An efficient strategy to supervise absorption, mobility, morphology of photovoltaic molecule by inserting a D-A unit

Article abstract of DOI:10.1016/j.dyepig.2019.03.050

The absorption, mobility and morphology of molecules in the active layer have a crucial influence on photovoltaic properties of organic solar cells. In this work, we design an A₁-(D-A)₂-D-A₁ type oligomer DRCN4FBT on the basis of an A₁-D-A-D-A₁ type small molecule DRCN2FBT by inserting D-A unit. It is found that oligomer DRCN4FBT exhibits wider absorption spectrum and higher carrier mobility and better morphology than small molecule DRCN2FBT in active layer using fullerene PC₇₁BM as acceptor. The absorption and energy level, mobility, morphology of DRCN4FBT in active layer are well supervised by inserting D-A unit in DRCN2FBT. As a result, its power conversion efficiency and short current density are increased by about 20% for the DRCN4FBT/PC₇₁BM based organic solar cells in comparison with the DRCN2FBT/PC₇₁BM based devices. It indicates that simply increasing D-A unit in small molecules to construct oligomer is an efficient strategy to improve photovoltaic properties.

Full text of DOI:10.1016/j.dyepig.2019.03.050

Accepted Manuscript
An efficient strategy to supervise absorption, mobility, morphology of photovoltaic
molecule by inserting a D-A unit
Ziqian Gu, Jiali Guo, Rulin Hao, Zixuan Lin, Yu Qian, Chao Ma, Yankai Fan, Xiaoyuan
Deng, Guangjun Zhang, Wenhong Peng, Hao Xia, Qiang Peng, Weiguo Zhu
PII:
S0143-7208(19)30262-1
https://doi.org/10.1016/j.dyepig.2019.03.050
DYPI 7439
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 February 2019
Revised Date: 11 March 2019
Accepted Date: 18 March 2019
Please cite this article as: Gu Z, Guo J, Hao R, Lin Z, Qian Y, Ma C, Fan Y, Deng X, Zhang G, Peng
W, Xia H, Peng Q, Zhu W, An efficient strategy to supervise absorption, mobility, morphology of
photovoltaic molecule by inserting a D-A unit, Dyes and Pigments (2019), doi: https://doi.org/10.1016/
j.dyepig.2019.03.050.
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ACCEPTED MANUSCRIPT
Graphical Abstract
An A1-(D-A)2-D-A1 type oligomer DRCN4FBT is designed and synthesized by simply
inserting a D-A unit in an A1-D-A-D-A1 type small molecule DRCN2FBT. Its absorption
and energy level, mobility, morphology are well supervised by inserting D-A unit. As a
result, both PCE and Jsc are increased by about 20% for the DRCN4FBT/PC71BM based
devices. It indicates that simply increasing D-A unitis an efficient strategy to improve
photovoltaic properties.

ACCEPTED MANUSCRIPT
An efficient strategy to supervise absorption, mobility, morphology of
photovoltaic molecule by inserting a D-A unit
Ziqian Gu,a Jiali Guo,a Rulin Hao,a Zixuan Lin,a Yu Qian,a Chao Ma,a Yankai Fan,a
Xiaoyuan Deng,a Guangjun Zhang,b Wenhong Peng,a Hao Xia,*a Qiang Peng,*b Weiguo
Zhu*a
aSchool of Materials Science and Engineering, Jiangsu Engineering Laboratory of
Light-Electricity-Heat Energy-Converting Materials and Applications, Jiangsu
Collaborative Innovation Center of Photovoltaic Science and Engineering, National
Experimental Demonstration Center for Materials Science and Engineering,
Changzhou University, Changzhou 213164, China.
bKey Laboratory of Green Chemistry and Technology of Ministry of Education, College
of Chemistry, and State Key Laboratory of Polymer Materials Engineering, Sichuan
University, Chengdu 610065, P. R. China
Email addresses:
zhuwg18@126.com
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Abstract:
The absorption, mobility and morphology of molecules in the active layer have a crucial
influence on photovoltaic properties of organic solar cells. In this work, we design an
A1-(D-A)2-D-A1 type oligomer DRCN4FBT on the basis of an A1-D-A-D-A1 type small
molecule DRCN2FBT by inserting D-A unit. It is found that oligomer DRCN4FBT
exhibits wider absorption spectrum and higher carrier mobility and better morphology
than small molecule DRCN2FBT in active layer using fullerene PC71BM as acceptor.
