Stabilization of the film morphology in polymer: Fullerene heterojunction solar cells with photocrosslinkable bromine-functionalized low-bandgap copolymers

Article abstract of DOI:10.1002/pola.26695

Novel bromine-functionalized photocrosslinkable low-bandgap copolymers, PBDTTT-Br25 and PBDTTT-Br50, are synthesized via Stille cross-coupling polymerization for the purpose of stabilizing the film morphology in polymer solar cells (PSCs). Photocrosslinking of PBDTTT-Br25 and PBDTTT-Br50 copolymers dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport, which is confirmed by the insolubility of the films washed by organic solvents and by their thermal behavior. As a result, the formation of large aggregations of fullerene is suppressed in polymer:fullerene blend films even after prolonged thermal annealing, and the stability of the device is enhanced when compared with cells based on noncrosslinkable PBDTTT. The power conversion efficiency of the PSCs based on PBDTTT-Br25 and PBDTTT-Br50 reaches 5.17% and 4.48%, respectively, which is improved obviously in comparison with that (4.26%) of the PSCs based on the control polymer PBDTTT.

Full text of DOI:10.1002/pola.26695

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Stabilization of the Film Morphology in Polymer:Fullerene Heterojunction
Solar Cells with Photocrosslinkable Bromine-Functionalized Low-Bandgap
Copolymers
Deping Qian,1 Qi Xu,1 Xuliang Hou,1 Fuzhi Wang,1 Jianhui Hou,2 Zhan’ao Tan1
1State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, The New and Renewable Energy of
Beijing Key Laboratory, North China Electric Power University, Beijing 102206, China
2Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Correspondence to: J. Hou (E-mail: hjhzlz@iccas.ac.cn) or Z. Tan (E-mail: tanzhanao@ncepu.edu.cn)
Received 31 January 2013; accepted 20 March 2013; published online 11 April 2013
DOI: 10.1002/pola.26695
ABSTRACT: Novel bromine-functionalized photocrosslinkable
low-bandgap copolymers, PBDTTT-Br25 and PBDTTT-Br50, are
synthesized via Stille cross-coupling polymerization for the
purpose of stabilizing the film morphology in polymer solar
cells (PSCs). Photocrosslinking of PBDTTT-Br25 and PBDTTT-
Br50 copolymers dramatically improves the solvent resistance
of the active layer without disrupting the molecular ordering
and charge transport, which is confirmed by the insolubility of
the films washed by organic solvents and by their thermal
behavior. As a result, the formation of large aggregations of
fullerene is suppressed in polymer:fullerene blend films even
after prolonged thermal annealing, and the stability of the
device is enhanced when compared with cells based on non-
crosslinkable PBDTTT. The power conversion efficiency of the
PSCs based on PBDTTT-Br25 and PBDTTT-Br50 reaches 5.17%
and 4.48%, respectively, which is improved obviously in com-
parison with that (4.26%) of the PSCs based on the control
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polymer PBDTTT. 2013 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2013, 51, 3123–3131
KEYWORDS: conjugated polymers; crosslinking; photophysics;
morphology stabilization; photocrosslinkable conjugated poly-
mers; polymer photovoltaic materials; polymer solar cells;
stabilization
long operation time, as the polymer and the PCBM ([6,6]-
phenyl-C61-butyric acid methyl ester) thermodynamically
prefer to segregate from each other.10–12 Improving the
robustness of the BHJ with respect to thermal stability is
critical, as any heat generated by solar irradiation could be
detrimental to the performance of these devices due to the
relatively low Tg of polymers and the strong immiscibility of
components in the active layer.13
INTRODUCTION Solution-processable polymer solar cells (PSCs)
have attracted considerable attention over the past two
decades because of the advantages of cost-effective fabrica-
tion and the potential physical flexibility.1 Bulk heterojunc-
tion (BHJ) is the most successful device structures, in
which the photoactive layer is a blend of a p-type conju-
