Improved energy conversion efficiency in wide bandgap Cu(In, Ga)Se<inf>2</inf> solar cells

Abstract

This report outlines improvements to the energy conversion efficiency in wide bandgap (E g >1.2 eV) solar cells based on CuIn 1−x Ga x Se 2 . Using (a) alkaline containing high temperature glass substrates, (b) elevated substrate temperatures 600°C-650°C and (c) high vacuum evaporation from elemental sources following NREL's three-stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2 g 18% for absorber bandgaps ∼1.30 eV and efficiencies ∼16% for bandgaps up to ∼1.45 eV. In comparing J-V parameters in similar materials, we establish gains in the open-circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures may be due to reduced recombination at the grain boundary of such absorber films. Solar cell results, absorber materials characterization, and experimental details are reported.