Quantification of surface ZnSe in Cu2ZnSnSe4-based solar cells by analysis of the spectral response

• CZTSe solar cells with different surface coverages of ZnSe were produced.
• Jsc and overall EQE decrease with increasing ZnSe mole fraction.
• EQE suggests additional carrier collection from underneath ZnSe.
• The modeled EQE provides qualitative description of the phenomena.

Absorber layers consisting of Cu2ZnSnSe4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6% power conversion efficiency. The loss of short circuit current density for samples with increasing ZnSe content is consistent with an overall reduction of spectral response, pointing to a ZnSe current blocking behavior. A feature in the spectral response centered around 3 eV was identified and attributed to light absorption by ZnSe. A model is proposed to account for additional collection of the carriers generated underneath ZnSe capable of diffusing across to the space charge region. The model satisfactorily reproduces the shape of the spectral response and the estimated ZnSe surface coverage is in good qualitative agreement with analysis of the Raman spectral mapping. The model emphasizes the importance of the ZnSe morphology on the spectral response, and its consequences on the solar cell device performance.