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

Abstract

Sulfide kesterite Cu₂ZnSnS₄ (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.