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

近年来，高效卤化铅钙钛矿太阳能电池的发展是第二代光伏领域的重大突破，向第三代迈进了一步。第一代太阳能电池是由晶硅制成的。它们可以作为商业产品使用，寿命可达数十年，效率约为26%。然而，它们的制造需要大量的能源和资本投资，并且它们的最高效率被限制在32%。第二代光伏是以薄膜的形式生产的，成本可能会降低，使用的材料数量更少，对结晶度的要求也不那么严格。机械灵活性开辟了更多的应用领域。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，初步研究证明了卤化铅钙钛矿在第三代光伏发电中的潜力。在未来的这一代光伏中，将使用新的概念来实现超过前两代适用的32%的基本限制的效率。卤化铅钙钛矿型太阳能电池的大规模应用受到有毒铅含量和长期稳定性问题的限制。因此，该项目遵循一种补充方法：将研究原子尺度上卤化铅钙钛矿的独特性质，以得出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，但迄今为止仍不清楚是什么物理机制让卤化铅钙钛矿太阳能电池如此出众。与合作伙伴一起，我发现了几种有助于提高太阳能电池效率的相关效应：由于拉什巴效应，相反自旋的电子在卤化铅钙钛矿中以不同的方式传播。自旋是电子的一种性质，在今天的常规设备中还没有得到利用。此外，卤化铅钙钛矿在阳光下保存蓝色光谱成分的高能量的时间是硅等更传统材料的1000倍。这也可能是拉什巴效应的结果。另外，还提出了形成大极化子的建议。极化子是可能稳定电子能量的晶体结构的扭曲。在这个项目中，这些效应将使用最先进的时间分辨光谱来量化。它们对器件性能的影响将通过并行执行的传输测量来调查。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，它们将有助于根据类似的工作原理确定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.