Bidentate Anchoring Enables Concurrent Grain Orientation and Lattice Strain Mitigation in Wide‐Bandgap Perovskites for High‐Performance All‐Perovskite Tandem Solar Cells

Abstract Wide‐bandgap perovskite solar cells (WBG‐PSCs) are essential for high‐performance all‐perovskite tandem solar cells. However, their efficiency and stability are limited by inhomogeneous crystallization, which induces disordered crystal orientation and detrimental lattice strain. Herein, malondiamidine hydrochloride (MAMCl) is introduced as a new ligand that simultaneously controls crystal nucleation orientation and passivates grain boundaries in WBG perovskites while relieving lattice strain. MAMCl's unique molecular structure – featuring amide and amidine terminal groups connected by a short carbon chain, exhibits strong binding affinity with lead ions, promoting preferential (100)‐oriented nucleation. The ligand's compact molecular structure, devoid of sterically hindering groups, facilitates charge extraction and transport at the perovskite/charge transport layer interface. During thermal processing, MAMCl preferentially anchors at grain boundaries through strong coordination bonding, effectively mitigating lattice strain and enhancing thermal stability. As a result, single‐junction 1.77 eV WBG‐PSCs achieve a champion power conversion efficiency (PCE) of 20.4% with an exceptional open‐circuit voltage ( V OC ) of 1.369 V. When incorporated into tandem devices, a high PCE of 29.0% (certified 28.06%) is obtained. Notably, the encapsulated all‐perovskite tandem devices retain 93% of initial efficiency after 700 h and over 80% after 1320 h of continuous maximum power point tracking (MPPT) under 1‐sun illumination in ambient conditions.