Monolithic perovskite/silicon tandem solar cells have demonstrated record power conversion efficiencies; however, most high-performing devices rely on spin-coated perovskite films. Translating these performances to large-area devices remains a major challenge, as scalable deposition processes must preserve both the optoelectronic quality of the perovskite absorber and efficient optical management.

In this work, we combine molecular interface engineering with ambient-air slot-die coating to develop scalable monolithic two-terminal perovskite/silicon tandem solar cells. A wide-bandgap double-cation perovskite absorber (E_g = 1.65 eV) was deposited by open-air slot-die coating followed by vacuum quenching to obtain uniform large-area films. To mitigate non-radiative recombination, a bimolecular ammonium treatment consisting of ethylenediammonium iodide (EDAI) and phenethylammonium chloride (PEACl) was introduced at the perovskite/C60 interface. This passivation increased the tandem open-circuit voltage (V_OC) from 1.75 V to 1.81 V by suppressing non-radiative recombination, resulting in planar monolithic tandem devices with a power conversion efficiency (PCE) of 24.2% on a 10.2 cm² device area.

Despite the improved voltage, the planar architecture remains photocurrent-limited owing to reflection losses in the planar silicon heterojunction bottom cell. To address this limitation, the process was transferred to the nanotextured silicon heterojunction solar cell, where achieving uniform perovskite coverage over micron-scale pyramids becomes the critical challenge. We systematically investigated the influence of slot-die dispense rate on film formation and found that coating dynamics strongly govern film quality, local thickness distribution, and grain evolution. Low dispense rates produce locally thin perovskite regions near the pyramid tips together with a high density of grain boundaries, whereas optimized coating conditions result in continuous films with larger crystalline domains across the textured surface. These microstructural changes suppress recombination, improve charge extraction, and simultaneously enhance the V_OC and fill factor (FF). Finally, optical optimization using an MgF₂ anti-reflection coating further increased the short-circuit current density (J_SC) by reducing optical reflection. The optimized nanotextured monolithic tandem achieved a PCE of 31.5% on a 4 cm² device, with V_OC of 1.916 V, J_SC of 20.1 mA cm^-2, and FF of 81.8%. These results demonstrate that molecular interface engineering and control of slot-die coating dynamics are both critical to achieving high-efficiency monolithic perovskite/silicon tandem solar cells, highlighting the relationship between scalable film formation, microstructure evolution, and device performance.