Research on the Preparation Technology of High Performance Perovskite Films via Vapor Phase Method

Author:Chen Hai Bin

Supervisor:dai song yuan


Degree Year:2019





Hybrid organic-inorganic perovskite solar cells(PSCs)have drawn enormous attention owing to their lowcost fabrication process and excellent photovoltaic performance.The quality of perovskite film is vital to obtain high-performance PSCs.During the preparation of perovskite films,there would arise many problems,which could limit the device performance.Optimizing the film preparation process,controlling the crystal growth rate and film morphology,and reducing the defect state recombination are very important to improve the device performance.Herein,the uniform perovskite films are prepared through vapor phase method,and a comprehensive study is conducted in preparation parameters combining with the internal mechanism.(1)A surface modification process is developed to optimize the surface layer morphology of PbI2 precursor films and lower the resistance of the induced period in crystallization.The vapor optimization time is shortened to several seconds,and the intermediate phase forms on the surface layer of PbI2 films.We achieved porous PbI2 surface with smaller grains through dimethyl sulfoxide vapor treatment,which promotes the migration and reaction rate between CH3NH3I vapor and PbI2 layer.The PbI2 precursor films undergo dramatic morphological evolution due to the formed intermediate phase on PbI2 surface layer.Taking advantage of the proposed surface modification process,we achieved high-quality uniform perovskite films with larger crystal grains and without residual PbI2.The repeatable PSCs with modified films exhibited power conversion efficiency(PCE)of up to 18.43%for planar structure.Moreover,the devices showed less hysteresis because of improved quality and reduced defect states of the films.Our work expands the application of morphology control through forming intermediate phase and demonstrates an effective way to enhance the performance of the PSCs.(2)To achieve high-quality PSCs,the morphology and carrier transportation of perovskite films need to be optimized.Herein,C60 is employed as nucleation sites in PbI2 precursor solution to optimize the morphology of perovskite films via vapor-assisted deposition process.Accompanying the homogeneous nucleation of PbI2,the incorporation of C60 as heterogeneous nucleation sites can lower the nucleation free energy of PbI2,which facilitates the diffusion and reaction between PbI2 and organic source.Meanwhile,C60 could enhance carrier transportation and reduce charge recombination in the perovskite layer due to its high electron mobility and conductivity In addition,the grain sizes of perovskite get larger with C60 optimizing,which can reduce the grain boundaries and voids in perovskite and prevent the corrosion because of moisture.As a result,we obtain PSCs with a PCE of 18.33%and excellent stability.(3)Spray coating as a deposition technique is explored to prepare perovskite films to break the bottleneck that plagued large scale production.A comprehensive study is conducted in fabricating perovskite films by air-brush spraying method combining with the internal mechanism.The balance between solvent evaporation and perovskite crystallization rate is the deeper factor for fabricating high quality perovskite films.The spraying process is divided into three stages,namely,atomization process,droplet flight and film deposition.Each of them is detailed discussed from principles to parameters.Integrating the above three aspects,high quality perovskite films comparable to spin coated films are obtained,and a best PCE of 15.33%is achieved for planar structure PSCs,which is high among the PSCs fabricated by sprayins method.In addition,the devices with spraying method show excellent optical and electrical properties,and the principles we discussed here could be applied in the actual condition for mass production.Herein,spray coating as a deposition technique is explored to prepare perovskite films to break the bottleneck that plagued large scale production.However,it is difficult to avoid the notorious "cofee ring effect" during the process of spraying perovskite films especially in one step spray method(OSSM).Thus,the vapor assisted spraying method(VASM),namely,fabricating perovskite films through vapor-solid in-situ reaction between CH3NH3I vapor and sprayed PbI2 films,is creatively applied to the preparation of dense and uniform perovskite films.The surfaces of spraved PbI2 films are optimized by adjusting the wettability,viscosity,and contact quality via various methods such as the selection of solvent,precursor solution concentration,and substrate temperature to inhibit the capillary flow and release the pinning contact line.The application of component solvent could effectively crush the dense structure of PbI2 layer,improving the morphology of PbI2 films as well as the reducing of coffee ring effect.Integrating the above aspects,the optimized PbI2 films could form uniform perovskite films via in-situ reaction with CH3NH3I vapor and a best PCE of 17.65%is achieved for planar structure PSCs,which is high among the PSCs fabricated by spraying method.In addition,the VASM could be applied in the actual condition for mass production,exhibiting excellent optical and electrical properties,and paving the way of the commercialization of PSCs.The growth mechanism and preparation process of perovskite crystal nucleation are investigated by gas phase method.In the preparation of perovskite films by vapor-assisted method,PbI2 precursor films are optimized to reduce the resistance during the induction period of reaction with MAI gas.and high-quality perovskite thin films are prepared.In the preparation of perovskite films by spraying method,the mechanism and process parameters of one-step spraying method are deeply explored,and uniform perovskite thin films are prepared.In order to reduce the "coffee ring" effect in the spraying process,high quality perovskite films are prepared by vapor assisted spraying.