gated polymer (electron donor) and an n-type fullerene de-
rivative (electron acceptor).2,3 Controlling the BHJ
morphology within the active layer is critical for achieving
and maintaining high performance. In optimized BHJs, the
network of the donor and acceptor materials should be
bicontinuous, and the domain size of the phase-separated
donor and acceptor network should be comparable with
that of the exciton diffusion length to facilitate charge sepa-
ration and transportation.4–6
To improve the thermal stability of the PSCs, several strat-
egies have been proposed, which include the use of diblock
copolymer compatibilizers,14 corsslinkable fullerene-attached
diblock copolymers,15 and crosslinkable acceptor16 and do-
nor13 materials within the active layer. Crosslinking has pro-
ven to be a promising approach to stabilize the nanoscale
morphology of the blended active layer. Most approaches of
crosslinking conjugated polymers focus on crosslinking the
specific functional groups such as pendant acrylates,17 pe-
ripheral oxetane groups,18 alkyl bromide,19 azide,20 vinyl,21
and diacetylene moieties,22 with the crosslinking reactions
being activated by either heat (thermal) or ultraviolet (UV)
light. Although the crosslinking concept is simple and
Thermal annealing,7 solvent annealing,5,8 and adding high-
boiling-temperature additives9 are the most used approaches
to control phase separation of the donor and the acceptor
materials. However, even if such a phase-separated nano-
structure is constructed, this morphology only represents a
metastable state and may gradually undergo changes over
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SCHEME 1 Synthetic route of the polymers.
EXPERIMENTAL
powerful, the development of materials with both stable
morphology and high performance remains a challenge. To
minimize the disturbance to the p–p stacking of the conju-
gated polymers thus maintaining high charge mobility and
light absorption, the crosslinking units should be small in
size and efficient enough to freeze the BHJ morphology.
Materials
Pd(PPh3)4 (tetrakis(triphenylphosphine)palladium(0)) was
purchased from Frontiers Scientific. 4,6-Dibromothieno[3,4-
b]thiophene-2-carboxylic acid and bis(trimethyltin)-BDT ((4,8-
bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)-
bis(trimethylstannane)) monomers were purchased from
Solarmer Materials. Patterned Indium Tin Oxide (ITO) glass
with a sheet resistance of 10 X/sq was purchased from CSG
Holding (China). PC70BM was purchased from Nano-C.
Nowadays, the copolymerization of electron-rich and elec-
tron-deficient monomers, building a conjugated backbone
with donor–acceptor (D–A) structure, has proven to be an
effective way to tune the optical and electronic properties of
low-bandgap conjugated polymers. The power conversion ef-
ficiency (PCE) of PSCs based on the low-bandgap D–A copol-
ymer has exceeded 7%.23 However, the morphology stability
of PSCs based on D–A copolymers has not been intensively
investigated.
PEDOT:PSS
(poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate) Clevious P VP AI 4083) was purchased from H. C.
Stark. 1,8-Diiodooctane was purchased from Sigma-Aldrich. All
these commercial available materials were used as received
without further purification.
In this study, we have developed novel photocrosslinkable
conjugated polymers with D–A conjugated backbone and
bromine-functionalized side group (see Scheme 1), which
can be crosslinked by UV light with minimal impact on the
packing of conjugated polymers and electronic properties,
thus enabling high-performing and thermally stable PSCs.
Devices based on copolymers with various amounts of Br
units [0% Br-units (PBDTTT), 25% Br-units (PBDTTT-Br25),
and 50% Br-units (PBDTTT-Br50)] were investigated. The
best PSC performance was obtained with photocrosslinked
PBDTTT-Br25. In contrast to the sharp PCE decrease
observed for PBDTTT devices, the PSCs based on PBDTTT-
Br25 demonstrated remarkable long-term thermal stability.
