Study On Two-Dimensional Absorption Layer And Defect Passivation Of Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials with three-dimensional（3D）ABX₃ crystal structure have excellent photoelectric properties.As the forefront of photovoltaic research,the power conversion efficiency（PCE）of perovskite solar cells（PSCs）has increased rapidly in a few years.Since 2012,the PCE of PSCs has increased from 9.7% to 25.5%.However,the instability of 3D perovskite hinders the further development of PSCs.In recent years,two-dimensional（2D）perovskites with large-scale amine cations as interlayer have shown good stability.However,the introduction of organic amine cations hinders the exciton separation and charge transfer in 2D perovskite,which makes it difficult for 2D-PSCs to obtain high PCE.On the one hand,this paper aims to study the effect of diamine cation structure on the photoelectric properties of 2D perovskite,on the other hand,to study the passivation effect and mechanism of 2D perovskite on 3D perovskite surface.（1）The 2D perovskite（DMEN）MA_n-1Pb_nI_3n+1 was synthesized by using2-dimethylaminoethylamine（DMEN）with asymmetric molecular structure as spacer cations.The effects of n value（n is the number of inorganic layers）on the properties of 2D perovskite thin films and 2D-PSCs devices were studied.The properties of 2D perovskite thin films with different n values（n=3,4,5）were studied by scanning electron microscope（SEM）and grazing-incidence wide-angle x-ray scattering（GIWAXS）.The results show that the 2D perovskite films（n=3）are more uniform,dense and have stronger optical absorption ability.Moreover,2D-PSCs（n=3）obtain the PCE up to 7.26% and show the better stability than traditional 3D-PSCs.On this basis,two diamines（DMEN and EDA）with two carbon atoms in the main chain but different symmetries were compared,to study the effects of the different structure on the properties of 2D perovskite films and the photoelectric properties of 2D-PSCs.The results indicate that the DMEN-based 2D perovskite films are compact and uniform,having better optical absorption properties and crystallinity.However,the PCE of EDA-based 2D-PSCs is only 2.85%.The results confirm that asymmetric organic diamines show more excellent photoelectric application potential than symmetrical organic diamines in DJ type 2D perovskite.（2）4-Chlorobenzylamine（4-CBA）cation was introduced on the 3D perovskite thin film to in-situ grow 2D perovskite,which can effectively passivate the surface and grain boundary defects of the 3D perovskite thin film.The structure and photoelectric properties of perovskite film,as well as the photovoltaic characteristics and stability of PSCs are systematically studied.The results show that the formation of 2D perovskite can passivate the surface/interface defects of 3D perovskite and suppress the internal non-radiative recombination of 3D perovskite film,thus improving the efficiency and stability of 2D/3D-PSCs devices.Compared with 3D-PSCs,the open-circuit voltage and fill factor of 2D/3D-PSCs are greatly improved,making PCE increase from 18.70% to 20.88%.In addition,the stability of the device was monitored.The results of thermal stability test at 85℃ displayed that the PCE of 2D/3D-PSCs maintained at 58% of the initial PCE after 240 h.Moreover,the 2D/3D-PSCs device could still maintain 82%of the initial PCE after 200 h continuous operation under the condition of under 1-sun illumination in N₂ atmosphere.Therefore,2D/3D perovskite can provide an effective reference for further improving the photoelectric performance and stability of PSCs.