Long-lived hot carriers, coherent spin transport, and the role of surfaces in lead halide perovskites

近年来，高效铅卤化物钙钛矿太阳能电池的发展是第二代光电池领域的重大突破，也是迈向第三代光电池的一步。第一代太阳能电池由晶体硅制成。 它们可作为商业产品使用，寿命长达数十年，效率约为26%。然而，它们的制造需要大量的能源和资金投资，并且它们的最大效率被限制在32%。第二代光致发光材料是以潜在降低的成本、使用对结晶度要求不太严格的较少量的材料来生产的薄膜。机械灵活性开辟了额外的应用领域。由卤化铅钙钛矿制成的薄膜太阳能电池使用丰富的材料，并在低温下加工，以降低其财务和能源成本。在实验室规模上，它们的效率达到了22%左右。商业化生产的研究已经在进行中。此外，最初的研究证明了卤化铅钙钛矿用于第三代光致发光的潜力。在未来一代的光伏电池中，新的概念将被用于实现超过前两代32%的基本限制的效率。卤化铅钙钛矿太阳能电池的大规模应用受到所含有毒铅和长期稳定性问题的限制。因此，该项目遵循一种互补的方法：将研究卤化铅钙钛矿在原子尺度上的独特性质，以获得高效太阳能电池材料的一般设计原则。尽管自2013年以来加强了基础研究，但迄今仍不清楚是什么物理机制使卤化铅钙钛矿太阳能电池如此出色。与合作伙伴一起，我发现了几个有助于提高太阳能电池效率的相关效应：由于Rashba效应，相反自旋的电子通过卤化铅钙钛矿的传播方式不同。自旋是电子的一种性质，在今天的传统器件中还没有被利用。此外，卤化铅钙钛矿在阳光下保持蓝色光谱成分的高能量比硅等传统材料长一千倍。这也可能是Rashba效应的结果。或者，提出了大极化子的形成。极化子是晶体结构的扭曲，它可能会稳定电子能量。在这个项目中，这些影响将使用最先进的时间分辨光谱进行量化。它们对器件性能的影响将通过并行执行的传输测量进行研究。一方面，这一结果可能为卤化铅钙钛矿开辟新的应用领域。另一方面，它们将有助于确定基于类似工作原理的高效太阳能电池的新材料。

The development of high-efficiency lead-halide perovskite solar cells in recent years poses a major breakthrough in the field of second generation photovoltaics and a step towards a third generation. First generation solar cells are made from crystalline silicon. They are available as commercial products with lifetimes lasting decades and efficiencies around 26%. Their fabrication, however, requires a significant investment of energy and capital, and their maximum efficiency is limited to 32%. Second generation photovoltaics are produced as thin films at a potentially reduced cost using smaller amounts of material with less strict requirements to crystallinity. Mechanical flexibility opens¬¬ up additional fields of application. Thin-film solar cells made from lead-halide perovskite use abundant materials and are processed at low temperature to reduce their financial and energetic cost. They have reached efficiencies around 22% at the laboratory scale. Research for commercial production is already underway. Moreover, initial research demonstrated the potential of lead halide perovskites for third generation photovoltaics. In this future generation of photovoltaics, novel concepts will be used to achieve efficiencies beyond the fundamental limit of 32% which applies to the first two generations.Large-scale applications of lead halide perovskite solar cells are limited by the contained toxic lead and problems with long-term stability. This project hence follows a complementary approach: The unique properties of lead halide perovskites on the atomic scale will be investigated to derive general design principles for high-efficiency solar cell materials. Despite intensifying basic research since 2013, it remains unclear to date what physical mechanism make lead halide perovskite solar cells so exceptional. Together with collaboration partners I found several relevant effects that can contribute to a high solar-cell efficiency: Due to the Rashba effect, electrons of opposite spin propagate differently through lead halide perovskites. Spin is a property of electrons that is not yet made use of in todays conventional devices. In addition, lead halide perovskites preserve the high energy of the blue spectral components in sunlight a thousand times longer than more traditional materials like silicon. This may also be a result of the Rashba effect. Alternatively, the formation of large polarons was proposed. Polarons are distortions of the crystal structure which might stabilize the electronic energy.Within this project, these effects will be quantified using state-of-the-art time-resolved spectroscopies. Their influence on device performance will be investigated by transport measurements performed in parallel. On the one hand, the results may open up new fields of application for lead halide perovskites. On the other, they will help to identify new materials for high-efficiency solar cells based on similar working principles.