Modifications On The Photoluminescence And Stability Properties Of White Light Emitting Diode Phosphors Through Atomic Layer Deposition Technique

Due to the advantages of energy-saving,environmental-friendly,high efficiency and long lifetime,white light-emitting diodes(wLEDs)has become a promising star for current lighting and display techinique.Phosphors play an important role in the luminous efficiency,color rendering,correlated color temperature and lifetime of wLEDs devices.During the practical application of wLEDs,a certain amount of heat will be generated,thus the phosphors will be aged under the effect of heat toghther with the surrounding air,water and other factors,leading to the decline of the luminescence intensity of phosphors and the performance and lifetime of the devices.Therefore,the aging of phosphors is a main problem that is urgent to be solved for wLEDs devices.The object of this study is to improve the anti-aging performance of phosphors by surface modification so as to improve the reliability and lifetime of the devices.Red Sr2Si5N8:Eu2+(SSN)nitride phosphor has been selected as an excellent commercial LED phosphor on account of its broad-band excitation and high quantum yield,however,its thermal stability(especially with the presence of moisture)is quite poor that needs to be further improved.Red K2GeF6:Mn4+(KGF)fluoride phosphor has the advantages of wide-band excitation and narrow-band emission,ultra-high quantum yield and superior thermal quenching property,which promise it great potentials to be used in high color rendering index and luminious efficiency wLEDs,however,its poor moisture resistance limits its practical application significantly.Meanwhile,cyan BaSi2N2:Eu2+(BSON)(oxy)nitride phosphor with high quantum yield and narrow half-width,make it sutiable to be used in super-high color rendering index wLEDs,yet it suffers from poor hydrothermal stability.The object of this work is to improve the stability and reliability of the above mentioned three kind of phosphors through surface modification technique.Coating an organic or inorganic layer on the phosphor surface has been proved that can improve the phosphor stability.However,it is difficult for conventional coating methods(such as sol-gel,coprecipitation,emulsion,etc.)to form uniform and completely closed layer on the phosphor particle surface and precisely control the coating thickness,which make it difficult to give full play to these surface modification technique.Therefore,in this work,atomic layer deposition based on a fluidized bed reactor(FB-ALD)was utilized to deposite uniform,closed and controllable layer on the phosphor particles,and the influence and mechanism of surface coating on the phosphor optical and stability properties and the reliability of the fabricated LED devices etc.were studied.Firstly,a nano-scale Al2O3 coating layer was successfully deposited on SSN particles through FB-ALD technique under different process conditions.The coating parameters were optimized using different oxidizers,reactant dosage,deposition temperature and number of coating cycles.A uniform and conformal coating layer was achieved when using O3 as oxidizer,and the thickness of the coating layer decreased with increasing the deposition temperature and increased with increasing the number of coating cycles.With increasing of the deposition temperature and number of coating cycles,the emission intensity and quantum yield of the coated sample decreased,and the sample surface became hydrophobic gradually.Thermal gravimetric analysis results showed that the oxidation temperature of the coated SSN phosphor increased from 700? to 850?,suggesting the coating layer has a function of anti-oxidation.Under 200? in air for 2 h,the uncoated SSN phosphor was degraded owing to the oxidation of the host lattice and forming an amorphous layer on the particle surface,leading to the decline of the luminescence intensity,while the dense Al2O3 coating layer can prevent the oxidation of the host efficiently and thus to improve the anti-thermal degradation property of coated SSN phosphor greatly.Based on the luminscence and thermal degradation properties of SSN phosphor,the optimum deposition temperature of Al2O3 was in the range of 100-150?,and the number of deposition cycles is in the range of 15-30 cycles(coating thickness:-8-10 nm)at 100 ?.Moreover,after 1500 h,the relative luminous flux of the fabricated LED with uncoated SSN decreased by 15%,while the luminous flux of the LED fabricated with coated sample remained stable.indicating that coating an Al2O3 layer on SSN phosphor surface can benefit the practical application of the fabricated LED devicesSecondly.a uniform inorganic Al2O3 coating was successfully deposited on the KGF phosphor surface using ALD technique.The thickness of the coating layer increased with incrasing the number of coating cycles,and the deposition of different thickness of Al2O3 layer has no obvious influence on the profiles of the excitation and emission spectra of KGF.The emission intensity and quantum yield decreased 5-9%after coating ascribed to the reduced absorption of light,and the thicker the coating layer,the lower emission intensity and quantum yield.Under the attack of water.the uncoated KGF phosphor was ionized,the ionized[MnF6]2-was hydrolyzed to MnO2 hydrate,and the morphology of the KGF particles was re-constructed,thus the emission intensity of KGF was decreased.With the protection of Al2O3 coating,the hydrolysis of KGF can be resisted,and the water resistance of KGF phosphor was improved remarkably.The quantum yield of the uncoated KGF decreased from 87%to 55%after immersed in water for 2 h,while it just decreased from 81%to 71%for the Al2O3 coated sample.Moreover,after 80 h,the relative luminious flux of the LED devices fabricated with Al2O3 coated KGF was improved from 35%to 72%compared with the uncoated one.Thirdly,different thickness of SiO2 and Al2O3 coating layers were successfully deposited on BSON particle surface through FB-ALD technique,respectively.After SiO2 coating,the emission intensity and quantum yield of BSON decreased 1-4%,the oxidation temperature of the coated phosphor increased from 550? to 630?,and the water-assisted thermal degradation was reduced.The oxidation temperature and the water-assisted thermal degradation property did not show big difference as the coating thickness increased from 4.5 nm to 7 nm.Compared with the uncoated BSON,the relative luminious flux of the LED device fabricated with SiO2 coated BSON was improved from 35%to 49%after 200 h.After Al2O3 coating,the emission intensity and quantum yield of BSON decreased 4-7%compared with the uncoated sample,while the oxidation temperature of the coated phosphor increased from 550? to 750?.Under 200?in high-preesure water steam for 24 h,the uncoated BSON phosphor host was destroyed,N was substituted by O,NH3 gas was released,and the Eu2+was oxidized,leading to the decline of the emission intensity,while the Al2O3 coating can prevent these to be happened and thus to improve the water-assisted thermal degradation property of BSON phosphor greatly.The oxidation temperature of BSON phosphor was increased and the water-assisted thermal degradation property was improved gradually with increasing the coating thickness.Moreover,compared with the uncoated BSON,the relative luminious flux of the LED device fabricated with Al2O3 coated BSON was improved from 22%to 73%after 300 h.