In-situ solution complexation for n-type surface-energetics reconstruction in 2.0 eV ultra-wide-bandgap perovskite solar cells

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.