Medium band gap conjugated polymers from thienoacene derivatives and pentacyclic aromatic lactam as promising alternatives of poly(3-hexylthiophene) in photovoltaic application

Article abstract of DOI:10.1002/pola.28874

Two alternating medium band gap conjugated polymers (PBDT-TPTI and PDTBDT-TPTI) derived from 4,8-bis(4,5-dioctylthien-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T) or 5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT-T) with pentacyclic aromatic lactam of N,N-didodecylthieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-iso-quinoline-5,11-dione (TPTI), are synthesized and characterized. The comparative investigation of the photostabilities of the copolymers revealed that the PDTBDT-TPTI film exhibited the comparable photostability in relative to P3HT. Meanwhile, the inverted photovoltaic cells (i-PVCs) from the blend films of PBDT-TPTI and/or PDTBDT-TPTI with PC₇₁BM, in which poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] were used as cathode modifying interlayer, presented higher power conversion efficiencies (PCEs) of 5.98% and 6.05% with photocurrent response ranging from 300 nm to 650 nm in contrast with the PCEs of 4.48% for the optimal inverted PVCs from P3HT/PC₇₁BM under AM 1.5 G 100 mW/cm². The PCEs of the i-PVCs from PBDT-TPTI and PDTBDT-TPTI were improved to 7.58% and 6.91% in contrast to that of 0.02% for the P3HT-based i-PVCs, and the photocurrent responses of the devices were extended to 300–792 nm, when the ITIC was used as electron acceptor materials. The results indicate that the PBDT-TPTI and PDTBDT-TPTI can be used as the promising alternatives of notable P3HT in the photovoltaic application.

Full text of DOI:10.1002/pola.28874

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Medium Band Gap Conjugated Polymers from Thienoacene Derivatives
and Pentacyclic Aromatic Lactam as Promising Alternatives of Poly(3-
hexylthiophene) in Photovoltaic Application
Peili Gao,1 Junfeng Tong,1 Pengzhi Guo,1 Jianfeng Li,1 Ningning Wang,1 Cheng Li,1
1,3
Xuying Ma,1 Peng Zhang,1 Chenglong Wang,2 Yangjun Xia
1Key Laboratory of Optoelectronic Technology and Intelligent Control of Ministry Education, Lanzhou Jiaotong University,
Lanzhou 730070, People’s Republic of China
2National Green Coating Technology and Equipment Research Center, Lanzhou Jiaotong University, Lanzhou 730070, People’s
Republic of China
3Department of Materials and Chemistry, Biochemistry University of California Santa Barbara, CA 93106-9510, USA
Correspondence to: Y. Xia (E-mail: yjxia73@126.com) or C. Wang (E-mail: clwang@mail.lzjtu.cn)
Received 26 July 2017; accepted 22 September 2017; published online 00 Month 2017
DOI: 10.1002/pola.28874
ABSTRACT: Two alternating medium band gap conjugated poly-
mers (PBDT-TPTI and PDTBDT-TPTI) derived from 4,8-bis(4,5-
dioctylthien-2-yl)benzo[1,2-b:4,5-b0]dithiophene (BDT-T) or 5,10-
bis(4,5-didecylthien-2-yl)dithieno[2,3-d:20,30-d0]benzo[1,2-b:4,5-b0]
dithiophene (DTBDT-T) with pentacyclic aromatic lactam of
N,N-didodecylthieno[20,30:5,6]pyrido[3,4-g]thieno[3,2-c]-iso-quin-
oline-5,11-dione (TPTI), are synthesized and characterized. The
comparative investigation of the photostabilities of the copoly-
mers revealed that the PDTBDT-TPTI film exhibited the compa-
rable photostability in relative to P3HT. Meanwhile, the inverted
photovoltaic cells (i-PVCs) from the blend films of PBDT-TPTI
and/or PDTBDT-TPTI with PC71BM, in which poly[(9,9-bis(30-
(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]
were used as cathode modifying interlayer, presented higher
power conversion efficiencies (PCEs) of 5.98% and 6.05% with
photocurrent response ranging from 300 nm to 650 nm in
contrast with the PCEs of 4.48% for the optimal inverted PVCs
from P3HT/PC71BM under AM 1.5 G 100 mW/cm2. The PCEs of
the i-PVCs from PBDT-TPTI and PDTBDT-TPTI were improved to
7.58% and 6.91% in contrast to that of 0.02% for the P3HT-based
i-PVCs, and the photocurrent responses of the devices were
extended to 300–792 nm, when the ITIC was used as electron
acceptor materials. The results indicate that the PBDT-TPTI and
PDTBDT-TPTI can be used as the promising alternatives of nota-
C
V
ble P3HT in the photovoltaic application. 2017 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: benzo[1,2-b:4,5-b0]dithiophene; conjugated poly-
mers;
crystallization;
dithieno[2,3-d:20,30-d0]benzo[1,2-b:4,5-
b0]dithiophene; medium band gap; morphology; pentacyclic
aromatic lactam; photovoltaic cells; synthesis; thin films
PVCs with higher Voc, the tandem PVCs assembled by two or
more cells in series, in which the rear cell is generally made
by LBG materials and the front one is made by medium (or
wide) band gap (MBG) CPs, have been considered as a prom-
ising way to further enhance the performance of PVCs.14–16 It
is obvious that the MBG CPs with excellent photovoltaic prop-
erties are highly desired in addition to high performance LBG
polymers for achievement of the efficient tandem PVCs. In
spite that many promising medium (or wide) CPs with the
optical band gap larger than 1.9 eV have been provided in
past few years, and the PCEs of 3.5%28.5% have been
achieved in the PVCs from the MBG CPs,17–21 poly(3-
INTRODUCTION In last decades, the development of novel
low band gap (LBG) donor conjugated polymers (CPs) has
been demonstrated to be the most vigorous and promising
approach to achieve high performance polymeric photovol-
taic cells (PVCs), and various high performance LBG CPs
have been presented.1–11 Generally, the improvement of the
short circuit current density (Jsc) can be achieved by narrow-
ing the band gaps of the LBG CPs. Unfortunately, this
approach usually compromised the trade-off problem such as
the decrease of the open circuit voltage (Voc) with the
increase of the Jsc of the PVCs from the LBG CPs.12,13 To use
solar radiation more effectively and achieve high performance
Peili Gao and Junfeng Tong contributed equally to the manuscript.
