Donor-acceptor-integrated conjugated polymers based on carbazole[3,4-c:5,6- c]bis[1,2,5]thiadiazole with tight π-π Stacking for photovoltaics

Article abstract of DOI:10.1002/pola.26438

An original strategy to construct a new donor-acceptor (D-A)-integrated structure by directly imposing "pull" unit on the "push" moiety to form fused ring architecture has been developed, and poly{N-alkyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole-alt-thiophene} (PCBTT) with D-A-integrated structure, in which two 1,2,5-thiadiazole rings are fixed on carbazole in 3-, 4- and 5-, 6-position symmetrically and thiophene is used as bridge, has been synthesized. The interaction between pull and push units has fine tuned the HOMO/LUMO energy levels, and the resulting copolymer covers the solar flux from 300 to 750 nm. The interaction between pull and push units is worth noting that due to the fused five rings inducing strong intermolecular interaction, an extremely short π-π stacking distance of 0.32 nm has been achieved for PCBTT both in powder and solid states. This is the shortest π-π stacking distance reported for conjugated polymers. Additionally, an obvious intramolecular charge transfer and energy transfer from donor units to acceptor units have been detected in this D-A integration. A moderate-to-high open-circuit voltage of ～0.7 V in PCBTT:[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) (w/w = 1/2) solar cells is achieved due to the low-lying HOMO energy level of PCBTT.

Full text of DOI:10.1002/pola.26438

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Donor–Acceptor-Integrated Conjugated Polymers Based on
Carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole with Tight p–p
Stacking for Photovoltaics
Daijun Zha,1 Lie Chen,1,2 Feiyan Wu,1 Hongming Wang,2 Yiwang Chen1,2
1Institute of Polymers, Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
2Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
Correspondence to: Y. Chen (E-mail: ywchen@ncu.edu.cn)
Received 17 August 2012; accepted 17 October 2012; published online 27 November 2012
DOI: 10.1002/pola.26438
ABSTRACT: An original strategy to construct
a
new donor–
0.32 nm has been achieved for PCBTT both in powder and solid
states. This is the shortest p–p stacking distance reported for
conjugated polymers. Additionally, an obvious intramolecular
charge transfer and energy transfer from donor units to
acceptor units have been detected in this D–A integration. A
moderate-to-high open-circuit voltage of ꢀ0.7 V in PCBTT:[6,6]-
phenyl-C61 butyric acid methyl ester (PCBM) (w/w ¼ 1/2) solar
acceptor (D–A)-integrated structure by directly imposing ‘‘pull’’
unit on the ‘‘push’’ moiety to form fused ring architecture has
been developed, and poly{N-alkyl-carbazole[3,4-c:5,6-c]bis[1,2,5]-
thiadiazole-alt-thiophene} (PCBTT) with D–A-integrated struc-
ture, in which two 1,2,5-thiadiazole rings are fixed on carbazole
in 3-, 4- and 5-, 6-position symmetrically and thiophene is used
as bridge, has been synthesized. The interaction between pull
and push units has fine tuned the HOMO/LUMO energy levels,
and the resulting copolymer covers the solar flux from 300 to
750 nm. The interaction between pull and push units is worth
noting that due to the fused five rings inducing strong intermo-
lecular interaction, an extremely short p–p stacking distance of
cells is achieved due to the low-lying HOMO energy level of
C
V
PCBTT.
