Preparation Of High-quality Mixed Halide Perovskite Thin Films For Perovskite Solar Cells

The photoelectric conversion efficiency?PCE?of perovskite solar cells?PSCs?has increased dramatically in the past decade,which has attracted great attention in photovoltaic field.Not only in the photovoltaic field,the perovskite has the enormous potential to be utilized in light emitting diodes,detectors and lasers.Although the PCE of PSCs has obtained 23.7%,which is on a par with polycrystalline silicon solar cells,Compared with MAPbI₃ perovskite,the mixed-ion perovskite materials?FAPbI₃?_0.85?MAPb Br3?_0.15 exhibit better photoelectric characteristics,adjustable direct band gap,higher optical absorption coefficient,longer carrier diffusion length,better stability and so on.However,the mechanism of crystallization quality and thermal stability of mixed perovskite films is still vague,which restricts the commercialization of PSCs.In this work,the effect of DMSO/DMF solvent ratio in perovskite precursor solution and substrate temperature of FTO/TiO₂ on the morphology and crystallization process of mixed perovskite films,as well as the effect of Cs ion doping concentration and substrate temperature of FTO/TiO₂ on the crystallization of mixed perovskite films were studied.The specific results are as follows:?1?The effect of DMSO/DMF solvent ratio in perovskite precursor solution and FTO/TiO₂ substrate temperature on the morphology and crystallization process of mixed perovskite films were studied.Here,perovskite precursor solutions with DMSO/DMF solvent ratios of 2:8,3:7,4:6,5:5 and 6:4 were prepared,respectively,and then spin-coated on FTO/TiO₂ substrates at temperatures of 30 ?,50? and 70?,respectively.It was found that when the substrate temperature was 30 ?,the morphology and crystallization of the films did not have obvious variation with the change of solvent ratio of DMSO/DMF,and the efficiency and stability of devices also had nearly no change.When the substrate temperature raised to 50 ?,the island structure occurred on the film's surface with a DMSO/DMF solvent ratio of 2:8.With the DMSO/DMF solvent ratio increasing,films became more uniform and denser,which effectively inhibited the formation of the island structure,and perovskite solar cells with high efficiency and good stability were obtained.If the substrate temperature further raised to 70 ?,all perovskite films presented the island structure on their surface,which would seriously damage the crystallization quality of the films.Due to the lower saturated vapor pressure of DMSO compared with that of DMF,increased proportion of DMSO in precursor solution at substrate temperature of 50 ? would effectively delay the volatilization rate of solvents,thus delaying the crystallization of perovskite films,which would contribute to high quality perovskite films.Based on the planar cell structure of FTO/TiO₂/?FAPbI₃?_0.85?MAPb Br₃?_0.15/Spiro-OMe TAD/Au,the perovskite device with a DMSO/DMF solvent ratio of 4:6 in precursor solution and a 50?substrate temperature presented an excellent PCE of 19.43%.After 800 hours in the N₂-filled glove box,the device could still maintain 90% of its initial efficiency.?2?The effect of Cs ions and FTO/TiO₂ substrate temperature on the crystallization process of mixed perovskite films were studied.Here,the doping ratio of Cs ions in perovskite precursor solution was set as 0%,5%,10%,15% and 20%,and then the precursor solutions were spin-coated on FTO/TiO₂ substrates at 30?,50? and 70?,respectively.It was found that the surface morphology of perovskite films doped with different ratios of Cs ions did not have obvious variation when the substrate temperature was 30 ?.Cs ions doping on substrates at 50? or even higher modified the morphology of perovskite films,prompt the transformation of delta phase?non-perovskite phase?and PBI₂ phase in island structure to alpha phase?perovskite phase?on the surface of perovskite films,thus inhibiting the formation of impurity phase and improving the crystallization quality of perovskite films.TThese results show that the doping of Cs ions decreased the defect density of perovskite films and improved the crystallization quality of perovskite films.Based on the planar cell structure of FTO/TiO₂/?FAPbI₃?_0.85?MAPb Br₃?_0.15/SpiroOMe TAD/Au,the optimum photoelectric conversion efficiency of perovskite solar cells doped with 20% Cs ions can reach 21.25% at substrate temperature of 30 ?.After 1560 hours in the N₂-filled glove box,the device could still maintain 90% of its initial efficiency.Cs doping mixed halide PSCs still obtained high PCE and staility at 50? substrate or even higher,which provides an important experimental basis for the commercialization of perovskite solar cells in near future.