The Origin of Non-Radiative Losses in Metal Halide Perovskites

由新型廉价材料制成的太阳能电池和发光二极管(LED)具有成为低成本、清洁和可扩展的解决方案的潜力，以满足我们日益增长的电力和照明需求。太阳能电池将阳光转化为电能，而LED则相反，电能转化为发射的光。金属卤化物钙钛矿材料在这两个方面都是非常有前途的材料。钙钛矿太阳能电池在短短三年内将电力转换效率从3%提高到22%，接近市场领先的技术硅(25%)。早期关于钙钛矿型发光二极管的报道也令人鼓舞，尽管相对来说还没有被探索出来。钙钛矿成分丰富，可以廉价地结合成具有类似硅晶体结构的薄膜。有一天，一卷薄薄、柔软的钙钛矿薄膜可能会从一台特殊的打印机上快速卷绕起来，制造出重量轻、可弯曲、五颜六色的太阳能和发光薄膜。然而，钙钛矿的全部潜力尚未实现。强烈的光发射对于太阳能电池和LED达到其理论效率极限是必不可少的，但发射和性能仍然受到寄生发射损失路径的限制，这些路径仍然知之甚少。薄膜由密集堆积的晶体(颗粒)组成，我们假设每个颗粒具有略有不同的局部化学和结构特性，其中一些是有缺陷的。这项工作的最终目的是通过阐明哪些是最优的化学和结构性质，并利用这些信息来获得最优的薄膜，来确定钙钛矿型薄膜和全器件中这些损失路径的根本来源。这一目标将通过使用一种新型的显微镜系统测量颗粒到颗粒的发射来实现，该系统将允许以高空间分辨率快速成像发射。到目前为止，对钙钛矿的大多数微观发射测量都使用了共焦显微镜，通过对相邻区域的光谱进行连续测量并逐点移动样品，直到覆盖了感兴趣的区域，来绘制发射图。另一方面，成像包括将样品的图像聚焦在探测器上，并测量每个像素在一个特定波长上的光强度，这很像拍照，但只是一个波长。在某些应用中，用于成像的激光功率可能比用于测绘的激光功率高几个数量级，因为功率分布在整个区域而不是单个点，从而允许在类似设备的条件下进行测量。成像还允许更高的分辨率，并将采集时间缩短数量级。在光(光致发光)和施加电偏压(电致发光)两种情况下都将获得发射图像。然后，同一扫描区域的发射图像将与使用电子显微镜技术(包括能量色散X射线(EDX)光谱)和使用钻石同步加速器的纳米X射线衍射(n-X射线)光束线的局部结构测量的局部颗粒到颗粒化学图直接相关。这项工作是非常及时的，结果将为努力使钙钛矿达到其效率极限提供一个平台。这项工作将揭示生长更高性能的钙钛矿膜必须针对的特定首选化学和结构特性，并揭示能够修复钙钛矿材料缺陷的潜在后处理方法。这将引起钙钛矿领域的一系列学术研究人员以及英国牛津光伏等工业实体的浓厚兴趣，牛津光伏目前正在领导这项令人兴奋的技术的商业化努力。最后，该项目将使PI能够建立他的团队，成为拥有尖端方案和工具集的世界领先团队。

Solar cells and light-emitting diodes (LEDs) made from novel, inexpensive materials have the potential to be low-cost, clean and scalable solutions to supply our growing electricity and lighting demands. While solar cells convert sunlight into electrical energy, LEDs are the reverse, with electrical energy transformed into emitted light. Metal halide perovskites are extremely promising materials for both applications. Perovskite solar cells have improved their power conversion efficiency from 3% to 22% in just three years, approaching that of the market-leading technology, silicon (25%). Early reports of perovskite LEDs are also encouraging though relatively unexplored. Perovskite ingredients are abundant and can be combined inexpensively into thin films with a crystalline structure similar to silicon. Rolls of thin, flexible perovskite film could one day be rapidly spooled from a special printer to make lightweight, bendable, and colourful solar and light-emitting sheets.Nevertheless, the full potential of perovskites has not yet been realised. Strong light emission is essential for both solar cells and LEDs to reach their theoretical efficiency limits, but emission and therefore performance is still limited by parasitic emission loss pathways that are still poorly understood. The films are made up of densely packed crystals (grains) and we hypothesise that each grain has slightly different local chemistry and structural properties, some of which are defective. The ultimate aim of this work is to determine the fundamental origin of these loss pathways in perovskite films and full devices by elucidating which are the optimal chemical and structural properties, and using this information to achieve optimal films.This aim will be achieved by measuring the grain-to-grain emission using a novel microscope system which will allow rapid imaging of the emission with high spatial resolution. Most microscopic emission measurements on perovskites to date have employed confocal microscopes in which the emission is mapped by taking sequential measurements of the spectra of adjacent regions and moving the sample point by point until the region of interest has been covered. On the other hand, imaging consists of focusing the image of a sample on a detector and measuring for each pixel the intensity of light at one particular wavelength, much like taking a photograph, but at a single wavelength. In some applications, the power of the laser used in imaging can be orders of magnitude higher than in mapping, since the power is spread over the whole region instead of a single point, thus allowing measurement under device-like conditions. Imaging also permits a higher resolution and reduces the acquisition time by orders of magnitude.Emission images under both light (photoluminescence) and when applying an electrical bias (electroluminescence) will be acquired. The emission images of the same scan area will then be directly correlated with maps of the local grain-to-grain chemistry using electron microscopy techniques including energy-dispersive X-Ray (EDX) spectroscopy and local structural measurements using a nano-X-Ray Diffraction (n-XRD) beamline at the Diamond synchrotron. The work is highly timely and the results will provide a platform for efforts to take perovskites to their efficiency limits. The work will reveal the specific preferred chemistry and structural properties which must be targeted for growth of higher performing perovskite films and also reveal insights into potential post-treatments capable of healing defects in the perovskite materials. This will be of strong interest to a range of academic researchers in the perovskite field as well as industrial entities such as UK-based Oxford PV, which is leading the current commercialisation efforts of this exciting technology. Finally, the project will allow the PI to establish his team as a world-leading group with a cutting-edge programme and toolset.