Photoluminescence properties of Cu-poor Cu₂Sn_1−xGe_xS₃ thin films with varying Ge/(Ge+Sn) ratio

This study investigates the photoluminescence (PL) spectra of Cu₂Sn_1−xGe_xS₃ (CTGS) thin films, which are currently the most suitable composition ratio for high-efficiency absorbers through low temperature-PL measurements to reveal the effects of the x ratio on defect properties of CTGS thin films. The PL spectrum of Cu₂SnS₃ (CTS) thin films with x= 0.00 exhibits five peaks at 0.782, 0.832, 0.862, 0.885, and 0.933 eV. Moreover, all PL peak positions in the CTGS thin films shift to higher energies with increasing x ratios because the defect levels in the films changed with an increase in the x ratio. Moreover, we obtain the estimated activation energy (E_a) values of the CTS thin films with x = 0.00 ranging from 6 to 20 meV. The E_a values of CTGS are similar to those of the CTGS thin films, even at x ratios of up to 0.19 in CTGS thin films. The increasing x ratio in CTGS thin films does not influence the acceptor in CTGS. Therefore, the CTGS is advantageous as an absorption layer in solar cells rather than a CTS because E _g can be large while maintaining a shallow acceptor. Hence, CTGS can be expected to be increasingly used like CuIn_1−xGa_xSe₂ and Cu₂ZnSnS₄ as next-generation absorption materials.