Synthesis
8-Bromooctan-1-ol (1)
To a mixture of octane-1,8-diol (7.3 g, 50 mmol) and tolu-
ene (150 mL), concentrated HBr [18 mL of a 48% (9 M)
aqueous solution, 0.162 mol] was added.24 The heteroge-
neous mixture was stirred and heated at reflux for 36 h.
The reaction mixture was allowed to cool to room temper-
ature, and the phases were separated. The organic layer
was diluted with ether and washed with 1 M NaOH, brine,
and phosphate buffer (3 M, pH 7). Drying (Na2SO4) and
concentration of the organic layer gave a yellow oil, which
was purified by silica gel column chromatography using
ethyl acetate/hexane (1:10) as eluent. After the removal of
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the solvent, 9.29 g (yield 89%) of Compound 1 was
obtained.
filtered through a Soxhlet thimble, which was then subjected
to Soxhlet extraction with methanol and hexane. The solid was
dissolved in chloroform (150 mL) and passed through a col-
umn packed with alumina, Celite, and silica gel. The column
was eluted with chloroform. The combined polymer solution
was concentrated and was poured into methanol.
1H NMR (CDCl3, 400 MHz), d (ppm): 3.65 (s, 1H), 3.62 (t,
2H), 3.40 (t, 2H), 1.87 (quintuple, 2H), 1.54 (d, 2H), 1.31–
1.41 (m, 8H).
8-Bromooctyl 4,6-dibromothieno[3,4-b]thiophene-2-
carboxylate (2)
Polymerization of PBDTTT
Without monomer (2), monomer (3) (0.114 g, 0.25 mmol),
yield 5 210 mg (54.7%).
The 4,6-dibromothieno[3,4-b]thiophene-2-carboxylic acid (5.13
g, 15 mmol), DCC (Dicyclohexylcarbodiimide, 3.7 g), and DMAP
(4-Dimethylaminopyridine, 640 mg) were added to a 250-mL
round-bottomed flask with CH2Cl2 (100 mL).25 8-Bromooctan-
1-ol (15.68 g, 75 mmol) was added to the flask and then stirred
for 20 h under N2 protection. The reaction mixture was poured
to 100 mL of water and extracted with CH2Cl2. The organic
phase was dried by sodium sulfate, and then the solvent was
removed. Column chromatography on silica gel using hexane/
CH2Cl2 5 10/1 yielded the title compound as red solid (3.39 g,
42%).
1H NMR (CDCl3, 400 MHz), d (ppm): 7.50–8.10 (br, 4H), 6.52–
7.16 (br, 2H), 4.42 (br, 2H), 3.96 (br, 4H), 0.88–1.92 (br, 45H).
Polymerization of PBDTTT-Br25
Monomer (2) (0.067 g, 0.125 mmol) and monomer (3)
(0.170 g, 0.375 mmol), yield 5 220 mg (57%).
1H NMR (CDCl3, 400 MHz), d (ppm): 7.50–8.10 (br, 4H),
6.52–7.16 (br, 2H), 4.42 (br, 4H), 4.02 (br, 8H), 3.47 (t, 1H),
0.70–2.49 (br, 89H).
1H NMR (CDCl3, 400 MHz), d (ppm): 7.53 (s, 1H), 4.33 (t,
2H), 3.43 (t, 2H), 1.87 (m, 2H), 1.76 (m, 2H), 1.36–1.47 (m,
8H). 13C NMR (CDCl3, 400 MHz), d (ppm): 162.49, 145.73,
141.22, 140.57, 123.33, 102.42, 97.30, 66.21, 34.06, 32.89,
29.15, 28.75, 28.67, 28.20, 25.95.
Polymerization of PBDTTT-Br50
Monomer (2) (0.133 g, 0.25 mmol) and monomer (3) (0.114
g, 0.25 mmol), yield 5 220 mg (56%).
