From two-dimensional electron gas to localized charge: Dynamics of polaron formation in organic semiconductors

Abstract

Electronic transport in organic semiconductors is mediated by localized polarons. However, the dynamics on how delocalized electrons collapse into polarons through electron-nuclear interaction is not well known. In this work, we use time- and angle-resolved photoemission spectroscopy to study polaron formation in titanyl phthalocyanine deposited on $\mathrm{Au}(111)$ surfaces. Electrons are optically excited from the metal to the organic layer via the image potential state, which evolves from a dispersive to a nondispersive state after photoexcitation. The spatial size of the electrons is determined from the band structure using a tight-binding model. It is observed that the two-dimensional electron wave collapses into a wave packet of size \ensuremath{\sim}3 nm within 100 fs after photoexcitation.