Bottom-contact poly(3,3′-didodecylquaterthiophene) thin-film transistors with gold source-drain electrodes modified by alkanethiol monolayers

A series of alkanethiol monolayers (CH₃(CH₂) _n-1SH, n = 4, 6, 8, 10, 12, 14, 16) were used to modify gold source-drain electrode surfaces for bottom-contact poly(3,3‴- didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs). The device mobilities of TFTs were significantly increased from ∼0.015 cm² V^-1 s^-1 for bare electrode TFTs to a maximum of ∼0.1 cm² V^-1 s^-1 for the n = 8 monolayer devices. The mobilities of devices with alkanethiolmodified Au electrodes varied parabolically with alkyl length with values of 0.06, 0.1, and 0.04 cm ² V^-1 s^-1 at n = 4, 8, and 16, respectively. Atomic force microscopy investigations reveal that alkanethiol electrode surface modifications promote polycrystalline PQT-12 morphologies at electrode/PQT-12 contacts, which probably increase the density of states of the PQT-12 semiconductor at the interfaces. The contact resistance of TFTs is strongly modulated by the surface modification and strongly varies with the alkanethiol chain length. The surface modifications of electrodes appear to significantly improve the charge injection, with consequent substantial improvement in device performance.