Jacket-like structure of donor-acceptor chromophores-based conducting polymers for photovoltaic cell applications

Article abstract of DOI:10.1002/pola.23921

Through the Stille coupling polymerization, a series of soluble acceptor/donor quinoxaline/thiophene alternating conducting polymers with a hole-transporting moiety of carbazole as a side chain (PCPQT) has been designed, synthesized, and investigated. The UV-vis measurement of the chargetransferred type PCPQTs of different molecular weights with low polydispersity exhibits a red shifting of their absorption maximum from 530 to 630 nm with increasing chain length (M_n: from 1100 to 19,200). The HOMO and LUMO energy levels of PCPQT can be determined from the cyclic voltammetry measurement to be -5.36 and -3.59 eV, respectively. Solar cells made from PCPQT/PCBM bulk heterojunction show a high open-circuit voltage, V_oc of ～0.75 V, which is significantly higher than that of a solar cell made from conventional poly(3- hexyl thiophene)/PCBM as the active polymer PCPQT has lower HOMO level. Further improvements are anticipated through a rational design of the new low band-gap and the structurally two-dimensional donor-acceptor conducting polymers.

Full text of DOI:10.1002/pola.23921

Jacket-Like Structure of Donor–Acceptor Chromophores-Based
Conducting Polymers for Photovoltaic Cell Applications
1
1
1
1
1
1
YI-LUNG YANG, YI-HUAN LEE, CHUN-JIE CHANG, ANG-JHIH LU, WEI-CHUN HSU, LEEYIH WANG,
1
1,2
MAN-KIT LEUNG, CHI-AN DAI
1
Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Roosevelt Rd., Sec. 4, Taipei 10617, Taiwan
2
Department of Chemical Engineering, National Taiwan University, No. 1, Roosevelt Rd., Sec. 4, Taipei 10617, Taiwan
Received 18 November 2009; accepted 31 December 2009
DOI: 10.1002/pola.23921
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Through the Stille coupling polymerization, a series
of soluble acceptor/donor quinoxaline/thiophene alternating
conducting polymers with a hole-transporting moiety of carba-
zole as a side chain (PCPQT) has been designed, synthesized,
and investigated. The UV–vis measurement of the charge-
transferred type PCPQTs of different molecular weights with
low polydispersity exhibits a red shifting of their absorption
maximum from 530 to 630 nm with increasing chain length
open-circuit voltage, Voc of ꢁ0.75 V, which is significantly
higher than that of a solar cell made from conventional poly(3-
hexyl thiophene)/PCBM as the active polymer PCPQT has lower
HOMO level. Further improvements are anticipated through a
rational design of the new low band-gap and the structurally
two-dimensional donor–acceptor conducting polymers. VC 2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
1607–1616, 2010
(
n
M : from 1100 to 19,200). The HOMO and LUMO energy levels
of PCPQT can be determined from the cyclic voltammetry mea-
surement to be ꢀ5.36 and ꢀ3.59 eV, respectively. Solar cells
made from PCPQT/PCBM bulk heterojunction show a high
KEYWORDS: conducting polymers; electrochemistry; fractiona-
tion of polymers; structure–property relations; UV–vis spectros-
copy; X-ray
2
6,27
INTRODUCTION Recently, conducting polymers or p-conju-
structure.
The highest efficiencies have been achieved
gated polymers have been extensively studied for their
using a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM), but its PV per-
formance depends critically on the material properties and
processing conditions. This variability is believed to be influ-
enced by the self-organizing properties of P3HT, which
means that both electrochemical and photophysical proper-
ties are sensitive to the molecular packing of the conducting
polymer. A recent study by Kim’s group has also demon-
strated that a high regioregularity (RR) P3HT exhibited a
high PCE, because of its enhanced optical absorption and
transport property resulted from the organization of the RR-
1
–4
5,6
7,8
potential applications in sensors,
and transistor/photovoltaic (PV) devices.
actuators, displays,
9
–11
Polymer solar
cells (PSCs) offer a combined advantage of low cost, light
weight, mechanical flexibility, and amenability to manufactur-
ing by high-throughput and large-area reel-to-reel coating
1
2
processes. For PV application, conjugated polymers with
low band-gap, broad and strong solar light absorption, and
high charge-carrier mobility are the key factor for improving
power conversion efficiency (PCE). To fulfill these require-
ments, various design strategies on the conducting polymers
have been pursued. One of the most popular methods is to
synthesize low band-gap conducting polymers containing
alternating donor (D) and acceptor (A) monomer units on
28
P3HT chains. We believe that the molecular nanostructure
and optoelectronic properties of the blend material in their
BHJ structure played important roles on the aforementioned
1
3–18
their polymer main-chain backbone.
Some of them were
29
results.
fabricated in PSCs with an improved efficiency of sunlight
1
9–23
harvest.
