaterial(s) in target BHJ films. Additionally, substituents are, inmost cases, also responsible for solution processability of SCs,
and the need for solubilization is highly dependent on the
structure of p-backbones. Hence, substituents must be optimized
on a case-by-case basis, which makes the substituent engineeringof OPV SCs quite complex and hard to generalize.
With this background in mind, this Review aims to provide a
broad picture of substituent impact by overviewing recent
prominent studies. We focus on the studies that directlycompared the photovoltaic performance of small-molecule(SM) SCs with diﬀerent substituents. Readers interested in
the substituent impact among polymer SCs are referred toprevious reviews and several recent papers on that topic.19–27
Also note that the substituents compared in this Review arelimited to those intended mainly for the modulation ofmorphological behavior of SMSCs rather than the modulationof their electronic characteristics at the molecular level.
As such, studies on the ‘halogen impact’ are not covered inthis Review, although it affects the BHJ morphology to acertain degree.28–33 The following sections overview substituent
impact on different types of SMSMs in the order of (1) phthalocyanines, (2) porphyrins, (3) perylene diimide derivatives,

Taehyun Won is an undergraduate
student in the Chemistry-Biology
Combined
Major
Program(CBCMP) at Osaka University. His
research interests involve organic
semiconductors, including newgeneration OLEDs and device
fabrication methods.

Hiroko Yamada received her PhD
degree in 1992 from Kyoto
University, under the guidance ofProf. Kazuhiro Maruyama and Prof.
Atsuhiro Osuka. In 1993, she did
her research under Prof. Michael R.
Wasielewski in Argonne National
Laboratory, USA as a JSPS
Research Fellow. After working in
Ciba–Geigy Ltd. and Ciba Specialty
Chemicals Inc., she was a postdoctoral researcher in Osaka
University. Since 2003, she was an
Hiroko Yamada
associate professor in the group of
Prof. Noboru Ono in Ehime University, and moved to an associate
professor of Nara Institute of Science and Technology in 2011. She waspromoted to a full professor at NAIST in 2012. In 2006–2010, she was a
researcher of PRESTO, JST. She received the Japanese Photochemistry
Association Award in 2012 and Chemical Society of Japan Award for
Creative Work in 2018. Her current research interests include design,synthesis, morphology control of organic functional materials.

Taehyun Won

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2. Substituents of phthalocyanines
Phthalocyanine (H2Pc) is a prototypical organic dye having
a two-dimensionally extended p-conjugated frameworkassociated with excellent stability and high light absorptivity.
These characteristics naturally prompted the use of this dye in
optoelectronic applications; indeed, pristine H2Pc and a Cu(II)complex (CuPc) were used as p-type SMSCs in the earliest
examples of heterojunction OPVs.34,35 Thereafter, H2Pc andits derivatives were used in a number of studies for elucidating
the working mechanism of OPVs and improving device
eﬃciencies.36 This section overviews substituent impact on thisclass of compounds. The chemical structures of the relevant
compounds and the corresponding photovoltaic data aresummarized in Fig. 2 and Table 1, respectively.
Derivatives used as p-type materials
Fig. 1

Structures of the p-conjugated core units discussed in this review.

(4) diketopyrrolopyrrole derivatives, (5) dithienosilole, (6)benzodithiophene-based systems, (7) indacenodithiophenebased systems (8) di(thienopyrrolo)benzothiadiazole-based
core, and (9) other systems (Fig. 1). Finally, we conclude withprospects regarding the substituent design of SMSCs for OPV
applications.

Non-substituted H2Pc is essentially insoluble, and flexible alkyl
chains are commonly appended on the benzo units to make itcompatible with solution processing. In 2013, Dao and co-workers
compared a series of liquid crystalline 1,4,8,11,15,18,22,25-octa(nalkyl)phthalocyanines 1-1a–e to identify the optimal length of
linear alkyl substituents.37 These compounds were evaluated as
p-type materials in BHJ OPVs in combination with PC71BM as ann-type material. The results clearly showed the superiority ofshorter alkyl chains over longer ones (entries 1-1–1-5). Specifically,

Fig. 2 Chemical structures of (a) SMSCs 1-1-1-4 and (b) the compounds employed with 1-1-1-4 in OPV active layers. The best PCEs obtained with eachphthalocyanine derivative are shown in parentheses (see Table 1 for full OPV parameters). The inset shows relevant substitution positions of thephthalocyanine core.

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Review

Journal of Materials Chemistry C

Table 1

Photovoltaic performance of SMOCs 1-1-1-4a

Entry

Active layer

Solvent

Additional conditions

VOCb (V)JSCb (mA cm2)

FFb (%)

PCEb (%)

Ref.

1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
1-9
1-10
1-11
1-12
1-13
1-14

1-1a : PC71BM (3 : 2)
1-1b : PC71BM (3 : 2)
1-1c : PC71BM (3 : 2)
1-1d : PC71BM (3 : 2)
1-1e : PC71BM (3 : 2)
P3HT : PC61BM : 1-2a (1 : 1 : 0.11)
P3HT : PC61BM : 1-2b (1 : 1 : 0.11)
P3HT : PC61BM : 1-2c (1 : 1 : 0.11)
PTB7 : 1-3a (1 : 1.5)
PTB7 : 1-3b (1 : 1.5)
PM6 : 1-4a (1 : 1)
PM6 : 1-4b (1 : 1)
PM6 : 1-4c (1 : 1)
PM6 : 1-4d (1 : 1)

CF
CF
CF
CF
CF
CB
CB
CB
CF
CF
CB
CB
CB
CB

0.2 vol% DIO
0.2 vol% DIO
0.2 vol% DIO
0.2 vol% DIO
0.2 vol% DIO
145 1C, 10 min
145 1C, 10 min
145 1C, 10 min
100 1C, 10 min
100 1C, 10 min
0.5% DIO; 100 1C, 10 min
0.5% DIO; 100 1C, 10 min
0.5% DIO; 100 1C, 10 min
0.5% DIO; 100 1C, 10 min

0.77
0.73
0.69
0.67
0.65
0.49
0.58
0.60
1.00
1.03
0.84
0.96
0.82
0.89

9.5
9.6
7.8
4.1
1.4
7.9
10.5
10.7
0.60
6.18
9.69
7.44
7.19
6.4

48
53
42
35
28
28
39
46
45
42
60
47
53
46

3.5
3.7
2.6
1.0
0.3
1.1
2.4
3.0
0.27
2.67
4.92
3.37
3.15
2.67

37
37
37
37
37
39
39
39
40
40
41
41
41
41

a
Structures of active-layer compounds are shown in Fig. 2. CF: chloroform, CB: chlorobenzene, DIO: 1,8-diiodooctane. b Data of the bestperforming devices.

the pentyl and hexyl derivatives 1-1a and 1-1b yielded PCEs of
3.5% and 3.7%, respectively, while the decyl derivative 1-1e gave a
PCE of 0.3%. The PCEs obtained with the heptyl and octyl
derivatives 1-1c and 1-1d were 2.6% and 1.0%. Based on X-ray
diffraction (XRD) data, the authors suggested that columnar
stacks of 1-1a and 1-1b adopted a pseudo-hexagonal
packing, while those of 1-1c–e formed 2D rectangular structures.
This alkyl-length dependent variation in the packing motif may be
the reason for the considerable difference in hole mobility(mh = 0.15 and 0.81 cm2 V1 s1 for 1-1a and 1-1b; 0.03 and0.02 m2 V1 s1 for 1-1d and 1-1e by the time-of-flight method),
and thus in JSC and FF (Table 1).
In the following year, the same group reported an extensionof this work to a ternary system in which a mixture of 1-1a and1-1b was used as the p-type material in BHJ OPVs with phenylC61-butylic acid methyl ester (PC61BM) as the n-type material(data not shown in Table 1).38 It turned out that the
25 : 75 mol% mixture of 1-1a and 1-1b aﬀorded a PCE of
3.8%, working better than pure 1-1a (PCE = 1.4%) or 1-1b
(3.1%). The improved performance of the mixture wasattributed to the decreased crystallite size and balanced
mobilities between holes and electrons in the BHJ active layer.
Derivatives used as co-sensitizers
The impact of linear alkyl chains was also examined with ap-extended phthalocyanine. Yamamoto and Kimura compared aseries of Zn(II) fluorenocyanines 1-2a–c as the third component
in ternary BHJ OPVs (entries 1-6–1-8).39 The frontier-orbitalenergy levels of 1-2a–c were thought to allow all these compounds to work as a co-sensitizer in the BHJ system comprisingpoly(3-hexyl)thiophene (P3HT) and PC61BM; however,improvement in PCE was confirmed only with the octahexyl
derivative 1-2c (3.0% and 2.6% for devices with and without 1-2c,
respectively). Indeed, PCE decreased when octamethyl derivative
1-2a or tetrahexyltetramethyl derivative 1-2b was doped to
P3HT:PC61BM films (1.1% and 2.4%). The authors concluded
that this observation was due to unfavourable aggregation of
1-2a and 1-2b in the BHJ films, based on light absorption spectraand surface topology observed using an atomic force microscopeThis journal is © The Royal Society of Chemistry 2022

(AFM). Thus, regardless of the presence of large substituents
perpendicular to the main p-framework, impact of linear alkyl
chains was noticeable.
Derivatives used as n-type materials
Phthalocyanines can form complexes with various main-group ortransition-metal ions, enabling the facile modulation of electronic
and optoelectronic characteristics of their p-conjugated systems.
Additionally, the inserted ions of these complexes provide
additional sites for structural modification. The group attachedto these ions generally occupies the axial position of a macrocyclic
p-framework and influences intermolecular p–p interactions andphotovoltaic processes. In this context, silicon phthalocyanine
derivatives 1-3a and 1-3b reported by Zysman-Colman and coworkers showed a significantly different performance as n-typematerials in BHJ OPVs.40 When used with a p-type polymer PTB7,
bis(eicosanoate) 1-3a afforded a PCE of merely 0.27%, whilebis(3,5-di-t-butylbenzoate) 1-3b gave a one-order higher value of
2.67% (entries 1-9 and 1-10). This large difference in PCE
could be almost fully ascribed to the difference in JSC (0.60 vs.
6.18 mA cm2), and the authors speculated that the lower JSC with
1-3a was due to the deterioration of charge-carrier mobility causedby the insulating eicosanoate groups.
Huang et al. reported in 2019 a systematic evaluation ofsubphthalocyanine triimides with diﬀerent groups on theimide nitrogen and central boron atoms.41 They directlycompared compounds 1-4a–d as n-type materials in BHJ OPVs
containing PM6 (PBDBT-2F or PBDB-TF) as the p-type material(entries 1-11–1-14). Among these four compounds, the B-fluoro/
N-butyl derivative 1-4a worked best yielding a 4.92% PCE. Whenits three butyl groups were all replaced with significantly larger
1-pentylhexyl groups (1-4b), PCE decreased to 3.37% mainly
due to the large drop in JSC from 9.69 to 7.44 mA cm2.
The same tendency was observed also for chlorides 1-4c and1-4d; namely, a higher JSC and PCE were obtained with the butyl
derivative (7.19 mA cm2 and 3.15%) than the 2-pentylhexyl
derivative (6.4 mA cm2 and 2.67%). In contrast to the cases of
1-1 and 1-3 series, the alkyl-size dependency in the 1-4 series
exists as an outlier because the bulkier 1-pentylhexyl group

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Journal of Materials Chemistry C
afforded higher electron mobilities (me) and better hole/electron
mobility ratios (me/me) (me = 1.39  105, 5.07  105, 7.76 
106, 4.70  105 cm2 V1 s1 and me/me = 13.38, 10.10, 19.72,
9.91 for 1-4a–d, respectively), which usually lead to higher JSC
and FF, thus higher PCE.
As exemplified by the 1-3 and 1-4 series, axial groups on thecentral atom of (sub)phthalocyanine complexes can eﬀectively
attenuate the excessive aggregation of molecules, so that
relatively small substituents may suﬃce the need for achieving
adequate solution processability and miscibility in BHJ layers.
Such molecular design is unique to this class of compounds; onthe other hand, covalent conjugation with other p-frameworks
is scarce for phthalocyanines largely due to their unfeasibility
to have meso substituents. The latter is in contrast quite
common with porphyrins as described in the next section.

3. Substituents of porphyrin derivatives
The parent porphyrin (porphine) has a cyclic p-conjugatedframework comprising four pyrrole(-like) units linked via
methine bridges. The methine units can serve as substitution
points, enabling an essentially diﬀerent mode of derivatization(i.e., meso substitution) as compared with phthalocyanines.
The structure and arrangement of meso substituents are indeed
crucial factors in tuning the solid-state packing and propertiesof porphyrin-based molecules.42–44 This section overviews
substituent impact on porphyrin-based SMSCs. The chemical
structures of the relevant compounds and the corresponding
photovoltaic data are summarized in Fig. 3, 4 and Table 2.
Derivatives used as p-type materials
Matsuo et al. investigated the diﬀerence between stereoisomersof 2 : 2 hetero-substituted Mg(II) porphyrins 2-1a and 2-1b as
p-type materials in BHJ OPVs (entries 2-1–2-7).45 The cis-isomer
2-1a showed relatively low PCEs (1.5% at best), but the device
performance was rather independent of the p : n-ratio between
1 : 2 and 1 : 4. In contrast, the trans-isomer 2-1b gave a relatively
high PCE of 2.5% at best, while the PCE is more strongly
dependent on the p : n-ratio when compared with the case of
2-1a.
So far, the most common molecular design of porphyrinbased SMSCs for OPVs is the trans-type fourfold meso substitution.
The p-conjugation is extended mainly along one axis to tune thefrontier-orbital energy levels and to achieve a wide lightabsorption range, while the other two meso positions are typically
substituted with bulky or flexible groups to control the moleculararrangement in active layers. According to this design, Wang andco-workers synthesized three acceptor–donor–acceptor (A–D–A)
conjugates 2-2a–c with Zn(II) porphyrin and 3-ethylrhodanine as
the D and A units, respectively.46 The three conjugates weredifferent in the 5,15-substituents of the porphyrin core: 3,5di(dodecyloxyl)phenyl for 2-2a, 4-dodecyloxylphenyl for 2-2b, and1-hexylnonyl for 2-2c. Among these compounds, 2-2c performed
the best as a p-type material in BHJ OPVs with the n-type PC71BM,
affording a PCE of 5.20% at best, while 2-2a and 2-2b gave lower

1166 | J. Mater. Chem. C, 2022, 10, 1162–1195

Review
PCEs of 3.21% and 5.07%, respectively, under the same
conditions (entries 2-8–2-10). Note that the device with 2-2c
was further improved to show 7.70% PCE after optimizing the
p:n-ratio and annealing conditions. The authors suggested thatthe alkyl substituents of 2-2c are superior to the perpendiculararomatic substituents of 2-2a and 2-2b in terms of providing
adequate miscibility with PC71BM and allowing effective
intermolecular p–p-stacking. This suggestion was supportedby the lower surface roughness and higher hole mobility for the
2-2c:PC71BM blend than 2-2a:PC71BM and 2-2b:PC71BM.
In 2018, the same group reported two modified derivatives of the
A–D–A system comprising Zn(II) porphyrin and 3-ethylrhodanine(2-3a and 2-3b).47 The structural difference of these derivatives
is only in the substituents on the phenylene moieties: hexylthio
for 2-3a and hexyl for 2-3b. However, the difference inperformance as p-type materials in OPVs was profound: while 23a afforded an excellent PCE of 8.04%, 2-3b gave only a 5.86% PCE(entries 2-11 and 2-12). The superiority of 2-3a was attributed to its
ability to self-assemble into J-aggregates through intermolecularS  S interactions. In contrast, compound 2-3a without sulphur
atoms did not show preferential formation of a specific selfassembly. It is worth pointing out that such relatively weakinteractions can induce a drastic change in the arrangement oflarge p-conjugated frameworks in BHJ blended films.
Substituent impact for another type of porphyrin-based
A–D–A system was reported by Ogumi et al. in 2017 (entries
2-13–2-17).48 They replaced the two triisopropylsilyl (TIPS)
groups of tetraethynylporphyrin 2-1b with dithienyldiketopyrrolopyrrole (DPP) in order to enhance the light absorption
capability, and introduced three different substituents (hexylphenyl, trifluoromethylphenyl, or dimethylaminophenyl) at the
4-positions of the two phenyl groups in order to modulate themorphological behaviour and frontier-orbital energies. Amongthe four derivatives 2-4a–d (including one without substituentson the phenyl rings), the 4-hexylphenyl derivative 2-4b worked
best as a p-type material in BHJ OPVs with PC61BM, affording areasonably high PCE of 4.83%. Compound 2-4b was found toafford a higher FF of 57% as compared to the non-substituted
derivative 2-4a (FF = 49%), probably due to a higher degreeof phase separation and thus more efficient charge-carriertransport (entries 2-13 and 2-14). Indeed, this advantage
allowed 2-4b to afford an improved PCE of 5.73% in a thicker
active layer deposited from a 30 mg mL1 solution instead of
the original 20 mg mL1 solution (entry 2-15). The electronwithdrawing trifluoromethyl groups of 2-4c and the electrondonating dimethylamino groups of 2-4d affected the VOC as
expected from the corresponding changes in the energy level ofthe highest-occupied MOs (HOMOs); however, a positive
impact on the overall photovoltaic process was not observedwith these derivatives (PCEs were 1.65% and 2.33% with 2-4c
and 2-4d, respectively, when the active layers were depositedfrom a 20 mg mL1 solution; entries 2-16 and 2-17).
Soon after, the same group expanded the scope of structural
screening of the Mg(II) porphyrin–DPP conjugate system toinclude asymmetrically substituted molecules 2-4e–h (entries
2-18–2-21).49 This study confirmed that asymmetricThis journal is © The Royal Society of Chemistry 2022

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Review

Fig. 3 Chemical structures of SMSCs 2-1-2-8. The best PCEs obtained with each compound are shown in parentheses (see Table 2 for full OPV
parameters). The inset shows relevant substitution positions of the porphyrin core.

