Stereoisomerism of ladder-type acceptor molecules and its effect on photovoltaic properties

Article abstract of DOI:10.1016/j.dyepig.2019.02.046

Asymmetrical side chains were widely used in conjugated molecules, but related stereoisomeric phenomenan is rarely investigated. Herein, two stereoisomers based on ladder-type acceptor IDT-BT-R with asymmetrical side chains were synthesized and their prop

Full text of DOI:10.1016/j.dyepig.2019.02.046

Accepted Manuscript
Stereoisomerism of ladder-type acceptor molecules and its effect on photovoltaic
properties
Jian Cao, Tong Shan, Ji-Kang Wang, Yun-Xiang Xu, Xiancheng Ren, Hongliang
Zhong
PII:
S0143-7208(18)32876-6
https://doi.org/10.1016/j.dyepig.2019.02.046
DYPI 7380
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 December 2018
Revised Date: 14 February 2019
Accepted Date: 25 February 2019
Please cite this article as: Cao J, Shan T, Wang J-K, Xu Y-X, Ren X, Zhong H, Stereoisomerism of
ladder-type acceptor molecules and its effect on photovoltaic properties, Dyes and Pigments (2019), doi:
https://doi.org/10.1016/j.dyepig.2019.02.046.
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ACCEPTED MANUSCRIPT
Graphical abstract
Two stereoisomers of ladder-type acceptor molecules were obtained and their
aggregation behaviors and photovoltaic performance were investigated.

ACCEPTED MANUSCRIPT
Stereoisomerism of ladder-type acceptor molecules and its effect on photovoltaic
properties
Jian Cao a, Tong Shan b, Ji-Kang Wang a, Yun-Xiang Xu a*, Xiancheng Ren a* Hongliang Zhong b*
a College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials
Engineering, Sichuan University, Chengdu 610065, China.
b School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai
200240, P. R. China.
* Correspondence author
E-mail: yxxu@scu.edu.cn (Y.-X. Xu), xiancren@sina.com (X. Ren), hlzhong@sjtu.edu.cn (H.
Zhong)
Abstract
Asymmetrical side chains were widely used in conjugated molecules, but related stereoisomeric
phenomenan is rarely investigated. Herein, two stereoisomers based on ladder-type acceptor
IDT-BT-R with asymmetrical side chains were synthesized and their properties were studied.
Compared to cis-IDT-BT-R, trans-IDT-BT-R with centrosymmetrical structure showed stronger
crystallinity and more ordered molecular packing. As a result, trans-IDT-BT-R based photovoltaic
devices exhibited higher charge transport properties and superior power conversion efficiency (PCE)
up to 9.43%.
Keywords: Stereoisomerism; Ladder-type acceptor; Crystallinity; Intermolecular packing;
Photovoltaic properties
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1. Introduction
Organic conjugated materials are actively used for optoelectronic applications such as organic
photovoltaics (OPVs) and organic filed-effect transistors (OFETs). Recent studies indicate that
isomerism has a significant effect on the device performance of OPVs and OFETs,[1-3] which is a
common phenomenon and is widely present in organic conjugated materials[4, 5]. Isomerism
usually results from positional exchange of functional groups or heteroatoms.[6, 7] Generally, there
are three types of isomerism: i) main chain isomerism; ii) side chain isomerism; iii)
stereoisomerism. Various isomeric structures usually exhibit different aggregation behaviours and
optoelectronic properties owing to their distinct electronic structures, dipole directions or molecular
packings.[8-10]
Among these three types of isomerism, researchers mainly focus on main chain isomerism[1, 2,
11-15] and side chain isomerism.[16-20] For example, Takimiya and co-workers reported a series
of isomeric small molecules and polymers for OFETs based on naphthodithiophenes (NDTs) which
present different electronic structures, packing mode and devices performance.[21-23] Forrest and
co-works revealed the isomeric effects on electronic structures and crystal properties of the IT-IC
and BDT-IC isomers used in OPVs.[15] Li and co-workers synthesized a new non-fullerene
acceptor m-ITIC with meta-alkylphenyl side chains.[17] Compared with its isomeric counterpart
ITIC, m-ITIC showed higher film absorption coefficient, larger crystalline coherence, and higher
electron mobility, resulting in a higher power conversion efficiency (PCE). It is obvious that
isomerism is an effective way to establish structure−property relationship and also a useful
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approach to extend the scope of conjugated molecules. However, only a small amount of work
revealed how the stereoisomerism influence the properties of conjugated molecules and their device
performance.[3, 24-26] There are still plenty of conjugated systems which utilize asymmetrical
branched alkyl chains such as 2-ethylhexyl chain[27] or asymmetrical combinational side
chains[28]. As reported by Nguyen and her co-workers, stereoisomerism exhibited huge impact on
the crystallization behaviour, morphology and FET properties of the films[3]. Therefore, it’s
valuable to develop new stereoisomeric system to probe the influence of stereoisomerism and
develop superior materials.
