Studies Of Quality Optimization And Stability Of Optoelectronic Thin Films And Devices Of Organic-Inorganic Hybrid Perovskite

Three-dimensional(3D)organic-inorganic halide perovskites of CH₃NH₃PbX₃(X=I,Br,Cl or their mixture)have emerged as among the most promising light absorbers for solar cell fabrication.Their power conversion efficiency(PCE)has rapidly improved from 3.8%to 25.2%within ten years.Though extraordinary progress has been made in perovskite solar cells,lead-halide cells still experience moisture instability,which hinders their further commercialization.Recently,two-dimensional(2D)hybrid perovskites have come into focus.It presents outstanding stability under ambient conditions,but the device efficiency based on 2D perovskite is smaller than that of the 3D one.Thus we should develop simple and effective methods to improve the quality of the perovskite films and we need to deeply understand the stability mechanism of perovskite materials.These behaviors are always the important and hot points for the research on perovskite.In this dissertation,we focused on the 3D perovskite and 2D perovskite to improve the quality of thin films by adding appropriate additives,resulting in increasing the efficiency of perovskite solar cells,and decreasing its J-V hysteresis.At the same time,some common characterization methods and optical measurements are utilized to investigate the processes on thermal stability and photostability.The results are divided into four parts,as shown below.(1)Thermal stability:We systematically measured the thermal stability of 3D perovskite films of CH₃NH₃Pb I_3-xCl_x on substrates of ITO,ITO/PEDOT:PSS,ITO/Ti O₂,at various temperatures.Comparing their counterparts on Glass,Glass/PEDOT:PSS,Glass/Ti O₂.At the same time,we also measured the thermal stability of quasi 2D perovskite films of PEA₂MA₂Pb₃I₁₀ and 2D perovskite films of PEA₂Pb I₄on different substrates of ITO and Glass.We found that the films on substrates with ITO are always less thermal stable,resulting from charge transfer between perovskite and ITO substrates,in particular at higher annealing temperatures.Therefore,it was believed that more thermal degradation can occur in perovskite film with less neutral electrical environment(ITO).It can also provide a good idea for further research on thermal stability of perovskite.(2)Improving the efficiency of 3D perovskite solar cells:The additive of NH₄Cl will have important effect on crystallization process and optical properties.With adding optiumu NH₄Cl,the perovskite films prepared by one-step method present better morphology than films without adding NH4Cl,namely,smoother surface and better coverage which result in uniform and much more stable photoluminescence intensities,longer lifetime of photoexcitations as well as less traps.More importantly,the photovoltaic cells fabricated with the addition of the NH₄Cl have far better performance with good reproducibility than the cells without additives.Therefore,we can use this one-step method to fabricate high quality perovskite films.And this method can supply an effective reference to volume-produce high efficient perovskite solar cells in the future.(3)Improving the efficiency of quasi 2D perovskite solar cells:We present a one-step solution process to fabricate the quasi 2D perovskite(PEA)₂(CH₃NH₃)₂Pb₃I₁₀ as light absorbers in solar cells by adding methylammonium acetate(MAAc)in a precursor solution.With an appropriate molar ratio of MAAc,the 2D perovskite films show a smoother surface,higher coverage,and better crystallinity,resulting in an average PCE of 5.3%and an optimal PCE of5.7%,and importantly,no J-V hysteresis.So MAAc is an effective additive to acquire high quality of quasi 2D perovskite film.And this method is very simple to further improve the efficiency of quasi 2D perovskite solar cells.(4)Photostability:We investigated the light-induced photoluminescence quenching and degradation of quasi 2D perovskite of PEA₂MA₂Pb₃I₁₀(PEA=C₆H₅(CH₂)₂NH₃,MA=CH₃NH₃).Light induced PL quenching generally happened with different speed,depending on the wavelength and intensity of laser as well as sample environment.With red light(635 nm)illumination,film decomposition did not happen even in ambient air with intensity below 500m W/cm²,although in general,higher laser intensity and/or higher photon energy(447 nm)could make the decomposition process easier and faster.On the other hand,when the sample was in vacuum,both light induced PL quenching and degradation were significantly suppressed.Furthermore,we found that multiphase of n=1,2,3 in PEA₂MA₂Pb₃I₁₀ film decomposed together,and degradation process may begin with the collapses of crystalline structures.Our work suggested effective encapsulation,appropriate long pass filter,as well as integrity of crystals could be helpful to increase the photostability of quasi 2D perovskite films and to prolong the life time of related devices.