Long-Lived Spin-Polarized Intermolecular Exciplex States in Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes

TL;DR

This study reveals that triplet exciplex states in TADF-based OLEDs are highly spin-polarized and have long relaxation times, suggesting that slow spin relaxation limits efficiency more than reverse intersystem crossing.

Contribution

The paper demonstrates that triplet exciplex states can be coherently manipulated and have relaxation times much longer than RISC, highlighting a new focus for improving TADF OLED efficiency.

Findings

Triplet exciplex states are highly spin-polarized.

Spin relaxation time T1 exceeds RISC time by a large margin.

Slow spin relaxation limits OLED efficiency.

Abstract

Spin-spin interactions in organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are pivotal because radiative recombination is largely determined by triplet-to-singlet conversion, also called reverse intersystem crossing (RISC). To explore the underlying process, we apply a spin-resonance spectral hole-burning technique to probe electroluminescence. We find that the triplet exciplex states in OLEDs are highly spin-polarized and show that these states can be decoupled from the heterogeneous nuclear environment as a source of spin dephasing and can even be coherently manipulated on a spin-spin relaxation time scale T2* of 30 ns. Crucially, we obtain the characteristic triplet exciplex spin-lattice relaxation time T1 in the range of 50 us, which far exceeds the RISC time. We conclude that slow spin relaxation rather than RISC is an efficiency-limiting step for intermolecular donor:acceptor systems. Finding TADF emitters with faster spin relaxation will benefit this type of TADF OLEDs.