Furthermore, since the discovery of PV effect in
Currently, a new class of donor–acceptor alternating p-conju-
gated heterocyclic polymers of poly(thiophene-alt-quinoxa-
line) (PTHQx) derivatives is explored both as hole- and elec-
solar cells with bulk heterojunction (BHJ) structure in which
electron-donating conducting polymers and electron-accept-
ing inorganic nanoparticles are randomly mixed, there has
been a considerable advancement in the PSC fabrication
3
0
tron-transporting materials.
The use of quinoxaline
segment was aimed to improve the electron injection and
transport of the p-type thiophene segment. Yamashita and
coworkers have reported a series of low band-gap polymers
consists of thiophene as aromatic-donor and 10p- or 14p-
2
4,25
technique.
So far, after extensive optimization in the fab-
rication process, these polymer/inorganic nanoparticle solar
cells with PCE up to 4–5% have been made using the BHJ
Additional Supporting Information may be found in the online version of this article. Correspondence to: C.-A. Dai (E-mail: polymer@ntu.edu.tw)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1607–1616 (2010) V
2010 Wiley Periodicals, Inc.
C
CHROMOPHORES-BASED CONDUCTING POLYMERS, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
electron-heterocycles as o-quinoid-acceptor, which involves
Characterization Methods
1
13
2
,1,3-benzothiadiazole (BT), thieno[3,4-b]pyrazine (TP), and
All new compounds were characterized by H and C Nu-
clear magnetic resonance (NMR). NMR spectra were taken
using a Bruker Avance 400 FT-NMR system, and the chemi-
cal shifts of NMR spectra were reported in ppm. Splitting
patterns were designated. IR spectra were recorded on a
Jasco FT/IF-480 spectrometer. EI-MS experiments were car-
ried out using a Finnigan MAT TSQ 700 triple stage quadru-
ple mass spectrometer equipped with an electron ionization
ion source. GPC was carried out on a Waters-2695 Alliance
HPLC system interfaced with a Waters 996 photodiode array
detector and a Waters 2414 refractive index detector. The
analytical GPC columns were used with THF as the eluent at
a flow rate of 1.0 mL/min, and the elution time was cali-
brated with narrow polystyrene standards. The preparative
GPC technique was conducted by using the above GPC sys-
tem fitted with an autosampler module and with a prepara-
tive column (Polymer Laboratories PLgel, 10 lm, 103 Å, Se-
rial No. 10M-3-69A-7) and by using THF as the eluent with a
3
1
quinoxaline (Qx). Leclerc and coworkers have successfully
prepared PTHQx copolymer containing carbazole donor unit
that had hole-transporting ability in the main-chain back-
bone. Ingan a¨ s and coworkers have also reported the syn-
thesis and characterization of a series of fluorine-based
PTHQx copolymers with a balanced electron and hole
3
2
3
3,34
mobility.
In this article, we report the synthesis, characterization, and
the solar cell performance of a 2,3-diphenyl-quinoxaline-
based donor–acceptor conjugated copolymers (PCPQT), in
which the donors include carbazole and thiophene units
while Qx unit serves as an acceptor. These copolymers were
synthesized by using palladium(0)-catalyzed Stille cross-cou-
pling reaction. The introduction of the bulky pendent carba-
zole groups as the side chain on the Qx moiety significantly
improves the polymer solubility for further characterization
and device fabrication. Such approach has also explored the
possibility for synthesizing a structurally two-dimensional
main-chain and side-chain donor–acceptor conjugated poly-
mer system. We have also used a fractionation method
ꢂ
flow rate of 4.8 mL/min at 25 C. The fractionation of P2
was achieved by accumulating 100 analytical GPC runs.
From each GPC run, five fractions were collected with a
Waters fraction collector III. Thermogravimetric analysis
[
preparative gel permeation chromatography (prep GPC)] to
(
TGA) was conducted using a PerkinElmer Pyris 1 TGA. The
prepare PCPQTs of different molecular weights with low
polydispersity. The effects of the average molecular weight
on the optoelectronic properties of PCPQTs were investi-
gated. Furthermore, powder X-ray diffraction (XRD) analysis
exhibited a different crystalline ordered structure between
the monodispersed and polydispersed PCPQTs. PSC devices
fabricated using PCPQT/PCBM blends, sandwiched between
a transparent anode (ITO/PEDOT:PSS) and a cathode (Al),
were also explored. The present study revealed that the new
polymers had a potential to be used for applications in flexi-
ble polymer electronics.
