Intrinsic Properties of Perovskite/New Materials Affecting Optoelectronic Devices

杂化卤化物钙钛矿自被证明可用于低成本沉积方法生产的高效光伏器件以来，引起了人们极大的关注。其特殊的特性，包括高载流子迁移率，可调的光谱吸收范围，长的扩散长度，以及制备的简单性和可实现性使其成为在光伏、发光二极管、光电探测器、激光器等方面应用最特殊和最具市场竞争力的光电材料之一。尽管杂化钙钛矿具有非凡的特性，但在其工业化规模之前，仍然需要解决几个关键问题，包括毒性、不稳定性和电流-电压曲线的异常滞后。虽然越来越多的研究集中在改善光电性能上，但仍迫切需要在更基础的层面上详细了解这些材料。例如，钙钛矿A位阳离子的性质(有机或无机)和离子大小导致这些材料的性能趋势截然不同。本项目就是在这样的背景下进行的，其主要目标是了解卤化物钙钛矿的内在性质以及它们对器件性能的影响。由于该项目以内在性质为目标，早期的目标将是生长和表征杂化或无机材料的高质量多晶和单晶样品，并了解导致良好材料稳定性的机理。例如，最近在FAPbI3化合物中加入少量的铯和溴，就可以获得更稳定的钙钛矿结构。第二个早期的挑战将是完善单晶的生长(目前仅处于早期阶段)，因为这些样品由于其低陷阱密度和没有晶界，非常适合于研究钙钛矿的内在性质。此外，利用中子和同步加速器源的一些最尖端的技术需要单晶。我们还将探索通过将密度泛函理论(与F.Giustino教授的团队合作)提供的洞察力与合成和表征相结合的方式，开发用于光电子应用的全新杂化或无机材料的可能性。我们将对这些样品进行的实验的主要目标是了解整个相图(温度、组成、压力)上杂化和全无机体系的结构-性质关系，并指导它们的合理设计和光电性能的微调。特别是，对于杂化材料，我们将研究有机和无机物种之间的相互作用，以更好地利用两者的优势。为此，我们将在多晶样品和单晶上部署一整套表征技术，包括：中子和X射线衍射(实验室和同步加速器)，铁电和介电光谱测量，相干和非相干非弹性中子散射，以及X射线光谱(XAS，EXAFS)，所有这些都与广泛用于研究这些材料中的光载流子的经典光谱学技术相结合。直接影响将是为器件架构提供高效的材料，并加深对材料性质和器件之间关系的理解。学术影响将是一些备受瞩目的科学论文。这项研究属于EPSRCS能源应用材料和太阳能技术的研究领域。

Hybrid halide perovskites have attracted huge attention ever since it was shown that they could be used in highly efficient photovoltaic devices produced via low-cost deposition methods. Their exceptional attributes, including high carrier mobility, an adjustable spectral absorption range, long diffusion lengths, and the simplicity and affordability of fabrication make them one of the most exceptional and market-competitive optoelectronic materials for applications in photovoltaic, light emitting diodes, photodetectors, lasers and more.Despite the phenomenal properties of hybrid perovskites, several crucial issues still need to be tackled before their industrial-scale, including toxicity, instability and an anomalous hysteresis in the current-voltage curves. Whereas an ever growing number of studies focus on improving optoelectronic properties, there is still an urgent need for a detailed understanding of these materials at a more basic level. For example, the nature (organic or inorganic) and ionic size of the perovskite A-site cation leads to strikingly different trends in the properties of these materials. This project is framed in this context, and its principle goal consists in understanding the intrinsic properties of halide perovskites and how they affect the devices performances.Since the project targets intrinsic properties, an early goal will be to grow and characterize high-quality polycrystalline and single-crystal samples of hybrid or inorganic materials, and to understand the mechanisms that lead to good materials stability. As an example, very recently the addition of a small percentage of caesium and bromine in the FAPbI3 compound allowed a more stable perovskite structure to be obtained. A second early challenge will be to perfect the growth of single crystals (presently just in the early stages), since these samples are highly desirable to investigate intrinsic properties of perovskites, due to their low trap density and absence of grain boundaries. Moreover, some of the most incisive techniques exploiting neutron and synchrotron sources require single crystals. We will also explore the possibility of growing entirely new hybrid or inorganic materials for optoelectronic applications, by combining the insight provided by Density Functional Theory (in collaboration with Prof. F. Giustino's group) with synthesis and characterisation.The overarching goal of the experiments we will perform on these samples is to understand structure-property relationships in both hybrid and fully inorganic systems over the entire phase diagrams (temperature, composition, pressure), and to guide their rational design and fine tuning of their optoelectronic properties. In particular, for hybrid materials, we will investigate the interaction between organic and inorganic species to better use the advantages of both components. With this intent, we will deploy a panoply of characterisation techniques on both polycrystalline samples and single crystals, including: neutron and x-ray diffraction (lab and synchrotron), ferroelectricity and dielectric spectroscopy measurements, coherent and incoherent inelastic neutron scattering, and X-ray spectroscopy (XAS, EXAFS), all in combination with the classic light spectroscopy techniques that are widely employed to study photo-carriers in these materials.The direct impact will be to deliver efficient materials for the device architecture and to gain a deeper understanding of the relationship between the material properties and the devices. The academic impact will be a number of high profile scientific papers. This research falls within the EPSRCS research areas of Materials for Energy Applications, and Solar Technologies.