The Preparation Of CsPbBr₃ Thin Film By Electron Beam Evaporation And Its Application In Solar Cells

Organic-inorganic hybrid Pb halide perovskite materials have excellent optical and electrical properties.As one of its applications,perovskite solar cells（PSCs）have been widely studied by researchers.The photoelectric conversion efficiency（PCE）has been increased to 25.7%in just a decade,which is almost comparable to silicon solar cells.However,the poor wet and thermal stability of the device seriously affects the commercialization progress of the product.Therefore,all-inorganic Pb-based perovskite materials,especially Cs Pb Br₃,have attracted attention due to their excellent thermal and wet stability.However,due to the low solubility of Cs Br in conventional polar organic solution,the uniformity and surface coverage of films prepared by traditional solution method are poor.The evaporation method does not need solvents,and it is easy to achieve the deposition of a large area,dense,uniform film.Therefore,how to vaporize and deposit high-quality Cs Pb Br₃ films has become one of the research hotspots.Double-source or multi-source evaporation method requires accurate control of the evaporation rate of each target material,while sequential deposition requires strict control of the thickness of each layer,which requires high equipment requirements.The single source method uses a crucible to carry out one-step thermal evaporation of the single or mixed powder,which has low requirements for equipment and relatively simple operation,and has strong application potential.In this paper,we used single-source electron beam evaporation to deposit dense Cs Pb Br₃ films,followed by post-treatment to improve the quality of the films and apply them to battery devices.The main contents and conclusions of this paper are as follows:1.The influence of target material composition and post-treatment on film performance:1)High Cs target material:After simple grinding of Cs Br:Pb Br₂=4:1（mole ratio）mixed powder,and then closed heat treatment as the target,the deposited film contains more Cs₄Pb Br₆ hybrid phase content.After Pb Br₂ solution is treated,the content of impurity phase can be significantly reduced.After annealing,with the increase of annealing temperature,the particle size increases gradually,and the surface of the film becomes smooth.The FTO/c-Ti O₂/Cs Pb Br₃/Carbon PSCs with planar structure were constructed.It was found that the devices annealed at 400℃had the best performance,with open circuit voltage（Voc）and PCE of 1.09V and 3.31%,respectively.2)Low Cs target material:When the Cs Br:Pb Br₂=2:1 mixed powder was simply ground and then treated as the evaporating material by closed heat treatment,the condition of Br-rich in the evaporation process was realized,and the formation of Cs Pb₂Br₅ phase was successfully inhibited.The effects of substrate temperature,annealing temperature and time on the properties of thin films were studied.The constructed FTO/c-Ti O₂/Cs Pb Br₃/Carbon PSCs had the best performance under the annealing temperature of 350℃and 10min,that is,the PCE was 7.81%and Voc was1.43 V,and had good stability and repeatability.In addition,the effect of steam on thin films is also studied,and it is found that proper steam treatment can promote the optimal growth of thin films,which is conducive to the improvement of device performance.2.Preparation of MAPb Br₃-Cs Pb Br₃ double absorption layer:For evaporative deposition of Cs Pb Br₃ film,high crystallization quality can be obtained only by annealing at a relatively high temperature,which is not suitable for flexible substrates with low softening temperature.In order to improve the interface contact characteristics and reduce the annealing temperature,MAPb Br₃ film was introduced to construct a double absorption layer.Meanwhile,Cs Pb Br₃film forms a protective layer over MAPb Br₃ film,which can improve the stability of MAPb Br₃.The results show that the absorption properties and surface morphology of the bilayer films do not change significantly after annealing at 150℃.Finally,the PCE and Voc of unannealed PSCs prepared on glass substrates were 6.53%and 1.43V,respectively.