Heat treatment in an oxygen-rich environment to suppress deep-level traps in Cu2ZnSnS4 solar cell with 11.51% certified efficiency

Sulfide kesterite Cu 2 ZnSnS 4 (CZTS) is a competitive photovoltaic material, especially for multijunction solar cells. However, the device power conversion efficiency has remained stagnant for years. Deep-level defects, such as sulfur vacancies (V S ), cause serious non-radiative recombination of charge carriers. Here we propose a passivation strategy for V S through the heat treatment of the CdS/CZTS heterojunction in an oxygen-rich environment. In this process, V S are occupied by oxygen atoms, suppressing V S defects. In addition, the diffusion of Cd ions to the CZTS absorber layer, and the formation of positive Na–O and Sn–O complexes can passivate related defects. These effects led to a reduced charge recombination and favourable band alignment. We demonstrate a certified efficiency of 11.51% for air-solution-processed CZTS solar cells (bandgap of 1.5 eV) without any extrinsic cation alloying. The study offers insights into defect passivation and performance improvement mechanism of kesterite solar cells.