Indacenodithiophene (IDT) based ladder-type non-fullerene acceptors exhibit good potential to
achieve high performance devices because of favorable charge transport properties, strong
absorption, and good balance of solubility and crystallinity.[29-39] Among them, electron acceptors
using IDT as core unit and rhodanine as electron withdrawing end group are widely used. And a
series of works about the effects of side chains and space units have been reported.[36, 40-42]
Furthermore, related wide bandgap acceptors with fused or unfused core have also been
developed.[43-48] In our previous work, IDT-BT-R based solar cells were systematically
investigated in terms of thermal stability[49], extent of backbone fusion[32] and end chain
effects[50]. These solar cell devices showed high open-circuit voltages (Voc) above 1.0 eV and
decent efficiency about 9.0%. Since stereoisomeric compounds may present some superior
properties, two stereoisomers of IDT-BT-R (namely cis-IDT-BT-R and trans-IDT-BT-R, Scheme 1)
with asymmetrical side chains were developed in this work. Two stereoisomers exhibited almost the
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same photo-physical and electrochemical properties owing to the same conjugated backbones.
However, they showed quite different crystallinity and packing behaviors due to different side chain
configuration. Compared with cis-IDT-BT-R, the centrosymmetric trans-IDT-BT-R led to enhanced
crystallinity and denser intermolecular packing thus better charge transport properties. As a result,
solar cells fabricated from the blend of the polymer donor PTB7-Th and trans-IDT-BT-R achieved
PCE up to 9.43% with a Voc of 1.05 V, a Jsc of 15.08 mA cm-2, and an FF of 59.6%, which is higher
than that of cis-IDT-BT-R. These results indicated that stereoisomerism is an effective way to
reveal the structure-property relationship and boost the material improvement.
Scheme 1. Synthetic routes of cis-IDT-BT-R and trans-IDT-BT-R. (A) i) CH3Li, THF, -78 °C; ii) BF3·etherate,
DCM, R.T; (B) BuLi, −78 °C then Me3SnCl, R.T; (C) pyridine, CHCl3, reflux.
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2. Experimental
2.1 Materials and Synthetic procedures
All chemicals, unless otherwise specified, were purchased from Adams or other commercial
resources and used as received. Br-BT-R were synthesized according to previously reported
procedures.[32] All reactions were performed under Argon atmosphere. Toluene, chloroform and
dichloromethane solvents were of AR grade and dried over CaH2, and tetrahydrofuran (THF) was
distilled from sodium benzophenone under Argon prior to use. Other reagents and solvents were
used as received.
Synthesis
of
(2,5-dibromo-1,4-phenylene)bis
((4-hexylphenyl)methanone):
2,
5-Dibromoterephthalic acid (7.01 g, 21.0 mmol) was added to 50 mL thionyl chloride, and then
reflux for 10 h. The solvent was then removed under vacuum to obtain 2, 5-dibromoterephthaloyl
dichloride, which was used in the next step without purification.
To a solution of 2,5-dibromoterephthaloyl dichloride in 50 mL dry CH2Cl2 was slowly added a
solution of anhydrous AlCl3 (7.29 g, 54.7 mmol) in dry CH2Cl2 (70 mL) at 0 °C, the mixture was
kept at 0 °C for 20 min, then a solution of n-hexylbenzene (13 mL, 69.0 mmol) was added slowly.
