Investigation of CsF - Treatment Effects on Cu(In,Ga)(S,Se)2 Solar Cells using Photothermal Atomic Force Microscopy under Various Photoexcitation Conditions

Abstract

Performance of a thin film Cu(In,Ga)(S,Se)2 [CIGSSe] solar cell has been well improved by CsF-treatment. In this study, we have investigated the effect of the CsF -treatment by photothermal atomic force microscopy [PT -AFM] through examination of non-radiative recombination properties. The used samples were CdS/CIGSSe materials with or without the CsF -treatment, and PT-AFM were performed under three excitation conditions: (1) standard condition, (2) high-photon-energy condition, where a light with high photon energy was used for shallow excitation near the surface by shortening a penetration depth of light, and (3) high-modulation-frequency condition, where an incident light was modulated at a high frequency to detect the fast thermal expansion caused by non-radiative recombination near the surface. Under conditions (2) and (3), the PT signals are considered to be dominated by the heat generation near the surface. Topographic and PT signal images were taken by PT-AFM on two samples under three excitation conditions, and we have compared the intensities and distributions of the PT signals among them to discuss the effects of the CsF -treatment. First, we found that the PT signal intensity on the CsF -treated sample was weaker than that on the as-grown sample under any excitation condition, indicating that the non-radiative recombination was suppressed by the CsF -treatment. Especially on the CsF -treated sample, the PT signals were weakened under conditions (2) and (3), compared with those under condition (1). Therefore, we can consider that the CsF -treatment effectively passivates the recombination centers near the surface. Moreover, the reduction of the PT signal was very apparent around GBs. From those results, we conclude that the non-radiative recombination centers distributed at the CdS/CIGSSe interface and along GB were effectively passivated by the CsF-treatment.