Fabrication Of Perovskite Films And Solar Cells Basing On Lewis Acid-base Reaction

Recently,methylammonium lead iodide?CH₃NH₃PbI₃,MAPbI₃?perovskite solar cells?PSCs?have attracted much attention due to their excellent performance.High quality perovskite film holds a key to fabricate highly efficient PSCs.The organic solvents play important roles in formation of perovskite films.In this thesis,an approach basing on Lewis acid-base reaction was proposed to fabricate high quality perovskite films.By optimizing the parameters during the film formation process,perovskite films containing large columnar grains with size exceeding 1?m were fabricated,and PSCs with power conversion efficiency of 17.26%were achieved.It reveals the formation mechanisms of perovskite films fabricated from the Lewis acid-base approach by introducing some DMA into PbI₂/DMF solution.Firstly,it forms a Lewis acid-base adduct of PbI₂?xSol through a dative Pb-O bond between PbI₂ and solvent.Then,it forms perovskite precursor films of MAI-PbI₂-Sol through molecular exchange between the embedded solvent and MAI.Finally,the perovskite precursor convert to perovskite films by removing the solvent through annealing.The?001?plane of the PbI₂ lattice is expanded from 6.98?to 9.82?when it forms a Lewis acid-base adduct of PbI₂?xDMF,which not only provides molecular channels for diffusion of MAI,but also eliminates the volume expansion during the formation of perovskite films.The volume of PbI₂ lattice expands about 2 times when the PbI₂ convert to perovskite by reacting with MAI.Mesoporous PbI₂ films with different porous structure were fabricated by introducing 30%DMSO,20%NMP,5%HMPA into the PbI₂/DMF solution and then annealing.The pores in PbI₂ films also provide large diffusion channels for MAI,and eliminates the volume expansion during the formation of perovskite films.High quality perovskite films with smooth surface were both fabricated by the above two methods.By optimizing the morphology of PbI₂films,PSCs with efficiency of 16.17%are thus fabricated.In the Lewis acid-base adduct of PbI₂?xSol,the solvent coordinates with Pb²⁺through a dative Pb-O bond.In this thesis,the strength of Pb-O bonds can be modulated by adding 10%NMP,8%DMSO,2%HMPA into PbI₂/DMF solution.As a result,the mean grain size increases to 340 nm,and PSCs with efficiency of 17.26%can be achieved from the samples adding with 8%DMSO.By adding 5 at.%PbCl₂ into the PbI₂/DMF solution,the mean grain size of perovskite films increases from 223 nm to353 nm.The efficiency of the PSCs thus increases significantly from 13.2%to 17.0%.Some solvent molecules are mbedded in the perovskite precursor of MAI-PbI₂-Sol.The embedded solvent molecule can induce dissolution-recrystallization during annealing.Here,a sandwich structure with a configuration of FTO/MAI-PbI₂-DMF/PC₆₁BM is proposed to retard the escape of DMF molecules during annealing.A thin PC₆₁BM film was spin coated immediately onto the perovskite precursor of MAI-PbI₂-DMF before annealing.By using the sandwich structure annealing method,columnar perovskite films with mean size of 718.6 nm were fabricated.The growth of large grains follows the Ostwald ripening model.Besides,perovskite precursor of MAI-PbI₂-DMF can also convert to columnar perovskite films composed of large grains by annealing at high temperature of 250oC for only 10 s.