After the addition, the mixture was stirred at room temperature overnight. Then ice water and 3 M
hydrochloric acid solution was added to the mixture until it became clear. The mixture was washed
with saturated sodium bicarbonate solution and extracted with CH2Cl2. The organic layer was dried
over anhydrous Na2SO4. After removal of the solvent, the crude product was washed with hexane to
give the product as a white solid (9.95 g, 75%).1H NMR (400 MHz, CDCl3, δ): 7.79-7.73 (m, 4H),
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7.58 (s, 2H), 7.32 (d, J = 8.4 Hz, 4H), 2.75-2.64 (m, 4H), 1.65 (dt, J = 15.4, 7.6 Hz, 4H), 1.40-1.24
(m, 12H), 0.93-0.84 (m, 6H); 13C NMR (100 MHz, CDCl3, δ): 193.48, 150.68, 143.35, 132.86,
130.54, 129.04, 118.45, 36.23, 31.67, 31.04, 29.00, 22.60, 14.13; HRMS (ESI) m/z: [M+Na]+,
calcd for C32H36Br2O2, 633.0974; found, 633.0973.
Synthesis of compound 1: To a solution of 2-(tributylstannyl)-thiophene (3.93 g, 9.08 mmol)
and (2,5-dibromo-1,4-phenylene)bis((4-hexylphenyl)methanone) (2.10 g, 3.44 mmol) in anhydrous
toluene (30 mL), then Pd(PPh3)4 (125 mg, 0.108 mmol) was added under argon. The resulting
mixture was kept at 110 °C for 40 h. After cooling to room temperature, KF aqueous solution was
added and stirred overnight. The mixture was filtered over celite, extracted with CH2Cl2. The
organic layer was dried over anhydrous Na2SO4. After removing the solvent, the crude compound 1
was purified by silica gel chromatography using a mixture of petroleum ether/CH2Cl2 (1:1) as the
1
eluent to afford a white solid (1.50 g, 71%). H NMR (400 MHz, CDCl3, δ) : 7.70 (d, J = 8.3 Hz,
4H), 7.61 (s, 2H), 7.19 – 7.14 (m, 6H), 6.99 (dd, J = 3.6, 1.2 Hz, 2H), 6.84 (q, J = 3.6 Hz, 2H), 2.60
13
(t, J = 7.7 Hz, 4H), 1.62 – 1.57 (m, 4H), 1.31 – 1.24 (m, 12H), 0.87 (t, J = 6.8 Hz, 6H); C NMR
(100 MHz, CDCl3, δ): 197.48, 149.62, 140.38, 140.04, 134.13, 131.88, 130.19, 129.45, 128.59,
127.93, 127.76, 126.84, 36.07, 31.64, 30.93, 28.87, 22.59, 14.10; HRMS (ESI) m/z: [M+Na]+, calcd
for C40H42Br2O2, 641.2518; found, 641.2520.
Synthesis of compound 2: To a solution of compound 2 (308 mg, 0.498 mmol) in anhydrous
THF at -78 °C (15 mL) was added CH3Li (2.1 mL, 1.6 M in hexane) dropwise under argon, was
kept at -78 °C for 1 h. The mixture was stirred overnight at room temperature. Then the reaction
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was quenched with water. The mixture was washed with brine and water, extracted with CH2Cl2.
The organic layer was dried over anhydrous Na2SO4. The solvent was then removed under vacuum
to obtain light yellow solid, which was used in the next step without purification.
The light yellow solid was dissolved in 10 mL dry CH2Cl2, then BF3·etherate (0.24 mL, 1.97
mmol) was added dropwise to the mixture at room temperature under argon. After 4 h, the reaction
was quenched with menthol washed with water, treated with brine, and extracted with CH2Cl2. The
organic layer was dried over anhydrous Na2SO4. After removing the solvent, the crude compound
was purified by silica gel chromatography using a mixture of petroleum ether/CH2Cl2 (20:1) as the
eluent to afford a yellow solid (147 mg, 48%).1H NMR (400 MHz, CDCl3, δ): 7.28 (d, J = 4.1 Hz,
2H), 7.23 (dd, J = 4.9, 2.5 Hz, 2H), 7.20-7.14 (m, 4H), 7.06 (dd, J = 8.3, 1.8 Hz, 4H), 6.88 (dd, J =
4.9, 2.3 Hz, 2H), 2.60-2.49 (m, 4H), 1.87 (d, J = 2.9 Hz, 6H), 1.64-1.51 (m, 4H), 1.40-1.22 (m,
13
12H), 0.87 (t, J = 6.8 Hz, 6H); C NMR (100 MHz, CDCl3, δ): 157.97, 157.94, 156.09, 141.24,
141.04, 140.95, 134.65, 128.46, 128.44, 127.28, 126.14, 126.08, 124.71, 114.92, 52.49, 35.55,
31.75, 31.40, 29.16, 24.65, 24.50, 22.63, 14.13; HRMS (ESI) m/z: [M+Na]+, calcd for C42H46S2,
637.2933; found, 637.2900.
