Bifurcation‐Driven Additive Redistribution from Bulk Doping Enables In‐Situ Modulation of Interfaces Toward >26% Efficient Perovskite Solar Cells

ABSTRACT Interfaces in perovskite solar cells govern crystallization kinetics, interfacial energetics, defect formation, charge transport dynamics, and nonradiative recombination of charge carriers, thereby critically determining device efficiency and operational stability. Here, we report an in situ dual‐interface engineering strategy using a multifunctional additive, maleimido propionic acid hydrazide hydrochloride (MPAH.HCl), incorporated directly into the perovskite precursor. During film formation, MPAH.HCl undergoes spontaneous bifurcation‐driven bidirectional segregation, with Cl − preferentially enriching at the buried interface and MPAH + accumulating at the near top surface. At the buried interface, Cl − modulates crystallization dynamics, promotes grain growth, passivates deep‐level defects, and improves energy‐level alignment. Simultaneously, surface‐segregated MPAH + effectively passivates dominant vacancy (V I , V Pb , V FA ) and antisite (Pb I , I Pb , I FA ) defects. This synergistic dual‐interface modulation suppresses nonradiative recombination while facilitating efficient charge extraction and transport. As a result, optimized inverted perovskite solar cells achieve a fill factor of 86.6% (vs. 80.6% for the control), with a champion power conversion efficiency of 26.10% for 0.09 cm 2 , and 24.28% for 1 cm 2 devices. Furthermore, enhanced crystallinity and stabilized interfacial chemistry significantly improve device stability under ambient, thermal, and operational stress conditions. This work introduces bifurcation additive segregation as an effective strategy for simultaneous buried and top‐interface optimization, enabling highly efficient and stable perovskite photovoltaics.