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

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.