2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2013, 51, 565–574
KEYWORDS: conjugated polymers; luminescence; molecular
stack; photovoltaics; synthesis
INTRODUCTION Over the past few decades, as a renewable
resource while minimizing detrimental effects on the envi-
ronment, photovoltaic cells that harvest energy directly from
sunlight have received a great deal of attention from both
academic and industrial laboratories.1,2 Bulk heterojunction
(BHJ) polymer solar cells (PSCs) based on composites of an
electron-donating semiconducting conjugated polymer and
an electron-accepting fullerene offer significant promise over
conventional inorganic materials for the numerous advan-
tages in terms of low cost, lightweight, ease of processing,
large area, and flexibility.2 The power conversion efficiencies
(PCEs) of BHJ solar cells of more than 8–9% have been
achieved in research laboratories by considerable efforts;
however, there is still a long way to catch up with the inor-
ganic solar cells or to achieve for commercial applications.3–8
synthesis of narrow bandgap donor–acceptor (D–A)-conju-
gated polymers with alternating ‘‘push’’ and ‘‘pull’’ units
along the polymer main chain or pull units as pendant cova-
lently linked to polymer backbone (Scheme 1), which can
easily tune the bandgap and energy levels as a result of
intramolecular charge transfer (ICT) from the push to the
pull moieties.9 Nowadays, 2,7-carbazole, 2,7-fluorene, and
benzodithiophene (BDT) are the most popular push units,
whereas 2,1,3-benzothiadiazole (BT), diketopyrrolopyrrole-
dione (DPP), and thienothiophene (TT) are the most efficient
pull units; many excellent D–A copolymers have been synthe-
sized by using different moieties, for example, PCDTBT,10
PBDTTT,11 and PTPTPTDPP.12
Although the bandgap and energy levels can be adjusted by
D–A-conjugated polymer through intramolecular interaction,
the charge-carrier mobilities in polymer devices are gener-
ally low due to the poor packing and the lack of macroscopic
order of those materials by intermolecular interactions.13
Polymer molecular designing for better intermolecular inter-
actions has been paid more attention to promote p–p stack-
ing and increased crystallinity of the polymer, which is
expected to enhance the charge-carrier mobility.14–17 Some
General drawbacks of the BHJ are the following two main
aspects: on one hand, the donor polymer in the photoactive
layer has a relatively large bandgap and only harvests a
small portion of the solar spectrum; on the other hand, the
transport and collection of charges in a disordered nanoscale
blend can be hindered by phase boundaries and discontinu-
ities. Recently, most polymer designing studies focus on the
Additional Supporting Information may be found in the online version of this article.
C
V
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thiadiazole moieties and a functional thiophene bridge,
whereas the still maintaining low-lying HOMO energy level
offered moderate-to-high open circuit voltage (Voc) of ꢀ0.7 V.
Particularity, an extremely short p–p stacking distance of 0.32
nm, the shortest distance for conjugated polymer reported to
date, was achieved due to pentafused ring inducing a strong
intermolecular interaction in polymer PCBTT at solid state. In
addition, obvious ICT and energy transfer from donor to
acceptor have been detected in this D–A integration.
EXPERIMENTAL
Materials
o-Phenylenediamine, n-butyllithium, Al (99.998%), LiF
(99.99%), sodium hydride, tetrakis(triphenylphosphine)palla-
dium, bromododecane, PC61BM (99.9%), and other materials
were purchased from Alfa or Aldrich and without further pu-
rification. Tetrahydrofuran (THF) was dried over sodium.
Other chemicals were obtained from Shanghai Reagent and
used as received. Indium tin oxide (ITO) glass was purchased
from Delta Technologies, and PEDOT:PSS (Baytron PAl4083)
was obtained from Bayer.
SCHEME 1 Illustration of the D–A integration structure, D–A
alternating structure, acceptor-pended D–A structure, and the
representational examples of PCBTT, PCDTBT,10 and PCzDCN.32
approaches like postprocessing annealing, controlling the
regioregularity, spacing, and size of the solubilizing side
chains have been developed to enhance the crystallinity.18
However, the most efficient way to enhance the intrinsic
local packing of materials is to increase the planarity of the
polymer backbone, which in turn can generate tight p–p
stacking.19 Because the fused ring structures are much more
flat than the conjugated ‘‘single’’ bond,20 many moieties with
bifused, trifused, and tetrafused ring structures have been
incorporated into polymers to obtain high charge-carrier
mobility.21,22
Characterization
1H NMR and 13C NMR spectra were recorded in deuterated
solvents on a Bruker ADVANCE 400 NMR spectrometer and a
Bruker ADVANCE 600 NMR spectrometer. 1H NMR chemical
shifts are reported in parts per million downfield from tetra-
methylsilane reference using the residual protonated solvent
as an internal standard. Mass spectra [matrix-assisted laser
desorption–ionization time-of-flight mass spectrometry
(MALDI-TOF-MS)] were determined on a Bruker BIFLEX III
mass spectrometer. Molecular weights of the polymers were
determined using a Waters 2410 gel permeation chromato-
graph (GPC) with a refractive index detector in THF using a
calibration curve of polystyrene standards. Thermogravimet-
ric analysis (TGA) was performed on a TA-600 for thermal
analysis at a heating rate of 10 ꢁC/min under nitrogen with
a sample weight of 8–10 mg. Differential scanning calorime-
try (DSC) was performed on Shimadzu DSC-60. UV–vis
absorption spectra were measured with Perkin Elmer
Lambda 750 UV–vis spectrophotometer. Cyclic voltammetry
(CV) was performed with a Zahner IM6e electrochemical
workstation. The X-ray diffraction (XRD) study of the sam-
ples was carried out on a Bruker D8 Focus X-ray diffractome-
ter operating at 30 kV and 20 mA with a copper target (k ¼
1.54 Å) and at a scanning rate of 1ꢁ/min. Two-dimensional
wide-angle X-ray scattering (2D WAXS) pattern was meas-
ured on Anton Paar SAXSess mc2. Atomic force microscopic
(AFM) images were measured on a Nanoscope III A (Digital
Instruments) scanning probe microscope using the tapping
mode. Current–voltage (J–V) characteristics were recorded
using a Keithley 2400 Source Meter in the dark and under
100 mW/cm2 simulated AM 1.5 G irradiation (Abet Solar
Simulator Sun2000). External quantum efficiencies (EQEs)
were detected under monochromatic illumination (Oriel Cor-
nerstone 260 1/4 m monochromator equipped with Oriel
70613NS QTH lamp), and the calibration of the incident light
was performed with a monocrystalline silicon diode.
