Asymmetric push-pull small molecules with auxiliary electron-accepting unit for bulk heterojunction organic solar cells

Article abstract of DOI:10.1016/j.jphotochem.2019.112139

Two small molecular donors (TPA-DPP and TPA-DPP-MDN) were designed with asymmetric push-pull structure, namely, donor-acceptor (D-A) and donor-acceptor-acceptor (D-A-A) systems. They were synthesized and investigated by thermogravimetric analysis, UV–vis spectra, X-ray diffraction, density functional theory (DFT) calculation, electrochemical and the photovoltaic (PV) measurement. The D-A-A architecture (TPA-DPP-MDN) exhibited lower highest occupied molecular orbital (HOMO) of –5.18 eV, narrower optical band gap of 1.52 eV, better thermal stability and higher degree of ordered aggregation than these of the D-A system (TPA-DPP). TPA-DPP-MDN based PV device showed better performance with higher open-circuit voltage (V_OC) and short-circuit current density (J_SC) than these of TPA-DPP based PV devices due to the low-lying HOMO level and wide spectral absorption range of TPA-DPP-MDN.

Full text of DOI:10.1016/j.jphotochem.2019.112139

Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Short note
Asymmetric push-pull small molecules with auxiliary electron-accepting
unit for bulk heterojunction organic solar cells
a
b
c
a
a
a,⁎
Ying Chen , Duo Yang , Wei Liu , Junfeng Tong , Chunyan Yang , Peng Zhang
a
School of Materials Science and Engineering, Lanzhou Jiaotong University, Lanzhou, 730070, China
b
Key Laboratory of Optoelectronic Technology and Intelligent Control of Ministry Education, Lanzhou Jiaotong University, Lanzhou, 730070, China
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry and College of
c
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two small molecular donors (TPA-DPP and TPA-DPP-MDN) were designed with asymmetric push-pull structure,
namely, donor-acceptor (D-A) and donor-acceptor-acceptor (D-A-A) systems. They were synthesized and in-
vestigated by thermogravimetric analysis, UV–vis spectra, X-ray diffraction, density functional theory (DFT)
calculation, electrochemical and the photovoltaic (PV) measurement. The D-A-A architecture (TPA-DPP-MDN)
exhibited lower highest occupied molecular orbital (HOMO) of –5.18 eV, narrower optical band gap of 1.52 eV,
better thermal stability and higher degree of ordered aggregation than these of the D-A system (TPA-DPP). TPA-
DPP-MDN based PV device showed better performance with higher open-circuit voltage (VOC) and short-circuit
current density (JSC) than these of TPA-DPP based PV devices due to the low-lying HOMO level and wide spectral
absorption range of TPA-DPP-MDN.
Small molecular donor
Asymmetrical push-pull
Donor-acceptor-acceptor
Photovoltaic properties
1. Introduction
[18,22]. Furthermore, theoretical research suggested several favourable
characteristics of the D-A-A structure as following [26,27]: (i) enhan-
The organic solar cells (OSCs) based on small molecular donors
SMDs) have benefitgained immense interests due to constantly in-
cing the intramolecular charge transfer; (ii) modulating the energy gap
and the response of the light-harvesting range expediently; Interest-
ingly, the SMDs with D-A-A structure may become a reliable route for
designing organic absorber toward highly efficient SMDs. However, the
SMDs based on D-A-A systems have been relatively less explored and
studied [23,25]. And thus, it was necessary to pursue the objective of
understanding the structure-property relationships in this class of mo-
lecules (D-A-A system) to provide an insight into the molecular design
of asymmetric push-pull SMDs.
(
creased efficiencies [1–3], and the uniques over conventional polymers
such as structural adjustability, high purity, batch-to-batch reproduci-
bility, and so on [4,5]. Among small molecular donors, SMDs with
push-pull systems have drawn more and more attention [6–9]. These
SMDs featured donor-acceptor (D-A) configuration typically, demon-
strated an important advantage in molecular design flexibly, which
allowed for fine and precise tuning of the photophysical properties,
such as frontier orbital level alignment, band gap (E
sorption ability [10–12].
