In‐Situ Solution Complexation for n‐Type Surface‐Energetics Reconstruction in 2.0 eV Ultra‐Wide‐Bandgap Perovskite Solar Cells

ABSTRACT Ultra‐wide‐bandgap (UWBG) perovskites (>2.0 eV) are essential for high‐efficiency triple‐junction tandem solar cells but suffer from photo‐induced phase segregation and open‐circuit voltage ( V OC ) deficits arising from surface defects and energetic misalignment. Here, we report an in situ solution complexation (ISC) strategy to reconstruct the surface of 2.0 eV perovskites. By exploiting a proton transfer reaction between phenethylammonium chloride and ethylenediamine, we activate the passivation agents to selectively deplete unstable surface iodine clusters and eliminate metallic lead defects. This chemical reconstruction induces a degenerate‐like n‐type surface with pronounced downward band bending, simultaneously forming a robust hole‐blocking barrier and enabling efficient electron extraction via an Ohmic tunneling contact. Consequently, the ISC‐treated 2.0 eV single‐junction device achieves a power conversion efficiency (PCE) of 15.7% with a high V OC of 1.41 V and a fill factor of 0.84, while exhibiting superior photostability by suppressing phase segregation. Leveraging this UWBG top cell together with a 1.5 eV perovskite bottom cell, we further demonstrate a monolithic all‐perovskite tandem solar cell delivering a PCE of 24.2% with a V OC of 2.58 V. This work provides a practical pathway to minimize voltage losses and stabilize UWBG perovskites, advancing perovskite tandems toward perovskite/perovskite/Si triple‐junction architecture.