Simulated admittance spectroscopy measurements of high concentration deep level defects in CdTe thin-film solar cells

Abstract

Computer simulations of thin-film p-CdTe∕n-CdS solar cells using the SCAPS software program are examined to offer explanations for a number of experimentally observed admittance spectroscopy results. We show that for a high concentration of deep level defects, the magnitude of the admittance spectroscopy signal which is an indicator of defect concentration is also affected by the cell thickness. For CdTe cells thinner than 3μm, the signal for defects within 0.25eV of the band edge can be weak and may not be detected at all. We also show that Fermi level pinning resulting from high concentrations of deep level defects can distort the measured activation energy and apparent capture cross section. Finally, we show that decreasing capacitance values with increasing temperature can be caused by the interaction between the CdTe cell back contact Schottky barrier, a defect concentration gradient adjacent to the back contact, and a small shallow acceptor concentration relative to the defect concentration.