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Review

Fig. 4 Chemical structures of the compounds employed with 2-1-2-7 in OPV active layers. PC61BM and PC71BM are shown in Fig. 2.

Table 2

Photovoltaic performance of SMSCs 2-1-2-8a

Entry Active layer
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
2-20
2-21
2-22
2-23
2-24
2-25
2-26
2-27
2-28
2-29
2-30
2-31
2-32
2-33
2-34
2-35
2-36
2-37

Solvent Additional conditionsCB
2-1a : PC61BM (1 : 2)CB
2-1a : PC61BM (1 : 3)CB
2-1a : PC61BM (1 : 4)CB
2-1b : PC61BM (1 : 1)CB
2-1b : PC61BM (1 : 3)CB
2-1b : PC61BM (1 : 4)CB
2-1b : PC61BM (1 : 5)CB
2-2a/PC71BM (1 : 1)CB
2-2b/PC71BM (1 : 1)CB
2-2c/PC71BM (1 : 1)
2-3a : PC71BM (1 : 1.2) CB
2-3b : PC71BM (1 : 1.2) CB
2-4a : PC61BM (1 : 1.5) CB
2-4b : PC61BM (1 : 1.5) CB
2-4b : PC61BM (1 : 1.5) CB
2-4c : PC61BM (1 : 1.5) CB
2-4d : PC61BM (1 : 1.5) CB
2-4e : PC61BM (1 : 1.5) CB
2-4f : PC61BM (1 : 1.5) CB
2-4g : PC61BM (1 : 1.5) CB
2-4h : PC61BM (1 : 1.5) CB
2-5a : IDIC (1 : 1.2)CF
2-5b : IDIC (1 : 1.2)CF
2-5c : IDIC (1 : 1.2)CF
2-5a : PC71BM (1 : 1.2) CF
2-5b : PC71BM (1 : 1.2) CF
2-5c : PC71BM (1 : 1.2) CF
PBDB-T : 2-6a (1 : 1)
CB
PBDB-T : 2-6b (1 : 1)
CB
PBDB-T : 2-6c (1 : 1)
CB
PBDB-T : 2-6d (1 : 1)
CB
PTB7-TH : 2-7a (1 : 1.5) CB
PTB7-TH : 2-7b (1 : 1.5) CB
PTB7-TH : 2-7c (1 : 1.5) CB
PTB7-Th : 2-8a (1 : 1)
CB
PTB7-Th : 2-8b (1 : 1)
CB
PTB7-Th : 2-8c (1 : 1)
CB

VOCb (V)JSCb (mA cm2) FFb (%)

—
0.83
4.8
—
0.83
4.5
—
0.85
3.9
—
0.89
3.2
—
0.89
4.5
—
0.92
6.4
—
0.91
5.8
1% Py; 90 1C, 10 min
0.90
7.20
1% Py; 90 1C, 10 min
0.90
10.14
1% Py; 90 1C, 5 min
0.91
13.32
2 vol% Py; 100 1C, 5 min; THF vapour 20 s 0.91  0.01 13.19  0.04
2 vol% Py; 100 1C, 5 min; THF vapour 20 s 0.87  0.01 11.53  0.07
1% Py, 20 mg mL1
0.74
13.27
1% Py, 20 mg mL1
0.70
12.04
1
1% Py, 30 mg mL
0.69
14.80
1% Py, 20 mg mL1
0.79
4.61
1% Py, 20 mg mL1
0.52
9.19
1% Py; THF vapour 20 s
0.73
12.01
—
0.60
12.26
100 1C, 10 min
0.63
10.02
80 1C, 10 min
0.69
14.86
0.8 vol% DIO, 0.5 vol% Py
0.71  0.01 15.46  0.19
0.8 vol% DIO, 0.5 vol% Py
0.71  0.01 14.03  0.34
0.8 vol% DIO, 0.5 vol% Py
0.70  0.01 11.46  0.01
0.8 vol% DIO, 0.5 vol% Py
0.72
10.56
0.8 vol% DIO, 0.5 vol% Py
0.70
8.64
0.8 vol% DIO, 0.5 vol% Py
0.69
14.54
10 vol% Py
0.88  0.01 4.10  0.01
10 vol% Py
0.79  0.00 9.56  0.12
10 vol% Py
0.80  0.01 9.48  0.06
10 vol% Py
0.81  0.01 11.02  0.64
0.5% Py
0.66
13.54
0.5% Py
0.67
10.55
0.5% Py
0.68
9.61
0.4 vol% Py; CF vapour, 5 min
0.78
9.88
0.4 vol% Py; CF vapour, 5 min
0.80
13.94
0.4 vol% Py; CF vapour, 5 min
0.77
12.55

PCEb (%)

Ref.

36
1.4
45
39
1.5
45
34
1.2
45
36
1.0
45
42
1.7
45
41
2.5
45
40
2.1
45
48.12
3.21
46
55.60
5.07
46
63.60
7.70
46
66.70  0.01 8.04  0.03
47
58.50  0.03 5.86  0.07
47
49
4.85
48
57
4.83
48
56
5.73
48
45
1.65
48
48
2.33
48
47
4.07
49
39
2.85
49
38
2.42
49
47
4.84
49
56  06.13  0.08
50
53  2
5.21  0.10
50
51  14.08  0.02
50
53
4.05
50
50
2.98
50
62
6.26
50
49  11.87 (Av. 1.78) 51
50  2
3.79 (Av. 3.75) 51
53  2
4.23 (Av. 3.99) 51
58  1
5.34 (Av. 5.07) 51
58.29
5.21
52
57.15
4.06
52
56.20
3.68
52
53.2
4.10
53
64.8
7.23
53
56.7
5.48
53

a

Structures of the active-layer compounds are shown in Fig. 3 and 4. CB: chlorobenzene, CF: chloroform, Py: pyridine, THF: tetrahydrofuran, DIO:
1,8-diiodooctane. b Data of the best-performing devices when available; otherwise, average of multiple devices.

substitution was beneficial not only for fine-tuning of themolecular electronic structure, but also for improving
solubility. The latter aspect is rather important because large
p-conjugated frameworks are intrinsically low in solubility
and thus require extensive decollation with large, flexible,
and insulating solubilizing substituents to be compatible withsolution processes. However, such solubilizing groups tend topose negative effects in terms of forming intermolecular

1168 | J. Mater. Chem. C, 2022, 10, 1162–1195

contacts necessary for efficient carrier transport. Therefore,
asymmetric substitution is an attractive approach to improvesolubility without relying too much on an insulating solubilizing
substituent. Among the four asymmetric derivatives, the4-hexylphenyl/4-hexylthienyl substituted 2-4h afforded the highest
PCE of 4.84%. Indeed, this compound was found to be the mostsoluble in the 2-4 family, enabling the deposition of a thick active
layer of 215 nm which afforded a 4.06% PCE.

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Review
Hadmojo et al. explored the substituent impact for an A–D–A
system comprising Zn(II) porphyrin, instead of Mg(II) porphyrinof the 2-4 series mentioned above, and DPP moieties.50 The
5,15-carbons of the central Zn(II) porphyrin were substituted
with 4-octyloxyphenyl, 4-(2-ethylhexyloxy)phenyl, or 5-(2ethylhexyloxy)-2-thienyl, and the resultant compounds 2-5a–c
were evaluated as p-type materials in BHJ OPVs with a nonfullerene acceptor IDIC. Compounds 2-5a–c are very similar infrontier-orbital energy levels to afford essentially the same VOCsof around 0.70 V (entries 2-22–2-24). On the other hand, the JSC
and FF largely depend on substituents: both parameters
decreased in the order of 2-5a 4 2-5b 4 2-5c. Accordingly,
PCEs also decreased in the order of 2-5a (6.13%) 4 2-5b(5.21%) 4 2-5c (4.08%). Based on the XRD data, the authors
reasoned that the higher performance of 2-5a could be attributed
to the higher tendency of molecules to adopt a face-on orientationin the 2-5a:IDIC layer. Interestingly, when PC71BM was employed
as the n-type material, a better PCE was obtained with compound 2-5c (6.26%) than 2-5a (4.05%) and 2-5b (2.98%) (entries
2-25–2-27). This result clearly demonstrates that the molecular
design of organic semiconductors for BHJ layers must consider
the structure of the partner compound as well.
Derivatives used as n-type materials
Another series of Zn(II) porphyrin-based A–D–A conjugates werereported by Guo et al. in 2018.51 The authors used a perylene
diimide (PDI) derivative as acceptor (A) units and introduced 2,6di(dodecyloxy)phenyl, (2-ethylhexyl)thiophen-2-yl, pentadecan-7yl, or 3,5-di(dodecyloxy)phenyl onto the central porphyrin core toobtain compounds 2-6a–d. Note that these compounds were
designed as n-type materials for OPVs, although porphyrinbased SMSCs have been commonly used as p-type materials.
In BHJ OPVs with PBDB-T as the p-type material, compound 2-6agave a considerably low PCE of 1.87%, while the other threecompounds afforded relatively similar PCEs of 3.79% (2-6b),4.23% (2-6c), and 5.34% (2-6d) (entries 2-28–2-31). The authors
suggested that the low PCE of 2-6a was due to its tendency toform large crystalline domains in the BHJ layer, in addition to
the negligible LUMO–LUMO offset against PBDB-T (LUMO: thelowest unoccupied MO). It is worth pointing out that the
LUMO energy levels (ELUMOs) of 2-6a–d were found to be largely
dependent on the substituents of the porphyrin unit, eventhough the LUMO was predicted by DFT to be localized on the
PDI moieties (ELUMO = 3.53, 3.68, 3.63, and 3.74 eV for
2-6a–d, respectively). Pan et al. recently reported similar A–D–A
conjugates based on Zn(II) porphyrin, but with a PDI dimeras the acceptor unit (2-7a–c).52 In this case, non-substituted
porphyrin in 2-7a gave better PCEs than substituted ones in
2-7b and 2-7c (entries 2-32–2-34).
In addition, Tsai et al. reported three Zn(II) porphyrin-based
A–D–A conjugates having 2-(5,6-difluoro-3-oxo-2,3-dihydro-1Hinden-1-ylidene)-malononitrile as the electron-accepting end
group.53 Similar to the above examples, the three conjugates
2-8a–c are different from each other only in the structure ofmeso substituents of the central porphyrin unit; namely, 3,4,5tridodecyloxyphenyl for 2-8a, 2,6-didodecyloxyphenyl for 2-8b,This journal is © The Royal Society of Chemistry 2022

Journal of Materials Chemistry C
and 3,5-di-tert-butylphenyl for 2-8c. These compounds were
evaluated as n-type materials in BHJ OPVs with a p-type polymer
PTB7-Th. Derivative 2-8b was determined to be the best n-typematerial among the three, affording a PCE of 7.23%, while thecorresponding values were 4.10% and 5.48% with 2-8a and 2-8c,
respectively (entries 2-35–2-37). Superiority in the performance of thedevice with 2-8b originated from its higher JSC and FF than those of
the other two derivatives, and the authors ascribed this to a slightly
shorter p–p stacking distance (4.06 Å) as compared to those of theother systems (4.10 and 4.15 Å for PTB7-Th:2-8a and PTB7-Th:2-8c)
as determined by the out-of-plane profiles of two-dimensionalgrazing-incident wide-angle X-ray scattering (2D-GIWAXS) data.
It is notable that all the porphyrin derivatives mentioned inthis section were constructed by taking full advantage of meso
substitution of the porphyrin core. These meso substituentswere introduced either through or not through an acetylene
linker depending on the need for coplanarity between theporphyrin framework and substituents. This aspect largely
aﬀects the molecular packing of the resultant compounds, andaccordingly the optimal structure of peripheral substituents.

4. Substituents of perylene diimide
derivatives
Perylene diimide (PDI) is characterized by its highly stable andstrongly electron deficient p-conjugated framework. It is one of themost traditional SMSCs, and indeed its derivatives were employed
as n-type materials in the earliest examples of heterojunction OPVs
along with phthalocyanines.34,35 Although they somewhat lost the
popularity in the OPV community after the emergence of fullerenebased n-type materials, recent development of non-fullerene
acceptors is built upon the consistent research eﬀorts on the designand evaluation of new derivatives of PDI and other members in therylene diimide family.54–61 This section overviews substituent
impact on PDI-based SMSCs. The chemical structures of therelevant compounds and the corresponding photovoltaic data
are summarized in Fig. 5, 6 and Table 3, respectively.
PDI monomers
The study by Guide et al. in 2013 reports on the systematic
evaluation of substituent impact on the photovoltaic performanceof PDI derivatives.62 The authors compared four diﬀerently substituted PDIs 3-1a–d in solution processed planar heterojunction(PHJ) OPVs comprising tetrabenzoporphyrin (BP) as the p-typematerial (entries 3-1–3-4). The PHJ structure was constructed bydepositing the insoluble BP via an indirect solution process called‘precursor approach’ (see description of 4-7 in Section 5 fordetails), and then spin-coating one of the PDI derivatives on topof the BP layer. The highest PCE of 2.0% was obtained with thenon-bay-substituted derivative 3-1a, associated with a JSC of
5.3 mA cm2 and an FF of 61%. Derivative 3-1d with relatively
slim substituents (4-methylphenyethynyl) at the 1,7-positionsof the perylene core gave a similar PCE of 1.7% with a JSC of
4.7 mA cm2 and an FF of 58%. In contrast, derivatives 3-1b and3-1c with bulky substituents (4-tert-octylphenoxy) at the bay

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Journal of Materials Chemistry C

Fig. 5 Chemical structures of SMSCs 3-1-3-8. The best PCEs obtained with each compound are shown in parentheses (see Table 3 for full OPV
parameters). The inset shows relevant substitution positions of the PDI unit.

positions of the perylene framework afforded considerably low
PCEs of 0.4% and 0.02% due to large decreases in JSC and FF. This
report clearly demonstrated the significance of the baysubstituent steric effects on charge transport in PDI derivatives,
which is important not only in PHJ, but also in BHJ OPVs.
Sun et al. reported the photovoltaic performance of PDI derivatives 3-2a–d which were different from each other in the structureof alkyl substituents on the imide nitrogens.63 In the comparativeevaluation of these PDIs, the N,N0 -di(3-pentyl) derivative 3-2ayielded the highest PCE of 3.21% when used as the n-type material
in BHJ OPVs with a p-type SMSC F-DTS. Derivatives 3-2b and 3-2d,

1170 | J. Mater. Chem. C, 2022, 10, 1162–1195with 4-heptyl and 8-pentadecyl groups, respectively, afforded considerably lower PCEs of 1.98% and 0.19%, while the 2-ethylhexyl
substituted derivative 3-2c gave only negligible PCEs (entries 3-5–3-8).
Thus, substitution with more compact alkyl groups resulted inhigher performance in BHJ OPVs. This observation is consistent with the above-mentioned case of PHJ OPVs with 3-1a–d.
Non-fused PDI oligomers
A common issue associated with PDI derivatives is their stronger tendency to (1) crystalize/aggregate and (2) induce excessive
phase separation in blended films, which leads to low JSCs andThis journal is © The Royal Society of Chemistry 2022

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Review

Journal of Materials Chemistry C

Chemical structures of the compounds employed with 3-1-3-8 in OPV active layers. P3HT and PTB-7Th are shown in Fig. 2 and 4, respectively.

Fig. 6

Table 3

Photovoltaic performance of SMSCs 3-1-3-9a

Entry Active layer

Solvent

Additional conditions

VOCb (V)JSCb (mA cm2) FFb (%)

PCEb (%)

Ref.

