Interdependence of absorber composition and recombination mechanism in Cu(In,Ga)(Se,S)2 heterojunction solar cells

Temperature-dependent current-voltage measurements are used to determine the dominant recombination path in thin-film heterojunction solar cells based on a variety of $\mathrm{Cu}(\mathrm{In,Ga}\mathrm{)(}\mathrm{Se,S}{)}_2$ alloys. The activation energy of recombination follows the band gap energy of the respective $\mathrm{Cu}(\mathrm{In,Ga}\mathrm{)(}\mathrm{Se,S}{)}_2$ alloy as long as the films are grown with a Cu-poor final composition. Thus, electronic loss in these devices is dominated by bulk recombination. In contrast, all devices based on absorber alloys with a Cu-rich composition prior to heterojunction formation are dominated by recombination at the heterointerface, with activation energies smaller than the band gap energy of the absorber material. These activation energies are independent from the S/Se ratio but increase with increasing Ga/In ratio.