Synthesis of compound 3: To a solution of compound 3 (210 mg, 0.341 mmol) in anhydrous
THF at -78 °C was added n-BuLi (0.4 mL, 2.5 M in hexane) dropwise under argon. The mixture
was kept at -78 °C for 1 h, then warm to room temperature for another 0.5 h. After cooling back to
-78 °C, trimethyltin chloride (1.1 mL, 1 M in hexane) was added dropwise. The resulting mixture
was kept at room temperature overnight. Then the mixture was poured into water and extracted with
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hexane. The organic layer was dried over anhydrous Na2SO4. The solvent was removed and ethanol
1
was added, and the precipitation was collected as a yellow solid (203 mg, 63%). H NMR (400
MHz, CDCl3, δ): 7.25 (d, J = 3.3 Hz, 2H), 7.18 (d, J = 8.2 Hz, 4H), 7.06 (d, J = 8.2 Hz, 4H),
6.97-6.89 (m, 2H), 2.61-2.48 (m, 4H), 1.86 (s, 6H), 1.66-1.51 (m, 4H), 1.31 (dd, J = 17.6, 6.0 Hz,
12H), 0.87 (dd, J = 9.4, 4.2 Hz, 6H), 0.46-0.23 (m, 18H); 13C NMR (100 MHz, CDCl3, δ): 159.81,
159.78, 156.29, 146.99, 141.45, 141.38, 141.07, 140.89, 134.40, 129.31, 129.31, 128.38, 126.26,
126.20, 115.21, 51.94, 35.58, 31.77, 31.41, 31.40, 29.21, 24.90, 24.76, 22.64, 14.15, -8.04.
Synthesis of cis-IDT-BT-R and trans-IDT-BT-R: To a solution of compound 4 (881 mg,
2.00 mmol) and Br-BT-R (777 mg, 0.82 mmol) in dry toluene (20 mL), Pd(PPh3)4 (55.0 mg, 0.0476
mmol) was added under argon. The resulting mixture was kept at 110 °C for 48 h. After cooling to
room temperature, KF aqueous solution was added and stirred overnight. The mixture was filtered
over celite, extracted with CH2Cl2. The organic layer was dried over anhydrous Na2SO4. After
removing the solvent, the crude compound was purified by silica gel chromatography using a
mixture of petroleum ether/CH2Cl2 (3:1 to 3:2) as the eluent to give cis-IDT-BT-R (361 mg, 33%)
and trans-IDT-BT-R (180 mg, 16%), respectively.
1
Cis-IDT-BT-R: H NMR (400 MHz, CDCl3, δ): 8.49 (s, 2H), 8.12 (s, 2H), 7.93(d, J = 7.8 Hz,
2H), 7.68 (d, J = 7.8 Hz, 2H), 7.42 (s, 2H), 7.27 (s, 4H), 7.12 (d, J = 8.2 Hz, 4H), 4.20-4.11 (m,
4H), 2.58-2.55 (m, 4H), 2.01 (s, 6H), 1.72 (dd, J = 15.0, 7.6 Hz, 4H), 1.63-1.53 (m, 8H), 1.44-1.23
(m, 24H), 0.96-0.79 (m, 12H); 13C NMR (100 MHz, CDCl3, δ): 193.16, 167.74, 159.63, 156.93,
154.45, 151.55, 145.14, 141.83, 141.72, 140.18, 135.29, 131.16, 130.09, 128.70, 127.03, 126.17,
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124.60, 124.45, 124.04, 123.89, 115.63, 53.08, 44.86, 35.57, 31.74, 31.42, 31.37, 29.17, 26.99,
26.48, 24.74, 22.62, 22.54, 14.14, 14.05. HRMS (MALDI-TOF) m/z: calcd for C74H76N6O2S8,
1336.380; found, 1336.338.
