Introduction Of Thiocyanate Into Formamidinium Based Perovskite Film Fabrication

The introduction of pseudo-halide thiocyanate ion（SCN-）is beneficial to improve the perovskite solar cell performance.In the MAPbI₃ system,the introduction of Pb（SCN）₂ tends to form layered 2-Dimensional perovskite（MA）2Pb（SCN）₂I2.To fabricate MAPbI₃ perovskite film,small amount of Pb（SCN）₂ would effectively improve the film quality and the solar cell performance.For now,studies about thiocyanate（SCN-）are limited to MAPbI₃.Herein,we focus on two thiocyanate additives,NH₄SCN and Pb（SCN）₂,and investigate their influence on the fabrication of formamidinium（FA）based perovskite films,as well as on the performance of corresponding perovskite solar cells（PVSCs）.We first introduce NH₄SCN additive into the one-step fabrication of FAPbI₃ perovskite film.NH₄SCN is found to suppress the formation of 8-FAPbI₃ and promote the formation and crystallinity of α-FAPbI₃.30 mol%of NH₄SCN additive makes the FAPbI₃ film constituted of pure α phase with larger crystals and improved morphology,so the corresponding film has better moisture stability.As a result,30 mol%of NH₄SCN additive improves the efficiency of the perovskite solar cell fabricated through one-step method from normal 5.94%to 11.44%,and simultaneously enhances the solar cell stability.Through evaporation of solvent,a thiocyanate-included three-dimensional perovskite FAPbI_3-x（SCN）_x powder is successfully fabricated,whose crystal structure is the same as α-FAPbI₃,and it will transform into δ phase orange powder under room temperature.The phenomenon that perovskite composition varies with different fabrication methods implies that fabrication methods and conditions will influence the final perovskite composition.In the fabrication of FAPbI₃ film through two-step method,Pb（SCN）₂ additive is applied to dope PbI₂.Small amount of Pb（SCN）₂ can dramatically enhance the crystallinity of FAPbI₃ and increase the crystal size of FAPbI₃.Meanwhile,the addition of Pb（SCN）₂ makes FAPbI₃ film become smoother,improving the efficiency of FAPbI₃ perovskite solar cell.However,more PbI₂ would be render in the film as the amount of Pb（SCN）₂ increases,so too much Pb（SCN）₂ would adversely degrade the solar cell performance.5%Pb（SCN）₂ dopant has the best effect of improving the efficiency of FAPbI₃ PVSCs,from initial 10.6%to 13.3%.For the FA-MA mixed perovskite MA_0.6FA_0.4PbW₃,with good performance,more effort is dedicated to investigate the role of NH₄SCN in the fabrication of its film through two-step method.The crystallinity of MA0.6FA0.4PbI₃ perovskite gradually increased as the amount of NH₄SCN increases.In the meantime,NH₄SCN would enlarge the crystal size and lower the roughness of the MA0.6FA0.4PbI₃ perovskite film.When the content of NH₄SCN is 2.5%,the corresponding PVSCs have the best efficiency of 16.0%.In conclusion,this dissertation investigated the role of SCN-in fabricating FA based perovskite including the mixed perovskite,obtained the new perovskite FAPbI₃.x（SCN）x,and extended the fabrication methods of FA based perovskite.This work would promote the development of FA-based PVSCs and new perovskite materials.