Stability Exploration Of（PEA）₂PbI₄/MAPbI₃ Perovskite Solar Cells And Synergistic Effect With TiO₂ Nanolayer

Organic-inorganic lead halide perovskites APb X₃[A=methylammonium（MA⁺）,formamidinium（FA⁺）,or Cs⁺;X=I^-,Br^-,Cl^-]have become a prevailing photovoltaic material for fabricating efficient solar cells due to their high absorption coefficient,ideal direct bandgap,excellent optical tunability and long-range charge transport,as well as economical fabrication technologies.Moreover,the power conversion efficiency（PCE）of perovskite solar cells（PSCs）has reached 25.5%with a third-part certification.However,the further commercialization of PSCs is hindered for the low stability（moisture,oxygen,heat and ultraviolet light）of the perovskite light absorbers.To solve these problems,some potential strategies have been proposed to improve the stability of organic-inorganic perovskites such as interface passivation,usage of carbon electrode to replace thin metal electrodes,and various ions doping.In this paper,the PCE and stability of perovskite solar cells can be improved by manufacturing two dimensional（2D）/three-dimensional（3D）perovskite film,carbon electrode and surface passivation layer.The main work contents are as follows:（1）It is effective to elevate the stability of perovskite films via altering the size and proportion of the A site organic ions,such as partially replacing MA⁺with larger organic hydrophobic cation（PEA⁺）to form a 2D/3D mixed perovskite structure.The recent studies widely believe that the stability of（PEA）₂Pb I₄/MAPb I₃ perovskite solar cells（PSCs）can be progressively increased with the addition of PEA⁺.Herein,we report on the stability variations after the addition of PEA⁺into the MAPb I₃ film and demonstrate that:1)（PEA）₂Pb I₄/MAPb I₃ perovskite films show the best stability and performance at appropriate concentration of PEA⁺（5%-MAPb I₃）;2)Excess PEA⁺has negative effect on the stability and PCE of（PEA）₂Pb I₄/MAPb I₃ perovskite film.X-ray photoelectron spectroscopy（XPS）,nuclear magnetic resonance（NMR）and X-ray absorption fine structure（XAFS）spectroscopy have been utilized to testify that the excess PEA⁺will damage the MAPb I₃ crystal structure and generate defects due to the strong coordination between NH₃⁺from PEA⁺and I^-from[Pb I₆]^4-octahedron.This work reveals that the stability of（PEA）₂Pb I₄/MAPb I₃ perovskite films is not proportional to the addition of PEA⁺,which is of interest for the developing of stable 2D/3D PSCs.（2）Carbon electrode has better hydrophobicity than traditional metal electrode,meanwhile,it also has simpler preparation technology and lower cost.Carbon electrodes can improve the stability of PSCs.On the basis of the previous work,the highest PCE of9.65%was obtained by combining 5%-MAPb I₃ perovskite film with the carbon electrode.In the meantime,the stability of 5%-MAPb I₃ PSCs with carbon electrode was tested.It still retained 63.2%of the original PCE under the ambient-air condition for 9 days.The carbon electrode prepared by simple blade coating improves the stability of perovskite solar cells,which has a positive effect on the realization of commercialization.（3）The arrangement of a Co-TiO₂ layer work as passivation layer on top of perovskite and promotes the increase of current density.X-ray diffractometer（XRD）,scanning electron microscope（SEM）and transmission electron microscope（TEM）spectroscopy have been utilized to testify that the synthesized Co-TiO₂ is a monolayer nanosheet structure.Co-TiO₂ nanolayer can reduce the composite effect of electrons in interlayer transport and increase the current density（J）of solar cells.The J-V results of PSCs exhibit that in different concentrations of Co-TiO₂ nanolayer,the sample 0.02（Co-TiO₂ was diluted 50 times）obtains excellent current density and photoelectric conversion efficiency.5%-MAPb I₃ perovskite films show better synergistic effect on Co-TiO₂nanolayer which compared with MAPb I₃.The 5%-MAPb I₃ carbon electrode PSC combined with 0.02 concentration of Co-TiO₂ nanolayer presents the best PCE of 10.27%.Our work provides a feasible method to improve the current density of PSCs and thus promote the improvement of PCE.