The efficiency of an OLED is fundamentally governed by the spin of the tightly-bound electron-hole pairs (excitons) which form upon electrical excitation. The ratio of emissive singlets to dark triplets is 1:3. This sets an upper limit of 25% internal quantum efficiency (IQE) for fluorescent OLEDs due to the exchange energy (the singlet-triplet energy gap) being generally large, so that triplets cannot intersystem cross to emissive singlets. Efficient OLEDs require materials that enable triplet harvesting. This can be achieved by phosphorescence, using heavy metals like platinum or iridium to render triplets emissive, or by thermally activated delayed fluorescence (TADF), which allows thermally-promoted triplet-to-singlet up-conversion for systems with low exchange energies. Here we present a new class of photoemitters that give effective emission by a different emission pathway, based on linear compounds (CAAC)M-amide, where CAAC = cyclic (alkyl)(amino)carbene, M = Cu or Au, and amide = anilide or carbazolate. In these compounds, intramolecular rotation of the amide ligand leads to stabilization of the excited singlet state so that ΔE(S1-T1) becomes negative, an example of spin-state inversion. During this twisting motion an angle must exist at which S1 and T1 are degenerate and inter-system crossing is very fast. In addition, these compunds are very suitable for cheap solution processing techniques. This has allowed us to construct OLEDs with up to 100% internal and over 27% external quantum efficiency, a record for solution-phase processed OLEDs. The current and power efficiencies (87 cd A<sup>-1</sup> and 75 lm W<sup>-1</sup>) are comparable to, or exceed, those of state-of-the-art vacuum-processed OLEDs and quantum dot LEDs.

Highly efficient emitters for Organic Light Emitting Diodes (OLEDs) based on intramolecular rotation and spin-state inversion

253rd National Meeting (2017)

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