Controlling The Crystallization And Morphology Of Perovskite To Improve The Performance In Solar Cells

The photovoltaic performance of perovskite solar cells is mainly determined by the crystallization and morphology of CH₃NH₃PbI₃ layer.Thus,a mass of research focused on fabricating CH₃NH₃PbI₃ films has been reported to control the crystallization of CH₃NH₃PbI₃films,including seeking a substitute for precursor solvent,antisolvent-induced precipitation,solvent annealing and so on.In this paper,to control the crystallization of CH₃NH₃PbI₃ has been realized by pre-treating the PbI₂ layer in the two-step perovskite precipitation,which also improve the morphology of CH₃NH₃PbI₃ films.XRD,SEM,UV-vis spectra,PL spectra are applied to characterize the CH₃NH₃PbI₃ films and I-V curves and external quantum efficiency are measured to observe the performance of perovskite solar cells.The main work in this paper are displayed as follow:（1）By adding NH₄I additive to the precursor solution for preparing PbI₂ films,NH₄PbI₃intermediate phase is realized by the conversion between NH₄I and PbI₂,and then decomposed into PbI₂ in the subsequent thermal annealing at 90℃,which the reversible transformation can retard the crystallization of PbI₂.The optimal PbI₂ precursor solution is prepared with a PbI₂/NH₄I mole ratio of 6:1.In this method,a dense,smooth and pinhole-free PbI₂ film has been achieved by the PbI₂/NH₄I precursor solution.Furthermore,the CH₃NH₃PbI₃ films fabricated by the retarded PbI₂ film show a longer charge lifetime compared to the pristine CH₃NH₃PbI₃ film,which indicates the reduced trap state density occurs in CH₃NH₃PbI₃ films.When the optimized CH₃NH₃PbI₃ film is applied to perovskite solar cells,both their V_OC and FF obtain enhancement.And at meanwhile,the device’s reproducibility can be dramatically improved.（2）The oriented growth of perovskite is achieved by a novel method.Depositing tetraphenylphosphonium chloride（TPPCl）on the top of the PbI₂ thin film in two-step perovskite deposition process can control the oriented growth of perovskite.The TPPCl-treated CH₃NH₃PbI₃ film exhibits larger and more inerratic grains.Besides,the prior growth orientation along with the（110）and（220）crystal planes is achieved in the TPPCl-treated CH₃NH₃PbI₃ thin film.By this method,the V_OC is increased from 1.04 V of the pristine perovskite solar cells（PSCs）to 1.12 V of the TPPCl-treated PSCs,and the TPPCl-treated PSCs obtained a maximum power conversion efficiency of over 18%.Moreover,the TPPCl-treated CH₃NH₃PbI₃ thin film are beneficial to suppress the device charge recombination process.Therefore,the results indicate our work provides a new strategy to fabricate the oriented growth of CH₃NH₃PbI₃ thin film for boosting the V_OC of PSCs.