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Proceedings Paper

High efficiency OLEDs enabled by molecular rotation (Conference Presentation)
Author(s): Dan Credgington

Paper Abstract

Excitonic spin plays a crucial role in the design of organic light emitting diodes (OLEDs). The random spin statistics of recombining charge sets a limit of 25% on the fraction of singlet (spin-0) excitons formed by electrical excitation. Without efficient emission from triplet (spin-1) excitons, the same limit applies to the internal quantum efficiency (IQE) of fluorescent OLEDs. Phosphorescent OLEDs, utilising the heavy atom effect to render triplets emissive, and TADF OLEDs, based on thermally-assisted triplet-to-singlet up-conversion, are currently the most promising routes for triplet emission. Here we demonstrate a different approach. The effective exchange energy in a family of linear copper and gold carbene metal amide compounds can be tuned via rotation about the metal-amide bond from positive to negative values. The energetic ordering of spin-states can therefore be inverted, enabling triplet-to-singlet down-conversion. The availability of degenerate states with high oscillator strength allows emission via triplets to occur on sub-microsecond timescales. Using such materials as emissive dopants in solution-processed OLEDs leads to extremely efficient devices with near 100% IQE (external quantum efficiencies >27%), and current efficiency, power efficiency and brightness comparable to or exceeding those of state-of-the-art vacuum-deposited OLEDs and quantum dot LEDs. We describe the experimental and theoretical evidence for rotationally accessed spin-state inversion. Using time-resolved spectroscopy we show how the resulting emission depends strongly in the interplay between rotational energetics, temperature, oscillator strength and the morphology of the emissive layer.

Paper Details

Date Published: 21 September 2017
Proc. SPIE 10348, Physical Chemistry of Semiconductor Materials and Interfaces XVI, 103480A (21 September 2017); doi: 10.1117/12.2273013
Show Author Affiliations
Dan Credgington, Univ. of Cambridge (United Kingdom)

Published in SPIE Proceedings Vol. 10348:
Physical Chemistry of Semiconductor Materials and Interfaces XVI
Hugo A. Bronstein; Felix Deschler, Editor(s)

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