Producing white light using near-UV LEDs requires the development of new phosphors, as well as the modification of certain existing ones. In this review, we discuss the luminescent properties of potential phosphors: oxides, silicates, phosphates and nitrides. We evaluate phosphors that employ 4f-5d transitions, line emission, the use of sensitizers and transition metal elements. We include information on the optical transitions and how these can limit the selection of a composition.

As the lighting industry transitions from traditional technologies to solid state lighting (SSL), it appears that the most preferred way to generate white light using SSL technology has been to use phosphor-converted light emitting diodes (pc-LEDs). There has been considerable debate in the literature whether near-UV LEDs or blue LEDs should be used to excite phosphors for white light. Quite often, in the phosphor literature, the efficiencies of LEDs from 365 nm to 470 nm are somewhat neglected in this debate. In this paper, we have provided data on external quantum efficiency of InxGa1-xN LEDs over the above spectral range, and use these data together with phosphor performance to compare near-UV and blue based approaches to making white light pc-LEDs. We also use simulations to discuss white light blends at two different correlated color temperatures (3000 K and 4000 K) for both LED configurations.

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An extension of a theorem for light extraction [Adv. Opt. Technol.2, 291 (2013)] from a higher index luminescent body (LED or phosphor) through an extracting surface into a lower index output medium is derived. The result is valid for both geometric and diffractive surface structures. Using this bound and radiation transport calculations, we show that extraction from LEDs or phosphors requires a combination of cavity effects to enhance radiance behind the extracting surface and scattering or diffraction to couple trapped total-internal-reflection modes to propagating modes. The treatment applies to macroscopic luminescent sources whose thickness exceeds the longitudinal coherence length of the luminescent radiation.

This paper reports on the synthesis of nano- and submicron sized (Ba1-xSrx)2SiO4:Eu2+ (0 ≤ x ≤ 1) green-yellow emitting phosphors prepared by the co-precipitation method. Instead of using water, N,N-dimethylformamide is used as a solvent. X-ray diffraction analysis shows single phase products after post-synthesis annealing at temperatures > 900°C for x = 0. The particles are nearly spherical with a narrow size distribution (100 nm–500 nm) depending on the annealing conditions. The photoluminescence emission spectra consist of a strong broad green band centered between 512 nm–570 nm depending on x. The shift in the peak wavelength is attributed to crystal field effects. The emission intensity was found to increase as the annealing temperature increases most likely due to increased crystallinity. The quantum efficiencies of the phosphors annealed at 1100°C (∼290 nm particle size) and 1150°C (∼460 nm particle size) are 82 and 84%, respectively. Thus, these phosphors have excellent applicability as the green- and/or yellow-emitting component in phosphor blends for white-emitting UV-LEDs.

Using the proper synthesis technique is an important consideration in the development of phosphors for white light emission from near UV LEDs (emission in the range 380 nm – 410 nm). In this review, synthesis methods are analyzed and compared, with the goal of obtaining the optimal particle size, morphology and purity. Post-synthesis treatments such as the fabrication of core/shell particles as well as annealing conditions are discussed. Finally, methods to incorporate phosphors with the diode are discussed, with an emphasis on electrophoretic deposition.

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Light extraction from luminescent materials, where luminescence generated in a high refractive index medium must be coupled to a lower index medium, is a complex problem with significant ramifications for efficient LED and phosphor converted LED lighting. We derive thermodynamic arguments which show that light transmission for incident Lambertian light through arbitrary structured or non-structured surfaces is always limited by the ratio of incident to output étendues. Numerical simulations of various strongly scattering surfaces in the wave-optic regime are made to confirm the surface extraction results. The results are also extended to arbitrary angular radiance distributions of incident light as in Lenef, et al. [Opt. Lett., 39, 3058 (2014)], but also generalized to cases of restricted angular output emission. Furthermore, we show how cavity effects and scattering are both needed to overcome the thermodynamic surface extraction limits. Examples of both surface scattering and a converted LED with volume scattering are given. Finally, a derivation of the Lambertian light transmission theorem is developed from scalar wave-optics to highlight the physics of the thermodynamic result and to provide limits of applicability of the surface extraction limit.