g
), and light-ab-
In this paper, two asymmetric push-pull molecules (TPA-DPP-MDN
and TPA-DPP) have been designed and synthesized, as shown in
Scheme 1. Specifically, these molecules were designed with the fol-
lowing structural characteristics: (i) the TPA-DPP with D-A structure
was consisting of phenyl-di-p-tolyl-amine (TPA) moiety as a D unit and
diketopyrrolopyrrole (DPP) as a A group; (ii) the 2-benzylidene-mal-
ononitrile (MDN) group as auxiliary electron-withdrawing unit was
introduced into the molecule to form D-A-A structure. In compare with
TPA-DPP, the TPA-DPP-MDN showed an extended spectral region with
In general, the SMDs with promising photovoltaic (PV) performance
based on the symmetrical D-A-D and A-D-A structures have been de-
veloped [6,8,13–17]. In contrast, the PV properties of the asymmetric
push-pull SMDs have lagged far behind these of the symmetrical
system-based OSCs [18–21]. Recently, Wong et al and Yin et al have
developed a novel asymmetric push-pull system in which an auxiliary
electron-withdrawing unit was introduced into the D-A structure to
propose a D-A-A structure [22–25]. These SMDs realized the high PV
performance possessing extended the spectral regions by reducing the
g
a E of 1.52 eV and relatively low-lying lower highest occupied mole-
cular orbital (HOMO) energy level. Furthermore, TPA-DPP-MDN pos-
band gap (E
g
) in compared with the SMDs featured D-A structure
sessed higher degree of crystallinity than TPA-DPP in pristine film
⁎
Corresponding author.
E-mail address: xiaozhangfei.fei@163.com (P. Zhang).
https://doi.org/10.1016/j.jphotochem.2019.112139
Received 13 June 2019; Received in revised form 10 September 2019; Accepted 2 October 2019
Available online 04 October 2019
1010-6030/ © 2019 Published by Elsevier B.V.

Y. Chen, et al.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
Scheme 1. Molecular structure and Synthetic rout of TPA-DPP and TPA-DPP-MDN.
according to the XRD data. Their PV properties were investigated,
employing these molecules as the electron donor and PC61BM as the
electron acceptor. The results showed that the D-A-A based device gave
a significantly improved performance with enhanced open-circuit vol-
tage (VOC) and short-circuit current density (JSC). This encouraging
result indicated a great potential of such D-A-A systems in developing
high-performance SMD materials.
2.2. Synthesis
2.2.1. Synthesis of compound 1
4,4′-Dimethyldiphenylamine (5 g, 14.2 mmol), 1-bromo-4-iodo-
benzene (7.90 g, 27.9 mmol), CuI (2.40 g, 12.6 mmol), KOH (6.93 g,
123 mmol) and 1,10-phenanthroline (1.3 g, 6.57 mmol) in anhydrous
(30 mL) xylene and then the solution was refluxed under argon for 20 h
at the temperature of 145 °C. After being cooled to the room tempera-
ture, the mixture was poured into 300 mL water and the organic layer
2
. Experimental
was separated. The aqueous layer was extracted with CH
3 × 40 mL) and the combined organic layers were dried over anhy-
drous Na SO . A white solid 1 (4.70 g) was obtained via column
chromatography (silica gel 200–300 mesh) using petroleum ether by
volume as eluent. Yield: 52.6%. H NMR (400 MHz, CDCl ), δ (ppm):
3
7.26 (d, J =8.0 Hz, 2 H), 7.06 (d, J =8.0 Hz, 2 H), 6.96 (d, J =8.0 Hz,
4 H), 6.87 (d, J =8.0 Hz, 4 H), 2.30 (s, 6 H).
2 2
Cl
(
2.1. Materials and general methods
2
4
1
All reagents were purchased from Aldrich, Acros, and TCI Chemical
Co. The solvents were further purified under nitrogen flow. Compound
3
and 4 were prepared according to reference [28]. The 1H NMR
spectra were attained on a Bruker DRX 400 spectrometer at 400 MHz.
