As perovskite/silicon tandem solar cells approach their practical efficiency limit (35.2% experimentally vs. a 39.5% practical potential),[1,2] research focus is increasingly shifting toward scalable manufacturing and operational stability. Residual tensile strain in the perovskite layer remains an intrinsic challenge, arising from the mismatch in thermal expansion coefficients between the perovskite absorber and the silicon substrate.[3] This strain lowers the defect formation energy of halide vacancies, thereby accelerating device degradation.[4] Buried‑layer engineering, specifically introducing a thin 3D inorganic perovskite layer with a smaller lattice parameter than the photoactive perovskite, has recently been shown to counteract this tensile strain by imposing an external compressive strain.[5] Yet, the underlying deposition processes remain challenging to reproduce: cumbersome pure single crystal synthesis, in-situ annealing during thermal evaporation, and need for high post-annealing temperatures (200°C); thus limiting the broader technology transfer.

In this work, we develop a post-treatment-free thermal co‑evaporation process to deposit a 10 nm CsPbCl3 inorganic 3D perovskite layer beneath a 1.68 eV FA_0.85Cs_0.15Pb(I_0.78Br_0.22)₃ photoactive absorber. The study systematically (1) maps the processing window required to obtain phase pure CsPbCl₃ by tuning the CsCl/PbCl₂ co evaporation rate ratio, assessed using XRD and steady-state PL; (2) investigates chlorine diffusion into the FA_0.85Cs_0.15Pb(I_0.78Br_0.22)₃ absorber as a function of the CsPbCl₃ post annealing temperature using ToF-SIMS; and (3) analyzes the charge extraction mechanism at the FACs/CsPbCl₃/hole-transport-layer interface by optimizing the buried‑layer thickness and applying a combination of UPS, KPFM, transient PL, and device-level characterization (Suns-V_OC and Suns-PL). In situ Bragg-Brentano XRD provides mechanistic insight into how the engineered inorganic layer modifies the crystallization pathway of the photoactive perovskite. Complementary grazing incidence XRD reveals the changes in residual strain within the target device stack. The derived design rules enable a scalable fabrication approach compatible with front side-textured silicon wafers featuring industry‑standard pyramid heights (> 2 µm), typically used in fully-textured perovskite/silicon tandem solar cells,[6] and presents a promising step towards stable and scalable next generation photovoltaics.