An appropriately combined triple interface modification, i.e. , post-annealing, O ₂ -plasma, and KCl treatments, is employed to ameliorate the optoelectronic properties of sputtered NiO _x films and achieve better device performance.

Bandgap gradient is a proven approach for improving the open-circuit voltages (V_OCs) in Cu(In,Ga)Se₂ and Cu(Zn,Sn)Se₂ thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers. We show that the introduction of the bandgap gradient reduces the hole density in the front junction region and introduces a small spike in the band alignment between this and the absorber regions, effectively suppressing the nonradiative recombination therein and leading to improved V_OCs in Cd(Se,Te) solar cells using commercial SnO₂ buffers. A champion device achieves an efficiency of 20.03% with a V_OC of 0.863 V.