To simultaneously obtain a polymer with optimal bandgap,
suitable energy levels, and ideal molecular conformation for
intramolecular and intermolecular interaction, we tuned the
traditional design concept of D–A alternating copolymer and
acceptor-pended D–A copolymer (Scheme 1) and originally
developed a new strategy to build a new D–A-integrated
structure by directly attaching the pull unit on the push moi-
ety to form fused ring architecture (Scheme 1). The high pla-
narity of D–A integration could ensure a close p–p stacking
to facilitate charge transportation, and the interaction
between pull and push units could fine tune the HOMO/
LUMO energy levels to fit the solar flux. It is worth noting
that ICT in such fused rings from D units to A units may ex-
hibit some particular and interesting characteristics relative
to those in the other D–A types.
Herein, we fuse a typical donor 2,7-carbazole as the push moi-
ety with low-lying HOMO energy level23–28 and the most
promising acceptor BT29 as the efficient pull moiety together
to create D–A-integrated architecture N-alkyl-carbazole[3,4-
c:5,6-c]bis[1,2,5]thiadiazole (CBT), in which two 1,2,5-thiadia-
zole rings are fixed on carbazole in 3-, 4- and 5-, 6-position
symmetrically. Using thiophene as bridge, its corresponding
polymer poly{N-alkyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadia-
zole-alt-thiophene} (PCBTT) is prepared successfully (see
Scheme 1). Polymer PCBTT obtained narrow bandgap origi-
nating from the strong electron-withdrawing capability of two
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Synthesis of Monomer and Polymer
1H NMR (400 MHz, CDCl3, ppm):
C6HBr2N3O2S (336.82): Calcd.: C, 21.26; H, 0.30; Br, 47.15; N,
12.40; S, 9.46; O, 9.44; Found: C, 21.36; H, 0.44; Br, 47.05; N,
11.98; S, 9.26; O, 9.64.
d
¼
8.279 (s, H).
The detailed synthesis of the novel monomer CBT is
described in Scheme 2. The copolymer PCBTT is synthesized
by CBT and bis(trimethylstannane)thiophene via Stille cross-
coupling polymerization using Pd(PPh3)4 catalyst. The corre-
sponding homopolymer PCBT is synthesized using Ni(COD)2
catalyst. The polymer is end-capped with bromobenzene and
phenylboronic acid, and the alkyl chain is chosen to enhance
the solubility of the resulting polymer. Because of the too
strong intermolecular interaction, the polymer PCBTT with
shorter alkyl chain (e.g., hexyl pendant) hardly dissolved in
common solvents, and thus, further investigation was not
carried out. After the number of carbon atom of alkyl chain
increased to 12, the solubility of the polymer has been
greatly improved, and the polymer can be dissolved in chlo-
roform and chlorobenzene; however, the solubility is still
poor with respect to P3HT. All compounds were character-
Synthesis of 4,7-Dibromo-2,2-dinitrobibenzothiadiazole (4)
Compound 3 (5 g, 15.0 mmol) and 5 g of Cu (powder, 37.6
mmol) were added to a round-bottomed flask purged with
N2 gas. Dimethylformamide (100 mL), degassed with N2
prior to use, was injected into the flask. The reaction was
stirred under reflux for 2 h. The cooled mixture was
extracted with dichloromethane and deionized water. The or-
ganic layer was collected, dried over anhydrous MgSO4, and
concentrated. Finally, the crude compound was purified by
column chromatography [silica gel; eluent: hexane/dichloro-
methane 10/1 (v/v)] and isolated as a yellow crystal (5.8 g,
70% yield).
1H NMR (400 MHz, CDCl3, ppm): d ¼ 8.934 (s, 2H).
1
ized by H NMR, 13C NMR, and MALDI-TOF-MS (see Support-
C12H2Br2N6O4S2 (515.79): Calcd.: C, 27.82; H, 0.39; Br, 30.84;
ing Information Figs. S1–S8).