3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
3-10
3-11
3-12
3-13
3-14
3-15
3-16
3-17
3-18
3-19
3-20
3-21
3-22
3-23
3-24
3-25
3-26
3-27
3-28
3-29
3-30
3-31
3-32
3-33
3-34

CF : CB = 1 : 2
CF : CB = 1 : 2
CF : CB = 1 : 2
CF : CB = 1 : 2
CB
CB
CB
CB
o-DCB
o-DCB
o-DCB
o-DCB
o-DCB
o-DCB
o-DCB
2Me-THF
2Me-THF
2Me-THF
2Me-THF
2Me-THF
2Me-THF
2Me-THF
2Me-THF
2Me-THF
2Me-THF
CB
CB
CB
1,2,4-TMB
1,2,4-TMB
o-DCB
o-DCB
o-DCB
o-DCB

180 1C, 20 min
180 1C, 20 min
180 1C, 20 min
180 1C, 20 min
0.4 vol% DIO
0.4 vol% DIO
0.4 vol% DIO
0.4 vol% DIO
3 wt% CN
8 wt% CN
1.75 wt% CN
3 vol% DIO
3 vol% DIO
3 vol% DIO
1.5 vol% CN + 1.5 vol% DIO
—
—
—
—
—
—
—
—
0.25 vol% DIO
180 1C, 10 min———
2.5% DIO
2.5% DIO
0.25% DIO; 100 1C, 5 min
0.25% DIO; 100 1C, 5 min
0.25% DIO; 100 1C, 5 min
0.25% DIO; 100 1C, 5 min

0.62
0.74
0.64
0.64
0.75
0.75
0.00
0.32
0.43
0.59
0.67
0.67
0.72
0.72
0.73
0.911  0.003
0.949  0.003
0.949  0.008
0.939  0.009
0.964  0.001
0.957  0.004
0.910  0.005
0.956  0.005
0.955  0.003
1.034
0.78
0.79
0.80
1.02  0.00
1.02  0.00
0.963  0.005
0.968  0.004
0.976  0.006
0.981  0.003

5.3
2.2
0.2
4.7
8.09
5.88
0.42
1.52
2.01
2.89
3.83
6.68
10.36
8.86
10.58
11.10  0.20
11.41  0.65
8.99  0.57
12.50  0.64
11.41  0.45
12.08  0.86
7.46  0.32
12.41  0.74
15.36  0.37
12.47
8.40
7.73
9.94
10.27  0.37
9.20  0.14
12.22  0.19
12.17  0.25
12.20  0.20
12.58  0.19

2.0
0.4
0.02
1.7
3.21
1.98
—
0.19
0.41
0.76
1.54
1.92
3.11
2.54
3.63
4.81 (4.27  0.28)
4.81 (4.64  0.10)
3.69 (2.99  0.23)
5.21 (4.77  0.24)
4.89 (4.62  0.13)
5.55 (5.16  0.14)
3.37 (2.80  0.27)
5.47 (5.33  0.10)
6.58 (6.24  0.19)
6.29
2.88
3.25
3.68
5.16 (4.92  0.11)
4.68 (4.40  0.26)
8.03 (7.97  0.05)
8.15 (8.02  0.14)
8.05 (7.94  0.10)
8.64 (8.54  0.09)

62
62
62
62
63
63
63
63
65
65
65
66
66
66
66
67
67
67
67
67
67
67
67
67
68
69
69
69
70
70
74
74
74
74

BP/3-1a (50 nm/20 nm)
BP/3-1b (50 nm/20 nm)
BP/3-1c (50 nm/20 nm)
BP/3-1d (50 nm/20 nm)
F-DTS : 3-2a (1 : 1)
F-DTS : 3-2b (1 : 1)
F-DTS : 3-2c (1 : 1)
F-DTS : 3-2d (1 : 1)
P3HT : 3-3a (1 : 1)
P3HT : 3-3b (1 : 1)
P3HT : 3-3c (1 : 1)
PBDTTT-C-T : 3-4a (1 : 1)
PBDTTT-C-T : 3-4b (1 : 1)
PBDTTT-C-T : 3-4c (1 : 1)
PBDTTT-C-T : 3-4b (1 : 1)
PTB7-Th : 3-5a (3 : 7)
PTB7-Th : 3-5b (3 : 7)
PTB7-Th : 3-5c (3 : 7)
PTB7-Th : 3-5d (3 : 7)
PTB7-Th : 3-5e (3 : 7)
PTB7-Th : 3-5f (3 : 7)
PTB7-Th : 3-5g (3 : 7)
PTB7-Th : 3-5h (3 : 7)
PTB7-Th : 3-5h (4 : 6)
TTFQx-T1 : 3-5f (4 : 6)
PTB7-Th : 3-6a (1 : 1)
PTB7-Th : 3-6b (1 : 1)
PTB7-Th : 3-6c (1 : 1)
P3TEA : 3-7a (1 : 1.5)
P3TEA : 3-7b (1 : 1.5)
PDBT-T1 : 3-8a (1 : 1)
PDBT-T1 : 3-8b (1 : 1)
PDBT-T1 : 3-8c (1 : 1)
PDBT-T1 : 3-8d (1 : 1)

61
21
16
58
53
45
—
38
47.2
44.8
60.0
42.94
42.08
39.75
46.80
42  343  235  141  342  345  241  245  243  1
48.8
45.2
53.1
46.4
47  247  367.8  0.7
68.1  1.4
66.7  1.1
69.2  0.3

aStructures of active-layer compounds are shown in Fig. 5 and 6. CF: chloroform, CB: chlorobenzene, o-DCB: o-dichlorobenzene, 2Me-THF:
2-methyltetrahydrofuran; 1,2,4-TMB: 1,2,4-trimethylbenzene, DIO: 1,8-diiodooctane; CN: chloronaphthalene. b Data of the best-performing devices
when available; otherwise, average of multiple devices.

FFs of the resulting BHJ OPVs.64 This issue can be circumvented by linking a pair of PDI cores at their bay-positions toform a twisted dimer. In 2013, Lu and co-workers reported PDIThis journal is © The Royal Society of Chemistry 2022

dimers with a thienylene bridge (3-3a–c).65 The dihedral angles
between the PDI and thienylene units were estimated to be aslarge as 50–651 by density-functional theory (DFT) calculations.

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The authors introduced different numbers of 2-methoxyethoxy
substituents on such a twisted p-framework and studied theireffect on the performance of BHJ OPVs with P3HT as the p-typematerial. The PCE was found to improve with the number of
2-methoxyethoxy groups: 0.41% with non-substituted 3-3a,
0.76% for di-substituted 3-3b, and 1.54% for tetra-substituted
3-3c (entries 3-9–3-11). The most favorable performance from
3-3c was explained by the adequate degree of phase separation
resulting from the four solvophobic 2-methoxylethoxy groups.
PDI dimers can be constructed by linking monomers
directly at bay positions. This way, the twist angle between
PDI units becomes much larger as compared to that of 3-3a–cwith a thienyl linker. Accordingly, crystallization or aggregationin BHJ films can be further attenuated. Jiang et al. studied thesubstituent impact on the morphological behaviour and photovoltaic performance of such a highly twisted dimeric system.
Specifically, they compared three PDI dimers 3-4a–c having
different alkyl substituents on the imide nitrogens.66 In BHJ
OPVs with PBDTTT-C-T as the p-type material, the N,N 0 -di(6undecyl) derivative 3-4b afforded a higher PCE of 3.11%
than the di(4-heptyl) derivative 3-4a (PCE = 1.92%) and di(8pentadecyl) derivative 3-4c (2.54%) when deposited under thesame conditions (entries 3-12–3-14). PCE in the PBDTTT-C-T:
3-4b system was further improved to 3.63% by optimizing thedeposition conditions (entry 3-15). These results show that
despite twist-shaped dimers’ low crystallinity, peripheral alkyl
groups and deposition conditions are critical.
Dayneko et al. synthesized a series of bridge-less PDI dimers
3-5a–h which were N-annulated at the bay positions of
PDI units and decollated with alkyl groups at the pyrrolicN-atoms.67 Their performance as n-type materials was evaluatedwith PTB7-Th as the p-type material (entries 3-16–3-23). The
observed photovoltaic parameters did not show an apparent
trend regarding the structure of alkyl groups; nonetheless, thesuperiority of hexyl and 2-ethylhexyl derivatives was noticeable.
Indeed, the 2-ethylhexyl derivative 3-5h afforded a 6.58% PCE
after optimization of the p:n ratio and solvent additive in the castsolution (entry 3-24). In addition, the PCE with hexyl derivative
3-5f was improved to 6.29% when a medium band-gap polymer
PPTFQx-T1 was used as the p-type material (entry 3-25).68
Twisted PDI dimers can also be constructed in the form of
N–N-linked benzo[ghi]perylenetriimides. Chen et al. investigated
the substituent effect among this class of dimers by comparing
three derivatives 3-6a–c with branched alkyl chains at the imide
nitrogen atoms.69 Their performance as n-type materials in BHJ
OPVs was studied using PTB7-Th as the p-type material, whereinderivative 3-6c with the longest alkyl chains was found to give thehighest PCE of 3.68% (entries 3-26–3-28). The authors attributed
the better performance of 3-6c to the formation of finer phaseseparation in the BHJ active layers compared to 3-6a and 3-6b.
Wang et al. compared two N-alkylated PDI tetramers 3-7a
and 3-7b as n-type materials in OPVs.70 These tetramers havepyrazino[2,3-g]quinoxaline (PQ) as the core unit, to which four
PDI units are linked at the bay positions through phenylene
linkers. The BHJ devices with P3TEA as the p-type material
showed PCEs as high as 5.16% when the N-6-undecyl derivative

1172 | J. Mater. Chem. C, 2022, 10, 1162–1195

Review
3-7a was used as the n-type material, while the PCE was thehighest at 4.68% with the N-7-tridecyl derivative 3-7b (entries
3-29 and 3-30). Namely, in contrast to the cases of dimers 3-6a–c
and trimers 3-8a–d (see below), substitution with a shorter alkyl
chain resulted in higher photovoltaic performance for this PDI
tetramer system. Based on 2D-GIWAXS and AFM analyses, theauthors suggested that the higher crystallinity and smaller
p-stack distance in the P3TEA:3-7a film would have contributed
to the higher JSC, thereby resulting in the higher PCE ascompared to P3TEA:3-7b.
Fused PDI oligomers
Several PDI trimers have been also designed for OPVs,71–73 andtheir substituent impact was investigated as an important partof structural optimization. In 2017, Fu et al. reported a series ofpropeller-shaped PDI trimers 3-8a–d as n-type materials in
OPVs.74 The imide nitrogens of these compounds were linked
to branched alkyl chains of different lengths from 5-nonyl of
3-8a to 8-pentadecyl of 3-8d. In BHJ OPVs with a p-type polymer
PDBT-T1, the derivative with the largest alkyl substituents(3-8d) afforded the highest PCE of 8.64% (entry 3-32). The otherthree derivatives were similar to each other in terms of theresulting PCEs: 8.03% for 3-8a, 8.15% for 3-8b, and 8.05% for
3-8c (entries 3-29–3-31). The authors attributed the superior
photovoltaic performance of 3-8d over other derivatives to its
higher degree of face-on orientation and a smaller domain
spacing as indicated by 2D-GIWAXS and resonate soft X-ray
scattering (R-SoXS) measurements, respectively.
The examples in this section highlight the fact that PDI
often need both twisted molecular conformation and branchedalkyl substituents, reflecting their strong tendency to aggregate.
Indeed, this class of chromophores generally requires longer
alkyl groups to form BHJ active layers of optimal morphology
as compared to the cases of phthalocyanine and porphyrinmentioned earlier.

5. Substituents of diketopyrrolopyrrole
derivatives
Diketopyrrolopyrrole (DPP) has been one of the most successful
dyes as a building block for organic semiconductors.75–79
Indeed, a ternary OPV with a DPP-based SMSC was reported
to show an excellent PCE of 15.6%.80 The study of substituent
impact on the electronic and semiconducting properties has
contributed greatly to the successful development of DPP-based
SMSCs. The chemical structures of the relevant compounds
and the corresponding photovoltaic data are summarized inFig. 7 and Table 4, respectively.
Seo pointed out in a paper published in 2009 that thestructure of alkyl substituents on lactam nitrogens exerted aconsiderable eﬀect on the performance of BHJ OPVs basedon the comparison between two 3,6-bis(500 -hexyl[2,2 0 :5 0 ,200 terthiophen]-5-yl)diketopyrrolopyrroles 4-1a and 4-1b as p-typematerials;81 specifically, N,N 0 -di(2-ethylhexyl) derivative 4-1a
showed a 3.0% PCE,82 while N,N 0 -di(2-ethylhexyl) derivative

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Review

Journal of Materials Chemistry C

Fig. 7 Chemical structures of SMSCs 4-1-4-7. The best PCEs obtained with each compound are shown in parentheses (see Table 4 for full OPV
parameters). The inset shows relevant substitution positions of the DPP unit.

4-1b aﬀorded PCEs of up to only 0.79% with PC71BM as then-type material (entries 4-1 and 4-2). The VOC of the 4-1b:PC71BM
device (0.47 V) was found to be much lower than that of the
4-1a:PC71BM device (0.75 V), which could be traced back to thediﬀerence in the ionization energies in the solid state (4.92 and4.61 eV for 4-1a and 4-1b, respectively). The author assumed that thisdissimilarity between the two compounds was related to the disruption of molecular packing due to branched alkyl (i.e., 2-ethlyhexyl)
chains. In relation to the steric eﬀect of the branched alkyl chains,
Zerdan et al. prepared compound 4-1a in a stereo-controlled fashion,and confirmed that the stereo-centres of the 2-ethylhexyl groups only
weakly aﬀected the overall photovoltaic process.83
Interestingly, 3,6-bis[4-(5 0 -hexyl[2,2 0 -bithiophen]-5-yl)phenyl]
diketopyrrolopyrroles 4-2a and 4-2b showed an opposite dependency on the structure of N-alkyl groups in terms of theperformance as a p-type material in BHJ OPVs. Lin et al. reported
that 4-2a and 4-2b gave PCEs of 3.45% and 0.76% at best,
respectively, with PC71BM as the n-type material (entries 4-3
and 4-4).84 Note that the PCE obtained with 4-2b was even lower
than that with 4-2c which did not have end-substituents (1.11%,
entry 4-5). The authors reasoned that both increasing thebulkiness of the solubilizing N-alkyl groups and removing thelinear end-alkyl groups induced micron-scale phase separations
at high p : n ratios, thereby requiring large excess of the n-typematerial (PC71BM) and resulting in limited photovoltaic eﬃciencies. The inconsistent results between 4-1 and 4-2 may be due toThis journal is © The Royal Society of Chemistry 2022

the steric eﬀects of the phenylene moieties which causes deviation from coplanarity with the neighbouring DPP andthienylene units.
The importance of end-alkyl groups was also demonstrated by
Shin et al. in 2013.85 In this case, the authors comparatively
evaluated two simple DPP derivatives 4-3a and 4-3b, the p-conjugated backbones of which were terminated with 4-hexylphenyl
and 4-dodecylphenyl groups, respectively. While 4-3b aﬀorded a
low PCE of 1.2%, 4-3a performed much better as a p-type material
to give a 4.2% PCE in BHJ OPVs with PC71BM (entries 4-6 and 4-7).
The hexyl derivative 4-3a was found to have liquid-crystalline (LC)properties, and the 4-3a:PC71BM active layer showed a significant
increase in JSC and FF upon thermal annealing via the LC
organization process. On the other hand, the dodecyl derivative
4-3b did not show LC characters and the photovoltaic
performance of the 4-3b:PC71BM system was essentiallyunchanged after thermal annealing. Thus, balancing the lengthsof alkyl chains and a rigid p-backbone is quite important forachieving a favourable molecular order in BHJ films and thus
improving photovoltaic performance.
Gevaerts et al. showed that the position of end-alkyl substituents was a critical factor in OPV performance by comparing
isomeric DPP derivatives 4-4a–c having hexyl chains at
different positions of the terminal thienyl groups.86 The threecompounds were found to be largely dissimilar in terms ofinteraction with PC71BM, crystallization kinetics, and crystallite

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Review

Table 4

Photovoltaic performance of SMSCs 4-1-4-7a

Entry

Active layer

Solvent

Additional conditions

VOCb (V)JSCb (mA cm2)

FFb (%)

PCEb (%)

Ref.

4-1
4-2
4-3
4-4
4-5
4-6
4-7
4-8
4-9
4-10
4-11
4-12
4-13
4-14
4-15
4-16
4-17
4-18
4-19
4-20
4-21
4-22

4-1a : PC71BM (1 : 1)
4-1b : PC71BM (1 : 1)
4-2a : PC71BM (3 : 2)
4-2b : PC71BM (1 : 9)4-2c : PC71BM (2 : 8)
4-3a : PC71BM (1 : 1)
4-3b : PC71BM (1 : 1)
4-4a : PC71BM (2 : 1)
4-4b : PC71BM (2 : 1)
4-4c : PC71BM (2 : 1)
4-5a : PC71BM (2 : 1)
4-5b : PC71BM (2 : 1)
4-5c : PC71BM (2 : 1)
4-5d : PC71BM (2 : 1)
4-6a : PC61BM (3 : 2)
4-6b : PC61BM (3 : 2)
4-6a : PC61BM (3 : 2)
4-6b : PC61BM (3 : 2)
4-7a : PC61BM (4 : 3)
4-7b : PC61BM (2 : 1)
4-7c : PC61BM (2 : 1)
4-7d : PC61BM (2 : 1)

CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF

—
—
120 1C, 10 min
80 1C, 10 min
80 1C, 10 min
140 1C, 10 min
120 1C, 10 min
100 1C, 1 min
100 1C, 1 min
100 1C, 1 min
CS2 vapour, 20 s
CS2 vapour, 20 s
CS2 vapour, 20 s
CS2 vapour, 20 s——
0.6 vol% DIO; 70 1C, 10 min
0.4 vol% DIO; 70 1C, 10 min
10 vol% CS2, 200 1C, 10 min
200 1C, 10 min
160 1C, 30 min
180 1C, 30 min

0.75
0.47
0.90
0.72
0.87
0.93
0.93
0.85
0.84
0.79
0.85
0.79
1.00
0.84
0.95
0.94
0.85
0.82
0.67
0.69
0.70
0.66

9.2
4.2
7.91
3.37
4.33
8.27  0.10
3.73  0.17
5.89
7.47
5.30
0.74
1.16
5.40
3.49
4.0
3.9
9.9
7.3
15.2
13.6
9.1
0.88

44
40
49
33
30
54  135  1
50
53
45
24
26
48
36
29.0
29.2
51.3
45.2
52
49
48
33

3.0
0.79
3.45
0.76
1.11
4.2  0.1
1.2  0.1
2.48
3.30
1.90
0.14
0.23
2.60
1.05
1.10
1.08
4.35
2.69
5.2
4.5
3.1
0.19

81 and 82
81
84
84
84
85
85
86
86
86
87
87
87
87
88
88
88
88
89
89
89
89

a
Structures of active-layer compounds are shown in Fig. 7. CF: chloroform, DIO: 1,8-diiodooctane. b Data of the best-performing devices whenavailable; otherwise, average of multiple devices.