1
Trans-IDT-BT-R: H NMR (400 MHz, CDCl3, δ): 8.49 (s, 2H), 8.11 (s, 2H), 7.93 (d, J = 7.7
Hz, 2H), 7.68 (d, J = 7.9 Hz, 2H), 7.41 (s, 2H), 7.28 (s, 4H), 7.13 (d, J = 8.2 Hz, 4H), 4.20-4.12 (m,
4H), 2.60-2.52 (m, 4H), 2.00 (s, 6H), 1.72 (dd, J = 15.0, 7.6 Hz, 4H), 1.64-1.49 (m, 15H), 1.40-1.23
(m, 24H), 0.91-0.85 (m, 12H). 13C NMR (100 MHz, CDCl3, δ): 193.23, 167.78, 159.64, 156.94,
154.48, 151.61, 145.18, 141.81, 141.71, 140.27, 135.31, 131.20, 130.15, 128.71, 127.11, 126.10,
124.65, 124.51, 123.98, 123.94, 115.61, 53.05, 44.86, 35.56, 31.73, 31.42, 1.37, 29.16, 26.99,
26.47, 24.56, 22.61, 22.52, 14.14, 14.04. HRMS (MALDI-TOF) m/z: calcd for C74H76N6O2S8,
1336.380; found, 1336.442.
2.2 Device Fabrication and Measurement
The patterned indium tin oxide glass (ITO) glass substrates (sheet resistance = 15 Ω sq-1) were
cleaned in detergent, de-ion water, acetone, chloroform, acetone, and isopropanol sequentially by
ultra-sonic bath for 15 min each and then dried by N2 gas. Further UV-Ozone treatment for 5 min
was applied before use.
The ZnO precursor solution was prepared by dissolving zinc acetate dihydrate
(Zn(CH3COO)2•2H2O, Sigam-Aldrich, 99.0%, 1 g) and ethanolamine (NH2CH2CH2OH, Acros, 99
%, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, Aladdin, 99.8%, 10 mL) under vigorous
stirring for 15 h for the hydrolysis reaction and aging in air. The ZnO precursor solution was
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spin-cast on top of the ITO-glass substrate. The films were annealed at 200 °C for 20 min in air to
form thickness of approximately 35 nm determined by an AFM. Then the ZnO-coated substrates
were transferred into a nitrogen-filled glove box. In order to fine tune the interfacial properties, a
thin
film
of
poly[(9,9-bis(3'-(N,N-dimethyl)
-nethylammoinium-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) film (ca. 5 nm) was
prepared by spin-coating PFN solution (0.4 mg/mL in methanol) onto ZnO. The PTB7-Th: acceptor
blends were dissolved in CB with 5 V% 1-chloronaphthalene (CN) as additive at 60 ℃ and stirred
overnight in a nitrogen-filled glove box. The active layers with optimized thicknesses (ca. 100 nm)
were obtained by spin-coating the blend solution at room temperature with polymer concentrations
of 12 mg mL-1 that was filtered by a PTFE (polytetrafluoroethylene) filter (pore size of 0.45 µm)
atop the PFN layer, and annealed at 100 °C for 10 min in glove box. Then, approximately 10 nm
MoO3 was evaporated on top of the active layer at a speed of 0.03 nm/s. Finally, the anode, 100 nm
Al was deposited at a speed of 0.3 nm/s through a shadow mask by thermal evaporation in a
vacuum chamber of under 3 × 10-6 Torr to complete the device fabrication. The active area of each
device was 3.68 mm2.
The device J-V characteristics were recorded by a Keithley 2420 SourceMeter unit in forward
direction under AM 1.5G 1 sun irradiance (100 mW cm-2) as generated by a 300W Xe lamp solar
simulator (Enlitech SS-F5-3A) at room temperature. The light intensity were calibrated using a
standard Si diode with KG-5 filter. The EQE spectra was characterized using an Enlitech EQE
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system (Enlitech QE-M110) with a Si diode as reference cell. Monochromatic light was generated
from an Enlitec lamp source with a monochromator.
