Reversing band bending at grain boundaries enables high-efficiency Cu2ZnSn(S,Se)4 solar cells

Kesterite solar cells show great potential for sustainable photovoltaic technology, attributed to their excellent semiconductor properties and earth abundant composition. However, undesirable band bending at the grain boundaries (GBs) in Cu₂ZnSn(S,Se)₄ (CZTSSe) films induces serious carrier recombination because of inhomogeneous distribution of S and Se in the grain interiors (GIs) and at GBs, which results in large open-circuit voltage deficit and overall poor performance of CZTSSe solar cells. Here, a robust hydrothermal sulfurization design has successfully inverted the band bending at the GBs, with advanced cathodoluminescence measurement confirming the transition of carrier collection pathways from the GBs to the GIs, thereby achieving efficient carrier collection within the GIs. Simultaneously, this design has effectively passivated the non-radiative recombination in the GIs, smoothing the way for carrier collection. Ultimately, a 13.7 % efficiency CZTSSe solar cell with 44 % improvement is realized by this process. This study discloses that reversing the band bending at GBs is practical to tailor the carrier collection, and thus pave the pathway for high-efficient photoelectronic devices.