1H NMR (CDCl3, 400 MHz), d (ppm): 7.50–8.10 (br, 4H),
6.52–7.16 (br, 2H), 4.42 (br, 4H), 4.02 (br, 8H), 3.47 (t, 2H),
0.70–2.49 (br, 88H).
Octyl 4,6-dibromothieno[3,4-b]thiophene-2-carboxylate (3)
The 4,6-dibromothieno[3,4-b]thiophene-2-carboxylic acid
(3.42 g, 10 mmol), DCC (2.47 g), and DMAP (427 mg) were
added to a 250-mL round-bottomed flask with CH2Cl2 (70
mL). 8-Bromooctan-1-ol (6.5 g, 50 mmol) was added to the
flask and then stirred for 20 h under N2 protection. The
reaction mixture was poured to 100 mL of water and
extracted with CH2Cl2. The organic phase was dried by so-
dium sulfate, and then the solvent was removed. Column
chromatography on silica gel using hexane/CH2Cl2 5 10/1
yielded the title compound as red solid (1.62 g, 35.7%).
Measurements
1H and 13C NMR spectra were measured on a Bruker Arx-
400 spectrometer using chloroform-d as a solvent and tet-
ramethylsilane as an internal standard. Absorption spectra
were taken on a Hitachi U-3010 UV–vis spectrophotometer.
The molecular weight of polymers was measured by the Gel
Permeation Chromatography (GPC) method, and polysty-
rene was used as a standard by using tetrahydrofuran
(THF) as eluent. Thermogravimetric Analysis (TGA) meas-
urements were performed on a TA Instruments TGA-2050.
The electrochemical cyclic voltammetry experiments were
conducted on a CHI650D Electrochemical Workstation with
glassy carbon disk, Pt wire, and a Ag/Ag1 electrode as the
working electrode, counter electrode, and reference elec-
trode, respectively, in a 0.1 mol/L tetrabutylammonium hex-
afluorophosphate (Bu4NPF6) acetonitrile solution. The hole
mobilities of the obtained polymers were derived from the
space-charge limited current model with a structure of
ITO/PEDOT:PSS/polymer:PC70BM/Au.26 The optical mi-
croscopy images of the blend films were obtained using an
Olympus Fluoview Fv1000.
1H NMR (CDCl3, 400 MHz), d (ppm): 7.53 (s, 1H), 4.33 (t,
2H), 1.87 (m, 2H), 1.75 (m, 2H), 1.26–1.44 (m, 8H), 0.87–
0.90 (t, 3H). 13C NMR (CDCl3, 400 MHz), d (ppm): 162.51,
145.75, 141.31, 140.60, 123.28, 102.37, 97.30, 66.21,
31.92.06, 29.32, 29.30, 28.71, 26.04, 22.78, 14.24.
General Procedure for the Stille Cross-Coupling
Polymerization
PBDTTT, PBDTTT-Br25, and PBDTTT-Br50 were prepared
with the same procedure as coupling dibromide compounds
with bis(tributylstannyl)-substituted compounds. In a 50-mL
flask, (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b0]dithiophene-
2,6-diyl)bis(trimethylstannane) (0.386 g, 0.5 mmol) and differ-
ent amount of monomer (2) and monomer (3) were dissolved
in toluene (10 mL) and flushed with argon for 10 min. Then,
Pd(PPh3)4 (30 mg) was added into the flask, which was then
flushed by argon for another 20 min. The solution was heated
to reflux and stirred for 15 h under inert atmosphere. Then,
the reactant was cooled to room temperature, and the polymer
was precipitated by the addition of 50 mL of methanol and
Fabrication and Characterization of PSCs
The PSCs were fabricated with the structure of glass/ITO/
PEDOT:PSS/blend layer/Ca/Al. The ITO glass was cleaned by
sequential ultrasonic treatment in detergent, deionized
water, acetone, and isopropanol. The precleaned ITO sub-
strate was treated in an UV–ozone chamber (Ultraviolet
Ozone Cleaner, Jelight Company) for 20 min. PEDOT:PSS
aqueous solution was filtered through a 0.45-lm filter and
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chlorobenzene, and dichlorobenzene at room temperature.