The absorption and energy level, mobility, morphology of DRCN4FBT in active layer are
well supervised by inserting D-A unit in DRCN2FBT. As a result, its power conversion
efficiency and short current density are increased by about 20% for the
DRCN4FBT/PC71BM based organic solar cells in comparison with the
DRCN2FBT/PC71BM based devices. It indicates that simply increasing D-A unit in small
molecules to construct oligomer is an efficient strategy to improve photovoltaic
properties.
KEYWORDS: Oligomer; Benzodithiophene; Benzodithiophene-4,8-dione; Fullerene;
Organic solar cells.
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1. Introduction
Organic solar cells (OSCs), compared with thin-film inorganic solar cells, have
attracted more and more attention due to their advantages, such as easy solution
processability, light weight, good mechanical flexibility and low cost.1-6 In the past
few years, due to the rapid development of non-fullerene receptors, power
conversion efficiencies (PCEs) for polymer based organic solar cells (P-OSCs)
have exceeded 14% in single-junction structure,7,8 and 17.3% in double-junction
structure.9 And PCEs of single-junction small molecules based organic solar cells
(SM-OSCs) are about 11% in a binary-component structure10-13 and 13% in a
three-component structure.14 It is still a challenge to obtain high-performance
organic photovoltaic materials and their organic solar cells.
As is known to all, the PCE value is determined by three key parameters, i.e.
open-circuit voltage (Voc), short-circuit current (Jsc), and filling factor (FF). These
factors are closely related to the absorption spectra, energy levels and morphology
of the photovoltaic materials in active layer.15 In general, low HOMO energy level
for donor material is available to obtain high Voc. And wide absorption spectrum
and high extinction coefficient for the donor and acceptor materials are available
16,17
to achieve high Jsc
.
However, to obtain a high FF level, good phase separation and interpenetrating net
work are simultaneously required between the donor and acceptor in the active
layer, which is a huge challenge for SM-OSCs, because the crystallinity of small
molecules is too strong.18 In order to solve this problem and obtain
high-performance bulk heterojunction (BHJ) OSCs using fullerene as acceptor, the
common strategy is to construct oligomers with bigger conjugated frame work by
introducing some electronic donor(D)–acceptor(A) elements into the main
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chain.19-25 For example, Bazan et al introduced a combined unit of
dithieno(3,2-b;2’,3’-d)-silole (DTS) and benzothiadiazole (BT) in main chain to
obtain a type of oligomers, which exhibited a PCE of 6.5% in these
oligomer-based organic solar cells (OM-OSCs).19 Wei et al reported another
oligomers with an impressive PCE up to 9.25% in their OSCs.22 The crystallinity
and dispersion in active layer is regulated by designing oligomer instead of small
molecules to some extent. However, the structure-property relationship has not
still uncovered for these oligomers completely.
In this work, we constructed an A1-(D-A)2-D-A1 type oligomer of DRCN4FBT
and its parent A1-D-A-D-A1 type small molecule of DRCN2FBT to study the
influence of oligomer structure on optoelectronic properties, in which bisfluoro-
benzothiadiazole (2FBT), thiophene and cyanorhodanine groups were used as
basic elements. Their molecular structures are shows in Figure 1. Inserting D-A
configuration is considered to strengthen the intra-molecular charge transfer (ICT)
effect and improve the molar extinction coefficient and film formation.
Introducing of electron-deficient cyanorhodanine terminal is suggested to reduce
the HOMO energy level, which is beneficial to increase the open-circuit voltage.26
Our results demonstrate that, with an inserting D-A framework, the oligomer
DRCN4FBT exhibits more intense absorption with a red-shifted absorption
spectrum, higher carrier mobility, better film formation in comparison with the
small molecule DRCN2FBT. Benefited from these qualities, DRCN4FBT exhibit
better photovoltaic properties with a higher power conversion efficiency (PCE) of
5.43%, which is increased by about 20% in comparison with the DRCN2FBT in
these fullerene-based OSCs. This work provides an efficient strategy to supervise
molecular absorption, carrier mobility, active layer morphology, further improve
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the photovoltaic performance by designing oligomer instead of its parent small
molecule.
Figure 1. Chemical structures of DRCN2FBT and DRCN4FBT
Scheme 1. Synthetic routs of DRCN2FBT and DRCN4FBT.
2. Experimental Section
2.1. Materials and Synthesis
All reagents were purchased. Tetrahydrofuran and toluene were distilled over sodium
before used. The other reagents were directly used without further purification. The
synthetic routes of DRCN2FBT and DRCN4FBT are depicted in Scheme 1. Their
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molecular structures are characterized by 1H, 13C NMR, high resolution mass
spectrometry (HRMS) and elemental analysis (EA), which are depicted in Electronic
Supporting Information (ESI). Good solubility in hot common solvents (such as
chloroform and chlorobenzene) is exhibited for both molecules.