Additional Supporting Information may be found in the online version of this article.
C
V
2017 Wiley Periodicals, Inc.
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hexylthiophene-2,5-diyl) (P3HT) is still the state-of-the-art
medium band gap polymer for tandem PVCs because of its
reliable photovoltaic performance so far. Unfortunately, the
relatively poor photo-stability of the P3HT should also hinder
the PVCs-based on the P3HT in practical application.22 Based
on the above considerations, development of novel MBG CPs
with enhanced performance and photo-stability, might be
helpful to pave the way of the PVCs into practical application.
tetramethylsilane. Analytical gel permeation chromatography
(GPC) was performed using a Waters GPC 2410 in THF rela-
tive to polystyrene standards. Thermal gravimetric analysis
(TGA) was conducted on a TGA 2050 (TA instruments) ther-
mal analyses system under a heating rate of 10 8C/min and
a nitrogen flow rate of 20 mL/min. UV–visible absorption
spectra were measured on a UV-2500 spectrophotometer (Shi-
madzu). The cyclic voltammetry (CV) was measured on CHI
600D electrochemical workstations (Shanghai Chenhua) at a
scan rate of 50 mV/s with a nitrogen-saturated solution of
0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in
acetonitrile (CH3CN) with glass carbon and Ag/AgNO3 electrode
as the working and reference electrode, respectively. Tapping-
mode atomic force microscopy (AFM) images were obtained on
a NanoScope NS3A system (Digital Instrument). The photo-
stabilities of the copolymers and P3HT in solid state were inves-
tigated as the following process, the copolymers and P3HT thin
films on glass substrates with thickness of 40 nm were firstly
irradiated by a solar simulator (Air mass 1.5 global, XES-70S1,
San-EI Electric Co) with irradiation of 100 mW cm22, then the
absorption spectra of the irradiated thin films were monitored
on a UV-2500 spectrophotometer. The photo-degradation of the
copolymers were carried on as following approaches: the dilute
chlorobenzene (CB) solution of copolymers were put into a sil-
ica glass tube, degassed and kept under argon, then irradiated
with UV-radiation ex-situ for varying amounts of time with a
Jinaopu UV lamp (UV-1800 Spectrophotomete Shimadzu). The
vial was placed within 15 cm of the UV lamp, and circumro-
tated of 400 cyclic per minute, ensuring that the entire solution
volume being uniformly irradiated. After that, the UV absorption
spectra of the copolymers in solution were monitored. The X-
ray diffraction (XRD) was carried out on a PANalytical X’Pert
PRO diffractometer equipped with a rotating anode (Cu Ka radi-
ation, k 5 1.54 Å).
Herein, we developed two novel MBG CPs (respectively, named
as PBDT-TPTI and PDTBDT-TPTI) by copolymerizing 4,8-
bis(4,5-dioctylthien-2-yl)benzo[1,2-b:4,5-b0]dithiophene (BDT-T)
or 5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:20,30-d0]benzo-
[1,2-b:4,5-b0]dithiophene (DTBDT-T) with the pentacyclic
aromatic lactam of N,N-didodecylthieno[20,30:5,6]pyrido[3,4-
g]thieno[3,2-c]-iso-quinoline-5,11-dione (TPTI) by Stille cross-
coupling reaction. The thermal stabilities, photo-stabilities,
optoelectronic properties, and the influence of the BDT-T and
DTBDT-T configurations on the optical properties etc. of the
copolymers, were investigated. The inverted photovoltaic cells
(i-PVCs) from the blend films of PBDT-TPTI and/or PDTBDT-
TPTI with PC71BM, in which poly[(9,9-bis(30-(N,N-dimethylami-
no)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)
was used as cathode modifying interlayer, have presented
the higher power conversion efficiencies (PCEs) of 5.98%
and 6.05% with photocurrent response ranging from 300
to 650 nm in relative to that of 4.48% for the optimal
inverted PVCs from P3HT/PC71BM blend films under AM
1.5G 100 mW/cm2. The PCEs of i-PVCs from PBDT-TPTI and
PDTBDT-TPTI were improved to 7.58 and 6.91%, and the
photocurrent responses of the devices were extended to
300–792 nm when the ITIC were was used as electron
acceptor materials. Moreover, the comparative investiga-
tions of the photo-stabilities of copolymers and P3HT (in
film state) under AM 1.5 sunlight illumination in air indi-
cated that the PDTBDT-TPTI exhibited comparable photo-
stability with the notable P3HT.
