Antimony selenide solar cells: non-ideal deep level response and study of trap-filling transients

In this paper, we present a detailed analysis of capacitance-based defect characterization techniques on antimony selenide solar cells. Based on capacitance deep level transient spectroscopy (C-DLTS) measurements, we detect two majority carrier traps; however, the common rate window analysis method is not able to provide physically valid values of the activation energy and apparent capture cross-section. In order to understand this effect, we carried out an extensive analysis based on the exponential fit method. We provide experimental evidence that (i) the Arrhenius plot extracted by the rate window analysis is not reliable, since the variation in capacitance transient amplitude is not caused by the variation in time constant of the exponential transient. This is because (ii) the amplitude of the capacitance transient is related to the amount of trapped charge, which changes significantly with temperature due to a temperature-assisted injection process, as confirmed by capacitance transient measurements carried out in the filling phase. Moreover, analysis of the transients with different filling time shows (iii) an additional contribution by minority carriers injected by the n-side of the junction. All the results are (iv) confirmed by analysis at various filling voltage.