Boost efficiency with buffer and bottom stack optimization in Cu2BaSn(S,Se)4 solar cells by simulation

Cu₂BaSn(S,Se)₄ is gaining tremendous attention as a potential absorber due to its non-toxicity, earth abundance, and low antisite defects opposing Cu₂ZnSn(S,Se)₄. However, the highest experimental efficiency of 6.17% is achieved with the device structure Al:ZnO/Mg:ZnO/Zn_1-xCd_xS/Cu₂BaSn(S,Se)₄/Mo where the drawback is large open circuit voltage (V_OC) loss stemming from the large cliff at the absorber/buffer interface and back contact recombination. Therefore, finding a suitable non-toxic buffer with modification of the bottom stack is crucial. In this regard, Cu₂BaSn(S,Se)₄ solar cells based on diverse buffers are numerically simulated using SCAPS-1D with Ni as back contact and introducing back surface field (BSF) at the absorber/Ni interface. At first, a baseline model validating the experimental efficiency is designed. The application of Ni enhanced efficiency to 7.92% due to the formation of ohmic contact. Further, it is improved to 9.91% with anti-reflection coating. Afterward, a total of 780 solar cells were designed with six buffer and eight BSF combinations by optimizing the absorber, buffer, and interface properties where the highest efficiency of 28.11% with incredible fill factor (90%) and less V_OC loss (0.24 V) is accomplished for the AZO/ZMO/TiO₂/Cu₂BaSn(S,Se)₄/CuI/Ni solar cell. Further, the importance of BSF is analyzed via energy band diagrams, Mott-Schottky and Nyquist plots. The outcomes disclosed that the BSF greatly influences the energy band alignment, built-in potential, depletion width, and recombination resistance of solar cells, underscoring its significance in improving performance. Overall, the present work proposes an efficient strategy to modify the device configuration of Cu₂BaSn(S,Se)₄ solar cells to enhance their efficiency.