Quasi-ordering in spontaneously associated surface dipoles: an intrinsic interfacial factor for high-kpolymer insulated organic field-effect transistors

Abstract

In this study, we report the observation of quasi-ordering in spontaneously associated highly polar surface functional groups (CN) in the high-kpolymer dielectric, cyanoethyl pullulan, and its impact on the organic field-effect transistor (OFET) characteristics. We find that the association originates from CN⋯H–C–CN hydrogen bonding as confirmed by XPS, NEXAFS experiments and molecular simulations. The quasi-ordered surface dipoles preferentially induce vertically well-stacked local semiconductor molecular clusters during the initial deposition process, which then promote large-area layer-by-layer growth. By maintaining sufficient quasi-ordering, high transistor performance (μ ≈ 6.5 cm² V⁻¹ s⁻¹, SS ≈ 0.062 V dec⁻¹) is obtained under low driving voltages (−3 to −5 V), while breakup of the association at higher baking temperatures leads to a dramatic drop in μ by a factor of ∼10. Our results demonstrate that local quasi-ordering of polymeric surface dipoles, which has a significant effect on the initial semiconductor molecular growth, represents a novel and sensitive factor affecting OFET characteristics.