Singlet and triplet to doublet energy transfer: improving organic light-emitting diodes with radicals

TL;DR

This paper demonstrates that using radicals in OLEDs allows direct energy transfer from both singlet and triplet excitons to doublet excitons, significantly improving efficiency and emission speed in organic light-emitting devices.

Contribution

It introduces a novel approach of employing radicals to enable energy transfer from all exciton types to doublet states, enhancing OLED performance.

Findings

Radicals facilitate direct energy transfer from singlet and triplet excitons.

Doublet exciton emission is faster and more efficient than traditional TADF systems.

Higher electroluminescent efficiency is maintained at increased current densities.

Abstract

Organic light-emitting diodes (OLEDs) must be engineered to circumvent the efficiency limit imposed by the 3:1 ratio of triplet to singlet exciton formation following electron-hole capture. Here we show the spin nature of luminescent radicals such as TTM-3PCz allows direct energy harvesting from both singlet and triplet excitons through energy transfer, with subsequent rapid and efficient light emission from the doublet excitons. This is demonstrated with a model Thermally-Activated Delayed Fluorescence (TADF) organic semiconductor, 4CzIPN, where reverse intersystem crossing from triplets is characteristically slow (50% emission by 1 microsecond). The radical:TADF combination shows much faster emission via the doublet channel (80% emission by 100 ns) than the comparable TADF-only system, and sustains higher electroluminescent efficiency with increasing current density than a radical-only device. By unlocking energy transfer channels between singlet, triplet and doublet excitons, further technology opportunities are enabled for optoelectronics using organic radicals.