Elemental analysis was performed on a Vario EL elemental analysis
instrument. The high-resolution mass spectra (HRMS) of TPA-DPP and
TPA-DPP-MDN were collected using Thermo Scientific Q Exactive. The
thermal stability of the material was achieved using thermal gravi-
metric analysis (TGA) 2050 with a heating rate of 10 °C min and a
range of 25 °C to 600 °C under nitrogen. The UV–vis absorption spectra
were carried out by Shimadzu UV-1800 spectrometer. The fluorescence
spectra of TPA-DPP (a) and TPA-DPP-MDN (b) in different solvents
have been measured on 970CRT fluorescence spectrophotometer. CHI
2.2.2. Synthesis of compound 2
Bromide
1 (2 g, 5.68 mmol), bis(pinacolato)diboron (3.12 g,
12.3 mmol), Pd(dppf)Cl2 (70 mg), CH COOK (2.4 g, 24.5 mmol), then
the mixture of toluene (20 mL) added into the two-neck flask and was
3
–
1
refluxed for 12 h at the temperature of 105 °C under argon. After being
cooled to the room temperature, the mixture was poured into 300 mL
water and the organic layer was separated. The aqueous layer was ex-
tracted with CH Cl (3 × 40 mL) and the combined organic layers were
2
2
dried over anhydrous Na SO . The organic solvent was evaporated, and
2
4
6
00D electrochemical workstation was employed for cyclic voltam-
the crude product was purified via column chromatography (silica gel
200–300 mesh) using petroleum ether by volume as the eluent to afford
metry (CV) measurements using a three-electrode cell of which
equipped with a Pt wire counter electrode, a glass carbon working
electrode and an Ag/AgNO
1
milky solid 3 (1.57 g), Yield: 58%. H NMR (400 MHz, CDCl ), δ (ppm):
3
3
reference electrode standardized with the
7.62 (d, J =8.0 Hz, 2 H), 7.07 (d, J =8.0 Hz, 4 H), 7.00 (d, J =8.0 Hz,
4 H), 6.96 (d, J =8.0 Hz, 2 H), 2.31 (m, 6 H), 1.32 (m, 12 H).
internal standard of ferrocene. The CV measurements were performed
at an electrolyte with 0.1 M tetrabutylammonium hexafluorophosphate
in anhydrous nitrogen-saturated acetonitrile with
1
a
scan rate
00 mV·s . The X-ray diffraction (XRD) was accomplished on a PA-
Nalytical X’Pert PRO diffractometer (Cu Kα radiation, λ =1.54 Å).
2.2.3. Synthesis of compound 5
–
1
Compound 4 (2 g, 2.42 mmol), 4-Formylphenylboronic acid (1.30 g,
8.66 mmol), by using Pd(PPh ) (200 mg), K CO (3.50 mg, 25.4 mmol)
3
4
2
3
2

Y. Chen, et al.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
as catalyst, then the THF (50 mL), H
2
O (5 mL), R336 (5 drops) added
3
(600 MHz, CDCl ) δ (ppm): 161.91, 161.54, 150.52, 149.00, 144.51,
into the two-neck flask and was refluxed for 15 h at the temperature of
0 °C under argon. After being cooled to the room temperature, the
mixture was poured into 300 mL water and the organic layer was se-
parated. The aqueous layer was extracted with CH Cl (3 × 40 mL) and
the combined organic layers were dried over anhydrous Na SO and the
organic solvent was removed under reduced pressure. A purple solid 5
1.70 g) was obtained via column chromatography (silica gel 200–300
mesh) using ethyl acetate:petroleum ether = 1:10 by volume as eluent.
140.75, 139.16, 137.52, 134.77, 133.47, 130.05, 130.02, 130.00,
128.32, 127.33, 126.82, 125.53, 125.22, 123.16, 121.46, 108.21,
107.47, 45.91, 45.87, 39.18, 39.08, 31.91, 30.90, 30.23, 29.68, 29.64,
29.34, 28.47, 28.36, 23.65, 23.55, 23.07, 23.05, 22.68, 20.86, 14.10,
14.04, 14.01 10.56, 10.50. TPA-DPP HRMS: 795.3879 [M ] (calcd for
50 57 2 3 2 57 3 2 2
C H O N S : 795.3887). Anal. Calcd for C50H N O S : C, 75.43; H,
7
2
2
+
2
4
(
7.22; N, 5.28; Found: C, 75.49; H, 7.31; N, 5.22.
1
Yield: 75.6%. H NMR (400 MHz, CDCl
3
), δ (ppm): 10.04 (s, 1 H), 8.92
(
d, J =4.0 Hz, 1 H), 8.90 (d, J =4.0 Hz, 1 H), 7.94 (d, J =8.0 Hz, 2 H),
2.2.7. Synthesis of compound TPA-DPP-MDN
7
1
1
.83 (d, J =8.0 Hz, 2 H), 7.65 (d, J =8.0 Hz, 1 H), 7.60 (d, J =4.0 Hz,
H), 7.28 (t, J =12.0 Hz, 1 H), 4.07–4.03 (m, 4 H), 1.95–1.91 (m, 2 H),
.33–1.22 (m, 48 H), 0.87–0.82 (m, 12 H).
