Numerical analysis of deep level defects in Cu2ZnSnS4 (CZTS) thin film solar cells

Cu₂ZnSnS₄ (CZTS) absorber layer has recently been put under extensive research as a potential replacement of CIGS absorber layer because of its excellent electrical and optical properties. In this work, CdS, ZnS, ZnSe, In₂S₃ and TiO₂ have been used as buffer layers in a CZTS/Buffer/i-ZnO structure. The use of i-ZnO as a Transparent Conducting Oxide (TCO) layer has been seen to enhance the performance of an ideal CZTS absorber layer without deep level defects. The effects of deep level defects on the performance of the cells have been numerically analyzed in terms of the energetic distribution and capture cross-section parameters. CZTS/In₂S₃/i-ZnO structure showed the best efficiency of 11.68% (with V_OC = 0.77V, J_SC = 26.66 mA/cm² and Fill Factor = 56.96%). A variation of impurity concentrations have been used to offset the deterioration of efficiency and an optimum acceptor concentration of 2×1016 cm^-3 was found for the enhancement of lower performance caused by electron and hole capture cross section parameters. The efficiency of the CZTS/In₂S₃/i-ZnO structure improved up to 14.56%. Furthermore, layer thickness has also been investigated as a potential way of compensating the effects of deep level defects. Finally, temperature dependence of various structures has been observed.