Study Of Solar Cells Based On Morphology Regulation And Defect Passivation Of CsSnI₃ Perovskite Films

Metal halide perovskite solar cells（PSCs）have attracted much attention in the photovoltaic field due to their excellent power conversion efficiency（PCE）.At present,high-efficiency PSCs use organic-inorganic hybrid lead-based perovskite films as the light-absorbing layer.Considering the volatility of organic components and the strong toxicity of lead（Pb）to human body and environment,the development of new inorganic lead-free perovskite materials has become an important research direction of PSCs.CsSnI₃perovskite is considered as an ideal inorganic lead-free perovskite candidate material due to its similar crystal structure and photoelectric properties to Pb-based perovskite,suitable optical bandgap,and high absorption coefficient.However,CsSnI₃ perovskite films face the challenges of easy oxidation of Sn²⁺,poor morphology quality and high surface defect density of states.Based on the above issues,the influences of the quality of CsSnI₃perovskite light-absorbing film on the photovoltaic efficiency of PSC were explored from the views of film oxidation inhibition,morphology regulation and surface defect passivation in this thesis,aiming to further enhance the PCE and stability of CsSnI₃ PSCs.The main research content and conclusions of this thesis are as follows:（1）The morphology homogeneity and oxidation inhibition of CsSnI₃ perovskite films were synergistically enhanced by the introduction of 2-Aminopyrazine containing pyrazine ring and amino functional group as a SnF₂ co-additive.The pyrazine ring induced the transformation of SnF₂ crystalline phase to amorphous phase through chelation and regulated the dynamic balance of crystallization precipitation between SnF₂ and CsSnI₃,which inhibited the surface segregation of SnF₂ and reduced the surface roughness of the film from 37.2 nm to 18.5 nm.The amino functional group induced a shift in the chemical equilibrium of the defects towards reduction through Lewis acid-base addition reactions,which further enhanced the oxidation inhibition ability of the film and inhibited the generation of tailing defect states induced by Sn vacancies on the basis of SnF₂ reduction,resulting in a sudden decrease of the Sn⁴⁺percentage from 68.2%to 4.7%on the surface of the film.After optimization based on the co-additive strategy,the efficiency of CsSnI₃PSCs using fully-printed Ti O₂/Al₂O₃/NiO/C mesoporous framework was greatly increased from 1.82%to 5.12%.After 100 hours of storage in the air environment,the efficiency only decreased by about 10%.This study has explored a new idea for improving the CsSnI₃ film morphology and device photovoltaic performance.（2）The controllable preparation of CsSnI₃ perovskite films with uniform morphology and high crystallinity was achieved through a dual regulation strategy involving phased two-step annealing and surface cationic coordination.The first stage of 40℃low-temperature annealing can slow down the grain growth rate and the second stage of70℃high-temperature annealing replenished sufficient energy to strengthen the crystallization process,reasonably solving the rate imbalance between crystallization growth and nucleation during film formation.In addition,multiple electron donor functional groups in 1-（4-Carboxyphenyl）-2-thiourea passivated the acceptor-type defects on the surface of the perovskite film,which effectively enhanced the carrier transport and collection efficiency and increased the carrier diffusion distance from 330 nm to 483 nm.Based on the optimization of the dual regulation strategy,the PCE of the mesoporous CsSnI₃ PSC was further increased to 8.03%,and the reverse saturation current density decreased from 8.2×10^-2 m A cm^-2 to 6.1×10^-3 m A cm^-2.This study has established a controllable method for balancing the crystallization process of CsSnI₃ films,which lays a solid foundation for further focusing on solving the problem of high defect density of states on CsSnI₃ film surface.（3）The deep level defect induced by undercoordinated Sn²⁺on the surface of CsSnI₃perovskite films were regulated by introducing 1-Ethyl-3-methylimidazolium acetate ionic liquid,which effectively reduced the loss of defect-assisted nonradiative recombination.The lone pair of electrons of carboxyl functional group andπelectrons of the electron-rich imidazole group in ionic liquid were used to generate strong electrostatic attraction and coordination interactions with the unsaturated dangling bonds exposed by the surface undercoordinated Sn²⁺to enhance the defect formation energy,prompting the deep level defect density of states decreased from 3.16×10¹⁷ cm^-3 to 4.25×10¹⁶ cm^-3 and the defect energy level position moved from 0.674 eV to 0.218 e V,implying a shift towards the shallow defect energy level.The mesoporous structure CsSnI₃ perovskite solar cells prepared by two-step temperature annealing process obtained a PCE of 10.11%,which is the highest efficiency of CsSnI₃ mesoporous solar cells reported so far,and the open circuit voltage loss was reduced by 0.34 V.This study has indicated that the passivation of film surface defects is of great practical importance for achieving performance enhancement of CsSnI₃ PSCs.