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Probing Local Heterogeneity in the Optoelectronic Properties of Organic-Inorganic Perovskites Using Fluorescence Microscopy

机译：使用荧光显微镜探测有机无机钙钛矿光电特性中的局部异质性

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摘要

Unregulated emission of carbon dioxide and greenhouse gases into our atmosphere has led to an increase in the average global surface air temperature, to a disruption of weather patterns, and to the acidification of oceans all of which threaten the continued prosperity of our race and our planet. The transition to renewable sources of energy is therefore one of, if not the most, important challenge that the 21st century faces. Solar energy is predicted to play a major role in global energy production in the coming century, as the amount of energy hitting the earth's surface is far greater than the energy demands of industrialized human activity. Many current photovoltaic technologies show promise in contributing to a large fraction of global energy production, but in order to reach terawatt-scale production the photovoltaic modules will need to be scalable, cheap, and efficient. Perovskite-based photovoltaics hold exceptional potential in contributing to solar energy production. Thus far, the unprecedented rise in power conversion efficiencies over the past few years can be primarily attributed to improvements in film processing and device engineering. Although effective, the fundamental photophysical processes that govern charge generation, transport, recombination, and collection in these materials is still in its infancy. Historically in semiconductor technologies, this understanding has been essential in the rational design of optimized materials.;Prior to these studies, much of the field had focused on bulk spectroscopic measurements to characterize the semiconducting properties of hybrid perovskite thin films. From our contributions as well as many others, microscopy has now given us a window into how this bulk behavior is composed of an ensemble of spatially varying structure and composition, which controls carrier transport and dynamics on the way to carrier extraction and power generation. This understanding has led to some exciting new discoveries on the rational design of materials and is leveraged to deploy chemical passivation techniques to improve the optoelectronic quality of the material, with the ultimate goal of improving photovoltaic power conversion efficiency.;Reducing non-radiative recombination in semiconducting materials is a prerequisite for achieving the highest performance in a host of light-emitting and photovoltaic applications. In the first study described herein, we used confocal fluorescence microscopy correlated with scanning electron microscopy to spatially resolve the photoluminescence (PL) decay dynamics from films of nonstoichiometric organic-inorganic perovskites, CH3NH 3PbI3(Cl). The PL intensities and lifetimes varied between different grains in the same film, even for films that exhibited long bulk lifetimes. The grain boundaries were dimmer and exhibited faster non-radiative decay. Energy-dispersive x-ray spectroscopy showed a positive correlation between chlorine concentration and regions of brighter PL, while PL imaging revealed that chemical treatment with pyridine could activate previously dark grains.;Next, to better elucidate the sources of these loss pathways, we performed a systematic study using confocal and widefield fluorescence microscopy to deconvolve the contributions from diffusion and non-radiative recombination which lead to the observed