Wide bandgap Cu(In,Ga)Se2 solar cells with improved energy conversion efficiency

We report on improvements to the energy conversion efficiency of wide bandgap (E-g > 1.2 eV) solar cells on the basis of CuIn1-xGaxSe2. Historically, attaining high efficiency (>16%) from these types of compound semiconductor thin films has been difficult. Nevertheless, by using (a) the alkaline-containing high-temperature EtaMax glass substrates from Schott AG, (b) elevated substrate temperatures of 600-650 degrees C, and (c) high vacuum evaporation from elemental sources following National Renewable Energy Laboratory's three-stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2 < E-g < 1.45 eV. The current densityvoltage (JV) data we present includes efficiencies >18% for absorber bandgaps of similar to 1.30 eV and efficiencies of similar to 16% for bandgaps up to similar to 1.45 eV. In comparing JV 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 are due to reduced recombination. We establish the existence of random and discrete grains within the CIGS absorbers that yield limited or no generation/collection of minority carriers. We also show that interfacial recombination is the main mechanism limiting additional enhancements to open-circuit voltage and therefore performance. Solar cell results, absorber materials characterization, and experimental details and discussion are presented. Copyright (c) 2012 John Wiley & Sons, Ltd.