Photon-assisted oxygen diffusion and oxygen-related traps in organic semiconductors.

Organic molecular crystals represent an excellent platform for fundamental studies of optoelectronic properties of organic semiconductors. [ 1 ] Rubrene, in particular, has attracted signifi cant attention due to a very low density of charge traps, recordhigh charge carrier mobility, and a very large micrometer-scale exciton diffusion length. [ 2 , 3 ] In this study, using rubrene as a model compound questions related to the interaction of crystalline organic semiconductors with oxygen are addressed. Although the chemistry of photo-oxidation of rubrene in solution and gas phases is understood based on self-sensitized photo-oxidation (see for example, Ref. [ 4 ] ), the mechanism of oxygen incorporation and its effect on the photophysical properties of crystalline rubrene still remain controversial. Mitrofanov et al. [ 5 ] identifi ed an oxygen-related in-gap band in photooxidized rubrene by means of a two-photon photoluminescence excitation spectroscopy and showed that oxygen is concentrated near the surface of the crystals. Krellner et al. [ 6 ] revealed that an oxygen-related impurity forms an acceptor-like in-gap state in rubrene with an energy ≈ 0.28 eV above the highest occupied molecular orbital (HOMO). X. Song et al. [ 7 ] used photoemission and X-ray absorption spectroscopies to show that the HOMO of oxidized rubrene is ≈ 1 eV deeper than that of pure rubrene and hence can not account for formation of acceptor-like states within the bandgap. Therefore, in this contribution, we set out to answer the following questions: What triggers oxygen incorporation in molecular crystals? What is its effect on the conductivity of these materials? What dictates the depth distribution of oxygen defects? In this paper, a highly sensitive trap characterization technique based on wavelengthand polarization-resolved photocurrent excitation spectroscopy of single crystalline organic semiconductors is developed and applied for the fi rst time. The method reveals that even a brief illumination of organic