TGA experiments were carried out on approximately 6–8 mg
film samples heated in a flowing nitrogen environment (flow
3
ꢂ
rate ¼ 20 cm /min) at a heating rate of 10 C/min. Differen-
tial scanning calorimetry (DSC) analysis was performed on a
ꢂ
Perkin-Elmer Pyris Diamond DSC at a scan rate of 20 C/
min in a flowing nitrogen environment at a flow rate of
3
2
0 cm /min. Ultraviolet–visible (UV–vis) analysis was per-
formed on a Jasco V-570 UV–vis spectrophotometer. Solid
state absorption spectrum was obtained on a thin film
coated on quartz prepared by spin-coating of the polymer in
dilute chloroform solutions. Photoluminescence (PL) spec-
trum was measured with a Hitachi-4500 spectrometer. Elec-
trochemical measurements were performed with a CHI
Model 405A electrochemical analyzer. Voltammograms are
presented with the positive potential pointing to the right
and with the increasing anodic currents pointing upward.
Cyclic voltammetry (CV) was conducted with the use of a
three-electrode cell in a dichloromethane solution, using
TBAP (0.10 M) as the supporting electrolyte at room temper-
ature with a platinum working electrode, a platinum wire
auxiliary electrode, and an Ag/AgCl, KCl (saturated) refer-
ence electrode. Powder XRD was performed in Taiwan in the
National Synchrotron Radiation Research Center (NSRRC) at
EXPERIMENTAL
Materials
3
5
3,6-Dibromobenzene-1,2-diamine (3),
5,8-dibromo-2,3-
3
6
diphenyl-quinoxaline (M1),
9-butyl-carbazole-3-boronic
3
7
38
acid (8), and 2,5-bis(tributylstannyl)thiophene (11) were
prepared according to the literature procedures. A previous
3
9
literature reported copolymer, PThQx(diPh) (P1),
pre-
pared via the Stille coupling method was also synthesized in
this study for comparison in its optical properties with those
of PCPQT (P2). A prep GPC technique with THF as the elu-
ent was used to remove monomers and oligomers of the as-
synthesized P2 after the polymerization. High molecular
weight PCPQT (Mw ¼ 18,000 g/mol) with a relatively nar-
row polydispersity (PDI ¼ 1.48) was thus obtained with the
prep GPC technique and designated as P2-P. P2 and P2-P
were used as the active materials for the fabrication of the
solar cells for the evaluation of their performance.
13A1 end station.
Fabrication of PV Devices
The solar cell devices were fabricated with a standard struc-
ture of ITO/PEDOT:PSS/active layer/Al. The ITO-glass sur-
face was precleaned and modified by a thin layer of poly (eth-
ylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:
PSS) (Bayer), which was spin-cast from its aqueous solution
Synthesis of Monomers and Polymers
The synthesis schematics of the monomers and polymers are
shown in Schemes 1 and 2. The detailed synthetic proce-
dures for all compounds are shown in the Supporting
Information.
ꢂ
on the ITO substrate and dried on a hot plate at 140 C for
100 min under vacuum. The active layer was subsequently
prepared by spin-coating a blend solution of PCPQT/PCBM in
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INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 Synthetic route of
monomers.
a weight ratio ranging from 1:0.5 to 1:1 in chlorobenzene on
the ITO-PEDOT:PSS electrode. Each polymer solutions were
filtered through a membrane filter with a channel size of 0.2
lm before spin-coating, and the coated active films were dried
(9), using Pd(PPh3)4 as the catalyst. IR and NMR spectro-
scopic techniques were used to verify the structure of the
1
new compounds 10 and M2. Figure S1 illustrates the H and
1
3
C NMR spectra of 10 and M2. The thermal property of
ꢂ
under reduced pressure for 10 min at 80 C. Then, an alumi-
monomer M2 was determined by the TGA measurements.
num cathode (ca. 1000-Å thick) was deposited on the polymer
The thermal decomposition temperature was recorded based
on a 5% weight loss. M2 is thermally stable up to 350 C, as
shown in Figure S2(a).
ꢀ
6
ꢂ
layer by vacuum evaporation under 4 ꢃ 10 torr. The solar
cell device characterization was carried out under AM1.5G so-
2
lar irradiation (100 mW/cm ) using a solar simulator.
2
,5-Bis(tributylstannyl) thiophene (11) is the organometallic
4
0
reagent for used in Stille cross-coupling reaction for P2.
RESULTS AND DISCUSSION
Compound 11 was synthesized with a commercially available
thiophene as the starting material and treated sequentially
with n-butyl lithium and tributylchlorostannane to give a
high quality organometallic reagent of the compound 11.
