EMRS 2018 Replacing rare earth ions in LEDs (?)
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Two decades after the development of the blue light-emitting diode (LED), LEDs have quickly established themselves as the lighting technology of the future. The high efficiency, spectral tunability, lack of toxic compounds and a small footprint makes them far more attractive than other lighting technologies. The high efficiency, now well exceeding 100 lum/W in commercial products, has still the margin to double, promising a strong reduction in electricity consumption. White LEDs are commonly based on a blue LED, combined with luminescent materials, or phosphors, which convert part of the blue light to longer wavelengths, the mixture providing white light. Besides the workhorse Y3Al5O12:Ce (YAG:Ce, yielding yellow emission), europium doped phosphors are used to provide e.g. the red emission required for warm-white LEDs. Six main requirements for LED phosphors are discussed and used to explain the discrepancy between the high number of compositions described in literature and the handful of actually used compounds, being almost uniquely based on rare earth ions as luminescent center [1]. Alternative materials avoiding the use of rare earth ions are discussed, including Mn4+ doped fluorides phosphors (e.g. K2SiF6:Mn4+ [2]) and quantum dots. Finally, the impact of phosphor geometries on phosphor use, including remote phosphor applications, are discussed. [1] Smet PF and Joos JJ, Nat. Mater. 16 (2017) 500. [2] Sijbom H et al, Opt. Mater. Exp. 7 (2017) 3332.
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EMRS 2018 Replacing rare earth ions in LEDs (?)
- 1. Are alternatives needed for the workhorses Eu2+ and Ce3+ in phosphor converted LEDs? June 18 2018 http://LumiLab.UGent.be Philippe Smet philippe.smet@ugent.be @pfsmet EMRS Spring meeting – Strasbourg – 18 to 22 June 2018 @UGentLumiLab Presentation can be downloaded at http://www.slideshare.net/pfsmet
- 3. Lighting accounts for an estimated 18% of the world’s electricity consumption. Fluorescent and LED lighting use rare earth elements. The RE elements regularly make headlines. Do we really need RE for lighting?
- 6. @pfsmet
- 7. Luminous efficacy of radiation (LER) – lumen/Watt V(λ) 400nm 700nm550nm
- 10. 185nm 405nm 365nm Discharge lamps: Na, Hg
- 11. Fluorescent lighting - principle Conversion by photoluminescent materials (hence “fluorescence lamps”)
- 12. Emission spectrum fluorescent lighting 0,0 0,2 0,4 0,6 0,8 1,0 350 450 550 650 750 Emission wavelength (nm) Intensity Tb3+ Tb3+ Eu3+ Hg 254nm (La,Ce)PO4:Tb3+ Y2O3:Eu3+ (BaMgAl10O17:Eu2+) @pfsmet
- 15. °1907 – SiC – yellow emission 1940s: theoretical framework (cfr. transistor, p-n junction) 1955: EL in III-V compounds 1962: IR emission in GaAs (+ laser) 1960s: green and red LEDs based on GaP Blue: predicted in GaN-based LEDs in 1950s! LEDs – direcht conversion of electricity into light
- 16. Nobel prize physics 2014 Isamu Akasaki Hiroshi Amano Shuji Nakamura "for the invention of efficient blue light-emitting diodes (LEDs) which has enabled bright and energy-saving white light sources"
- 17. Light emitting diodes (LEDs) Nobel prize communication p n Direct bandgap needed! Nearly monochromatic light.
- 18. RGB LEDs – the green gap. Source: OSRAM (In,Ga)N (Al,Ga,In)P Green gap
- 21. Materials (2010) 3, 2834 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 400 500 600 700 Normalizedintensity Wavelength (nm) Y3Al5O12:Ce yellow phosphor .new application ..new requirements ...new phosphors Phosphors for LEDs Blue LED
- 23. Tunability of Eu2+ based phosphors Phosphors for LEDs Journal of Luminescence (2003) 104, p. 239@pfsmet
- 24. Phosphors for LEDs Thousands of different phosphors can be made... ... but only a handful are actually used. • garnets (YAG:Ce, LuAG:Ce,...) • silicates ((Ca,Sr,Ba)2SiO4:Eu,...) • oxynitrides (SrSi2O2N2:Eu, SiAlONs,...) • nitrides ((Ba,Sr)2Si5N8:Eu, CaAlSiN3:Eu...)
