Admittance spectroscopy of thin-film photovoltaics

We propose a phenomenological theory of admittance characterization of diode structures with resistive electrodes, including photovoltaic cells and Schottky junctions. The concept of decay length is introduced which describes how far an ac signal propagates through the resistive electrode in the lateral direction. The measured capacitance and conductance strongly depend on the decay length and the electrode configuration of the device. We show that properly arranged admittance circuitry and adequate characterization allow one to extract much more information from the data than previously believed.