Isomer-Engineering of Pyridine Amidinium for Efficient Bifacial Perovskite/Silicon Tandem Solar Cells

Bifacial perovskite/silicon tandem solar cells (TSCs) exhibit significant performance advantages over their monofacial counterparts under real-world operating conditions, yet their progress is contingent on the development of high-quality wide-bandgap perovskites. Herein, we integrate the benefits of Lewis bases and oversized cations by designing a series of structurally tailored pyridine-functionalized amidinium additives (2PyFA⁺, 3PyFA⁺, and 4PyFA⁺) to systematically explore the impact of ionic configuration on perovskite film properties and device performance. Among them, the meta-configured 3PyFA⁺ demonstrates a unique bidentate anchoring capability: it simultaneously coordinates with Pb²⁺ and forms hydrogen bonds with I⁻, enabled by its optimal spatial geometry and inductive modulation. This dual-site interaction not only promotes oriented perovskite crystal growth and relieves residual lattice strain, but also induces the formation of low-dimensional perovskites that reduce residual PbI₂ and passivate defects. As a result, inverted single-junction 1.6 eV wide-bandgap perovskite solar cells achieved a remarkable power conversion efficiency of 23.53%, alongside excellent photo- and thermal stability. Notably, monolithic bifacial perovskite/silicon TSCs incorporating 3PyFA⁺ attain a record-high power generation density of 32.51 mW cm^-2 under typical bifacial illumination - the highest reported value to date for bifacial perovskite/silicon TSCs. This study establishes a new paradigm for additive design via structural isomer engineering and offers practical guidance for the development of efficient and durable bifacial tandem photovoltaics.