Synthesis of compound 1[27]
A solution of 4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole (1.63 g, 4.94 mmol),
tributyl(4-(2-ethyl-hexyl)thiophen-2-yl)stannane (6.0 g, 12.40 mmol), and Pd2(dba)3
(226.2 mg, 0.62 mmol), tri(o-tolyl)phosphine (300.7 mg, 2.48 mmol) and toluene (50 mL)
was degassed in a 100 mL two-neck round-bottom flask and stirred at refluxing
temperature for 4 h under the N2 atmosphere. After being cooled to room temperature, the
mixture was poured into water (150 mL) and extracted with CH2Cl2 (30 mL×3). The
organic phases were combined and dried over anhydrous MgSO4. The solvents were then
removed off under reduced pressure and there was purified by silica gel chromato-
1
graphy using PE as eluent to give yellow solid with a yield of 86% (2.38 g). H NMR
(400 MHz, CDCl3) δ 8.09 (s, 2H), 7.19 (s, 2H), 2.66 (d, J = 6.9 Hz, 4H), 1.65-1.60 (m,
2H), 1.40-1.25 (m, 16H), 0.94-0.90 (m, 12H).
Synthesis of compound 2[27]
To a solution of compound 1 (2.3 g, 4.10 mmol) in 50 mL THF, was added N-bromo-
succinimide (1.68 g, 9.43 mmol) in portions over a course of 30 min without light. The
mixture was stirred for another 12 h at room temperature and 100 mL water was added in
it to quench the reaction. The mixture was extracted with CH2Cl2 and the resulting
organic phase was washed with brine and dried over anhydrous MgSO4. The solvent was
removed off under reduced pressure and the residue was purified by silica gel
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chromato-graphy using PE as an eluent to give orange red solid with a yield of 82% (2.42
1
g). H NMR (400 MHz, CDCl3) δ 7.95 (s, 2H), 2.62 (d, J = 7.2 Hz, 4H), 1.80-1.66 (m,
2H), 1.45-1.23 (m, 16H), 0.99-0.85 (m, 12H).
Synthesis of compound 3
A solution of compound 2 (1.0 g, 1.39 mmol), (5-(1,3-dioxolan-2-yl) tributylstannane
(1.86 g, 4.17 mmol) and Pd2(dba)3 (63.6 mg, 0.21 mmol), tri(o-tolyl)phosphine (84.6 mg,
0.84 mmol) and toluene (50 mL) was degased with N2 flew in a 100 mL two-neck
round-bottom flask, and was stirred at refluxing temperature for 4 h under the N2
atmosphere. After being cooled to room temperature, 30 mL THF solution with 10%
dilute hydrochloric acid was added. The mixture was stirred overnight at room
temperature and poured into water (150 mL), then extracted with CH2Cl2 (30 mL×3). The
organic phases were collected and dried over anhydrous MgSO4. The solvents were then
removed off under reduced pressure and the residue was purified by silica gel
chromatography using PE-DCM(1:1, v/v) as an eluent to give amaranth solid with a yield
1
of 70%(760 mg). H NMR (400 MHz, CDCl3) δ 9.93 (s, 2H), 8.14 (s, 2H), 7.76 (d, J =
4.0 Hz, 2H), 7.39 (d, J = 4.0 Hz, 2H), 2.87 (d, J = 7.3 Hz, 4H), 1.91-1.68 (m, 2H),
1.42-1.25 (m, 16H), 0.92-0.87 (m, 12H). 13C NMR (101 MHz, CDCl3) δ 182.68, 148.63,
145.60, 142.99, 141.47, 136.75, 135.00, 133.90, 131.24, 127.02, 40.12, 37.13, 33.88,
32.77, 32.50, 31.96, 29.91, 29.28, 29.22, 28.65, 27.12, 25.79, 22.91, 14.17, 10.80.
Elemental analysis for [C40H42F2N2O2S5], calculated: C 61.51%, H 5.42%, N 3.59%;
found: C 61.39%, H 5.67%, N 3.72%.