Preparation and Characterization of the Photovoltaic
Solar Cells
A patterned indium tin oxide (ITO) coated glass with a sheet
resistance of 10–15 X/square, was cleaned by a surfactant
scrub, followed by a wet-cleaning process inside an ultra-
sonic bath, beginning with deionized water, followed by ace-
tone and iso-propanol (iPrOH). After oxygen plasma cleaning
for 5 min, a 5 nm thick PFN layers were spin-casted onto
the ITO. The active layers with a thickness ranging in the
100–110 nm, were then deposited on the top of the PFN-
modified ITO by spin-casting from the CB solution containing
PDTBDT-TPTI/PC61BM, PBDT-TPTI/PC61BM, PDTBDT-TPTI/
PC71BM, PBDT-TPTI/PC71BM with and without DIO as sol-
vent additives. Then, a 8 nm MoO3 and 100 nm silver layer
were evaporated with a shadow mask under vacuum of (1–
5) 3 1025 Pa. The same procedures were applied to the
devices with ITIC as electron acceptor materials expect that
the PCBM were replaced by ITIC. The overlapping area
between the cathode and anode defined a pixel size of device
of 0.1 cm2. The thickness of the active layers was deter-
mined by a Profile system (BRUKER VDS-9400 QS). The
thickness of the evaporated cathode was monitored by a
quartz crystal thickness/ratio monitor (SI-TM206, Shenyang
EXPERIMENTAL
Materials
All reagents, unless otherwise specified, were obtained from
Aldrich, Acros, and TCI Chemical, and used as received.
P3HT was purchased from J&K. The tetrahydrofuran (THF)
and diethyl ether (Et2O) were dried by sodium with benzo-
phenone as indicator under N2 flow. Benzo[1,2-b:4,5-
b0]dithiophene-4,8-diketone,23 2,7-bis(trimethylstannyl)-5,10-
bis(4,5-didecylthien-2-yl)dithieno[2,3-d:20,30-d0]benzo[1,2-b:4,
5-b0]dithiophene (DTBDT-TSn),24 2,8-dibromo-N,N-didode-
cylthieno[20,30:5,6]pyrido[3,4-g] thieno[3,2-c]-iso-quinoline-
5,11-dione (TPTIBr),25–28 PFN29 and ITIC,30,31 were synthe-
sized as the procedures reported in the reference. The
1
DTBDT-TSn, TPTIBr, and ITIC were characterized by H NMR
before use.
General Method
1H NMR spectra were recorded on a Bruker DRX 400 spec-
trometer operating at 400 MHz and were referred to
2
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Sciens). Except for the deposition of the PFN layers, all the
fabrication processes were carried on inside a controlled
atmosphere in a nitrogen drybox (Etelux) containing <1
ppm oxygen and moisture. The PCEs of the resulting poly-
mer solar cells were measured under 1 sun, AM 1.5 G (Air
mass 1.5 global) condition using a AAA class solar simulator
(XES-70S1, San-EI Electric) with irradiation of 100
mW cm22. The current density-voltage (J-V) characteristics
were recorded with a Keithley 2400 source-measurement
unit. The spectral responses of the devices were measured
with a commercial EQE/incident photon to charge carrier
efficiency (IPCE) setup (7-SCSpecIII, Bejing 7-star Opt). A cal-
ibrated silicon detector was used to determine the absolute
photosensitivity.
dehydrobenzo[l,2-b:4,5-b0]dithiophene-4,8-dione (2.69 g, 12.2
mmol) was quickly added, and the reaction mixture was
stirred at 50 8C for 1 h. After the reaction mixture was
cooled to r.t., a solution of SnCl2ꢀ2H2O (22.60 g, 99.8 mmol)
in 10% HCl (30 mL) was added and stirred for additional
1.5 h and then poured into the ice water. The mixture was
extracted twice with petroleum ether (PE). The organic
phase was dried over Na2SO4 and concentrated. The crude
product was purified by silica gel chromatography using PE
as the eluent to yield BDT-T as a yellow solid (6.40 g, 65%
1
yield). M.p. 65–67 8C. H NMR (CDCl3, 600 MHz), d (ppm): d
7.68 (d, J 5 5.4 Hz, 2H), 7.43 (d, J 5 5.4 Hz, 2H), 7.21 (s, 2H),
2.81 (t, J 5 7.8 Hz, 4H), 2.60 (t, J 5 7.2 Hz, 4H), 1.62–1.75 (m,
8H), 1.45–1.20 (m, 40H), 0.89 (t, J 5 6.6 Hz, 12H). Anal.
Calcd for C50H74S4: C, 74.75%; H, 9.28%. Found, C, 74.55%;
H, 9.20%.
