Research On Phase Purity And Crystallinity Control Of Mixed-organic-cation Perovskite Thin Films

Perovskite materials have become star materials in the fields of photovoltaics and electroluminescence due to their unique properties such as direct band gap,high absorption coefficient,narrow band gap and high defect tolerance.The power conversion efficiency(PCE)of perovskite solar cells(PSCs)increased rapidly from 3.8%to 25.2%in past decade,mainly based on component engineering,perovskite film growth control and perovskite/transport layer interface engineering.It could be said that phase purity and crystallinity play a decisive role on the efficiency and stability of the device.Improving film phase purity and crystallinity is the key to improve the comprehensive performance of perovskite solar cells.In order to improve the phase purity of the perovskite thin film,we systematically studied the stability of the perovskite precursor solution,which was easily neglected before.We found that in mixed-organic-cation perovskite precursor solution,methylamine iodide(MAI)is prone to deprotonation to form methylamine(MA);however,only a small amount of MA gas volatilizes away from the solution,instead,most of the remainder MA immediately condensates with formamidinium iodide(FAI)to form N-methyl formamidinium iodide(MFAI)and N,N’-dimethyl formamidinium iodide(DMFAI).Based on the above understanding,we have further developed a general method to solve this problem.This method uses triethyl borate(TEB)to restrict the deprotonation of MAI,thus preventing the occurrence of side reactions.This strategy not only improves the power conversion efficiency(PCE),but also improves the repeatability of high-efficiency device fabrication.Based on the new discovery of the interaction between B-I,we further introduced a more stable additive system,so that while stabilizing the solution,it can be uniformly dispersed to the grain boundaries of the perovskite film after film formation,thereby limiting the I ion Migration to achieve the purpose of improving device stability.Here,we choose boric acid as an additive.The boron atom is sp~2 hybridized,and it can easily accept a pair of electrons into its vacant unhybridized p orbital.It has been shown that strong B-I interaction can completely hinder side reactions,including the deprotonation of MAI and the subsequent condensation reaction between MA and FAI.The increase in the purity of the perovskite film phase leads to an expansion of the absorption range,so the conversion efficiency is increased from 20%to 21%.At the same time,in the solid perovskite film,this B-I interaction in the perovskite grain boundary inhibits the migration of iodide ions,which also makes the long-term stability of the perovskite device significantly improved.This method opens a new direction for improving the working life of perovskite devices in the future.