Temperature dependence of VOC in CdTe and Cu(InGa)(SeS)2-based solar cells

The temperature and intensity dependence of VOC in CdTe and Cu(InGa)(SeS)2 polycrystalline thin film solar cells was examined. VOC was measured from 100–330K and from 0.1 to 1 sun illumination. Two distinct regimes of temperature dependence are commonly observed: a linear regime at higher temperatures with slope −0.5 to −3 mV/K and a logarithmic intensity dependence; and a saturation regime at lower temperatures, with little intensity or temperature dependence. The T=0 K intercept extrapolated from the linear regime around 300K is related to the activation energy of the dominant recombination mechanism and is equal to the absorber bandgap for Shockley-Read-Hall recombination, or in some cases, from heterojunction interface recombination, which is less than the absorber bandgap. In this work, the temperature dependence of VOC will be characterized for CdTe and Cu(InGa)Se2 devices with differences in composition and processing conditions. Analysis will focus on the activation energy of the recombination mechanism and saturation at lower temperatures which indicates a maximum separation of the quasi Fermi levels as thermally activated SRH recombination is frozen out. The saturation voltage is ∼1 V for a typical CdTe device (Eg=1.45 eV), ∼1V for low bandgap Cu(InGa)Se2 (Eg=1.15), and ∼1.1 V for wider bandgap Cu(InGa)Se2 (Eg=1.38 eV).