Designing MoO_X/Ag/MoO_X sandwich structured buffer layer for four-terminal CsPbI₃/TOPCon tandem minimodules

CsPbI₃ perovskites with suitable bandgaps ∼1.70 eV present distinct advantages for top-cell photovoltaic materials in tandem solar cells, however, relevant work has been rarely reported. This work designed a sandwich-like MoO_X/Ag/MoO_X (MAM) buffer layer as the front cell window layer to maximize incident light utilization efficiency for semi-transparent CsPbI₃ solar cells and four terminal (4-T) stacked cells. Further investigation revealed that the MAM has to experience in-situ reaction between Ag metal and fresh MoO_X, which can simultaneously ensure the transparency of the buffer layer and improve the carrier transportation and collection, except for protecting the underlying spiro-OMeTAD layer from the bombardment of magnetron sputtering. Thanks to this MAM buffer layer and a tunnel oxide passivating contact (TOPCon) bottom cell with edge passivation, semi-transparent CsPbI₃ devices (aperture area: 0.50 cm²) demonstrated a power conversion efficiency (PCE) of 18.86% while corresponding 4-T CsPbI₃/TOPCon tandem solar cells (PSTSCs) presented the PCE of 26.55%. Besides, we also fabricated semi-transparent CsPbI₃ minimodules with 16.67% PCE and 4-T PSTSCs with 26.41% PCE (aperture area: 6.62 cm²). This work provided a new scalable strategy for transparent buffer layers by constructing in-situ generated sandwich structured buffer layer, which is suitable for perovskite tandem solar cells.