Nanoscale photophysics at defects and interfaces in organic semiconductors

硅电子学的成功取决于生产高纯度、近乎完美的硅晶体，这些晶体在微芯片不同组件之间的边界（或界面）具有良好的控制性能。今天，人们正在寻找一种具有类似或附加功能的材料，这种材料既要达到顶级性能，又要灵活、轻便、易于加工和机械耐用。这些理想的特性通常存在于全部或部分由含有碳-碳键的分子构成的材料中。这些基于有机半导体的材料中，有许多已经在市场上用于屏幕和传感器。然而，人们对这些材料中的缺陷所起的确切作用知之甚少。非常小的缺陷，或缺陷，会阻止这些材料中电荷的运动，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巨大进步，这种工具用于直接测量材料结构，直至单个原子的位置。目前，大多数研究依赖于仅描述有机半导体中分子和原子的平均位置的测量。有时可以推断，由于存在的无序和缺陷，诸如光发射或光吸收等目标特性是更好还是更差。但是只有通过直接观察缺陷并测量它们在适当长度范围内的行为，才能完全理解缺陷。确定分子和原子在物质中的位置是一个很大的部分，但只是整个故事的一部分。缺失的部分是对特定缺陷“如何”和“为什么”控制半导体发射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可穿戴式光传感器的使用寿命。这些“如何”和“为什么”的问题从根本上取决于缺陷造成的能源格局；电子会滚下山。如果电子在材料中遇到“上坡”或能量更高的材料区域，它们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流技术，可以在单个原子的维度上观察景观中“在哪里”和“发生了什么”。挑战在于有机半导体很容易被电子束破坏。这项研究计划将创造创新的方法，包括使用机器学习和数据科学技术以及新的显微镜硬件，使用电子束来测量有机半导体的能量景观。首先，光学特性——材料如何吸收或散射光——将在发光二极管（led）中使用的有机半导体的小“上坡”缺陷上进行检查。有了尖端的电子显微镜，现在可以直接看到一种材料是如何吸收或发射可见光能量的。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一种避免将样品损坏到无法识别的方式进行。接下来，将分析分子材料中单个原子的位置。在材料中有缺陷的区域，单个原子从预期的位置错位。在这项工作中，原子在这些错误处的位置将被非常精确地测量，并与观察到的附近地形的“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。通过切割这些多层器件的横截面，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过程相结合。

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.