Nanoscale photophysics at defects and interfaces in organic semiconductors

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.