Phosphors Trends

8/10/2019 Phosphors Trends

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L. Krishna BharatElectronics and Radio Engineering Department

Optoelectronics and Nanodevice Lab


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Introduction

Phosphor, most generally, is a substance that exhibits the phenomenon of

luminescence. Somewhat confusingly, this includes both phosphorescentmaterials, which show a slow decay in brightness (> 1 ms), and fluorescentmaterials, where the emission decay takes place over tens of nanoseconds.

Phosphors are often transition metal compounds or rare earth compounds ofvarious types.


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Fundamental Aspects

Photoluminescence

Energy transfer

CIE chromaticity coordinates and color temperature

CRI (Color Rendering Index)


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Photoluminescence

Luminescence is defined as the emission ofelectromagnetic radiation in excess of thermalradiation

The PL in a material usually involves the followingprocesses

excitation of the activator to a higher energy state (10-11s);

relaxation of the activator ion to the lowest energy level of theexcited state (10-8s); and

emission of a lower-energy photon as the activator ion returnsfrom its excited state to the ground state (10-910 s).


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Energy Transfer

Donor

Acceptor

Transfer Probability

PDA= 2/||2 gD(E)gA(E)dE


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CIE Chromaticity Coordinatesand Color Temperature

The Commission Internationale de lEclairage(CIE) system.

By a piece of mathematic legerdemain, it is necessaryonly to quote the quantity of two of the referencestimuli to define a color since the three quantities(x, y, z) are made always to sum to 1.

Color temperature (Kelvin) is the absolute temperatureat which a blackbody radiator must be operated to have achromaticity equal to that of the light source.

Incandescent bulb color temperature is the temperature of the filament.

Color temperatures of the pc-WLEDs is Correlated Color Temperature (CCT).

Low color temperature - warmer(more yellow-red) light.

High color temperature colder(more blue) light.


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CRI (Color Rendering Index)

CRI (Ra) is a measure of the degree to which theperceived colors of objects illuminated by the sourceconform to those of the same objects illuminated by a

reference source for specified conditions.


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Material Synthesis and Fabrication

Solid StateReaction

SolutionBasedChemicalSynthesis

Hydrothermal Synthesis

Sol-Gel Synthesis

Chemical Co-Precipitation

Combustion Synthesis

Spray Pyrolysis

Simple and suitable for mass productionLimitations Poor homogeneity and high calcination temperatures Relatively large size and less controllable

Difficulty in preparation of single phase compounds Easily introduce additional impurities and defects

which greatly reduces luminous efficacy


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Lanthanides (or) Rare Earths andits applications


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Emissions from rare earths

Europium red(3+), blue(2+)

Samarium red

Terbium green

Erbium green

Dysprosium blue

Thulium blue

Cerium blue


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Different host materials with rareearth doping


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YAG:Ce phosphor particles preparedby ultrasonic spray pyrolysis

1400oC


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Phosphors excited by blue-LEDs


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Blue LED Blue LED Blue LED


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Phosphors excited by UV-LEDs


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Comparison


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conclusion

Documents

Phosphors Trends