LAUSR

Light-emitting diodes (LEDs) are key for the development of next-generation displays for ultra-high-definition television. Alternative materials beyond organic LEDs are required to meet the color purity standards, while retaining low processing cost and environmental friendliness. Liang and colleagues report in Advanced Science that two-dimensional (2D) tin halide perovskite—efficiently stabilized by H3PO2 incorporation—has great promise for ultra-pure red LEDs. The emphasis of the new Rec. 2020 ultra-high-definition television (UHDTV) standard is not on resolution but on color reproduction. To represent the natural objects faithfully and accurately, wide color gamut is necessary, which relies on the following primary colors: 630 nm for red, 532 nm for green, 467 nm for blue, and, importantly, the full width at half maximum (FWHM) for all colors is \ 20 nm [1]. The ideal light sources to meet this standard are monochromatic lasers; however, their commercialization is hindered by the high-cost and speckle problem. Exploring alternative substitutes, non-monochromatic light sources with precisely defined emission profiles, is highly desirable but challenging. Among all kinds of light-emission materials, colloidal chalcogenide quantum dots and metal halide perovskites show great promise, due to their low-cost, facile processing, color tunability and narrow FWHM. But the toxicity caused by the heavy metal (Cd, Pb) leakage cannot currently be mitigated [2, 3]. Moreover, despite excellent external quantum efficiencies (EQE) and brightness of QD-based or lead halide perovskite lightemitting diodes (LEDs), their color purities cannot meet the Rec. 2020 standard [2–6]. The standard requires the x, y coordinates for red luminescence to be (0.708, 0.292) on the Commission Internationale de l’Eclairage (CIE) scale, but the current Pb-based perovskite devices can only reach (0.71, 0.28). Recently in Advanced Science, Liang and colleagues reported the successful realization of ultra-pure red emission with Sn-based perovskites. The CIE of the lead-free LED is (0.706, 0.294) (Fig. 1), matching closely the Rec. 2020 standard for red emission (0.708, 0.292) [7]. The authors chose the Sn-based perovskites with natural quantum well structure as active layers for light emission. Benefitting from the strong confinement effect of the bulky organic ligands, the LEDs show emission centered at 633 nm with a FWHM of only 24 nm (Fig. 1b). The authors also evaluated the color stability under changing temperatures, power and applied potential, finding that the emission shift was \ 3 nm at low temperature and high power, and less than 0.3 nm V at working voltage (Fig. 1c). To avoid oxidation, the authors investigated the oxidation pathway of Sn-based perovskite film and determined that the formation of SnI4 was the crucial step. With this knowledge, preventing the formation of SnI4 was proposed as effective strategy to retard the oxidation of Sn. Therefore, the H3PO2 (HPA) was introduced into Sn precursors as a mild reducing agent with good coordinating ability with Sn intermediates. It not only impedes the oxidation through inhibiting SnI4 formation but also improves film quality via seeding the crystal growth. Upon optimizing the device preparation parameters, a champion luminance of 70 cd m under 5.8 V with an EQE of 0.3% has been achieved, which is the record brightness and efficiency among red Pb-free LEDs. O. Voznyy* Department of Physical and Environmental Sciences, University of Toronto Scarborough, Scarborough, ON M1C 1A4, Canada e-mail: [email protected]