In this work, the impact of the Ge introduction into Cu₂SnS₃ (CTS) on photovoltaic performances is investigated. By Ge introduction, non-radiative recombination near the absorber surface and the voids of absorber are decreased. Consequently, the interface carrier recombination of Cu₂Sn_1-xGe_xS₃ solar cell is lower than that of CTS solar cell, thereby increasing open-circuit voltage and fill factor. The conversion efficiency is consequently enhanced through Ge introduction.

Tl-based compound semiconductor shows low-dimensional crystal structure due to the arrangement of Tl atoms. In TlInS₂, TlGaSe₂ and TlGaS₂, Tl atoms show two-dimensional layered structure. However, It is unknown how the result of the optical properties originating from the Incommensurate phase due to the band structure and density of state. Therefore, we clarify the electronic structure and optical characteristics in Tl compound of two-dimensional layered structure. In the band structure, TlInS₂ shows a direct transition type at the Γ point. TlGsSe₂, TlGaS₂ shows the indirect transition type. Differences in electronic structure occur due to changes in the Valence band structure. As a result of density of state, it was found that the density of state increases at the upper region of the valence band, with common property. Those results explain the optical experiments.

The relationship between defects in Cu₂ZnSnS₄ and post annealing are discussed in terms of optical properties by photoluminescence (PL). Comparing the PL spectra of samples grown at 500 and 600 ℃ before and after post annealing, it was found that the spectrum shifted to the higher energy side. Therefore, it can be predicted that Cu_Zn was successfully reduced by post-annealing.

CZTS is attracting attention as an absorption layer material of solar cells, because constituent elements are abundant in the crust and those are non-toxic. CZTS thin films are prepared in non-vacuum by the sol-gel sulfurization method. Removing the secondary phases on the surface is necessary to improve of conversion efficiency of CZTS-based solar cells. In previous our study, the secondary phase on the surface was removed by hydrochloric acid. In this study effect of etching by bromine water was investigated.

Tin Sulfide (SnS) absorber layers were annealed with the sulfur at 550℃ to enlarge their crystalline grains. As a result, SnS layers with large crystalline grains were obtained. However, SnS solar cells comprised of the layers showed extremely low efficiency. Therefore, future experiments at the lower annealing temperature may be needed.