Rear-Surface Passivation by Melaminium Iodide Additive for Stable and Hysteresis-less Perovskite Solar Cells

Surface passivation of perovskite grains is one of the promising methods to reduce recombination and improve stability of perovskite solar cells (PSCs). We herein report the effect of a melaminium iodide additive on the photovoltaic performance of PSCs based on (FAPbI₃)_0.875(CsPbBr₃)_0.125 perovskite. Cyclic -C═N- and primary amine in melamine are a good hydrogen bond acceptor and Lewis base, which can interact with both the organic cation and Lewis acidic lead iodide in the perovskite film. Melaminium iodide is synthesized and added to the precursor solution, which is directly spin-coated to form the perovskite film. The presence of melaminium iodide additive reduces the trap density from 1.02 × 10¹⁶ to 0.645 × 10¹⁶ cm^-3, which leads to the reduction of nonradiative recombination and thereby improving the mean open-circuit voltage and the fill factor from 1.054 to 1.095 V and from 0.693 to 0.725 V, receptively. In addition, photocurrent-voltage hysteresis is reduced by the melaminium iodide additive, which results in an enhanced average power conversion efficiency, obtained from reverse and forward scanned data, from 15.86 to 17.32%. Time-resolved photoluminescence confirms that melaminium iodide plays a more important role in passivating the rear surface of the perovskite layer contacting the hole transporting spiro-MeOTAD layer. An aging test under a relative humidity of 65% reveals that melaminium iodide improves stability because of the suppression of the defect evolved by moisture.