Role of PCBM in the Suppression of Hysteresis in Perovskite Solar Cells

The power conversion efficiency of inorganic-organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical-field-induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in-situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl-C61-butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter-diffusion. By employing wide-field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature-dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X-ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built-in field modulation and interfacial barriers.