Nanoscale photophysics at defects and interfaces in organic semiconductors

硅电子学的成功取决于高纯度、近乎完美的硅晶体的生产，这些晶体在微芯片的不同组件之间的边界（或界面）具有良好的控制性能。如今，人们正在寻找具有类似或附加功能的材料，这些材料不仅具有最佳性能，而且还具有柔性、重量轻、易于加工和机械耐用性。这些理想的性质最典型地存在于全部或部分由结合碳-碳键的分子构建的材料中。其中许多基于有机半导体的材料已经在屏幕和传感器中上市。然而，人们对这些材料中缺陷所起的确切作用知之甚少。非常小的缺陷或缺陷会阻止这些材料中的电荷运动，正是这种受控的电荷运动决定了它们的工作效果。精确定位微小缺陷的能力需要电子显微镜的重大进步，电子显微镜是一种用于直接测量材料结构直至单个原子位置的工具。目前，大多数研究依赖于仅描述分子和原子在有机半导体内的平均位置的测量。有时可以推断出，由于存在的无序和缺陷，目标性质如光发射或光吸收更好或更差。但是，只有通过直接观察缺陷，并在适当的长度尺度上测量它们的行为，才能完全理解它们。确定分子和原子在材料中的位置是整个故事的很大一部分，但只是一部分。缺少的部分是对特定缺陷“如何”和“为什么”控制半导体发光或吸收光的效率的直接实验观察。这种与光的相互作用决定了显示屏的工作效果，或者可穿戴式光传感器的寿命。这些“如何”和“为什么”的问题从根本上取决于缺陷产生的能量景观;电子将向下滚动。如果电子遇到“上坡”或能量更高的材料区域，穿过材料的电子可能会遇到不可逾越的障碍。使用电子束的显微镜是一种主要的技术，可以在单个原子的维度上看到景观中发生的“位置”和“事情”。挑战在于有机半导体很容易被电子束破坏。该研究计划将创造创新方法，包括使用机器学习和数据科学技术以及新的显微镜硬件，使用电子束测量有机半导体的能量景观。首先，将在发光二极管（LED）中使用的有机半导体的小“上坡”缺陷处检查光学特性-材料如何吸收或散射光。有了尖端的电子显微镜，现在可以直接看到材料如何吸收或发射可见光能量的光。与这些电子束相互作用相关的物理学意味着这些测量也可以以避免对样品造成无法识别的损坏的方式进行。接下来，将分析分子材料中单个原子的位置。在材料中的缺陷区域，单个原子与其预期位置错位。在这项工作中，原子位于这些错误的位置将被非常精确地测量，并与附近景观的“上坡”或“下坡”进行比较。在最后阶段，新工具将扩展到由许多层组成的完全可操作的设备内部。通过从这些多层器件中切割出横截面，电子显微镜的新见解最终将与制造开发和质量控制平台上使用的工艺相结合。

The success of silicon electronics hinged on the production of high-purity, near-perfect crystals of silicon with well-controlled properties at the boundary (or interface) between different components of a micro-chip. Today, the search is well underway for materials that will serve analogous or additional functions but that achieve top performance while also being flexible, light-weight, easily processed, and mechanically durable. These desirable properties are most typically found in materials built in whole or part from molecules incorporating carbon-carbon bonds. Many of these materials, based on organic semiconductors, are already on the market in screens and sensors. And yet little is understood about the exact role played by imperfections in these materials. Very small imperfections, or defects, stop the movement of charges in these materials, and it is this controlled movement of charge that governs how well they work.The ability to pinpoint tiny imperfections requires substantial advances in electron microscopy, the tool used to directly measure materials structure down to the positions of individual atoms. At the moment, most research relies on measurements that describe only the average position of where molecules and atoms are inside an organic semiconductor. It can sometimes be inferred that targeted properties like light emission or light absorption are better or worse as a result of disorder and defects that are present. But only by seeing defects directly and measuring how they behave at the appropriate length scale can they be understood fully. Identifying where the molecules and atoms are in a material is a large part, but only a part, of the full story. The missing piece is the direct experimental observation of 'how' and 'why' particular defects govern how efficiently a semiconductor emits or absorbs light. This interaction with light then determines how well a display screen works or how long a wearable light-based sensor will last.These 'how' and 'why' questions depend fundamentally on the energy landscape created by defects; electrons will roll downhill. Electrons travelling through a material may run into an insurmountable obstacle if they encounter a region of material that is 'uphill' or at higher energy. Microscopy using electron beams is a mainstay technique for seeing 'where' and 'what' is happening in the landscape at the dimension of single atoms. The challenge is that organic semiconductors are easily damaged by electron beams. This research programme will create innovative approaches, including the use of machine learning and data science techniques as well as new microscope hardware, for using electrons beams to measure the energy landscape of organic semiconductors.First, the optical properties - how a material absorbs or scatters light - will be examined at the small 'uphill' defects in organic semiconductors used in light emitting diodes (LEDs). With cutting edge electron microscopes, it is now possible to directly see how a material absorbs or emits light at visible light energies. The physics associated with these electron beam interactions means these measurements can also be carried out in a way that avoids damaging the sample beyond recognition. Next, the positions of individual atoms in molecular materials will be analysed. At defective regions in a material, individual atoms are misplaced from their expected positions. In this work, where the atoms sit at these mistakes will be measured very precisely and compared with observations about how 'uphill' or 'downhill' the landscape is in the vicinity. In the final stage, the new tools will be extended to look inside fully operational devices consisting of many layers. By cutting out cross-sections from these multi-layered devices, the new insights from electron microscopy, ultimately, will be integrated with processes in use on manufacturing development and quality control platforms.