Rational Design and Synthesis of a New Class of Non-Fullerene Acceptors and Matched Donor Polymers for Highly Efficient Organic Photovoltaics

Description

Organic photovoltaics (OPV) have unique potential to provide low cost solar energy due to their excellent power/weight ratios, good flexibility and conformality, free of heavy metals, and are scalable through solution processes. In the active layer of OPV, donor and acceptor are blended to facilitate efficient light absorption, charge generation, transport, and collection to convert light to electricity. Recently, power conversion efficiencies (PCEs) of OPV have improved dramatically exceeding 17% in single junction devices after the introduction of new non-fullerene acceptors (NFAs). Nevertheless, these values are still below that can be obtained theoretically. The voltage deficit, which is the difference between the experimental open circuit voltage (VOC) and the theoretical VOCbased on bandgap, needs to be improved, which represents a major challenge for OPVs.Besides introducing newly improved NFAs, understanding factors that affect non-radiative exciton decay of blend materials and developing methods to suppress the electronic tail states and have optimal morphology should improve efficiencies further. The presence of long tail states and non-ideal morphology cause significant energy loss in OPV since charge carriers are trapped into these states and defects, which will reduce their mobilities and increase bimolecular recombination. The tail states and ordered packing of NFAs are associated with their conformational rigidity and uniformity in solid state. Thus, by suppressing the tail states and enhancing crystallinity of donor/acceptor blends, it will enhance charge carrier dynamics and reduce charge trapping. Moreover, the performance of device is also limited by the kinetic competition between hole injection and exciton decay of acceptors.In this proposal, we will develop a new class of NFAs with matched donor polymers that possess minimized tail states and enhanced molecular interactions via rational structural modifications. Additionally, these NFAs should possess improved excitons delocalization and hole injection into donor polymers. Their optical, electrochemical, and photophysical properties will be systematically studied by collaborating with Professor David Ginger at the University of Washington who is a world expert in spectroscopic and microscopic studies. Morphological study of donor/acceptor blend films will also be conducted in conjunction with crystallographic study on molecular components to understand the correlation between charge transport and molecular structures.By pairing suitable new NFAs and donor polymers, we aim at enhancing their optical absorption and VOC to enable devices to achieve high PCEs beyond 20%. The results from this study will greatly contribute to the development of next generation OPV and other applications for organic electronics.