N, 16.22; S, 12.38; O, 12.35; Found: C, 26.36; H, 0.54; Br,
30.05; N, 16.98; S, 13.26; O, 13.64.
Synthesis of 4,7-Dibromo-5-nitrobenzothiadiazole (3)
The nitration mixture (60 mL) was added dropwise very
slowly to the sulfuric acid (30 mL) solution of 4,7-dibromo-
benzothiadiazole (15.0 g), and the mixture was reacted
under 0 ꢁC. After the reaction mixture was left to stand at
room temperature for 0.5 h, it was poured into 500 mL of
ice water. The precipitate was filtered off and washed with
water for three times. The product was recrystallized with
ethanol to give Compound 3 (7.5 g, 0.02 mol; 29.0% yield).
Synthesis of 4,7-Dibromo-2,2-diaminobibenzothiadiazole (5)
Compound 4 (5 g, 9.69 mmol), tin powder (20 equiv, 5 g),
20 mL of hydrochloric acid, and 100 mL of alcohol were
added to a round-bottomed flask. The reaction was stirred at
80 ꢁC for 4 h. Then the mixture was poured into cold 5%
NaOH solution (250 mL) and extracted with diethyl ether for
three times. The ether phase was washed with NaHCO3 solu-
tion, dried over anhydrous MgSO4, and concentrated. The
SCHEME 2 Synthetic route for the monomer CBT and copolymer PCBTT.
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TABLE 1 Characterization of PCBTT
Mn (kDa)a
PDIa
2.3
Sol. kmax (nm)b
Film kmax (nm)c
359, 602
Eogpt (eV)d
E
(V)e
HOMO (eV)f
LUMO (eV)g
onset
ox
14
352, 547
1.65
0.9
ꢂ5.30
ꢂ3.65
a
e
f
Number–average molecular weight and polydispersity index.
Solution absorption in chloroform.
Film absorption spin-cast from chlorobenzene solution.
Optical energy gap estimated from the onset of UV curve measured in
CV measurement of thin film versus SCE electrode.
b
c
onset
ox
Estimated from HOMO ¼ ꢂ(E
þ 4.4 eV).
g
Estimated from LUMO ¼ HOMO þ Eg.
d
film.
ꢁ
residue was purified by column chromatography [silica gel;
eluent: hexane/dichloromethane 10/1 (v/v)] and isolated as
a yellow powder (3.32 g, 50% yield).
stirred at 130 C for 12 h. Then the mixture was poured into
cold water, filtered, and collected as filter cake.
Synthesis of N-Dodecyl-2,7-dibromo-dithiadiazole
[3,4-b;7,8-b0]carbazole
1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.741 (s, 2H), d ¼
5.774 (s, 3.86H). C12H6Br2N6S2 (458.15): Calcd.: C, 31.46; H,
1.32; Br, 34.88; N, 18.34; S, 14.00; Found: C, 31.36; H, 1.06;
Br, 35.05; N, 18.98; S, 13.55.
Compound 6 (1.32 g, 4.10 mmol) in 10 mL of anhydrous
THF was mixed with sodium hydride (0.20 g, 8.20 mmol) in
10 mL of anhydrous THF under an argon atmosphere. The
ꢁ
Synthesis of 2,7-Dibromo-dithiadiazole
[3,4-b;7,8-b0]carbazole (6)
Compound 5 (2 g, 4.38 mmol) and 50 mL of phosphoric acid
were added to a round-bottomed flask. The reaction was
reaction mixture was stirred at 68 C for 1 h and then added
to bromododecane (3.00 g, 12.30 mmol) in 50 mL of anhy-
drous THF. The reaction mixture was refluxed for 18 h. After
cooling to room temperature, the solvent was removed
FIGURE 1 The UV–vis absorption spectrum of PCBTT: (a) in dilute (1 ꢃ 10ꢂ6 mol/L) chloroform solutions and as film of PCBT and
PCBTT spin-cast from chlorobenzene solution; (b) the simulated UV–vis absorption spectra of dimer of CBT and CBTT by TD-SCF/
DFT-D/B3LYP/6-31G*; and (c) as cast film and after annealed at 80, 110, and 150 ꢁC.
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FIGURE 2 The optimized electronic cloud structure, HOMO and LUMO location of carbazole, CBT, repeat unit, and polymer
oligomer (n ¼ 3) imitated by DFT-D/B3LYP/6-31G*.
under reduced pressure, and the oil phase was extracted
with dichloromethane (100 mL), washed successively with
water, and then dried over anhydrous MgSO4. The residue
was purified by column chromatography with 1:5 (v/v)
dichloromethane/petroleum ether as the eluent to harvest
the yellow green crystalline compound (1.40 g, 62% yield).