Synthesis of the Monomers and Copolymers
2-Bromo-3-octylthiophene (I)
To a solution of 3-octylthiophene (20 g, 0.1 mol) in CC14
(40 mL), a 20 mL CCl4 solution of Br2 (16 g, 0.1 mol) was
added slowly at –20 8C and stirred for 2 h. The reaction mix-
ture was washed with water (3 3 50 mL) and dried over
Na2SO4, and the CCl4 was removed to give light brown crude
product, which was distilled to afford I (20.6 g, 75%) as a
2,6-Di(trimethylstannyl)-4,8-bis(4,5-dioctyl-thien-2-
Yl)benzo[1,2-b:4,5-B0]dithiophene (BDT-TSn)
To the 60 mL of anhydrous THF solution of BDT-T (2.00 g,
n
2.49 mmol), BuLi (1.6 M in hexane, 3.9 mL, 6.2 mmol) was
added dropwise under Ar at 0 8C. Then, the reaction mixture
was warmed to r. t. and stirred for 2 h. Then the reaction
mixture was cooled to 0 8C again, and hexane solution of
Me3SnCl (1.50 g, 7.5 mmol) was added in one portion. The
reaction mixture was stirred at 0 8C for 0.5 h and then
warmed to r. t. for 6 h. Subsequently, the reaction mixture
was poured into water and extracted twice with PE. Then,
the organic layer was dried over Na2SO4 and concentrated to
afford the yellow crude product, which was recrystallized
1
colorless liquid. H NMR (400 MHz, CDCl3) d 7.18 (d, J 5 5.6
Hz, 1H), 6.78 (d, J 5 5.6 Hz, 1H), 2.56 (t, J 5 7.6 Hz, 2H),
1.58–1.54 (m, 2H), 1.33–1.31 (m, 10H), 0. 88 (t, J 5 6.8 Hz,
3H). Anal. Calcd for C12H19BrS: C, 52.36%; H, 6.96%. Found,
C, 52.21%; H, 6.72%.
2,3-Dioctylthiophene (II)
To a suspension of magnesium turnings (2.90 g, 122.0 mmol)
in 10 mL anhydrous Et2O solution, 40 mL Et2O solution of 1-
bromooctane (16.8 g, 87.3 mmol) containing several drops of
1,2-dibromoethane as an initiator was added slowly. And the
reaction mixture was then refluxed for 1.5 h. The Grignard
solution was transferred via cannula to an addition funnel
and added dropwise to a mixture of [1,3-bis(diphenylpho-
sphino)propane]nickel(II) chloride (Ni(dppp)Cl2, 82.0 mg, 0.15
mmol) and 2-bromo-3-octylthiophene (20.00 g, 72.7 mmol) at
0 8C. The resulting mixture was refluxed overnight, then
cooled to room temperature (r. t.), hydrolyzed with 1 M HCl
(30 mL) and 70 mL ice water followed by extraction with
Et2O. The obtained combined phase was dried with Na2SO4,
and the solvent was removed under reduced-pressure to
afford a yellow oil, which was purified by distillation under
high vacuum to give 14.40 g (64%) as pale-yellow liquid. 1H
NMR (400 MHz, CDCl3) d 7.02 (d, J 5 4.8 Hz, 1H), 6.80 (d,
J 5 5.2 Hz, 1H), 2.71 (t, J 5 7.6 Hz, 2H), 2.50 (t, J 5 7.6 Hz,
2H), 1.62–1.50 (m, 8H), 1.41–1.26 (m, 20H), 0.88 (t, J 5 6.4
Hz, 6H). Anal. Calcd for C20H36S: C, 77.85%; H, 11.76%.
Found, C, 77.71%; H, 11.66%.
i
from PrOH and finally afforded yellow sheet-shaped crystals
1
(1.80 g, 60%). H NMR (CDCl3, 400 MHz), d (ppm): d 7.72 (t,
J 5 15.6 Hz, 2H), 7.23 (s, 2H), 2.83 (t, J 5 7.6 Hz, 4H), 2.61 (t,
J 5 7.2 Hz, 4H), 1.64–1.76 (m, 8H), 1.22–1.28 (m, 40H), 0.89
(m, 12H), 0.40 (t, J 5 27.6 Hz, 18H). Anal. Calcd for
C56H90S4Sn2: C, 59.58%; H, 8.03%. Found, C, 59.45%; H,
7.92%.
Synthesis of Polymer PDTBDT-TPTI
A mixture of toluene (6 mL) and N,N-dimethyl-formamide
(DMF, 0.5 mL) was added to a 55 mL microwave tube con-
taining DTBDT-TSn (270.7 mg, 0.2 mmol), TPTIBr (163.8 mg,
0.2 mmol), Pd2(dba)3 (2.0 mg), and P(o-tol)3 (3.5 mg) in a
glove box with moisture and oxygen under 1 ppm. Then the
tube was subjected to the following reaction conditions in a
microwave reactor: 120 8C for 5 min, 140 8C for 5 min, and
160 8C for 20 min. At the end of polymerization, the polymer
was end-capped with 2-tributyl-stannylthiophene and 2-
bromothiophene to remove bromo and trimethylstannyl end
groups. The mixture was then poured into methanol. The
precipitated material was collected and extracted with etha-
nol, acetone, hexane, and toluene in a Soxhlet extractor. The
solution of the copolymer in toluene was condensed to
15 mL and then poured into methanol (300 mL). The precip-
itation was collected and dried under vacuum overnight
(yield: 75%). Mn 5 29,200 g/mol with a polydispersity index
(PDI 5 Mw/Mn) of 2.78. Anal. Calcd for C102H142N2O2S8: C,
4,8-Bis(4,5-dioctylthien-2-Yl)benzo[1,2-b:4,5-B0]dithiophene
(BDT-T)
To an anhydrous THF (30 mL) solution of compound II
n
(10.40 g, 34.2 mmol) at an ice bath, BuLi (2.5 M in hexane,
13.7 mL, 34.2 mmol) was added dropwise. The reaction mix-
ture was then heated to 50 8C for 2 h. Subsequently, 4,8-
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SCHEME 1 Synthesis routes of the monomers and copolymers.
