Fabrication of Bendable Perovskite/Silicon Heterojunction Bifacial Tandem Solar Cells

We report the fabrication of a flexible bifacial monolithic perovskite (PVK)/silicon heterojunction (SHJ) tandem solar cell consisting of a Cs₀.₀₅FA₀.₉₅PbI₃ top cell (bandgap ~1.54 eV) and an ~80 μm-thick SHJ bottom cell. External quantum efficiency (EQE) measurements showed a top-cell current of 22.1 mA (1 cm² aperture) and a current mismatch of 8.0 mA between the sub-cells, which can be compensated by rear-side illumination of ~27 mW/cm², as indicated by bifacial SHJ single-cell results. Current–voltage characterization showed a linear increase in short-circuit current up to ~12.3 mW/cm², followed by saturation without achieving current balance with the top cell, accompanied by pronounced hysteresis. Extrapolation of the linear regime yielded a balancing irradiance of ~29 mW/cm², in good agreement with both the EQE-based estimate and reported simulations. In contrast, the four-terminal tandem exhibited no saturation, with the bottom-cell current scaling linearly with rear-side irradiance. These findings indicate that interfacial imperfections at the PVK/hole-transport layer interface may play a critical role in hindering current balancing in monolithic PVK/SHJ tandems.