For next-generation high-definition displays, the newly recommendation 2020 (Rec. 2020) standard calls for the emission wavelength of green emitters to be kept at 525−535 nm with narrow emission line width below 25 nm. All-inorganic lead halide perovskite CsPbBr 3 nanocrystals (NCs) with sharp emission seem as the prefect one to match the call, however, it is a great challenge to tune their emission wavelength to the target window of 525−535 nm because of the intrinsic band gap of CsPbBr 3 . Herein, we report a facile strategy to prepare CsPbBr 3 NCs with finely tunable emissions in the narrow range of 525−535 nm by establishing a CsPbBr 3 −Cs 4 PbBr 6 :Na Type II structure. The doping of the Na ions is the key factor to construct the Type II structure, which induces the formation of the Cs 4 PbBr 6 :Na phase and builds CsPbBr 3 −Cs 4 PbBr 6 :Na Type II structure, leading to the tunable emission wavelength of CsPbBr 3 NCs. The as-synthesized CsPbBr 3 NCs exhibit an ideal emission at 525−535 nm with a narrow emission line width of 23 nm and a high photoluminescence quantum yield (PLQY) of 85%. As a green downconverter, an ultrapure green backlight covers 97% of the Rec. 2020 standard in the CIE 1931 color space, representing the "greenest" backlight up to now.

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