Abstract

Copper(I) emitters have long been hoped to become practical alternatives to noble metal phosphors in organic light-emitting diodes (OLEDs).1 A critical step in advancing the practical application of copper-based OLEDs is to bridge the insurmountable gap between device efficiency and operational stability at practical luminance.

To improve the device stability, both high emission quantum yield and short emission decay lifetime should be realized to enlarge the radiative decay rate constants (i.e., 106 s-1), so that those unnecessary side reactions can be suppressed. The recent discovery of the two-coordinate carbene–copper–amide (CMA(Cu)) emitters based on non-conventional bulky N-heterocyclic carbene (NHC) ligands,2 as exemplified by those depicted in Figure 1a, show emission quantum yields and radiative rate constants (kr) of up to unity and 1×106 s-1,2,3 respectively, and high external quantum efficiencies (EQEs) of ca. 20% were achieved with their as-fabricated OLEDs.2c,3 However, report on the stability of OLEDs with CMA(Cu) emitters is lacking. In addition, the host-guest devices with CAACAd-Au-amide emitters gave LT95 at 100 cd m-2 of several minutes to two hours for green and blue emitters respectively,4 which was still still far inferior from those noble-metal-based (i.e., Ir and Pt) OLEDs.

Figure 1. (a) Selected examples of Cu(I) emitters in the literature and (b) chemical structure of CMA(Cu) complexes in this work.

Described here a panel of air- and thermally stable two-coordinate Cu(I) emitters featuring bulky pyrazine- (PzIPr) or pyridine-fused N-heterocyclic carbene (PyIPr*) and carbazole (Cz) ligands with enhanced amide–Cu–carbene bonding interactions (Figure 1b).5 These Cu(I) emitters display thermally activated delayed fluorescence (TADF) from the 1LL’CT(Cz🡪PzIPr/PyIPr*) excited states across the blue to red regions with exceptional radiative rate constants of 1.1–2.2×106 s-1. Vapour-deposited OLEDs based on these Cu(I) emitters showed excellent external quantum efficiencies and luminance up to 23.6% and 222,200 cd m-2, respectively, alongside record device lifetimes (LT90) up to 1,300 h at 1,000 cd m-2 under our laboratory conditions, highlighting the practicality of the Cu(I)-TADF emitters.

(a) Czerwieniec, R. et al., Coord. Chem. Rev. 2016, 325, 2–8.
(a) Di, D. et al., Science 2017, 356, 159–163; (b) Hamze, R. et al., Science 2019, 363, 601–606; (c) Shi, S. et al. J. Am. Chem. Soc. 2019, 141, 3576–3588.
Ying, A. et al., ACS Appl. Mater. Interfaces 2021, 13, 13478–13486.
Conaghan, P. J. et al., Nat. Commun. 2020, 11, 1758.
Tang, R.; Xu, S.; Lam, T.-L.; Cheng, G.; Du, L.; Wan, Q.; Yang, J.; Hung, F.-F.; Low, K.-H.; Phillips, D. L.; Che, C.-M. Angew. Chem. Int. Ed., 2022, 61, e202203982.

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