White LEDs based on dye converters
White LEDs can also be fabricated using organic dye molecules as wavelength converter materials. The dyes can be incorporated in the epoxy encapsulant (Schlotter et al., 1997). Dyes can also be incorporated in optically transparent polymers.
A drawback of organic dyes is their finite lifetime. Dye molecules “bleach out”, i. e. become optically inactive, after a certain number of photon absorption events. Typically a dye molecule is stable for about 104-106 optical transitions (Jones, 2000). The lack of high molecular stability of dyes is a serious drawback. The lifetime of dyes is considerably shorter than the lifetime of semiconductor or phosphor wavelength converters.
Dyes have a relatively small difference between the absorption and the emission band (Stokes shift). For example, the Stokes shift for the dye Coumarin 6 is just 50 nm, as discussed earlier in this chapter. This shift is smaller than the Stokes shift required for dichromatic white LEDs that need typical wavelength shifts of 100 nm or more, as inferred from the separation of complementary wavelengths.