Impact of defect states on the capacitance voltage characteristics of space charge limited organic diodes, and determination of defect states

Abstract

We investigate the frequency dependent capacitance–voltage (C–V) characteristics of space charge limited organic diodes based on m-MTDATA, which is a well characterized model material system. The low frequency C–V curves show an intense peak in forward bias operation, and its height decreases with an increase in modulation frequency. Under the reverse bias condition, the capacitance is observed to be voltage dependent, which is a feature at variance with reports in the literature. A numerical study is performed using the drift-diffusion model by incorporating the defect density of states (DOS) within the energy gap in order to understand the observed C–V features. We show that the low carrier mobility severely affects the capacitance frequency (C–f) features and also governs the frequency dependence of the peak height in the C–V curve. The zero bias C–f characteristics are modelled to extract the input defect DOS obtained at different mobility values. We show that the use of a thermal velocity dependent capture coefficient as usually invoked in the case of inorganic semiconductors to interpret the defect DOS derived from the C–f curves leads to artifacts owing to low carrier mobility in organic semiconductors. We demonstrate that the mobility dependent attempt-to-escape frequency should be considered while dealing with low mobility organic solids for the correct determination of the defect energy distribution. Finally, the experimental C–f curves are modelled to estimate the defect parameters including the energy level and distribution.