Influence of Surface Energy and Roughness on Hole Mobility in Solution-Processed Hybrid Organic Thin Film Transistors

We investigate the influence of the surface energy and roughness on the hole mobility in organic thin-film transistors where a poly(3,3'-dialkylquarterthiophene) (PQT-12) hybridized with an octyltrichlorosilane self-assembled monolayer is employed as the semiconducting layer on a silicon nitride (SiNx) gate insulator. Here, these surface properties are modified with varying the duration of oxygen plasma treatments on the SiNx surface, eluding to a different surface roughness and energy. From our analysis coupled with the experimental results, it is found that the surface roughness (R-a) controls the degree of surface roughness scattering (chi SR) while the surface energy (E-S) determines the reference mobility (mu b), yielding the effective hole mobility expressed as a product of two terms associated with mu b and chi SR, respectively. It is found that mu b follows a power law as a function of ES, and chi SR grows exponentially with increasing R-a. In addition, a characteristics scattering length (lambda(c)) appears in the mobility expression, which turns out to be a demarcation, suggesting that Ra is required to be at least smaller than lambda c to minimize chi SR.