[Paper Review] Efficient near-infrared organic light-emitting diodes with emission from spin doublet excitons

Hwan‐Hee Cho, Sebastian Gorgon+12 more|arXiv (Cornell University)|Aug 4, 2023

Organic Light-Emitting Diodes ResearchEngineering3 citations

TL;DR

This study demonstrates efficient near-infrared organic light-emitting diodes (OLEDs) by leveraging spin doublet excitons in organic radicals, specifically using the TTM-TPA radical as a guest emitter. By engineering a host:guest system with energy-matched triplet states in an anthracene-based host, the authors achieve up to 9.6% external quantum efficiency at 800 nm through reversible host-guest triplet-doublet energy transfer, overcoming non-radiative losses.

ABSTRACT

The development of luminescent organic radicals has resulted in materials with excellent optical properties for near-infrared (NIR) emission. Applications of light generation in this range span from bioimaging to surveillance. Whilst the unpaired electron arrangements of radicals enable efficient radiative transitions within the doublet-spin manifold in organic light-emitting diodes (OLEDs), their performance is limited by non-radiative pathways introduced in electroluminescence. Here, we present a host:guest design for OLEDs that exploits energy transfer with demonstration of up to 9.6% external quantum efficiency (EQE) for 800 nm emission. The tris(2,4,6-trichlorophenyl)methyl-triphenylamine (TTM-TPA) radical guest is energy-matched to the triplet state in a charge-transporting anthracene-derivative host. We show from optical spectroscopy and quantum-chemical modelling that reversible host-guest triplet-doublet energy transfer allows efficient harvesting of host triplet excitons.

Motivation & Objective

• To develop efficient near-infrared OLEDs for applications in bioimaging and surveillance.
• To overcome non-radiative decay pathways that limit the performance of organic radical-based OLEDs.
• To exploit spin doublet excitons in organic radicals for enhanced radiative transition efficiency.
• To design a host:guest system with optimal energy alignment for efficient triplet energy transfer from host to radical guest.

Proposed method

• Employing tris(2,4,6-trichlorophenyl)methyl-triphenylamine (TTM-TPA) as a radical guest emitter with strong near-infrared emission at 800 nm.
• Using a charge-transporting anthracene-derivative host with a triplet energy level matched to the TTM-TPA radical's doublet state.
• Enabling reversible triplet-doublet energy transfer between host and guest via precise energy level engineering.
• Conducting optical spectroscopy and quantum-chemical modeling to confirm the energy transfer mechanism and exciton dynamics.
• Measuring external quantum efficiency (EQE) under electroluminescent operation to evaluate device performance.
• Validating the mechanism through correlation of spectroscopic data with device efficiency metrics.

Experimental results

Research questions

• RQ1 Can spin doublet excitons in organic radicals enable efficient near-infrared emission in OLEDs?
• RQ2 What role does reversible triplet-doublet energy transfer play in enhancing device efficiency?
• RQ3 How does energy level alignment between host and guest materials influence exciton harvesting and EQE?
• RQ4 To what extent do non-radiative pathways limit the performance of radical-based OLEDs, and how can they be mitigated?
• RQ5 Can a host:guest system with matched triplet and doublet states achieve high EQE in the near-infrared range?

Key findings

• The TTM-TPA radical exhibits strong near-infrared emission at 800 nm with favorable optical properties for OLED applications.
• Reversible host-guest triplet-doublet energy transfer was confirmed via optical spectroscopy and quantum-chemical modeling.
• The device achieved a maximum external quantum efficiency (EQE) of 9.6% at 800 nm, representing a significant advance in NIR OLED performance.
• Energy transfer from the host’s triplet excitons to the radical’s doublet state enables efficient harvesting of otherwise non-emissive triplet excitons.
• The host:guest energy alignment minimizes non-radiative losses and enhances radiative transition efficiency.
• The combination of experimental characterization and theoretical modeling validates the mechanism of efficient exciton utilization in radical-based OLEDs.