White LEDs based on semiconductor converters (PRS-LED)
Light-emitting diodes using semiconductor wavelength converters have been demonstrated by Guo et al. (1999). The schematic structure of the photon-recycling semiconductor LED (PRS- LED) is shown in Fig. 21.13. The figure indicates that a fraction of the light emitted by the blue GaInN LED is absorbed by a AlGaInP secondary active region and re-emitted (or “recycled”) as lower-energy photons. In order to obtain white light, the intensity of the two light sources must have a certain ratio that will be calculated below. The schematic power budget of the device is shown in Fig. 21.14. It is assumed that the electrical input power is P0, and the output powers in the blue and amber spectral range are P1 and P2, respectively. The power-conversion efficiency of the blue LED and the photon-recycling semiconductor are assumed to be n and n2, respectively. The efficiency and luminous efficiency of the device are calculated below.
The energy loss occurring in the photon-recycling process must be taken into account when determining the optimum choice of wavelengths for highest efficiency. Note that energy is lost even if the recycling process occurs with unit quantum efficiency. To calculate the optimum wavelength of operation, we represent white light by the Illuminant C standard, for which the chromaticity coordinates are xc = 0.3101, yc = 0.3163, zc = 0.3736. Using these chromaticity coordinates, the pairs of complementary wavelengths can be determined.