Effect Of Phosphor-Silica-Gel On Phosphor-Converted LED Light Property By Monte-Carlo Method

Phosphor-converted white light emitting diode (LED) is the mainstream of whiteLEDs due to its high efficiency, high reliability and long lifetime. Among which thecomposition of YAG yellow phosphor and blue LED chip are particllarly attractive. As akey converting material, phosphor silica-gel involves the parameters as its geometry,location, inner particle size, distribution and concentration that can directly determine thefinal LED optical qualities. This paper investigated the effect of phosphor-silica-gel onLED efficiency and optical properties.A Monte-Carlo program has been compiled to tracing photons in homogenousmaterials like phosphor-silica-gel. The correctness has been verified by comparing thesimulation results with former experimental data and the error analysis has also beenconducted.Influence of inner particle concentration, phosphor-silica-gel thickness and boundarymaterial on luminous efficiency has been investigated using homogenous Monte-Carlomethod. The results showed an opposite influence when increasing the particleconcentration and phosphor-silica-gel layer thickness. Besides, a great back scattingphenomenon appeared under the situation that no reflector was fixed along thephosphor-silica-gel low boundary. It is also found that covering the phosphor layer directlyon the silica-gel rather than the LED die can effectively reduce the back scatteringphenomenon, thereby can effectively improve the LED color rendering as well as theefficiency.By integrating Mie scattering theory and the algorithm of phosphor-silica-gel opticalparameters, a continuous spectrum Monte-Carlo simulation model aiming at tracingphotons in heterogeneous phosphor-silica-gel has been built.Scattering properties of single particle as well as the whole phosphor-silica-gel hasbeen analyzed using Mie theory. It was found that the scattering cross section for blue lightis about one order of magnitude greater than yellow light. Meanwhile, larger SiO2particlesize would lead to larger scattering coefficient while larger phosphor particle size would result in poorer scattering properties. But both SiO2and phosphor particle sizes have fewimpacts on absorbing coefficient.Effect of phosphor-silica-gel inner particles on LED has been studied. A generalfinding was that with increase of inner SiO2particle concentration, the output light showeddecreased blue-emission intensity but improved yellow-emission intensity. Both thequantum efficiency and color temperature showed obvious downtrend. The similarvariation trend was presents when increase the SiO2size. But when increasing the phosphorparticle size, it showed stronger blue-emission intensity, higher color temperature andefficiency.