Oxidation of In2Se3 precursor films and its effects on preparation of CuInSe2 based thin film solar cells

Abstract

The present work deals with the air-annealing effects on In2Se3 precursor layers and the related CISe based heterojunction solar cell devices. CISe films were grown based on a modified 3-stage co-evaporation process that enabled the oxidation of In2Se3 precursor layer at the end of the first stage. To study the role of grain boundaries on oxidation, precursor layers were prepared at high and low temperatures. In2Se3 precursor thin film grown at high temperature shows a gamma-phase with (110) preferential orientation and grain size of 0.5-1 micrometer. Precursor layer prepared at low temperature showed amorphous structure with grains size around 300 nm. CISe films prepared with both precursor layers (high and low temperature) exhibit chalcopyrite structure with a (112) preferential orientation. Comparison between samples prepared with and without air-annealing do not exhibit clear morphological or structural changes. The effect of oxidation process on electrical properties of the solar cells was studied with current-voltage and external quantum efficiency measurements. These results showed that, as compared to devices with non-oxidized CISe, the device with 1h-oxidized CISe film exhibit a decrement in open circuit voltage of ~65mV. This could be related to passivation of interface states on the CdS/CISe interface. Comparing oxidized-CISe cells with different grain boundary density, more degradation of electrical parameters were observed on samples with high number of grain boundaries. Our result show that oxygen introduction to CISe films through the air-annealing of In2Se3 precursors is detrimental to the CISe based solar cell performance.