Synthesis of compound 4
A solution of compound 2 (1.0 g, 1.39 mmol), (5-(1,3-dioxolan-2-yl) tributylstannane
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(0.62 g, 1.39 mmol) and Pd(PPh3)4 (80.3 mg, 0.07 mmol) and toluene (20 mL) was
degased with N2 flew in a 100 mL two-neck round-bottom flask, and was stirred at
refluxing temperature for 4 h under the N2 atmosphere. After being cooled to room
temperature, 30 mL THF solution with 10% dilute hydrochloric acid was added. The
mixture was then stirred overnight at room temperature and poured into water (150 mL),
then extracted with CH2Cl2 (30 mL×3). The organic phases were combined and dried
over anhydrous MgSO4. The solvents were then removed off under reduced pressure and
the residue was purified by silica gel chromatography using with PE-DCM(1:1, v/v) as an
1
eluent to give red solid with a yield of 42% (438 mg). H NMR (400 MHz, CDCl3) δ
9.95 (s, 1H), 8.12 (s, 1H), 7.96 (s, 1H), 7.78 (d, J = 4.0 Hz, 1H), 7.40 (d, J = 4.0 Hz, 1H),
2.88 (d, J = 7.3 Hz, 2H), 2.62 (d, J = 7.2 Hz, 2H), 1.77 (d, J = 25.4 Hz, 2H), 1.36 (s,
16H), 0.93 (d, J = 4.4 Hz, 12H). 13C NMR (101 MHz, CDCl3) δ 182.63, 148.43, 145.63,
142.93, 141.84, 141.40, 136.69, 134.79, 133.67, 132.47, 131.19, 126.94, 115.35, 111.24,
40.05, 33.81, 32.49, 28.72, 25.75, 23.09, 14.16, 10.83. Elemental analysis for
[C35H39BrF2N2OS4], calculated: C 56.06%, H 5.24%, N 3.74%; found: C 56.01%, H
5.30%, N 3.93%.
Synthesis of compound 5
A solution of compounds 4 (438 mg, 0.58 mmol), 2,5-bis(trimethylstannyl)thiophene
(119 mg, 0.29 mmol), and Pd2(dba)3 (13.3 mg, 0.67 mmol), tri(o-tolyl)phosphine (17.7
mg, 2.66 mmol) in toluene (20 mL) was degased with N2 flew in a 100 mL two-neck
round-bottom flask and was stirred at refluxing temperature for 12 h under the N2
atmosphere. After being cooled to room temperature, the mixture was poured into water
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(150 mL) and extracted with CH2Cl2 (30 mL×3). The organic phases were combined and
then dried over anhydrous MgSO4. The solvents were then removed off under reduced
pressure and the residue was purified by silica gel chromatography using PE-DCM(1:3,
v/v) as an eluent to give purple solid with a yield of 67% (276 mg). 1H NMR (400 MHz,
CDCl3) δ 9.87 (s, 2H), 8.05 (d, J = 8.5 Hz, 4H), 7.69 (s, 2H), 7.33 (s, 2H), 7.19 (s, 2H),
2.80 (t, J = 9.3 Hz, 8H), 1.78 (s, 4H), 1.34 (s, 32H), 0.88 (s, 24H). 13C NMR (101 MHz,
CDCl3) δ 182.38, 148.31, 145.57, 142.51, 141.08, 138.29, 136.55, 135.83, 135.56,
134.60, 134.30, 134.27, 133.43, 131.30, 128.80, 126.41, 126.03, 111.31, 109.85, 39.84,
33.95, 32.52, 29.08, 28.63, 26.93, 25.76, 23.20, 22.64, 19.46, 14.24, 11.46, 10.81.
Elemental analysis for [C74H80F4N4O2S9], calculated: C 62.50%, H 5.67%, N 3.94%;
found: C 62.32%, H 5.61%, N 4.05%.
Synthesis of compound DRCN2FBT
Compound 3 (250 mg, 0.32 mmol), 2-(3-ethyl-4-oxo-thiazolidine-2-methylene) propane
(620 mg, 3.2 mmol), triethylamine (0.25 mL) and chloroform (30 mL)were added in 50
o
mL two-neck bottles in turn. The mixture was reacted at 60 C for 12 h under nitrogen
protection. After cooled to room temperature, the reaction liquid was poured into
anhydrous methanol (200 mL) to form precipitation. The precipitation was filtered and
purified by silica gel chromatography using dichloromethane as an eluent to obtain
1
DRCN2FBT as purple black solid with a yield of 52% (190 mg). H NMR (400 MHz,
CDCl3) δ 8.16 (s, 2H), 8.09 (s, 2H), 7.50 (d, J = 4.1 Hz, 2H), 7.42 (d, J = 4.1 Hz, 2H),
4.39-4.29 (m, 4H), 2.92-2.83 (m, 4H), 1.81-1.74 (m, 2H), 1.48-1.19 (m, 16H), 1.15-0.79
(m, 18H). HRMS (m/z) for C56H52F2N8O2S7, calculated: [M]+ 1130.2226, found: [M]+
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1130.2376. Elemental analysis for [C56H52F2N8O2S7], calculated: C 59.44%, H 4.63%, N
9.90%; found: C 59.26%, H 4.51%, N 10.02%.
