Study of defect properties and recombination mechanism in rubidium treated Cu(In, Ga)Se2 solar cells

Heavy alkali-metal treatment of Cu(In,Ga)Se-2 (CIGSe) absorbers has been emerging as a key process for achieving over 23% high conversion efficiencies in CIGSe solar cells. Here, we investigate the effect of rubidium fluoride post-deposition treatment (RbF-PDT) on the electronic and carrier recombination properties of narrow bandgap (narrowgap) gap and wide bandgap (widegap) CIGSe solar cells using thermal admittance spectroscopy (TAS), transient photocapacitance spectroscopy (TPC), as well as time-resolved photoluminescence (TRPL). We find that the activation energy of the main capacitance step in TAS spectra of narrowgap and widegap CIGSe solar cells reduces after RbF-PDT. On the other hand, capacitance-voltage (C-V) and temperature-dependent current-voltage (IVT) measurements demonstrate that the built-in potential, as well as the activation energy E-a, increases upon RbF-PDT both for narrowgap and widegap samples, pointing to reduced interface recombination. TPC revealed an appreciable reduction of the optical response of bulk defects in the narrowgap and widegap CIGSe, suggesting improvement of bulk properties after RbF treatment. TRPL confirmed that RbF-PDT significantly reduces carrier recombination in the bulk of narrowgap and widegap CIGSe absorbers and at the surface, leading to extended carrier lifetimes. Analysis of open-circuit voltage (V-OC) losses due to nonradiative recombination in the bulk of the CIGSe showed a strong correlation between enhanced carrier lifetime and improved V-OC for narrow gap CIGSe cells. In contrast, although we observed a substantial decrease of V-OC losses in widegap CIGSe bulk, the analysis indicated that the key to photovoltaic performance enhancement is improved interface quality.

Automated summary

The post-deposition treatment of rubidium fluoride has been found to significantly influence the electronic and carrier recombination properties of narrowgap and widegap CIGSe solar cells, resulting in improved interface quality and reduced optical response of bulk defects after treatment. The treatment also leads to extended carrier lifetimes and a correlation between enhanced carrier lifetime and improved open-circuit voltage for narrow gap CIGSe cells. Overall, the key to photovoltaic performance enhancement seems to lie in the improved interface quality.