image heterogeneity. We showed that, in addition to local variations in non-radiative loss, carriers diffuse anisotropically due to heterogeneous intergrain connectivity. In addition to non-radiative recombination impeding material performance, we also showed that the materials exhibit a range of complex dynamic phenomena under illumination. We used a unique combination of confocal PL microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH 3PbI3 films under illumination. We demonstrated that the photo-induced "brightening" of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density.;Next, we studied the effects of a series of post-deposition ligand treatments on the PL of polycrystalline methylammonium lead triiodide perovskite thin films. Using glow discharge optical emission spectroscopy (GDOES) and nuclear magnetic resonance (NMR) spectroscopy, we showed that the ligands are incorporated primarily at the film surface and are acting as electron donors. These results indicate it is possible to obtain thin film PL lifetime and PLQE values that are comparable to those from single crystals by control over surface chemistry.;Finally, we further characterized these TOPO treated films to show, with respect to material bandgap, these passivated films could demonstrate quasi-Fermi level splittings comparable to the highest performing GaAs solar cells, reaching 96% of the Shockley-Queisser limit. Importantly, we reported internal photoluminescence quantum efficiency values of 92% under one sun illumination intensity, which are the highest values achieved to date. These results suggest that the material optoelectronic quality has been nearly optimized and further increases in voltage and device efficiency will be obtained by integrating these types of surface passivation schemes into charge carrier selective interfaces. (Abstract shortened by ProQuest.).
机译：二氧化碳和温室气体向大气的无节制排放已导致全球平均地表气温升高，天气模式中断以及海洋酸化，所有这些威胁着我们的种族和地球的持续繁荣。因此，向可再生能源的过渡是21世纪面临的，即使不是最重大的挑战之一。预计到下个世纪，太阳能将在全球能源生产中发挥重要作用，因为撞击地球表面的能量远大于工业化人类活动对能源的需求。当前许多光伏技术都显示出有望在全球能源生产中占很大比重的希望，但是为了达到兆瓦规模的生产，光伏模块将需要可扩展，廉价和高效。钙钛矿型光伏电池在促进太阳能生产方面具有非凡的潜力。到目前为止，过去几年功率转换效率的空前提高主要归因于胶片处理和设备工程方面的改进。尽管有效，但控制这些材料中电荷的产生，传输，重组和收集的基本光物理过程仍处于起步阶段。从历史上看，在半导体技术中，这种理解对于优化材料的合理设计是必不可少的。在这些研究之前，许多领域都集中于体光谱测量以表征杂化钙钛矿薄膜的半导体性能。从我们以及其他许多人的贡献中，显微镜现在为我们提供了一个窗口，让我们了解这种整体行为是如何由空间变化的结构和组成的整体组成的，该结构在载流子提取和发电的过程中控制载流子的运输和动力学。这种理解在材料的合理设计方面带来了一些令人振奋的新发现，并被用于部署化学钝化技术以提高材料的光电质量，最终目的是提高光伏功率转换效率。半导体材料是在众多发光和光伏应用中实现最高性能的先决条件。在本文所述的第一个研究中，我们使用了与扫描电子显微镜相关的共聚焦荧光显微镜技术，从空间上解析了非化学计量有机无机钙钛矿，CH3NH 3PbI3（Cl）薄膜的光致发光（PL）衰减动力学。即使在显示出长寿命的薄膜中，同一薄膜中不同晶粒之间的PL强度和寿命也不同。晶界较暗并且表现出更快的非辐射衰减。能量色散X射线光谱显示氯浓度与较亮PL的区域呈正相关，而PL成像显示用吡啶进行化学处理可以激活先前的暗颗粒;接下来，为了更好地阐明这些损失途径的来源，我们进行了研究使用共焦和宽场荧光显微镜对卷积的扩散和非辐射重组的贡献进行反卷积的系统研究，导致观察到的图像异质性。我们表明，除了非辐射损耗的局部变化之外，由于异质颗粒间的连通性，载流子也各向异性地扩散。除了阻碍材料的非辐射重组外，我们还表明，材料在光照下会表现出一系列复杂的动态现象。我们使用了共焦PL显微镜和化学成像的独特组合，将光物理的局部变化与CH3NH 3PbI3膜在光照下的成分相关联。我们证明了钙钛矿PL的光诱导“增亮”可以归因于陷阱态密度的数量级降低。；接下来，我们研究了一系列沉积后配体处理对PL的PL的影响。多晶甲基铵三碘化铅钙钛矿薄膜。使用辉光放电发射光谱（GDOES）和核磁共振（NMR）光谱，我们表明配体主要掺入薄膜表面并充当电子给体。这些结果表明，通过控制表面化学性质，有可能获得与单晶相当的薄膜PL寿命和PLQE值。最后，我们进一步表征了这些TOPO处理过的薄膜，以显示它们在材料带隙方面的钝化效果。薄膜可能显示出与最高性能的砷化镓太阳能电池相当的准费米能级分裂，达到了肖克利-奎塞尔极限的96％。重要的是，我们报道了一种太阳光照强度下内部光致发光量子效率值为92％，这是迄今为止达到的最高值。这些结果表明，通过将这些类型的表面钝化方案集成到电荷载流子选择界面中，材料的光电质量已接近最佳化，电压和器件效率将进一步提高。 （摘要由ProQuest缩短。）。

著录项

  • 作者

    De Quilettes, Dane W.;

  • 作者单位

    University of Washington.;

  • 授予单位 University of Washington.;
  • 学科 Chemistry.;Physical chemistry.;Materials science.
  • 学位 Ph.D.
  • 年度 2017
  • 页码 236 p.
  • 总页数 236
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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