Mitigating diffusion-length limitations in back-contact CZTSSe solar cells via synergistic field-effect engineering:...

Interdigitated back-contact (IBC) architectures can eliminate front-surface parasitic absorption, yet their application to kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) absorbers is strongly constrained by the mismatch between the minority-carrier diffusion length typically on the order of 1 µm and industrially relevant electrode pitches of 15-25 µm. Here, we use two-dimensional TCAD simulations, calibrated against a certified 14.9%-efficiency front-contact CZTSSe device, to numerically investigate a synergistic field-effect engineering strategy for quasi-interdigitated back-contact (QIBC) CZTSSe solar cells. Negative fixed charges of -3 × 10¹² cm^-2 at the front Al₂O₃/CZTSSe interface induce a hole accumulation layer for lateral majority-carrier transport. Highly asymmetric electrode geometry with 95% n-electrode coverage minimizes minority-electron diffusion distances. Full-thickness bandgap grading from 1.45 to 1.11 eV generates a quasi-electric field of approximately 2.4 kV cm^-1 that drives vertical electron drift. The simulated optimized quasi-IBC device achieves a power conversion efficiency of 15.75% with short-circuit current density of 38.48 mA cm^-2, open-circuit voltage of 606 mV, and fill factor of 67.5%, surpassing the calibrated front-contact reference of 14.9%. Sensitivity analysis establishes threshold material parameters (bulk defect density below 10¹⁴ cm^-3 and electron mobility above 40 cm² V^-1 s^-1) that are within reach of current fabrication technology. These simulation results suggest that synergistic field-effect engineering offers a promising design route toward back-contact kesterite solar cells.