The polymers P1 and P2 were synthesized through the Pd-
catalyzed Stille cross-coupling from 5,8-dibromo-2,3-di-
phenyl-quinoxaline (M1)/M2, respectively, and 11. The
yields of all the polymers were higher than 95%. P1 is a pre-
vious literature reported material. High molecular weight P1
was found to be insoluble in common organic solvents
including THF, CH Cl , and toluene but was found to be
Synthesis and Characterization
Scheme 1 illustrates the synthetic routes for the new Qx-
and carbazole-based monomer M2. The monomer M2 can be
prepared by a reaction from the compound 3, 6-dibromoben-
zene-1,2-diamine (3) and 1-(4-(9-butyl-9H-carbazol-3-yl)
phenyl)-2-(4-(9-butyl-9H-carbazol-6-yl) phenyl) ethane-1,2-
dione (10) in high yield of 83%. 1-(4-(9-Butyl-9H-carbazol-
3
1
9
-yl) phenyl)-2-(4-(9-butyl-9H-carbazol-6-yl)phenyl)ethane-
,2-dione (10) was prepared by the Suzuki coupling between
-butyl-carbazole-3-boronic acid (8) and 4,4 -dibromobenzil
0
2
2
CHROMOPHORES-BASED CONDUCTING POLYMERS, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Synthetic route of P1
and P2 polymers.
soluble in CF COOH; in contrast, P2 exhibits excellent solu-
However, the polydispersity index of P2 is still large (PDI ¼
1.96), as large amount of oligomers resulted from the poly-
merization still presents in P2. We applied a prep GPC frac-
tionation technique to separate the high molecular weight
polymers from the oligomers. The resulting polymer was
obtained with Mw ¼ 18,000 g/mol and with lower PDI ¼
3
bility in many common organic solvents such as CH Cl , tolu-
2
2
ene, dichlorobenzene, chlorobenzene, and THF. The insolubil-
ity of the high molecular weight P1 might have been due to
a tightly packed structure of P1, which induces crystalliza-
1
tion during polymerization. IR and H NMR spectroscopic
techniques were used to identify the structure of the new
polymer P2. The average molecular weights were deter-
mined by GPC with polystyrene standards, using THF as the
eluting solvent. The molecular weight distribution profile of
P2 is showed in Figure 1(a). To improve the solubility of P1,
bulky carbazoyl substituents with a butyl chain on the nitro-
gen were introduced on the pendant phenyl ring connected
to the Qx unit of P2 to increase the steric hindrance and to
prevent a close packing of the main chains. In addition, car-
bazole groups have been known for its electron donating
property. The average molecular weight of dicarbazoyl sub-
1.48 and it was designated as P2-P. P2 showed a T at 108
g
ꢂ
C from the DSC measurement and its thermogram is shown
in Figure S2(b). The thermal stability of polymers P1 and P2
was determined by TGA measurements. P1 and P2 are ther-
ꢂ
mally stable up to 425 and 430 C, respectively. The thermal
stability between P1 and P2 are comparable to each other,
yet, the incorporation of the planar carbazole groups in P2
significantly improves its solubility. The thermal stability of
conducting polymers is found to be a very important prop-
erty for solar cell performance as well as for other
applications.
stituted P2 is sufficiently high with M
w
¼ 11,000 g/mol.
Optical Properties
ꢀ
5
The UV–vis absorption spectra of M2, P1, and P2 in 10
M
THF solution and in the film state, respectively, are shown in
FIGURE 2 Comparison of UV–vis absorption spectra of M2 &,
ꢀ
5
FIGURE 1 GPC trace of (a) P2 and P2-P and (b) different molec-
ular weights (M
¼ 19,200 &, 8800 *, 4900 ~, 2800 !, and
100 ^) of P2s.
P1 *, and P2 ~ in 10 M THF; PL spectra of M2 & and P2 (~
excitation wavelength at 339 nm; ~ excitation wavelength at
n
ꢀ5
1
599 nm) in 10 M THF (PL: photoluminescence).
1610
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ARTICLE
M2 has a good overlap with the absorption of P2 around
600 nm. Therefore, F o¨ rster resonance energy transfer
(
FRET) from the excitation states of M2 segment of P2 (PL
wavelength ꢁ589 nm; donor) to the intervalence CT (IVCT)
band of P2 (excited wavelength at 599 nm; acceptor) could
be favored. Moreover, the emission spectrum of P2 shows a
single emission around 720 nm, when P2 is separately
excited at 339 or 599 nm. Hence, FRET can therefore occur
4
3,44
in P2.
In addition, we prepared different molecule
weight of P2s, by using a preparative GPC fractionation tech-
nique. The UV–vis absorption spectra of the thus fractionated
polymers with different molecular weights are showed in
Figure 3. In the case of P2s, the wavelength of the absorp-
tion maxima (from 530 to 630 nm) increased with increas-
n
ing molecule weight (M : from 1100 to 19,200). The conju-
gation length of the thiophene/Qx alternating copolymers,
therefore, increases with increasing molecular weight. This
increase in the conjugation length is remarkably enhanced
by the extensive p-delocalization in the polymer backbone
containing the donor–acceptor system as well as by the
bulky carbazole side chain to prevent the main chain from
FIGURE 3 (a) UV–vis absorption spectra of different molecule
weight [M
¼ (a) 19,200; (b) 8800; (c) 4900; (d) 2800; (e) 1100] of
P2s (in THE solution) prepared from Prep-GPC method. (b)
Relationship of E and 1/n for P2s.
