Study On Controlling The Defects And Photovoltaic Performance Of CsPbI₂Br Perovskite Films

CsPbI₂Br perovskite is a photoactive material for solar cells with great development potential.At the same time,compared with traditional devices,the carbon-electrode-based perovskite solar cells without a hole transport layer have better long-term stability and lower manufacturing costs.Therefore,carbon-electrode-based CsPbI₂Br perovskite solar cells have received extensive attention in recent years.However,the power conversion efficiency of carbon-electrode-based perovskite solar cells is still a certain distance from that of traditional devices.On the one hand,there are a large number of defects inside the perovskite films and its surface,resulting in serious charge carrier loss;on the other hand,the carbon electrode has no reflection effect on the incident light,and the perovskite film thickness（about 400 nm）in the carbon-electrode-based perovskite solar cells is not enough to achieve saturation absorption of the incident light.In order to further improve the power conversion efficiency of carbon-electrode-based CsPbI₂Br perovskite solar cells,a CsPbI₂Br film with a thickness of about 600nm was prepared by increasing the concentration of perovskite precursor solution and optimizing the anti-solvent process in this thesis.Then,the formation mechanism of stripes microstructures in the perovskite films was investigated,and the preparation process of stripe-free perovskite films was explored.Finally,based on the stripe-free CsPbI₂Br films,the effect of oleic acid surface treatment on the characteristics of perovskite films and solar cells was investigated.The main research questions of this thesis are as follows:（1）The concentration of perovskite precursor solution was increased to 1.0 M,and isopropanol was used as the anti-solvent to investigate the effects of anti-solvent drop-adding time points and drop-dropping dosage on the morphology,crystallinity and photovoltaic properties of CsPbI₂Br films.The results show that the appropriate anti-solvent dropping time and dosage can help to increase the average size of grains in perovskite films and enhance the crystallinity of the films,thereby obtaining perovskite films with fewer defects.The optimal anti-solvent drop-adding process is 150μL of isopropanol dropwise 13 s before the end of spin coating.Under this condition,a CsPbI₂Br film with a thickness of about 600 nm and a carbon-electrode-based CsPbI₂Br perovskite solar cells of power conversion efficiency of 12.52%was achieved.（2）The formation mechanism of stripes microstructures in about 600 nm thick CsPbI₂Br films was explored,and it was found that the formation of this microstructure was related to the I/Br homogenization process of I ion insertion into Br-rich perovskite in the high-temperature annealing stage.By introducing 50°C pre-annealing treatment,the precursor films before high-temperature annealing were changed from the Br-rich phase to the I-rich phase,and a stripe-free CsPbI₂Br film was obtained.In addition,pre-annealing treatment helps to improve the crystallinity of CsPbI₂Br films,reduce defects inside the films,and improve charge carrier transport at the perovskite/carbon electrode interface.Finally,carbon-electrode-based CsPbI₂Br perovskite solar cells with a power conversion efficiency of 13.99%was achieved.（3）Based on the work of the first two parts,by introducing oleic acid treatment into the surface of CsPbI₂Br films,combined with a series of thin film characterization methods,it is proved that the oleic acid molecule contains a carboxyl group which can bind with undercoordinated Pb ions through the interaction on the perovskite films surface to achieve the purpose of reducing surface defects.At the same time,the hydrophobic alkyl chains in oleic acid can enhance the hydrophobicity of the perovskite films surfaces and improve the environmental stability of the perovskite films.Thanks to the defect passivation effect of oleic acid treatment on the surface of perovskite films,the open circuit voltage and filling factor of the perovskite solar cells were significantly improved,and the power conversion efficiency of15.57%was achieved,which is the highest efficiency of carbon-electrode-based CsPbI₂Br perovskite solar cells to date.