The weight–average molecular weight (Mw) of PBDTTT,
PBDTTT-Br25, and PBDTTT-Br50 are 3.3 3 104, 3.1 3 104,
and 3.0 3 104 with the polydispersity index of 2.3, 2.0, and
1.8, respectively. The TGA curves of the polymers reveal a
relatively high thermal stability as shown in Figure 1. The
initial weight loss (5%) temperatures of PBDTTT, PBDTTT-
Br25, and PBDTTT-Br50 are found to be 327, 335, and 345
ꢀC, respectively. The high thermal stability of the photovol-
taic polymers is closely related to the long-term stability of
the PSCs, which prevents morphological change, deformation,
and degradation of the photoactive layer by heat during the
operation of photovoltaic devices.10
Optical and Electrochemical Properties
As shown in Figure 2(a), the ultraviolet–visible (UV–vis)
absorption spectra of PBDTTT-Br25 and PBDTTT-Br50 in
dilute chloroform solution exhibit three maxima (412, 623,
FIGURE 1 TGA thermograms of the polymers.
spin-coated at 2000 rpm for 60 s on the ITO electrode. Sub-
sequently, the PEDOT:PSS film was baked at 150 ꢀC for 10
min in air. The thickness of the PEDOT:PSS was around 30
nm. Subsequently, the photosensitive blend layer was pre-
pared by spin coating (2000 rpm) the 1,2-dichlorobenzene
solution of polymer and PC70BM (1:1.5 weight ratio, poly-
mer concentration of 10 mg/mL) with 3% volume ratio of
1,8-diiodooctane additive on the ITO/PEDOT:PSS electrode
for 60 s. Photocrosslinking was carried out for PBDTTT-
Br25- and PBDTTT-Br50-based films in a nitrogen-filled
glove box using 254 nm UV light from a low-power hand-
held lamp for 10 min. The photoactive layer thickness of all
these devices was about 90 nm as determined by Ambios
XP-2. Finally, a 10 nm Ca and 100 nm Al was thermally de-
posited in vacuum under a pressure of 5 3 1025 Pa. The
active area of the device is about 4 mm2.
The current density–voltage (J–V) measurement of the devi-
ces was conducted on a computer-controlled Keithley 236
Source Measure Unit. Device characterization was done in a
glove box under simulated AM 1.5 G irradiation (100 mW/
cm2) using a xenon lamp-based solar simulator (from New-
port). The input photon to converted current efficiency
(IPCE) was measured using a Stanford Research Systems
model SR830 DSP lock-in amplifier coupled with a WDG3
monochromator and 500-W xenon lamp. The light intensity
at each wavelength was calibrated with a standard single-
crystal Si photovoltaic cell. The IPCE measurement was per-
formed under ambient atmosphere at room temperature.
RESULTS AND DISCUSSION
Synthesis and Structural Characterization
The obtained polymers, PBDTTT, PBDTTT-Br25, and
PBDTTT-Br50, were easily prepared by Stille cross-coupling
method as shown in Scheme 1 and identified by 1H NMR
spectroscopy. The synthesized polymers are easily soluble in
common organic solvents, such as chloroform, toluene,
FIGURE 2 Absorption spectra of the polymers (a) in diluted
chloroform solution and (b) in solid-state film as cast.
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From solution state to solid film, a red shift of about 10 nm
can be observed from the absorption peaks of PBDTTTs [Fig.
2(b)]. These changes indicate the extension of the p-conjuga-
tion lengths and the interchain interaction between the back-
bones in the films.27 Furthermore, no major differences can
be observed between the absorption spectra of the three
polymers, indicating that the addition of bromine units to
the polymer does not significantly affect its optical proper-
ties in solid films.
