Tailoring Perovskite Crystallization Via Solvent And Compositional Engineering For Highly Efficient Perovskite Solar Cells

Organic-inorganic hybrid perovskite Solar Cells（PSCs）have attracted many researchers’attention due to their rapid improvement in photoelectric conversion efficiency.The high efficiency of perovskite solar cells is fundamentally due to the excellent photoelectronic properties of perovskite materials,but technically the performance of perovskite solar cells is also affected by many factors,such as perovskite film quality and device structures.Therefore,the preparation of high-quality perovskite thin films with high grain size,high phase purity,high crystallinity,high coverage and suitable bandgap width is always an important prerequisite for obtaining high-performance perovskite solar cells.Therefore,this thesis is devoted to the development of high-quality perovskite thin film preparation methods and understanding of perovskite thin film nucleation growth law in order to further improve the performance of perovskite solar cells.The main research contents and results are as follows:（1）In order to solve the problem that PbI₂ could not be converted into perovskite rapidly and completely when the traditional two-step method was used to prepare double-layer mesoporous PSCs,the porous PbI₂ films were prepared by using a green antisolvent(ethanol（ET）and 2-butanol（2BA）extraction method,which were recorded as ET-PbI₂ and 2BA-PbI₂,respectively.The results show that the porosity of 2BA-PbI₂ film is 14%,while that of ET-PbI₂ film is 24%.This is due to the higher polarity and volatility of ET compared with 2BA,which is more conducive to extracting N,N-dimethylformamide（DMF）and dimethyl sulfoxide（DMSO）solvent molecules in the film and reducing the amount of the PbI₂-DMSO intermediate phase.Using 2BA-PbI₂ as template,the complete conversion time of MAPbI₃perovskite phase（2BA-PVK）is reduced to 3 minutes,while it only takes 10seconds when using ET-PbI₂ as template to obtain high phase purity MAPbI₃perovskite films（ET-PVK）,which indicates that different porosity of PbI₂ films can be obtained by using antisolvent extraction with different chemical properties,and then the complete conversion time of perovskite phase can be regulated.Both perovskite conversion time and porosity of PbI₂ affect the morphology of perovskite films.By combining with solvent annealing process,perovskite films with good surface morphology can be prepared without affecting the conversion rate.The optimum efficiency of PSCs fabricated with2BA-PVK as active layer is 12.31%,while that of ET-PVK devices is 14.18%,which is due to the denser and smoother annealed ET-PVK films composed of larger grains.（2）Aiming at the problems of the green anti-solvent extraction method which requires subsequent solvent annealing to obtain high-quality perovskite films,an in situ solvent-assisted crystallization（ISAC）method was developed to improve the conversion rate and the film quality of MAPbI₃.The results show that the low crystallinity,good wettability and in situ solvent molecules（DMF,DMSO and PbI₂-DMSO intermediates）of ISAC-prepared PbI₂ films promote the phase transformation of PbI₂ to MAPbI₃ perovskite,and the in-situ DMSO molecules and PbI₂-DMSO mesophase are helpful to improve the morphology of perovskite films.When the molar ratio of PbI₂ to DMSO is 1 and the loading time of methamidoiodine（MAI）solution is 60 s,the cell efficiency is the highest.The efficiency of the double-layer mesoporous perovskite solar cell based on the ISAC-MAPbI₃ film reaches 14.23%.（3）In order to expand the optical absorption range of perovskite materials and further improve the photovoltaic performance of devices,a small amount of FAI was introduced into MAI.The mixed cationic perovskite films(ISAC-FA_xMA_1-x-x PbI₃)and their photovoltaic devices were prepared by the ISAC method.The effects of the ISAC method on the nucleation and growth of perovskite in two different material systems of MAPbI₃ and FA_x MA_1-xPbI₃ as well as their underlying mechanism were studied.The results show that the introduction of FA⁺not only reduces the band gap of perovskite materials,broadens the optical absorption range,but also improves the crystallinity and average grain size of the films.For ISAC-MAPbI₃ films,because there are not enough in situ（MA）₂Pb₃I₈（DMSO）₂ intermediates in the films,the role of in situ solvent molecules is mainly to accelerate the transformation of PbI₂ into perovskite phase;for ISAC-MA_x FA_1-xPbI₃ films,the formation and decomposition of in situ（MA）₂Pb₃I₈（DMSO）₂ intermediates in the films during annealing can not only accelerate the formation of perovskite,but also promote the growth of perovskite grains.The average grain size of ISAC-MAPbI₃ film is only 210 nm,while that of ISAC-MA_xFA_1-xPbI₃ film is 450 nm.The photoelectric conversion efficiency of ISAC-MA_xFA_1-xPbI₃ devices is up to17.31%,which is much higher than that of ISAC-MAPbI₃ devices（14.30%）.The main reason is that the quality of ISAC-MA_xFA_1-x-x PbI₃ films is improved（with larger grains,better crystallinity and fewer defects）,rather than the expanded optical absorption range of the films.