Cyclohexyl-substituted non-fullerene small-molecule acceptors for organic solar cells

Article abstract of DOI:10.1039/d0nj04240d

In this paper, two cyclohexyl-substituted non-fullerene small molecules, T2-Cy6MRH and T2-Cy6PRH, are designed to have the same molecular backbone of a bithiophene (T2) core and rhodanine (RH) end groups. T2-Cy6MRH and T2-Cy6PRH can be synthesized via two

Full text of DOI:10.1039/d0nj04240d

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Cyclohexyl-substituted non-fullerene small-
molecule acceptors for organic solar cells†
Cite this: New J. Chem., 2021,
45, 10373
Seunggyun Hong,‡a Chang Eun Song, ‡b Du Hyeon Ryu,b Sang Kyu Lee,
b
b
c
Won Suk Shin
and Eunhee Lim
*
In this paper, two cyclohexyl-substituted non-fullerene small molecules, T2-Cy6MRH and T2-Cy6PRH,
are designed to have the same molecular backbone of a bithiophene (T2) core and rhodanine (RH) end
groups. T2-Cy6MRH and T2-Cy6PRH can be synthesized via two and four synthesis steps, respectively,
from commercially available starting materials. Thermogravimetric analysis shows that both small
molecules are thermally stable at temperatures greater than 380 1C. Compared with T2-Cy6PRH,
T2-Cy6MRH exhibits a higher thermal-transition temperature and a more negative enthalpy change,
indicating that it exhibits greater crystallinity. Unlike T2-Cy6MRH, T2-Cy6PRH can form a uniform film
via solution-processing, because the introduction of longer flexible alkyl chains endows T2-Cy6PRH
with better solubility than T2-Cy6MRH. The T2-Cy6PRH film (lmax = 490 nm) exhibits complementary
UV-vis absorption with the small-bandgap polymer donor PTB7-Th (lmax = 700 nm). In addition, the
highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels of
T2-Cy6PRH are relatively low-lying compared with those of PTB7-Th. An organic solar cell device based
on T2-Cy6PRH and PTB7-Th exhibited a power conversion efficiency (PCE) of 4.60% with a high open-
circuit voltage of 1.03 V. The addition of [6,6]-phenyl-C61-butyric acid methyl ester substantially
improved the device performance, owing to the improved film morphology and the cascade-energy-
level alignment, resulting in a PCE of 6.91% under additive- and annealing-free conditions.
Received 24th August 2020,
Accepted 12th May 2021
DOI: 10.1039/d0nj04240d
rsc.li/njc
polymer donor to enhance the light-harvesting ability of the
active layer, (2) energy levels compatible with those of the
1. Introduction
Organic solar cells (OSCs) have drawn considerable attention polymer donor to ensure efficient exciton separation and to
because of great advances in the fabrication of low-cost, maximize the open-circuit voltage (VOC), and (3) a suitable
lightweight, flexible, transparent, and large-area printable crystallinity to realize a favorable D/A film morphology for
devices.1–5 Recently, researchers have achieved remarkable efficient charge transport. The modulation of the molecular
progress in the development of non-fullerene acceptors (NFAs), structure—in particular, p-conjugated core units—has been
and typical acceptor–donor–acceptor (A–D–A)-type NFAs have used as an effective approach to achieving the aforementioned
increased the power conversion efficiencies (PCEs) of OSCs to properties because it enables modification of molecular energy
greater than 15%.6–9 The realization of high-performance OSCs levels, increased intermolecular interaction, and improved
necessitates the selection of an appropriate combination of charge mobility.10
donor and acceptor materials to enable the fabrication of blend
In the development of new organic semiconducting materi-
films with well-matched physical properties. NFAs should be als, side-chain engineering is also a very useful method to fine-
designed to exhibit (1) absorption that complements that of the tune the physical properties of the materials and the morphol-
ogy of the corresponding films. Various side chains introduced
onto the donor or acceptor materials in OSCs include linear/
cyclic/branched alkyl side chains, electron-donating/withdraw-
a Department of Chemistry, Kyonggi University, 154-42 Gwanggyosan-ro,
Yeongtong-gu, Suwon 16227, Republic of Korea
b Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro,
ing side chains, conjugated side chains, and oligoether side
chains.11 Linear or branched (i.e., 2-ethylhexyl) alkyl or p-
conjugated 2D alkylaryl (i.e., alkylphenyl or alkylthienyl) side
chains have been widely used in the active materials of OSCs.4,5
A few examples of organic semiconducting materials contain-
ing cyclic side chains have been reported.12–16 The effect
of cyclohexyl groups on intermolecular aggregation has been
Yuseong-gu, Daejeon 34114, Republic of Korea
c Department of Applied Chemistry, University of Seoul, 163 Seoulsiripdae-ro,
Dongdaemun-gu, Seoul 02504, Republic of Korea. E-mail: ehlim@uos.ac.kr;
Tel: +82-2-6490-2465
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04240d
‡ S. Hong and C. E. Song contributed equally to this work.
