Fostering Charge Carrier Transport and Absorber Growth Properties in CZTSSe Thin Films with an ALD-SnO₂ Capping Layer

The present study demonstrates that precursor passivation is an effective approach for improving the crystallization process and controlling the detrimental defect density in high-efficiency Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films. It is achieved by applying the atomic layer deposition (ALD) of the tin oxide (ALD-SnO₂) capping layer onto the precursor (Cu-Zn-Sn) thin films. The ALD-SnO₂ capping layer was observed to facilitate the homogeneous growth of crystalline grains and mitigate defects prior to sulfo-selenization in CZTSSe thin films. Particularly, the Cu_Zn and Sn_Zn defects and deep defects associated with Sn were effectively mitigated due to the reduction of Sn²⁺ and the increase in Sn⁴⁺ levels in the kesterite CZTSSe film after introducing ALD-SnO₂ on the precursor films. Subsequently, devices integrating the ALD-SnO₂ layer exhibited significantly reduced recombination and efficient charge transport at the heterojunction interface and within the bulk CZTSSe absorber bulk properties. Finally, the CZTSSe device showed improved power conversion efficiency (PCE) from 8.46% to 10.1%. The incorporation of ALD-SnO₂ revealed reduced defect sites, grain boundaries, and surface roughness, improving the performance. This study offers a systematic examination of the correlation between the incorporation of the ALD-SnO₂ layer and the improved PCE of CZTSSe thin film solar cells (TFSCs), in addition to innovative approaches for improving absorber quality and defect control to advance the performance of kesterite CZTSSe devices.