At this point the researcher needs to learn how to produce uniform films of the
new polymer mixed with the chosen fullerene without using too much of the
valuable polymer. With the mixture of P3HT:PCBM, the PCE can increase tenfold
with no change in layer thickness, concentration ratio, or deposition solvent
[42]. This means that even if wise coating choices are made about the polymer
solution and a highly uniform film is coated, the resulting PCE may still be much
lower than that for an optimized morphology. The PCE increase for P3HT:PCBM
comes from briefly annealing the film at 150
C, which causes a change in the
nanoscale morphology and the altered morphology leads to an increase in PCE. The
fabrication toolkit can therefore be further subdivided into fabrication steps that
occur before, during, and after coating. The following sections attempt to describe
the many interrelated but independently controlled fabrication parameters that
affect morphology formation. We focus on how fabrication choices affect device
morphology and thus efficiency.
2.1.1 Molecular Weight
Unlike small molecules, polymers do not have a well-defined M W and a polymer
sample contains a distribution of molecular weights so it is difficult to compare the
molar ratio with a small molecule such as C 60 or PCBM. Instead, one typically
reports a weight ratio or wt%, so the molar ratio of polymer repeat units with respect
to PCBM is fixed. However, the polymer M W has a large effect on the solubility of
the polymer and the miscibility of the polymer with the fullerene. Smaller M W
polymers or oligomers are in general more soluble and so can be processed with a
wider variety of solvents. However, the smaller M W means that the polymer chains
are less entangled and that all species in both solution and melt phases can diffuse
more quickly. As a result, low M W polymers and oligomers tend to make less
viscous solutions, phase separate on a larger length scale, and low M W films are
more likely to de-wet or form voids.
The relationship between polymer M W and PCE has been exhaustively studied in
P3HT and mixtures with PCBM [49–61]. We will attempt to briefly summarize the
lessons here. First, a lower M W polymer is more soluble and more miscible with
PCBM. Also, in pure P3HT samples, it has been shown that aggregated domain
content (for a given solvent and temperature) increases with M W up to ~20 kDa as a
result of increased regioregularity and reduced sample entropy [54, 62]. Even
higher M W (~70 kDa) P3HT has been shown to form highly crystalline nanofibers
that can be solution processed [63, 64]. In mixed samples, the PCBM interferes with
the formation of pure P3HT domains, so the P3HT is less crystalline. The P3HT
crystallinity can be reintroduced by annealing the sample to allow phase separation.
For a high regioregularity and M W , the solubility of PCBM in P3HT is ~30% [65].
Another important result was the realization that charge mobility did not necessarily scale with crystallinity [58]. Kline et al. and others showed that for pure
P3HT, lower M W polymer formed samples with higher apparent crystalline content
but that the crystals had few connecting polymer strands leading to poor
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
191
new polymer mixed with the chosen fullerene without using too much of the
valuable polymer. With the mixture of P3HT:PCBM, the PCE can increase tenfold
with no change in layer thickness, concentration ratio, or deposition solvent
[42]. This means that even if wise coating choices are made about the polymer
solution and a highly uniform film is coated, the resulting PCE may still be much
lower than that for an optimized morphology. The PCE increase for P3HT:PCBM
comes from briefly annealing the film at 150
C, which causes a change in the
nanoscale morphology and the altered morphology leads to an increase in PCE. The
fabrication toolkit can therefore be further subdivided into fabrication steps that
occur before, during, and after coating. The following sections attempt to describe
the many interrelated but independently controlled fabrication parameters that
affect morphology formation. We focus on how fabrication choices affect device
morphology and thus efficiency.
2.1.1 Molecular Weight
Unlike small molecules, polymers do not have a well-defined M W and a polymer
sample contains a distribution of molecular weights so it is difficult to compare the
molar ratio with a small molecule such as C 60 or PCBM. Instead, one typically
reports a weight ratio or wt%, so the molar ratio of polymer repeat units with respect
to PCBM is fixed. However, the polymer M W has a large effect on the solubility of
the polymer and the miscibility of the polymer with the fullerene. Smaller M W
polymers or oligomers are in general more soluble and so can be processed with a
wider variety of solvents. However, the smaller M W means that the polymer chains
are less entangled and that all species in both solution and melt phases can diffuse
more quickly. As a result, low M W polymers and oligomers tend to make less
viscous solutions, phase separate on a larger length scale, and low M W films are
more likely to de-wet or form voids.
The relationship between polymer M W and PCE has been exhaustively studied in
P3HT and mixtures with PCBM [49–61]. We will attempt to briefly summarize the
lessons here. First, a lower M W polymer is more soluble and more miscible with
PCBM. Also, in pure P3HT samples, it has been shown that aggregated domain
content (for a given solvent and temperature) increases with M W up to ~20 kDa as a
result of increased regioregularity and reduced sample entropy [54, 62]. Even
higher M W (~70 kDa) P3HT has been shown to form highly crystalline nanofibers
that can be solution processed [63, 64]. In mixed samples, the PCBM interferes with
the formation of pure P3HT domains, so the P3HT is less crystalline. The P3HT
crystallinity can be reintroduced by annealing the sample to allow phase separation.
For a high regioregularity and M W , the solubility of PCBM in P3HT is ~30% [65].
Another important result was the realization that charge mobility did not necessarily scale with crystallinity [58]. Kline et al. and others showed that for pure
P3HT, lower M W polymer formed samples with higher apparent crystalline content
but that the crystals had few connecting polymer strands leading to poor
P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance
191