Emerging substance class with narrow-band blue/green-emitting rare earth phosphors for backlight display application

Abstract

Solid state lighting (SSL) on the basis of phosphorconverted white light-emitting diodes (pc-wLEDs) is rapidly changing the lighting and display industry. New materials and concepts provide light sources and systems that have unmatched performance and/or functionality [1,2]. In addition to the well-known application of pc-wLEDs in home and office lighting, these efficient white light sources are also increasingly applied as alternative backlight units in liquid-crystal displays (LCDs), where there is a strong focus on improving resolution, wider color gamut, and decreased power consumption [3]. Especially, the improvement of color gamut is a hot issue as it enables more vivid colors that appeal to consumers. The color gamut is determined by the color coordinates of red, green, and blue (RGB) emissions from white LEDs that pass through the corresponding RGB color filters in the LCD. Compared to competing technologies for display backlights, such as individual RGB LED chips and multi-color quantum dot (QD) emitters, pc-wLEDs have the advantage of a higher color stability, being more robust, and are simpler in design [4][5][6].The discovery of new rare earth (RE) phosphor materials is recognized as a key enabler for emerging applications in lighting and displays [7]. The development of narrow-band emitting phosphors in the RGB spectral region is a tremendous challenge. The operating conditions for phosphors in pc-wLEDs are extremely related to the high operating temperatures and photon fluxes. Nowadays, the commercial pc-wLEDs backlights combine a blue-emitting (In,Ga)N chip (λ=440-460 nm) with narrow-band green-emitting β- SiAlON:Eu 2+ that has an emission band centered at~540 nm with a full width at half maximum (FWHM) of~55 nm and red-emitting K 2 SiF 6 :Mn 4+ (KSF:Mn 4+ ) with sharp emission lines around 630 nm [5]. For example, the green phosphor β-SiAlON:Eu 2+ still limits the maximum accessible color gamut and energy efficiency of LCDs. Moreover, the harsh synthesis condition of this oxynitride phosphor is a serious drawback. Accordingly, the discovery and development of alternative high efficiency narrowemitting phosphors in the RGB spectral region is particularly important and a major challenge [8]. In this context, recent discoveries in the field of phosphor materials are worth to be highlighted. Based on the insight that famous nitridoaluminates like Ca[LiAl 3 N 4 ] [9] and Sr[LiAl 3 N 4 ] [2] crystallize in the Na[Li 3 SiO 4 ] (Fig. 1a) and K[Li 3 SiO 4 ] structure type, respectively, Prof. Hubert Huppertz and co-workers together with OSRAM Opto Semiconductors issued the patent WO 2018/029304 A1 on the alkali lithosilicate phosphors in Germany [10], where a number of additional narrow-band representatives for this material class are described, and then they also reported in Angew. Chem. Int. Ed. that alkali metal silicates such as Na[Li 3 SiO 4 ] and K[Li 3 SiO 4 ] can unexpectedly be doped with Eu 2+ to yield highly efficient narrow band emission [11]. The fact that the substance class of alkali metal s...

Na Replaces Rb towards High‐Performance Narrow‐Band Green Phosphors for Backlight Display Applications

“…Phosphor‐converted white light‐emitting diodes (PC‐wLEDs) have been widely used for liquid crystal display (LCD) as backlights due to their low power consumption, long life and low cost. [ 1–3 ] For backlight display applications, wide color gamut and high luminous efficiency are particularly important. [ 4–6 ] The key to achieving this is that the phosphor has a suitable peak position, a narrow emission band, high photoluminescence quantum efficiency, and excellent thermal stability.…”

Chasing Down the Eu2+ Ions: The Delicate Structure−Property Relationships in the Ultra‐Narrow Band Phosphor K1.6Na2.1Li0.3[Li3SiO4]4:Eu2+

“…A reason why they might have not been initially considered as a host material was presumably the fact that the structure built from monovalent cations was considered unsuitable for doping with strongly absorbing higher valent activator ions with electric dipole‐allowed 4f n → 4f n −1 5d 1 transitions such as Eu 2+ ( n = 7) or Ce 3+ ( n = 1). [ 40 ] However, already in the 1980s, Hoppe's research group was able to synthesize several representatives and examined the substance class in detail. [ 41–45 ] Their structure partially also derives from the previously mentioned UCr 4 C 4 aristotype.…”

Thermal quenching properties of narrow-band blue-emitting MBe2(PO4)2:Eu2+(M = Ca, Sr) phosphors towards backlight display applications

“…6,7 Accordingly, many efforts have been devoted to the discovery of narrow-band phosphors. 8,9 Nevertheless, rare earth Eu 2+ activated narrow-band phosphors are quite scarce and encounter tremendous challenges in exploration, as evident from there being only a few known narrow-band phosphor families (mainly limited to the UCr 4 C 4 family), [10][11][12][13][14] hindering state-of-the-art backlights from further advancing.…”