orientation. These factors were considered responsible for theobserved variation in device performance, in which PCEs were
2.48%, 3.30%, and 1.90% for 4-4a, 4-4b, and 4-4c, respectively(entries 4-8–4-10). Similarly, Más-Montoya and Janssen
compared thienyl end-capped di(2-pyridyl)diketopyrrolopyrroles
4-5a–d.87 Among the four compounds, only 4-5a had no end-alkyl
chain, and the other three were substituted with hexyl groups atdifferent positions of the thiophene units. In BHJ OPVs comprisinga DPP derivative and PC71BM, derivatives 4-5c and 4-5d gave
relatively high PCEs of 2.60% and 1.05%, whereas 4-5a and 4-5b
showed significantly lower PCEs of 0.14% and 0.23% (entries 4-11–
4-14). Spectroscopic data clearly showed that the former two
derivatives formed J-aggregates, and the latter two H-aggregates.
The authors assumed that more efficient exciton diffusion in Jaggregates caused the observed faster charge generation and thus
higher photovoltaic efficiency with 4-5c and 4-5d.
The impact of N-alkyl substituents was also studied regarding
A–D–A-type conjugates with DPP as the acceptor unit. In 2015,
Jung et al. reported two such compounds, 4-6a and 4-6b, bearing
2-ethylhexyl or 2-butyloctyl groups, respectively, on the DPP
nitrogens.88 The two compounds showed very similar photovoltaic
performance as p-type materials in BHJ OPVs when deposited
without a solvent additive (1,8-diiodooctane, DIO) to afford essentially the same PCEs of 1.10% and 1.08% (entries 4-15 and 4-16).
In contrast, when a small amount of DIO was added to the castsolution, the PCE obtained with 4-6a was considerably higher
than that of 4-6b (4.35% vs. 2.69%, entries 4-17 and 4-18). The
authors concluded that compound 4-6b with longer alkyl groups
suffered from excessive phase separation that limited JSC and FF,
while 4-6a with shorter alkyl groups formed bicontinuous
morphology with favorable domain sizes for the photovoltaic
process. In relation to this report, it is worth pointing out that mostof the successful DPP-based SMSCs in BHJ OPVs are equipped

1174 | J. Mater. Chem. C, 2022, 10, 1162–1195with relatively short branched alkyls, typically 2-ethylhexyl, as
N-substituents and linear alkyl groups as end substituents.
Meanwhile, Takahashi et al. prepared DPP-based A–D–A
conjugates 4-7a–d with tetrabenzoporphyrin (BP) as the central
donor unit, and studied the effect of N-alkyl chains on theperformance of these conjugates as p-type materials.89 A uniqueaspect of this series of compounds is that they are essentiallyinsoluble because of the large, considerably rigid p-frameworkand the minimal alkyl substituents. Accordingly, compounds
4-7a–d were deposited via a thermal precursor approach,wherein soluble precursor compounds 4-8a–d were deposited
as a solution then transformed to the target compounds bythermally induced retro-Diels–Alder reactions in the solid state(Scheme 1). The solubilizing dimethylbicyclo[2.2.2]octadieno
units of the precursors can be quantitatively converted to the
benzo units to from a BP framework, and the by-produced
isobutene molecules escape from the film as a gas. By employing
this approach, one can design SMSCs with a minimum of bulky
flexible solubilizing substituents if not at all, and thus the role ofsubstituents becomes largely focused on the control of molecular
packing and phase-separation behaviors in the solid state.90
Comparative evaluation of 4-7a–d in BHJ OPVs with PC61BM
revealed that the PCE was higher when the N-alkyl groups wereshorter: 5.2% for 4-7a with butyl, 4.5% for 4-7b with hexyl, 3.1%
for 4-7c with octyl, and 0.19% for 4-7d with decyl (entries 4-19–4-22). One of the problems associated with longer alkyl chains
was that they induced excessive phase separation with then-type material PC61BM during the solid-state thermal reactions
for generating 4-7a–d from precursors 4-8a–d. Note that theblended films before thermal reactions (i.e., 4-8:PC61BM films)
were very smooth and highly homogeneous, probably because
the steric effect of the bulky solubilizing units prevented theprecursor compounds from self-aggregation. Another issue withThis journal is © The Royal Society of Chemistry 2022

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Review

Scheme 1 Thermal conversion of precursors 4-8a–d to 4-7a–d. (* Mixtures ofstereoisomers regarding the orientation of dimethylbicyclo[2.2.2]octadieno units.)

longer alkyl chains was that they induced edge-on arrangements
of molecules during the solid-state thermal reactions, whichsignificantly limits carrier transport in the out-of-plane direction
and thus the overall photovoltaic process. The dependence ofmolecular arrangement on alkyl-chain lengths was clearly observedin the2D-GIWAXS patterns of neat films (Fig. 8a–d); namely, butyl
and hexyl derivatives 4-7a and 4-7b showed clear diffractions on theqz axis around qz = 1.79 Å which indicated that these compounds
formed p–p stacking in the out-of-plane direction with a face-on
molecular orientation. The data of 4-7c showed an arc-shaped
diffraction around q = 1.75 Å, indicating a rather random orientation of p–p stacking. On the other hand, it was apparent from thestrong diffraction on the qxy axis around qxy = 1.74 Å that 4-7d had ahigh tendency to form p–p stacking in the in-plane direction withedge-on molecular orientation. The difference in molecular orientation became somewhat less distinct, but was generally preservedin BHJ films with PC61BM (Fig. 8e–h). These results demonstrated
well the cruciality of substituent design also for those SMSCs
processed via the thermal precursor approach.

6. Substituents of p-conjugatesbearing dithienosilole as a core unit
Rigid fused-ring p-systems have been widely employed as a coreunit in donor–acceptor (D–A)-linked SMSCs for BHJ OPVs.

Journal of Materials Chemistry C
Among various synthetically accessible structures, 5–5–5-tricyclic
systems such as dithienosilole (DTS) and dithienopyrrole (DTP)
are the commonly employed types of electron-donating core units.
A typical example of this class of compounds is a DTS-based small
molecule called F-DTS or p-DTS(FBTTh2)2 which afforded PCEs of
up to 7.0% in BHJ OPVs with PC71BM.91 This section overviews
the substituent impact on DTS-based SMSCs. The chemical
structures of the relevant compounds and the corresponding
photovoltaic data are summarized in Fig. 9 and Table 5,
respectively.
Several papers have reported the substituent eﬀects on theperformance of DTS-based p-type SMSCs. For example, Ye et al.
reported three A–D–A systems 5-1a–c bearing a DTS frameworkas the central D unit in 2013.92 In BHJ OPVs with PC61BM,
compound 5-1a performed better as a p-type material than thecorresponding tetramethyl derivative 5-1b (PCEs = 3.27% and2.88%, entries 5-1 and 5-2). This difference was mainly attributed
to the change in FF (56% vs. 49%), and the authors speculated
that the four methyl groups on the thiophene rings in 5-1b
induced an unfavorable BHJ morphology for the photovoltaic
process. Accordingly, the BHJ films of 5-1a:PC61BM and5-1b:PC61BM showed opposite responses to thermal annealing;
namely, the former systems gave the best device results without
thermal annealing, while the latter system required thermal
annealing to exhibit its optimal performance. The othercompound 5-1c afforded an even higher PCE (3.81%, entry 5-3)
than 5-1a. The authors mentioned that this improvement with
5-1c was not only due to its wider light-absorption range causedby the replacement of the ester groups with cyano groups, butalso due to a rougher morphology most likely caused by its lower
solubility. Note that certain degrees of film roughening has been
known to induce improvements in charge-transport efficiency.93–95
In addition, Min and co-workers reported substituent eﬀects
on the photovoltaic performance of a similar series of DTSbased p-type small molecules 5-2a and 5-2b.96 Evaluationof these compounds in BHJ OPVs with PC71BM revealed
much higher performance with the methyl derivative 5-2aFig. 8 2D-GIWAXS patterns of (a) 4-7a, (b) 4-7b, (c) 4-7c, (d) 4-7d, (e) 4-7a:PC61BM, (f) 4-7b:PC61BM, (g) 4-7c:PC61BM, and (h) 4-7d:PC61BM. All sample
films were prepared by the thermal precursor approach. Adopted from ref. 89 with permission. Copyright 2017, Royal Society of Chemistry.

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Fig. 9 Chemical structures of (a) SMSCs 6-1-6-4 and (b) n-type polymer N2200. The best PCEs obtained with each DTS derivative are shown inparentheses (see Table 5 for full OPV parameters). The inset shows relevant substitution positions of DTS-based SMSCs.

(PCE = 5.30%) than the hexyl derivative 5-2b (1.25%) because ofa higher JSC and FF (entries 5-4 and 5-5). The authors showedthat the terminal alkyl groups (i.e., the methyl or hexyl chains)
had a significant eﬀect on the intermolecular interaction andsolubility, and thus the morphological characteristics andsemiconducting properties of these compounds in the BHJ
active layers. In particular, the 5-2b system showed a relatively
ineﬃcient and unbalanced charge-carrier transport which
resulted in considerable non-geminated recombination. On
the other hand, the 5-2a system did not show signs of chargemobility limitation and enhanced bimolecular recombination.

Jung et al. compared three A–D–A conjugates comprising the
DTS and DPP frameworks (5-3a–c).97 These compounds were
substituted with octyl or 2-ethylhexyl, respectively, at the silicon
and nitrogen atoms, and showed considerably different
performances in BHJ OPVs with PC61BM (entries 5-6–5-8; notethat the corresponding fully octyl substituted compound 5-3d
was also prepared but not evaluated in an OPV because of its
insufficient solubility). In thermally annealed BHJ active layers,
the Si-octyl/N-2-ethylhexyl derivative 5-3c afforded a considerably
higher PCE of 3.5% than the Si-2-etheylhexyl/N-2-ethylhexy
derivative 5-3a (0.5%) and the Si-octyl/N-2-ethylhexy derivative

Table 5Photovoltaic performance of SMSCs 5-1-5-4a

Entry

Active layer

Solvent

Additional conditions

VOCb (V)JSCb (mA cm2)

FFb (%)

PCEb (%)

Ref.

5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
5-10
5-11
5-12

5-1a : PC61BM (1 : 0.8)
5-1b : PC61BM (1 : 0.8)
5-1c : PC61BM (1 : 0.8)
5-2a : PC71BM (1 : 0.8)
5-2b : PC71BM (1 : 0.8)
5-3a : PC61BM (1 : 1)
5-3b : PC61BM (1 : 1)
5-3c : PC61BM (1 : 1)
5-4a : N2200 (3 : 2)
5-4b : N2200 (3 : 2)
5-4c : N2200 (3 : 2)
5-4d : N2200 (3 : 2)

CF
CF
CF
CF
CF
CF
CF
CF
CB
CB
CB
CB

—
Thermal annealing
Thermal annealing––
110 1C, 10 min
110 1C, 10 min
110 1C, 10 min
0.2 vol% DIO; 120 1C, 10 min
0.2 vol% DIO; 120 1C, 10 min
0.2 vol% DIO; 120 1C, 10 min
0.2 vol% DIO; 120 1C, 10 min

0.92
0.89
0.92
0.85
0.91
0.82
0.70
0.77
0.825
0.784
0.807
0.787

6.37
6.61
8.73
10.05
4.37
2.1
3.7
7.5
5.51
5.65
4.18
6.08

56
49
48
62.0
31.5
30
47
60
56.4
60.0
57.7
63.3

3.27
2.88
3.81
5.30
1.25
0.5
1.2
3.5
2.45
2.68
1.95
3.04

92
92
92
96
96
97
97
97
98
98
98
98

a
Structures of active-layer compounds are shown in Fig. 9. CF: chloroform, CB: chlorobenzene, DIO: 1,8-diiodobenzene. b Data of the bestperforming devices.

1176 | J. Mater. Chem. C, 2022, 10, 1162–1195

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Review
5-3b (1.2%). This superiority in the performance of 5-3c wasattributed to its isotropic arrangement of intermolecular p–p
stacks in the active layer, which was more favorable for chargecarrier transport in OPVs than the edge-on-preferred orientationof 5-3a and 5-3b.
In 2018, Han et al. reported a comparison between 5-4a(p-DTS(FBTTh2)2) and its cyclohexyl end-capped derivative 5-4b.98
The two compounds were evaluated as p-type materials in BHJ
OPVs with an n-type polymer N2200 (entries 5-9 and 5-10).
Analyses with 2D-GIWAXS and AFM revealed that the 5-4b:
N2200 film had a more pronounced face-on molecular orientation
and a more coarse morphology than the 5-4a:N2200 film. The
authors attributed these diﬀerences in the microstructure ofactive layers to the higher JSC and FF obtained with 5-4b.
Additionally, the same trend was observed with the corresponding
dithienogermoles 5-4c and 5-4d in terms of both photovoltaic
performance (entries 5-11 and 5-12) and film microstructure.
As observed also for other DTS derivatives and similar linearlyextended p-systems in the following sections, it seems generally
advantageous to keep the end substituents relatively compact inorder to obtain high PCEs.

7. Substituents of p-conjugates bearing
benzodithiophene as the core unit
The benzo[1,2-b:4,5-b 0 ]dithiophene (BDT) unit has been a
popular building unit of organic semiconductors for OPV
applications because of the rigid p-conjugated framework witha suitable electron-donating nature.99–101 For example, BDT
was used as the donor unit in PTB7 which has been one of
the most studied high-performance p-type polymers for OPVs
since its first report in 2010.102 This section reviews recent
prominent papers reporting the substituent impact on theperformance of BDT-based SMSCs in OPVs. The compounds
are separately discussed depending on the position of thesubstituents of interest, namely, core, end, or side substituents(Fig. 10). The chemical structures of the relevant compounds
are summarized in Fig. 10 and 11, while their photovoltaic
performance is listed in Table 6. The examples below demonstrate that 5-(thio)alkyl-2-thienyl is the preferred choice as core
substituents, while linear alkyls are common among highperformance BDT-based conjugates.
Core substituents
Wang et al. evaluated a series of BDT–DPP conjugates for theirperformance as p-type materials in BHJ OPVs.103 A pair ofdifferently core substituted derivatives 6-1a and 6-1b were apart of the series, and the di(2-ethylhexyl) derivative 6-1a wasfound to afford a higher PCE than the tetrahexyl derivative 6-1b
(4.4% vs. 0.6%) in thermally annealed BHJ films with PC61BM(entries 6-1 and 6-2). The considerably lower performance of
6-1b was attributed to its high crystallinity which induced
excessive phase separation with PC61BM upon thermal annealing
as confirmed by TEM analysis of the BHJ films. Without thermal
annealing, 6-1a and 6-1b gave PCEs of 2.4% and 1.1%,

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Journal of Materials Chemistry C
respectively; thus, the difference between the two BHJ systems
was much smaller (entries 6-3 and 6-4).
Similarly, Zhang et al. highlighted the correlation between
core substituents, molecular crystallinity, and photovoltaic
performance in a comparative evaluation of BDT–DPP conjugates
6-2a and 6-2b (in addition to another derivative which is not
mentioned here because its p-conjugated backbone is different
from those of 6-2a and 6-2b).104 In BHJ OPVs with PC61BM, thedithienyl-BDT derivative 6-2a afforded a PCE of 5.15%, while thebis(bithienyl) derivative 6-2b gave a PCE of 6.04% at best (entries
6-5 and 6-6). The authors showed that compound 6-2b had a lower
crystallinity and finer phase separation than 6-2a in the BHJ films,
which could lead to a more effective exciton diffusion and chargecarrier transport, thus higher JSC and FF. The crystallinity andphase separation in the 6-2a:PC61BM film were considered
excessive for smooth photovoltaic process.
Meanwhile, the 2-ethylhexyloxy-substituted derivative 6-3a
showed a lower performance than the corresponding 5-(2ethylhexyl)thienyl substituted derivative 6-3b.105 The substitution
with relatively small 2-ethylhexyloxy groups led to a higher degreeof crystallization of 6-3a in BHJ films, but the film morphology
was fixed such that the annealing eﬀect was rather limited.
In contrast, the morphology of the 6-3b:PC61BM film wassignificantly improved upon the use of a molecular additive(DIO) and thermal annealing to form well-developed phaseseparation. Accordingly, the best PCEs were 4.19% and 7.12%
with 6-3a and 6-3b, respectively (entries 6-7 and 6-8). The 7.12%
PCE was the highest at that time among those obtained withsmall-molecule DBT–DPP conjugates. Similar comparisons were
performed between BDT–DTP conjugates 6-4a and 6-4b,106 as
well as between BDT–difluorobenzotriazole conjugates 6-5a
and 6-5b (entries 6-9–6-12).107 In these cases again, the 5-(2ethylhexyl)thienyl substituted derivatives 6-4a and 6-5a performed
better (PCE = 5.90% and 9.73%) than the corresponding
2-ethylhexyloxy substituted derivatives 6-4b and 6-5b (5.12% and5.51%) in BHJ OPVs.
The same trend was further observed in a comparativeevaluation of BDT–rhodanine conjugates by Yin and co-workers,
wherein the dodecyloxy derivative 6-6a and 5-dodecylthienyl
derivative 6-6b afforded PCEs of 4.25% and 6.99%, respectively,
in BHJ OPVs with PC71BM (entries 6-13 and 6-14).108 Denser
molecular packing and higher hole mobility of the alkylthienyl
derivative were highlighted in this study similar to the cases of
6-3-6-5; thus, the superiority of alkylthienyl over alkyloxy as core
substituents of the BDT unit seems broad in scope.
Yin et al. also evaluated the corresponding alkylthiothienyl
derivative 6-6c in the same paper, which aﬀorded a slightly
lower PCE (6.78%) than the alkylthienyl derivative 6-6b (entry
6-15). This result contrasted with the study by Min and co-workers
in 2016 which reported that the 5-(2-ethylhexyl)thiothienyl derivative 6-7b aﬀorded a PCE of 9.20%, while the 5-(2-ethylhexyl)thienyl
derivative 6-7a gave only 7.44% at best in BHJ OPVs with PC71BM(entries 6-16 and 6-17).109 These opposite trends between 6-6 and6-7 should be related to the structure of alkyl groups (i.e., dodecyl
vs. 2-ethylhexyl), demonstrating the subtle nature of substituent
impact. Note here that the advantage of 5-(2-ethylhexylthio)thienyl

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Fig. 10 Chemical structures of (a) SMSCs 6-1-6-9 and (b) the compounds employed with 6-1-6-9 in OPV active layers. PC61BM and PC71BM are shownin Fig. 2, and IDIC is shown in Fig. 4. The best PCEs obtained with each BDT derivative are shown in parentheses (see Table 6 for full OPV parameters). The
inset shows relevant substitution positions of BDT-based SMSCs.

