Development Of Transmission-type Measuring System For The Temperature-dependent Characteristics Of Phosphors In LEDs

With the phosphor as an integral part of phosphor-converted LEDs(pc-LEDs),its performance has greatly affected the light color quality of the LED lamp.In the actual working process of pc-LEDs,the phosphor is often coated on the LED chip through a dispensing process.Therefore,the phosphor may be thermally quenched due to the increasing junction temperature of the LED chip,which seriously affects LED lamps'parameters such as the light efficiency,the color quality,and the device lifetime.Based on this fact,it is of great significance to study the temperature-dependent performance of the phosphor-silicone mixture.However,most of current systems for testing the temperature-dependent performance of phosphors have generally adopted a reflection-type structure,which do not coincide with the actual working mode of phosphors in pc-LEDs.Therefore,this paper proposes a transmission-type measuring system for determining the temperature-dependent spectral characteristics of phosphors.By using this system,the temperature-dependent spectral characteristics of the phosphor-silicone mixture based on YAG:Ce3+ and CaAlSiN3:Eu2+ have been studied,and the effect of different contents of the silicone gel in the mixture is also analyzed.The research content involves:1.A transmission-type measuring system and the related measuring method for the temperature-dependent spectral characteristics of phosphors have been proposed for the first time.The test system adopts a transmission-type structure,which is quite in line with the working mode of pc-LEDs.The spectral distribution of the phosphor or the phosphor-silicone mixture at different temperatures can be accurately measured by employing this system.In the comparative experiments,parameters of the light conversion efficiency and the external quantum efficiency are used as two indicators,and the comparison shows that the reflection one has overestimated the true luminescent performance of the fluorescent materials.In the transmission-type testing system,we have found that both the light conversion efficiency and the external quantum efficiency of the sample with high phosphor concentrations at the same temperature are lower,indicating that the different phosphor concentrations in the phosphor-silicone mixture have different influences on the emitting light.In addition,when the heating temperature rises,the light conversion efficiency and the external quantum efficiency of both YAG:Ce3+-based mixture and CaAlSiN3:Eu2+-based mixture slightly decrease.2.The testing system proposed in this paper is used for temperature-dependent spectral tests of the YAG:Ce3+ yellow phosphor layers with 20wt%,33wt%and 50wt%phosphor concentrations.The experimental data show that in the process of heating-temperature rise,three concentrations of YAG:Ce3+ phosphor-silicone mixture have all undergone the temperature attenuation,mainly manifested as the drop of the spectral peak and the luminous intensity of the mixture.This is because the increase of the temperature enhances the probability of the non-radiative transition of the luminescent-center ions,so that the phosphor emission is suppressed.The decrease of the luminous intensity curve of the samples with low phosphor concentration shows that the silicone-gel content does influence the thermal stability of the mixture.And the spectral parameters of the mixture have also changed,such as the red shift of the peak wavelength and the broadening of the full-width at half-maximum.Because the color coordinates of YAG:Ce3+phosphor-silicone mixture drift relatively slowly with the increasing temperature,it iFplies that the YAG:Ce3+-based mixture has shown excellent thermal stability.3.The CaAlSiN3:Eu2+ red phosphor-silicone mixture with the same concentration gradients as mentioned above are used as samples,and the transmission-type testing system is used for carrying out the experiment.Their temperature-dependent characteristics similar to those of the YAG:Ce3+ samples are found.Compared with the YAG:Ce3+ yellow phosphor-silicone mixture,their luminescence intensities have shown greater decrease with the increasing temperature.When samples'temperature increases,the luminescence spectra's peak wavelength of CaAlSiN3:Eu2+ phosphor-silicone mixture has shown blue shift,which is opposite to the trend for the YAG:Ce3+ phosphor samples.This phenomenon is caused by the tunneling effect of activated ions.When the temperature rises,the internal electrons are more likely to catch the thermal phonons.The transition of internal electrons between the lower energy level and the higher energy level of the excited state occurs,eventually resulting in a blue shift of the peak wavelength.From the viewpoint of color coordinates,as samples' temperature changes,the CaAlSiN3:Eu2+ phosphor sample has a larger color coordinate shift than the YAG:Ce3+ phosphor sample does.This fact tell us that when nitride phosphors are used to encapsulate LED devices,the influence of the color drift caused by this phosphor should be carefully considered.In brief,this paper puts forward a kind of transmission-type testing system that measures temperature-dependent spectral characteristics of phosphors or phosphor-silicone mixtures for the first time.The system is of great significance for accurately characterizing the spectral characteristics of phosphors and other light color characteristics.