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investigated. In some cases, intermolecular aggregation has OSCs were fabricated with a device structure of indium tin
been found to be enhanced by the introduction of cyclohexyl oxide (ITO)/ZnO nanoparticles (NPs)/polyethyleneimine ethoxy-
groups because of their rigidity and preferential chair confor- lated (PEIE)/PTB7-Th:T2-Cy6PRH/MoOx/Ag. A PCE of 4.60%
mation;17,18 in other cases, cyclohexyl groups have led to weak with a high VOC of 1.04 V was obtained for an OSC fabricated
aggregation between molecules by functioning as a bulky steric under the additive-free and annealing-free condition.
side chain.15 Therefore, the effect of cyclohexyl groups on
molecular aggregation depends on the substitution position
and the type of the molecular backbone. The grafting of
cyclohexyl groups as side-chains into polymer donors has been
shown to strengthen side-chain interdigitation, resulting in an
overall tighter lamellar packing in the film state and a PCE
2. Result and discussion
2.1. Synthesis
The two small molecules, T2-Cy6MRH and T2-Cy6PRH, con-
sisted of a bithiophene core and cyclohexylalkyl-substituted RH
more than twice that of the corresponding device with a
reference polymer.17 In addition, most of the studies on the
ends designed with different alkyl spacer lengths (methylene
and propylene). These RH-substituted bithiophenes, T2-
Cy6MRH and T2-Cy6PRH, were synthesized via two and
four steps, respectively, from commercially available starting
materials (i.e., cyclohexanemethylamine (Cy6MNH2) and
effects of cyclohexyl groups have been limited to polymer
donors; the literature contains few studies on acceptors with
a cyclohexyl group.
Recently, we reported a series of high-performance NFAs,
composed of bithiophene (T2) and alkylrhodanine dye.19–22
3-cyclohexyl-1-propanol, respectively). The alkyl-substituted
RH derivatives, cyclohexylmethylrhodanine (Cy6MRH) and
N-(3-cyclohexyl)propylrhodanine (Cy6PRH), were synthesized
using bis(carboxymethyl)trithiocarbonate though
The relatively short p-conjugation of these NFAs resulted in
optical properties that complement those of the small-bandgap
a
ring-
polymer donor poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo
[1,2-b:4,5-b0]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-
carboxylate] (PTB7-Th). Moreover, the most promising physical
properties of this series, the high-lying lowest unoccupied
molecular orbital (LUMO) energy levels (for example, ꢀ3.6 eV
for T2-ORH) provided high VOC values (41 V) and a remarkably
low energy loss. Because of the non-fused core structure of
bithiophene, the synthesis procedures for this series could be
quite simple (e.g., two steps for T2-ORH). The structural sim-
plicity of this series enables the chemical structure to be
modified in various ways. In the present work, we introduced
cyclohexyl groups onto both ends as alkyl side chains to
investigate the effect of these groups on the physical properties
of a series of these non-fullerene small molecules. Two
cyclohexylalkyl-substituted rod-shaped small molecules, T2-
Cy6MRH and T2-Cy6PRH, were designed to have the same
molecular backbone of a rhodanine (RH)-flanked bithiophene.