n
g
4
5
Figure 2 and S3. P1 and P2 have a similar average molecular
weight of ꢁ5500. The P2 solution exhibited two major UV–
vis absorption bands centered roughly around 339 and 599
nm, which were assigned to the p–p* transition and the
charge-transfer (CT) complex, respectively. The absorption
maximum peak of P2 was red-shifted with respect to that of
P1. This result indicates that P2 has a longer p-conjugation
length, which can be attributed to the existence of the addi-
tional carbazole unit with its electron donating effect. In the
solid state, the polymers showed absorption profiles similar
to those of their solution analogs, as illustrated in Figure S3.
However, the konset of P1 in the solid state showed a batho-
coiling up. That is, the bulky carbazole side chains might
form a jacket-like structure along the polymer main-chain
and forced the main-chain in adopting a more rigid-rod
structure. The steric side-chain effect on the main-chain
structure can be further examined by plotting the band-gap
energy E of the fractionated polymers, as a function of its
g
reciprocal chain length (1/n). The limiting value for E
g
of
PCPQT is estimated to be about 1.75 eV for n ¼ 1, as
4
6
shown in the insert of Figure 3. Even for the highest molec-
ular weight P2 (n ¼ 24), its band-gap energy still follows the
linear relationship on chain length, indicating that the main
chain of the current polymers takes an extended rigid-rod
structure and exhibits a better p-delocalization in the polymer
backbone. Detailed elucidation on the conformation of the
polymers can be examined using XRD analysis discussed later.
The smaller band-gap and higher absorption intensity (log e ¼
4.2) in the visible region of P2 are comparable with various
ꢀ
5
chromic shift (ꢁ30 nm) compared with that of P1 in 10
M
THF solution. This indicates the presence of strong inter-
chain interactions due to the self-p-stacking of P1 in the
2
8
solid state, leading to an increase in its conjugation length.
P2 also showed a red-shift of konset (ca. 20 nm) in solid film
ꢀ
5
compared with that observed from its 10 M THF solution.
The reason for the decrease in the red-shifting between P2
and P1 in the solution and solid state is probably because of
the bulky carbazole side chain affecting p–p planner stacking
of P2 in the solid state. The UV–visible absorption maxima
of P1 in the solid states shows a longer wavelength (kmax
50 nm) than those of films of a corresponding homo-
polymers polythiophene (kmax ¼ 420–490 nm) and poly(2,
¼
5
4
1
3
-diphenylquinoxaline-5,8-diyl) (kmax ¼ 420 nm). The rea-
sons that P1 and P2 exhibit lower band-gaps than their cor-
responding homopolymer analogs (polythiophenes or poly-
quinoxalines) are due to the intramolecular charge transfer
(
ICT) between the carbazole/thiophene donor and the Qx
acceptor. Hence, P1 and P2 both have two UV–visible
absorption maxima, which are associated to the p–p* transi-
4
2
tion and ICT. Furthermore, the PL spectra of M2 and P2 in
ꢀ
5
10
M THF solutions showed PL bands centered around
589 and 719 nm, respectively. As shown in Figure 2, a strong
luminescence quenching might be resulted from a highly effi-
cient intramolecular energy transfer, because the emission of
FIGURE 4 Cyclic voltammogram of P2.
CHROMOPHORES-BASED CONDUCTING POLYMERS, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 3 The anodic oxidation and cathodic reduction pathways for P2.
low band-gap conducting polymers reported or used as an
active material for PV applications in the literature.
