Experimentally informed drift-diffusion assessment of a lead-free four-terminal BaZr0.85Ti0.15S3/CZTSSe tandem solar cell

A lead-free four-terminal tandem architecture is assessed by optically coupling a semi-transparent BaZr 0.85 Ti 0.15 S 3 chalcogenide-perovskite top sub-cell to a Cu 2 ZnSn(S 0.25 Se 0.75 ) 4 (CZTSSe) bottom sub-cell. The assessment includes a 33.93% idealized design target only as an upper bound because the assumed p-type, low-defect top absorber has not been demonstrated in a photovoltaic device. The device physics are evaluated with a one-dimensional drift-diffusion framework based on the coupled Poisson and electron/hole continuity equations. A finite-volume Python implementation using Scharfetter-Gummel fluxes was benchmarked against a fresh SCAPS 3.3.08 single-junction calculation, giving differences of 0.842 mV in open-circuit voltage, 0.049 mA cm − 2 in short-circuit current density, and 0.056% point in fill factor; the J-V root-mean-square error was 0.169 mA cm − 2 . The tandem parameter set was anchored to measured BaZrS 3 thin-film transport and defect ranges and to a recent high-performance CZTSSe device. Under the favorable n-type top-absorber boundary, optical/electrical co-optimization moves the BaZr 0.85 Ti 0.15 S 3 thickness from 0.90 μm to 0.05 μm and gives 3.30% from the top cell plus 13.56% from the filtered bottom cell, for a 16.86% four-terminal efficiency. A 1200-sample material/contact uncertainty analysis at this thickness gives P 5 , median, and P 95 efficiencies of 8.07%, 10.73%, and 15.49%, respectively. An additional 55-case direct SCAPS sweep over donor density, bulk defect density, minority-hole mobility, and trap capture cross section, together with maximum-power-point depth profiles, identifies defect-mediated recombination and minority-hole collection as the controlling top-cell constraints. The results show that the proposed absorber pair remains scientifically interesting, but its performance is controlled by minority-hole transport, sulfur-vacancy-related n-type doping, top-cell thickness, bottom-cell material quality, and contact resistance. Accordingly, 33.93% should be interpreted as a long-term material-quality ceiling rather than an experimentally credible near-term prediction.