Improved Energy Conversion Efficiency in Wide-Bandgap Cu(In,Ga)Se2 Solar Cells: Preprint

This report outlines improvements to the energy conversion efficiency in wide bandgap (Eg>1.2 eV) solar cells based on CuIn1-xGaxSe2. Using (a) alkaline containing high temperature glass substrates, (b) elevated substrate temperatures 600degC-650degC and (c) high vacuum evaporation from elemental sources following NREL's three-stage process, we have been able to improve the performance ofwider bandgap solar cells with 1.2<Eg<1.45 eV. Initial results of this work have led to efficiencies >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 voltagesseen 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.