Diffusion transport over grain-boundary barriers as the origin of N1 deep level transient spectroscopy signal in Cu ( In , Ga ) Se 2 solar cells

Highlights: • Influence of grain boundaries on deep level transient spectra. • Diffusion of carriers over grain boundary barriers as an origin of capacitance transients. • Effective mobility in thin film solar cells. In this work, we explore the hypothesis that the N1 peak in Cu(In,Ga)Se${}_2$ (CIGS) solar cells is caused by secondary barriers. We indicate, through numerical modelling of the corresponding Deep Level Transient Spectroscopy (DLTS) signal, that these barriers may be located at grain boundaries. Moreover, the proper reconstruction of the N1 signal using the double-barrier model requires an assumption that corresponding capacitance transients are due to the diffusion limited transport of holes over grain boundaries. Only then we were able to achieve sufficiently small values of the saturation current for realistic values of material parameters. We show that the position of the N1 DLTS peak and its activation energy depends on the effective density and the effective mobility of holes which can be calculated based on grain size and the properties of grain boundaries. This interpretation opens a room for the explanation of the unusual properties of the N1 signal in CIGS solar cells.