Renewal: Overcoming Energy Loss in Organic Bulk Heterojunctions

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Non-technical Description. Solar technologies are increasingly providing energy across the US at costs well below even that of fossil fuels. In short, solar energy is delivering on its promise as a source of low cost, clean and renewable energy. However, solar solutions have largely been based on silicon, which is far from an optimal solution. New solutions must have the objective of making solar power ubiquitous, helping to fulfill our ever-expanding energy needs. These include solar power generating windows and building-integrated photovoltaics as well as devices that operate at very low light levels to scavenge waste illumination power. This is an urgent and fundamental technological challenge. Recently, there have been dramatic increases in the efficiency of potentially low-cost organic solar cells to over 19%, approaching that of silicon cells. This project is directed at determining the ultimate power conversion efficiency of organic solar cells. The investigators will study new organic materials with state-of-the-art optical spectroscopy to understand the power generating mechanisms that limit the efficiency of organic solar cells. The principles derived from these studies can provide molecular design rules and guide the improvement of organic solar cells towards their theoretical limit of ~25% efficiency. The project supports training of a diverse workforce through the education of graduate and undergraduate students in materials design, synthesis, and characterization, coupled with device engineering and scientific communication. The PIs will recruit and retain a diverse next generation of students in STEM fields through diversity, equity and inclusion efforts at the University of Michigan, including outreach to underrepresented groups and hosting a Conference for Undergraduate Women in Physics.Technical Description. The primary goal of this project is to understand and improve organic photovoltaic (OPV) devices through improved materials and device design strategies based on quantum mechanical models. Dramatically reduced energy losses in the charge photogeneration process may ultimately provide a pathway towards ultralow cost solar power in situations where established, mature solar technologies are less effective. Beyond solar energy harvesting, these systems open new avenues for engineering materials for charge and energy transport at the atomistic level, and for their exploitation in applications as light emission, energy and charge transfer over exceptional distances, and may even result in extending electronic technology well beyond its current limits. This project combines the investigators’ extensive expertise in OPV materials, design and characterization with state-of-the-art and emerging multidimensional spectroscopies to understand the energy loss mechanisms that currently limit single junction organic solar cell device efficiencies. The principles derived from these fundamental studies provide molecular design rules to guide the improvement of cell efficiencies towards their thermodynamic limit of ~25%. The work significantly expands the spectroscopic toolbox for probing OPVs, providing transformative opportunities for understanding the mechanisms of charge generation and concomitant energy losses. The research has the following primary goals: (i) Gain a fundamental understanding of the mechanisms governing charge generation and energy loss at organic heterojunctions (HJs) to increase the solar-to-electrical power conversion efficiency to near the thermodynamic limit; (ii) Map the complete HJ charge photogeneration process using multidimensional spectroscopy to probe the mechanisms of charge generation and the origins of energy loss; (iii) Exploit ultrastrong coupling in unique light harvesting architectures to realize exciton-polariton transfer with near-zero energy loss.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.