【作者】 孙浩；

【导师】 陆亚林； 黄秋萍；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2021， 硕士

【摘要】 受益于有机无机杂化钙钛矿(hybrid organic-inorganicperovskites,HOIPs)突出的光电性能和易于制备的特点,近些年来HOIPs已经被广泛地应用于光电和纳米光子器件,并在光电转化效率方面取得了一个又一个的突破。目前,阻碍HOIPs商业化的主要问题是其在正常工作环境下表现的不稳定性。比如,纯的FAPbI3(FAPI)钙钛矿在水分影响下表现出较差的稳定性,黑色的α光电钙钛矿相非常容易转变成非光敏的黄色δ相。这种结构的转变严重降低了 HOIPs的光电性能。为了解决这一问题,添加剂和组分调控被广泛地研究以提高HOIPs的光电性能和环境稳定性。添加剂工程通常将定量的有机物添加到前驱体溶液中,添加的有机物有助于降低样品的缺陷态密度,同时也能改善样品的形貌。有时候具有疏水性的添加剂将自发地沿晶体表面和晶界分布,从而保护钙钛矿免受水分干扰。相比之下,组分调控可以在钙钛矿结构中引入不同的离子,从而改善钙钛矿材料的本质特性,因此在近十年来被广泛学习。为了揭示添加剂和组分工程对HOIPs性能的影响,各种各样的方法已经被用于研究其衰减过程。最大功率点追踪方法被用于表征基于HOIPs的太阳能电池的性能衰减过程。但是,将HOIPs结合到层状太阳能电池时,其相邻层和界面都会对钙钛矿层的稳定性施加额外效果,这对清楚地观察衰减过程是非常不利的。因此其他的方法比如弹性和介电测量、荧光和吸收谱都被用来观察HOIPs的衰减过程。此外,瞬态吸收光谱和光泵浦太赫兹探测谱也用于研究不同钙钛矿结构的载流子动力学变化过程。然而,目前的研究重心更加集中在HOIPs的性能优化上,在衰减周期的载流子动力学和光物理性质变化依然是知之甚少的。在本论文中,我们使用了非接触的光泵浦太赫兹探测光谱观察了 HOIPs的衰减过程。选择了二维PEA2PbI4钝化的和Cs稳定的样品与纯FAPbI3样品进行对比观察,着重研究了衰减周期内载流子动力学和光物理性质的变化。本论文主要分为四章,具体内容如下:第一章是绪论,主要分为两部分。第一部分介绍了太赫兹波的基本特性、主要的太赫兹波产生与探测方法以及目前常用的三种太赫兹光谱,最后简单描述了太赫兹波的几种应用前景。第二部分我们首先介绍了有机无机杂化卤化铅钙钛矿体系,并对其进行了简单的分类。在末尾介绍了论文的选题依据以及工作的出发点。第二章是实验方法和数据处理,主要分为样品制备、表征手段和数据处理。介绍了几种常用的HOIPs合成方法,包括溶剂型生长,胶体型生长,气相与液相外延生长,机械球磨法。在本节最后部分,也简单介绍了几种常用的单晶合成策略。第二部分主要介绍几种样品表征方法,重点集中在光泵浦太赫兹探测光谱技术。在本章末尾,详细介绍了太赫兹谱解析的依据以及具体方法。第三章是光泵浦太赫兹探测光谱在有机无机杂化钙钛矿衰减周期的应用,是文章的核心部分,我们对实验结果进行了总结和诠释。我们发现,FA0.9Cs0.1PbI3薄膜的光电导率和载流子迁移率稳定衰减,而二维钙钛矿PEA2PbI4钝化的FAPbI3薄膜的光电导率和载流子迁移率却显示出一个临界时间点,超过该临界点,其光电导和载流子迁移率将会呈现出一个加速的衰减。此外,根据Drude-Smith模型的拟合结果,我们还发现载流子的局域化随着相变过程的进行而增加,结合前人的文献报告,当引入较小含量的PEA分子时,PEA分子将在晶界和表面形成二维PEA2PbI4并抑制水在晶界上的聚集以及离子迁移。将Cs掺杂进入钙钛矿结构中有利于优化其结构的Goldschmidt容忍因子并使其能量上不利于α→δ相变。这些结果表明,Cs掺入的稳定作用是连续的,而PEA2PbI4的钝化作用将会被新出现的相界削弱。该报告从一个新的角度阐明了HOIPs的降解过程。第四章总结了论文的主要内容,提出了课题研究的创新之处。并对通过HOIPs实现太赫兹超快调控方向进行了展望。更多还原