72.72%; H, 8.50%; N, 1.66%. Found, C, 72.61%; H, 8.47%; N,
1.69%.
copolymers PDTBDT-TPTI and PBDT-TPTI were synthesized
between DTBDT-TSn or BDT-TSn and TPTIBr via the
palladium-catalyzed Stille cross-coupling reaction (Scheme
1).31 The copolymers were end-capped with 2-tributyl-
stannylthiophene and 2-bromothiophene to remove bromo
and trimethylstannyl end groups.17 After the polymerization,
the reaction mixture was poured into methanol, and the pre-
cipitated copolymers were collected and extracted with etha-
nol, acetone, hexane and toluene in a Soxhlet extractor to
remove the residue catalyst and oligomers. Then, the concen-
trated solutions of the copolymers in toluene were poured
into methanol again, and the precipitated copolymers were
collected with the yields of 75–85%. The number-average
molecular weight (Mn) of the PDTBDT-TPTI and PBDT-TPTI
determined by GPC in THF with polystyrene standards, were
about 29,202 g/mol with PDI of 2.78 and 34,456 g/mol with
PDI of 3.16, respectively (Table 1). The decomposed temper-
ature (Td, 5% weight-loss) of the PDTBDT-DPTI and PBDT-
TPTI were about 410.3 and 378.9 8C under N2 flow (Table 1,
Supporting Information Fig. S7), indicating that two copoly-
mers exhibited good thermal-stability.
Synthesis of Polymer PBDT-TPTI
The PBDT-TPTI was synthesized as the procedure of
PDTBDT-TPTI, except that the polymerization was carried
out with BDT-TSn of (270.7 mg, 0.2 mmol), TPTIBr
(163.8 mg, 0.2 mmol). Yield: 85%. Mn 5 34,500 g/mol with
PDI of 3.16. Anal. Calcd for C90H126N2O2S6: C, 74.02%; H,
8.70%; N, 1.92%. Found, C, 73.95%; H, 8.59%; N, 1.99%.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Copolymers
The general synthetic routes of monomers and the copoly-
mers are outlined in Scheme 1. BDT-TSn was started from
the bromination of 3-octylthiophene in CCl4 with bromine at
220 8C under dark condition. Following that, the reaction of
2-bromo-3-octylthiophene with octylmagnesium bromide
with Ni(dppp)Cl2 as catalyst led to 2,3-dioctylthiophene.
Then, the pre-prepared solution of 2,3-dioctylthiophene with
n-BuLi in anhydrous THF under 0 8C, was firstly added into
the suspending solution of benzo[1,2-b:4,5-b0]dithiophene-
4,8-diketone, and treated with SnCl2ꢀ2H2O and HCl (10%) to
generate BDT-T. The treatment of BDT-T with nBuLi at 0 8C
and Me3SnCl in anhydrous THF, yielded the bisstannyl como-
nomer BDT-TSn. DTBDT-TSn24 and TPTIBr25–27 were pre-
pared following the reported procedure. The structures of
Optical Properties of the Copolymers and Aggregation of
the Copolymers in Solution and Solid Thin Film
The UV–vis absorption spectra of the copolymers in dilute
CB solution and solid thin film were monitored on a UV-
2500 spectrophotometer, and the normalized UV–vis absorp-
tion spectra of PBDT-TPTI and PDTBDT-TPTI were presented
in Figure 1 and Table 1. Both copolymers exhibited the weak
1
the monomers were confirmed by H NMR before they were
used (Supporting Information Fig. S1–S6). The obtained
4
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TABLE 1 Molecular and Optoelectronic Parameters of the PDTBDT-TPTI and PBDT-TPTI
Egopta
u
ox
HOMOb
(eV)
LUMOc
(eV)
onset
lh (cm2 V21 s21
)
film
max
Copolymer
Mn (kg/mol) Mw (kg/mol)
PDI
Td (8C)
k
(nm)
(eV)
(eV)
PDTBDT-TPTI 29.2
81.2
2.78
3.16
2.00d
410.3
378.9
–
587
575
522
1.98
2.02
1.91
0.62
0.69
0.26
25.32
25.39
24.96
23.34
23.37
23.05
6.58 3 1024
1.92 3 1023
–
PBDT-TPTI
P3HT
34.5
50.0d
109.0
100.0d
a
c
Calculated from the onset of the film absorption (Eg 5 1240/kfoinlmset).
Calculated from the optical band gap (Egopt) and HOMO levels of the
b
Calculated from oxidation potential of the copolymer [EHOMO 5 –
copolymers ELUMO 5 –(jEHOMOj–Eg) (eV).
d
onset
ox
e(u
1 4.70) (eV)].
The data are obtained from the J&K Com.
charge transfer transition absorption peaks (k0 2 1) around
527 nm for PBDT-TPTI and 538 nm for PDTBDT-TPTI,
together with strong vibrational absorption peaks (k0 2 0) of
571 nm for PBDT-TPTI and 587 nm for PDTBDT-TPTI in the
solid state and solution.32–35 The k0 2 1, k0 2 0 peaks and
onset band gap wavelengths (konset) for PBDT-TPTI were
changed to 532, 575, and 615 nm ongoing from solution to
solid state, respectively. The optical band gap (Egopt) of PBDT-
TPTI in film was about 2.02 eV. The PDTBDT-TPTI also
exhibited two main absorption peaks at around 538 and
587 nm with konset of about 629 nm in solution and solid
thin film, corresponding to the optical band gap of 1.97 eV.
Interestingly, the bathochromic shift of konset with only 2 nm
for PDTBDT-TPTI was observed ongoing from solution to
solid state.