of the repeating unit of P2 was initially optimized, using Mo-
lecular Mechanics with MMþ force field based on the Allin-
4
7
ger’s method. The geometry optimization was carried out
using the Polak-Ribiere conjugate gradient, set to terminate
at an RMS gradient of 0.01 kcal/(Å mol). Then, the repeating
unit of P2 was optimized with the semiempirical molecular
orbital method using the parameterized model number 3
Electrochemical Properties
The redox behavior of P2 was investigated, by using CV. The
electrochemical properties of P2 were measured in a dilute
ꢀ
3
P2 solution (10 M) with TBAP dichloromethane (0.1 M),
as the electrolyte, by using CV at a scanning rate of 100 mV/
s. Figure 4 shows a representative cyclic voltammogram of
P2. It revealed two clear redox couples at half-wave poten-
tial (E1/2) of 1.18 and 1.43 V for the oxidation and an irre-
versible reduction wave at Eca,p ¼ ꢀ1.12 V under an cathodic
sweep. The anodic oxidation and cathodic reduction path-
ways for P2 are postulated, as shown in Scheme 3. Com-
pared with the electro-oxidation activities, the oxidation of
thiophene ring requires a higher oxidative potential than
that of the carbazole group. It is because of the direct car-
bon–carbon linkage between the nearby thiophene ring and
the electron-withdrawing Qx. Furthermore, the effect of neg-
ative electron affinity is stronger in the quinoxaline–thio-
phene main-chain than the quinoxaline–carbazole side-chain,
because the linking phenyl group is twisted in connecting
the carbazole ring with the Qx core. The onset potentials
(
PM3) Hamiltonian on the wavefunction in the gas state and
4
8
used for the prediction as well as to save CPU time. The
molecular model was implemented in the hyperchem molec-
ular graphics to obtain the information on the conjugation of
P2. As each of the phenyl carbazole group in P2 is twisted
from the Qx unit at around 42.9 , it suggests that there can
49
ꢂ
be some electronic delocalization between the twisted carba-
zole and Qx units. Indeed, from the UV–vis measurement, the
broad absorption bands in P2 suggest a relatively good elec-
tronic delocalization throughout the whole main-chain and
side-chain donor–acceptor system. As shown in Figure 5, the
HOMO level of P2 is primarily located on the p-framework
of the phenyl carbazole; whereas, the electron density of
LUMO is extended over the whole main chain through the
Qx moiety. From Figure 5, it also suggests that the second
oxidation of the main-chain thiophene ring requires a higher
potential than that of the carbazole side-chain. Although only
a single conformation is observed for the optimized molecu-
lar geometry of P2, it is possible that the relaxation of pack-
ing constraints in solution might allow other conformations
(Eonset) of the oxidation and reduction levels for P2 are
observed at þ 1.04 and ꢀ0.73 V, respectively, versus Ag/
AgCl in dichloromethane, and the external Ag/AgCl redox
standard E1/2 is 0.48 V. The electrochemical band-gap value
(1.77 eV) of P2 is, therefore, closely matched with the opti-
5
0
to be present. Therefore, a molecular mechanics study was
undertaken to determine the potential energy (in kcal/mol)
cal band-gap value of P2 (1.76 eV) shown in Figure 2.
Assuming that the HOMO energy for the Ag/AgCl standard is
0
of P2 s repeating unit, as a function of the dihedral angles
ꢀ
4.8 eV with respect to the zero vacuum level, the HOMO
energy level for P2 can be calculated according to the fol-
/1
, / , / , / , and / . The dihedral angles / –/ are
2 3 4 5 1 5
lowing equation:
ox;polymer
ox;ferrocent
EHOMO ¼ ꢀeðEonset
ꢀ E1=2
Þ þ ðꢀ4:8 eVÞ
(1)
Therefore, the HOMO energy for P2 can be evaluated to be
ꢀ
5.36 eV. For P2, the electrochemical band-gap (E , 1.77 eV)
g
is the difference with the onset potential of the oxidation
level and the onset potential of the reduction level. There-
fore, the LUMO value (ꢀ3.59 eV) can then be obtained from
the electrochemical band-gap plus the HOMO value.
Optimized Geometry Properties
We have also used a molecular modeling method to simulate
the conformational behavior of P2. The molecular geometry
FIGURE 5 HOMO and LUMO of the P2 repeating unit as
indicated.
1612
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ARTICLE
FIGURE 6 Potential energy of
the P2 repeating unit as func-
tions of the dihedral angles /
1
,
/
2
, / , / , and /
3
4
5
.
5
2
defined, and the potential energy results of the P2 unit are
plotted in Figure 6. Comparatively, the dihedral angles / and
/3 exhibit a larger potential energy, probably because of the
pendant phenyl ring for increasing its steric hindrance. We
believe the effect of the side-chain steric hindrance is to lock
on the backbone conformation and to reduce the torsional
motion of the backbone about the axis defined by the main-
chain. Moreover, the bulky carbazoyl substituents were intro-
duced on the pendant phenyl ring to prevent a close packing
of the main chains and to increase the solubility of P2.
the polymer. The diffraction peak at d ¼ 3.42 Å is consid-
5
2
ered to be the layer-to-layer p-stacking distance between the
coplanar backbones. The powder XRD pattern of PCPQT is
also showed in Figure 7. P2 and P2-P reveal the same pow-
5
3
der XRD peak at d ¼ 25.7 Å, but P2 does not exhibits any
1
other peaks at higher angles. The estimated length of the
rigid portion of the bulky carbazole side-chain is about 15 Å.