Compound
0.188 mmol), piperidine (2 drops), then the CHCl
the 25 mL two-neck flask and was refluxed for 7 h at the temperature of
5 °C under argon. After being cooled to the room temperature, the
solution was poured into 100 mL water and extracted with CH Cl
(3 × 20 mL). The combined organic layers were dried over anhydrous
Na SO . The organic solvent was evaporated, and the crude product
7
(150 mg, 0.125 mmol), malononitrile (13 mg,
3
(7 mL) added into
6
2.2.4. Synthesis of compound 6
2
2
Compound 5 (1.50 g, 1.61 mmol) was dissolved in 40 mL CHCl
3
and
a solution of NBS (300 mg, 1.69 mmol) in 30 mL CHCl was added
3
2
4
dropwise to the solution. Then the mixture was stirred under at room
temperature without light for 15 h. The organic solvent was evaporated,
and the crude product was purified via column chromatography (silica
gel 200–300 mesh) using ethyl acetate:petroleum ether = 1:10 by vo-
was purified via column chromatography (silica gel 200–300 mesh)
using dichloromethane: petroleum ether = 3:1 by volume as the eluent
to afford purple black solid. Then, the solid was dissolved with tri-
chlormethane, which was poured into methanol. The precipitate was
1
1
lume as the eluent to afford solid 6 (0.940 g), Yield: 57.5%. H NMR
collected and dried under vacuum over night (yield: 57.5%). H NMR
(
8
2
=
(
400 MHz, CDCl
3
), δ (ppm): 10.04 (s, 1 H), 8.91 (d, J =4.0 Hz, 1 H),
.66 (d, J =4.0 Hz, 1 H), 7.94 (d, J =8.0 Hz, 2 H), 7.83 (d, J =8.0 Hz,
H), 7.60 (d, J =4.0 Hz, 1 H), 7.23 (d, J =8.0 Hz, 1 H), 4.05 (d, J
8.0 Hz, 2 H), 3.95 (d, J =8.0 Hz, 2 H), 1.95–1.91 (m, 2 H), 1.31–1.23
m, 48 H), 0.87–0.80 (m, 12 H).
3
(400 MHz, CDCl ), δ (ppm): 9.08 (d, J =4.0 Hz, 1 H), 8.87 (d, J
=4.0 Hz, 1 H), 7.96 (d, J =8.0 Hz, 2 H), 7.81 (d, J =8.0 Hz, 2 H), 7.72
(s, 1 H), 7.61 (d, J =8.0 Hz, 1 H), 7.49 (d, J =8.0 Hz, 2 H), 7.37 (d, J
=4.0 Hz, 1 H), 7.11 (d, J =8.0 Hz, 4 H), 7.05–6.99 (m, 6 H), 4.07–4.04
(m, 4 H), 2.34 (s, 6 H), 2.03–1.94 (m, 2 H), 1.34–1.22 (m, 48 H),
0
.85–0.79 (m, 12 H). 13C NMR (400 MHz, CDCl
) δ (ppm): 161.86,
3
2
.2.5. Synthesis of compound 7
Compound (400 mg, 0.396 mmol), compound
.501 mmol), using Pd(PPh (25 mg), Na CO (110 mg, 1.04 mmol),
O (2 mL), R336 (2 drops) as catalyst then the
161.28, 158.09, 151.32, 149.19, 145.59, 144.39, 141.44, 138.95,
138.25, 137.51, 135.84, 133.62, 131.59, 130.41, 130.07, 127.05,
126.83, 126.63, 125.31, 125.20, 123.18, 121.21, 113.83, 112.75,
109.48, 107.58, 81.96, 46.33, 37.95, 37.88, 31.83, 31.80, 31.27, 30.91,
2
6 (200 mg,
0
3
)
4
2
3
the toluene (20 mL), H
2
solution was refluxed Suzuki coupling for 12 h under argon atmosphere
at the temperature of 105 °C. After being cooled to the room tempera-
ture, the solution was poured into 100 mL water and extracted with
30.02, 29.71, 29.68, 29.54, 29.27, 26.30, 22.60, 20.87, 14.08. TPA-
+
DPP-MDN HRMS: 1171.6762 [M ] (calcd for
76 93 2 5 2
C H O N S :
1171.6765). Anal. Calcd for C76
93 5 2 2
H N O S : C, 77.84; H, 7.99; N, 5.97;
CH
anhydrous Na
pressure. A solid 7 (300 mg) was obtained via column chromatography
silica gel 200–300 mesh) using petroleum ether by volume as eluent.
2
Cl
2
(3 × 20 mL). The combined organic layers were dried over
Found: C, 77.88 H, 7.93; N, 5.94.
