Impact of Lithium as Interfacial Treatment for CZTSSe Solar Cells

Cu2ZnSn(S,Se)4 (CZTSSe), despite its slow progress in terms of efficiency, stands out as a viable option for solar cell absorber materials due to its comparatively low-cost fabrication process, and abundance of constituent elements. The efficiency of CZTSSe faces challenges, particularly a notable Voc deficit arising from its non-stoichiometric nature and interface recombination between CZTSSe and the buffer layer, thus deteriorating the performance of the device. Lithium doping has been known to be effective in tuning the band gap and modifying grain boundaries, enhancing device performance. It is essential to identify a direct and efficient method to address the challenges associated with significant lithium losses in in-situ doping processes. Herein, we demonstrate an effective approach to significantly increase the open-circuit voltage (VOC) and efficiency of the device via a simple impurity inclusion process: the surface of the precursor film is treated with chlorine-based compounds containing Li and Ge. This modification results in an efficiency of up to 7.2 %, with a significant improvement in Voc, indicating that the use of Ge and Li is a viable means of increasing efficiency by reducing defect density. The structural modifications are addressed by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Current-voltage, capacitance-voltage and drive-level capacitance profiling are used to analyze the performance and defect density of the finished devices.