The common normal or branched alkyl groups (e.g., the n-octyl
group in T2-ORH or 2-ethylhexyl group in T2-OEHRH)19,20
attached to both RH ends were replaced with cyclohexylalkyl
groups. Methylene (–CH2–) and propylene (–CH2CH2CH2–), as
alkyl spacers with different lengths, are located between the RH
and cyclohexyl groups, resulting in T2-Cy6MRH and T2-
Cy6PRH, respectively. The introduction of the cyclohexyl side
chains resulted in higher thermal transition temperatures
compared with those of the n-alkyl-containing small molecules.
Such a substantial increase in molecular interaction resulted in
very poor solubility of T2-Cy6MRH; however, the molecular
packing ability of T2-Cy6PRH was suppressed by the introduc-
tion of the longer alkyl spacer (propylene), resulting in better
closing reaction with the corresponding amine derivatives of
Cy6MNH2 and N-(3-cyclohexyl)propylamine (Cy6PNH2), respec-
tively. The Cy6MNH2 was acquired from a commercial source
(US$12 per gram), and Cy6PNH2 was easily synthesized from
3-cyclohexyl-1-propanol (US$5 per gram) via the Gabriel reac-
tion (yield: 470%)23,24 Finally, T2-Cy6MRH and T2-Cy6PRH
were obtained in high yield (492%) via a Knoevenagel con-
densation between 2,20-bithiophene-5,50-dicarbaldehyde and
the corresponding RH derivatives of Cy6MRH and Cy6PRH,
respectively, followed by washing with methyl alcohol. No
further purification steps were needed. The synthetic routes
to T2-Cy6MRH and T2-Cy6PRH are shown in Scheme 1. The
chemical structures of all of the intermediates and the final
products were confirmed by 1H-NMR spectroscopy. The final
products of T2-Cy6MRH and T2-Cy6PRH were analyzed by
13C-NMR spectroscopy, elemental analysis (EA), and high-
resolution mass spectrometry (HRMS). However, the 13C-NMR
spectrum of T2-Cy6MRH could not be obtained because of its
poor solubility. The UV-vis spectra and cyclic voltammograms
of T2-Cy6MRH could also not be obtained for the same reason.
By contrast, T2-Cy6PRH was soluble in organic solvents includ-
ing chloroform. T2-Cy6MRH was soluble in chloroform in
concentrations of less than 4 mg mLꢀ1, whereas T2-Cy6PRH
exhibited improved solubility in chloroform (20 mg mLꢀ1) at
room temperature. The introduction of a longer flexible alkyl
spacer resulted in T2-Cy6PRH exhibiting better solubility than
T2-Cy6MRH.
2.2. Physical properties
solubility suitable for film formation. The optical and electro- Both T2-Cy6MRH and T2-Cy6PRH displayed excellent thermal
chemical properties of the small molecules were investigated stability, losing less than 5% of their weight when heated to
and found to be complementary to those of the representative 380 1C, as measured by thermogravimetry (TGA) (Fig. 1a).
low bandgap polymer donor PTB7-Th. T2-Cy6PRH could be The thermal transition behaviors of the two small molecules
used as an acceptor for OSCs because of the strong electron- were studied by differential scanning calorimetry (DSC). The
withdrawing ability of the two RHs at both ends. The inverted endothermic melting temperature (Tm) of T2-Cy6MRH (327 1C)
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Scheme 1 Synthetic routes to T2-Cy6MRH and T2-Cy6PRH.
