Overcoming Energy Loss in Organic Bulk Heterojunctions

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Mature solar technologies, in particular silicon, are now providing energy in a growing number of locales across the US at costs well below even that of fossil fuels. In short, solar energy is delivering on its promise of over 60 years as a source of low cost, clean and literally infinitely renewable energy. However, basing all solar solutions on this single materials system is still far from an optimal solution. To be impactful, new solutions must have the objective of making solar power generation a ubiquitous presence, ultimately filling all of our ever-expanding energy needs. Recently there have been dramatic increases in organic photovoltaics (OPVs) that have led to the demonstration of 15% solar power conversion efficiencies (PCE) in cells containing new non-fullerene acceptors (NFAs) and unusual combinations of liquid and solution-processed organic and metallic materials that leverage our increased understanding of charge and exciton transport in films with heterogeneous phases at the nanometer scale. The primary impact of the research will be to understand and improve OPVs to ultimately provide ultralow cost solar power in situations where established, mature solar technologies are less effective, such as in solar power generating windows and building-integrated photovoltaics, and generation at very low light levels to scavenge waste illumination power. The project will support the education of a diverse group of graduate and undergraduate students in experimental and material design and synthesis, spectroscopic measurement, data-taking and analysis, and scientific communication. The PIs will seek to host a Conference for Undergraduate Women in Physics (CUWiP) at the University of Michigan in 2021. They will also develop strong ties to local minority serving institutions to give seminars and short lectures discussing state of the art OPV design and spectroscopic measurements.The project will combine extensive expertise in OPV design and characterization with state-of-the-art and emerging multidimensional coherent spectroscopies (MDCS) to understand the energy loss mechanisms that currently limit single junction OPV device efficiencies. The research aims to improve our fundamental understanding of the mechanisms governing charge and excited state transport and sources of energy loss in NFA-based OPVs. This will be facilitated by the development and use of new spectroscopic and visualization tools that can precisely quantify, on femtosecond - second time scales across the UV and into the infrared, the dynamics of photogenerated charge transfer across heterogeneous interfaces and transport away from their points of origin. The principles derived from these fundamental studies will provide molecular design rules to guide the improvement of NFA-based OPVs towards their thermodynamically limited efficiencies of ~25%.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.