Higher efficiency perovskite solar cells using additives of LiI, LiTFSI and BMImI in the PbI2 precursor

Abstract

The efficiencies of perovskite solar cells have been significantly increased to 18%, 17.01% and 15.6% for the cells containing the additives BMImI, LiI and LiTFSI in the PbI₂ precursor solutions, respectively, from 11.3% for the devices without any additives. Incorporation of these additives led to the formation of perovskites with larger grain size and higher crystallinity with reduced PbI₂ residue as indicated by X-ray diffraction (XRD) and atomic force microscopy (AFM) results. Kelvin Probe Force Microscopy (KPFM) and current sensing (CS)-AFM results were in good agreement with external quantum efficiency (EQE) measurements and proved the great enhancement in short circuit current density (J_sc) as a result of doping. Transient photovoltage measurement results exhibited longer charge carrier lifetimes for the additive incorporated perovskites than those without additives, thus improving the fill factor (FF) and open circuit voltage (V_oc). In addition to the improved efficiency, the incorporation of these additives led to higher stability of the CH₃NH₃PbI₃ perovskite solar cells. The new additive BMImI, LiI and LiTFSI incorporated CH₃NH₃PbI₃ perovskite solar cells exhibited a reduced degradation with a 57%, 60%, and 91% decrease in performance respectively after exposure to air for 70 days compared to a 93% decrease for the pristine cell after only 24 days. The lithium salt additives can serve as desiccants to absorb moisture preventing perovskite degradation. Further, the BMImI additive can prevent the formation of free radicals in perovskites upon exposure to light and heat.