Electrochemical cyclic voltammetry is used to measure the mo-
lecular energy levels of the polymers.28 The cyclic voltammo-
gram curves of PBDTTT, PBDTTT-Br25, and PBDTTT-Br50 are
shown in Figure 3. Based on the onset potentials, the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) levels of the control polymer,
PBDTTT, are estimated to be 25.02 and 22.92 eV, respectively.
After functionalized with bromine groups at the side chains,
the LUMO levels for PBDTTT-Br25 and PBDTTT-Br50 are
almost unchanged, whereas the HOMO levels are slightly
changed to 25.06 eV for PBDTTT-Br25 and 25.04 eV for
PBDTTT-Br50, indicating that the addition of bromine units to
the polymer does not significantly affect p–p coupling between
polymer backbones. These findings are in good agreement
with the aforementioned optical absorption studies.
FIGURE 3 Cyclic voltammograms of PBDTTT, PBDTTT-Br25,
and PBDTTT-Br50 drop-cast on glassy carbon working elec-
trode in 0.1 M Bu4NPF6 in CH3CN at a scan rate of 50 mV/s.
and 673 nm) between 400 and 700 nm, which are almost
identical to that of PBDTTT. It illuminates that the conjuga-
tion length of the main chain in the solution was not affected
by the introduction of bromine groups at the side chains.
FIGURE 4 Photocrosslinking behaviors of the polymers. Absorption spectra of (a) PBDTTT, (b) PBDTTT-Br25, and (c) PBDTTT-Br50
films under UV treatment for 5 min before and after flushing with chlorobenzene at 2000 rpm for 1 min. (d) Insoluble film fraction
as a function of UV exposure times is measured for PBDTTT, PBDTTT-Br25, and PBDTTT-Br50.
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absorption spectra of as-cast and photocrosslinked films
are observed, indicating that the photocrosslinking process
does not significantly affect the optical properties of the
solid-state polymer.
To calculate the insoluble fraction after photocrosslinking,
the irradiated polymer films were flushed with chloroben-
zene by spin coating at 2000 rpm for 1 min. We compared
the integrated optical absorbance of the films before and af-
ter washing with chlorobenzene, as shown in Figure 4(b,c).
After 10 min of UV irradiation, the films of PBDTTT-Br25
and PBDTTT-Br50 are insoluble, whereas the control
PBDTTT is not photocrosslinked at all. The trend of photo-
crosslinking behavior as a function of Br content is clearly
evident in Figure 4(d).
Photovoltaic Properties
The performance of the corresponding PSC devices was
investigated using the ITO/PEDOT:PSS/polymer:PC70BM/Ca/
Al device architecture. Photocrosslinking was carried out for
PBDTTT-Br25- and PBDTTT-Br50-based films after spin coat-
ing in a nitrogen-filled glove box using 254 nm UV light
from a low-power hand-held lamp for 10 min. Figure 5(a)
shows the J–V characteristics of devices made from PBDTTT,
crosslinked PBDTTT-Br25, and crosslinked PBDTTT-Br50
copolymers, which achieve PCE (average of 10 devices) of
4.26, 5.17, and 4.48%, respectively. The devices based on
PBDTTT-Br25 and PBDTTT-Br50 without photocrosslinking
show similar Voc, Jsc, FF, and PCE as PBDTTT-based devices,
indicating that introduction of bromine groups at the side
chains does not affect the electron properties of PBDTTT.
The photovoltaic performances of the devices are summar-
ized in Table 1. Because of the small difference of the HOMO
levels (Fig. 3), all devices based on PBDTTT, PBDTTT-Br25,
and PBDTTT-Br50 show similar (around 0.60 V) open-circuit
voltage (Voc), which is determined by the gap between the
HOMO level of the polymer and the LUMO level of the fuller-
ene in the blend film.29,30 In comparison with PBDTTT-based
devices (12.97 mA/cm2), crosslinked PBDTTT-Br25- and
PBDTTT-Br50-based devices show higher short-circuit cur-
rent density (Jsc) of 14.88 and 14.97 mA/cm2, respectively.
