Consistent double Gaussian model with non-symmetric potential barriers at contacts for organic diodes

Abstract

A transport model with double Gaussian density of state (DOS) for organic semiconductors is proposed, with one Gaussian DOS for free carriers and one for trapped carriers. The variations of the density of trapped carriers with the density of free carriers, the effective mobility and the ratio of diffusion coefficient to effective mobility with the density of trapped carriers are analyzed. It is shown that the ratio of diffusion coefficient to effective mobility is the Einstein type for free carriers, but the ratio for total carriers is the non-Einstein type, and is an increasing function of the density of trapped carriers at low density, and a decreasing function at high density. The importance of non-symmetric barriers at contacts is emphasized to quantitatively describe the current–voltage relationships of typical organic layers sandwiched between two metallic electrodes. It is shown that slopes of current–voltage curves at low bias are very sensitive to the values of right barriers. The slopes in all bias are sensitive to the values of left barriers, and increase as the left barrier decreases. When applying the modified model to three organic diodes, an excellent agreement is obtained between theoretical results and experimental data.