Synthesis of compound DRCN4FBT
Compounds 5 (270 mg, 0.19 mmol), 2-(3-ethyl-4-oxo-thiazolidine-2-methylene) propane
(367 mg, 1.9 mmol), triethylamine (0.2 ml) and chloroform (30 mL) were added in 50
o
mL two-neck bottles in turn. The mixture was reacted at 60 C for 12 hunder nitrogen
protection. After cooled to room temperature, the reaction liquid was poured into
anhydrous methanol (200 mL) to form precipitation. The precipitation was filtered and
purified by silica gel chromatography using dichloromethane as an eluent to obtain
1
DRCN4FBT as purple black solid with a yield of 48%(206 mg). H NMR (400 MHz,
CDCl3) δ 7.58-7.60 (m, 6H), 7.00 (s, 2H), 6.88 (s, 2H), 6.43 (s, 2H), 4.05 (s, 4H), 2.47 (s,
8H), 1.56 (s, 4H), 1.32 (m,32H), 0.96 (m, 30H). HRMS (m/z) of C90H90F4N10O2S11
calculated [M]+ 1772.4520, found: [M]+ 1772.4022. Elemental analysis for
[C90H90F4N10O2S11], calculated: C 60.99%, H 5.12%, N 7.90%; found: C 60.74%, H
5.07%, N 8.12%.
3. Results and Discussion
3.1. Thermal Properties and Crystallinity
The thermo-gravimetry (TGA) curves of DRCN2FBT and DRCN4FBT are shown in
Figure S1. Under a nitrogen atmosphere, both molecules exhibit good thermal stability
o
with high decomposition temperature over 390 C at 5% weight loss. Their differential
scanning calorimetry (DSC) curves are further depicted in Figure S2. It is observed that
both molecules have obvious melting temperature (Tm) upon heating and crystallization
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peak (Tc) upon cooling. Furthermore, DRCN2FBT has higher Tc and Tm values,
indicating that oligomer has a subdued crystallinity in comparison with small molecule.28
3.2. Optoelectronic Properties
The UV-vis absorption spectra of DRCN2FBT and DRCN4FBT in 10−5 mol L−1dilute
chloroform solution and their thin films are depicted in Figure 2. The detailed optical
data are listed in Table 1. In dilute chloroform solution, DRCN4FBT delivers a
red-shifted absorption profile with a π-π* absorption peak in 300-400 nm and an ICT
absorption peak in 400-650 nm in comparison with DRCN2FBT. Moreover, the
maximum extinction coefficient is increased from 9.1×104 M-1cm-1 of DRCN2FBT to
1.23×105 M-1cm-1 of DRCN4FBT. It indicates that DRCN4FBT has more intense absorp
-tion than DRCN2FBT. Oligomer is more benefited to increase absorption than small
molecule. In addition, from the solution to the thin film, the absorption spectra of both
molecules exhibit significant bathochromic profile due to their closer intermolecular
stacking in the thin films.29 The maximum absorption peak locates at 585 nm for the
DRCN2FBT film and 623 nm for the DRCN4FBT film. Thus, their optical band gaps
(Egopt) are 1.70 eV and 1.61 eV, respectively, estimated from the onset of the film
absorption. It indicates that absorption can be improved by introducing a D-A building
into small molecule.
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Figure 2. (a) UV-vis absorption spectra in chloroform solution and (b) UV-vis
absorption spectrain chloroform solution and in their neat films by normalized treatment
for DRCN2FBT and DRCN4FBT.
3.3. Electrochemical Properties
Figure S3 displays the electrochemical cyclic voltammetry (CV) curves for DRCN2-
FBT and DRCN4FBT. The onset oxidation and reduction potentials (Eox/Ered) are 1.10
V/-0.89 V and 0.98 V/-0.82 V vs Ag/Ag+ for DRCN2FBT and DRCN4FBT, respectively.
The calculated electrochemical data are listed in Table 1. The highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels (EHOMO
/ELUMO) are estimated to be -5.46 eV/ -3.47 eV for DRCN2FBT and -5.28 eV /-3.54 eV
for DRCN4FBT based on empirical equation.27 Here, oligomer DRCN4FBT deliversan
increasing HOMO energy level compared to small molecule DRCN2FBT. And both
molecules show appropriate HOMO and LUMO energy levels to the PC71BM acceptor,
which have enough force to promote exciton dissociation in the PC71BM-based OSCs.
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Figure 3. (a) The device structure and (b) Energy levels of DRCN2FBT, DRCN4FBT
and PC71BM.
