Metal-induced Urbach tail at the gold∕pentacene interface of top-contact organic field effect transistors

We investigated the photocurrent-voltage characteristics of pentacene field effect transistors in order to study the gap state distribution and carrier generation mechanism. In top-contact devices, the photocurrent spectrum versus photon energy exhibited the typical Urbach tail, while such a tail was not observed in bottom-contact devices. The formation of this Urbach tail can be explained by assuming that diffused gold nanoparticles at the gold∕pentacene interface cause microfields and stabilize the excitons. This mechanism also provides an explanation for the reduced barrier height and Ohmic contact formation at "dirty" interfaces fabricated by metal deposition.