Channels of oxygen diffusion in single crystal rubrene revealed

Abstract

Electronic devices made from organic materials have the potential to support a more ecologically friendly and affordable future. However, the ability to fabricate devices with well-defined and reproducible electrical and optical properties is hindered by the sensitivity to the presence of chemical impurities. Oxygen in particular is an impurity that can trap electrons and modify conductive properties of some organic materials. Until now the 3-dimensional profiling of oxygen species in organic semiconductors has been elusive and the effect of oxygen remains disputed. In this study we map out high-spatial resolution 3-dimensional distributions of oxygen inclusions near the surface of single crystal rubrene, using Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). Channels of diffused oxygen are found extending from uniform oxygen inclusion layers at the surface. These channels extend to depths in excess of 1.8 μm and act as an entry point for oxygen to diffuse along the ab-plane of the crystal with at least some of the diffused oxygen molecularly binding to rubrene. Our investigation of surfaces at different stages of evolution reveals the extent of oxygen inclusion, which affects rubrene's optical and transport properties, and is consequently of importance for the reliability and longevity of devices.

Local anodic oxidation lithography on organic semiconductor crystals: Oxide depth revealed by conductance tomography

“…Rubrene is considered the prototype single crystal organic semiconductor because of its relatively large charge carrier mobility (10-40 cm 2 /(V•s)) [10][11][12] and long exciton diffusion lengths (~4 microns) [13,14]. Oxidation properties of the single crystal have been studied in detail; in addition to evidence of an epitaxial native oxide layer [15], oxygen-related species have been observed uniformly on the surface and preferentially at structural defects [16], as well as under the surface [17], having accessed preferential channels into the bulk before spreading laterally [18]. Meanwhile, multiple oxygen incorporation mechanisms have been proposed, each with unique oxygen exposure methods, suggesting varying effects on rubrene device performance.…”