2
SO and the organic solvent was removed under reduced
4
(
2.3. Photovoltaic cells devices fabrication and characterization
1
Yield: 62.9%. H NMR (400 MHz, CDCl
3
), δ (ppm): 10.06 (s, 1 H), 9.05
(
d, J =8.0 Hz, 1 H), 8.87 (d, J =4.0 Hz, 1 H), 7.93 (d, J =8.0 Hz, 2 H),
Prior to use, a patterned indium tin oxide (ITO) glass substrate with
a resistance of 10–15 Ω/square were rinsed with ultrasonication in
deionized water, acetone and i-propanol, sequentially. The inverted
devices were fabricated with the configuration ITO/PFN/
7.83 (d, J =8.0 Hz, 2 H), 7.59 (d, J =4.0 Hz, 1 H), 7.49 (d, J =8.0 Hz,
2 H), 7.36 (d, J =4.0 Hz, 1 H), 7.11 (d, J =8.0 Hz, 4 H), 7.05–6.99 (m,
6 H), 4.07 (t, J =12.0 Hz, 4 H), 2.34 (s, 6 H), 2.03–1.94 (m, 2 H),
1.34–1.22 (m, 48 H), 0.85–0.79 (m, 12 H).
SMDs:PC61BM/MoO
3
/Ag. The PFN was dissolved in methanol with tiny
−
1
amounts of acetic acid to prepare 1 mg mL
solution. Then the PFN
2
.2.6. Synthesis of TPA-DPP
solution were spun on the cleaned ITO substrate. The thickness (10 nm-
thick) of the interlayer was monitored by a surface Tencor Alpha-500
profilometer. The active layer (60–70 nm) films of TPA-DPP (or TPA-
DPP-MDN)/PC61BM (1:3 Wt. ratio) were prepared by the spin-coating
method on the top of interlayer in chloroform solution, respectively.
Borate ester
.196 mmol) using Pd(PPh
2
(150 mg, 0.376 mmol), bromide
(20 mg), Na CO (106 mg, 1.00 mmol) as
O (1 mL), R336 (2 drops) added
3 (140 mg,
0
3
)
4
2
3
catalyst, then the toluene (7 mL), H
2
into the 25 ml two-neck flask and was refluxed for 5 h at the tem-
perature of 105 °C under argon. After being cooled to the room tem-
perature, the solution was poured into 100 mL water and extracted with
After drying the solvent, a mask was attached, and the MoO
3
(8 nm) and
Ag (80 nm) layer were deposited by thermal evaporation under a
–4
CH
2
Cl
2
(3 × 20 mL). The combined organic layers were dried over
SO . The organic solvent was evaporated, and the crude
pressure of 5 × 10 Pa, successively. The effective area of device was
2
anhydrous Na
2
4
3
defined by masks about 0.10 cm . The thickness of the MoO and Ag
product was purified via column chromatography (silica gel 200–300
mesh) using dichloromethane:petroleum ether = 1.5:1 by volume as
the eluent to afford purple black solid. Then, the solid was dissolved
with trichlormethane, which was poured into methanol. The precipitate
was recorded by a Shenyang Sciens quartz SI-TM206 crystal thickness/
ratio monitor. All the fabrication steps were performed in a Etelux glove
box filled with high-purity nitrogen. The PCEs of the PSCs were mea-
sured using a San-EI Electric XEC-300M2 solar simulator, under a sun
1
–2
was collected and dried under vacuum over night (yield: 65%). H NMR
AM 1.5 G irradiation with the intensity of 100 mW·cm . The current
(
=
(
7
400 MHz, CDCl
4.0 Hz, 1 H), 7.60 (d, J =4.0 Hz, 1 H), 7.49 (d, J =8.0 Hz, 2 H), 7.35
d, J =4.0 Hz, 1 H), 7.27 (t, J =12.0 Hz, 1 H), 7.11 (d, J =8.0 Hz, 4 H),
.05–7.00 (m, 6 H), 4.04 (t, J =12.0 Hz, 4 H), 2.34 (s, 6 H), 1.94–1.87
3
), δ (ppm): 9.02 (d, J =4.0 Hz, 1 H), 8.86 (d, J
density-voltage (J–V) characteristics were carried out using a Keithley
2400 source-measurement unit. Incident photon to charge carrier effi-
ciency (IPCE) of the devices were characterzied by a Beijing 7-star Opt
7-SCSpecIII commercial incident photon. A calibrated silicon detector
was used to determine the absolute photo sensitivity.