Table 1. Optical and electrical properties of DRCN2FBT and DRCN4FBT.
opt
Donor
λmax (nm)
λonset.film
(nm)
720
Eg
EHOMO
(eV)b
-5.46
-5.28
ELUMO
(eV)b
-3.47
-3.54
Solution
Film
(eV)a
1.72
1.60
DRCN2FBT
DRCN4FBT
548
550
585,653
623
777
aCalculated from the absorption band edge of the films, Egopt=1240/λonset
bCalculated from empirical equation: EHOMO = −(Eox + 4.8) eV, ELUMO = (Ered + 4.8)
eV(The formal potential of Fc/Fc+ is 0.44 V vs. Ag/AgCl measured in this work)
3.4. Theoretical Calculations
Figure 4 shows the optimized conformations (front and side view) and their calculated
LUMO and HOMO surfaces of DRCN2FBT and DRCN4FBT using the hybrid density
functional theory (DFT) at the B3LYP/6-31G* level using Gaussian 09W, in which all
alkyl side chains in thiophene or rhodanine units are replaced with shorter methyl in
order to simplify their calculation. It is seen that both molecules have good planarities
from the front and side view of the molecular skeleton. Their torsion angle between
elements is less than 10 degrees. The small torsion angle is beneficial to the π-π stacking
between molecules and can improve the hopping charge transfer ability. However, due to
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longer molecular skeleton, oligomer DRCN4FBT exhibits more widely distributed
electron clouds for HOMO and LUMO and more electron cloud overlap between HOMO
and LUMO than small molecule DRCN2FBT. Here, this tunable planar structure and
extended electron clouds distribution are available to further result in higher extinction
coefficient and promote charge transmission.22
Figure 4. Optimized geometries of the front and side views, HOMO and LUMO orbitals
calculated for DRCN2FBT and DRCN4FBT using Gaussian at the B3LYP/6-31G* level.
3.5. XRD Analysis
The ordered structures of DRCN2FBT and DRCN4FBT in the solid state were
investigated by powder X-ray diffraction (XRD) measurement. Figure 5 shows the XRD
patterns of the DRCN2FBT and DRCN4FBT powders. It is clear that, in the small-angle
o
region, an intense diffraction peak from (100) at 2θ = 5.45 (d-spacing of 16.2 Å), but
o
only a weak diffraction peak at 2θ = 5.65 (d-spacing of 15.23 Å) is exhibited for the
DRCN4FBT powder. At the same time, in the wide-angle region, an intense diffraction
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o
peak from (010) at 2θ = 25.07 (d-spacing of 3.55 Å) for the DRCN2FBT solid and a
weak diffraction peak at 2θ = 25.13 o (d-spacing of 3.54 Å) for the DRCN4FBT solid are
observed. In comparison with their different diffraction peaks and their intensity in XRD
patterns of both molecules, DRCN2FBT has more ordered lamellar structures and higher
crystallinity than DRCN4FBT.21,22 This is coincident with more planar structure of
DRCN2FBT.
Figure 5. XRD patterns of DRCN2FBT and DRCN4FBT pristine solid powders.
3.6. Photovoltaic Properties
Figure 3 shows the bulk heterojunction architecture and energy levels of the devices. The
donor/acceptor ratios of 1:0.8, 1:1 and 1:1.2, as well as concentrations of 1,8-diiod
-octane (DIO) from 0.2 wt% to 0.6 wt% were selected to optimize device performances.
The measured photovoltaic parameters of these solar cells are summarized in Table S1,
S2, S3 and S4. The current density–voltage (J–V) curves are shown in Figure S4, S5, S6
and S7 under an illumination of AM1.5G simulated solar light at 100 mW/cm2. It is
found that the donor/acceptor ratio and the additives amount have a significant effect on
the photovoltaic properties. The optimum donor-acceptor ratio and the additive dosage
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are 1:1 and 0.4%, respectively for these cells.
Under the optimal conditions, the corresponding current density-voltage (J-V) curves of
the devices are shown in Figure 6, and their photovoltaic parameters are summarized in
Table 2. It is observed that the optimal DRCN4FBT:PC71BM based solar cell exhibit
better photovoltaic properties than the optimal DRCN2FBT:PC71BM based solar cell.
The maximum PCE value is 5.43% for the DRCN4FBT:PC71BM based solar cell with a
circuit current density (Jsc) of 13.43 mA cm−2, a Voc of 0.72 V, and a fill factor (FF) of
56.24%. Obviously, the DRCN4FBT:PC71BM cell exhibits increasing PCE, Jsc, and FF
values except for Voc in comparison with the DRCN2FBT:PC71BM cell. Furthermore,
The PCE and Jsc values are increased by 20% for the device using oligomer DRCN4FBT
instead of small molecule DRCN2FBT as an electron-donating material. These results
indicates that using oligomer instead of small molecule, simply with inserting a D-A
building, is efficient approach to improve photovoltaic properties for these fullerene
based OSCs.
