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–PbI₂ complex. X-ray diffraction and scanning electron microscopy results demonstrate that MDHI effectively suppressed the formation of PbI₂, 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 cm²) 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 cm²). This study provides a new strategy to improve the efficiency and stability of WBG PVK in tandem photovoltaics.