Unified Dynamic Approach for Simulating Quantum Tunneling and Thermionic Emission at Metal-Organic Interfaces

Abstract

Injection from metallic electrodes serves as a main channel of charge generation in organic semiconducting devices and the quantum effect is normally regarded as essential. We develop a dynamic approach based upon the surface-hopping (SH) algorithm and classical device modeling, by which both quantum tunneling and thermionic emission of charge-carrier injection at metal-organic interfaces are concurrently investigated. The injected charges from a metallic electrode are observed to spread quickly onto the organic molecules, followed by an accumulation close to the interface, induced by the built-in electric field. We compare the Ehrenfest dynamics on the mean-field level and the SH algorithm by simulating the temperature dependence of the charge-injection dynamics and it is found that the former leads to an improper result, that the injection efficiency decreases with increasing temperature in the room-temperature regime while the SH results are credible. The relationship between the injected charges and the applied bias voltage suggests that it is the quantum tunneling that dominates the low-threshold injection characteristics in molecular crystals, which is further supported by the calculation results of a small entropy change during the injection process. An optimum interfacial width for the charge-injection efficiency at the interface is also quantified and can be utilized to understand the role of the interfacial buffer layer in practical devices.