86% yield). GPC: Mn ¼ 14,000, polydispersity index (PDI) ¼
2.3.
The polymer PCBTT with hexyl side chain is hardly dissolved
in common solvents, and thus, further investigation was not
carried out.
1H NMR (400 MHz, CDCl3) d ¼ 8.34 (br, 2H), d ¼ 8.01 (br,
2H), d ¼ 4.67 (br, 2H), d ¼ 2.04 (br, 4H), d ¼ 0.89–1.87 (m,
20H).
1H NMR (400 MHz, CDCl3, ppm): d ¼ 8.052 (s, 2H), d ¼
4.494–4.459 (t, 2H), d ¼ 1.950–1.933 (d, 2H), d ¼ 1.236–
1.379 (m, 18H), d ¼ 0.848–0.881 (t, 3H). C24H27Br2N5S2
(607.01): Calcd.: C, 47.30; H, 4.47; Br, 26.22; N, 11.49; S,
10.52; Found: C, 46.36; H, 5.04; Br, 25.95; N, 11.98; S, 10.26.
RESULTS AND DISCUSSION
The number–average molecular weight of PCBTT achieves 14
kDa with PDI of 2.3 measured by GPC (Table 1), but that of
PCBT only is 11 kDa with PDI of 2.0. According to the TGA
(see Supporting Information Fig. S9), the polymer exhibits
good thermal stability with degradation temperature (Td)
Synthesis of the Copolymer PCBTT
Monomer N-dodecyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadia-
zole (621 mg, 0.5 mmol) and 2,5-bis(trimethylstannyl)thio-
phene (367 mg, 0.5 mmol) were added in 10 mL of dry tolu-
ene in a flask under argon. The reaction mixture was purged
with argon for 15 min. The catalyst Pd2(dba)3 (14 mg, 3%)
and ligand P(o-Tol)3 (28 mg) were added quickly under a
stream of argon, and then the reaction mixture was purged
with argon again for 15 min. Subsequently, the reaction mix-
ture was heated to reflux for 48 h with stirring. Next, the
reaction mixture was cooled to ambient temperature and
poured into methanol. The polymer was precipitated and
then collected by filtration. After drying in the vacuum dry-
ing oven, the polymer was dissolved in toluene and purified
by silica-gel chromatography with toluene eluent. The poly-
mer was again precipitated using methanol and dried.
Finally, PCBTT was obtained as a dark green solid (640 mg,
ꢁ
above 200 C and just 3.5% weight loss at 300 ꢁC, which is
an important characteristic for device fabrication and appli-
cation. DSC performed on copolymer does not reveal any no-
ꢁ
ticeable thermal degradation signals below 200 C.
The ultraviolet and visible (UV–vis) absorption of the homo-
polymer PCBT (homopolymer of CBT monomer) shows two
absorption bands at 300–400 nm and 400–600 nm [see Fig.
1(a)]. For copolymerizing with thiophene, the absorption
band of polymer PCBTT has been further fine tuned to cover
almost the whole visible solar spectrum from 300 to 750 nm
with two absorption bands and the more red-shifted low-
energy band that implies the stronger ICT effect.30 Through
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shoulder at 660 nm, which can be attributed to the stronger
intermolecular packing in the solid state caused by their pla-
nar and rigid backbone.31 In addition, different from the con-
spicuous bathochromic shift for most conjugated polymer on
annealing, no significant red shift is observed, and the inten-
sity of peak around 600–750 nm only slightly increased
along with the film annealing temperature enhanced [shown
in Fig. 1(c)], indicating that the inherent strong intermolecu-
lar interaction has existed in PCBTT drop-cast film, irrespec-
tive of thermal treatment.
The optical bandgap of PCBTT is determined from the UV–vis
absorption onsets in the solid state to be 1.65 eV, which is
consistent with the most intense solar photon flux well. In
particular, the bandgap of PCBTT is much lower than that of
homolog PCDTBT (1.9 eV), a famous D/A alternating copoly-
mer based on carbazole and DTBT building blocks (Scheme
1).9 It is further inferred that a more efficient ICT happened
in the unique D–A-integrated structure. As estimated by CV
(Supporting Information Fig. S11), PCBTT possesses low-lying
HOMO level of ꢂ5.30 eV to ensure relatively high Voc and suit-
able LUMO energy of ꢂ3.65 eV for charge separation from
polymer to [6,6]-phenyl-C61 butyric acid methyl ester
(PCBM), and the map of HOMO and LUMO energy levels of the
materials are presented in Supporting Information Figure
S12. The electronic properties and energy levels of carbazole,
CBT, repeat unit, and polymer oligomer (n ¼ 3) are further
evaluated using DFT at the B3LYP/6-31G* level of theory; pic-
torial representations of the frontier molecular orbitals of
materials are shown in Figure 2. The trend of the calculated
HOMO and LUMO energy levels correlates well with that
obtained from the electrochemical measurement. With respect
to the unmodified polycarbazole, D–A integrations show quiet
lower LUMO and HOMO levels and lead to much narrower
bandgap. The electron density of HOMO and LUMO of CBT,
repeat unit, and polymer oligomer is fully localized on the
whole molecule, as explained by high coplanarity of the struc-
tures. Thus, ICT can also be understood by the distribution
difference of electron density in HOMO and LUMO orbits.
