Correlating optimal electrode buffer layer thickness with the surface roughness of the active layer in organic phototransistors

•The C60 buffer layer thicknesses of the modified pentacene-OPTs were further optimized.•The optimized buffer layer thickness increases with the increasing of the surface roughness of the active layer in OPTs.•An important parameter, saturated photoresponsivity, which can be used for comparing the performances of OPTs was introduced.