Fig. 1 (a) TGA and (b) DSC curves of T2-Cy6MRH and T2-Cy6PRH.
was substantially higher than that of T2-Cy6PRH (232 1C), and The Tms gradually increased from octyl to hexyl to ethyl sub-
the corresponding enthalpy change was also more negative for stitution, indicating increased molecular interaction. In the
T2-Cy6MRH (ꢀ94.4 J gꢀ1) than for T2-Cy6PRH (ꢀ83.4 J gꢀ1). A present study, because the molecular aggregation behaviors
similar trend was observed in the exothermic recrystallization can be controlled by the shape of the substituents as well as
temperatures (Tcr) upon cooling. The higher thermal-transition their length, as an example of modifying the shape of the
temperature and more negative enthalpy change of T2-Cy6MRH substituents, we introduced a cyclohexyl group as a terminal
indicate that it exhibits greater molecular aggregation and side chain, which resulted in a much higher phase-transition
greater crystallinity than T2-Cy6PRH, consistent with the poor temperature in T2-Cy6MRH (Tm = 327 1C and Tcr = 280 1C). That
solubility of T2-Cy6MRH. In addition, T2-Cy6PRH exhibited is, very strong molecular interactions were induced by the
liquid-crystalline characteristics at 193 1C according to its cyclohexyl group, which, according to the literature, is attribu-
DSC thermogram. The phase-transition temperatures of mole- table to the rigid and preferential chair conformation of the
cules with the same backbone increase gradually with decreas- cyclohexyl group.17,18 The Tm of T2-Cy6MRH (327 1C) was more
ing length of the alkyl side chain.25,26 These trends were than 120 1C higher than that of T2-ORH (204 1C), resulting in
also observed in a series of these bithiophene-RH-based rod- very poor solubility of the T2-Cy6MRH. Notably, T2-Cy6MRH
shaped small molecules. In our previous report about the had a higher Tm than the ethyl-containing T2-ERH, despite the
alkylrhodanine-flanked bithiophenes, ethyl-substituted T2- longer side-chain length of T2-Cy6MRH. Again, such excessive
ERH (Tm = 305 1C and Tcr = 265 1C)21 exhibited a much higher molecular aggregation could be weakened by the introduction
transition temperature than octyl-substituted T2-ORH (Tm
=
of a longer n-alkyl spacer of propylene in T2-Cy6PRH. The Tm of
204 1C and Tcr = 175 1C).20 The phase-transition properties of T2-Cy6PRH was lowered to 232 1C, still higher than that of T2-
the hexyl-substituted T2-HRH (Tm = 243 1C and Tcr = 213 1C)22 ORH but lower than that of T2-HRH. Thus, the insertion of a
were intermediate between those of T2-ERH and T2-ORH. longer flexible alkyl spacer imparted T2-Cy6PRH with sufficient
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Table 1 Thermal properties of T2-Cy6MRH and T2-Cy6PRH
(lmax = 510 nm). A greater blue-shift in the absorption max-
imum indicates stronger H-type aggregation induced by ther-
mal annealing (TA). The intensity of the absorption shoulder
observed at B560 nm was greatly reduced. That is, the aggrega-
tion behavior of the T2-Cy6PRH films was changed by anneal-
ing. Interestingly, the phenomenon observed in T2-Cy6PRH
films was reversed compared with that observed in the pre-
a
b
c
d
e
Td
Tm
DHm
(J gꢀ1
Tcr
DHcr
(1C)
(1C)
)
(1C)
(J gꢀ1
)
T2-Cy6MRH
T2-Cy6PRH
383
380
327
232
ꢀ94.4
ꢀ83.4
280
184
91.2
58.4
a
Temperature resulting in 5% weight loss based on the initial weight.
Temperature at the melting endothermal peak. Enthalpic changes at Tm.
b
c
d Temperature at the recrystallization exothermal peak. Enthalpic viously reported T2-ORH case, where predominant H-type
e
changes at Tcr.
aggregation in the as-cast T2-ORH film was shifted to J-type
aggregation by annealing. The change in the aggregation
solubility for film formation via solution processing. The behavior of the T2-Cy6PRH film could significantly influence
thermal characteristics of T2-Cy6MRH and T2-Cy6PRH are the OSC device performance, as discussed in Section 2.3.
summarized in Table 1.
