Effects of cadmium alloying on solution-processed CZTSSe solar cells: morphology, band alignment and efficiency improvement

Abstract

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells are considered promising candidates for sustainable photovoltaic applications due to their high theoretical efficiency and the abundance of low-toxicity elements. However, their performance is hindered by Cu_Zn antisite defects and associated defect clusters, which contribute to harmful band tailing and non-radiative recombination. Substituting Zn with Cd, which has a larger ionic radius, has been shown to effectively suppress non-radiative recombination, achieving efficiencies of up to 11.73%. However, the effect of Cd alloying on absorber morphology, electronic properties, and device performance has not yet been fully explored. In this work, using the Cu⁺-Sn⁴⁺-dimethyl sulfoxide (DMSO) solution as a platform, we have successfully incorporated Cd²⁺ into the lattice of CZTSSe across the full concentration range, enabling a systematic investigation of the Cd alloying effect. The results show that Cd alloying promotes grain growth, resulting in a flat and compact film at low concentration, but excessive grain growth at high concentration. Furthermore, low Cd content inhibits lattice disorder, thereby reducing band tailing. Incorporation of Cd is found to linearly reduce the band gap by raising the valence band maximum and lowering the conduction band minimum, which increases the conduction band offset (CBO) at the heterojunction, but the CBO remains a relatively ideal value (<0.3 eV) at low Cd content (<50%). Notably, the 5% Cd alloyed CZTSSe solar cell achieved 13.31% efficiency without an anti-reflective coating, owing to the improved film properties, ideal CBO (<0.24 eV), and suppressed interface recombination. This represents an over 7.0% improvement compared to the intrinsic device performance without Cd alloying. However, high-concentration Cd alloying can damage the absorber, leading to void formation within the grains and the crystal structure transition from kesterite to stannite at 50% Cd, resulting in significant device performance degradation.