The improvement of Jsc should attribute to the enhanced
hole motilities (Table 1) via crosslinking between the side
chains, in which the CAH bond energies in alkyl chains are
weaker than those in the conjugated rings,31 thereby pre-
serving the main polymer chain backbone integrity and poly-
mer conjugation length, increasing transverse hole
conductivity in the solid films.32 In comparison with
PBDTTT-Br25, PBDTTT-Br50 with more Br units of 50 mol
% shows similar Voc and Jsc but lower FF of 50.0%. The pos-
sible reason should attribute to the higher crosslinking sites
in PBDTTT-Br50 to create higher conformational defects,
which give rise to localized charge traps.33
FIGURE 5 (a) J–V curves of the polymer solar cells based on
PBDTTT, PBDTTT-Br25, and PBDTTT-Br50 under the illumina-
tion of AM 1.5 G, 100 mW/cm2. (b) Absorbance of the blend
films and IPCE curves of the corresponding PSCs.
Photocrosslinking Behavior of the Polymers
After spin coating of the active layer, photocrosslinking
was carried out in a nitrogen-filled glove box using 254
nm UV light from a low-power hand-held lamp with expo-
sure times ranging from 0 to 30 min. The photocrosslink-
ing behaviors of the polymers are shown in Figure 4. As
given in Figure 4(a), the control polymer PBDTTT without
crosslink group shows no solvent resistance, and the film
retains less than 3% of the initial integrated optical ab-
sorbance after flushing with chlorobenzene at 2000 rpm
for
1 min. In contrast, the functionalized polymers,
PBDTTT-Br25 and PBDTTT-Br50, which were crosslinked
via a radical mechanism initiated by the photochemical
cleavage of the C-Br bonds under deep-UV irradiation,19
show excellent solvent resistance with UV treatment for 5
min, as shown in Figure 4(b,c). No major differences in the
Figure 5(b) shows the IPCE of the PSCs based on the
PBDTTT, PBDTTT-Br25, and PBDTTT-Br50. The shape of the
IPCE curves of the devices based on the three polymers is
very similar to their absorption spectra, which indicates that
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respectively. The major difference between the three poly-
mers in IPCE curves located at the region of 500–800 nm
might be attributed to the difference of the charge mobility
and the absorbance of the films.
Long-Term Thermal Stability of the Devices
To examine the use of photocrosslinkable conjugated poly-
mers for enhancing the stability of BHJ devices, devices
made from PBDTTT and PBDTTT-Br25 (UV exposed for 10
min) undergoing postproduction annealing at 150 ꢀC in
nitrogen-filled glove box for 0.5–24 h were compared. The J–
V output characteristics and the evolution of device perform-
ance with annealing time for the two systems are shown in
Figure 6(a,b), respectively. At early stage, an increase in the
Voc is observed for both devices after annealing at high tem-
perature. The increased Voc should attribute to that thermal
annealing potentially causes the morphological rearrange-
ment of the fullerene molecules adjacent to the polymer
chains, which induces the change in the energy of the inter-
facial charge-transfer states between the polymer and fuller-
ene.34,35 However, the control device made from PBDTTT
without crosslinking units undergoes a significant decrease
in Jsc and Voc after long-time annealing, which results in a
sharp decrease (60%) in PCE with respect to the initial val-
ues. Conversely, the Jsc and Voc of PBDTTT-Br25 device
remain 80% and 95% of its initial value even after 24 h of
annealing at 150 ꢀC. This result clearly shows that the pho-
tocrosslinking is a promising approach for thermally stable
high-performance devices.