1
3
(
m, 2 H), 1.40–1.26 (m, 16 H), 0.92–0.84 (m, 12 H).
C NMR
3

Y. Chen, et al.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
Fig. 1. Optimized geometries and surface plots of TPA-DPP and TPA-DPP-MDN by DFT at the PBE0/6-311G*.
3. Results and discussion
3.1. Theoretical calculations
Calculations were carried on the two small molecules by using
density functional theory (DFT) calculation with PBE0/6-311G* basis
set, as implemented in Gaussian 09 [29]. To simplify the calculation,
the side alkyl chains were modified to methyl groups. The calculated
energy levels and electronic-density distribution were shown in Fig. 1.
The computational analysis showed that the dihedral angle of the
thiophene and MDN linked around the single bond was 17.6°, which
presented a near coplanar conformations. The MDN moiety not only
extended electronic conjugation across successive aromatic segments
along the conjugated backbone, but also led to an increase in facil-
itating the intramolecular charge transfer (ICT) from donor group to the
acceptors. Fig. 1 showed the molecular orbital distributions of HOMO
and lowest unoccupied molecular orbital (LUMO) isosurfaces of two
models. The HOMO orbital distribution for TPA-DPP was equally dis-
tributed the whole conjugated backbones, while the LUMO was sig-
nificantly localized on the electron accepting moieties. In the case of D-
A-A system, the HOMO was distributed on the donor and DPP group,
whereas LUMO was located on the acceptors, mainly in MDN moiety,
indicating an apparent ICT characteristic. In addition, the calculation
results gave the HOMO/LUMO levels of –5.05/–2.52 eV for TPA-DPP
and –5.26/–3.24 eV for TPA-DPP-MDN, corresponding to energy gaps
Fig. 2. TGA plots at a heating rate of 10 °C min–1 under a nitrogen atmosphere
of TPA-DPP and TPA-DPP-MDN.
stability, the onset decompositions of TPA-DPP and TPA-DPP-MDN
were 275 °C and 388 °C, respectively. Obviously, TPA-DPP-MDN has a
better thermal stability for long-term applications with respect to TPA-
DPP.
g
(E ) of 2.53 eV for TPA-DPP and 2.02 eV for TPA-DPP-MDN. Compared
3
.3. X-ray diffraction
with the TPA-DPP, the TPA-DPP-MDN exhibited a decrease in both the
HOMO and LUMO energy levels. Moreover, the LUMO level was low-
X-ray diff ;raction (XRD) was performed to investigate the ag-
opt
ered by a larger amount than the LUMO level. The increased Eg for D-
gregation ordered structures of the TPA-DPP and TPA-DPP-MDN in the
pristine films. Fig. 3 showed the XRD patterns of the corresponding
films. The XRD pattern of TPA-DPP-MDN featured a relatively sharp
peak at 2θ of 5.56° with a d-spacing of 15.71 Å, which revealed a high
degree of crystallinity and a lamellar structure of the films. TPA-DPP
film exhibited weak diffraction peaks at 2θ of 5.74° and 5.30°, corre-
sponding to the d-spacings of 15.38 Å and 16.65 Å. It was clear that
TPA-DPP-MDN exhibited a second order diff ;raction peak at 11.82° (a
d-spacing of 7.48 Å). Furthermore, two additional sets of reflections
were at 2θ of 17.52° and 20.75° with d-spacings of 5.06 Å, and 4.28 Å.
According to the XRD information, the TPA-DPP-MDN film possessed
higher degree of crystallinity than the TPA-DPP film.
A system may indicate that the low-energy absorption band dominated
by HOMO to LUMO transition of TPA-DPP was blue-shifted compared
to that of TPA-DPP-MDN.
3.2. Thermal properties
Thermal stabilities of the small molecules were important for the
fabrication process of the photovoltaic device. Their thermal stabilities
were studied by thermogravimetric-analysis (TGA), which was shown
in Fig. 2. The temperatures at 5% weight loss were selected as onset
point of decompositions. Both the SMDs exhibited high thermal
4

Y. Chen, et al.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
Table 1
UV–vis absorption data of TPA-DPP and TPA-DPP-MDN in different solvent.