【Abstract】 Benefited from the outstanding optoelectronic properties and easy fabrication techniques,hybrid organic-inorganic lead halide perovskites(HOIPs)have been widely studied in photovoltaic and nano-photonic applications,with the power conversion efficiency(PCE)of perovskite solar cells reaching a phenomenal advancement.One primary problem which hinders the commercialization of HOIPs is their environmental instability.For example,it has been reported that pure FAPbI3(FAPI)perovskite exhibited poor stability after exposure to humidity,and the black α phase readily turned into the non-photoactive yellow δ phase.This unpleasant transformation severely hinders their further applications.In order to improve the stability of HOIPs,additive and compositional engineering has been actively explored.Additive engineering usually introduces a controlled amount of organics into the precursor.The added component can contribute to the reduction of trap-state density and improvement of morphology,and sometimes the hydrophobic additives are prone to distribute along grain boundaries and surfaces,which can protect the perovskite from moisture.By contrast,compositional engineering incorporates different ions into the structure and thus provides more freedom to improve the intrinsic properties of perovskite materials.Hence great strides developing the compositional engineering have been widely made in recent years.To reveal the effect of additive and compositional engineering on the performance of HOIPs,various methods have been employed.The maximum power point tracking method is frequently adopted to demonstrate the degradation process of solar cells based on HOIPs.However,when a perovskite film is incorporated in a layered solar cell,it has been shown that the adjacent layers and their interfaces also have an effect on the perovskite stability.It’s unfriendly for us to observe the degradation process clearly.Therefore,some other methods,including the anelastic and dielectric measurements,photoluminescence and absorbance spectra,are also introduced to observe the degradation process.Moreover,the ultrafast transient absorption spectrum and optical pump THz probe spectroscopy were performed to provide information on carrier dynamics of HOIPs.However,in a large number of studies,more attention is focused on the optimization of optoelectronic performance by additive and compositional engineering.Little understanding has been established on the charge carrier dynamics and photo-physical changes in the natural degradation period.In this report,the time-resolved spectroscopy,that is,optical pump THz probe spectroscopy,was employed to observe the long term degradation process of HOIPs.we compared the evolution of 2D PEA2PbI4 passivated(FAPEAPI)and Cs stabilized(FACsPI)films with pure FAPI perovskite film,and focused on the charge carrier dynamics and photo-physical changes in the natural degradation period.This thesis is mainly divided into four chapters,the specific content is as follows.The first chapter is the introduction,which is mainly divided into two parts.The first part introduces the essential properties of terahertz,the generation and detection methods and three commonly used terahertz spectra systems.Meanwhile,we also briefly describe several application prospects of terahertz.In the second part,organic-inorganic hybrid perovskite system is briefly introduced and we make a simple classification.In the end,the basis and foothold of our work are introduced.The second chapter is about experimental methods and data processing,which is mainly divided into sample preparation,characterization methods and data processing.Several commonly used HOIPs synthesis methods are introduced,including solution-based growth,colloidal growth,vapor and liquid-phase epitaxial growth and mechanical ball milling.Besides,several commonly used single crystal synthesis strategies are also briefly introduced.The next section mainly introduces several characterization techniques,focusing on the optical pump terahertz probe spectra.At the end of this chapter,the basis and specific methods for terahertz spectrum analysis are demonstrated in detail.The third chapter is the application of optical pump terahertz probe spectra in the decay period of HOIPs.It is the core part of the article.We find that photoconductivity and carrier mobility of the FA0.9Cs0.1PbI3 film decay steadily,while these of the 2D phenylethylammonium lead iodide(PEA2PbI4)passivated FAPbI3 film show an accelerated decay rate.Moreover,it’s also revealed that the localization of charge carriers increases with the transformation process from the fitted results by the Drude-Smith model.It has been pointed out that at small to moderate concentrations,the 2D PEA2PbI4 perovskite spontaneously forms at grain boundaries and surfaces to passivate the ingression pathway of moisture and suppress ion migration.Meanwhile,the incorporation of Cs optimizes the Goldschmidt tolerance factor,which favors the formation of a perovskite phase thermodynamically.What’s more,it’s energetically unfavorable for the unpleasant transformation reaction for the FACsPI film.These results indicate that the stabilized effect of Cs incorporation is continuous,while the passivated effect of PEA2PbI4 can be weakened by the newly emerged phase boundaries.This report proposes a new perspective and sheds light on the degradation process of HOIPs.The fourth chapter is the summary of this thesis,we raise the innovation points of the subject research,and prospects for the realization of terahertz ultrafast regulation by HOIPs are given.更多还原