Ionic Liquid‐Driven Intermediate Phase Engineering for Surface‐Reconstruction of CsPbI ₂ Br Toward High‐Efficiency Perovskite/Organic Tandem Solar Cells

ABSTRACT All‐inorganic CsPbI 2 Br perovskites are promising wide‐bandgap front‐cell candidates for tandem photovoltaics due to their outstanding thermal stability and optimal bandgap. However, their performance is significantly constrained by surface defects and imperfect film morphology, which promote non‐radiative recombination and energy losses. In this work, we propose a surface reconstruction (SRC) strategy employing the ionic liquid dimethylammonium acetate (DMAAc) to fundamentally reconstruct the CsPbI 2 Br surface. Moving beyond conventional passivation, this approach triggers the formation of an intermediate DMAPb(I 2 Br) 1‐ x Ac 3 x phase on the perovskite surface, followed by its in situ recrystallization into high‐quality CsPbI 2 Br crystals. This phase transformation effectively improves surface morphology, heals ionic defects, and optimizes interfacial energy level alignment. Consequently, SRC‐treated CsPbI 2 Br single‐junction solar cells achieve a champion power conversion efficiency (PCE) of 17.54% with a high open‐circuit voltage ( V OC ) of 1.37 V. Furthermore, by integrating this optimized wide‐bandgap subcell with a narrow‐bandgap organic solar cell, we demonstrate a monolithic all‐inorganic perovskite/organic tandem device that delivers an impressive PCE of 24.20% under AM 1.5G illumination. This study presents a generalized surface reconstruction route to mitigate interfacial losses, offering a viable pathway toward highly efficient and stable multi‐junction photovoltaics.