Exciton Transport in Nonfullerene Acceptors and Semiconducting Polymers Boosted by Transient Quantum Delocalization

The transport of excitons in molecular systems is essential to the function of flexible optoelectronic devices (including organic solar cells) and biological light-harvestings. It is remarkable, however, that the nature of excitons and related transport mechanism in these soft materials have puzzled the community for so many years. Even for apparently simple systems such as organic single crystals: some experiments would better fit a localized particle picture, while others a coherent wave-like interpretation.<br/>Our non-adiabatic molecular dynamics simulations show that energy carriers in OSs are neither waves nor particles, as previously assumed. By solving the coupled nuclear-electronic motion, we observe that in molecular OSs and non-fullerene acceptors (NFAs), including Y6, exhibiting some of the highest measured (singlet) exciton diffusion length to date [1], excitons undergo a transient quantum delocalization/localization mechanism that underpins the larger spatial displacements and diffusivity of the excitonic wavefunction in these materials [1] (in agreement with what found experimentally). This mechanistic scenario, by which the excitons can thermally access delocalized states within the excitonic band giving rise to long-range exciton transport, is similar to what we found for charge carrier transport in high charge carrier mobility OSs [3,4]. Similarities and differences will be discussed.<br/>Remarkably, we also found that an analog transient delocalization mechanism takes place when simulating (singlet) exciton diffusion in highly ordered conjugated polymer nanofiber of P3HT [5,6]. Importantly, in the latter case, the mechanism is highly sensitive to the interplay between delocalization along the polymer chains and long-range interactions along the stacks [6]. Surprisingly, the diffusion coefficient is predicted to rocket by 3 orders of magnitude when going beyond nearest-neighbour intermolecular interactions in fibers of extended (30-mer) polymer chains (in line with transient absorption microscopy measurements [5]) and to be resilient to interchain energetic and positional disorders.<br/>On the basis of our simulations we discuss a path for improving the exciton diffusion constant in organic semiconductors.<br/><br/>[1] Firdaus, Y. et al. Long-range exciton diffusion in molecular non-fullerene acceptors. <i>Nat. Commun.</i> 11, 5220 (2020)<br/>&lt;!--StartFragment--&gt;[2] Giannini, S.<i> </i>et al. Exciton transport in molecular organic semiconductors boosted by transient quantum delocalization. <i>Nat. Commun.</i> <b>13</b>, 2755 (2022).&lt;!--EndFragment--&gt;<br/>[3] Giannini, S. et al. Quantum localization and delocalization of charge carriers in organic semiconducting crystals. <i>Nat. Commun.</i> 10, 3843 (2019).<br/>[4] Giannini, S., et al. Flickering Polarons Extending over Ten Nanometres Mediate Charge Transport in High-Mobility Organic Crystals. <i>Adv. Theory Simul.</i> 3, 2070021 (2020).<br/>[5] Sneyd, A. J. et al. Efficient energy transport in an organic semiconductor mediated by transient exciton delocalization. <i>Sci. Adv.</i> 7, eabh4232 (2021).<br/>[6] Prodhan, S., Giannini, S., Wang, L. and Beljonne, D. Long-Range Interactions Boost Singlet Exciton Diffusion in Nanofibers of π-Extended Polymer Chains. <i>J. Phys. Chem. Lett.</i> 12, 8188–8193 (2021).