Widebandgap (WBG) perovskites are critical for highefficiency tandem solar cells but suffer from rapid crystallization, photoinduced halide segregation, and interfacial losses. We introduce methyl 2(4,4,5,5tetramethyl1,3,2dioxaborolan2yl)1Hindole7carboxylate (MHIC) as a multifunctional modulator for 1.68 eV WBG per...