1178 | J. Mater. Chem. C, 2022, 10, 1162–1195

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Fig. 11 Chemical structures of (a) SMSCs 6-10-6-13 and (b) the compounds employed with 6-10-6-13 in OPV active layers. PC 61BM and
PC 71BM are shown in Fig. 2. The best PCEs obtained with each BDT derivative are shown in parentheses (see Table 6 for full OPV
parameters).

over 5-(2-ethylhexyl)thienyl was also observed between dimericporphyrin compounds 6-8a (PCE = 10.03%) and 6-8b (8.29%) asreported by Piradi et al. in 2020 (entries 6-19 and 6-20).110
Additionally, Min et al. also reported that the corresponding5-(2-ethylhexyloxyl)thienyl derivative 6-7c gave a good 6.50%
PCE (entry 6-18). Although this PCE was somewhat lower thanthose of 6-7a and 6-7b, a significant decrease in JSC as in
the case of 6-6a was not observed. Therefore, the generaleﬀectiveness of thienyl substitution, rather than direct alkyloxy
substitution, on BDT was also supported in this case.
Concerning branched alkyl substituents such as 2ethylhexyl, Zhou et al. studied the eﬀect of the branching pointby comparing dithienobenzodithiophene (DTBDT)-based
SMSCs 6-9a–c.111 Their evaluation revealed that, by moving
the branching point away from the DTBDT core, the crystallinity and intermolecular p–p stacking were enhanced in BHJThis journal is © The Royal Society of Chemistry 2022

films with a non-fullerene n-type compound IDIC-4F or Y6. Thischange in molecular arrangement resulted in the improvementof JSC and FF. Accordingly, PCEs of the corresponding BHJ
OPVs showed monotonic increase for 6-9a–c from 11.79% to12.84% and to 14.78% with IDIC-4F, or from 9.27% to 9.91%
and to 10.35% with Y6 (entries 6-21–26).
End substituents
Patra et al. compared cyanoacetate and dicyanovinyl units aselectron-accepting end substituents of BDT-based A–D–A p-typemolecules 6-10a and 6-10b.112 In their evaluation, it was foundthat the n-octyl cyanoacetate derivative 6-10a (PCE = 5.42%)
performed better than the dicyanovinyl derivative 6-10b (2.13%)
as a p-type material in BHJ OPVs with PC61BM (entries 6-27 and6-28). While the 6-10a:PC61BM and 6-10b:PC61BM systems were
essentially the same in terms of VOC, JSC and FF were found to

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Table 6

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Entry Active layer
6-1
6-2
6-3
6-4
6-5
6-6
6-7
6-8
6-9
6-10
6-11
6-12
6-13
6-14
6-15
6-16
6-17
6-18
6-19
6-20
6-21
6-22
6-23
6-24
6-25
6-26
6-27
6-28
6-29
6-30
6-31
6-32
6-33
6-34
6-35
6-36
6-37
6-38
6-39
6-40
6-41
6-42

Review

Solvent Additional conditionsCF
6-1a : PC61BM (1 : 1)CF
6-1b : PC61BM (1 : 1)CF
6-1a : PC61BM (1 : 1)CF
6-1b : PC61BM (1 : 1)CF
6-2a : PC61BM (1 : 1)CF
6-2b : PC61BM (1 : 1)CB
6-3a : PC71BM (1 : 1)CB
6-3b : PC71BM (1 : 1)CF
6-4a : PC71BM (1 : 2)CF
6-4b : PC71BM (1 : 2)
6-5a : IDIC (1 : 1)CF
6-5b : IDIC (1 : 1)CF
CF
6-6a : PC71BM (1 : 1)CF
6-6b : PC71BM (1 : 1)CF
6-6c : PC71BM (1 : 1)CF
6-7a : PC71BM (1 : 0.8)CF
6-7b : PC71BM (1 : 0.8)CF
6-7c : PC71BM (1 : 0.8)
6-8a : 6TIC (1 : 0.6)CF
6-8b : 6TIC (1 : 0.6)CF
6-9a : Y6 (1 : 0.55)
CF
6-9b : Y6 (1 : 0.55)
CF
6-9c : Y6 (1 : 0.55)
CF
6-9a : IDIC-4F (1 : 0.6)CF
6-9b : IDIC-4F (1 : 0.6)CF
6-9c : IDIC-4F (1 : 0.6)CF
6-10a : PC61BM (1 : 0.40) CF
6-10b : PC61BM (1 : 0.60) CF
6-11a : PC71BM (1 : 1.5) CF
6-11b : PC71BM (1 : 1.5) CFCF
6-12a : PC71BM (1 : 1)CF
6-12b : PC71BM (1 : 1)
6-13a : IT-4F (1 : 1)CF
6-13b : IT-4F (1 : 1)CF
6-13c : IT-4F (1 : 1)CF
6-13d : IT-4F (1 : 1)CF
6-14a : IDIC-4Cl (1.5 : 1) —c
6-14b : IDIC-4Cl (1.5 : 1) —c
6-14c : IDIC-4Cl (1.5 : 1) —c
6-15a : PC71BM (1.5 : 1) CF
6-15b : PC71BM (1.5 : 1) CF
6-15b : PC71BM (1.25 : 1) CF

VOCb (V)JSCb (mA cm2) FFb (%)

0.2% CN; 110 1C, 10 min
0.83
9.36
0.2% CN; 110 1C, 10 min
0.86
2.43
0.2% CN
0.92
6.76
0.2% CN
0.97
3.59
110 1C, 10 min
0.837
11.00
0.75% CN; 110 1C, 10 min
0.799
12.17
0.4 vol% DIO; 100 1C, 10 min
0.88
7.87
0.4 vol% DIO; 100 1C, 10 min
0.89
10.9
TSA
0.92
10.68
TSA
0.87
10.15
120 1C, 10 min
0.977
15.21
120 1C, 10 min
0.955
10.51
CF vapour, 1 min
0.88
7.38
CF vapour, 1 min
0.90
11.69
CF vapour, 1 min
0.94
11.70
—
0.93
11.75
—
0.97
13.45
—
0.90
11.03
2.0 vol% Py + 0.5 vol% DIO; 120 1C, 10 min 0.79  0.01 19.36  0.13
2.0 vol% Py + 0.5 vol% DIO; 120 1C, 10 min 0.75  0.01 17.27  0.16
120 1C, 10 min
0.848
21.35
120 1C, 10 min
0.852
23.03
120 1C, 10 min
0.854
24.69
120 1C, 10 min
0.796
18.15
120 1C, 10 min
0.783
18.27
120 1C, 10 min
0.776
18.95
—
0.90
9.08
—
0.91
5.17
1.0 vol% DIO
0.62
15.64
0.8 vol% DIO
0.67
8.35
0.5% CN; 120 1C, 10 min
0.69
13.39
0.3% CN; 120 1C, 10 min
0.65
6.08
THF vapour
0.893
16.66
THF vapour
0.909
18.27
THF vapour
0.929
17.92
THF vapour
0.928
16.15
120 1C; THF vapour, 40 s
0.564
4.9
120 1C; THF vapour, 40 s
0.865
20.1
120 1C; THF vapour, 40 s
0.870
14.2
80 1C, 30 min
0.75
4.0
80 1C, 30 min
0.87
8.82
80 1C, 30 min
0.87
10.54

PCEb (%)

Ref.

56
4.4
103
29
0.6
103
39
2.4
103
30
1.1
103
55.8
5.15
104
62.1
6.04
104
60.4
4.19
105
73.6
7.12
105
60
5.90
106
58
5.12
106
65.46
9.73
107
54.89
5.51
107
65.39
4.25
108
66.48
6.99
108
61.61
6.78
108
68.1
7.44
109
70.5
9.20
109
65.5
6.50
109
65.6  0.6 10.03  0.29 110
64.0  0.5 8.29  0.25 110
65.12
11.79
111
65.43
12.84
111
70.06
14.78
111
64.16
9.27
111
69.29
9.91
111
70.41
10.35
111
66
5.42
112
46
2.13
112
59.4
5.77
113
57.0
3.20
113
56
5.12
114
60
2.36
114
64
9.52
115
68
11.24
115
63
10.52
115
61
9.14
115
33.9
0.90
116
71.3
12.40
116
67.0
8.25
116
35.3
1.06
117
68.9
5.30
117
71.4
6.55
117

a

Structures of active-layer compounds are shown in Fig. 10 and 11. CF: chloroform, CN: 1-chloronaphthralene, DIO: 1,8-diiodooctane, TSA:
thermal followed by solvent annealing; Py: pyridine; THF: tetrahydrofuran. b Data of the best-performing devices when available; otherwise,
average of multiple devices. c Information not provided.

be considerably higher in the former system. This result could
be traced back to the favorable morphological characteristics of
6-10a, namely, its higher miscibility with PC61BM that led tofine nanoscale phase separation and higher crystallinity,
enabling the formation of effective p–p staking in the BHJ film.
Both factors should bring about more efficient transport andreduced recombination of charge carriers, and thus improvements
in JSC and FF.
The eﬀects of diﬀerent end substituents were also studied
with BDT–DPP conjugates 6-11a and 6-11b.113 The N,Ndiphenylamino group of 6-11a affected the electronic characteristics of the p-conjugated system, resulting in an enhanced
light absorbance by a factor of 46% when compared to that ofbutyl substituted 6-11b. However, this effect did not fully
explain the large difference in JSC: 15.64 vs. 8.35 mA cm2 with
6-11a and 6-11b, respectively (entries 6-29 and 6-30). Further

1180 | J. Mater. Chem. C, 2022, 10, 1162–1195

analyses revealed that the aromatic branched diphenylamino
end group brought about a higher compatibility with PC61BM
to form finer phase separation that led to more efficientphoton-to-charge conversion in the BHJ film. In addition,
charge recombination probability was reduced. With all thesefactors combined, 6-11a afforded a significantly higher JSC andthus PCE (5.77%) than 6-11b (3.20%).
In relation to the latter work, Hoang et al. reported theeﬀect of a hexyl end group on the photovoltaic performanceof a similar BDT–DPP system.114 The authors compared two
derivatives 6-12a and 6-12b as p-type materials in BHJ OPVs
with PC71BM, and found that the non-substituted derivative
6-12a yielded a higher PCE (5.12%) than the hexyl-substituted
derivative 6-12b (2.36%) (entries 6-31 and 6-32). Again, thedifference in PCE could be traced back to the morphologicalbehavior of the two compounds, namely, the introduction ofThis journal is © The Royal Society of Chemistry 2022

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Review
end-hexyl groups led to excessive phase separation in
the 6-12b:PC71BM blend film which significantly deteriorated charge-carrier generation and transport, and thus the
J SC (13.39 and 6.08 mA cm2 with 6-12a and 6-12b,
respectively).
Yang et al. compared rhodanine-appended BDT trimers
6-13a–d (entries 6-33–6-36).115 These four derivatives aredifferent from each other in the length of N-alkyl chains of
the rhodanine units, and the N-butyl derivative 6-13b was foundto perform the best as a p-type material in BHJ OPVs with IT-4F,
affording a PCE of 11.24% associated with a high JSC of
18.27 mA cm2. Notably, the 2D-GIWAXS analysis revealed
that molecular orientation in thin films shifted from edge-on
to face-on with the elongation of N-alkyls from ethyl (6-13a)
to octyl (6-13d). The authors explained the superiority of 6-13b
to other derivatives based on the molecular orientation, crystallinity, and phase-separation behavior (i.e., domain sizes),
which all affected charge-carrier mobility in BHJ layers.
In addition, the preferable morphology brought abouthigh tolerance of the 6-13b:IT-4F system to film thickness;
specifically, it retained 10% PCE with an active-layer thicknessof up to 300 nm.
The eﬀect of the branching point in end alkyl groups wasinvestigated with a series of BDT–rhodanine conjugates
6-14a–c.116 Indeed, the difference in PCE was drastic in BHJ
OPVs with IDIC-4Cl, that is, merely 0.90% with 3-heptyl substituted 6-14a, 12.40% with 2-ethylhexyl derivative 6-14b, and8.25% with 3-ethylpentyl derivative 6-14c (entries 6-37–6-39).
Red-shifted absorption and enhanced crystallinity wereobserved as the branching point of end alkyls shifted awayfrom the p-conjugated backbone, indicating that outerbranching induced stronger intermolecular interactions among
this series of compounds. An excellent PCE of 12.40% with
6-14b could be due to its favourable degree of phase separation
against the n-type material (IDIC-4Cl) to form a large heterojunction area with sufficient charge-carrier transport paths.

Side substituents
Zhang et al. reported a comparison between diﬀerently sidesubstituted BDT derivatives. They prepared two A–D–A-typep-conjugates 6-15a and 6-15b comprising BDT and indanedione
units linked via terthiophene linkers, and evaluated them in
BHJ OPVs with PC71BM as the n-type material.117 The structuraldifference between the two conjugates was the alky substitution
pattern at the terthiophene linkers: 3,300 -dihexyl (inward
substitution) for 6-15a and 4,400 -dihexyl (outward substitution)for 6-15b. The outward substitution resulted in a slightly redshifted absorption with a higher molar absorbance in solution
and smoother, a more homogeneous morphology of BHJ films,
which collectively contributed to the significantly higher photovoltaic performance of the 6-15b:PC71BM blend (PCE = 5.30%)
than that of the 6-15a:PC71BM blend (1.06%) in thecomparative evaluation under the same conditions (entries
6-40 and 41). Note that 6-15b:PC71BM was further optimized
to afford a 6.55% PCE (entry 6-42).

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Journal of Materials Chemistry C

8. Substituents ofindacenodithiophene-based
p-conjugates
The five-ring-fused dihydro-s-indaceno[1,2-b:5,6-b 0 ]dithiophene(ID) framework and the seven-ring-fused dihydrodithieno[2,3d:2 0 ,3 0 -d 0 ]-s-indaceno[1,2-b:5,6-b 0 ]dithiophene (IT) framework
stand out as building blocks of high-performance n-type molecules.
In particular, the IT and ID frameworks are the core units of ITIC
and IDIC, respectively, which are both iconic non-fullerene n-typematerials for OPVs (Fig. 12).118,119 This section reviews recent
studies which investigated substituent impact for n-type materials
bearing an IT- or ID-based core unit. The corresponding chemical
structures and photovoltaic data are shown in Fig. 13 and Table 7,
respectively.
Molecules with a dithienoindacenodithiophene core
Substituents at the 6- and 12-positions of the IT units protrude
from the p-plane, and thus play important roles in regulatingintermolecular interactions and morphology in BHJ films.
Yang et al. comparatively evaluated ITIC and its isomer 7-1,
which had hexyl groups on the four phenyl rings at the 6- and12-carbons of the IT core.120 Both compounds performed wellin BHJ OPVs with a medium bandgap polymer J61121 as thep-type material, aﬀording PCEs of 10.57% with ITIC and11.77% with 7-1 (entries 7-1 and 7-2). The higher performanceof the m-hexylphenyl derivative 7-1 was attributed to its higher
electron mobility (2.45  104 cm2 V1 s1) than that of thep-hexylphenyl derivative ITIC (1.60  104 cm2 V1 s1), which
could be further traced back to higher crystallinity and stronger
preference for face-on molecular orientation of 7-1 in the active
layers. These factors corresponded well with the improvements
in JSC (17.97 - 18.31 mA cm2) and FF (65.49 - 70.55%).
On the other hand, Liu et al. reported that replacement of all
the p-hexylphenyl groups of ITIC with p-(2-methoxyethoxy)
methylphenyl to form derivative 7-2 resulted in lowering of the
PCE from 10.4% to 8.5% (entries 7-3 and 7-4).122 The polar
substituents of 7-2 led to decreased crystallinity and excessive
mixing with the p-type polymer PBDB-T,123 which induced lower
mobilities and enhanced recombination of charge carriers.
Liu et al. compared end-chlorinated ITIC derivatives 7-3a–cwhich differed in their alkyl-chain length in the four p-alkylphenyl groups.124 The performance of the corresponding BHJ
OPVs with a p-type polymer PBDB-TF (or PM6)125 was increased

Fig. 12 Chemical structures of the ID and IT frameworks, and the nonfullerene n-type molecule ITIC. The structure of IDIC is shown in Fig. 4.

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Fig. 13 Chemical structures of SMSCs 7-1-7-17. The best PCEs obtained with each compound are shown in parentheses (see Table 7 for full OPV
parameters). The inset shows relevant substitution positions of BDT-based SMSCs.

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Journal of Materials Chemistry C

Table 7

Photovoltaic performance of SMSCs 7-1-7-17a

Entry

Active layer

Solvent

Additional conditions

VOCb (V)JSCb (mA cm2)

FFb (%)

PCEb (%)

Ref.

