Band gap shift of Cu2ZnSnS4 thin film by residual stress

Abstract

The band gap tuning technology can contribute to improvement of power-conversion efficiency (PCE) of solar cells. In this study, the band gap of the Cu2ZnSnS4 (CZTS) thin film, which was changed by residual strain, has been reported. When the thin film was deposited on soda-lime glasses, the substrate temperatures was fixed by an electric heater. Through the analysis of XRD patterns and Raman spectra, the presence of a second phase, of SnS2, ZnS, or Cu4Sn7S16, was confirmed. Using XRD patterns, the lattice constants and residual strains of the CZTS thin films were determined. The CZTS thin film which had the smallest lattice constant c was under the largest residual compressive stress. The residual strain made Raman peaks and band gaps of CZTS photoabsorbers change linearly. The line slopes in the plots of Raman peak position and band gap as function of residual strain were determined to be 572.7 cm−1 per unit strain and 93.51 eV per unit strain, respectively. This result indicates that the band gap of CZTS photoabsorbers can be tuned by controlling residual stress.