Hole-only diode configuration: ITO/PEDOT-PSS/blend films/MoO3/Al. Firstly, PEDOT-PSS
was spin-coated onto the ITO-glass substrate, and annealed in ambient atmosphere for 15 min at
140 ℃. Then the following layer were deposited by the same route as OPV devices. Electron-only
diode configuration: ITO/ZnO/blend films/Al.
The mobility in blend films were determined by fitting the dark current hole/electron-only diodes
to the space-charge limited current (SCLC) model. The mobility was determined by the equation:
9
ε0εr µ0V 2
J =
3
8L
where J is current density, µ0 is the hole or electron mobility, ε0 is the dielectric permittivity of
the active layer (generally taken to be about 3 for organic materials), ε0 is the dielectric permittivity
of free space, L is the film thickness, and V is the voltage, which is defined as V = Vappl - Vbi, where
Vappl is the applied voltage, Vbi is the built-in voltage which is related to the difference in the work
function of the electrodes.
3. Results and discussion
3.1 Synthesis and characterization
The chemical structures and synthetic details of cis-IDT-BT-R and trans-IDT-BT-R are shown in
Scheme 1. The key steps lie in the introduction of asymmetric side chains. Methyl groups were
attached by nuclearphilic addition (Step A, i). Then ladder-type IDT core was obtained by
intramolecular annulation catalyzed by boron trifluoride etherate (Step A, ii). In this way, two
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bridging chiral carbon centers were generated. IDT-BT-R were obtained by Stille coupling reaction
which consists of a racemate and a mesomer, named cis-IDT-BT-R and trans-IDT-BT-R. The ratio
of cis-IDBR and trans-IDT-BT-R generated in the reaction is about 2:1. The nonequal ratio may be
attributed to some stable intermediate during the cyclization reaction. Both compounds can be
separated by column chromatography due to the difference in polarity (trans-IDT-BT-R shows
1
smaller polarity in silica gel). The mass spectrometry, H NMR and 13C NMR spectra of
trans-IDT-BT-R and cis-IDT-BT-R were measured. Both compounds showed almost the same
signals in 1H NMR and 13C NMR spectra. Because cis-IDT-BT-R possessed two chiral enantiomers
as shown in Scheme S1, the assignment of cis-IDT-BT-R was confirmed by chiral
high-performance liquid chromatography (HPLC) (Figure S1) which showed double peaks
corresponding to two chiral enantiomers. And another product was trans-IDT-BT-R which didn’t
possess enantiomers. Unfortunately, the enantiomers of cis-IDT-BT-R could not be separated
because of poor resolution. The solubility of trans-IDT-BT-R and cis-IDT-BT-R in different
solvents were measured and summarized in Table S1 (Supporting Information). Compared to
cis-IDT-BT-R, trans-IDT-BT-R showed lower solubilities due to its ordered molecular packing.
Their thermal stability properties were studied by thermogravimetric analysis (TGA) (Figure S2,
Supporting Information). From the curves, two stereoisomers have the same decomposition
temperature (5% weight loss) of 388 °C.
3.2 Molecular properties
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The UV-vis absorption spectra of two stereoisomers were shown in Figure 1(a) and 1(b) and the
optical data were summarized in Table 1. Both acceptors had almost the same absorption profile in
chloroform solutions and as thin films. However, trans-IDT-BT-R displayed slightly higher molar
extinction coefficient (ε = 1.05 × 105 M-1 cm-1) than cis-IDT-BT-R (ε = 1.01 × 105 M-1 cm-1),
suggesting its stronger light harvesting ability. Compared to their maximum absorption peaks at
638nm in solution, stereoisomers exhibited maximum absorption peaks at 677 nm in thin films due
to intermolecular electronic coupling in solid state. The optical bandgaps (Egopt) calculated from the
onset of film absorption curves were determined to be 1.64 eV for cis-IDT-BT-R and 1.63 eV for
trans-IDT-BT-R. The energy levels of high occupied molecular orbital and the lowest unoccupied
molecular orbital (HOMO/LUMO) of two stereoisomers were measured by electrochemical cyclic
voltammetry (Figure S3, Supporting Information) and were summarized in Table 1. The
HOMO/LUMO energy levels of cis-IDT-BT-R calculated from onset oxidation/reduction potentials
were -5.44/-3.62 eV, which are similar to those of trans-IDT-BT-R (-5.43/-3.61 eV). Thus, the
electrochemical bandgaps of both compounds were calculated to be 1.82 eV. It should be noted that
slightly different oxidation/reduction potential of two stereoisomers could due to the influence of
side chain configuration on the charge transfer process [17].