To further understand the extreme contrast in thermal
stabilities observed above, we examined the active layer
morphology via optical microscopy. Figure 7(a–c) shows the
optical microscopy images of as-prepared polymer:PC70BM
(1:1.5) blend film for PBDTTT, PBDTTT-Br25, and PBDTTT-
Br50, respectively. Obviously, all the films are uniform with-
out fullerene aggregation, indicating no large-scale phase
separation between the polymer and the fullerene. The mi-
croscopy images for the blended films without UV crosslink-
ing after 24 h of annealing at 150 ꢀC are shown in Figure
7(d–f) for PBDTTT, PBDTTT-Br25, and PBDTTT-Br50 blend
films, respectively. These images demonstrate that thermal
annealing induces the formation of many dot-like PCBM
crystals that are near 1 mm in diameter in all blend film,
indicating that there is severe macrophase separation
FIGURE 6 (a) The J–V output characteristics and (b) the evolu-
tion of device performance with annealing time for the
PBDTTT- and PBDTTT-Br25-based devices.
all the absorption of the polymers contributed to the photo-
voltaic conversion. Furthermore, the IPCE curves can be sim-
ply divided into two regions of 300–500 nm and 500–800
nm due to the contribution of PC70BM and polymers,
TABLE 1 Photovoltaic Performance and Hole Mobility of PBDTTT-, PBDTTT-Br25-, and PBDTTT-Br50-Based PSCs
Polymer
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
Mobility [cm2/(V s)]
PBDTTT
0.59
0.61
0.61
0.60
0.60
12.97
13.13
14.88
12.85
14.97
55.7
53.8
57.0
53.5
50.0
4.26
4.31
5.17
4.13
4.48
1.56 3 1024
1.87 3 1024
4.67 3 1024
1.68 3 1024
3.74 3 1024
PBDTTT-Br25a
PBDTTT-Br25b
PBDTTT-Br50a
PBDTTT-Br50b
a
b
Without UV crosslinking.
UV crosslinking for 10 min.
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FIGURE 7 Optical microscopy images of polymer:PC70BM (1:1.5) blend film for PBDTTT (a, d, and g), PBDTTT-Br25 (b, e, and h),
and PBDTTT-Br50 (c, f, and i) as prepared, as prepared-after thermal annealing, and UV treated after thermal annealing,
respectively.
driven by the crystallization of the polymer and PCBM mol-
ecule. Figure 7(g) shows the morphology of control sample
PBDTTT blend film with UV treatment after 24 h of anneal-
ing at 150 ꢀC, in which severe phase separation of PC70BM
crystals is observed. Uncrosslinked films undergo macro-
phase separation on further thermal annealing, and thus
their performance is quickly degraded. In contrast, the opti-
cal micrograph of the PBDTTT-Br25 [Fig. 7(h)] and
PBDTTT-Br50 [Fig. 7(i)] blend films crosslinked by UV
treatment show homogeneous films with only a few dark
PC70BM crystals. These optical micrographs confirm that
the photocrosslinking of the polymers dramatically
suppresses phase segregation, thus producing stable per-
formance in solar cell devices.
CONCLUSIONS
Novel photocrosslinkable low-bandgap D–A copolymers,
PBDTTT-Br25 and PBDTTT-Br50, for using in PSCs were
developed. The UV-sensitive Br unit attached at the side
chain does not appear to disturb the p–p stacking of the
backbone. Careful adjust of the photocrosslinking Br content
in the side chain was found to be critical in order to achieve
optimal device performance. The formation of large fullerene
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13 S. Miyanishi, K. Tajima, K. Hashimoto, Macromolecules
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This work was supported by the National Natural Science Foun-
dation of China (nos. 21004019, 51173040, 91023039, and
21021091), the Ministry of Science and Technology of China
(863 project, no. 2011AA050523), and the Chinese Academy of
Sciences. Z. Tan thanks the financial support from the Beijing
NOVA Program (no. 2010B038), Program for New Century
Excellent Talents in University (NCET-12-0848), SRFDP (no.
20100036120007), and Fundamental Research Funds for the
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