Impedance Spectroscopy Approach for a 2T Perovskite/Silicon Tandem Solar Cell

Impedance spectroscopy (IS) is a powerful tool for analyzing the physical mechanisms occurring in single-junction solar cells. However, identifying the impedance responses of the top and bottom sub-cells within a tandem solar cell remains challenging. This is because the series connection of the two sub-cells results in an overall tandem impedance in which the contributions of the individual sub-cells are combined and may even overlap, making their individual impedance contributions difficult to distinguish and investigate. This work proposes a diagnostic approach for monolithic perovskite/silicon tandem solar cells. The proposed approach requires a preliminary numerical or experimental characterization of the individual sub-cells before analyzing the monolithic tandem device. In particular, static and dynamic characterization techniques, including J-V curve tracing, C-V and C-f analyses, relaxation-time measurements, and IS, can be performed on each sub-cell as a standalone device to identify the frequency ranges in which its contribution to the overall tandem impedance is expected to be dominant. Subsequently, J-V curve tracing and IS are performed on the monolithic tandem device. The regions of the Nyquist plot in which the contribution of each sub-cell is expected to be dominant are then analyzed to extract information on the actual operating conditions of the individual sub-cells. The proposed methodology is demonstrated through TCAD numerical simulations and applied to case studies involving a tandem cell operating under near-current-matched and current-mismatched conditions.