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(a)
(b)
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
cis-IDT-BT-R
trans-IDT-BT-R
cis-IDT-BT-R
trans-IDT-BT-R
1.0
0.8
0.6
0.4
0.2
0.0
300 400 500 600 700 800 900
300 400 500 600 700
Wavelength(nm)
Wavelength(nm)
Figure 1. (a) UV–vis absorption spectra of cis-IDT-BT-R and trans-IDT-BT-R in chloroform solutions and (b) as thin
films.
Table 1. Optical and electrochemical properties of stereoisomers.
solu a
a
film
film
opt b
λmax
(nm)
εmax
λmax
λedge
(nm)
Eg
HOMO/LUMO c
(eV)
Molecules
(105 M−1 cm−1)
(nm)
(nm)
cis-IDT-BTR
638
639
b
1.01
1.05
677
677
756
762
1.64
1.63
-5.44/-3.62
-5.43/-3.61
trans-IDT-BTR
a
c
Measured in chloroform.
Calculated from the absorption edges of thin films. Measured by electrochemical
cyclic voltammetry.
(a)
(b)
Figure 2. (a) DSC and (b) XRD curves of (a) cis-IDT-BT-R and (b) trans-IDT-BT-R.
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Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were used to investigate
the crystallinity of two stereoisomers (Figure 2(a) and (b)). The DSC curves showed quite different
thermal transitions for two stereoisomers. Cis-IDT-BT-R exhibited a tiny melting peak at 128 °C
and two very weak crystallization peaks at 89°C and 123°C, indicating weak polycrystalline
structure in cis-IDT-BT-R. While trans-IDT-BT-R exhibited a significant melting peak at 210 °C
and an obvious crystallization peak at 198 °C, implying that trans-IDT-BT-R lead to a higher
degree of crystallinity in solid state. Indeed, X-ray diffraction results (Figure 2(b)) showed that no
obvious diffraction patterns were observed for cis-IDT-BT-R. On the contrary, a set of distinct
diffraction signals were observed for trans-IDT-BT-R, indicating enhanced molecular ordering. The
(100) diffraction peak of trans-IDT-BT-R was located at 2θ = 7.32°, corresponding to lamellar
distance of 12.07 Å. Moreover, the (200) diffraction peak of trans-IDT-BT-R was also observed at
2θ = 14.64°, implying a more ordered lamellar packing. Furthermore, the diffraction peaks between
3.5 Å to 4.0 Å (2θ = 24.26°, 23.20°, 21.66°) demonstrated strong π-π stacking.
The aggregation behaviours of two stereoisomers were also revealed by atomic force microscopy
(AFM). Figure 3 showed AFM phase and height images acquired from films of stereoisomers
which were fabricated from chlorobenzene (CB) and chloroform (CF) solutions. The cis-IDT-BT-R
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films from both CB and CF solutions showed very smooth surfaces with root mean square (RMS)
roughness of 0.398 nm and 1.38 nm, respectively. At the same time, the trans-IDT-BT-R films
prepared from CB and CF solutions exhibited very large RMS roughness of 4.90 nm and 6.38 nm,
indicating a stronger aggregation in solid state.
Figure 3. a-b) AFM height and e-f) phase images of trans-IDT-BT-R films; c-d) AFM height and g-h) phase
images of cis-IDT-BT-R films made from chlorobenzene (CB) and chloroform (CF) solutions. (2×2 µm2)
3.3 Photovoltaic Device Performance
Photovoltaic properties based on two acceptors were investigated with an inverted device structure
of ITO/ZnO/PFN/PTB7-Th: acceptors/MoO3 (10 nm)/Al (100 nm). The current density-voltage
curves were shown in Figure 4 and photovoltaic parameters were summarized in Table 2. The
devices based on PTB7-Th: trans-IDT-BT-R (1:1.3 by weight) showed a higher PCE of 9.43% with
a Voc of 1.05 V, a Jsc of 15.08 mA cm-2, and an FF of 59.6%. In comparison, the devices based on
PTB7-Th: cis-IDT-BT-R (1:1.5 by weight) exhibited a lower PCE of 7.86%, with a slightly higher
Voc of 1.08 V, a lower Jsc of 13.79 mA cm-2 and an FF of 53.8%. The significant improvement in Jsc
and FF values of trans-IDT-BT-R based devices could be attributed to higher electron mobility and
proper phase morphology, leading to higher PCE. Figure 4b shows the external quantum
efficiencies (EQEs) of the cis-IDT-BT-R and trans-IDT-BT-R based PSCs. All devices displayed
wide photocurrent responses from 300 to 800 nm. But the trans-IDT-BT-R based devices showed a
stronger response in the region of 300-750 nm, leading to a higher Jsc value.
