Quantum Efficiency Measurements and Modeling as Tools to Monitor Air Annealing of Cu2SnS3 Solar Cells

Abstract

Air annealing of chalcopyrite solar cells at either 200 degrees C or higher is often known to increase their power conversion efficiency. In this paper, we investigate the nature of this effect for Cu2SnS3 (CTS) solar cells by modeling the experimental external quantum efficiency. We find that the cell efficiency increase stems from increased diffusion length and depletion width and decreased interface recombination at the p-n junction. The increased diffusion length is also reproduced when only the absorber layer is air annealed. When solar cells are annealed in N-2, no increase in diffusion length is measured. Hence, we attribute the increase in diffusion length to passivation of the grain boundaries in the bulk by oxygen. The larger depletion width on air and N-2 annealing in the devices is independent of the CdS buffer layer thickness and occurs in its absence. We ascribe it to copper diffusion from the absorber layer to the n-type buffer and window layers. Interface recombination positively correlates with increasing buffer layer thickness. Based on our modeling, we conclude that the CTS absorber layer is still too highly doped to obtain large depletion widths and is highly recombination active at the p-n interface.