7-1
7-2
7-3
7-4
7-5
7-6
7-7
7-8
7-9
7-10
7-11
7-12
7-13
7-14
7-15
7-16
7-17
7-18
7-19
7-20
7-21
7-22
7-23
7-24
7-25
7-26

J61 : ITIC (1 : 1)
J61 : 7-1 (1 : 1)
PBDB-T : ITIC (1 : 1)
PBDB-T : 7-2 (1 : 1)
PM6 : 7-3a (1 : 1)
PM6 : 7-3b (1 : 1)
PM6 : 7-3c (1 : 1)
PBT1-C : 7-4a (1 : 1.3)
PBT1-C : 7-4b (1 : 1.3)
PBT1-C : 7-4c (1 : 1.3)
PBDB-T : 7-5 (1 : 1)
PBDB-T : 7-5 (1 : 1)
PBDB-T : ITIC (1 : 1)
PBDB-T : 7-6a (1 : 1)
PBDB-T : 7-6b (1 : 1)
PBDB-T : 7-6c (1 : 1)
PBDB-T : 7-6d (1 : 1)
FTAZ : 7-7a (1 : 1.5)
FTAZ : 7-7b (1 : 1.5)
FTAZ : 7-7c (1 : 1.5)
FTAZ : 7-7d (1 : 1.5)
PBDB-T : 7-8a (1 : 1)
PBDB-T : 7-8b (1 : 1)
PBDB-T : 7-8c (1 : 1)
PBDB-T : 7-8d (1 : 1)
PBDB-T : 7-8c (1 : 1)

CF
CF
CB
CB
CB
CB
CB
CF
CF
CF
CB
CB
CB
CB
CB
CB
CB
CF
CF
CF
CF
CB
CB
CB
CB
CB

0.898
0.912
0.89
0.85
0.78
0.83
0.87
0.85  0.01
0.88  0.01
0.98  0.01
0.97
0.97
0.91
1.01
0.93
0.97
0.96
0.915
0.849
0.751
0.962
0.89
0.90
0.89
0.90
0.88

17.97
18.31
16.5
14.8
18.85
20.44
20.32
17.0  0.3
20.3  0.2
18.1  0.3
15.85
16.41
16.27
12.31
17.53
16.38
14.69
15.84
19.33
17.19
16.34
13.47
14.94
14.94
14.88
16.30

65.49
70.55
71
67
45
76
77
66.7  2.4
74.6  1.1
71.3  1.3
68
73
69
51
73
68
56
61.26
73.73
70.07
68.33
59.03
55.09
58.21
59.35
64.18

10.57
11.77
10.4
8.5
6.67
12.88
13.54
10.0 (9.6  0.5)
13.7 (13.4  0.2)
12.7 (12.5  0.4)
10.45
11.61
10.21
6.3
11.9
10.8
7.9
8.88
12.1
9.06
10.7
7.04
7.43
8.26
7.93
9.29

120
120
122
122
124
124
124
126
126
126
128
128
128
129
129
129
129
130
130
130
130
132
132
132
132
132

7-27
7-28
7-29
7-30
7-31
7-32
7-33
7-34
7-35
7-36
7-37
7-38
7-39
7-40
7-41
7-42
7-43
7-44
7-45
7-46
7-47
7-48
7-49
7-50
7-51
7-52
7-53
7-54

PBDB-T : ITIC (1 : 1)
PBDB-T : 7-9a (1 : 1)
PBDB-T : 7-9b (1 : 1)
PM6 : 7-9b (1 : 1)
PBDB-T : 7-10a (1 : 0.5)
PBDB-T : 7-10b (1 : 1)
PBDB-T : 7-10c (1 : 1)
PBDB-T : IDIC (1 : 1)
PBDB-T : 7-11a (1 : 1)
PBDB-T : 7-11b (1 : 1)
PM6 : 7-11b (1 : 1)
PM6 : IDIC (1 : 1)
PM6 : IDIC (1 : 1)
PM6 : IDIC (1 : 1)
PM6 : 7-12 (1 : 1)
PM6 : 7-12 (1 : 1)
PM6 : 7-12 (1 : 1)
PTB7-Th : 7-13a (1 : 1.5)
PTB7-Th : 7-13b (1 : 1.5)
PBDB-T : 7-14 (1.5 : 1)
PBDB-T : IDIC (1.5 : 1)
PBDB-T : 7-15a (1 : 1)
PBDB-T : 7-15b (1 : 1)
PBDB-T : IDIC (1 : 1)
PBDB-T : 7-16a (1 : 1)
PBDB-T : 7-16b (1 : 1)
PTQ10 : IDIC-2F
PTQ10 : 7-17

CB
CB
CB
CB
o-DCB
o-DCB
o-DCB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
CB
o-DCB
o-DCB
o-DCB
CF
CF
CF
CF

130 1C, 5 min
130 1C, 5 min
0.5 vol% DIO; 120 1C, 10 min
0.5 vol% DIO; 120 1C, 10 min
100 1C, 10 min
100 1C, 10 min
100 1C, 10 min
0.5% DIO
0.5% DIO
0.5% DIO—
125 1C, 10 min
0.5% DIO
0.5 vol% DIO; 150 1C, 30 min
0.5 vol% DIO; 150 1C, 30 min
0.5 vol% DIO; 150 1C, 30 min
0.5 vol% DIO; 150 1C, 30 min
0.25 vol% DIO
0.25 vol% DIO
0.25 vol% DIO
0.25 vol% DIO
0.3% DIO
0.3% DIO
0.3% DIO
0.3% DIO
0.3% DIO; THF vapour,
1 min; 120 1C, 10 min
0.5% DPE
0.5% DPE
0.5% DPE
0.5% DPE———
130 1C, 5 min
130 1C, 5 min
130 1C, 5 min
130 1C, 30 s—
130 1C, 10 min
0.5 vol% DIO, 130 1C, 10 min—
130 1C, 10 min
0.5 vol% DIO, 130 1C, 10 min
0.5% DIO, 120 1C, 5 min
0.5% DIO, 120 1C, 5 min
0.5% DIO, 120 1C, 5 min
0.5% DIO, 120 1C, 5 min———
160 1C, 2 min
160 1C, 2 min
120 1C, 5 min
110 1C, 5 min

0.87
0.89
0.87
0.96
0.89
0.88
0.86
0.819
0.869
0.822
0.941
0.963
0.928
0.921
0.963
0.961
0.926
0.940
0.940
0.93
0.88
0.92  0.01
0.92  0.01
0.85  0.01
0.917
0.943
0.892
0.906

15.45
16.71
18.04
17.78
13.30
16.18
15.64
17.27
12.20
18.08
19.06
17.94
17.21
16.66
18.55
18.91
18.87
16.9
17.0
14.43
11.94
15.1  0.47
14.8  0.24
11.5  0.35
16.56
16.25
18.94
19.87

57
70
70
73
53.9
71.1
72.3
73.60
57.90
77.42
78.32
71.60
70.13
69.34
73.26
74.45
75.58
57
59
76.41
57.90
68  269  271  1
61.61
65.41
71.7
74.8

7.8
10.4
11.0
12.5
6.42
10.12
9.68
10.41
6.14
11.50
14.04
12.37
11.20
10.64
13.10
13.53
13.21
9.1
9.4
10.22
6.11
9.60 (9.40  0.16)
9.64 (9.35  0.24)
7.23 (6.96  0.16)
9.35
10.02
12.09
13.46

133
133
133
133
134
134
134
135
135
135
135
136
136
136
136
136
136
137
137
138
138
139
139
139
140
140
141
141

a
Structures of active-layer compounds are shown in Fig. 13 and 14. CF: chloroform, CB: chlorobenzene, o-DCB: o-dichlorobenzene, DIO: 1,8diiodooctane, THF: tetrahydrofuran, DPE: diphenylether. b Data of the best-performing devices when available; otherwise, average of multiple
devices.

as the alkyl chains were elongated from n-hexyl of 7-3a (PCE =6.67%) to n-octyl of 7-3b (12.88%), and to n-decyl of 7-3c
(13.54%) (entries 7-5–7-7). The authors concluded that the shortalkyl chains of 7-3a induced the formation of tight molecularaggregations and excessive phase separation in BHJ films with
PM6, leading to an increased non-radiative energy loss. The
longer alkyl chains in 7-3b and 7-3c, in contrast, loosened

This journal is © The Royal Society of Chemistry 2022

intermolecular contacts and enabled finer phase separation,
thereby suppressing the non-radiative energy loss.
Similarly, Ye et al. compared a series of A–D–A conjugates
7-4a–c comprising a 6,6,12,12-tetraalkylated IT core and 2-(5methylene-6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (TIC) end units.126 Single-crystal X-ray structure
analysis revealed that molecular stacking was largely affected by

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Journal of Materials Chemistry C
the length of alkyl chains on the IT core, namely, molecules ofn-hexyl derivative 7-4a crystallized in a p–p stacking mode, n-octyl
derivative 7-4b in a mixed p–p/no p–p-stacking mode, and n-decyl
derivative 7-4c in a no p–p stacking mode. Meanwhile, BHJ OPVs
with PBT1-C127 as the p-type material showed maximum PCEs of
10.0% with 7-4a, 13.7% with 7-4b, and 12.7% with 7-4c (entries
7-8–7-10). These results indicated that the end-group p–p staking
could be non-critical in achieving sufficient charge-carriertransport and high photovoltaic efficiency; however, close sideatom contacts in non-p-staking modes could provide effective
charge-carrier paths in the BHJ films. The authors also pointed
out that energy loss was reduced with longer-alkyl derivatives.
Zhang et al. investigated the eﬀect of outer substituents bycomparing n-hexyl substituted derivative 7-5 with ITIC.128 The
steric hinderance of the outer n-hexyl groups was expected tolock the orientation of the end electron withdrawing units,
leading to enhanced planarity of the overall p-conjugatedsystem. In addition, these alkyl chains were found to weaken
the p–p stacking, enhancing the compatibility of thiscompound with the donor polymer PBDB-T. These effects
brought about a PCE of 10.45% which is slightly higher thanthat with the original ITIC (10.21%, entries 7-11 and 7-13).
The efficiency was further improved to 11.61% (entry 7-12) afterthermal annealing at 125 1C which induced a higher degree ofphase separation and crystallization of the polymer to form
fibrous morphology.
The eﬀect of end-unit substituents in this class of n-type
compounds has also been studied by several groups. For
example, Li et al. evaluated four methoxy-substituted ITIC
derivatives 7-6a–d for their performance in BHJ OPVs with
PBDB-T (entries 7-14–7-17).129 Higher PCEs were obtained with
7-6b (11.9%) and 7-6c (10.8%) with methoxy groups at the
6- and 5-positions, respectively, of the two 2-(2-methylene-3oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile (IC) units.
In contrast, the derivatives with 7- and 4-methoxy IC units(7-6a and 7-6d) afforded considerably lower PCEs of 6.3% and7.9%. The superiority of 7-6b and 7-6c was explained withhigher planarity of their IC units due to lower steric hinderance, which led to a more efficient intermolecular p–p stacking
and higher charge-carrier mobilities. The effect of these factors
was observed as enhanced JSC and FF.
In a related vein, Li et al. compared four 6,6,12,12-tetra(5hexylthiophen-2-yl) IT derivatives diﬀerently substituted at the
benzo moiety of the IC end-capping units: non-substituted 7-7a(known as ITIC-Th), 5- or 6-fluoro derivative 7-7b, 5,6-difluoro
derivative 7-7c, and 6-methoxy derivative 7-7d.130 These substituents altered not only the electronic structures of the main pconjugated backbone, but also the intermolecular arrangement
and the morphological behavior. Monosubstituted derivatives
7-7b and 7-7c showed a more ordered molecular packing,associated with higher crystallinity and longer coherence
length than 7-7a and 7-7d in blended films with a p-type
polymer FTAZ131 as revealed by the 2D-GIWAXS data. Additionally,
the monofluorinated derivative 7-7b showed smaller-sizeddomains in the blended film with FTAZ, resulting in a higher
PCE (12.1%) of the FTAZ:7-7b system than those of FTAZ:7-7a1184 | J. Mater. Chem. C, 2022, 10, 1162–1195

Review(8.88%), FTAZ:7-7c (9.06%), and FTAZ:7-7d (10.7%) (entries
7-18–7-21).
Qu et al. investigated the optimal length of end-alkyl chainson an A–D–A system comprising IT as the donor, rhodanine as
the acceptor, and benzothiadiazole as the linker.132 Among thefour end-alkylated derivatives, the hexyl derivative 7-8c afforded
a higher PCE of 8.26% than the ethyl (7-8a, 7.04%), butyl (7-8b,
7.43%), and octyl (7-8d, 7.93%) derivatives in BHJ OPVs
with PBDB-T as the p-type material (entries 7-22–7-25). The
2D-GIWAXS data of these BHJ films suggested that the size of
crystallites increased in the order of PBDB-T:7-8a o PBDB-T:
7-8b o PBDB-T:7-8d o PBDB-T:7-8c, roughly matching with theorder of the JSCs as well as PCEs. Note that derivative 7-8c
afforded an improved PCE of 9.29% after solvent-vapor annealing
followed by thermal annealing (entry 7-26). In addition, derivative
7-8a showed a higher crystallinity than the other derivatives inneat films, demonstrating that the morphology of blended films
cannot be predicted based on the structure of neat films alone.
Hydrogen-bond-directed manipulation of molecular packing and film morphology was attempted with hydroxyfunctionalized ITIC derivatives by Liu et al.133 By comparing
ITIC with end-hydroxyl derivatives 7-9a and 7-9b, the authors
demonstrated that hydroxy substitution at the end-capping IC
units brought about a higher packing order and crystallinity in
BHJ films with PBDB-T, which was attributed to the formationof intermolecular hydrogen-bonding. Predictably, the dihydroxy
derivative 7-9b showed a higher crystallinity than the monohydroxy
derivative 7-9a. Consequently, the BHJ systems of PBDB-T:7-9a
and PBDB-T:7-9b aﬀorded PCEs of 10.4% and 11.0%, while
PBDB-T:ITIC gave a somewhat lower PCE of 7.8% (entries 7-27–729). Furthermore, the diol 7-9b gave an even higher PCE of 12.5%
when combined with PM6 (entry 7-30).
Molecules with an indacenodithiophene core
IDIC has four hexyl chains directly attached onto the 4- and 9carbons of the ID framework. As in the case of four 4-hexylphenylgroups on ITIC, these substituents are electrically insulating but
still exert a significant impact on the photovoltaic process by
preventing IDIC molecules from excessive p–p stacking and ensuring the formation of sufficient p–n junction areas in BHJ films.
In 2017, Feng et al. reported that nonsymmetric substitutionof the ID core at the 4- and 9-carbons led to improved photovoltaic performance.134 The authors evaluated three IDIC
derivatives 7-10a–c, and demonstrated that the nonsymmetricdi(4-hexylpheny)–dioctyl derivative 7-10b afforded a higher PCE(10.12%) than the tetra(4-hexylphenyl) derivative 7-10a (6.42%)
and the tetraoctyl derivative 7-10c (9.68%) in BHJ OPVs with
PBDB-T (entries 7-31–7-33). While the devices with symmetricderivatives suffered from either excessive phase separation(7-10a) or lower electron mobility (7-10c), the nonsymmetrically
substituted 7-10b yielded adequately attenuated crystallinity
and improved charge-carrier mobilities in the BHJ film.
Interestingly, photovoltaic efficiency of the PBDB-T:7-10b
system was found to be less sensitive to active-layer thickness
than other systems, giving a 9.17% PCE even in a 4200 nmthick film.

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Review

Fig. 14

Journal of Materials Chemistry C

Chemical structures of the compounds employed with 7-1-7-17 in OPV active layers. PBDB-T, PTB7-Th, and PM6 are shown in Fig. 2 or Fig. 4.

