Measurement and control of exciton spin in organic light emitting devices

Abstract

Organic semiconductors are a promising new material set for electronic and optoelectronic devices. Their properties can be precisely controlled through chemistry, and they are well-suited for large-area, flexible, and low-cost devices. Optical emission and absorption in these materials is mediated by strongly-bound electron-hole pairs called "excitons". While the function of many organic electronic devices depends on excitons, exciton formation is incompletely understood. This thesis presents a general rate model for exciton formation, and studies formation through three different experimental approaches, in the context of the rate model. First, a novel method for measuring exciton spin statistics is described and implemented. This method avoids several drawbacks common to existing methods, and shows completely randomized exciton spin statistics in two archetypal organic semiconductors: one that is a small molecule, and another that is a polymer. Second, optically-detected magnetic resonance effects in organic semiconductors are shown to be unrelated to exciton formation processes, contrary to the current understanding. A quenching-based model is developed and shown to completely describe the data. Both of these experimental results suggest an absence of spin mixing of exciton precursor states. In the third section of this thesis, this lack of mixing is confirmed both experimentally and through calculation. It is then "turned on" through the introduction of spin-orbit coupling. An approximately three-fold increase in the fluorescent efficiency of an organic light emitting device results.