- 25. 6 (scientific) requirements for LED phosphors • Suitable emission spectrum (peak, width) • High (quantum) efficiency (QY, QE) • Strong absorption at +/- 460nm (EQE) • Chemically stable • Short luminescence lifetime • Thermal stability Phosphors for LEDs + cost, embedding,… ECS Journal of the Electrochemical Society (2011) 158, p. R37@pfsmet
- 26. ECS Journal of the Electrochemical Society (2011) 158, p. R37 Only handful of phosphors are suitable for high brightness LEDs. Two dopants: Ce3+ and Eu2+ 6 (scientific) requirements for LED phosphors • Suitable emission spectrum (peak, width) • Strong absorption at +/- 460nm • High (quantum) efficiency • Chemically stable • Short luminescence lifetime (no saturation) • Thermal stability LED Heat sink Phosphor Phosphors for LEDs
- 27. Nature Materials (2017) 16, p. 500 (Thermal) losses for phosphor converted LEDs Assuming QE for phosphor of 90%, overall luminous efficacy 180 lum/W. @pfsmet
- 29. Lab record – CREE 303 lum/watt IKEA PHILIPS NICHIA Luminous efficacy (LE) Fluorescent lighting threshold DoE
- 31. RE usage • Host: yttrium (YAG), dopants: cerium, europium. • Phosphors and pigments: 7% of RE usage (2013, Roskill) • Key applications (CRT displays, fluorescent (+ Tb3+)) + LEDs • On chip: very low phosphor quantities • In first order, phosphor use does not scale with luminance
- 32. • Narrow red phosphor Chemically stable, thermally stable, with high saturation level. • Narrow banded cyan/green phosphors. • Phosphors for high brightness applications Headlights, projectors. • Improved thermal quenching (quantum deficit) • Rare earth free (?) Key issues in phosphor research
- 33. Red components: Eu2+ doped http://dx.doi.org/10.1155/2014/290952 CaAlSiN3:Eu2+
- 34. Red Mn4+ doped fluoride phosphors K2SiF6:Mn4+
- 36. Synthesis methods HF + KMnO4 @pfsmet
- 38. ECS Journal of Solid State Science and Technology (2016) 5, p. R3040
- 39. Quantum dots as an alternative?
- 40. Colloidal Quantum Dots | what? ̶ 1983, first discovery in Bell Labs ̶ Since 1990’s, accelerated development Size-Tunable Optical Properties 5nm diameter wavelength produced as printable inks CdSe/CdS, InP/ZnSe, perovskites,…
- 41. Lowering synthesis cost of InP QDs Aminophosphines – a new phosphorous precursor Song, J. Nanoparticle Res. 2013 Tessier, Chem. Mater. 2015 Kim, Angew. Chemie Int. Ed. 2016 Buffard, Chem. Mater. 2016 Full Green-to-red color range accessible
- 42. Samsung KS7500 “25% more colour”
- 43. QDs for displays Colour filtering: balance between color gamut and efficiency ߣ (nm) 350 450 550 650 transmission 100% Liquid crystal display Colour filters
- 44. Displays • (AM)OLEDs vs LED backlight Blue OLEDs degrade faster • Main drivers: colour gamut and efficiency • Quantum dots (Cd-free…) vs phosphors (Mn4+) • Electroluminescence from QDs (no filtering, printing,…) Trends
- 45. General lighting • Further efficiency increase needed • Long lifetime required (>50 khrs) • Color quality after brightness increase • Blue LED flux strongly increased • Very competitive market Trends Displays Other applications
- 46. Conclusions • Phosphor converted LEDs : relevant energy saving technology • RE – based phosphors are key elements (Y, Ce, Eu, Tb) • Mass to luminance ratio is decreasing • Some alternative materials available (red Mn4+ fluorides) • Display backlighting: competitive non-RE technologies • Phosphors: recycling is an issue!
- 47. Contact & resources Philippe Smet philippe.smet@ugent.be @pfsmet http://LumiLab.UGent.be @UGentLumiLab Presentation can be downloaded at http://www.slideshare.net/pfsmet Looking forward to your feedback!