Kinetic Modeling of Transient Electroluminescence reveals TTA as Efficiency-Limiting Process in Exciplex-Based TADF OLEDs

TL;DR

This study uses kinetic modeling of transient electroluminescence to identify triplet-triplet annihilation as a key efficiency-limiting process in exciplex-based TADF OLEDs, providing insights into non-radiative decay and exciton dynamics.

Contribution

It introduces a temperature-dependent kinetic model for TADF OLEDs that quantifies the impact of TTA and other processes on device efficiency.

Findings

TTA significantly depopulates triplet states and limits efficiency.

RISC contributes to exciton upconversion but is affected by TTA.

Non-radiative decay processes also impact overall device performance.

Abstract

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) show increased efficiencies due to efficient upconversion of non-emissive triplet states to emissive singlets states via reverse intersystem crossing (RISC). To assess the influence of the characteristic efficiency-enhancing RISC process as well as possible efficiency-limiting effects in operational OLEDs, we performed temperature-dependent measurements of transient electroluminescence (trEL). With kinetic modeling, we quantify and separate the impact of different temperature-dependent depopulation processes and contributions to EL in the established donor:acceptor model system m-MTDATA:3TPYMB. The underlying rate equations adapted for EL measurements on TADF systems include radiative and non-radiative first- and second-order effects. In this way, we are able to evaluate the non-radiative recombination and annihilation processes with respect to their efficiency-limiting effects on these OLEDs. On the one hand, we evaluate the depopulation of intermolecular exciplex triplet states via non-radiative direct triplet decay, RISC and triplet-triplet annihilation (TTA). On the other hand, we determine the contribution to EL from the formation of singlet exciplex states via polarons, RISC and TTA. Our results show that TTA accounts for a significant part to triplet depopulation and contributes to EL while limiting the overall device quantum efficiency.