Exciton Delocalization Induced by Aggregation for Efficient Non-Fullerene Organic Photovoltaics

Intimate π-stacking in organic semiconductors is known to form aggregates, which drive dissociation of photogenerated excitons through delocalization of exciton wavefunctions [1-3]. In particular, the onset of non-fullerene acceptors (NFAs) such as Y6 has seen a dramatic rise in discussion of exciton delocalization and its relation to (acceptor-donor) hole transfer [4-6]. Here, we explore the concept of exciton delocalization and show that such treatment is necessary to accurately describe charge separation in high performance OPV blends based on D18:Y6. Specifically, we evaluate how π-interactions in aggregates contribute to delocalization strength, revealing that the formation of delocalized excitons in strongly π-interacting materials opens a new pathway for free carrier generation. This pathway partially bypasses the formation of performance-limiting singlet and triplet charge-transfer states in OPV blends and improves the internal quantum efficiency in OPVs to realize power conversion efficiencies of &gt;19%. We further show that the extent of delocalization can be tuned with precise structural modifications of organic donor and acceptor molecules. Ultimately, we provide an insight into overcoming the fundamental limit of OPVs associated with intrinsic material properties. Designing materials with more pronounced delocalization character should maximize the exciton dissociation efficiency and minimize the terminal back recombination, pushing OPVs forward closer to theoretical efficiency limits [7].<br/><br/>[1] Rao, A.<i> et al.</i> The role of spin in the kinetic control of recombination in organic photovoltaics. <i>Nature</i> <b>500</b>, 435-439 (2013)<br/>[2] Gélinas, S.<i> et al.</i> Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. <i>Science</i> <b>343</b>, 512-516 (2014)<br/>[3] Tamai, Y.<i> et al.</i> Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells. <i>ACS Nano</i> <b>11</b>, 12473-12481 (2017)<br/>[4] Price, M. B.<i> et al.</i> Free charge photogeneration in a single component high photovoltaic efficiency organic semiconductor. <i>Nat. Commun.</i> <b>13</b>, 2827 (2022)<br/>[5] Zhang, G.<i> et al.</i> Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells. <i>Nat. Commun.</i> <b>11</b>, 3943 (2020)<br/>[6] Wang, R., Zhang, C., Li, Q., Zhang, Z., Wang, X., Xiao, M. Charge Separation from an Intra-Moiety Intermediate State in the High-Performance PM6:Y6 Organic Photovoltaic Blend. <i>J. Am. Chem. Soc.</i> <b>142</b>, 12751-12759 (2020)<br/>[7] R. Westbrook, K. Jiang <i>et al</i>, Exciton Delocalization Induced by Aggregation in Polymer Donor for Efficient Non-fullerene Organic Photovoltaics. <i>Under Review</i>