Admittance spectroscopy of polycrystalline and epitaxially grown CuGaSe2

Using either single crystalline, epitaxially grown p-type CuGaSe2 (CGSe) films in Schottky diodes or polycrystalline p-CuGaSe2/n-CdS single-junction solar cells, we employed thermal admittance spectroscopy (TAS) to gain insight into the electronic transport mechanisms of CGSe. In both types of devices, the capacitance decreases about 50% to its geometrical value in a frequency dependent step between 250 and 150 K. For the Schottky diodes, this capacitance step reflects the response of the shallowest acceptors whose energy level is located 150 meV above the valence band. In the solar cells, a comparable response occurs but is superposed by carrier freeze-out outside the space-charge region.