Wide bandgap Cu(In,Ga)Se ₂ 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 CuIn _1-xGa _xSe ₂. 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 °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 density-voltage (J-V) data we present includes efficiencies >18% for absorber bandgaps of ∼1.30 eV and efficiencies of ∼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 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.