Figure 6(b) shows the external quantum efficiency (EQE) spectra of the optimal
DRCN2FBT:PC71BM and DRCN4FBT:PC71BM based solar devices. A broader spectral
response region from 300 nm to 800 nm, but with a reduced maximum EQE of 65%, is
observed in the DRCN4FBT:PC71BM blend film rather than the DRCN2FBT:PC71BM
blend film. It is suggested that, as a whole, DRCN4FBT:PC71BM based cell should have
better photon response. Thus, this cell delivers a higher Jsc than the DRCN2FBT:PC71BM
based cell.
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Figure 6. (a) J–V curves of the DRCN2FBT:PC71BM and DRCN4FBT:PC71BM based
solar cells at an optimal processing condition under a simulated AM 1.5 G irradiation
(100 mW cm-2) and (b) EQE curves of the optimized devices based on
DRCN2FBT:PC71BM and DRCN4FBT:PC71BM.
Table 2. Photovoltaic parameters of the solar cells based on DRCN2FBT:PC71BM and
DRCN4FBT:PC71BM at an optimal processing condition.
DIO Voc
Jsc
PCEmax(%)
Rs
Rsh
Donor
FF (%)
(V) (mA cm-2)
PCEave (%) (Ω.cm2) (Ω.cm2)
NO 0.75
8.48
11.93
10.15
13.43
39.7
53.0
42.8
56.2
2.52(2.31)
4.53(4.26)
3.13(2.82)
5.43(5.16)
24.2
18.4
21.8
14.4
172.4
195.1
185.3
260.5
DRCN2FBT
DRCN4FBT
0.4% 0.75
NO 0.72
0.4% 0.72
The best PCE values, and the average PCE values are provided in parentheses from 15
devices.
3.7. Film Morphology and Charge Mobility
Figure 7 shows the height and phase images of the DRCN2FBT:PC71BM and DRCN2-
FBT:PC71BM blend films recorded by the atomic force microscopy (AFM) using the
trapping mode. It is found that the root-mean-square (RMS) roughnesses are 3.96 and
2.76 nm for the DRCN2FBT:PC71BM blend films before and after adding the DIO
additive, respectively. However, the RMS roughnesses are much decreased to 3.04 and
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2.26 nm for the DRCN4FBT: PC71BM blend films at the corresponding condition,
respectively. It is distinct that the DIO additive has a great impact on the morphology of
the active layer. From the reduced RMS value, it indicates that adding DIO additive can
reduce the aggregation of the DRCN2FBT:PC71BM and DRCN4FBT:PC71BM blend
film,[31] and is beneficial to form appropriate phase separation size of the active layer,
thereby improve the charge mobility.[32] Compared to the phase images in Figure 7,
DRCN4FBT:PC71BM blend film has a more uniform morphology of the active layer. It is
further found that the DRCN4FBT:PC71BM blend film has more uniform morphology
than the DRCN2FBT:PC71BM blend film. After a treatment of the DIO additive, a
well-organized morphology with nano-fibrils is observed in the DRCN4FBT:PC71BM
blend film. Therefore, introducing oligomer of DRCN4FBT and adding the DIOtreat-
ment areavailable to improve the active layer morphology. It is considered that, with
inserting D-A unit, the crystallinity should be reduced and the self-aggregation ability
should be weakened for the oligomer, which are available for DRCN4FBT to form
appropriate phase separation with the receptor PC71BM in the their blend films and make
its device exhibit much increase for Jsc and FF values.31-40
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Figure 7. AFM height images (a, b, c, d) and phase images (e, f, g, h) of the active blends.
(a. e) DRCN2FBT:PC71BM without DIO; (b. f) DRCN2FBT:PC71BM with DIO; (c, g)
DRCN4FBT:PC71BM without DIO; (d, h) DRCN4FBT:PC71BM with DIO.
In addition, by the equation: Eloss = Eg − qVoc, where Eloss is the minimum energy loss, Eg
is the optical gap of donor, it is found that the Eloss of DRCN2FBT is 0.97 eV, which is
higher than 0.09 eV for that ofDRCN4FBT. The high Eloss is related to the poor phase
separation of the active layer and should result in excessive non-radiation energy
loss.41-45 Therefore, moderately inserting the D-A unit in the molecular main chain can
effectively adjust the morphology of the blend film and decrease the Eloss of devices.
