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The fundamental scientific challenge for organic light emitting diodes (OLEDs) is to successfully manage electronic spin. The excited state can only give out light if it can decay to the spin-0 ground state. Finding ways of harvesting light from spin-1 excitations has shaped OLED technology for the last three decades, since the choice of strategy to achieve this necessarily impacts materials design, device architecture, and the processes limiting device lifetime.
Here we demonstrate a new approach to rapid triplet harvesting. We introduce a novel class of linear donor-bridge-acceptor light-emitting molecules which twist in their excited states, changing the coupling between electron and hole.(1) These enable doped polymer LEDs with near-100% internal quantum efficiency even at high brightness.(2) Our solution-processed OLEDs achieve current efficiency, power efficiency and brightness comparable to or exceeding those of state-of-the-art vacuum-deposited OLEDs and quantum dot LEDs.
Using time-resolved spectroscopy, we establish that luminescence via triplets occurs on 100s of ns timescales at ambient temperature, after reverse intersystem crossing to singlets. We find this occurs because molecular geometries exist at which the singlet-triplet energy gap (exchange energy) is close to zero, such that rapid interconversion is possible. Unlike other low exchange energy systems, substantial oscillator strength is sustained at this point.
We describe recent experimental and theoretical evidence for emission from these materials and show how it depends strongly in the interplay between rotational energetics, temperature, oscillator strength and the nanomorphology of the emissive layer. This gives us new tools to control emission colour and rate. Doing so, we tune emission from green to sky-blue, and achieve EQE at 1000 cdm-2 of nearly 30%. Based on this molecular motif, we realise new designs for molecular emitters realising sub-microsecond triplet emission and low roll-off in devices across the visible spectral range.
1. A. S. Romanov et al., Copper and Gold Cyclic (Alkyl)(amino)carbene Complexes with Sub-Microsecond Photoemissions: Structure and Substituent Effects on Redox and Luminescent Properties. Chem. - A Eur. J. 23, 4625–4637 (2017).
2. D. Di et al., High-performance light-emitting diodes based on carbene-metal-amides. Science. 356, 159–163 (2017).
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