Investigation of near-stoichiometric polycrystalline CuInSe2 thin films by photoreflectance spectroscopy

Abstract

The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe2/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.