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(a)
(b)
Figure 4. (a) J-V curves and (b) EQE spectra and integrated current densities of PTB7-Th: cis-IDT-BT-R and
PTB7-Th: trans-IDT-BT-R based devices.
To investigate the charge transport properties of the active layers, hole and electron mobilities
were
measured by the space-charge-limited current (SCLC) method using hole- and electron- only
devices with configurations of ITO/PEDOT:PSS/blend films/MoO3/Al and ITO/ZnO/blend
films/Al, respectively. For PTB7-Th: cis-IDT-BT-R blend film, the hole/electron mobilities (µh/µe)
were estimated to be 5.92 × 10-4/1.02 × 10-6 cm2 V-1 s-1, respectively. Whereas for the PTB7-Th:
trans-IDT-BT-R film, the corresponding µh/µe values were 8.88 × 10-4/2.18 × 10-6 cm2 V-1 s-1,
respectively. A higher and more balanced charge mobilities of PTB7-Th: trans-IDT-BT-R film
could facilitate charge transport and reduce the probability of charge recombination, leading to
higher Jsc, FF and PCE. The morphologies of optimized blend films were measured by AFM. As
shown in Figure S4 (c) and S4 (d) (Supporting Information), both films showed homogeneous
surfaces and no large aggregation could be observed from the phase imagines. And trans-IDT-BT-R
exhibited smaller domain size. The corresponding RMS roughness values of PTB7-Th:
trans-IDT-BT-R film and PTB7-Th: cis-IDT-BT-R film were 2.06 nm and 2.29 nm, respectively.
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The slightly lower RMS value and smaller domain size of PTB7-Th: trans-IDT-BT-R indicated
good miscibility between donor and acceptor components within the blend film.
Table 2. The optimal photovoltaic parameters of PTB7-Th: cis-IDT-BT-R and PTB7-Th: trans-IDT-BT-R based
devices.
b
Voc
Jsc
Cal Jsc
FF
PCE(best)
(%)
acceptors
[D]:[A]a
1:1.3
(V)
(mA cm-2)
(mA cm-2)
(%)
14.59± 0.48
(15.08)
58.2± 1.4
(59.6)
8.92 ± 0.49
(9.43)
trans-IDT-BT-R
cis-IDT-BT-R
1.05± 0.00
1.08 ±0.00
14.57
13.30
13.73± 0.20
(13.79)
53.0 ±1.0
(57.8)
7.86 ± 0.12
(8.02)
1:1.5
a with 5% CN as solvent additive. D represents PTB7-Th and A represents acceptors. b Calculated from the
integration of EQE spectra.
4. Conclusions
In summary, two stereoisomeric ladder-type acceptors with asymmetrical side chains based on
IDT-BT-R were designed and synthesized. And their structures were characterized by various
methods. Compared with cis-IDT-BT-R, the centrosymmetric trans-IDT-BT-R showed stronger
intermolecular packing and higher degree of crystallinity in aggregation states. The solar cells based
trans-IDT-BT-R exhibited a high PCE of 9.43% which is one of the highest values for IDT-BT-R
based non-fullerene solar cells. These results will help to understand the structure-property
relationship of ladder-type non-fullerene acceptors and further guide the molecular design of high
efficiency photovoltaic materials.
Acknowledgements
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This work was supported by the National Natural Science Foundation of China (51403135).
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Highlights
•
•
•
Two stereoisomers with asymmetrical side chains based on IDT-BT-R were
successfully synthesized.
Centrosymmetric trans-IDT-BT-R showed enhanced crystallinity and denser
intermolecular packing.
OSCs based on trans-IDT-BT-R showed high PCE of 9.43%.