UV-pumped phosphor-based white LEDs
White LEDs can also be fabricated with optical excitation of phosphor in the ultraviolet (UV) wavelength range (Karlicek, 1999). Semiconductor sources emitting in the near-UV (320390 nm) and in the violet, close the edge of the visible spectrum (390-410 nm) are frequently used for such white sources. Semiconductor diodes emitting near 400 nm with remarkably high efficiencies have been reported (Morita et al., 2004).
For deep-UV semiconductor sources (200-320 nm), conventional phosphors, as used in fluorescent lighting, can be used for wavelength conversion. However, the large Stokes shift associated with deep-UV sources is a significant drawback for such sources. Furthermore the development of deep-UV LEDs is challenging due to the low p-type and n-type doping efficiency in AlGaN with high Al content and the difficulties encountered in epitaxially growing high-quality AlGaN with low dislocation and defect densities.
In UV-pumped white LEDs, the entire visible emission originates in the phosphor. Phosphors excited in the deep UV have been used since the 1950s in fluorescent light tubes and since the 1980s in compact fluorescent lamps (CFLs). Phosphors in fluorescent light sources are pumped by the UV emission coming from the low-pressure mercury-vapor discharge occurring inside the tube. The dominant emission of low-pressure mercury-vapor discharge lamps (Hg lamps) occurs in the UV at 254 nm. Phosphors with strong absorption in this wavelength range are readily available. The color rendering properties of such phosphors are very suitable for most applications.
A white LED using a UV AlGaInN LED pump source and a tricolor phosphor blend was reported by Kaufmann et al. (2001). The LED pump source emitted at 380-400 nm, that is, near the boundary between the visible and UV spectrum. The phosphor blend consisted of three phosphors emitting in the red, green, and blue parts of the spectrum. A color-rendering index of 78 was reported for the lamp.
The color-rendering index (CRI) of UV-excited phosphor mixes ranges between 60 and 100. Excellent CRIs as high as 97 were reported by Radkov et al. (2003) for phosphor blends excited near 400 nm. Furthermore, such UV-LED based sources exhibit independence of the phosphor - emission spectrum on the exact UV-LED excitation wavelength, because the visible emission is solely due to the phosphor. Consequently, UV-pumped white lamps are expected to have a highly reproducible optical spectrum so that “binning” will likely not be required. Monte Carlo simulations reported by Radkov et al. (2004) indeed showed a very low chromaticity point variation (entirely within the first MacAdam ellipse) for phosphor sources excited with a variety of LEDs coming from chip bins with peak wavelengths ranging between 400 nm and 410 nm. The chromaticity variation was shown to be much lower for UV-LED/phosphor sources than for blue-LED/phosphor sources.
A fundamental drawback of UV-pumped white LEDs is the energy loss (Stokes shift) incurred when converting UV light to white light. The potential luminous efficiency of UV - pumped white LED lamps is therefore markedly lower than that of white sources based on a blue
LED exciting a yellow phosphor.