Investigation of microstructural and optical properties of Cu(In, Al)Se2 thin films with various copper content

• CIAS have been prepared via the selenization of sputtered precursor thin films.
• The pure phase purity can be obtained by manipulating the Cu content.
• Microstructural parameters of CIAS films have little relevance with the Cu content.
• The band gap of CIAS films shows a declining trend with the increase of Cu content.

Cu(In, Al)Se2 (CIAS) thin films are deposited onto the soda-lime glass substrates via the selenization of sputtered Cu–In–Al precursors, have been investigated by means of microstructural and optical characterization. The influence of Cu content ranges from 0.61 to 1.05 on the microstructure and optical properties of CIAS thin films is assessed. X-ray diffraction patterns and Raman scattering spectra reveal that the phase purity of thin films is strongly relied on the Cu content. The secondary phases, such as Cu(In, Al)3Se5 and CuSe have been detected in the Cu-poor and Cu-rich films, respectively. However, the microstructural parameters (e.g. tetragonal distortion, crystal strain and dislocation density) of thin films have little relevance with the Cu content. X-ray photoelectron spectroscopy analysis confirms the pure phase formation of CIAS was located at the Cu/(In + Al) ratio of 0.83. The corresponding scanning electronic microscopy image shows that the film exhibits a relatively flat-grained and compact morphology. Additionally, optical transmittance measurement indicates that the band gap of CIAS thin films shows a declining trend with the increase of Cu/(In + Al) ratios. The results are significant to gain insight into the Cu–In–Al–Se system materials for thin film solar cell application.