Study On The Morphology Control And Properties Of (CH₃NH₃)₃Bi₂I₉ Light-Absorbing Layer Film By Organic Additives

Since 2009,the photoelectric conversion efficiency of perovskite solar cells(PSCs)has rapidly increased from the original 3.8%to 25.02%.The toxicity and inherent instability of the widely studied lead-based perovskite have limited its commercial development,thus promoting the research of non-lead perovskite materials.More and more researchers are beginning to search for perovskites that can replace lead material.The advantages of low toxicity and long-term stability in air of bismuth-based perovskite have attracted wide attention from researchers.However,bismuth-based perovskite((CH₃NH₃)₃Bi₂I₉,MBI)thin films are formed by disorderly accumulation of hexagonal crystals.This disordered stacking results in low density of the MBI film and many pinhole defects.The defects in the MBI film will become the recombination center of carriers,thereby reducing the photoelectric conversion efficiency of MBI PSCs.It is necessary to improve the quality of the MBI light absorption layer film.The uniform and dense MBI film can improve the performance of bismuth-based PSCs.The research contents of this paper are as follows:(1)Polyethylene glycol(PEG)was added as an additive to the bismuth-based perovskite film,and the device was fabricated for related tests.Through the interaction between the three-dimensional frame of PEG and the bismuth-based perovskite,defects of the MBI thin film were successfully decreased.When the doping amount of PEG was 30 mg/m L,the obtained champion photoelectric conversion efficiency was 0.14%;XRD patterns showed that the intensity of the diffraction peaks at each diffraction angle was stronger than the diffraction peaks of other doping contents;SEM images showed that the MBI film has almost no defects,where the vertically grown hexagonal crystals are uniformly distributed at the grain boundaries,with the largest grain size,and the most dense film morphology;UV-vis results showed that the absorption range is 300-560 nm,with the highest absorption peak at 510 nm,and the largest absorption intensity at this doping content.It has revealed that with this doping concentration,the film has relatively best quality,which can effectively transport electrons and reduce carrier recombination at the interface.The doping of PEG not only resulted in a dense MBI film with almost no defects,but also significantly improved the photoelectric performance of MBI PSCs.(2)The acidified conductive polyaniline(PAN)was used to improve the quality of the MBI film.In the first system,it was found that the non-conductive polymer would inhibit the carrier transport.The acidified conductive PAN can provide a large number of-NH₂⁺-groups which can connect with I^-in MBI through ionic bonds.The MBI crystals can grow along the polymer framework of PAN.SEM images showed that PAN doping did improve the morphology of the MBI film and reduced the pinhole defects of the film.The XPS and XRD results showed that the doping of PAN does not affect the MBI crystal formation.The UV-vis and XRD characterization results showed that when the doping amount of PAN was 20 mg/m L,the light absorption intensity and crystal strength of MBI were the relatively largest,and the relatively best photoelectric conversion efficiency of 0.11%was obtained.(3)Small organic molecule chloromethylamine(MACl)was used to improve MBI film quality.The synergistic effect of dimethyl sulfoxide(DMSO)and MACl additive can assist the crystallization of perovskite.SEM image showed that a single crystal film with a smooth surface and low defect density was formed.XRD and XPS spectra showed that the incorporation of MACl did not affect the formation of MBI crystals.The UV-vis and XRD characterization showed that when the doping amount of MACl was 30%mmol,the light absorption intensity and crystalline intensity of MBI were the relatively largest,and the relatively best photoelectric conversion efficiency of 0.16%was obtained.