Experimental And First-Principle Calculation Study On The Reaction Mechanism Between Lead Iodide And Small Polar Molecules Containing Nitrogen

Organic-Inorganic hybrid perovskite solar cells?PSCs?are one of the most exciting advances in photovoltaics in recent years.Most of PSCs are based on methyl ammonium lead iodide?MAPbI₃?that has a perovskite structure.MAPbI₃ is synthesized by the reaction between PbI₂ and MAI.One of the issues in the synthesis is that MAPbI₃ is largely influenced by the amount of MAI.Excess amount of MAI will bleach the as-synthesized MAPbI₃,lowering the light absorption of the films and deteriorating the performance of fabricated devices.Similar phenomena are observed on other small polar molecules containing nitrogen like ammonia and formamidinium iodide.However,until now,the reaction mechanism between PbI₂ and small polar molecules containing nitrogen is yet to be explored.In this thesis,we addressed this issue by combining experimental study and first-principle study.We first studied the reaction between PbI₂ films and MAI.The reaction was monitored by an in-situ resistance measurement method.Based on the time evolution of resistance,we studied the time evolution of morphology and structures of thin films during the reaction.Two new XRD peaks at 11.5° and 13.5° were observed during the reaction between PbI₂ and MAI,meaning a new perovskite structure was formed.A stepwise reaction model was built to elucidate the observed experimental results.This model can explain the formation mechanisms of pervoskite films in vapor-solid reactions.We further studied the reaction between PbI₂ and ammonia using first-principle calculation.Thermal gravimetric analysis reveals two distinct ammonia adsorption sites in PbI₂ and MAPbI₃ lattices.The intercalation of ammonia into PbI₂ lattice was found to be an exothermal process,with a formation energy of about-0.55 eV.The intercalation was facilitated by the formation of hydrogen bonds between H in ammonia and I in PbI₂,as well as the coordination from N in ammonia to Pb in PbI₂.The intercalation was a reversible process.Finally,we found that the intercalation of ammonia into PbI₂ leads to an increase in the band gap of PbI₂,in agreement with the observed bleaching.Calculation suggests that ammonia does not participate in the formation of the band gap.