To investigate the aggregation behaviors of the copolymers
in solution, temperature-dependent absorption spectra (TD-
Abs) of the copolymers in CB solution were monitored (Fig.
2). The k0 2 0 peaks of two copolymers were gradually
decreased and blue-shifted while the temperature was ele-
vated from 25 to 115 8C, except that the blue-shifted value
of PDTBDT-TPTI was slightly larger than that of PBDT-TPTI.
FIGURE 1 Normalized UV–vis spectra of the PBDT-TPTI (a) and
PDTBDT-TPTI (b) in dilute CB solution and solid thin film.
[Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 2 TD-Abs spectra of PBDT-TPTI (a) and PDTBDT-TPTI
(b) in diluted CB solution. [Color figure can be viewed at
wileyonlinelibrary.com]
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Bragg’s law (i.e., k 5 2dsinh). It could be also found that the
peak of PBDT-TPTI was much stronger than that of PDTBDT-
TPTI. Meanwhile, the two copolymers also provided the
peaks at about 2h 5 4.068, corresponding to interlayer d-
spacing of 21.74 Å. Notably, the comparison between the
XRD patterns of the two films clearly indicated that a more
ordered structure could be formed in the film of PBDT-TPTI
in contrast to that of PDTBDT-TPTI film.
Computational Consideration of DTBDT-T and BDT-T
Units
Considering that the electron acceptor units of TPTI in the
two copolymers are same, the difference between the copoly-
mers should be come from the electron donor moieties of
BDT-T and DTBDT-T. To disclose the effect of the donor moie-
ties on the aggregation characteristics of the two copolymers,
computational calculations were implemented on the models
of the DTBDT-T and BDT-T by using density functional theory
(DFT) calculation with B3LYP/6–31G** set, as implemented
in Gaussian 09,40 and the side alkyl chains were modified to
methyl groups to simplify the calculation. As shown in Fig-
ure 4, both bonding geometries for extending the conjugated
system through the two non-substituted a-positions of
DTBDT-T and BDT-T are associated with about 1808 bend,
and the optimized geometries of the models of BDT-T and
DTBDT-T reveal that the torsion angles between the flanked
substituent of 4,5-dialkylthien-2-yl and planar core of DTBDT
is about 75.58, which is much larger than that of 52.58 for
BDT-T unit. It is reasonable that the larger twisting of the
flanked substituent of DTBDT-T moieties might attribute to
the weaker aggregation of PDTBDT-TPTI backbone in relative
to that for PBDT-TPTI.
FIGURE 3 XRD patterns of the pristine PBDT-TPTI and
PDTBDT-TPTI films casted from CB onto a glass substrate.
[Color figure can be viewed at wileyonlinelibrary.com]
Meanwhile, it was also observed that the k0 2 1 peaks of
PBDT-TPTI almost kept stable, whereas those of PDTBDT-
TPTI were gradually increased at the same process. It has
been demonstrated that the intensity ratio of I0 2 0/I0 2 1
between k0 2 0 and k0 2 1 could be applied to further evaluate
the aggregation degree.36 For PDTBDT-TPTI, the I0 2 0/I0 2 1
value was <1 when heated to 85–95 8C, indicating that the
aggregation was slowly broken, whereas the I0 2 0/I0 2 1 value
of PBDT-TPTI still was far >1 even if heated to 115 8C.
According to the above-mentioned facts, we could find that
PBDT-TPTI exhibited more pronounced aggregations in CB
solutions than that of PDTBDT-TPTI.37–39
To explore the aggregation characteristics of PDTBDT-TPTI
and PBDT-TPTI in solid state, XRD analyses of the polymers
films were carried out, and the thin films were casted from
CB solution onto glass substrate. As shown in Figure 3. The
p-p stacking peaks of PBDT-TPTI and PDTBDT-TPTI were,
respectively, located at 2h 5 24.508 and 2h 5 24.078, corre-
sponding to the d-spacings of 3.63 and 3.69 Å based on the
Photo-Stabilities of the PBDT-TPTI and PDTBDT-TPTI
In spite of that researches on the PVCs have mainly focused
on the development of novel high performance CPs to boost
up the PCEs of the PVCs so far,1–11 it is inevitable that the
photo-stabilities of the CPs should be addressed due to the
influence of them on the stability of the PVCs. To investigate
FIGURE 4 Optimized geometries of the models for DTBDT-T and BDT-T by DFT at the B3LYP/6–31G**. [Color figure can be viewed
at wileyonlinelibrary.com]
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FIGURE 5 CV curves of copolymers (a) and energy levels
diagram of the copolymers, P3HT, PC61BM and ITIC (b). [Color
figure can be viewed at wileyonlinelibrary.com]
FIGURE 7 J-V (a) and IPCEs (b) characteristics of the i-PVCs
from the copolymers. [Color figure can be viewed at wileyonline-
library.com]
the photo-stabilities of the copolymers, we inspected the
photo-stabilities of the copolymers by exposing the copoly-
mers’ films with thickness around 40 nm under AM 1.5
sunlight irradiation in air at ambient temperature, and
absorption spectra of the irradiated P3HT films were also
measured for comparison (Supporting Information Fig. S8).
