Quantifying and Improving Structure-Function Relationships of All-Small-Molecule Organic-Solar-Cells

Solar cells are an effective way to reduce greenhouse gas emissions from the generation of electricity. Apart from contributing to the major societal challenge that climate change poses, organic solar cells (OSCs) have many exciting new applications resulting from their remarkable physical properties that sets them apart from other solar cell technologies. Their mechanical flexibility allows the integration in wearable textiles and electronic appliances, lightweight and semitransparent designs allow the deployment and retrofitting as facades for greenhouses, and low costs combined with efficient indoor operation makes OSCs feasible to supply low-power sensors for the internet of things (IoT). Overall, OSCs offer a cost-effective, scalable, and environmentally friendly way of generating renewable energy. Wide commercial success of OSCs requires further improvements in efficiency, and a stronger focus in research on industrially relevant technologies. The proposed research will identify and improve critical physical processes in OSCs. The applied materials are highly relevant to industrial production. I thereby pursue pathways to break today's limits in power conversion efficiency (PCE) and seek to push the commercialization of the technology. To identify routes towards real-world economic impact, it is worth looking at the precedent established by organic light emitting diodes (OLEDs). The commercial success of OLEDs was stimulated by so-called 'small molecules' that offer reproducible synthesis and purification, as well as longterm device stability over several years. Similarly, small molecules (SMs) rather than polymers are the most likely material choice for upscaled industrial OSC production. In terms of device function, OSCs apply an intimately mixed blend of two molecular species to generate electrical power from incoming light. The complex influence on the efficiency by the structural arrangement of molecules relative to each other is a flourishing field of research. Recently, the intermixing of the two species has been identified as the key structural property to affect OSC performance. The proposed research focuses on polymer-free All-Small-Molecule OSCs (ASM-OSCs). The core objective of my work is to build quantitative models that relate the mixing behaviour in an OSC blend to its optoelectronic properties and the resulting performance. From there, guidelines for the design of novel molecules and the deposition process are drawn and put into practice. Central to achieving these objectives are advanced optoelectronic measurements to characterize the energetic landscape and the transport and recombination dynamics of charge carriers. The holistic study of ASM-OSCs deposited from solution and in vacuum yields comprehensive and widely applicable quantitative descriptions of structure-function-performance relationships. The developed models, guidelines, and improved efficiency contribute to the advancement of solution- and vacuum processed OSC technology. Both deposition routes are highly relevant to industrial production. The proposed work will result in unprecedented high PCEs for ASM-OSCs and thereby facilitate the technology's commercial success. Ultimately, the undertaken research aims at reducing global CO2 emissions to tackle climate change, and to foster manufacturing and innovative applications in the UK and worldwide. The Department of Condensed Matter Physics at the University of Oxford offers the ideal environment for my research with excellent facilities for optoelectronic characterization and outstanding fabrication tools such as the EPSRC-awarded national thin-film cluster. National and international partners from academia and industry will support my research through synchrotron-based structural characterization, ultrafast spectroscopy, molecular simulations, synthesis of new molecules, and identification of ways to transfer research findings into commercial applications.