So the polymer is considered to assume an end-to-end pack-
ing structure, as illustrated in Figure 8. In this case, it is
likely that large amount of monomers and oligomers remain-
ing in P2 greatly affect its crystallinity and crystalline struc-
X-Ray Diffraction
ture. The P2-P showed a sharp peak at d ¼ 25.7 Å and a
1
The powder XRD patterns of P1 have been reported by
broad peak at d ¼ 8.74 Å. The former peak at the low angle
5
1
2
Yamamoto et al. In this study, we have also synthesized P1
and its XRD pattern is shown in Figure 7 and compared with
that of P2. P1 exhibits several sharp powder XRD peaks at
d ¼ 15.8 Å, d ¼ 7.93 Å, d ¼ 5.24 Å, d ¼ 4.73 Å, and d
region is usually assigned to the distance between p-conju-
gated main-chains separated by the bulky side chain. The lat-
ter peak at d2 ¼ 8.74 Å is considered to be the distance
between the coplanar p-conjugated main chains. This result
suggests a higher structural organization for PCPQT with
narrow polydispersity presents in the solid state. As dis-
cussed previously, the bulky carbazole side-chain may force
the polymer main-chain to take a rigid-rod conformation,
which may enhance a crystalline ordered structure between
PCPQT chains.
1
2
3
4
5
¼
3.42 Å, revealing that the polymer forms a highly crystal-
line ordered structure. The diffraction peak at 2h of 3.7 ,
ꢂ
corresponding to spacing d ¼ 15.8 Å, is assigned to a dis-
1
tance between the polymer backbone separated by the side
2 1
chains. The diffraction peak d is equals to the half of d ,
and it might be the second-order peak of the diffraction at
d . The diffraction peaks d and d reveals that there maybe
1
3
4
some interesting, yet, so far unknown microstructure within
PV Properties
The solar cells containing P2/PCBM were fabricated with
a
structure of ITO/PEDOT:PSS/active layer/Al. The PV
FIGURE 8 Schematic representation of the proposed packing
structure of P2-P in solid state. (a) Top view and (b) side view.
FIGURE 7 Powder X-ray diffraction patterns of P1, P2, and P2-P.
CHROMOPHORES-BASED CONDUCTING POLYMERS, YANG ET AL.
1613

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
sity PCPQT, P2-P, blended with PCBM as an active layer for
the PV device fabrication. Figures 10 and 11 show the cur-
rent–voltage (J–V) curves and the dark current, respectively,
for both P2 and P2-P solar cells. The best Voc of the P2/
PCBM solar cells reaches 0.75 V, which is comparable with
the P2-P/PCBM devices. However, a lower Jsc and a lower
FF of the P2-P/PCBM device limit its efficiency to a value of
0.34%. The best P2-P device performance was obtained,
using a solution with P2-P:PCBM weight ratio of 1:0.7. The
device, thus, made has g ¼ 0.40% under AM 1.5 illumination
2
(
100 mW/cm ). The improvement in the device performance
for low PDI P2-P may be because of the following reasons.
First, as mentioned before, as the bulky carbazole side
chains in PCPQT may force the main-chain in adopting a
more rigid-rod structure, we expect that the jacket-like struc-
ture of the bulky carbazole side chains form an ordered p-
type pathway around the main-chain for efficient charge
transport. Second, the unpurified P2 is polydispersed and
multimodal whereas the fractionated P2-P has low polydis-
persity and unimodal. The difference in their molecular
weight distribution may significantly alter the way the rod-
like polymer packed in the solar cell device. As shown in
their XRD patterns, P2 has no significant sharp diffraction
peaks whereas P2-P exhibits a better ordered structure.
Upon mixing with PCBM, P2-P device with well ordered
structure may have larger donor/acceptor interfacial area for
change separation, therefore, better PCE, g. The p/n junction
structure of well ordered p-type P2-P rigid-rod with n-type
PCBM dispersed around the rod resembles that of recently
developed ‘‘double-cable’’ polymers, in which donor conduct-
ing polymer and acceptor PCBM are in close proximity in
FIGURE 9 Energy level diagrams of the components in P2 de-
vice referenced to the vacuum level.
performance measurements were carried out under an ambi-
ent atmosphere to demonstrate the air stability of P2. The
cell structure employs P2 as the electron-donor and PCBM
as the electron-acceptor material with the corresponding
band-gap energy levels shown in Figure 9. Table 1 lists the
P2 device performance for blends of P2/PCBM. Note that
with decreasing P2:PCBM weight ratio from 1:1 to 1:0.7, the
open-circuit voltage (V ), the short-circuit current (J ), fill
oc
sc
factor (FF), and the PCE (g) of the P2/PCBM devices
increased. Generally, the PCE of polymer PV device is related
to the weight ratio of acceptor to donor in its BHJ structure.
In the case of the P3HT/PCBM blend, the best efficiency was
2
6
observed when the weight ratio of P3HT to PCBM was 1:1
2
7
or 1:0.8. We also noted that a lower concentration of the
conducting polymer in the chlorobenzene around 25 mg/mL
and a higher spin-coating around 1400 rpm (corresponding
to a thinner film of ꢁ120 nm) give higher efficiency. The
condition for the best performance of the P2/PCBM devices
was obtained based on the solution with P2:PCBM ratio of
5
4,55
their nanodomain.
