Enhancing Photovoltaically Preferred Orientation in Wide‐Bandgap Perovskite for Efficient All‐Perovskite Tandem Solar Cells

Wide‐bandgap perovskite solar cells (WBG PSCs) have promising applications in tandem devices yet suffer from low open‐circuit voltages ( V OC s) and less stability. To address these issues, the study introduces multifunctional nicotinamide derivatives into WBG PSCs, leveraging the regulation on photovoltaically preferential orientation and optoelectronic properties via diverse functional groups, e.g., carbonyl, amino. Isonicotinamide (IA) molecule emerges as the most effective agent, enhancing crystallization kinetics and defect passivation due to its unique planar spatial configuration. Incorporating IA into WBG perovskites improves the (100) preferred crystal orientation, reduces trap density, and enables well‐matched energy band alignment. High‐performance 1.77 eV WBG PSCs are achieved with a champion power conversion efficiency of 19.34% and a V OC of 1.342 V, leading to the fabrication of the best‐performing all‐perovskite tandem solar cell with a PCE of 28.53% (certified 28.27%) and excellent operational stability, maintaining over 90% of the initial efficiency under 1 sun illumination for 600 h.