Development And Characteristics Of Novel Phosphate Red Emitting Phosphors For Ultraviolet-near Ultraviolet Based White LEDs

The advent of white light LEDs is a revolution in the field of lighting. Light conversion type white LEDs have attracted extensive attention of the researchers in the field due to the low cost, simple technique, and so on. Thus white LEDs phosphors also become a research hot issue in the field of luminescence. Currently, the lack of efficient and stable red phosphor excited by near ultraviolet and blue light leads to the low color rendering of white LEDs, and thus the popularization of white LEDs is greatly restricted in the field of lighting. This dissertation focuses on the study of red phosphors with SrZn2(PO4)2 and Sr3La(PO4)3 as the host material to improve the luminescent properties of white LEDs by using good thermal, chemical and physical stability of phosphate. A variety of new red phosphate phosphors for white LEDs were developed, such as SrZn2(PO4)2:Eu2+, Sm3+, Sr3La(PO4)3:Eu2+, Sm3+, Sr3La(PO4)3:Sm3+, Eu3+, Sr3La(PO4)3:Eu3+, A+(A= Li, Na, K), Sr3La(PO4)3:Ce3+, Mn2+, Sr3La(PO4)3:Eu2+, Mn2+, which have a wide absorption band in ultraviolet and near ultraviolet region and a strong luminescence intensity. The research results are of a great significance to improve the property of white LEDs so as to promote its popularization in the field of lighting. The main achievements of this dissertation are summarized as follows:(1) An orange red phosphor SrZn2(PO4)2:Eu2+, Sm3+was developed for the first time with SrZn2(PO4)2 as host material by co-doping Eu2+and Sm3+. The phosphor has a wide absorption band in ultraviolet and near ultraviolet region and a strong orange red luminescence intensity. The luminescence properties of the phosphor were deeply investigated. The results show that not only the orange red luminescence intensity is enhanced by the sensitization of Eu2+to Sm3+, but also the absorption capacity of the UV band of 350～380 nm is improved, and moreover the half peak width of the excitation spectrum of 400 nm is widened to about 20 nm. The broadband excitation spectrum is benefit to solve the problem of the instability of the red luminescence intensity caused by the emission wavelength drift of LED chip.(2) An orange red phosphor Sr3La(PO4)3:Eu2+, Sm3+was developed for the first time with Sr3La(PO4)3 as host material by the way of Eu2+and Sm3+co-doped. The phosphor has a wide absorption band in ultraviolet and near ultraviolet region and a strong orange red luminescence intensity. Comparing with SrZn2(PO4)2:Eu2+, Sm3+, Sr3La(PO4)3:Eu2+, Sm3+ has a stronger absorption intensity in the band of 300～380 nm, and a longer excitation wavelength as well as a smaller Stokes shift. Therefore, the luminescent properties of Sr3La(PO4)3:Eu2+, Sm3+ are better than that of SrZn2(PO4)2:Eu2+, Sm3+ from the point of the excitation wavelength and the Stokes shift. In addition, the color coordinates of Sr3La(PO4)3:3%Eu2+,ySm3+(y=5%,7% and 10%) are located at white light area, and the correlated color temperature (CCT) are 4248 K, 4075 K and 3598 K, respectively. The rarely warm whitelight with low color temperature make it potential application in the field of indoor lighting white LEDs.(3) A red phosphor Sr3La(PO4)3:Sm3+, Eu3+ was developed for the first time with Sr3La(PO4)3 as host material by the way of Sm3+ and Eu3+ co-doped. The phosphor has a wide absorption band in near ultraviolet band and a strong red luminescence intensity. The luminescence properties of the phosphor were deeply investigated. The results show that the absorption band (especially 395～405nm) of the phosphor are widened in the near ultraviolet band by the sensitization of Sm3+ to Eu3+. This result demonstrate that the red phosphor has a good match with the existent 400 nm high-power NUV chip. The luminescence intensity of Sr3La(PO4)3:5%Sm3+,7%Eu3+ and Sr3La(PO4)3:7%Sm3+, 5%Eu3+ are stronger than that of the commercial red phosphor Y2O2S:Eu3+. The external quantum efficiency are 41.2% and 45.3%, respectively, which has a great increase comparing with that of Sr3La(PO4)3:Eu2+, Sm3+. Therefore, Sr3La(PO4)3:7%Sm3+, 5%Eu3+ can be used as a new phosphate red phosphor in the field of "near UV+tricolor" white LEDs.(4) In the unequilibrium system of Sr3La(PO4)3:Eu3+, the luminescence intensity of the red phosphor was effectively improved by adding alkali metal ions A+(A= Li, Na, K). The effect of A+ on the unequilibrium substituted luminescence system was clarified by studying the effect of A+ with different radius on the luminescence properties and lattice parameters of the materials. Compared with the red phosphor without A+-doped, the luminescence intensity of the red phosphor with doping 5%Li+,5%Na+,5%K+ is increased to 3.558,1.768 and 1.215 times, respectively. Among them, Sr3La(PO4)3:5%Eu3+,5%Li+ has the strongest luminescence intensity. Its color coordinate is close to that of the commercial red phosphor, such as Sr2Si5N8:Eu2+ and u3+, and a relatively high external quantum efficiency of 33.1% is achieved. Therefore, Sr3La(PO4)3:5%Eu3+,5% Li+ can be used as a new phosphate red phosphor in the field of "near UV+tricolor" white LEDs.(5) A orange red phosphor Sr3La(PO4)3:Ce3+, Mn2+was developed for the first time with Sr3La(PO4)3 as host material by the way of Ce3+and Mn2+co-doped. The phosphor has a wide absorption band in ultraviolet region and a strong orange red luminescence intensity. The excitation peak of Sr3La(PO4)3:Ce3+, Mn2+located at 298nm; The luminescence intensity of Sr3La(PO4)3:0.07Ce3+,0.25Mn2+is significantly stronger than that of Sr3La(PO4)3:0.25Mn2+, which demonstrates that an effective energy transfer from Ce3+to Mn2+occuring; The energy transfer mechanism from Ce3+to Mn2+is deeply studied by measuring the spectral properties of the phosphors. The external quantum efficiencies of the phosphors are measured, and the corresponding maximum is 38.6%. The results show that Sr3La(PO4)3:Ce3+, Mn2+will have a potential application in the field of " UV+tricolor" white LEDs.