【作者】 陈红；

【导师】 张材荣；

【作者基本信息】 兰州理工大学 ， 凝聚态物理， 2021， 硕士

【摘要】 太阳能电池被认为是解决未来能源危机的重要方向之一,因此,研究低成本高效率的太阳能电池对商业化生产至关重要。目前研究发现钙钛矿结构的卤化物具有优越的光电性质,但是传统的钙钛矿太阳能电池仍然具有环境中的不稳定性以及铅的毒性等缺点。无机无铅卤化物双钙钛矿是一大类具有A₂B⁺B³⁺X ₆通式的化合物。双钙钛矿材料A₂B⁺B³⁺X ₆与钙钛矿材料ABX₃的晶体结构类似,这些双钙钛矿具有良好的光电性质,包括载流子寿命较长,对空气和湿气的良好稳定性和载流子有效质量较低等特点,以Cs₂AgBiBr ₆为代表的无铅双钙钛矿材料在太阳能光吸收层的应用具有巨大的潜力。在半导体光电器件的生产中,往往会引入缺陷。缺陷可以作为调节光电材料光电性能的一种合适方法。近年来,双钙钛矿Cs₂AgBiBr₆因其无毒且增强了化学和热力学稳定性而作为新型钙钛矿的光电材料引起了大家的广泛关注。本文基于密度泛函理论的计算方法,研究了空位缺陷和杂质缺陷对于无机双钙钛矿Cs₂AgBiBr₆光电性质的影响,分别探讨了在这两种不同缺陷下对双钙钛矿Cs₂AgBiBr₆材料的结构、电子结构、光学性质、激子结合能和载流子迁移率等性质的影响。本论文的主要工作如下:（1）为了理解空位缺陷对双钙钛矿Cs₂AgBiBr₆光电性能的影响,基于密度泛函理论和超胞模型,系统地研究了Cs,Ag,Bi和Br的单原子空位,Cs-Br和Ag-Br原子对空位对双钙钛矿Cs₂AgBiBr₆晶体结构,电子结构,光吸收,载流子和激子结合能的影响。结果发现双钙钛矿Cs₂AgBiBr₆中的空位缺陷引起晶格的畸变较小;空位缺陷不能在带隙中产生额外的缺陷态,Cs,Ag,Bi,Ag-Br和Cs-Br缺陷将带隙从未掺杂体系的间接带隙变为了直接带隙,并减小了带隙值;引入Cs,Ag,Bi,Ag-Br和Cs-Br空位缺陷增强了光吸收能力,并导致低能区吸收光谱的红移;就电子和空穴有效质量而言,引入Cs,Ag,Bi,Ag-Br和Cs-Br空位缺陷会产生不平衡的电荷传输性质,并增加激子结合能。这项工作的结果为我们了解空位缺陷如何影响双钙钛矿Cs₂AgBiBr₆的光电性能提供新的理论视角。（2）掺杂是解决无机双钙钛矿Cs₂AgBiBr₆太阳能电池结构稳定性和提高光电转换效率的相对有效的方法。基于超级晶胞模型,研究了Rb取代双钙钛矿Cs₂AgBiBr₆超胞中一个Ag和Cs原子,掺杂浓度为6.25%。容差因子（tolerance factors）和八面体因子（octahedral factors）表明掺Rb的钙钛矿体系是稳定的;杂质形成能（取代Ag和Cs的能量分别为-5.37和-3.00 eV）表明引入Rb掺杂是放热过程;掺Rb将双钙钛矿Cs₂AgBiBr₆的间接带隙（2.25 eV）变为直接带隙（取代Ag和Cs体系的带隙值分别为2.27和2.19 eV）,并且在带隙中没有引入杂质态;掺杂还拓展了可见光的吸收范围,减小（增加）了电子（空穴）的有效质量,并增大了弹性常数。（3）CH（NH₂）₂⁺（FA）掺杂钙钛矿太阳能电池材料Cs Pb I₃的光电性质对有机阳离子的取向具有很强的敏感性,有机-无机杂化双钙钛矿（FA）₂Bi Cu I₆材料具有优异的光学和电学性能,在光伏和光电器件应用方面有很好的前景。为理解FA掺杂对双钙钛矿Cs₂AgBiBr₆光电性质的影响,本文研究了有机阳离子FA在五种不同的取向下取代超级晶胞中的一个Cs原子,其对应的原子坐标为（0.5,0.5,0.5）,探究并详细分析了FA杂质的引入对双钙钛矿Cs₂AgBiBr₆的几何结构、电子结构、光学性质、激子解离和载流子迁移率的影响。几何结构结果表明FA掺杂后使双钙钛矿Cs_1.9375FA_0.0625AgBiBr₆的超胞体积增大,分别增加了0.41%,0.50%,0.50%,0.39%和0.34%;电子结构表明FA掺杂后带隙为间接带隙,带隙值增大(Cs₂AgBiBr₆的带隙值为2.25eV,Cs_1.9375FA_0.0625AgBiBr₆的带隙值分别为2.39,2.38,2.40,2.41和2.37eV),带隙中没有杂质态;光吸收谱图表明在0-3eV和3.9-4.6eV能量范围内表现出比纯相双钙钛矿Cs₂AgBiBr₆更好的光吸收性能;FA的引入使电子有效质量增大,空穴有效质量减少,从而导致了空穴载流子迁移率比电子载流子迁移率大。希望本研究能为开发稳定的有机-无机杂化卤化物双钙钛矿提供理论依据。更多还原

