**8. Conclusion**

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 potential applications for the next-generation full-color display and solid-state lighting. Owing to the square planar and rigid molecular configurations, platinum(II) complexes have many unique and exciting photophysical properties. On the one hand, easy molecular modification enables tunable emission spectra, and the FWHM of the pyrazole- or NHC-carbene-based complexes can achieve no more than 20 nm and can be as narrow as 15 nm. This facilitates them to serve as efficient deep-blue emitters, and device-doped NHC-carbene-based complex successfully realized "pure" blue emitting with CIE coordinates of (0.148, 0.079) and peak EQE of 24.8%. On the other hand, some planar d8 platinum(II) complexes can form intermolecular Pt-Pt bond to achieve 18e structure in their excited state, making them serve as single-doped white OLEDs with high CRI values. Besides, tetradentate platinum(II)-based green, especially for the red OLEDs, demonstrated superlong operational lifetime and satisfied the requirements of the initial commercialization. What's more is that sky-blue OLEDs also achieved encouraging performances, indicating their bright future for the development of the efficient and stable blue OLEDs.

Despite great progress that has been made for the tetradentate platinum(II) complexes, a challenge remains for the development of the stable deep-blue OLEDs, and more work still be needed. To overcome this challenge, it is important to develop stable host materials with a high enough T1 state level and highly balanced charge carrier ability. However, through continued efforts of the academia and industry, we believe that these critical issues can be solved and the platinum(II)-based OLEDs will be one candidate for display and lighting applications.