The solar cell based on P2-P/PCBM
in the current study has an efficiency comparable or better
than that of a solar cell based on double-cable polymers.
However, an elaborate chemistry was often involved for the
synthesis of double-cable polymers while facile synthesis
procedure and device fabrication technique were used in
this study. In Figure 12, the results for the incident photon
to current efficiency (IPCE) measurements were made on
1:0.7 and with the spin-coating rate at 1400 rpm. The best
PCE, g was measured to be ꢁ0.27% under AM 1.5 illumina-
2
tor (100 mW/cm ). Furthermore, we use the low polydisper-
TABLE 1 Device Characteristics Assembled in ITO/PEDOT:PSS/Active Layer/Al
P2:PCBM Weight
Concentration of
Rate of
Film
J
sc
2
Ratio
Chlorobenzene (mg/mL)
Spin-Coated (rpm)
Thickness (nm)
V
oc (V)
(mA/cm )
FF (%)
g (%)
1
1
1
1
1
1
1
1
1
1
a
:1.0
:1.0
:1.0
:1.0
:0.8
:0.7
:0.7
:0.7
:0.7
:0.5
24
24
24
36
36
26
26
26
26
22
800
1,100
1,400
1,400
1,400
1,000
1,200
1,400
1,400
1,400
410
360
110
380
380
250
180
120
120
100
0.58
0.59
0.60
0.56
0.61
0.63
0.61
0.62
0.74
0.75
1.30
1.60
1.65
1.36
1.46
1.59
1.60
1.77
2.22
1.98
21.56
22.00
22.38
22.45
22.80
23.63
24.20
24.24
24.22
22.80
0.16
0.20
0.22
0.18
0.20
0.23
0.23
0.27
0.40
0.34
a
a
P2-P, PCBM weight ratio.
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ARTICLE
FIGURE 12 IPCE versus wavelength for blends consisting of
P2-P/PCBM (1:07, 25 mg/mL in chlorobenzene at 1400 rpm).
FIGURE 10 Current–voltage (J–V characteristics of the ITO/
PEDOT:PSS/active layer/Al solar cells with different composi-
tion of P2/PCBM under AM 1.5 of simulated sun light.
side chain and examined their photophysical and electro-
chemical properties. The new conducting polymer PCPQT
consists of alternating electron-donating thiophene/carbazole
units and electron-accepting Qx units and shows interesting
optical properties, because of the formation of a two-dimen-
sional CT electronic structure along main chain as well as
along side chain to the copolymer main-chain. Furthermore,
we prepared different molecule weights of P2s and found a
solar cells based on a P2-P/PCBM (1:0.7) blend. For a de-
vice consisting of P2-P/PCBM, the maxima in the IPCE are
observed at 400 and 600 nm, both with an efficiency of
ꢁ
16%. The photocurrent dependence on wavelength
matches well with the UV–vis absorption spectra of P2-P.
This IPCE value of the P2-P/PCBM device is found to be
high compared with that of many ‘‘double-cable’’ polymer-
based devices. The reason for a low IPCE value maybe
because of an insufficient fullerene loading in their double-
cable polymer systems whereas in our case an optimized
amount of fullerene can be easily added in the jacket-like do-
nor-acceptor rigid-rod system, which significantly improves
limiting value (1.75 eV) of its band-gap energy E
g
for n ¼ 1.
P2 absorbed light in a wavelength range wider than the cor-
responding monomers M2, and this result is brought about
by the ICT between the carbazole/thiophene donor and the
Qx acceptor. P2-P is electrochemically active in both oxida-
tion and reduction regions, and its CV measurements showed
that the HOMO value of the P2 was of ꢀ5.36 eV. Compared
to the solar cell devices made from the broad PDI P2, the
device with low PDI P2-P showed higher solar conversion
efficiency. The present PCPQT polymers showed the best
5
6
electron transportation.
CONCLUSIONS
In this article, we have demonstrated the synthesis of a
novel soluble Qx/thiophene alternating conducting polymers
with a hole transporting moiety of carbazole group as the
PCE up to 0.40% for P2-P/PCBM hybrid system under AM
2
1
.5 simulated sun light (100 mW/cm ). Notably, the solar
cell device made from PCPQT/PCBM exhibits a higher V
oc
3
7
(
ꢁ0.75 V) mainly due to the lower HOMO level of P2. Fur-
ther improvements are anticipated through a rational design
of the new low band-gap Qx/thiophene/carbazole jacket-like
donor–acceptor conducting polymers.
The financial support of this work from the National Science Coun-
cil of Taiwan (Grant No. NSC97-2221-E-002-064-MY3) is greatly
appreciated. The authors would also like to thank all beamline staff
and technical support at the National Synchrotron Radiation
Research Center of Taiwan for the X-ray measurements.
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