Influence of NaF and KF Post-Deposition Treatment on the Sub-Band Gap Absorption of Cu(In,Ga)Se2 Absorber Layers

Urbach tails describe a density of states that decays exponentially into the band gap. Non-radiative and radiative recombinations in the Urbach tails contributes to voltage loss in the Cu(In, Ga)Se2 CIGSe solar cells. In the CIGSe semiconductor, the Urbach tails can be suppressed by alkali fluoride post-deposition treatment (PDT). We report the impact of NaF and KF-PDT on the quasi-Fermi level splitting (QFLS) and Urbach energy of the CIGSe absorbers, which are evaluated by absolute photoluminescence measurement. Our data reveal that both NaF and KF PDT have beneficial impacts on the QFLS. Samples with a low Na concentration show high Urbach energies, which are decreased by NaF-PDT. Sufficient KF PDT on the samples deposited on Mo-coated soda lime glass (i.e., with a high concentration of Na) further decreases the Urbach energy. The improvement of the QFLS is not solely due to the reduction of tail states, but it can also be due to surface passivation, grain boundary passivation or increase in the doping concentration.