【Abstract】 Solar cells are considered to be one of the important directions to solve the future energy crisis.Therefore,research on low-cost and high-efficiency solar cells is essential for commercial production.Current studies have found that the perovskite structure halide has superior photoelectric properties,but traditional perovskite solar cells still have disadvantages such as environmental instability and lead toxicity.Inorganic lead-free halide double perovskite is a large class of compounds with the general formula A₂B⁺B³⁺X₆.The crystal structure of the double perovskite material A₂B⁺B³⁺X₆ is similar to that of the perovskite material ABX ₃.These double perovskite materials have good photoelectric properties,including long carrier life,good stability to air and moisture,and with the characteristics of low effective carrier mass,lead-free double perovskite materials represented by Cs₂AgBiBr₆ have great potential in the application of solar light absorption layers.In the production of semiconductor optoelectronic devices,defects are often introduced.Defects can be used as a suitable method to adjust the optoelectronic properties of optoelectronic materials.In recent years,double perovskite Cs₂AgBiBr₆ has attracted widespread attention as a new type of perovskite photoelectric material because of its non-toxicity and enhanced chemical and thermodynamic stability.In this paper,based on the calculation method of density functional theory,the influence of vacancy defects and impurity defects on the photoelectric properties of inorganic double perovskite Cs₂AgBiBr₆ is studied,the effects of these two different defects on the structure,electronic structure,optical properties,exciton binding energy and carrier mobility of the double perovskite Cs₂AgBiBr₆ material are discussed separately.The main work of this paper is as follows:（1）In order to understand the effect of vacancy defects on the optical and electrical properties of double perovskite Cs₂AgBiBr₆,based on density functional theory and supercell model,the single-atom vacancies of Cs,Ag,Bi and Br,and Cs-Br and Ag-Br atom pairs were systematically studied.The effect of vacancy on double perovskite Cs₂AgBiBr₆ crystal structure,electronic structure,light absorption,binding energy of carriers and excitons.It is found that the vacancy defects in the double perovskite Cs₂AgBiBr₆ cause less distortion of the crystal lattice;the vacancy defects cannot generate additional defect states in the band gap,and the Cs,Ag,Bi,Ag-Br and Cs-Br defects will change the band gap from The indirect band gap of the undoped system becomes the direct band gap,and the band gap value is reduced;the introduction of Cs,Ag,Bi,Ag-Br and Cs-Br vacancy defects enhances the light absorption capacity and causes the red shift of the absorption spectrum in the low energy region;in terms of the effective mass of electrons and holes,the introduction of Cs,Ag,Bi,Ag-Br and Cs-Br vacancy defects will produce unbalanced charge transport properties and increase exciton binding energy.The results of this work provide us with a new theoretical perspective on how vacancy defects affect the photoelectric properties of double perovskite Cs₂AgBiBr₆.（2）Doping is a relatively effective method to solve the structural stability of the inorganic double perovskite Cs ₂AgBiBr₆ solar cell and improve the photoelectric conversion efficiency.Based on the supercell model,the substitution of Rb for one of the Ag and Cs atoms in the double perovskite Cs₂AgBiBr₆supercell was studied,and the doping concentration was 6.25%.Tolerance factors and octahedral factors indicate that the Rb-doped perovskite system is stable;the impurity formation energy（energy to replace Ag and Cs is-5.37 and-3.00 eV）indicates the introduction of Rb doping Impurity is an exothermic process;Rb doping changes the indirect band gap（2.25 eV）of the double perovskite Cs₂AgBiBr₆ into a direct band gap（the band gap values of the substituted Ag and Cs systems are 2.27 and2.19 eV）,and in the band gap no impurity state is introduced;doping also expands the absorption range of visible light,reduces（increases）the effective mass of electrons（holes）,and increases the elastic constant.（3）The photoelectric properties of CH（NH₂）₂⁺（FA）doped perovskite solar cell material Cs Pb I₃ are highly sensitive to the orientation of organic cations.The organic-inorganic hybrid double perovskite（FA）₂Bi Cu I₆material has excellent optical and electrical properties.There are good prospects in the application of photovoltaic and optoelectronic devices.In order to understand the effect of FA doping on the photoelectric properties of double perovskite Cs₂AgBiBr₆,this paper studied the organic cation FA in five different molecular orientations to replace a Cs atom in the super unit cell,the corresponding atomic coordinates is（0.5,0.5,0.5）.The influence of the introduction of FA impurities on the geometric structure,electronic structure,optical properties,exciton dissociation and carrier mobility of the double perovskite Cs₂AgBiBr₆ was explored and analyzed in detail.The geometric structure results show that FA doping increases the supercell volume of the double perovskite Cs_1.9375FA_0.0625AgBiBr₆by 0.41%,0.50%,0.50%,0.39%and 0.34%;the electronic structure shows that the band gap after FA doping is an indirect band gap,and the band gap value increases(the band gap value of Cs₂AgBiBr₆ is 2.25eV,and the band gap value of Cs_1.9375FA_0.0625AgBiBr₆ are 2.39,2.38,2.40,2.41 and 2.37eV),there is no impurity state in the band gap;the light absorption spectrum shows that in the energy range of 0-3eV and 3.9-4.6eV,it exhibits better light absorption performance than the pure phase double perovskite Cs₂AgBiBr₆;the introduction of FA increases the effective mass of electrons and reduces the effective mass of holes.As a result,the hole carrier mobility is greater than the electron carrier mobility.It is hoped that this research can provide a theoretical basis for the development of stable organic-inorganic hybrid halide double perovskite.更多还原