Optical spectroscopy studies of Cu₂ZnSnSe₄ thin films

Cu₂ZnSnSe₄ thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor-acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis. - Highlights: • We determine the band gap of Cu₂ZnSnSe₄ thin films using 4 K absorption spectra. • We compare photoluminescence (PL) spectra of different structural quality films. • We analyse temperature and excitation intensity dependencies of the PL spectra. • We clarify the nature of defects. • We propose a model of radiative recombination