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 (SiN_x) gate insulator. Here, these surface properties are modified with varying the duration of oxygen plasma treatments on the SiN_x 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 (χ_SR) while the surface energy (E_S) determines the reference mobility (μ_b), yielding the effective hole mobility expressed as a product of two terms associated with μ_b and χ_SR, respectively. It is found that μ_b follows a power law as a function of E_S, and χ_SR grows exponentially with increasing R_a. In addition, a characteristics scattering length (λ_c) appears in the mobility expression, which turns out to be a demarcation, suggesting that R_a is required to be at least smaller than λ_c to minimize χ_SR.