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Effects of back contact resistance, depletion width and relaxation time distributions in admittance spectroscopy of CZTSe devices

We present an updated equivalent circuit model based in the physics of heterojunction devices and apply it in the interpretation of admittance spectroscopy from CZTSe photovoltaic devices. We investigate the effects of fundamental device parameters on capacitance-frequency profiles and show that the widely-used derivative analysis method is only accurate for secondary capacitance steps, which may not be easily distinguished from the main junction capacitance-frequency step. We investigate whether a distribution of depletion widths or other temperature dependencies in the capacitance can account for the behavior of the main capacitance step and find that only a strong temperature-dependent series resistance can explain the frequency shift of the main capacitance step. For the samples studied, this resistance appears to be non-monotonic. We discuss the validity of this result within a model of thermionic emission over a back contact having a distribution of barrier heights.