Versatile Two‐Step Process for Perovskite‐Based Tandem Photovoltaics

Perovskite photovoltaics promise high power conversion efficiencies (PCEs) and cost-effective fabrication, making them a transformative solar technology. Among deposition methods, the solution-based two-step process has emerged as a promising approach for integrating high-quality perovskite layers onto silicon (Si) bottom cells, enabling dense and pinhole-free films. However, achieving both high efficiency and long-term stability remains underexplored for solution-based two-step-processed perovskite solar cells (PSCs). This study introduces a versatile solution-based two-step method, demonstrating a seamless transition from a triple-cation (CsMAFA) to a more stable double-cation (CsFA) perovskite composition. Implementing a novel dual bimolecular passivation strategy with propane-1,3-diammonium iodide (PDAI$_2$) and n-butylammonium iodide (BAI) for both bulk and surface passivation effectively addresses defects at grain boundaries and interfaces. This approach minimizes nonradiative recombination, enhances film crystallization, and promotes efficient charge extraction. The resulting PSCs demonstrate a stable power output of 20.9%, representing the highest reported efficiency for a solution-based two-step processed PSC with a bandgap of 1.67 eV. Laboratory-scale monolithic perovskite/Si tandem solar cells (1 cm$^2$ active area) achieve PCEs exceeding 26% on small-textured Si bottom cells (<2 μm). This emphasizes the potential of the solution-based two-step process for practical implementation in high-performance photovoltaic systems.