FIGURE 3 Photoluminescence emission spectrum of the
PCBTT: (a) chloroform solution with concentration of 8.12 ꢃ
10ꢂ7, 1.62 ꢃ 10ꢂ6, 3.25 ꢃ 10ꢂ6, 6.50 ꢃ 10ꢂ6, 8.12 ꢃ 10ꢂ6, 3.25 ꢃ
10ꢂ5, and 3.25 ꢃ 10ꢂ5 mol/L (6.50 ꢃ 10ꢂ5, 1.30 ꢃ 10ꢂ4, and 2.60
ꢃ 10ꢂ4 mol/L in the inset; arrow on the direction of concentra-
tion enhancement and (b) as thin film (as-cast from chloroben-
zene) and after annealed at 80, 110, and 150 ꢁC.
UV–vis absorption spectra simulated by TD-SCF/DFT-D/
B3LYP/6-31G*, as shown in Figure 1(b), for both polymers,
the former band around 300–350 nm is mainly assigned to
the charge transfer (CT 1) from the HOMO orbit to the fifth
excited state, and the other low-energy band around 500–
600 nm is the charge transfer (CT 2) from the HOMO orbit
to the LUMO orbit. When compared with homopolymer
PCBT, the CT 2 for PCBTT is more easily surmised from the
red-shifted band and the almost doubled oscillator strengths,
which benefit from the more coplanar backbone accompany-
ing the incorporation of thiophene bridge. As seen from the
relating information of calculation in Supporting Information
Figure S10, an obvious ICT from benzene ring to thiadiazole
has been detected in this D–A integration.
When compared with PCBTT in solution, PCBTT film (spin-
coated from chlorobenzene solution) shows a significant red
shift (ꢀ55 nm) and an obvious enhanced absorption
FIGURE 4 XRD diffraction patterns of the PCBTT film and film
annealed at 80 and 150 ꢁC.
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FIGURE 5 Two-dimensional wide-angle X-ray scattering pattern of PCBTT: (a) power; (b) film drop-cast from chlorobenzene; (c)
film 80 ꢁC annealed; (d) film 150 ꢁC annealed; (e) one-dimensional WAXS data of PCBTT power; and (f) one-dimensional WAXS
data of (b–d).
The photoluminescence (PL) spectroscopy of PCBTT in chlo-
roform solution has also been measured, as shown in Figure
3(a). Interestingly, when the concentration of PCBTT
increased from 8.12 ꢃ 10ꢂ7 to 2.6 ꢃ 10ꢂ4 mol/L, the PL in-
tensity first increases to reach the maximum and then drops
to nearly no emission [see the inset in Fig. 3(a)]. The saddle-
like trend of the intensity is indicative of a stronger inter-
chain interaction in the condensed state resulting in the self-
quench fluorescent. The thiadiazole moiety absorbs in a
spectral region (ꢀ460) exactly located where carbazole back-
bone emits (ꢀ450 nm),9 and consequently, carbazole emis-
sion at 300–500 nm decreases and green emission increases
with concentrating solution. The excitation pumps carbazole
to the excited state, and the UV light emitted from the back-
bone is reabsorbed by the thiadiazole moiety. Therefore,
with the increase in the concentration, more energy transfer
from the carbazole to the thiadiazole dramatically favors the
stronger green emission. Nevertheless, when concentration is
beyond to a critical value, the generated strong molecular
packing leads to self-quench fluorescent of the whole emis-
sion spectra. The molecular aggregation can be further veri-
fied by PL emission spectroscopy of the film of PCBTT, as
shown in Figure 3(b). There is almost no PL emission as
film before and after thermal annealing when compared with
that in solution. This predicts that the strong interchain
interaction in the film, even without annealing, induces mo-
lecular aggregation with a close p–p stacking, which is in
good agreement with the UV–vis spectra.
