Study On Synthesis,processing And Application Of High Quality Perovskite Thin Films

In recent years,the rapid rise of organic-inorganic metal halide perovskite materials has changed the research status of optoelectronics in many aspects.Due to their excellent optoelectronic properties,such as low defect concentration,tunable band gap,high defect tolerance,high carrier mobility,long carrier lifetime and long carrier diffusion distance,perovskite materials are widely used in solar cell,photodetectors,light-emitting diodes and other fields demonstrated excellent performance.Perovskite materials have various morphologies（such as polycrystal,single crystal,quantum dots,nanowires,etc.）,but polycrystalline thin films and single crystal thin films are the most widely used in optoelectronic devices.Polycrystalline thin films are widely used because they can be prepared with large area.Single crystal thin films can reflect the intrinsic structural characteristics and physicochemical properties of the material so that it is an ideal carrier for exploring the power conversion efficiency of perovskite materials.Therefore,from the perspective of preparing high-performance perovskite optoelectronic devices,both perovskite polycrystalline thin films and single crystal thin films are irreplaceable.We focus on processing and synthesizing high-quality perovskite thin films,and take the optimization strategy of perovskite thin films as a means to prepare high-performance perovskite solar cells and photodetectors with high-quality thin films.In order to achieve the purpose of preparing high-performance perovskite optoelectronic devices,we have formulated three technical solutions from three directions（micromachining,doping strategy,and preparation technology）:femtosecond laser polishing technology,lattice strain release strategy,thickness thinning techniques seek to optimize perovskite thin films.The main research contents and conclusions of the paper are as follows:1.Enhancing perovskite solar performance through femtosecond laser polishingUsing the femtosecond laser micromachining system,the influence of different defocus distances（580μm,600μm,630μm,650μm）on the polycrystalline thin films was studied,and the optimal processing parameters were determined.The effect of femtosecond laser on the surface polishing of polycrystalline thin films was finally realized,and an inverted structure perovskite solar cells was fabricated based on the polycrystalline thin films before and after polishing.First of all,through the characterization of the surface roughness of the unpolished and polished polycrystalline thin films,it was found that the roughness of the thin films surface after polishing was significantly reduced,and the smooth thin films surface after polishing would increase the electron extraction and the shunt resistance of the solar cell.Next,by characterizing the optoelectronic properties of the unpolished and polished polycrystalline thin films after secondary crystallization,it was found that both the fluorescence intensity and the fluorescence lifetime of the processed polycrystalline thin films showed a significant increase,which was due to the increase in grain size after secondary crystallization,resulting in reduced non-radiative recombination losses.Femtosecond laser polishing improves the surface morphology of polycrystalline thin films,which is of great significance for the development of high-performance solar cells.The improved morphology increases the solar cell performance parameters,such as short-circuit current,open-circuit voltage,and fill factor,which in turn increases the power conversion efficiency of the solar cell by 1.6%.2.Accurate adjusting the lattice strain of triple-cation and mixed-halide perovskite for high performance photodetectorIn order to fabricate long-term stable and high-performance photodetectors,we synthesized triple-cation and mixed-halide（FAPb I₃）_0.79（MAPb Br₃）_y（Cs Pb I₃）_0.21-y（y=0.19,0.17,0.15,0.13,0.11）perovskite single crystal thin films by spatially confined inverse temperature crystallization method.The effect of different contents of small radius ion Cs⁺on the lattice strain of single crystal thin films was studied.The ion ratio of single crystal thin films with the best optoelectronic properties was determined,and the photodetectors with horizontal structure were prepared based on single crystal thin films with different composition ratios.First of all,through quantitative XRD and qualitative Raman characterization,it was found that the lattice strain of the single crystal thin film gradually decreased with the increase of Cs content,which was caused by the release of the lattice strain in the single crystal thin film due to the addition of small radius ion Cs⁺and the release of lattice strain has a positive effect on the stability of the photodetector.Next,through space charge limited current and dynamic fluorescence characterization,it was found that the single crystal thin film has a low defect density and the longest fluorescence lifetime at y=0.13,which is attributed to the release of lattice strain in single crystal thin films.The high-quality thin films with low defect density are favorable for the fabrication of high-performance photodetectors.The release of lattice strain improves the quality of single crystal thin films and enhances the photodetectors performance.The release of the lattice strain in the single crystal thin film improves the responsivity（40 A/W）,detectivity(1.9?10¹³ Jones)and stability of the photodetector,thereby achieving the goal of long-term stability and high performance of the photodetector.3.Ultrathin perovskite monocrystals boost the solar cell performanceIn order to improve the power conversion efficiency of single crystal solar cells,we synthesized single crystal thin films MAPb I₃ with a controllable thickness ranging from hundred nanometers to several micrometers by using a combination of anti-solvent vapor-assisted and spatial confinement growth strategies.The similarities and differences in the morphology and optoelectronic properties of the single crystal and polycrystalline thin films MAPb I₃were demonstrated,and the inverted structure perovskite solar cells were prepared based on the single crystal and polycrystalline thin films with 300 nm.First of all,through AFM and cross-sectional SEM characterization of the single crystal thin films,it was found that the thickness of the single crystal thin films decreased from 550 nm to 60 nm with the increase of pressure,indicating that the ultrathin single crystal thin films was successfully synthesized.The ultra-thin absorber layer leads to the shortening of the carrier transport time and the reduction of the recombination probability of photogenerated carriers,which improves the carrier collection efficiency.Next,the thin film was characterized by SEM and transient photovoltage,and it was found that the surface of the single crystal thin films has almost no grain boundaries while the polycrystalline thin films has many grain boundaries.The carrier lifetime of the single crystal thin film is longer,which is attributed to almost no grain boundaries.The single crystal thin films suppress the non-radiative recombination losses caused by grain boundaries.The ultrathin single crystal thickness improves the carrier collection efficiency,which is of great significance for improving the power conversion efficiency of single crystal solar cell.The solar cell prepared by using single crystal thin films with 300 nm with higher short-circuit current,open circuit voltage,and fill factor than that of the polycrystalline solar cell,and the power conversion efficiency is 3%higher than that of the polycrystalline solar cell.