Solvents
n-Hexane
Toluene
Chlorobenzene
Chloroform
TPA-DPP λonset(nm)
TPA-DPP-MDN λonset(nm)
624
715
635
725
642
739
646
745
Fig. 4. (b) and (c), with the increase of the solvent polarity gradually
from hexane to chloroform, TPA-DPP exhibited a little red-shifted by
22 nm from peak of 624 nm (in n-hexane) to 646 nm (in chloroform). In
contrast, the absorption behavior of TPA-DPP-MDN with increase of the
solvent polarity showed red-shifted of 30 nm with broader shape. The
difference of the absorption bands registered could be ascribed to the
different structures of the push-pull system in which the auxiliary
electron-withdrawing unit (MDN) extended the conjugation length in
the backbone facilitated the intramolecular electron transfer, and thus
modulated the response of the light-harvesting range. Fig. 4. (a) showed
the UV–vis absorption spectra of two SMDs in thin film. The absorption
peak and band edge of TPA-DPP film were at 600 nm and 675 nm,
which was red-shifted by 10 nm and 29 nm with respect to these in
chloroform. In comparison with the spectra of TPA-DPP-MDN in
chloroform, its spectra were greatly broadened and red-shifted with
absorption peak and band edge at 657 nm and 818 nm in film. Ob-
viously, the absorption band edge of TPA-DPP-MDN film was red-
shifted by 73 nm with regard to its absorption in chloroform, which was
consistent with the XRD information indicating the strong inter-
Fig. 3. XRD patterns of the pristine TPA-DPP-MDN and TPA-DPP thin films.
3.4. Optical propertie
The UV–vis absorption spectra of two SMDs in solution and solid
thin films were displayed in Fig. 4, and the related data were sum-
marized in Table 1. In comparison with TPA-DPP, TPA-DPP-MDN both
in solution and in film (Fig S14) are greatly broadened and red-shifted,
respectively. Moreover, the TPA-DPP-MDN exhibits a higher molar
extinction coeffcient (ε) than that of TPA-DPP (Fig. S14). The TPA-DPP
showed main absorption peak at 590 nm with the absorption edge
g
molecular interaction in the solid-state. The optical E of TPA-DPP and
TPA-DPP-MDN was 1.89 eV and 1.52 eV, respectively, corresponding to
their absorption onset in film. Compared with D-A structure, the D-A-A
system showed an extended the spectral region with a low energy gap.
The fluorescence spectra of TPA-DPP (a) and TPA-DPP-MDN (b) in
different solvents were shown in Fig. 5. The Photoluminescence (PL)
behaviors of TPA-DPP and TPA-DPP-MDN were compared to emphasize
the Push-Pull character. TPA-DPP exhibited a little red-shift of 24 nm
with increase of the solvent polarity gradually from Hexane to Chlor-
obenzene (CB). It was noted that all of the PL spectra of TPA-DPP
showed the similar shape in solutions. Moreover, full width at half
maximum (FWHM) of TPA-DPP seemed to be independent with solvent
polarity. However, compared with these of TPA-DPP, the fluorescent
spectra of TPA-DPP-MDN showed the larger red-shifted of 35 nm,
broader shape with increase of the solvent polarity, a FWHM of 129 nm
in CB. This change was consistent with a variety of the excited state
characteristics, which indicated that D-A-A compound presented the
remarkable push-pull effect.
(
λ
onset) located at 646 nm in chloroform solution. The absorption peak
and edge of TPA-DPP-MDN in chloroform solution were red shifted of
1 nm and 99 nm relative to these of TPA-DPP. As shown in Table 1 and
5
3.5. Electrochemical properties
The energy levels of the SDMs were determined by cyclic voltam-
metry (CV). The CV curves were shown in Fig. 6 (a), and the results of
the electrochemical measurements were summarized in Table 2. The CV
+
curves were determined by the reference electrode (Ag/Ag ), which
+
was referenced to ferrocene/ferrocenium (Fc/Fc ) redox couple (4.8 eV
below the vacuum level) [30]. TPA-DPP exhibited the onset reduction
potential at –1.14 V. It was noted that TPA-DPP-MDN shown two onset
reduction at –1.05 and –1.45 V, which were ascribed to the reduction of
the DPP and the MDN group, respectively. The onset oxidation poten-
tials (EOX) of TPA-DPP and TPA-DPP-MDN were around 0.43 V and
0
.48 V. The HOMO and LUMO energy levels were calculated by em-
pirical formula [31] EHOMO = – (EOX + 4.70) (eV) and ELUMO = – (ERed
4.70) (eV), –5.13 and –3.56 eV for TPA-DPP, –5.18 and –3.65 eV for
+
TPA-DPP-MDN, respectively. As shown in the energy levels of the SMDs
and acceptor (Fig. 5. (b)). It was clear that the HOMO energy levels
decreased slightly in the order: D-A and D-A-A. The LUMO energy level
of D-A-A system was 0.09 eV lower than the level of D-A system. The
low-lying HOMO level of TPA-DPP-MDN would ensure a relatively
Fig. 4. (a) Normalized UV–vis absorption spectra of TPA-DPP and TPA-DPP-
MDN in thin film; Normalized UV–vis absorption spectra of TPA-DPP-MDN (b)
and TPA-DPP (c) in different solvent.
