Single-crystal Cu(In,Ga)Se2solar cells grown on GaAs substrates

Single-crystal Cu(In,Ga)Se2 (CIGS) solar cells were produced with techniques developed for high-efficiency polycrystalline CIGS solar cells. The CIGS layers of a lattice match with GaAs were grown on GaAs(001) substrates by co-evaporation. The presence of a single-crystal CIGS layer without dislocations was confirmed by transmission electron microscopy. Alkaline metal incorporations were achieved by doping and postdeposition treatments. Ga grading structures were fabricated by two-layer deposition with different Ga contents. The Ga grading significantly increased the fill factor and open-circuit voltage. The best efficiency of 20% was achieved after heat–light soaking.