the end groups of the self-assembled monolayer (e.g., –CN, –F versus CH 3 ) and
thus the surface dipole, it is possible to shift the threshold voltage and alter the
orientation of organic molecules at the interface [73].
Early studies on P3HT employed HMDS-treated SiO 2 surfaces and found
substantial enhancement of mobility compared to bare SiO 2 [4, 5]. This was
attributed to the promotion of edge-on orientation and thus π–π stacking of the
P3HT molecules. Subsequent experiments showed that using alkylsilanes, in particular octadecyltrichlorosilane (OTS), significantly improved the crystallinity and
order of P3HT close to the interface [74–76]. Modern X-ray characterization
techniques such as WAXS, SAXS, NEXAFS, etc. played an important role in
analyzing the buried P3HT interface in these devices [77]. Overall, it has emerged
that self-assembled monolayers such as OTS are crucial for the nucleation of P3HT
crystallites at the dielectric surface where charge transport takes place. Kline
et al. first showed that for low molecular weight P3HT, surface treatment with
OTS instead of HMDS increased mobility by orders of magnitude and explained
this by improved edge-on orientation and thus more shallow-angle grain boundaries
between crystallites. The effect was less pronounced for high molecular weight
P3HT [75]. Jimison et al. later found that the crystallinity of spincoated P3HT films
was actually lower at the surface than in the bulk, whether the surface was treated or
not. However, the interaction of the alkyl chains of the polymer with those of the
OTS apparently promoted nucleation and thus the density of interface-nucleated
crystallites was 20 times larger than on bare SiO 2 , leading to higher carrier
mobilities [76].
It is important to realize that these results are to a large degree specific to the
particular bottom gate device structure. The mobility in top-gated FETs, which are
common in flexible electronics, depends on the roughness of the P3HT surface and
the microstructure of the film at the top surface. Spincoating or printing a gate
dielectric on top also means that the semiconductor film is exposed to another
solvent. Even if an orthogonal solvent is chosen that does not dissolve the P3HT, it
may swell the film and thus alter its structure. In addition, the dielectric constant of
the dielectric plays a role and influences the charge carrier mobility due to dipolar
disorder and the associated broadening of the density of states [78, 79].
3.5 Alignment and Grain Boundaries
Detailed control and characterization of the microstructure in P3HT thin films
enables inference as to the nature of charge transport in this conjugated polymer.
Early studies had suggested that the π–π stacking in P3HT is responsible for fast
charge transport [5]. Although this is true, because interchain transport ultimately
limits the overall mobility in P3HT thin films, the π–π stacking direction is not the
direction of highest mobility. According to theoretical calculations [80, 81], the
fastest charge transport should still be that along the polymer backbone (i.e., the
intrachain transport), as is the case for many other conjugated polymers [82, 83]. In
120
J. Zaumseil
thus the surface dipole, it is possible to shift the threshold voltage and alter the
orientation of organic molecules at the interface [73].
Early studies on P3HT employed HMDS-treated SiO 2 surfaces and found
substantial enhancement of mobility compared to bare SiO 2 [4, 5]. This was
attributed to the promotion of edge-on orientation and thus π–π stacking of the
P3HT molecules. Subsequent experiments showed that using alkylsilanes, in particular octadecyltrichlorosilane (OTS), significantly improved the crystallinity and
order of P3HT close to the interface [74–76]. Modern X-ray characterization
techniques such as WAXS, SAXS, NEXAFS, etc. played an important role in
analyzing the buried P3HT interface in these devices [77]. Overall, it has emerged
that self-assembled monolayers such as OTS are crucial for the nucleation of P3HT
crystallites at the dielectric surface where charge transport takes place. Kline
et al. first showed that for low molecular weight P3HT, surface treatment with
OTS instead of HMDS increased mobility by orders of magnitude and explained
this by improved edge-on orientation and thus more shallow-angle grain boundaries
between crystallites. The effect was less pronounced for high molecular weight
P3HT [75]. Jimison et al. later found that the crystallinity of spincoated P3HT films
was actually lower at the surface than in the bulk, whether the surface was treated or
not. However, the interaction of the alkyl chains of the polymer with those of the
OTS apparently promoted nucleation and thus the density of interface-nucleated
crystallites was 20 times larger than on bare SiO 2 , leading to higher carrier
mobilities [76].
It is important to realize that these results are to a large degree specific to the
particular bottom gate device structure. The mobility in top-gated FETs, which are
common in flexible electronics, depends on the roughness of the P3HT surface and
the microstructure of the film at the top surface. Spincoating or printing a gate
dielectric on top also means that the semiconductor film is exposed to another
solvent. Even if an orthogonal solvent is chosen that does not dissolve the P3HT, it
may swell the film and thus alter its structure. In addition, the dielectric constant of
the dielectric plays a role and influences the charge carrier mobility due to dipolar
disorder and the associated broadening of the density of states [78, 79].
3.5 Alignment and Grain Boundaries
Detailed control and characterization of the microstructure in P3HT thin films
enables inference as to the nature of charge transport in this conjugated polymer.
Early studies had suggested that the π–π stacking in P3HT is responsible for fast
charge transport [5]. Although this is true, because interchain transport ultimately
limits the overall mobility in P3HT thin films, the π–π stacking direction is not the
direction of highest mobility. According to theoretical calculations [80, 81], the
fastest charge transport should still be that along the polymer backbone (i.e., the
intrachain transport), as is the case for many other conjugated polymers [82, 83]. In
120
J. Zaumseil