Interfaces, Stability and Energy Efficiency: Photochemical Characterisation of Perovskites for Printable Photovoltaics

清洁能源发电是21世纪人类面临的最重要的科技挑战。对气候变化、能源独立和不可再生能源储量枯竭的担忧，正在推动世界各国政府开发和实施替代能源政策和技术。发展低成本、大面积、稳定的光伏发电是满足人类当前日益增长的能源需求的必然要求。目前的光伏(PV)市场由晶体硅组件主导(占目前光伏市场的85%)。尽管主流硅光伏的制造成本最近大幅降低，但光伏技术仍存在明显的机会，这些技术有望显著提高电能转换效率(PCE)或显著降低加工成本(无论是在财务方面还是在具体能源方面)。钙钛矿型太阳能电池在这两个方面都提供了前景。钙钛矿基太阳能电池是一项相对年轻的技术，2009年首次报道，比喻地说，它开启了一种令人兴奋的、新的、高效的固态光伏技术的大门，这种技术可以与需要真空沉积和/或昂贵的非平凡工艺的硅和薄膜技术竞争。然而，稳定性和寿命问题必须被理解和克服，才能使该领域取得进展。在这里，我们的目标是观察可打印的钙钛矿型光伏电池的光化学和稳定性，目的是加深对适合大规模生产的材料的理解。本项目将致力于详细了解控制钙钛矿材料的光致发光(PL)特性的基本过程。钙钛矿薄膜的发光并不像最初想象的那么简单，这突显了这些材料有时令人惊讶的性质，在这里，我们试图解开发光数据，并讨论它可以告诉我们关于这些材料的什么。我们将通过结合光纤光谱仪的荧光显微镜(FM)来研究一系列钙钛矿结构。这使我们能够精确控制测量环境(温度和大气控制)，提供有关整体和局部光致发光(PL)的信息，并使我们能够绘制薄膜表面的地图，并监控暴露在各种受控环境下的光致发光随时间的演变。钙钛矿材料往往对空气/湿气、光线和氧气敏感。分子氧可能是特别有问题的，因为半导体表面的光生电子将氧还原为自由基物种和空穴，这些物种和空穴对器件非常活跃，并可能导致器件的快速退化。我们的目标是将光致发光以及光致发光随时间的变化与光伏器件的整体效率和稳定性相关联。这是一种快速、直接的筛选方法，不需要制造完整的设备来评估和优化钙钛矿性能。这将为了解这些器件的稳定性提供非常必要的信息，并为优化和提高器件稳定性提供一条明确的途径。时间分辨PL提供了关于电荷载流子动力学和提取(在存在电荷选择性接触的情况下)的重要信息，这是性能的关键指标。其目的是通过严格控制吸收体-电荷选择性接触界面的所有组件，并评估通过这种界面发生的电荷转移过程的动力学，实现对电荷载流子寿命和引出的全面了解。然后，可以设计/定制电荷触点，以最大限度地提高导电性，同时最大限度地减少反向电子转移和复合，从而提高器件性能。

The clean generation of energy is the most important scientific and technological challenge that faces humankind in the 21st century. Concerns about climate change, energy independence, and depletion of non-renewable reserves, are pushing governments around the world to develop and implement alternative-energy policies and technologies. The development of low-cost, large area, stable photovoltaics is an absolute must to meet humankinds' current, growing, energy need. The current photovoltaic (PV) market, is dominated by crystalline silicon modules (> 85 % of the present PV market). Despite the recent substantial reductions in the manufacturing cost of mainstream silicon PV, there exists clear opportunities for PV technologies that promise either significant higher power energy conversion efficiencies (PCE) or significantly lower processing costs (both in financial and embodied energy terms). Perovskite-based solar cells, offer prospects on both fronts. Perovskite based solar cells are a relatively young technology, first reported in 2009, and have metaphorically opened the door to an exciting, new, highly efficient solid-state photovoltaic technology which could compete with silicon and thin film technologies that require vacuum deposition and/or expensive non-trivial processing. However, stability and lifetime issues must be understood and overcome to progress the field.Here we aim to look at the photochemistry and stability of printable perovskite photovoltaics with the aim to develop an understanding of materials that are suitable to manufacture at a large scale. This project will focus on developing a detailed understanding of the fundamental processes that govern the photoluminescence (PL) properties of perovskite materials. PL of perovskite thin-films is not as straightforward as initially thought highlighting the sometimes-surprising nature of these materials, here we attempt to unravel the PL data and discuss what this can tell us about these materials. We will study a series of perovskites via fluorescence microscopy (FM) coupled with an optical fibre spectrometer. This allows precise control of the measurement environment (temperature and atmosphere control) and provides information on the bulk and local photoluminescence (PL) and allows us to map the surface of the films and monitor the evolution of photoluminescence with time exposed to various controlled environments. Perovskite materials tend to be sensitive to air/moisture, light and oxygen. Molecular oxygen can be particularly problematic as photo-generated electrons at a semiconductor surface reduce oxygen to radical species and holes which are extremely reactive towards and can result in rapid degradation of devices. We aim to correlate the PL, and the changes in PL with time, with the overall PV device efficiency and stability. This is a rapid and straight forward screening method that does not need the manufacture of a complete device to evaluate and optimise the perovskite properties. This will provide much needed understanding of the stability of these devices and deliver a clear route for the optimisation and improvement of device stability.Time resolved PL provides vital information on the charge carrier kinetics and extraction (in the presence of charge selective contacts) which is a key indicator of performance. The aim is to achieve a global understanding of the charge carrier lifetime and extraction, achieved through a rigorous control of all the components of the absorber-charge selective contact interface, and an evaluation of the kinetics of charge transfer processes occurring through such interfaces. Charge contacts can then be designed/tailored to maximise conductivity, while minimising back electron transfer and recombination and thus improving device performance.