The maximal absorption peaks around 569 nm for PBDT-
TPTI and 587 nm for PDTBDT-TPTI were decreased as the
exposure times were prolonged. After 10 h of irradiation, the
PBDT-TPTI and PDTBDT-TPTI maintained 51 and 64% of its
initial light absorption at the maximal absorption peaks,
respectively. And the P3HT maintained 66% of its initial light
absorption at the maximal absorption peaks (Supporting
Information Fig. S8).41 In addition, the photo-stabilities of
these copolymers in solution under the 365 nm UV light
irradiation in air were also investigated and the results were
provided (Supporting Information Fig. S9). It was shown that
after exposing 2 h of irradiation, PBDT-TPTI, PDTBDT-TPTI,
and P3HT maintained 52, 65, and 44% of its initial light
absorption at the maximal absorption peaks (Supporting
Information Fig. S9), respectively. It could be found that the
PDTBDT-TPTI exhibited comparable photo-stability in solid
state, and better photo-stability in solution as comparable
with those for P3HT.
FIGURE 6 J0.5–V characteristics of the copolymers in the hole-
only devices with configuration of ITO/PEDOT:PSS/copolymers/
Au. [Color figure can be viewed at wileyonlinelibrary.com]
Electrochemical Characteristics of the Copolymers
The electrochemical characteristics of the copolymers were
investigated by CV in a nitrogen-saturated solution of 0.1 M
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TABLE 2 Parameters of Inverted PVCs with PFN as Cathode Layers from the Copolymers and P3HT
Polymers/Acceptors
Solvent Additives
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
P3HT/PC61BM (W: W, 1:1)
0
0.60
0.60
0.86
0.86
0.81
0.81
0.25
0.94
0.86
9.60
65.8
69.9
56.5
67.5
65.6
62.5
27.2
61.1
60.0
3.79
4.48
3.98
5.98
4.79
6.05
0.02
7.58
6.91
P3HT/PC71BM (W: W, 1:1)
0
10.68
8.19
PBDT-TPTI/PC61BM (W: W, 1:1.5)
PBDT-TPTI/PC71BM (W: W, 1:1.5)
PDTBDT-TPTI/PC61BM (W: W, 1:1.5)
PDTBDT-TPTI/PC71BM (W: W, 1:1.5)
P3HT/ITIC (W: W, 1:1)
3%
3%
3%
3%
0
10.30
9.02
11.96
0.31
PBDT-TPTI/ITIC (W: W, 1:1.5)
PDTBDT-TPTI/ITIC (W: W, 1:1.5)
0
13.19
13.34
0
Bu4NPF6 in CH3CN with glass carbon and Ag/AgNO3 elec-
trode as the working and reference electrode, respectively.
The onset oxidation potential of PDTBDT-TPTI and PBDT-
TPTI are 0.62 and 0.69 V, respectively [Fig. 5(a)]. The redox
potential of Fc/Fc1 in the above-mentioned condition is
10.10 V, which is assumed to have an absolute energy level
of 24.8 eV to vacuum for calibration, LUMO levels calculated
by empirical formulas [EHOMO 5 –(Eox 1 4.70) (eV)42 and
as compared with that for P3HT [Fig. 5(b)]. Therefore, the
higher Voc would be anticipated to achieve in the PVCs from
PBDT-TPTI and PDTBDT-TPTI as compared with that for the
PVCs from P3HT.43
Hole Mobility of the Copolymers
The carrier charge mobility is an important parameter, which
could affect charge separation of the exciton, charge carrier
transport and recombination in the photovoltaic devices.44,45
To investigate charge carrier transport in PDTBDT-TPTI and
PBDT-TPTI films, the hole mobilities of the copolymers have
been measured by using space-charge-limited current (SCLC)
ELUMO 5 –(jEHOMOj–Eg) (eV)], were about 25.32 and 23.34
eV for PDTBDT-TPTI and 25.39 and 23.37 eV for PBDT-
TPTI. For comparison, the HOMO and LUMO energy levels
for P3HT were also measured under the same condition
(Supporting Information Fig. S10). It was noted that the
resultant copolymers exhibited the obviously deeper EHOMO
method with
a
device structure of ITO/PEDOT:PSS/
FIGURE 8 Tapping AFM height images (a, b) and phase (c, d) images of the active layers PBDT-TPTI/PC61BM and PDTBDT-TPTI/
PC61BM: without 3% DIO (a, c) and with 3% DIO (b, d). Image size: 5 3 5 lm. [Color figure can be viewed at wileyonlinelibrary.
com]
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copolymer/Au. The mobilities of the copolymers were calcu-
lated by SCLC model, which is described by eq 1.
8L3J
9E0ErV2
l5
(1)
where J stands for current density, E0 is the permittivity of
free space, Er is the relative dielectric constant of the trans-
port medium, l is the hole mobility, V is the internal poten-
tial in the devices, and L is the thickness of the active layers.
And the thickness of our devices from the copolymers were
fixed about 60–70 nm. The J-V characteristics of the devices
from the PBDT-TPTI and PDTBDT-TPTI were presented in
Figure 6. The hole mobilities of PBDT-TPTI and PDTBDT-TPTI
were about 1.92 3 1023 and 6.58 3 1024 cm2 V21 s21
,
respectively.