Photoluminescence (PL) spectra of the DRCN2FBT and DRCN4FBT neat films, as well
as their blend films with PC71BM were measured to investigate charge transfer in the
photoactive layers. As shown in Figure S8, the DRCN2FBT and DRCN4FBT neat films
show emission maxima at 585 nm and 623 nm, respectively. However, the PL emissions
of DRCN2FBT and DRCN4FBT are quickly quenched by PC71BM with a quenching
efficiency of 30.8% and 71.8%, respectively. It implies that the DRCN4FBT/PC71BM
blend has higher efficient electron transfer than the DRCN2FBT/PC71BM blend. This
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further explains the reason of higher Jsc and FF for the DRCN4FBT:PC71BM based cells.
To investigate the charge transport performance of both active layer under optimal
conditions, the hole (µh) and electron mobility (µe) of blend films were measured by
space-charge-limited current (SCLC) method with the Mott–Gurney equation.41 Thus, the
hole-only device with a structure of Glass/ITO/ PEDOT:PSS/active layer/ MoO3 /Au and
the electron-only device with a structure of Glass/Al/active layer/Al were made. Figure 8
shows the J1/2-V characteristics of these devices. The detail results are summarized in
Table 3. The higher hole mobility of 1.11×10-4 cm-2V-1s-1 and electron mobility of
7.32×10-5 cm-2V-1s-1 are achieved for the DRCN4FBT:PC71BM based devices. In contrast,
the hole and electron mobility of the DRCN2FBT:PC71BM based devices are only
6.38×10-5 and 3.73×10-5 cm-2V-1s-1, respectively. By comparison, it is found that the ratio
of hole/electron mobility for the DRCN4FBT:PC71BM based devices is 1.52, which is
more balance than that value of 1.72 for the DRCN2FBT:PC71BM based devices. It is
well known that the balance between the hole and electron mobility in the active layer
plays an important role in the improvement of FF and PCE46,47. Therefore, higher and
more balanced hole and electron mobility is the reason why the DRCN4FBT:PC71BM
based devices have higher Jsc and FF.
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Figure 8. (a) J1/2-V characteristics of the hole-only devices based on DRCN2FBT:
PC71BM and DRCN4FBT:PC71BM; (b) J1/2-V characteristics of the electron-only devices
based on DRCN2FBT:PC71BM and DRCN4FBT:PC71BM.
Table 3. Hole and electron mobilities of the devices based on DRCN2FBT:PC71BM and
DRCN4FBT:PC71BM blend films under optimal conditions.
µh
µe
Active layer
µh/µe
(cm2 V-1 s-1)
6.38×10-5
(cm2 V-1 s-1)
3.73×10-5
DRCN2FBT:PC71BM
DRCN4FBT:PC71BM
1.71
1.52
1.11×10-4
7.32×10-5
4. Conclusions
In summary, an A1-D-A-D-A-D-A1 type oligomer DRCN4FBT and its parent
A1-D-A-D-A1 type small molecule DRCN2FBT were obtained. By a simple inserting
D-A unit, oligomer DRCN4FBT exhibits a stronger and wider absorption spectrum,
higher and more balanced carrier mobility in comparison with its parent small molecule
DRCN2FBT. What’s more, the slightly reduced crystallinity of DRCN4FBT improves
the blending property and makes its PC71BM based active layer form a better nano
phase separation structure. As a result, the DRCN4FBT:PC71BM based solar cells
present a significantly improved photovoltaic properties with a higher PCE of 5.43%.
Acknowledgement
Thanks to the financial supports from the National Natural Science Foundation of China
(51673031, 51573154), the Major Program of the Natural Science Research of Jiangsu
Higher Education Institutions (18KJA480001), the Top-notch Academic Programs
Project (TAPP) for Polymeric Materials Science and Technology & the Priority
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Academic Program Development (PAPD) of Jiangsu Higher Education Institutions,
Jiangsu Provincial Talents Project of High-Level Innovation and Entrepreneurship, the
Talent Project of Jiangsu Specially-Appointed Professor.
Supporting Information
The characterization and measurement, 1H, 13C NMR spectra and HRMS data, TGA,
DSC, CV and PL measurement, fabrication and characterization of the solar cells and the
detail J-V curves of the solar cells at different processing conditions.
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Research Highlights
>An A1-(D-A)2-D-A1 type oligomer DRCN4FBT and its prototypical small molecule
DRCN2FBT are synthesized.
>The strategy to supervise molecular absorption, mobility, crystallinity is provided.
>Their optical, electrochemical, morphological and photovoltaic properties are
studied.
>DRCN4FBT shows better photovoltaic properties than DRCN2FBT in the PC71BM
based solar cells.