The role of shallow traps in dynamic characterization of organic semiconductor devices

We present an analysis of charge mobility determination methods for the steady as well as the transient state and investigate shallow charge traps with respect to their dynamic behavior. We distinguish between fast and slow trap states in our numerical model corresponding to two characteristic regimes. The two regimes manifest themselves in both impedance spectroscopy and dark injection transient currents (DITC). Further we investigate the charge mobility obtained from dynamic simulations and relate it to the extracted charge mobility from steady-state current-voltage curves. To demonstrate the practical impact of these regimes, we apply our numerical model to the DITC that have commonly been used to determine the charge mobility in organic semiconductor devices. The obtained results from DITC studies strongly depend on the measurement conditions. Therefore we analyze the measurements of reference [Esward et al., J. Appl. Phys. 109, 093707 (2011)] and reproduce the effects of varying pulse off-times on the transient current qualitatively. Thus, our simulations are able to explain the experimental observations with the help of relaxation effects due to shallow traps