Importantly, T2-Cy6PRH exhibited complementary UV-vis
The UV-vis absorption spectra of T2-Cy6MRH and T2- absorption with the small-bandgap polymer PTB7-Th. The
Cy6PRH in chloroform solution are shown in Fig. 2a and absorption spectra for the PTB7-Th film are shown in Fig. S8
appear to be approximately the same; this result is reasonable (ESI†) for comparison. T2-Cy6PRH can absorb light with wave-
given the similarity of their molecular backbones, both of lengths shorter than 600 nm, while PTB7-Th can absorb light
which consist of bithiophene and RH. The UV-vis absorption with wavelengths longer than 600 nm, which is beneficial to
maxima (lmax) of the two small molecules were observed at light-harvesting in OSCs based on blend films composed of T2-
510 nm, irrespective of the introduced alkyl chains. Fig. 2b Cy6PRH and PTB7-Th as an electron acceptor and an electron
shows the UV-vis absorption spectra of the T2-Cy6PRH films. donor, respectively. The optical band gaps (Eg,opts) were esti-
(As previously noted, a film with a proper morphology could not mated from the absorption onset wavelengths (lonset) of the T2-
be obtained from T2-Cy6MRH because of its very poor solubi- Cy6PRH and PTB7-Th films according to the equation Eg
=
lity.) The spectrum of the as-cast T2-Cy6PRH film shows two 1240/lonset (eV). T2-Cy6PRH exhibited a relatively wider Eg,opt
absorption peaks. The lmax of the T2-Cy6PRH film was 490 nm, (2.02 eV) compared with that of PTB7-Th (1.67 eV).
which was blue-shifted by B20 nm compared with the value
The electrochemical properties of T2-Cy6PRH were investi-
corresponding to the solution-state (lmax = 510 nm), suggesting gated by CV measurements (Fig. 3a). The highest occupied
H-aggregation.20 Another absorption peak centered at 510 nm molecular orbital (HOMO) and LUMO energy levels of
exhibited nearly the same intensity as that observed at 490 nm T2-Cy6PRH were calculated using the empirical relationship
together with an additional shoulder peak at approximately EHOMO = ꢀ(Eonset,ox ꢀ E1/2,Fc + 4.8) eV and ELUMO = ꢀ(Eonset,red
ꢀ
563 nm. This relatively red-shifted and broadened absorption E1/2,Fc + 4.8) eV, where Eonset,ox and Eonset,red are the onset
compared with that in the solution spectrum indicated the potentials of oxidation and reduction, respectively, assuming
existence of J-type molecular aggregation. Thus, the T2-Cy6PRH that the energy level of Fc is 4.8 eV below the vacuum level.30,31
molecules formed both J- and H-type aggregates in the film The HOMO and LUMO levels were calculated to be ꢀ5.67 and
state. The coexistence of these different types of crystalline ꢀ3.62 eV, respectively. The electrochemical bandgap (Eg,CV
)
domains in films of pentacene or other rod-shaped small corresponding to the difference between the HOMO and LUMO
molecules has been widely reported.27–29 In addition, the T2- energy levels was 2.05 eV, which is similar to the optically
Cy6PRH film was thermally annealed at 90 1C for 10 min. The obtained Eg,opt value (2.02 eV). The energy-level diagram of
spectrum of the TA-treated T2-Cy6PRH film shows an appar- T2-Cy6PRH is shown in Fig. 3b, together with that of PTB7-Th
ently blue-shifted absorption (lmax = 445 nm) compared with for comparison. The energetic offsets between the HOMOs
the spectra of the as-cast film (lmax = 490 nm) and the solution (0.5 eV) and LUMOs (0.12 eV) of PTB7-Th and T2-Cy6PRH are
Fig. 2 UV-vis absorption spectra of (a) T2-Cy6MRH and T2-Cy6PRH in solution and (b) T2-Cy6PRH films in the as-cast and TA conditions.
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