**Conflict of interest**

*Liquid Crystals and Display Technology*

.

standards required for commercialization.

100 cd m<sup>−</sup><sup>2</sup>

of 1000 cd m<sup>−</sup><sup>2</sup>

**4. Conclusions**

and the LT70 was significantly prolonged to 635 h at 1000 cd m<sup>−</sup><sup>2</sup>

. Considering the advantages of TrisPCz (EBL) and mCBT (HTL) in the

above devices (i.e., structures 2 and 3), these materials were employed in structure 4. As expected, this device achieved the best efficiency, namely, a peak EQE of 17.8%. However, the operational lifetime of LT70 decreased to 482 h at a luminance

Overall, tetradentate cyclometalated Pt(II) emitters have been demonstrated to exhibit high versatility in emission color tuning across RGB colors and white light, as well as superior photophysical and electroluminescent efficiencies and respectable operational lifetimes at practical luminance levels. While the performance metrics of this class of Pt(II) emitters are comparable to that of the best reported Ir(III) emitters in many aspects, more focused efforts should be directed at reducing the radiative lifetimes of these emitters by careful molecular design, which will be instrumental in further improving the operational stability of these complexes to meet the stringent

Substantial room for innovation remains in OLED materials research, and the development of robust, high efficiency emitters for diverse applications remains a challenge both in academia and industry. While in the past decade, tris-(bidentate chelate) iridium(III) complexes have been seemingly edging out other classes of metal phosphors, it is remarkable that tetradentate platinum(II) emitters have demonstrated high performance and are being increasingly recognized by academia and industry as a competitive alternative. Importantly, the unique aggregation behavior and the associated photophysical properties afforded by their planar coordiantion geometry distinguish platinum(II) emitters from octahedral iridium(III) emitters. The unique photophysical properties of platinum(II) emitters render them well suited for some OLED applications using simple device structures such single-dopant WOLEDs and aggregation-based red and NIR OLEDs as covered in this review. In addition, appropriate molecular design of the ligand scaffold allows the regulation of the emissive excited states and the intermolecular interactions, which consequently offers flexibility in manipulating the emission characteristics of platinum(II) emitters to cater to various OLED applications. Indeed, sustained and concerted efforts between academia and industry have already realized successful application of tetradentate Pt(II) emitters in OLED devices in an industrial setting. It is without doubt that Pt(II) emitters, after full optimization, will meet the technical specifications including operational stability, required for commercialization. We hope the perspective described herein will spur interest among stakeholders and drive further development

of tetradentate Pt(II) emitters for display and lighting applications.

and Robotics cluster under InnoHK (AIR@InnoHK).

This work was supported by the Major Program of Guangdong Basic and Applied Research (2019B030302009), Innovation and Technology Fund (ITS/224/17FP), Hong Kong Research Grants Council (HKU 17330416), the Basic Research Program of Shenzhen (JCYJ20170412140251576, JCYJ20170818141858021, and JCYJ20180508162429786), the National Key Basic Research Program of China (2013CB834802), Innovation and Technology Commission, Centre of Machine Learning for Energy Materials and Devices, a major initiative—Artificial Intelligence

and 31,806 h at

**180**

**Acknowledgements**

The authors declare no conflict of interest.
