Preparation And Photoelectric Properties Of CsPbBr₃ And Cs₂SnI_6-xBr_x Thin Films

High efficiency,long lifetime and low cost of solar cells are pursuing goals of solar photovoltaic industries.The organic/inorganic hybrid perovskite solar cell,a new type of solar cells,has advantages of high efficiency,flexibility and low manufacturing cost,exhibiting great potential in photovoltaic applications.The organic/inorgantic material,however,due to the existence of organic components,also has poor photostability and thermal stability,hindering its industrialization.In contrast,the cesium-based inorganic perovskite possesses a better stability,thus a great value in applications.In the inorganic perovskite system,the film quality has a direct effect on the performance of solar cells.Therefore,it is of great significance to prepare high-quality inorganic perovskite films and perform systematic investions into these film-based solar cell devices.In this thesis,researches on inorganic perovskite systems of Cs Pb Br₃ and Cs₂Sn I_6-xBr_x including thin film fabrications and device optimizations have been carried out.Following innovative results have been obtained:1.The farication and properties of Cs Pb Br₃ films have been studied.New processes of inorganic perovskite solar cells based on Cs Pb Br₃ materials have been developed,which significantly improve the performance of the solar cells.The performance of the device reaches international advanced level and can be kept in operation for more than 4,000 hours.First,a two-step film preparation method for Cs Pb Br₃-based solar cells has been proposed,based on previously reported various preparation processes.This method combines infiltration and spin-coating processes together.The coarse Pb Br₂ substrate has been prepared by an anti-solvent process,in order to promote the adequate reaction in the following two-step preparation process.By adjusting the infiltration time of the Cs Br solution in the Pb Br₂ substrate,the Cs Pb Br₃ film can be obtained,which becomes smooth and dense after spin-coating and annealing processes.The phase transition,Cs B+Pb Br₂?Cs Pb₂Br₅?Cs Pb Br₃?Cs₄Pb Br₆,can be achieved by increasing the contact time between the Cs Br solution and the Pb Br₂ substrate.According to the high temperature stability of Cs Pb Br₃,a high temperature post-treatment process has been proposed,which can effectively optimize the film quality and the device performance.It is shown that with increase in the post-treatment temperature,the grain size of the Cs Pb Br₃ thin film increases,inducing a prefered orientation in lattice and a significant enhancement of photoluminescence.When the film is annealed at 400°C,the efficiency of the FTO/c-Ti O₂/m-Ti O₂/Cs Pb Br₃/carbon structure-based device can reache 6.99%,and the V_OC and J_SC increase to 1.47 V and6.68 m A/cm²,respectively.Furthermore,based on the high-temperature post-treatment process,a photoannealing process has also been proposed to improve the device efficiency.The Cs Pb Br₃ film have been passivated by the photoannealing process during the heat treatment at 400 ^oC.The solar-cell efficiency can increase by 23%to 8.61%when the Cs Pb Br₃ film has been illuminated(1.5AM)at 400 ^oC for 10 min.The other parameters of the champion solar cell are V_OC of 1.55 V,J_SC of 6.97 m A/cm² and FF of 79.27.2.A series of lead-free dual perovskite derivative semiconductor Cs₂Sn I_6-xBr_x(x=0-6)materials with an optical band gap adjustable from 1.26 e V to 2.93e V have been obtained.The Cs₂Sn I₆ films have been prepared and studied.The structural evolution and optoelectronic properties of the Cs₂Sn I_6-xBr_x(x=0-6)materials under the regulation of halogens have been discussed in this thesis.The pure-phase Cs₂Sn I_6-xBr_x(x=0-6)materials have been synthesized by liquid-phase reaction and solid-phase annealing.X-ray diffraction(XRD)and Raman experiments have been used to characterize the structural evolution of the materials under the control of halogens.The tunable optical band gap of the Cs₂Sn I_6-xBr_x(x=0-6)system has been obtained by UV-Vis absorption spectroscopy,valence band spectroscopy experiments and the first-principles calculations.The band gap increases from 1.26 e V to 2.93 e V with the increase in Br concentration,which covers the visible light range.Next,Cs₂Sn I₆ thin films have been successfully prepared by various film preparation processes.It was found that the one-step spin-coating method is not suitable for the preparation of Cs₂Sn I₆ thin films due to the solubility problem.Although dendritic films and wafer-like crystals can be obtained by spin-coating method,the film coverage is poor,which restricts the later device fabrication.The thermal evaporation method can produce flat Cs₂Sn I₆ films with good coverage,and the process parameters can be further adjusted and optimized,which is promising for better devices.In the thesis,a thermal evaporation-assisted two-step method is designed,which allows the preparation of Cs₂Sn I₆ films with a single orientation.The film has high texture,smooth surface and good light absorption in the visible range.