macroscopic charge mobility due to a high activation barrier to charge hopping. In
contrast, in high M W P3HT the crystalline domains were well connected to adjoining crystalline domains by individual polymer chains, and so the activation barrier
for charge hopping is much lower [54, 58]. A more detailed discussion of the
morphology of P3HT thin films can be found in the chapter “Morphology of P3HT
in Thin Films” by Kim Tremel and Sabine Ludwigs in this book [66]. Various
studies have shown that a M W of less than 20,000 kD is too low for high efficiency
OPV devices [52, 54, 57]. High M W P3HT is also undesirable because it requires
higher boiling point solvents and longer equilibration times for morphology formation [52]. The “Goldilocks” region for M W for P3HT used for OPV is 20,000–
75,000 kD with a maximum possible regioregularity [67, 68].
2.1.2 Solvent Choice
The choice of casting solvents became a clear issue in 2001 when Shaheen
et al. published an article showing a PCE increase from 1.1 to 2.5 % for MDMOPPV:PCBM OPV devices cast from toluene and chlorobenzene, respectively
[36]. Subsequent studies showed that toluene was a better solvent for the
MDMO-PPV than PCBM, so the PCBM crystallized out of solution with large
domain sizes (>1 μm) that reduced the device quality [69]. However, chlorobenzene and dichlorobenzene are equally good solvents for both polymer and fullerene
so the polymer and fullerene remain miscible to higher concentrations and the
resulting domain sizes are much smaller (tens of nanometers) [69].
A zero-order statement about solvent choice is that the solvent must be equally
good for each component to avoid large-scale phase separation. The next thing to
consider is the absolute solubility of the polymer and fullerene. Polythiophene and
C 60 have negligible solubility in any solvent, whereas P3HT and PCBM have side
chains that greatly increase their solubility. Nevertheless, concentrations of at least
10 mg/mL are necessary to obtain highly planar films of >80 nm thickness without
defects using spin-coating. In general, a higher solubility is desired so that more
concentrated solutions can be made and a wider variety of coating techniques can
be employed. P3HT and PCBM have shown the highest solubility in polar aromatic
solvents such as chlorobenzene, 1,2 dichlorobenzene, and 1-chloronaphthalene.
Another important consideration is the boiling point (BP) of the solvents. The
BP determines how quickly the solvent evaporates and thereby the formation rate
for polymer and fullerene domains. Polymer domains that equilibrate slowly in high
BP solvents tend to be more crystalline (thermodynamic product) whereas low BP
solvents evaporate quickly and yield mixed amorphous films (kinetic product).
When spin-coating at 1,000 rpm, a 20 mg/mL solution of 1:1 P3HT:PCBM condenses to a film in 1–3 s when processed with CHCl 3 (BP ¼ 61
C), 5–10 s when
processed in chlorobenzene (BP ¼ 132
C), and the film remains wet after 60 s when
processed with 1,2 dichlorobenzene (BP ¼ 182
C) (Moule ´, personal observation)
[70]. The change in structural order in P3HT is observed in a red shift of the
absorption spectrum and formation of a clearly defined vibronic structure [71].
192
A.J. Moule ´ et al.
contrast, in high M W P3HT the crystalline domains were well connected to adjoining crystalline domains by individual polymer chains, and so the activation barrier
for charge hopping is much lower [54, 58]. A more detailed discussion of the
morphology of P3HT thin films can be found in the chapter “Morphology of P3HT
in Thin Films” by Kim Tremel and Sabine Ludwigs in this book [66]. Various
studies have shown that a M W of less than 20,000 kD is too low for high efficiency
OPV devices [52, 54, 57]. High M W P3HT is also undesirable because it requires
higher boiling point solvents and longer equilibration times for morphology formation [52]. The “Goldilocks” region for M W for P3HT used for OPV is 20,000–
75,000 kD with a maximum possible regioregularity [67, 68].
2.1.2 Solvent Choice
The choice of casting solvents became a clear issue in 2001 when Shaheen
et al. published an article showing a PCE increase from 1.1 to 2.5 % for MDMOPPV:PCBM OPV devices cast from toluene and chlorobenzene, respectively
[36]. Subsequent studies showed that toluene was a better solvent for the
MDMO-PPV than PCBM, so the PCBM crystallized out of solution with large
domain sizes (>1 μm) that reduced the device quality [69]. However, chlorobenzene and dichlorobenzene are equally good solvents for both polymer and fullerene
so the polymer and fullerene remain miscible to higher concentrations and the
resulting domain sizes are much smaller (tens of nanometers) [69].
A zero-order statement about solvent choice is that the solvent must be equally
good for each component to avoid large-scale phase separation. The next thing to
consider is the absolute solubility of the polymer and fullerene. Polythiophene and
C 60 have negligible solubility in any solvent, whereas P3HT and PCBM have side
chains that greatly increase their solubility. Nevertheless, concentrations of at least
10 mg/mL are necessary to obtain highly planar films of >80 nm thickness without
defects using spin-coating. In general, a higher solubility is desired so that more
concentrated solutions can be made and a wider variety of coating techniques can
be employed. P3HT and PCBM have shown the highest solubility in polar aromatic
solvents such as chlorobenzene, 1,2 dichlorobenzene, and 1-chloronaphthalene.
Another important consideration is the boiling point (BP) of the solvents. The
BP determines how quickly the solvent evaporates and thereby the formation rate
for polymer and fullerene domains. Polymer domains that equilibrate slowly in high
BP solvents tend to be more crystalline (thermodynamic product) whereas low BP
solvents evaporate quickly and yield mixed amorphous films (kinetic product).
When spin-coating at 1,000 rpm, a 20 mg/mL solution of 1:1 P3HT:PCBM condenses to a film in 1–3 s when processed with CHCl 3 (BP ¼ 61
C), 5–10 s when
processed in chlorobenzene (BP ¼ 132
C), and the film remains wet after 60 s when
processed with 1,2 dichlorobenzene (BP ¼ 182
C) (Moule ´, personal observation)
[70]. The change in structural order in P3HT is observed in a red shift of the
absorption spectrum and formation of a clearly defined vibronic structure [71].
192
A.J. Moule ´ et al.