Physics and simulation of transport processes in hybrid organic semiconductor devices

Abstract

Organic semiconductors and nanomaterials promise to potentially form the basis for future efficient and cost-effective large area optoelectronic devices, such as lightemitting diodes and solar cells. Although these materials' amorphous nature allow utilization of cheap, high-throughput manufacturing techniques, it poses a unique challenge: the physics of carrier and excitation transport in amorphous semiconductors is fundamentally different from their crystalline semiconductor counterparts. Excitations remain localized on single molecules or nanocrystals; the drift-diffusion equations, which describe carrier transport in delocalized states near thermal equilibrium, are no longer valid. A computational model for device operation would give researchers a powerful tool to design and improve devices. This work presents a novel one-dimensional discrete model that combines the computational speed of simulations based on the drift-diffusion equations with the accuracy and flexibility of Monte Carlo simulations. The one-dimensional model is shown to be exactly equivalent to the drift-diffusion model in the limits of small applied field, narrow densities of state, and low carrier concentrations. In this limit, the Einstein relation for Brownian motion holds and the transport parameters in the one-dimensional discrete model can be directly estimated from experimentally-measurable quantities. The model is implemented in an object-oriented Python computational framework. Finally, two test cases are numerically studied: an initial, test device with fictitious parameters and a well-known organic light-emitting diode. Preliminary results demonstrate reproduce experimental current-voltage characteristics over a wide range of bias voltages.