5

Y. Chen, et al.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
Fig. 5. Normalized fluorescence spectra of TPA-DPP (a) and TPA-DPP-MDN (b) in different solvents.
Fig. 6. (a) CV curves of compound TPA-DPP and TPA-DPP-MDN in 0.1 mol·L–1 Bu
NPF
6
acetonitrile solution at a sweep rate of 100 mV·s–1; (b) Energy levels of the
4
components.
Table 2
Optical and electrochemical characteristics of the TPA-DPP and TPA-DPP-MDN.
2
3
Compound
film
opt
1(eV)
E
oxonset(V)
E
redonset(V)
ECV g(eV)
ECV HOMO (eV)
ECV LUMO (eV)
λonset(nm)
E
g
TPA-DPP
TPA-DPP-MDN
657
818
1.89
1.52
0.43
0.48
−1.14
−1.05
1.57
1.53
−5.13
−5.18
−3.56
−3.65
1
Calculated from the onset of the film absorption (E = 1240/λonset).
g
2
3
Calculated from oxidation potential of the copolymer (ECV HOMO = – (EOX+ 4.70) (eV)).
Calculated from reduction potential of the copolymer (ECV LUMO = – (ERed+ 4.70) (eV)).
higher VOC than that of TPA-DPP. In addition, compared with the
counterpart TPA-DPP, TPA-DPP-MDN exhibited higher oxidation peak
potential. The electrochemical data were in agreement with the com-
putational results which clearly indicate that the HOMO level of the
TPA-DPP-MDN was lower than that of the TPA-DPP attributed to the
electron-withdrawing effect of the auxiliary unit. In fact, the MDN unit
afforded also a decrease of the LUMO level, which was lowered by a
larger amount than the HOMO level, resulting in a decrease of the
energy gap of D-A-A compound.
Blends/MoO
been fabricated by spin-casting a solution of SMDs and [6,6]-phenyl-
61-butyric acid methyl ester (PC61BM) mixtures dissolved in chloro-
form. The OSCs were tested under AM 1.5 G (Air mass 1.5 global,
3
/Ag. The active layers containing various D/A ratios have
C
−
2
100 mW·cm ). The current density-voltage (J-V) characteristics were
shown in Fig. 7. (a), and detailed parameters of the devices were listed
in Table 3. The OSC based on TPA-DPP-MDN/PC61BM (w:w, 1:3) blend
showed the power conversion efficiency (PCE) of 1.12%, with a VOC of
–
2
0.74 V, a JSC of 4.61 mA·cm , and a fill factor (FF) of 32.84%. The OSCs
based on TPA-DPP/PC61BM (w:w, 1:3) blend showed the PCE of 0.35%,
–
2
with a VOC of 0.65 V, a JSC of 1.80 mA·cm , and a FF of 29.71%. Evi-
dently, the high VOC of D-A-A system based device was originated from
the low-lying HOMO level of TPA-DPP-MDN, due to the electron-
withdrawing effect of the auxiliary unit. The IPCE curves of the devices
3
.6. Photovoltaic properties of two molecules
The PV properties of the SMDs were examined by fabricating bulk
heterojunction (BHJ) OSCs with device architecture of ITO/PFN/
6

Y. Chen, et al.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112139
Fig. 7. (a) J–V curves of BHJ devices with a configuration of ITO/PFN/Blends/MoO
3
/Ag under an illumination of AM 1.5 G at 100 mW·cm–2; (b) The IPCE curves of
the OSCs based on the SMDs:PC61BM.
Table 3
online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.
112139.
Photovoltaic parameters of the OSCs based on the SMDs and PC61BM.
Active layer
w:w
V
OC (v)
Jsc (mA·cm–2)
FF (%)
PCE (%)
References
TPA-DPP-MDN/PC61BM
TPA-TDPP/PC61BM
1:3
1:3
0.74
0.65
4.61
1.80
32.84
29.71
1.12
0.35
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Acknowledgements
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Supplementary material related to this article can be found, in the
7