Bottom directional deposition perovskite heterojunctions for efficient and stable lead halide perovskite/silicon tandem solar cells

Perovskite/silicon (PSC/Si) tandem solar cells are promising for high-efficiency photovoltaics, yet wide-bandgap (WBG) perovskites face challenges including poor charge transport, phase segregation, and poor conformality on textured silicon. Here, we engineered directional deposition 2D perovskite assemblies at the perovskite bottom interface to establish heterojunctions, enhancing charge transport, improving band alignment, and boosting operational stability by introducing 3,3-difluoropyrrolidinium hydrochloride (DFPHCl) and guanidinium thiocyanate (GASCN) into the WBG perovskite precursor. Leveraging the strong binding affinity of DFPHCl within the precursor solution for indium tin oxide (ITO), we converted excess lead iodide (PbI₂) into directional deposition 2D perovskite (DFP)₂PbI_xCl_4−x, which accumulates at bottom grain boundaries and can compromise stability. The process facilitated by GASCN as a crystallization promoter concurrently enhanced (100)-oriented crystal growth and further optimized the band alignment. The resultant 1.67 eV perovskite solar cell achieves a high open-circuit voltage (V_OC) of 1.284 V and a power conversion efficiency (PCE) of 23.29%, maintaining 90% of its initial performance after 983 hours of continuous illumination. The optimized tandem device delivers a V_OC of 1.913 V and a stabilized PCE of 31.37%, establishing a good pathway toward efficient and stable tandem photovoltaic devices.