To better understand the solid-state crystallinity and molecu-
lar packing of the D–A integration, the XRD and 2D WAXS
analyses were performed on thin drop-cast and annealed
films of PCBTT. From the XRD patterns presented in Figure
4, the local packing and crystalline nature of drop-cast films
is evident, and three highly resolved diffraction peaks arising
from the lamellar structure are observed at 2y ¼ 5.05ꢁ,
13.81ꢁ, and 27.83ꢁ. The peak at 2y ¼ 5.05ꢁ corresponds to
an interlayer lamellar d-spacing of 1.73 nm and can be
assigned to the (100) reflection. A distinct diffraction peak at
high angle (2y ¼ 27.83ꢁ) is attributed to p–p stacking reflec-
tion, and its corresponding d(010)-spacing is extremely short
to 0.32 nm, which is much smaller than that of the classical
material P3HT (0.38 nm).33 Note that, to the best of our
knowledge, it is found to be the closest p–p stacking for con-
jugated polymer in the literature to date. Strong interchain
interactions originating from the planar molecular and the
D–A polar interaction should be responsible for the extreme
short distance of p–p stacking. The highly organized polymer
chains in the thin films are expected to be particularly favor-
able for charge transport via hopping through p–p stacking
in a PSC device. The film annealed from 80 ꢁC displays a
similar XRD pattern to the drop-cast one, except a little bit
enhanced intensity at lamellar (100) reflection, due to more
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FIGURE 6 AFM images of thin-film PCBTT (5 ꢃ 5 lm): height images of film (a) not annealed, (b) annealed at 80 ꢁC, and (c)
annealed at 150 ꢁC; phase images of films (d) not annealed, (e) annealed at 80 ꢁC, and (f) annealed at 150 ꢁC.
ordered organization in the lamellae. Distinct from the for-
mer, on annealing from higher temperatures (150 ꢁC), the
diffraction peak of p–p stacking disappeared, probably
resulting from a transition in molecular packing from a pref-
erentially orientation of conjugated plane to another one.16,34
accordance with its molecular arrangement [Fig. 5(a–d)].
Conclusively, such tight p–p stacking promotes us to
believe that an intrinsic intermolecular p–p stacking has al-
ready been achieved by constructing D–A-integrated struc-
ture with high coplanarity.
The deeper insight of the highly ordered lamellar structure
with tight p–p stacking is also given by 2D WAXS (Fig. 5).
All the PCBTT films, including drop-cast and annealed
films, show a obvious peak of lamellar (100) at q ¼ 17.90
nmꢂ1; however, the d(100)-spacing for polymer powder is
much larger (ꢀ0.64 nm) than that for the drop-cast films
due to the loose molecular packing [Fig. 5(e,f)]. Similar to
the XRD observation, a scattering peak of p–p stacking
appears in 2D WAXS patterns for all the samples, even
when the sample is in the powder state, and the thermal
annealing also exerts some influence on the molecular
packing. The p–p stacking distance, calculated to be 0.348
nm, is a slightly larger value than that (0.320 nm) obtained
from XRD; however, it is also less than the values reported
in the literature and is within the experimental error. In
addition, the image of 2D WAXS for each sample is in good
Besides the previously presented structural analysis of thin
film crystallinity, film morphologies are also essential for
understanding the microstructure of the thin film and the
transition behavior. The solid-state morphology and supramo-
lecular organization of the PCBTT are revealed by AFM. As
seen from the AFM height images [Fig. 6(a–c)] and the phase
images [Fig. 6(d–f)] of the drop-cast PCBTT thin film, a con-
spicuous fibrillar feature of morphology, in which a large
amount of small fibers aggregate together in parallel manner,
reveals the strong intermolecular interaction inducing a high
crystallinity. The morphology converts to more homogeneous
after annealed at 80 ꢁC; however, the fibrillar domains
become obscure when annealed at higher temperature.
To illustrate the property of PCBTT as the donor materials in
the active layer, BHJ solar cells were fabricated with typical
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device structures based on glass/ITO/PEDOT:PSS/Poly-
mer:PCBM (1:2 w/w)/LiF/Al. The details of device fabrica-
tion and characterization are summarized in the Supporting
Information. Figure 7(a) shows the current density–voltage
(I–V) curves for solar cells under simulated AM 1.5 G radia-
tion (100 mW/cm2), and the performance data are presented
in Table 2. The EQE spectrum of the PCBTT:PCBM (1:2) solar
cell is about 19% in the broad wavelength range of 400–800
nm and a peak value of 20% at 300–400 nm [Fig. 7(b)]. The
shape of the EQE curve is similar to its absorption spectrum
(Supporting Information Fig. S13), indicating that absorption
in the visible region for the photovoltaic conversion com-
pletely comes from the contribution of the polymer absorp-
tion. Without any optimization, the device shows a low
short-circuit current density (Jsc) of 2.45 mA/cm, a fill factor
of (FF) 25.9%, and a PCE value of only 0.39%. A relatively
high open–circuit voltage (Voc) of 0.62 V is obtained because
of the low-lying HOMO energy level. As expected, the ther-
mal annealing only slightly affects the values of Jsc, FF, Voc,
and PCE, and the highest Voc of 0.67 V and the best efficiency
of 0.48% are achieved for the device annealed from 80 ꢁC.
