Chemical Bath Solution In‐Addition Enabled Grain Boundary and Heterojunction Interface Defects Mitigation Achieving 12.1% Efficiency CZTS Solar Cell

ABSTRACT The performance of wide bandgap Cu ₂ ZnSnS ₄ (CZTS) thin film solar cells is limited by serious recombination due to high concentration of defects and “cliff‐like” conduction band offset (CBO) at the heterojunction. The state‐of‐the‐art strategies tackling these issues are post heat treatment and alternative buffer layer. Here, we address these issues by adding InCl ₃ to the chemical bath deposition solution (CBD‐In‐addition) for CdS buffer layer. At early CBD stage, In ³⁺ sufficiently substitute Zn ²⁺ near CZTS absorber surface and along the grain boundaries (GBs), enabled by smaller size than Cd ²⁺ and no complexation with NH ₃ , resulting in highly conductive GBs with beneficial upward band bending. Further, the incorporation of In ³⁺ into CdS ((Cd,In)S) improved buffer layer n‐type conductivity, alleviated heterojunction lattice mismatch and enlarged buffer layer bandgap, resulting in a coherent CZTS/(Cd,In)S interface, a “spike‐like” CBO, and a more transparent buffer layer. These effects lead to 12.14% efficiency CZTS solar cell, improved by 31% compared to reference device (9.29%). This is the highest efficiency sulfide CZTS solar cell with chemical bath deposited buffer layer without post heat‐treatment. The simple and synergetic defects mitigation strategy is expected to advance other wide bandgap chalcogenide thin film solar cells.