Salleo and coworkers in a recent publication [84]. On large length scales, polymer
chains are flexible, whereas the backbone is rather rigid at the length scale of the
persistence length. Therefore, the authors propose a weak bending of the polymer
chains if the distance between adjacent crystallites is in the region of a few
persistence lengths. These tie-chains with extended conjugation serve as an efficient charge transport pathway through disordered domains in high molecular
weight P3HT.
Processing Conditions
Not only molecular parameters, but also the processing conditions strongly affect
the thin film morphology. The literature on different deposition protocols and postprocessing protocols is huge. In this section, we highlight some representative
examples.
Typical solution deposition techniques are dip-coating, drop casting, and spincoating, with the evaporation rate of the solvent increasing in the given order. The
nanostructured morphology is most distinctive in the case of a slow drying of the
film, when the polymer chains have sufficient time to self-assemble into nanocrystalline structures. Changes in the thin film morphology as a function of the solvent
evaporation rate are illustrated in Fig. 14, which shows AFM images of P3HT films
of four different molecular weights cast by three different methods under identical
conditions. Dip-coating and spin-coating were performed from chloroform solution, whereas drop-cast films were obtained from a solvent mixture of chloroform
and tetrahydrofurane (14:3, v/v). Fast solvent evaporation during spin-coating leads
to rod-like structures only for low molecular weight samples, whereas layers of
M n > 10 kg/mol remain almost featureless. By contrast, slow solvent evaporation
achieved by dip-coating results in well-defined nanostructures that are independent
of the molecular weight. During the slow film drying process, the polymer chains
can stack into ordered nanofibrils, even in the case of long chains. The impact of
such a slow solvent evaporation achieved by dip-coating was further demonstrated
by Heeger and coworkers, who studied charge transport in ultrathin dip-coated
films with a thickness of a few nanometers [70]. The latter exhibit highly ordered
structures, as evidenced by well-resolved bands in the optical absorption spectrum,
which correlates with high mobilities reaching 0.2 cm
2 /V s in accumulation mode.
In spin-coated layers, the microcrystalline order is typically low, especially
when low boiling point solvents such as chloroform are used. Order can be
significantly improved by using high boiling point solvents such as trichlorobenzene (TCB), as evidenced by X-ray diffraction and AFM [69]. This also causes
a significant enhancement of the charge carrier mobility to 0.12 cm
2 /V s, which is
around one order of magnitude higher than for layers cast from chloroform. Another
approach to slow solidification was demonstrated by Cho and coworkers, who
reported the formation of highly ordered 1D nanofibrils via spin-coating in a solvent
vapor atmosphere [85]. Spin-coating was carried out from chloroform, and the
amount of chloroform within the spin-coating chamber determined the solvent
60
K. Tremel and S. Ludwigs
chains are flexible, whereas the backbone is rather rigid at the length scale of the
persistence length. Therefore, the authors propose a weak bending of the polymer
chains if the distance between adjacent crystallites is in the region of a few
persistence lengths. These tie-chains with extended conjugation serve as an efficient charge transport pathway through disordered domains in high molecular
weight P3HT.
Processing Conditions
Not only molecular parameters, but also the processing conditions strongly affect
the thin film morphology. The literature on different deposition protocols and postprocessing protocols is huge. In this section, we highlight some representative
examples.
Typical solution deposition techniques are dip-coating, drop casting, and spincoating, with the evaporation rate of the solvent increasing in the given order. The
nanostructured morphology is most distinctive in the case of a slow drying of the
film, when the polymer chains have sufficient time to self-assemble into nanocrystalline structures. Changes in the thin film morphology as a function of the solvent
evaporation rate are illustrated in Fig. 14, which shows AFM images of P3HT films
of four different molecular weights cast by three different methods under identical
conditions. Dip-coating and spin-coating were performed from chloroform solution, whereas drop-cast films were obtained from a solvent mixture of chloroform
and tetrahydrofurane (14:3, v/v). Fast solvent evaporation during spin-coating leads
to rod-like structures only for low molecular weight samples, whereas layers of
M n > 10 kg/mol remain almost featureless. By contrast, slow solvent evaporation
achieved by dip-coating results in well-defined nanostructures that are independent
of the molecular weight. During the slow film drying process, the polymer chains
can stack into ordered nanofibrils, even in the case of long chains. The impact of
such a slow solvent evaporation achieved by dip-coating was further demonstrated
by Heeger and coworkers, who studied charge transport in ultrathin dip-coated
films with a thickness of a few nanometers [70]. The latter exhibit highly ordered
structures, as evidenced by well-resolved bands in the optical absorption spectrum,
which correlates with high mobilities reaching 0.2 cm
2 /V s in accumulation mode.
In spin-coated layers, the microcrystalline order is typically low, especially
when low boiling point solvents such as chloroform are used. Order can be
significantly improved by using high boiling point solvents such as trichlorobenzene (TCB), as evidenced by X-ray diffraction and AFM [69]. This also causes
a significant enhancement of the charge carrier mobility to 0.12 cm
2 /V s, which is
around one order of magnitude higher than for layers cast from chloroform. Another
approach to slow solidification was demonstrated by Cho and coworkers, who
reported the formation of highly ordered 1D nanofibrils via spin-coating in a solvent
vapor atmosphere [85]. Spin-coating was carried out from chloroform, and the
amount of chloroform within the spin-coating chamber determined the solvent
60
K. Tremel and S. Ludwigs