The origin behind the quiet low PCE is assumed to the poor
solubility of the PCBTT. Due to the too close p–p stacking, it
is difficult for the solvent molecules to diffuse into the inter-
chains which lead to nonuniform morphology.
A large
amount of charge carrier would readily be trapped, and this
may also be the reason for the S-shape of I–V curves. The re-
markable reduced charge transportation resulting from the
morphology nonuniformity is confirmed by space charge-lim-
ited current method based on hole-only devices with the
configuration of ITO/PEDOT:PSS/Polymer/Au. PCBTT film
shows a very low hole mobility of 2.5 ꢃ 10ꢂ5 cm2/(V s) rel-
ative to its such close p–p stacking, indicating that the path-
way for the charge transportation has been interrupted to
discontinuous phase. The discontinuous phase owning to the
poor solubility of PCBTT has been further worsened by mix-
ing with the PCBM, and the well-defined bicontinuous nano-
structure critical to charge mobility is hard to be developed
even after thermal annealing. This hypothesis has been
approved by the AFM images shown in Supporting Informa-
tion Figure S14.
FIGURE 7 OPV device characteristics of PCBTT:PCBM. (a) Cur-
rent–voltage curves of film not annealed, 80 ꢁC annealed, and
150 ꢁC annealed, with
a
structure of ITO/PEDOT/
PCBTBDT:PCBM/LiF/Al under 100 mW/cm2 at AM 1.5 G sunlight
illumination. The thicknesses of the active layer are about 80–90
nm. (b) EQE spectrum of PCBTT:PCBM (1:2) annealed at 80 ꢁC.
CONCLUSIONS
In summary, an original concept of D–A-integrated structure
developed by directly imposing the pull unit on the push
moiety to form fused ring architecture is demonstrated, and
the D–A-integrated polymer PCBTT with high planarity is
synthesized by fusing carbazole with two 1,2,5-thiadiazole
rings and using thiophene as bridge. The absorption band of
polymer PCBTT has been fine tuned to cover almost the
whole visible solar spectrum from 300 to 750 nm, and the
pronounced ICT and energy transfer have been detected in
the D–A integration; however, their mechanisms need to be
further studied. Manifested by UV–vis, PL, XRD, 2D WAXS,
and AFM, an extremely short p–p stacking distance of 0.32
nm is achieved for the polymer, even for powder sample, in
virtue of the inherent strong intermolecular interaction gen-
erated by high planarity. However, the too tight packing
between the molecules results in the poor solubility of
PCBTT, and the nonuniform morphology of the films
TABLE 2 Characteristic Current–Voltage Parameters of Polymer
Solar Cells Based on PCBTT:PCBM 5 1:2 at Various Thermal
Treatments
Voc (V)a Jsc (mA/cm2)b FFc Averaged PCE (%)
PCBTT
PCBTT(80)d 0.67
0.62
2.45
2.69
2.56
0.26 0.39
0.27 0.48
0.27 0.42
PCBTT(150) 0.62
a
Voc is the open-circuit voltage.
Jsc is the short-circuit current.
The fill factor (FF) is a graphic measure of the squareness of the I–V
b
c
curve.
d
The number in the bracket following the copolymer represents the
annealing temperature.
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16 J. Kim, B. Lim, K.-J. Baeg, Y.-Y. Noh, D. Khim, H.-G. Jeong,
J.-M. Yun, D.-Y. Kim, Chem. Mater. 2011, 23, 4663–4665.
consequently leads to the poor performance of PSCs. Never-
theless, the concept of D–A-integrated structure offers an
attractive means to design D–A copolymers, which boost the
intermolecular interaction and extended p-conjugated system
for optimized absorption spectrum and charge transport.
17 X. Zhang, L. J. Richter, D. M. DeLongchamp, R. J. Kline, M.
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ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (51073076, 51273088, 51003045, and
51263016).
19 B. Lim, K.-J. Baeg, H.-G. Jeong, J. Jo, H. Kim, J.-W. Park,
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