Communications
On the other hand, the choice of the p-type polymers
range from 680 nm for PDBS–PDI to 940 nm for PDTP–PDI,
thus proving that the absorption spectra could be tuned over a
wide range by using donor segments with different electron-
donating abilities. All these polymers show similar absorption
combined with the acceptor is also crucial for all-PSCs.
Among the wide variety of p-type photovoltaic polymers,
regioregular P3HT has been extensively studied by blending it
[
5]
[7]
with fullerene derivatives or n-type polymers. However,
for combination with n-type polymers, P3HT might not
necessarily be the best choice. Li et al. developed a series of
conjugated side chain polythiophenes and applied them in
spectra in CHCl (Figure S3); the tuning over a wide range is
3
highly useful for designing n-type polymers to complement
the absorption of p-type polymers for all-PSC application.
The absorption spectra of PT1 and P3HT, both in
chlorobenzene and in film, are shown in Figure S4. In
solution, PT1 shows an absorption maximum at 476 nm,
which is 20 nm red-shifted compared to that of P3HT, because
of the enhanced conjugation caused by the tris(thienylenevi-
nylene) side chain. In film, P3HT shows a large red-shift with
an absorption maximum at 553 nm and a shoulder at
approximately 600 nm, which corresponds to strong p–p
stacking. In film, PT1, however, only shows a relatively small
red-shift (21 nm), without any shoulder, probably because of
its regiorandom structure. Although PT1 shows no improve-
ment in its absorption spectra compared with P3HT, the
introduction of a conjugated side chain may contribute to the
[10]
both polymer/fullerene and all-polymer devices.
Herein, we used P3HT and a polythiophene derivative
with conjugated side chains (PT1) as p-type semiconducting
polymers (Scheme 1). We systematically investigated the
photovoltaic properties by using all donor/acceptor combina-
tions and analyzed the results from the viewpoints of the
absorption spectra, the alignment of the energy levels, and the
mixed morphology of the films.
The synthesis of six PX–PDIs is shown in Figure S1 in the
Supporting Information. PV–PDI, PT–PDI, and PDTP–PDI
were synthesized by the Stille coupling reaction and PF–PDI,
PDBS–PDI, and PC–PDI were synthesized by the Suzuki
[
9]
[10]
coupling reaction as previously reported. For the donor
material, P3HT was purchased from Merck Chemicals; PT1
was synthesized by the Stille reaction (Figure S2). All
polymers show good solubility in common organic solvents,
such as chloroform (CF), chlorobenzene (CB), o-dichloro-
benzene (DCB), toluene, and xylene.
charge transport to the corresponding electrodes.
The electrochemical properties of the six n-type polymers
and two p-type polymers are also investigated by cyclic
voltammetry (CV, Figure S5 and S6). All PDI-based polymers
undergo reversible reductive n-doping/dedoping (reduction/
reoxidation) processes, while only PDTP–PDI showed rever-
sible p-doping/dedoping (oxidation/rereduction) processes.
The reason for this behavior is not yet clear; however, it is
speculated to be due to the high oxidation potential of the
other five polymers. The highest occupied molecular orbital
As mentioned earlier, the modulation of the absorption
spectra of PDI-based polymers is crucial for the application of
these polymers in all-PSCs. The absorption spectra of six PX–
PDIs in film are shown in Figure 1. PV–PDI, PT–PDI, and
(HOMO) and LUMO energy levels of the conjugated
polymers are calculated from the onset oxidation potential
and onset reduction potential, respectively. By changing the
electron-donating segments, we successfully tailored the
LUMO energy levels of the PDI-based polymers; the
LUMO energy levels varied from 4.05 eV for PV–PDI to
3
.61 eV for PF–PDI, which will help in the choice of
appropriate donor materials for effective charge separation
at the donor/acceptor interface and at the same time for
higher VOC
.
For donor materials, the two polymers show similar
electrochemical band gaps. However, the HOMO of PT1
decreases to 5.08 eV (P3HT: 4.91 eV). It is understood that
the VOC of PSCs is related to the energy difference between
the LUMO of the acceptor (A) and the HOMO of the donor
[
4]
(
D; [LUMO(A)-HOMO(D)]). The CV results indicate that
Figure 1. UV/Vis absorption spectra of six PX–PDI films on quartz
plates.
we can successfully tune this difference by inducing either an
upward shift in energy of the LUMO of PDI-based polymers
or a downward shift in energy of the HOMO of polythio-
phene derivatives (Figure 2).
Prior to their use in all-PSC devices, it is necessary to
confirm that the PDI-based polymers actually function as
electron-transporting materials. To investigate the electron-
transport properties of PX–PDIs in solid films, bottom-gate,
top-contact field-effect transistors (FETs) were fabricated by
PDTP–PDI show absorption in the relatively long-wave-
length region probably because of the strong electron-
donating ability of these donor units used, especially for
PDTP–PDI, and/or the small steric hindrance of vinylene for
PV–PDI. However, PDBS–PDI, PF–PDI, and PC–PDI
absorb at relatively short wavelengths, which is attributable
to the relatively weak electron-donating ability and large
steric hindrance of bulky side chains in these donor building
blocks, thus disturbing the planarity of the backbones. The
absorption onsets of the six polymers in film also show a wide
spin-coating solutions of PX–PDIs in chloroform onto SiO /Si
2
substrates modified with octadecyltrichlorosilane (OTS). All
the transistors based on PX–PDIs exhibited typical n-type
behavior, and distinct field effects were observed from the
2
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2799 –2803
Zhou, Erjun
