Phosphor

A phosphor is a luminescent solid, usually an inorganic host crystal doped with a small concentration of an activator ion, that absorbs energy and re-emits it as light at a longer wavelength. The excitation may be ultraviolet or blue light, an electron beam (cathode-ray tubes), or X-rays (scintillator screens). The most widely used white-LED phosphor is cerium-doped yttrium aluminium garnet, Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce), which converts part of the 450 nm emission of a gallium nitride LED into a broad yellow band peaking near 555 nm; the mixture appears white. Good commercial phosphors have internal quantum yields above 90%, and the Ce³⁺ emission decays in about 60 ns.

Despite the name, most modern phosphors are fluorescent in the photophysical sense: LED phosphors based on Ce³⁺ and Eu²⁺ emit through allowed transitions within nanoseconds to microseconds. Their emission is a form of photoluminescence, or cathodoluminescence under electron excitation. The name comes from the Greek phosphoros, light-bearer, applied in the 17th century to glowing materials such as the Bologna stone before the element phosphorus was named; the element's own glow is chemiluminescence and unrelated.

Activators and emission

The emission colour is set by the activator and tuned by the host lattice:

• Ce³⁺ and Eu²⁺: allowed 5d→4f transitions give broad bands (around 100 nm wide) with lifetimes of tens of nanoseconds to about a microsecond. The crystal field of the host shifts the band; Eu²⁺ in CaAlSiN₃ emits near 650 nm, in other hosts in the blue or green.
• Eu³⁺ and Tb³⁺: shielded 4f→4f transitions give narrow lines, red near 611 nm for Eu³⁺ and green near 545 nm for Tb³⁺, with millisecond lifetimes. These were the workhorses of fluorescent lamps and CRT screens.
• Mn⁴⁺: narrow red lines near 630 nm in fluoride hosts such as K₂SiF₆ (KSF), used in wide-gamut displays, with decay times of several milliseconds.

Conversion efficiency

A converted photon always carries less energy than the absorbed one. The Stokes efficiency is the ratio of wavelengths:

$$\eta_{\text{Stokes}} = \frac{\lambda_{\text{abs}}}{\lambda_{\text{em}}} = \frac{450}{555} = 0.81.$$

Each 2.76 eV blue photon becomes a 2.23 eV yellow one, and the 0.52 eV difference becomes heat in the phosphor. The total conversion efficiency multiplies this by the internal quantum efficiency $\Phi$ of the phosphor. As a worked example, if a phosphor layer absorbs 60% of 1 W of blue light and has $\Phi = 0.95$, it emits $0.6 \times 0.81 \times 0.95 = 0.46$ W of yellow light, 0.40 W of blue passes through, and 0.14 W is dissipated as heat in the layer. The blue-to-yellow ratio, and so the colour temperature, is set by the phosphor thickness and concentration.

Measurement

The absolute quantum yield of a phosphor powder or film is measured in an integrating sphere: the sample is illuminated with monochromatic light, and a calibrated spectrometer records both the reduced excitation peak (giving absorbed photons) and the emission band (giving emitted photons). Excitation and emission spectra are taken with a spectrofluorometer, the decay time with pulsed excitation and time-correlated photon counting or a fast photodiode, and thermal quenching by repeating the measurement on a heated stage from room temperature to 200 °C or more.

Where phosphors are used

Phosphor-converted white LEDs dominate general lighting and display backlights; blue-pumped YAG:Ce alone gives a colour rendering index typically in the 70s, and adding a red nitride phosphor raises it above 90 at some cost in efficacy. Laser-excited remote phosphors, pumped by a blue laser diode, give very high luminance for automotive headlamps and projectors. Other uses include X-ray intensifying screens (Gd₂O₂S:Tb), persistent afterglow materials such as SrAl₂O₄:Eu²⁺,Dy³⁺ that glow for hours after exposure, and infrared viewer cards that make otherwise invisible laser beams visible for alignment.

Pitfalls

Phosphor efficiency drops with temperature (thermal quenching), and the Stokes heat is generated in the phosphor itself, so high-power and laser-pumped designs are often limited by phosphor temperature. At high excitation density, long-lifetime phosphors saturate because a large fraction of activators are already excited: a millisecond Mn⁴⁺ phosphor saturates at far lower irradiance than a 60 ns Ce³⁺ one. Some nitride and fluoride phosphors degrade in humidity. Emission spectra shift and broaden with temperature, so colour measured at room temperature may differ from colour in operation.

Common questions

How does a white LED make white light?

A blue InGaN chip excites a yellow-emitting phosphor such as YAG:Ce. The unconverted blue and the yellow emission together stimulate all three cone types and look white. High-colour-rendering LEDs add green and red phosphors.

What is the difference between a phosphor and a fluorescent dye?

A phosphor is usually an inorganic crystal with dopant ions; a fluorescent dye is an organic molecule, usually in solution or a polymer. Phosphors are generally more stable under intense light and heat, while dyes offer a wider choice of absorption bands and are used for labelling.

Why do some phosphors glow in the dark?

Persistent phosphors contain traps that capture excited electrons; thermal energy releases them slowly over minutes to hours, and each release produces emission from the activator. Ordinary lighting phosphors have no such traps and stop emitting within microseconds to milliseconds.

References: S. Shionoya, W. M. Yen (eds.), Phosphor Handbook (CRC Press, 1998); G. Blasse, B. C. Grabmaier, Luminescent Materials (Springer, 1994); T. Matsuzawa et al., J. Electrochem. Soc. 143, 2670 (1996).