Photovoltaic Properties of the PVCs from the
Copolymers
To explore the photovoltaic properties of the two copoly-
mers, inverted photovoltaic devices based on the blend film
of copolymers with configuration of ITO/PFN/copoly-
mer:PC61BM/MoO3/Ag were fabricated due to the inverted
PVCs with PFN as cathode modified layers exhibiting
enhanced stability and performance.46 The weight ratios of
the copolymers to PC61BM in the blend films, were firstly
varied from 1:1 to 1:1.5 and then to 1:2., the PCEs of the i-
PVCs from PBDT-TPTI were increased from 0.99 to 1.11%
then dropped to 0.96%, and the PCEs of the i-PVCs from
PDTBDT-TPTI were increased from 0.59 to 1.29% and then
dropped to 0.80% for PBDT-TPTI. It could be found that the
optimum weight ratios of the copolymers and PC61BM were
both about 1:1.5. (Supporting Information Table S1, Fig. S11).
Following that, the DIO was used to optimize the inverted
PVCs due to potential modification of the morphology of the
blend active layer from the CPs and fullerene deriva-
tives.47–49 The PCEs of the i-PVCs from PBDT-TPTI/PC61BM
and PDTBDT-TPTI/PC61BM with optimum weight ratios,
were correspondingly increased from 1.11% to 3.98% and
1.29% to 4.79% while 3% DIO (VDIO/VCB) was used as sol-
vent additives [Fig. 7(a)] (Table 2, Supporting Information
Table S1 and Fig. S11). The pronounced enhancements of
PCEs were mainly attributed to the corresponding increase
for JSC and FF. As demonstrated by the AFM investigations in
Figure 8, the root-mean-square surface roughness (Rq) of
PBDT-TPTI/PC61BM and PDTBDT-TPTI/PC61BM blends were
respectively decreased from 7.65 to 6.86 nm, and 14.52 to
2.14 nm while 3% DIO were used as solvent additives. It
indicated that the modification of the morphologies for the
copolymers and PC61BM blend films with weight ratios of
1:1.5, were achieved by the inclusion of DIO as solvent addi-
tives, and the improvement of the performance of the i-PVCs
from the blend films of the copolymers and PC61BM should
be contributed to the modification of the morphologies for
the blend active layers from the copolymers and PC61BM.
FIGURE 9 J-V (a) and IPCEs (b) characteristics of the i-PVCs
from the blend films of the copolymers and ITIC. [Color figure
can be viewed at wileyonlinelibrary.com]
stronger absorption of PC71BM in the visible region in rela-
tive to that of PC61BM.50 The weight ratios of the copolymers
and PC71BM were fixed at 1:1.5, and 3% DIO as solvent addi-
tives. Meanwhile, the optimal i-PVCs from P3HT/PC71BM
(the active layer were about 180 nm, and heat annealing
process were employed to modify the devices) were also
provided for comparison. As shown in Figure 7(a), the PCEs
of the i-PVCs from PBDT-TPTI and PDTBDT-TPTI were
enhanced from 3.98 to 5.98% and 4.79 to 6.05% (Table 2),
respectively. The IPCE of the i-PVCs based on the copoly-
mers/PC71BM between 400 and 500 nm were clearly
increased when the PC61BM was replaced by PC71BM as
electron acceptor materials [Fig. 7(b)]. The expected
enhancements of the PCEs and Jsc of the i-PVCs from PBDT-
TPTI/PC71BM and PDTBDT-TPTI/PC71BM might be attributed
to the stronger absorption of PC71BM in relative to that of
PC61BM. And the higher Voc of the i-PVCs from the copoly-
mers were contributed to the deeper HOMO energy levels of
the copolymers in contrast to that for P3HT [Fig. 5(b)].
To further modify the inverted PVCs from copolymers, the
PC71BM were used as electron acceptor materials due to the
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Furthermore, we also fabricated fullerene-free PVCs by
employing PBDT-TPTI and PDTBDT-TPTI as electron donor
materials, and a notable non-fullerene-based small molecular
acceptor named as ITIC (Supporting Information, Fig. S12),
were used as electron acceptor materials. The weight ratios
of the copolymers and ITIC were fixed to 1:1.5. As shown in
Figure 9, the PCEs of the i-PVCs from the blend film of
PBDT-TPTI and ITIC were increased to 7.58% with the
enhancement of the Voc from 0.86 to 0.94 V, Jsc from 10.30
to 13.19 mA/cm2. The PCEs of the i-PVCs from the PDTBDT-
TPTI and ITIC were increased to 6.91% with the enhance-
ment of the Voc from 0.81 to 0.86 V, and Jsc from 11.96 to
13.34 mA/cm2. It could be found that the photocurrent
responses of the i-PVCs from the PBDT-TPTI/ITIC and/or
PDTBDT-TPTI/ITIC were extended to 300–780 nm, when the
PC71BM was replaced by ITIC. The enhancement of the Voc
and Jsc of the i-PVCs from the PBDT-TPTI/ITIC and PDTBDT-
TPTI/ITIC should be contributed to the higher LUMO energy
level and broadening absorption in contrast to those for
PC71BM (Fig. 5).30 Notably, the PCEs of the i-PVCs from the
copolymers and ITIC were higher than that of 0.02% for the
i-PVCs from P3HT/ITIC blend films (Supporting Information
Fig. S13).
experiments, P. Zhang did the DFT calculation, J. Li, C. Li, and
X. Ma have finished the photovoltaic devices, C. Wang and Y.
Xia Y supervised the research and wrote the manuscript.
ACKNOWLEDGMENTS
The authors are deeply grateful to National Nature Science Foun-
dation of China (51463011, 61404067), open fund of State Key
Laboratory of Infrared Physics (Z201302), China Scholarship
Council, the Foundation of A Hundred Youth Talents Training
(152022), and Excellent Team of Scientific Research in Lanzhou
Jiaotong University (201703, 201705) for financial support.
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