Reinterpretation of defect levels derived from capacitance spectroscopy of CIGSe solar cells

In this work we make an attempt to clarify ambiguities and to present our present understanding of defects and defect-related phenomena affecting the capacitance characteristics of Cu(In,Ga)Se2-based solar cells. We discuss deep defect levels derived from admittance and deep level transient spectroscopy, as well as shallow levels affecting the charge distributions by capacitance–voltage profiling. The discussion includes two types of metastable effects affecting capacitance characteristics: one induced at room temperature by light or voltage bias, and one created at low temperature by red illumination of reverse-biased junction (ROB effect). Recent theoretical achievements on negative-U properties of such intrinsic defects as selenium vacancies and InCu antisites are used to explain the experimental data. We show that the most prominent level in the admittance spectra is due to the response of interface states combined with contribution of deep VSe–VCu−/2− acceptor level. We attribute the ROB metastability to the relaxation of InCu defects upon electron capture. Finally we discuss the influence of these defects on the device efficiency.