Li et al. proposed a diﬀerent molecular design to avoid
excessive aggregation while at the same time achieve eﬃcientcharge-carrier transport. The authors demonstrated the superiorityof phenylbutyl over hexyl and 4-hexylpheny as 4,4,9,9-substituentson the ID core by comparing IDIC and its two derivatives 7-11a and7-11b.135 Evaluation of the three compounds resulted in PCEs of
10.41% (IDIC), 6.14% (7-11a), and 11.50% (7-11b) in BHJ OPVs with
PBDB-T (entries 7-34–7-36). Here, the FF of the PBDB-T:7-11b device
was as high as 77.42%, and impressively, it was further optimized to78.32% when PM6 was used instead of PBDB-T (entry 7-37). Thus,
introduction of a slightly bulky phenyl group at the tail, rather thanthe head, of core alkyl substituents would be an effective molecular
design for obtaining optimal morphology in BHJ films.
In connection to the work by Li et al., Chen et al. demonstrated
that an even subtler diﬀerence in substituent structure could bringabout appreciable diﬀerences in the morphology and performanceof BHJ films.136 They evaluated compound 7-12 which was
diﬀerentiated from IDIC only by the alkyl substituents on the ID
core (4-methylpentyl for 7-12, hexyl for IDIC), and demonstrated that
7-12 aﬀorded a higher PCE of 13.53% than IDIC (11.20%) inthermally annealed BHJ layers with PM6 (entries 7-39, 42). In asimilar manner to the phenylbutyl groups of 7-11b, the alkyl chainswith a branched terminal in 7-12 modestly attenuated crystallizationand self-aggregation without sacrificing p–p stacking and chargecarrier mobilities. The difference in morphological behavior between
the PM6:7-12 and PM6:IDIC systems could be noted when comparing their response to annealing treatments (entries 7-38–7-43).
Additionally, Chen et al. reported a related study with ID–
benzothiadiazole–rhodanine conjugates 7-13a and 7-13b.137 In thiscase, again, substitution with more extended alkyl chains (octyl) on
the ID core adequately attenuated self-aggregation and phaseseparation when compared to more compact (hexyl) alkyl chainsin BHJ films. Accordingly, higher photovoltaic performance
resulted from 7-13b (PCE = 9.4%) than 7-13a (9.1%) in BHJ OPVs
with PTB7-Th as the p-type material (entries 7-44 and 7-45).
Here, it would be worth pointing out that the ID corerequires smaller substituents as compared to the IT core forobtaining BHJ layers with optimal morphology. Specifically,
alkyl groups up to octyl are common for ID, while the considerably larger hexylphenyl is typical for IT. This diﬀerence should
be a reflection of the size of these p-cores.
Regarding the end IC units, the positive eﬀects of alkyl
substitution have been reported in several papers. For example, LiThis journal is © The Royal Society of Chemistry 2022

et al. showed that cyclohexyl-fused derivative 7-14 performed better
than IDIC, providing a PCE of 10.22% (6.11% with IDIC) in a BHJ
OPV with PBDB-T (entries 7-46 and 7-47).138 Also, Ryu et al. reported
that derivatives 7-15a and 7-15b with end-octyl substituents aﬀorded
similarly high PCEs of 9.60% and 9.64%, respectively, while thatwith IDIC was only 7.23% at the best (entries 7-48–7-50).139 In both
examples, end-alkyl-substitution led to higher crystallinity of thecorresponding BHJ films without excessive phase separation, which
contributed to more balanced, higher charge-carrier mobilities andthus enhancement in JSC. Note that the improvement in FF was alsosignificant in the PBDB-T:7-14 system, while the resultant increaseof both JSC and VOC was achieved in the PBDB-T:7-15 systems.
Luo et al. studied the eﬀect of substituents at linker units in
an ID–TIC-conjugate system by comparing 2-ethylhexyl substituted 7-16a and 2-ethylhexyloxy substituted 7-16b.140 As can be
expected from the alkoxy substituents’ more electron-donating
nature than that of alkyl ones, 7-16b showed a higher LUMO
level resulting in an enhanced VOC in a BHJ device with PBDB-T(entries 7-51 and 7-52). In addition, the authors pointed outthat more favorable phase separation and balanced chargecarrier mobilities were observed in 7-16b, which contributed tothe higher FF than that of the PBDB-T:7-16a BHJ system.
Li et al. reported in their article in 2019 that the introduction of
methoxy groups as the core substituents led to higher photovoltaic eﬃciency (entries 7-53 and 7-54).141 The best PCE obtainedwith the methoxy-substituted compound 7-17a was 13.46% whenused as the n-type material in BHJ OPVs in combination with thep-type polymer PTQ10. On the other hand, the PCE with theparent compound 7-17b (commonly called IDIC-2F) remained
12.09% at the best. The methoxy derivative was designed in thecontext of simplifying the synthesis to realize easily accessible,
cost-eﬀective SMSCs for OPVs; at the same time, this work
revealed the eﬀectiveness of core substitution for achieving
high-performance IDIC-based materials.

9. Substituents ofdi(thienothienopyrrolo)benzothiadiazole-based p-conjugates
A benzothiadiazole-based fused-ring system 12,13dihydrodi(thieno[200 ,300 :4 0 ,5 0 ]thieno[2 0 ,3 0 :4,5]pyrrolo[3,2-e:2 0 ,3 0 -g])benzothiadiazole (TPBT, Fig. 15) is another outstanding core

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Journal of Materials Chemistry C

Fig. 15

Chemical structure of the TPBP framework.

unit for non-fullerene n-type molecules. The report of Y6(Fig. 10b) by Yuan et al.142 in 2019, which demonstrated an
excellent PCE of 15.7% with the PM6:Y6 BHJ OPV, brought about
the development of this class of molecules,143 and optimizationof peripheral substituents has been an important part of theresearch as in the case of other SMSCs. This section reviews
relevant papers on this topic. The corresponding chemical
structures and the photovoltaic data are summarized in Fig. 16
and Table 8, respectively.
The TPBT framework oﬀers three types of positions to
introduce substituents: pyrrolic nitrogens (inner), thieno b-carbons(outer), and end-benzo carbons (end) as depicted in the inset of
Fig. 15. Soon after the first paper on Y6, Jiang et al. reported thephotovoltaic performance of three derivatives 8-1a–c whichwere differently substituted at the inner and outer positions.144

ReviewAmong the three derivatives, inner-3-ethylheptyl/outer-dodecyl 8-1b
performed better (PCE = 15.98%) than the original inner-2ethylhexyl/outer-dodecyl Y6 (15.20%) in BHJ OPVs with PM6, while
inner-dodecyl/outer-2-ethylhexyl 8-1a and inner-4-ethyloctyl/outerdodecyl 8-1c afforded lower PCEs of 12.91% and 14.31%,
respectively (entries 8-1–8-4). Derivative 8-1a was found have lower
solubility than the other three compounds, forming large domains
in the BHJ film with PM6. Combined with the considerably lower
PCE from 8-1a, this result indicated that a certain degree ofbulkiness of inner substituents would be necessary for the TPBT
framework to form an adequate BHJ morphology. Meanwhile, theeffect of branching positions in inner alkyl chains was found to berelatively minor, but still non-negligible. In relation to this work,
Yu et al. compared 2-ethylhexyl and 3-ethylhexyl as the innersubstituents of tetrachloro derivatives 8-2a and 8-2b, respectively.
They found the latter worked slightly better in devices with PM6 toafford a PCE of 16.532%, while the former provided PCEs of up to
16.061% (entries 8-5 and 8-6).145
There have been several other reports regarding the
optimization of the inner substituents of Y6-type systems.
Han et al. introduced 4-phenylbutyl and 6-phenylhexyl insteadof branched alkyl chains, and compared the performance of theresulting compounds 8-3a and 8-3b in BHJ OPVs bearing PM6

Fig. 16 Chemical structures of SMSCs 8-1–8-7. The best PCEs obtained with each compound are shown in parentheses (see Table 8 for full OPV
parameters). The inset shows relevant substitution positions of Y6 and its derivatives. Structures of the partner material in the photovoltaic active layer(PM6) are shown in Fig. 2.

1186 | J. Mater. Chem. C, 2022, 10, 1162–1195

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Review

Journal of Materials Chemistry C

Table 8

Photovoltaic parameters of SMSCs 8-1-8-7a

Entry

Active layer

Solvent

Additional conditions

VOCb (V)JSCb (mA cm2)

FFb (%)

PCEb (%)

Ref.

8-1
8-2
8-3
8-4
8-5
8-6
8-7
8-8
8-9
8-10
8-11
8-12
8-13
8-14
8-15
8-16
8-17
8-18
8-19
8-20
8-21
8-22
8-23
8-24
8-25
8-26
8-27
8-28
8-29

PM6 : Y6 (1 : 1.2)
PM6 : 8-1a (1 : 1.2)
PM6 : 8-1b (1 : 1.2)
PM6 : 8-1c (1 : 1.2)
PM6 : 8-2a (1 : : 1.2)
PM6 : 8-2b (1 : 1.2)
PM6 : 8-3a (1 : 1.2)
PM6 : 8-3b (1 : 1.2)
PM6 : 8-3a (1 : 1.2)
PM6 : 8-3b (1 : 1.2)
PM6 : 8-4a (1 : 1.5)
PM6 : 8-4b (1 : 1.5)
PM6 : 8-4c (1 : 1.5)
PM6 : 8-5a (1 : 1.2)
PM6 : 8-5b (1 : 1.2)
PM6 : 8-5c (1 : 1.2)
PM6 : 8-5d (1 : 1.2)
PM6 : 8-5b (1 : 1.2)
PM6 : 8-5c (1 : 1.2)
PM6 : Y6 (1 : 1.2)
PM6 : 8-6a (1 : 1.2)
PM6 : 8-6b (1 : 1.2)
PM6 : Y6 (1 : 1.2)
PM6 : 8-6c (1 : 1.2)
PM6 : 8-6d (1 : 1.2)
PM6 : 8-6e (1 : 1.2)
PM6 : 8-7a (1 : 1.2)
PM6 : 8-7b (1 : 1.2)
PM6 : 8-7c (1 : 1.2)

CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF
CB
CB
CF
CF
CF
CF
CF
CF
CF
CF
CF
CF

0.05% CN; 90 1C, 5 min
0.05% CN; 90 1C, 5 min
0.05% CN, 90 1C, 5 min
0.05% CN, 90 1C, 5 min
90 1C, 5 min
90 1C, 5 min
0.5 vol% CN
0.5 vol% CN
0.5 vol% CN; 100 1C
0.5 vol% CN; 100 1C
100 1C, 5 min––
0.5 vol% DIO; 100 1C
0.5 vol% DIO; 100 1C
0.5 vol% DIO; 100 1C
0.5 vol% DIO; 100 1C
0.5 vol% DIO; 100 1C
0.5 vol% DIO; 100 1C
0.5 vol% CN; 100 1C, 5 min
0.5 vol% CN; 100 1C, 5 min
0.5 vol% CN; 100 1C, 5 min
0.25% DIO; 100 1C, 10 min
0.25% DIO; 100 1C, 10 min
0.25% DIO; 100 1C, 10 min
0.25% DIO; 100 1C, 10 min
0.5% CN; 100 1C, 10 min
0.5% CN; 100 1C, 10 min
0.5% CN; 100 1C, 10 min

0.847
0.852
0.837
0.819
0.839
0.852
0.689
0.84
0.709
0.804
0.803
0.844
0.863
0.84
0.88
0.85
0.86
0.83
0.85
0.82
0.92
0.89
0.84
0.87
0.88
0.89
0.94
0.92
0.95

24.02
21.47
25.81
25.01
25.84925.901
15.74
23.82
16.43
25.11
21.80
21.31
19.07
19.82
18.90
24.80
23.24
25.13
25.26
25.3
13.3
23.2
25.91
25.72
25.08
24.57
20.31
21.38
18.79

74.7
70.6
73.9
69.9
74.1
74.9
67.01
72.68
68.00
67.25
67.3
61.6
63.1
68.27
56.24
72.20
69.78
68.94
76.25
75.5
53.5
78.5
76.0
81.5
78.8
74.6
62.53
68.25
61.79

15.20
12.91
15.98
14.31
16.061
16.532
7.28
14.54
7.92
13.58
11.77
11.07
10.38
11.36
9.38
15.13
13.95
14.28
16.43
15.7
6.6
16.1
16.61
18.32
17.39
16.26
11.92
13.43
11.08

144
144
144
144
145
145
146
146
146
146
147
147
147
148
148
148
148
148
148
149
149
149
150
150
150
150
151
151
151

a
b

Structures of active-layer compounds are shown in Fig. 16. CF: chloroform, CB: chlorobenzene, DIO: 1,8-diiodooctane, CN: chloronaphthalene.
Data of the best-performing devices.

as the p-type material.146 Indeed, the diﬀerence between thetwo compounds turned out to be significant, that is, the4-phenylbutyl derivative 8-3a gave a 7.28% PCE, while the6-phenylhexyl derivative 8-3b aﬀorded a 14.54% PCE without
annealing treatment (entries 8-7 and 8-8). With thermal annealing,
PCEs were 7.92% for 8-3a and 13.58% for 8-3b (entries 8-9 and8-10). The authors pointed out that the PM6:8-3b BHJ film showedstronger tendency for face-on orientation, tighter molecular
stacking, and a larger crystallite coherence length than thePM6:8-3a blend. The superior performance of 8-3b could be linked
to the synergistic contribution of these morphological factors.
In addition, Yi and co-workers demonstrated that 6-phenylhexyl
was a better choice than 6-phenoxyhexyl and octyl as the innersubstituent by comparing compounds 8-4a–8-4c (8-4a = 8-3b) in
BHJ OPVs with PM6 (PCE = 11.77%, 11.07%, and 10.38%),
although the difference was less significant as compared with thecase of the 8-3 series (entries 8-11–8-13).147
Mo et al. comparatively evaluated 8-5a–d in BHJ OPVs with
PM6.148 All these four compounds had chlorine atoms insteadof fluorine atoms of Y6 at the end benzo units and weredifferentiated from each other by the structure of inner alkyl
chains (dodecyl for 8-5a, 2-ethylhexyl for 8-5b, 2-butyloctyl for
8-5c, and 2-hexyldecyl for 8-5d). Interestingly, the authors
successfully obtained a single-crystal X-ray structure of 8-5c
and revealed that the two inner alkyl chains extruded in the
opposite directions from each other from the p-framework,
indicating strong influence of the alkyl chains in determining
p-stacking motif in the solid state. The OPV performance wasThis journal is © The Royal Society of Chemistry 2022

indeed largely dependent on alkyl-chain lengths; specifically,
PCEs were 11.36%, 9.38%. 15.13%, and 13.95% for 8-5a–d,
respectively, when the active layers were cast from chloroform
solutions (entries 8-14–8-17). The unexpectedly low PCE withthe 2-ethylhexyl derivative 8-5b was due to its insufficientsolubility in chloroform, which was improved to 14.28% whenchlorobenzene was used to replace chloroform (entry 8-18). In
addition, the 2-butyloctyl derivative 8-5c afforded an impressive
PCE of 16.43% when deposited from chlorobenzene (entry
8-19). The superiority of 8-5c was partly attributed to the higher
and more balanced charge-carrier mobilities in thecorresponding BHJ film, which should be related to the shorter
p–p stacking distance revealed by the 2D GIWAXS analysis.
In general, the branched alkyl chains yielded more positiveeffects than the linear dodecyl chain regarding photovoltaic
performance.
The impact of the outer alkyl groups was investigated byChen et al.149 They compared Y6 with symmetric dialkoxy
derivative 8-6a and non-symmetric alkyl/alkoxy derivative 8-6b
in BHJ OPVs with PM6 as the p-type polymer (entries 8-20–8-22).
As could be expected from the more electron-donating nature ofalkoxy groups as compared to that of alkyl groups, the LUMO
level was increased upon replacing the dodecyl groups of Y6
with dodecyloxy to form 8-6a (3.89 - 3.76 eV). The increasein the LUMO level of the n-type material is in general favourable
for obtaining a high VOC, and this was the case in 8-6a (0.82 0.92 V). However, the PCE with this didodecyloxy derivative wasconsiderably lower than that with Y6 (6.6% vs. 15.7%). This

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Journal of Materials Chemistry C
result was attributed to the poor solubility of 8-6a in the castsolvent (chloroform) which induced excessive crystallinity anddomain sizes. The non-symmetric alkyl/alkoxy derivative 8-6b, on
the other hand, showed both sufficient solubility and moderately
improved VOC, affording the best PCE (16.1%) among the threecompounds.
In a similar vein, Li et al. comparatively evaluated Y6 and its
three derivatives 8-6c–e having 2-butyloctyl, 2-hexyldecyl, and2-octyldodexyl, respectively, as the outer substituents.150 The
highest efficiency was obtained with 8-6c (PCE = 18.32%) in a
BHJ OPV using Y6 as the p-type material, while Y6, 8-6d, and8-6e afforded PCEs of 16.61%, 17.39%, and 16.26% (entries
8-23–8-26). The better performance of the 2-butyloctyl derivative
8-6c was attributed to its effective p–p contacts and an adequate
degree of phase separation/crystallization in the BHJ active
layer as indicated by XRD and microscopic analyses. The
authors pointed out that the outer substituents are near the
major p–p stack position of the Y6 derivative and thus aninteresting structural factor to consider for tuning the moleculararrangement in the solid state.
Chen et al. studied the eﬀect of end groups by evaluating
compounds 8-7a–c for performance in BHJ OPVs with PM6(entries 8-27–8-29).151 All the three compounds were substituted with a methyl group and a bromine atom at each end
benzo moiety, with different substitution positions. Among thethree isomers, 8-7b afforded a higher PCE (13.43%) than 8-7a
(11.92%) and 8-7c (11.08%), which was ascribed to the higher
crystallinity of the PM6:8-7b film.
Overall, the currently available data point to the superiorityof branched alkyls as both the inner and outer substituents, aswell as the need for halogen end groups, for achieving the stateof-the-art Y6-derived acceptors. Nonsymmetric substitution
may bring about additional improvement, which requires
further systematic investigation.

