**Acknowledgements**

complexes have high ϕ of 49–90% and τ of 4.1–17.7 μs in CH<sup>2</sup>

these complexes that showed very high brightness, even up to 66,000 cd/m<sup>2</sup>

94 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

**7. Other types of tetradentate platinum(II) complexes**

ture above 400°C, and one device exhibited a peak EQE of 22.3% [59].

Ir(mppy)<sup>3</sup>

**8. Conclusion**

parable to that of the Ir(mppy)<sup>3</sup>

the planar rigid molecular configuration enabled the fluorine-containing complexes **54**–**56** to have strong excimer emission at 616–624 nm, making them serve as ideal emitters for singledoped white OLEDs with CRI of up to 76 [52]. However, after introducing sterically bulky 3,5-di-*tert*-butylphenyl group to the pyridine ring, excimer formation was suppressed for the complexes **57**–**62**. Devices doped with complex **58** bridging phenylamine or complex **59** with a spiro linkage demonstrated peak EQEs over 25% and maximal PEs up to 109.4 lm/W using TmPyPB as ETL. The maximal PE of complex **59** could further be improved to 126.0 lm/W if Tm3PyBPZ as ETL, which were the highest among the reported platinum(II)-based OLEDs [53]. In the same year, Che's group developed another series of tetradentate platinum(II) complexes containing carbazole (**60**), phenoxazine (**61**), and phenothiazine (**62**) moieties, which served as

yellow phosphorescent emitters combined with blue emitter to make white OLEDs [54].

Besides the four series of tetradentate platinum(II) complexes discussed above, there were also some other new types. In 2015, a series of sky-blue emitters based on 3-(trifluoromethyl)- 5-(2-pyridyl)pyrazole or 3-(trifluoromethyl)-5-(2-pyridyl)-1,2,4-triazole containing spiroarranged tetradentate ligands were developed. The peak EQE of one blue OLED could reach 15.3% and CIE values of (0.190, 0.342) [58]. In 2017, Liao, Fan, and co-workers developed three 1-isopropyl-2-phenyl-benzo[*d*]imidazole-based emitters with decomposition tempera-

Very recently, Fukagawa and co-workers reported great progress in ultrapure green OLEDs based on a NHC emitter PtN7N [60], which was developed by Li′s group before 2014 [61]. The optimized OLED showed CIE coordinates of (0.18, 0.74) using a top-emitting OLED with a microcavity structure and also using a boron-based host material [60]. Fukagawa's work demonstrated that the narrowband emitter PtN7N was superior to the iridium(II)-based emitter

definition displays [60], owing to the very small vibrational structures of PtN7N that could be well suppressed by microcavity technology. Similar phenomenon was also observed in the previous report of narrowband green emitter PtN1N vs. PtOO3 [62]. Moreover, Fukagawa's work also demonstrated that the operational stability of PtN7N-based OLEDs could be com-

In summary, after over 10 years of development, the emission spectra of the tetradentate platinum(II)-based OLEDs can cover the whole visible spectrum, they also exhibit high efficiency, and some show high color purity and long operational lifetime, demonstrating their

tion of PtN7N by employing suitable host and charge-transporting materials [60, 63].

for the development of ultrapure green emitter to satisfy the BT.2020 for ultrahigh-


Cl<sup>2</sup>

solutions, and the OLEDs doped

at 10.5 eV. Moreover,

The authors thank the National Natural Science Foundation of China (21602198, 21776259, 21476270), the "Qianjiang Talents Plan" (QJD1602017), and AAC Technologies for their financial support. The authors also thank Dr. Tyler Fleetham from the University of Southern California for the measurements of the quantum efficiency and luminescent lifetime of the PtON3.
