parameters of P3HT thin films actually increase with molecular weight from about
3 to 7% and saturate at around the same molecular weight as the mobility. This
suggests that for long polymer chains, which guarantee good connectivity between
the P3HT aggregates, the paracrystallinity starts to limit charge transport and
therefore carrier mobility.
3.3 Solvents, Deposition Conditions, and Post-deposition
Treatment
The order and crystallinity of P3HT also depend on the film deposition conditions.
Spincoating, dip-coating, and dropcasting are the most popular techniques for
research. In general, slow evaporation of the solvent improves aggregation/crystallinity and thus mobility. Dropcast films tend to show superior performance over
spincoated samples. A detailed study by Chang et al. showed that the use of high
boiling point solvents such as trichlorobenzene instead of chloroform for
spincoating P3HT led to higher crystallinity and, consequently, improvement in
mobility by a factor of ten from 0.012 to 0.12 cm
2 V
À1 s
À1 for medium molecular
weight P3HT (see Fig. 6) [36]. Interestingly, the dependence of the transconductance and thus the mobility on gate voltage also changed with the boiling
point of the solvent. Clark et al. showed that this behavior can be correlated with
the free exciton bandwidth of the P3HT aggregates, as determined from simple
absorption measurements [37]. Within the multiple trapping and release model
[38], a shallow distribution of localized states (i.e., narrow exciton bandwidth as
seen for films spincoated from high boiling point solvent) leads to a weakly gate
voltage-dependent mobility, whereas a broad distribution of localized states (i.e.,
also broad exciton bandwidth in films spincoated from low boiling point solvents)
results in strongly gate voltage-dependent mobilities.
The spin speed also has an effect on the orientation of the P3HT chains within a
spincoated film and thus on the obtained mobility because it affects the drying time.
DeLongchamp et al. showed that fast spinning speeds (4,000 rpm) resulted in more
plane-on orientation [determined by near-edge X-ray absorption fine structure
(NEXAFS) spectroscopy] and lower mobilities, whereas low spinning speeds
(250 rpm) led to edge-on orientation and higher mobilities for P3HT thin films
deposited from chloroform [39].
Dropcasting is another way to ensure pre-aggregation of the polymer chains in
solution and thus preferential ordering during a slow drying process. Film crystallinity as determined by AFM and X-ray diffraction is generally higher than for
spincoated films [40]. Lowering the temperature during dropcasting enhances
crystallinity further [41]. However, dropcast films tend to be less uniform than
spincoated films. They usually show a coffee-stain effect. Similarly, long drying
times and more uniform films over large areas can be obtained by doctor-blading
and wire bar-coating of polymer solutions [42, 43].
116
J. Zaumseil
3 to 7% and saturate at around the same molecular weight as the mobility. This
suggests that for long polymer chains, which guarantee good connectivity between
the P3HT aggregates, the paracrystallinity starts to limit charge transport and
therefore carrier mobility.
3.3 Solvents, Deposition Conditions, and Post-deposition
Treatment
The order and crystallinity of P3HT also depend on the film deposition conditions.
Spincoating, dip-coating, and dropcasting are the most popular techniques for
research. In general, slow evaporation of the solvent improves aggregation/crystallinity and thus mobility. Dropcast films tend to show superior performance over
spincoated samples. A detailed study by Chang et al. showed that the use of high
boiling point solvents such as trichlorobenzene instead of chloroform for
spincoating P3HT led to higher crystallinity and, consequently, improvement in
mobility by a factor of ten from 0.012 to 0.12 cm
2 V
À1 s
À1 for medium molecular
weight P3HT (see Fig. 6) [36]. Interestingly, the dependence of the transconductance and thus the mobility on gate voltage also changed with the boiling
point of the solvent. Clark et al. showed that this behavior can be correlated with
the free exciton bandwidth of the P3HT aggregates, as determined from simple
absorption measurements [37]. Within the multiple trapping and release model
[38], a shallow distribution of localized states (i.e., narrow exciton bandwidth as
seen for films spincoated from high boiling point solvent) leads to a weakly gate
voltage-dependent mobility, whereas a broad distribution of localized states (i.e.,
also broad exciton bandwidth in films spincoated from low boiling point solvents)
results in strongly gate voltage-dependent mobilities.
The spin speed also has an effect on the orientation of the P3HT chains within a
spincoated film and thus on the obtained mobility because it affects the drying time.
DeLongchamp et al. showed that fast spinning speeds (4,000 rpm) resulted in more
plane-on orientation [determined by near-edge X-ray absorption fine structure
(NEXAFS) spectroscopy] and lower mobilities, whereas low spinning speeds
(250 rpm) led to edge-on orientation and higher mobilities for P3HT thin films
deposited from chloroform [39].
Dropcasting is another way to ensure pre-aggregation of the polymer chains in
solution and thus preferential ordering during a slow drying process. Film crystallinity as determined by AFM and X-ray diffraction is generally higher than for
spincoated films [40]. Lowering the temperature during dropcasting enhances
crystallinity further [41]. However, dropcast films tend to be less uniform than
spincoated films. They usually show a coffee-stain effect. Similarly, long drying
times and more uniform films over large areas can be obtained by doctor-blading
and wire bar-coating of polymer solutions [42, 43].
116
J. Zaumseil