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.
Dan Credgington, "High efficiency OLEDs enabled by molecular rotation (Conference Presentation)," Proc. SPIE 10348, Physical Chemistry of Semiconductor Materials and Interfaces XVI, 103480A (Presented at SPIE Nanoscience + Engineering: August 07, 2017; Published: 21 September 2017); https://doi.org/10.1117/12.2273013.5583353012001.
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