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 (Δ V _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 Δ V _nr .