Synthesis and characterization of Cu-based selenide photovoltaic materials: Cu₂FeSnSe₄ and Cu(In, Al)Se₂

Two Cu-based selenide semiconductors Cu₂FeSnSe₄ (CFTSe) and Cu(In, Al)Se₂ (CIAS) have been fabricated using radio-frequency magnetron sputtering followed rapid thermal processing. For CFTSe, the elevated selenization temperatures ranging from 460 to 520 °C result in a shortened trend of bond-length of cationic-anionic, corresponding to more formation heat released during the selenization process, which favors the grain growth. However, according to the strain calculation, the lattice strain of the thin film selenized at 500 °C presents a minimum value, meaning the fewer defects in thin films. As for the other Cu-based material CIAS, the band-gap (E_g) is determined by the band-to-band electron transition from valence band to conduction band directly. The Al content will affect the hybridization degree of Al-Se which will drive the redistribution of electrons in the conduction band, and then optimize the band-structure. The conduction band minimum shifts into band-gap due to the less amount of Al during the selenization process at higher temperature. In addition, both of CFTSe and CIAS transform into single-phase with large grain size and dense morphologies when selenized at 500 and 540 °C, respectively. The electrical properties of CFTSe (E_g ∼ 1.16 eV) and CIAS (E_g ∼ 1.34 eV), which open circuit voltage is 76 mV and 353 mV, respectively, are also researched.