Abstract
Abstract Adding so-called oversized organic cations to lead-halide perovskites has emerged as a promising route to enhance the stability and efficiency of perovskite solar cells (PSCs). These additives can induce quasi-2D phases, yielding “2D–3D” hybrids. Here, we study methylammonium (MA) lead iodide PSCs incorporating guanidinium (GA) cations using ultrafast transient absorption, open-circuit voltage decay and X-ray diffraction, and we probe their robustness under proton, electron, and UV irradiation. In our best devices, those with limited GA+ content, nominally GA(MA)5Pb5I16, and employing SnO2 as the electron-transport layer, no quasi-2D signatures are detected. Instead, GA is incorporated into a 3D perovskite lattice without phase segregation. Devices built on TiO2 and/or exhibiting quasi-2D character show inferior performance, which we attribute to enhanced nonradiative charge recombination. Finally, irradiation studies reveal that GA-containing PSCs reveal remarkable resistance to ionizing damage, preserving optoelectronic quality and voltage retention, thereby underscoring their potential for space applications.