How the Ge Incorporation Approach Affects Non-radiative Recombination Losses in CZTSSe Thin-Film Solar Cells

Kesterite-based thin-film solar cells (TFSCs) have regained attention in the photovoltaic community in recent times. However, their device performance still lags behind CdTe and Cu(In,Ga)Se₂-based TFSCs because of dominant non-radiative recombination losses. The unstable nature of Sn ionic states is the main cause of poor performance. The present study employs the strategy of Ge doping at different positions in stacked metallic precursor layers and investigates real-time external radiative efficiency (ERE) mapping to clarify the role of Ge in kesterite-based compounds. The position of the Ge within the stacked metallic precursor shows distinct alloying behavior and variations in the density of Sn-related defects and associated defect clusters. Consequently, it offers improved grain growth and a reduced density of Sn-related deep-level defects in kesterite thin films. Furthermore, spatial ERE mapping reveals that the stacking order of the Ge significantly influences the average signal and uniformity of ERE. The Ge incorporation in metallic precursors results in an increased open-circuit voltage owing to significantly reduced non-radiative recombination. This study provides valuable insights into the assessment of non-radiative losses using a powerful tool, such as ERE, which assists in the development of these devices and contributes to achieving higher power conversion efficiency in TFSCs.