10. Substituents of other p-conjugatedsystems
There have been several studies involving less common
p-conjugated frameworks, but they still presented considerable
substituent impact on the photovoltaic performance. Thesestudies will serve as valuable references for further optimizationof the currently available systems and can be even more so fordeveloping new SMSCs based on novel p-conjugated backbones.
This section reviews papers of this sort, roughly in the order ofpublication. The chemical structures and the photovoltaic dataof the corresponding compounds are summarized in Fig. 17 andTable 9.
Compounds used as p-type
Ouchi et al. studied the eﬀect of alkyl substituents on the
supramolecular and photovoltaic behaviour of barbiturated
oligo(butylthiophene)s 9-1a and 9-1b.152 While similar tapelike hydrogen-bonded supramolecules were assembled fromthese isomeric compounds, their higher-order structures

1188 | J. Mater. Chem. C, 2022, 10, 1162–1195

Review
diﬀered significantly; specifically, 9-1a formed a helicallytwisted structure, and 9-2b formed a lamellar packing(Fig. 18). These nano-architectures were generated even in
blended films with PC71BM, aﬀording PCEs of 4.50% (9-1a)and 1.39% (9-1b) in the corresponding devices, respectively(entries 9-1 and 9-2). This example clearly demonstrates theimportance of regulating higher order structures for obtaining
optimal photovoltaic performance with supramolecularmaterials.
Suzuki and co-workers compared four alkylated derivativesof di(bithienyl)anthracene, 9-2a–d.153 Depending on the structureof end-alkyl groups, these compounds afforded considerably
different VOCs in BHJ OPVs: 0.966 V with 9-2a, 0.419 V with
9-2b, 0.499 V with 9-2c, and 0.602 V with 9-2d. Accordingly, thecorresponding PCEs ranged widely from 1.46% to 4.02% (entries
9-3–9-6). The authors reasoned, based on single-crystal X-ray
analysis and quantum chemical calculations, that the higher
VOC from 9-2a was the result of its branched 2-ethylhexyl chainsthat effectively inhibited the formation of the non-slippedstacking of p-frameworks that would have otherwise led to anincrease in the HOMO level and thus a decrease in VOC. On theother hand, the linear alkyl chains in 9-2b–d partially allowed theformation of the non-slipped stacking, and this tendency wasstronger when alkyl chains were shorter. Note that compounds
9-2a–d were deposited through a precursor approach similarly tothe case of the 4-7 series in Section 5. The difference is thatcompounds 9-2a–d were formed by a light-induced reaction,rather than thermally induce ones for 4-7, from the corresponding
a-diketone-type precursors 9-3a–d (Scheme 2).154 The relationshipbetween the substituent structure, molecular packing, and photovoltaic properties was revealed in this study using the precursor
approach which relieved the concerns regarding solubility.
Compounds used as n-type
In 2020, He et al. reported three near infrared-active n-type
compounds comprising a di(thienopyrrolo)benzothiadiazole
core, two IC end units, and two thienylene linkers (9-4a–c).155
The three compounds differ in their alkyl chains on the
b-positions of the thienylene linkers: inward decyl chains for
9-4a, outward 2-ethylhexyl chains 9-4b, and outward decyl chains
9-4c. Evaluation in BHJ OPVs bearing PBDB-T as the p-type
polymer showed that derivative 9-4c (PCE = 13.75%) performed
better than 9-4a (6.98%) and 9-4b (13.15%); thus, outward chainswere better than inward, and linear chains were better thanbranched (entries 9-7–9-9). The authors pointed out that PBDB-T:
9-4c showed the strongest intensity and the closest p–p
stacking distance (3.55 Å) among the three BHJ systems, whichbrought about its superior photovoltaic performance.
An extensive screening of side alkyl substituents on theseven-ring-fused system was reported by Kan et al.156 The
authors introduced linear alkyl chains of diﬀerent lengths tothe outer side position of the fused-ring system to form
compounds 9-5b–g (9-5a was a non-substituted version), andevaluated them as n-type materials in BHJ OPVs with PM6(entries 9-10–9-16). The octyl derivative 9-5e was found to
perform best among the seven compounds, providing a PCE

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Fig. 17 Chemical structures of (a) SMSCs 9-1-9-9 and (b) the compounds employed with 9-1-9-9 in photovoltaic active layers. PC71BM, PBDB-T, PM6,
and PTB7-Th are shown in Fig. 2 or Fig. 4.

of 13.95%. Note that ternary system PM6:9-5e:F-Br gave an
excellent PCE up to 15.34%.
Relatively simple, non-fused conjugates 9-6a and 9-6b werecompared by Lee et al.157 These compounds were flanked by
diﬀerent alkyl chains (2-butyloctyl for 9-6a and octyl for 9-6b) andtheir eﬀects were examined in BHJ OPVs with PTB7-Th. The result
was that 9-6b (PCE = 6.21%) performed better than 9-6a (5.97%) in
the as-cast active layers, 9-6a (8.33%) outperformed 9-6b (7.60%)
after solvent-vapour annealing (entries 9-17–9-20). It was foundthat the hole and electron mobilities were more balanced in theannealed PTB7-Th:9-6a blend as compared with PTB7-Th:9-6b.

This journal is © The Royal Society of Chemistry 2022

A comparison between diﬀerent alkyl side chains was also
performed with a dialkoxyquinoxaline–cyclopentadithiopheneconjugate system bearing chlorinated version of ICs (IC-4F) as
end groups (9-7a–c).158 The three compounds had different
alkyl chains on the alkoxy units and the cyclopentadiene
moieties. A general trend observed among them was thatthe introduction of longer branched alkyl chains induceda more favourable p–p stacking and phase separation in
PBDB-T:9-7 films. Accordingly, the 2-ethylhexyloxy/2-butyloctyl
derivative 9-7b afforded a higher PCE (10.55%) than the2-ethylhexyloxy/2-ethylhexyl derivative 9-7a (9.09%) and hexyl/

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Journal of Materials Chemistry C
Table 9Photovoltaic performance of SMSCs 9-1-9-8a

Entry Active layer

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Review

Solvent Additional conditions

9-1
9-2
9-3
9-4
9-5
9-6
9-7
9-8
9-9
9-10
9-11
9-12
9-13
9-14
9-15
9-16
9-17
9-18
9-19
9-20
9-21
9-22
9-23
9-24
9-25
9-26
9-27
9-28
9-29
9-30
9-31

CF
9-1a : PC71BM (1 : 1)CF
9-1b : PC71BM (1 : 1)
PhBADT/9-2a : PC71BM (2 : 1)/PC71BM CFPhBADT/9-2b : PC71BM (2 : 1)/PC71BM CFPhBADT/9-2c : PC71BM(2 : 1)/PC71BM CFPhBADT/9-2d : PC71BM (2 : 1)/PC71BM CF
PBDB-T : 9-4a (1 : 1.2)
CF
PBDB-T : 9-4b (1 : 1.2)
CF
PBDB-T : 9-4c (1 : 1.2)
CF
PM6 : 9-5a (1 : 1)
CB
PM6 : 9-5b (1 : 1)
CB
PM6 : 9-5c (1 : 1)
CB
PM6 : 9-5d (1 : 1)
CB
PM6 : 9-5e (1 : 1)
CB
PM6 : 9-5f (1 : 1)
CB
PM6 : 9-5g (1 : 1)
CB
PTBT-Th : 9-6a (1 : 2)
CF
PTBT-Th : 9-6a (1 : 2)
CF
PTBT-Th : 9-6b (1 : 2)
CF
PTBT-Th : 9-6b (1 : 2)
CF
PBDB-T : 9-7a (1 : 1)
CF
PBDB-T : 9-7b (1 : 1)
CF
PBDB-T : 9-7c (1 : 1)
CF
J52 : 9-8a (1 : 1.2)
CFJ52 : 9-8b (1 : 1.5)
CFPM6 : 9-9a (1 : 1)
CF
PM6 : 9-9b (1 : 1)
CF
PM6 : 9-9c (1 : 1)
CF
PM6 : 9-9d (1 : 1)
CF
PM6 : 9-9e (1 : 1)
CF
PM6 : 9-9e (1 : 1)
CF

9-32
9-33
9-34
9-35

PM6 : 9-9f (1 : 1)
PM6 : 9-9g (1 : 1)
PM6 : 9-9h (1 : 1)
PM6 : 9-9h (1 : 1)

CF
CF
CF
CF

VOCb (V)JSC (mA cm2) FFb (%)

CS2 vapour, 80 s
0.74  0.00 9.73  0.20
CS2 vapour, 120 s
0.70  0.02 3.95  0.13
THF vapour, 60 s
0.966
6.71
THF vapour, 60 s
0.419
7.20
THF vapour, 60 s
0.499
7.19
THF vapour, 60 s
0.602
6.83
0.5% CN; 150 1C, 10 min
0.784
16.81
0.5% CN; 150 1C, 10 min
0.781
24.40
0.5% CN; 150 1C, 10 min
0.757
25.84
0.5 vol% DIO; 140 1C, 10 min
0.806
19.49
0.5 vol% DIO; 140 1C, 10 min
0.836
20.30
0.5 vol% DIO; 140 1C, 10 min
0.883
21.03
0.5 vol% DIO; 140 1C, 10 min
0.887
21.70
0.5 vol% DIO; 140 1C, 10 min
0.901
22.36
0.5 vol% DIO; 140 1C, 10 min
0.896
22.61
0.5 vol% DIO; 140 1C, 10 min
0.909
22.32
—
1.02
13.30
THF vapour, 15 s
1.02
15.27
—
0.92
14.99
THF vapour, 15 s
0.95
15.82
10 min, 0.5% CN; 140 1C
0.807
18.65
10 min, 0.5% CN; 140 1C
0.816
19.39
10 min, 0.5% CN; 140 1C
0.780
19.16
0.5% CN; 120 1C
0.769
24.69
0.5% CN; 120 1C
0.814
26.02
0.5% CN; 90 1C, 5 min
0.88
19.76
0.5% CN; 90 1C, 5 min
0.90
24.63
0.87
18.28
0.5% CN; 90 1C, 5 min
0.5% CN; 90 1C, 5 min; PDIN buﬀer 0.96
11.34
0.5% CN; 90 1C, 5 min; PDIN buﬀer 0.92
22.39
0.5% CN; 90 1C, 5 min; PFNDI-Br
0.91
24.03
buﬀer
0.5% CN; 100 1C, 5 min; Ag electrode 0.73
6.47
0.5% CN; 100 1C, 5 min; Ag electrode 0.88
21.67
0.5% CN; 100 1C, 5 min; Ag electrode 0.88
22.34
0.5% CN; 100 1C, 5 min; Al electrode 0.87
25.10

PCEb (%)

Ref.

62.2  0.8 4.50  0.09 152
50.2  0.4 1.39  0.01 152
62.0
4.02
153
48.3
1.46
153
56.2
2.02
153
54.1
2.22
153
53
6.98
155
69
13.15
155
70
13.75
155
67.39
10.59
156
67.38
11.43
156
68.31
12.68
156
68.79
13.24
156
69.23
13.95
156
65.16
13.20
156
61.77
12.53
156
44
5.97
157
54
8.33
157
45
6.21
157
51
7.60
157
60.3
9.09
158
66.9
10.55
158
63.8
9.53
158
61.69
11.71
159
69.96
14.82
159
63.84
11.16
160
72.09
16.00
160
55.74
8.89
160
51.96
5.67
161
70.10
14.50
161
76.22
16.66
161
47.60
60.27
66.95
71.60

2.26
11.55
13.14
15.63

162
162
162
162

a

Structures of active-layer compounds are shown in Fig. 17. CF: chloroform, CB: chlorobenzene, THF: tetrahydrofuran, CN: chloronaphthalene;
DIO: 1,8-diiodooctane; PDIN: N,N 0 -di[3-(N00 ,N00 -dimethylamino)propyl]perylene-3,4:9,10-tetracarboxylic diimide; PFNDI-Br: poly[(9,9-bis(3 0 -((N,Ndimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-5,5 0 -bis(2,2 0 -thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N 0 -di(2-ethylhexyl)
imide]dibromide. b Data of the best-performing devices when available; otherwise, average of multiple devices.

2-butyloxtyl derivative 9-7c (9.53%) as shown in entries
9-21–9-23.
Zhan et al. compared two benzotriazole–cyclopentadithiophene–IC conjugates 9-8a and 9-8b as n-type materials in BHJ
OPVs with p-type polymer J52.159 The effect of the octyl chainson the outer side of cyclopentadithiophene was significant;compound 9-8b afforded a PCE of 14.82%, while 9-8a gave only
11.71% (entries 9-24 and 9-25). Addition of electron donatingalkyl chains resulted in not only a higher LUMO energy level asreflected in a larger VOC, but also higher crystallinity, improved
conformational rigidity, and a smaller reorganization energy.
In the BHJ film with J52, 9-8b demonstrated balanced chargecarrier mobility, smaller energy loss, and more favorable
morphology as compared to the non-substituted 9-8a.
Recently, substituent impact was intensively studied on aseries of A–D–A non-fullerene acceptors comprising theladder type benzobis(dithienopyrrole) core as the central D unit
(9-9a–h). When all the four alkyl substituents were branched(2-ethylhexyl, 2-butyloctyl, and 2-decyltetradexy for 9-9a, b, and c),the derivative with medium-length chains (9-9b) performed thebest to afford PCEs up to 16.00% in BHJ OPVs with PM61190 | J. Mater. Chem. C, 2022, 10, 1162–1195(entries 9-26–28).160 Either shorter alkyl (9-9a, PCE = 11.16%)
or longer alkyl chains (9-9c, 8.89%) resulted in inferior
performance. The authors pointed out that 9-9b formed p–p
stacking with a spacing of 3.45 Å, which was shorter than thecorresponding values of 9-9a (3.51 Å) and 9-9c (4.08 Å). This
shorter p–p stack distance was regarded as a cause for thehigher charge-carrier mobilities, JSCs, and thus PCEs with 9-9b.
In a different study, the same group compared derivatives 9-9d
and 9-9e, which were differentiated by alkyl chains on the pyrrole
nitrogens: linear n-octyl for 9-9d and branched 2-hexyldecyl for 99e.161 Notably, the latter was found to preferably adopt a face-on
orientation, while the former preferred an edge-on arrangement.
Accordingly, 9-9e afforded a considerably higher PCE (14.50%)
than 9-9d (5.67%) (entries 9-29, 30). Replacement of the cathodebuffer material from PDIN to PFNDI-Br resulted in an evenhigher PCE of 16.66% with 9-9e. A similar trend between
molecular orientation and structure of alkyl substituents wasobserved with derivatives 9-9f–h,162 namely, when all the four
alkyl groups are branched 2-butyloctyl, the p-frameworkadopted a face-on arrangement to show a higher PCE (13.14%
with 9-9h) as compared to those with linear dodecyl groups

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Review

Journal of Materials Chemistry C

Fig. 18 Supramolecular structures and photovoltaic performanceobtained with barbiturated oligo(butylthiophene)s 9-1a and 9-1b. Adopted
from ref. 116 with permission. Copyright 2018, Royal Society of Chemistry.

Scheme 2

Light-induced conversion of precursors 10-3a-d to 10-2a-d.

(2.26% with 9-9f, 11.55% with 9-9g) (entries 9-32–34). The PCE
with 9-9h was further improved to 15.63% by changing thecathode material from Ag to Al.

11. Summary and outlook
We have reviewed the impact of substituents on the photovoltaic performance of various types of SMSCs ranging from
simple, single-p-core molecules to large, highly extended D–A
conjugates. Although the substituents concerned herein played
rather auxiliary roles in determining the electronic characteristics
of isolated molecules, their impact was found to be significant,affecting all photovoltaic parameters JSC, VOC, FF, and thus PCE inmany examples. This is because substituents generally dictate
molecular packing and film morphology, which critically affectthe efficacy of OPVs by altering orbital energy levels, exciton
dynamics, or charge-carrier mobilities in BHJ active layers. Evena seemingly subtle change in the substituent structure or position
induced a large difference in PCE as observed for, among manyothers, the 9-2 series.
Not surprisingly, the eﬀects of substituents appeared very
specific to each p-conjugated backbone. For example, 2-ethylhexyl
performed better than hexyl as N-substituents of the DPP coreThis journal is © The Royal Society of Chemistry 2022

between dithienyl-DPP derivatives 4-1a (N-2-ethylhexyl, PCE = 3.0%)
and 4-1b (N-hexyl, 0.79%), while an opposite result was obtainedwith diphenyl-DPP derivatives 4-2a (N-hexyl, 3.45%) and 4-2b
(N-2-ethylhexyl, 0.76%). The degree of substituent impact was also
found case-by-case. In particular, the diﬀerence in the branch
point of alkyl chains showed a relatively minor impact on the
PCE in the case of core substituents of DTBDT–rhodanine
conjugates 6-9a–c (11.79–14.78%), while it significantly affectedin the case of end substituents of BDT–rhodanine conjugates
6-14a–c (0.90–12.40%). Another important aspect is that themodification of the substituent structure often substantiallyalters the morphological response of BHJ films to different
types or conditions of the cast solvent, solvent additive, andannealing treatment. Furthermore, a partner material or materials in BHJ films regulate the outcome of substituent impact
for a compound in question.
Such a complex, multifactorial nature of the substituent
impact makes it diﬃcult to extract general rules from previous
examples. Indeed, accurate prediction of optimal substituent
design for achieving ideal BHJ morphology remains quite achallenge even after decades of research. This is especially true
for those molecules with highly extended p-conjugated frameworks that require many flexible solubilizing substituents.
At the same time, with the substantial amount of knowledge
and experience accumulated so far, we can now optimize
substituent design through a systematic screening with arelatively narrow target distribution, rather than arduous
examination of wide-range targets. In this context, this review
is intended to provide an overview of the currently available
knowledge and clues for an optimal substituent design of new
SMSCs. For example, the state-of-the-art Y6-type SMSCs
generally possess relatively bulky alkyls (typically the size ofwithin 12 carbons) on the central ladder unit and small groups(typically halogen atoms) as the end substituents. This
combination of substituents allows the resultant molecules tobe suﬃciently soluble and miscible, while maintaining eﬀective
intermolecular p–p contacts between end groups and certain
degrees of packing order in OPV active layers. This substituent
design is common also for other systems including IDIC and
ITIC derivatives and can be a good starting point in optimization ofsubstituents for novel SMSCs. One can then fine-tune the structure
and position of substituents depending on the nature of the
p-framework or partner compounds, and synthetic accessibility.
On the other hand, substituent engineering of simple model
systems is still of considerable importance because it elucidates
general rules hidden behind the complexity of experimental
data. In addition, active control of solid-state moleculararrangement via the use of highly directional substituents(e.g., hydrogen-bonding groups) should be useful in achieving
an optimal BHJ morphology with high reliability. The precursor
approach89,153 and supramolecular approach152 mentioned in
Section 10 may be valuable in these contexts. Another importantresearch direction would be to unravel the evolution of activelayer morphology during material deposition and annealing.
Toward this end, in situ analysis of morphology should be ofcritical importance; indeed, in situ light absorption,

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photoluminescence, and X-ray scattering analyses are becoming
increasingly common in OPV research.163–166 We also note thatthe use of computer-aided approaches should be widelyextended to accelerate the understanding of the structure–morphology–performance relationship of OPV materials. There
have been already several studies in this context,167–173 andresearch efforts in this area will aid achieving PCEs nearing
the theoretical limit.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
HY and MS thank Japan Society of the Promotion of Science(JSPS) for financial support (KAKENHI No. JP20H00379,
JP20H05833 (HY), and 21K05213 (MS)).

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