Directed Regulation of Intermolecular Excitonic Couplings to Minimize Non-Radiative Recombination of Excited States in NIR-Absorbing Non-Fullerene Acceptors.

Abstract

Suppression of excited-state non-radiative recombination is pivotal for overcoming efficiency bottleneck in organic optoelectronics. However, in film, mechanism of aggregates' excited-state non-radiative recombination and how to suppress its rate (k_nr) remain unclear. Here, taking classical Y6-type acceptors as an example, we investigate how change in their molecular packing modes impacts aggregates' excited-state properties and k_nr. We find that k_nr decreases with an increased population of the compact EECC (end-end and core-core) packing mode. Our results reveal that the EECC mode enhances the electronic coupling between intermolecular charge-transfer (iCT) and locally excited (LE) exciton states, facilitating aggregates' excited-state wavefunction delocalization and lowering the aggregates' exciton-phonon coupling, which compensates for the energy-gap-law effect. This intermolecular excitonic-coupling regulation strategy is further supported in the L8BO series through an increased population of the EE packing mode and enhanced LE-LE excitonic coupling. The corresponding D18:L8BO:HDL8 ternary OPV devices achieved a high efficiency of 20.63% (certified as 20.40%) with reduced non-radiative voltage loss (ΔV_nr). Our work has not only uncovered the underlying mechanism of how molecular packing mode impacts aggregates' electronic structures and k_nr, but also provided a molecule-design strategy for improving NIR luminescent efficiencies/exciton lifetimes of films and OPV device efficiencies with reduced ΔV_nr.