Modification Of Surface And Bulk For Perovskite Layer In Invertered Perovskite Solar Cells

The excellent optoelectronic properties of organo-metal halides perovskite materials have attracted the intensive attention of worldwide researchers.In just a decade,the photoelectric conversion efficiency of perovskite solar cells has increased from the initial3.8%to over 23%.Fullerene(C₆₀)is a commonly used electron transport material for inverted perovskite solar cells.It is low in price and suitable for vacuum evaporation.It plays an important role in the uniformity and consistency for large-area device manufacturing in the industry.However,there is a certain loss in the energy level matching between C₆₀ and perovskite,which results in lower open circuit voltage of C₆₀-based device than that PCBM-based under the same working condition,therefore,the photoelectric conversion efficiency is affected.In view of this,it is necessary to develop a method for surface modification and interface energy level control of perovskite which can be uniformly prepared over a large area to improve the open circuit voltage of a perovskite solar cell.In addition,the perovskite la yer is easily destroyed by water vapor in the air,resulting in poor stability of the corresponding device.Therefore,it is necessary to enhance the stability of the perovskite layer by bulk modification.In order to resolve the above two issues,this paper mainly carried out the following research work:（1）Lithium fluoride and sodium fluoride were deposited on a perovskite film by vacuum evaporationtoprepareperovskitesolarcellswiththestructureof Glass/FTO/NiMgLiO/MAPbI₃/XF/C₆₀/BCP/Ag.By comparison study,it is found that the optimal thickness of the modification layer of sodium fluoride is 0.5 nm.The interfacial charge transfer at this thickness is the most efficient by effective passivation of the surface traps of perovskite.Accordingly,the open circuit voltage of the device is increased from1.03V to 1.09V,and the photoelectric conversion efficiency of 18.03%is obtained,which is superior to the PCBM-based reference device.（2）Certain molar ratios of polyethyleneimine（PEI）were mixed into the MAPbI₃perovskitefilms,andaseriesofdeviceswiththestructureof Glass/FTO/NiMgLiO/MAPbI₃（PEI）/NaF/C₆₀/BCP/Ag have been compared and studied.Through a series of characterizations by XRD,SEM,PL and J-V curve,it was found that0.5%molar ratio of PEI dopant to MAPbI₃ is enough to effectively passivate the defects of MAPbI₃ perovskite film and results in the doped film with smooth surface and pin-hole-free morphology.The corresponding device obtained an improved photoelectric conversion efficiency of 18.41%;at the same time,the hydrophobicity of the PEI polymer gives the device superior environmental stability,which can maintain 99%of its initial efficiency after keeping in ambient air for 200 hours without humidity control.