Buffer Layers, Defects, and the Capacitance Step in the Admittance Spectrum of a Thin-Film Solar Cell

Capacitance-based measurements are key to the study of defects in semiconductor devices, and are used extensively in the field of thin-film photovoltaics. In a multilayered thin-film device, however, the mere presence of the layers can result in capacitance spectra identical to those due to deep defects. This study demonstrates that bias- and illumination-dependent impedance spectra reveal differences between effects from layers and defects. For Cu(In,Ga)Se₂ thin-film solar cells, the dominant ``N1'' signature is difficult to reconcile with the presence of deep defects, but follows naturally from considering a transport barrier caused by a buffer layer in the device.