conditions established by the co-solvent. DIO is by far the most popular co-solvent
in terms of citation frequency. It has been used with most of the new low band gap
alternating push–pull copolymers and often yields a superior PCE than films cast
without DIO.
2.2.4 Pre-formed Polymer Particles
A further fabrication technique that has been used for OPV devices is the use of
preformed polymer nanoparticles. Berson et al. published an article that examined
the formation of P3HT nanofibers in solution, followed by coating the nanofibers
with PCBM to form a BHJ layer [64, 139]. The idea behind the experiment was that
highly crystalline P3HT fibers could be formed in solution that would have fewer
defects than P3HT domains in a typical BHJ film. The principle is correct and the
spectrum of P3HT fibers shows considerably more vibronic character than P3HT
films [64, 139]. Subsequent work on P3HT fibers showed that P3HT forms highly
fluorescent J-aggregates in solution under slow cooling conditions [63, 72,
140]. However, OPV devices cast from P3HT fibers have lower PCE than normal
BHJ devices of the same thickness [64, 141]. It is known that the P3HT fibers lie
within the plane of the film, so it is possible that the fibers are poor conductors of
holes. In this case, we expect the J sc to be lower due to increased recombination at
unattached fibers. Also, the FF is expected to be lower because it probably requires
an activation potential for a hole to hop from one fiber to the next. This activation
barrier would appear as a series resistance and reduce the FF.
The mechanism was tested by adding a small amount of amorphous P3HT to the
P3HT fibers and re-fabricating OPV devices [64]. The OPV devices with some
amorphous P3HT had increased J sc and FF. The authors concluded that the P3HT
fibers lacked connectivity and that addition of some amorphous P3HT reestablished
the connectivity between P3HT domains [64]. Several other thiophenes were tested
for polymer nanoparticle-based OPV devices. It was found that poly
(quaterthiophene) (PQ12T) was too insoluble and that polymer domains with
extended sizes formed in solution [141]. Oosterbaan et al. fabricated nanofibers
from a series of thiophenes with side chain length from 3 to 9 [139]. They concluded that P3HT has the ideal side chain length for polymer nanofiber OPV.
Shorter side chains leave a less-soluble polymer that is immiscible with PCBM so
OPV device layers do not have enough donor–acceptor interface, which reduces the
charge separation probability. Longer side chains destabilize the polymer fibers and
allow defects within the fiber to form. For long side chains, the miscibility with
PCBM is too high and too much intermixed donor–acceptor phase forms, leading to
increased recombination [139]. Another study of polymer nanofiber OPV was
published by Xin et al., who worked on characterization of P3BT:PC 71 BM
nanofiber OPV devices (Fig. 10) [142]. P3BT:PC 71 BM formed OPV devices with
high EQE and increased efficiency compared with the Oosterbaan study. Analysis
of the fabrication method shows that a combination of nanofiber formation (solvent
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
201
in terms of citation frequency. It has been used with most of the new low band gap
alternating push–pull copolymers and often yields a superior PCE than films cast
without DIO.
2.2.4 Pre-formed Polymer Particles
A further fabrication technique that has been used for OPV devices is the use of
preformed polymer nanoparticles. Berson et al. published an article that examined
the formation of P3HT nanofibers in solution, followed by coating the nanofibers
with PCBM to form a BHJ layer [64, 139]. The idea behind the experiment was that
highly crystalline P3HT fibers could be formed in solution that would have fewer
defects than P3HT domains in a typical BHJ film. The principle is correct and the
spectrum of P3HT fibers shows considerably more vibronic character than P3HT
films [64, 139]. Subsequent work on P3HT fibers showed that P3HT forms highly
fluorescent J-aggregates in solution under slow cooling conditions [63, 72,
140]. However, OPV devices cast from P3HT fibers have lower PCE than normal
BHJ devices of the same thickness [64, 141]. It is known that the P3HT fibers lie
within the plane of the film, so it is possible that the fibers are poor conductors of
holes. In this case, we expect the J sc to be lower due to increased recombination at
unattached fibers. Also, the FF is expected to be lower because it probably requires
an activation potential for a hole to hop from one fiber to the next. This activation
barrier would appear as a series resistance and reduce the FF.
The mechanism was tested by adding a small amount of amorphous P3HT to the
P3HT fibers and re-fabricating OPV devices [64]. The OPV devices with some
amorphous P3HT had increased J sc and FF. The authors concluded that the P3HT
fibers lacked connectivity and that addition of some amorphous P3HT reestablished
the connectivity between P3HT domains [64]. Several other thiophenes were tested
for polymer nanoparticle-based OPV devices. It was found that poly
(quaterthiophene) (PQ12T) was too insoluble and that polymer domains with
extended sizes formed in solution [141]. Oosterbaan et al. fabricated nanofibers
from a series of thiophenes with side chain length from 3 to 9 [139]. They concluded that P3HT has the ideal side chain length for polymer nanofiber OPV.
Shorter side chains leave a less-soluble polymer that is immiscible with PCBM so
OPV device layers do not have enough donor–acceptor interface, which reduces the
charge separation probability. Longer side chains destabilize the polymer fibers and
allow defects within the fiber to form. For long side chains, the miscibility with
PCBM is too high and too much intermixed donor–acceptor phase forms, leading to
increased recombination [139]. Another study of polymer nanofiber OPV was
published by Xin et al., who worked on characterization of P3BT:PC 71 BM
nanofiber OPV devices (Fig. 10) [142]. P3BT:PC 71 BM formed OPV devices with
high EQE and increased efficiency compared with the Oosterbaan study. Analysis
of the fabrication method shows that a combination of nanofiber formation (solvent
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
201