Doubled Grain Size Originated From Suppression of PbI2 for Improved Performance of Wide‐Bandgap Perovskite and Tandem Devices

Wide‐bandgap (WBG) perovskites (PVK), known for their superior ability of validly capturing visible light, are critical for achieving high‐efficiency VK–silicon tandem solar cells. However, challenges such as high‐defect densities at grain boundaries, nonradiative recombination, and instability severely limit their performance. In this study, we introduce a molecule of 2‐methylthio‐2‐imidazoline hydriodide (MDHI) as a passivator to address the grain boundary defects by forming MDHI–PbI 2 complex. X‐ray diffraction and scanning electron microscopy results demonstrate that MDHI effectively suppressed the formation of PbI 2 , thereby greatly enhanced the crystallinity of PVKs with doubled grain size, which validly reduced defect densities and minimized recombination losses. As a result, an enhanced power conversion efficiency (PCE) of 20.19% (0.09 cm 2 ) is observed for a bandgap of 1.68 eV PVK solar cells (PSCs) with MDHI modification, which can retain over 90% of its initial efficiency after 1000 h of continuous illumination, demonstrating significantly promoted operational stability, while the control one only shows 17.96% PCE and remains 60% of its pristine efficiency. Eventually, the MDHI‐modified WBG PVK–silicon tandem devices achieve a champion PCE of 30.46% (1.07 cm 2 ). This study provides a new strategy to improve the efficiency and stability of WBG PVK in tandem photovoltaics.