Phosphor Converted White Led with Improved CRI

Most of the commercial white LED lamps use blue chip coated with yellow emitting phosphor. YAG:Ce phosphor is coated on blue chip to obtain white light. Though this is commercially successful, there are several drawbacks such as “halo effect”, poor colour rendition, etc. In recent years several efforts have been made to improve LED lamp performance. In this paper modification of YAG:Ce phosphor for improving CRI, by introducing Gd, Pr or Tb at Ce site is reported.


Introduction
Solid state lighting based on high-brightness LEDs has emerged as a new potentially revolutionary technology that could save up to half the energy used for lighting applications.Compared with the traditional lighting, white LED has the following advantages; small (use of many kinds of associations and flexible array device), robust, long lifetime (more than 10,000 hours), low power consumption and low pollution.Thus, White LED would be the most important light source in the 21 st century [1].
White LEDs, based on blue LED chips coated with a yellow emitting phosphor (YAG:Ce), were first reported in 1997 [2].The blue chip/YAG:Ce system has many advantages.But the lamps fabricated in this manner give a poor colour rendering because the resulting light is typically deficient in the green and red colours.There are two approaches which are being followed to overcome this problem.In the first approach, white LEDs are made by coating near ultraviolet (n-UV) emitting LED with a mixture of high efficiency red, green and blue emitting phosphors [3], analogous to the fluorescent lamp.This method yields lamps with better colour rendition.Addition of a yellow emitting phosphor improves colour rendition index (CRI) further.Recently, much progress has been made for the emission efficiency of LED chips in the near UV-to-deep blue range [4] [5] [6] [7].However, cost and light output wise, near UV chips are much inferior to blue chips.Thus, the second approach [8] comprising of coating blue excitable, green and red phosphors on blue chip, is considered to be more practicable for the time being.At present, LED lamps with YAG:Ce phosphor coated on the blue chip are dominating the market.In this paper modification of YAG:Ce phosphor for improving CRI, by introducing Gd 3+ , Pr 3+ or Tb 3+ at Ce 3+ site is reported.

Experimental
Instead of the conventional solution combustion synthesis, we used the modified procedure which led to the formation of the desired compounds in a single step.
Reagent grade (Indian Rare Earths, Ltd.) rare earth oxides/carbonates were converted to the corresponding nitrates by dissolving in nitric acid.The nitrates were dried by prolonged, gentle warming.Stoichiometric amounts of hydrated nitrates of yttrium, aluminium and cerium were thoroughly mixed with urea/ glycine.The nitrate to fuel ratios were calculated by the method described earlier [9] [10].Table 1 gives the details of the ingredients used in syntheses of various phosphors.
Due to the presence of large water of crystallization in aluminium nitrate, a thick paste was formed.A china dish containing the paste was inserted in a furnace preheated to 500˚C.Within minutes the paste foamed and a flame was produced which lasted for several seconds.The china dish was immediately removed from the furnace.X-ray diffraction patterns were recorded on Philips PANalytical X'pert Pro diffractometer.PL characteristics in the range of 200 -700 nm, at room temperature were studied using a Hitachi F-4000 spectrofluorimeter, with 1.5 nm spectral slit width.

Results and Discussion
Yang et al. [11] prepared YAG using combustion synthesis with urea as a fuel.
However, the combustion product was admixture of YAG and YAP.Repeated sintering for 5 hrs at 1000 C was necessary to obtain YAG.More or less similar results were obtained by Pan et al. [12].Fu et al. [13] used carbohydrazide fuel, but annealing at 1150 C for 6 hours was required to achieve good crystallinity and PL intensity.In our experiments, when urea was used as a fuel, the combustion products were poorly crystallized.Yttrium nitrate does not have exothermic reaction with urea.Hence we tried mixed (Glycine + urea) fuel.Glycine has exothermic reaction with Yttrium nitrate and urea with aluminum nitrate.XRD pattern of the product obtained with this fuel is shown in Figure 1.
Phase pure YAG is thus obtained in a single step combustion process without any additional thermal treatment when mixed fuel (Glycine + urea) was used.
Activation with Ce 3+ was also attempted by adding cerous nitrate to the combustion mixture.
Figure 2 shows PL results for the phosphor so prepared.For 465 nm excitation, intense emission was observed with a broad maximum at 528 nm and  The shift is related to the lattice expansion caused by substituting smaller Y ion by bigger rare earth ions.In Pr 3+ co-doped sample, there is partial energy transfer from Ce 3+ to Pr 3+ .Pr 3+ emission corresponding to

Conclusion
Using Gd or Tb co-dopants, Ce 3+ emission in YAG:Ce could be shifted to longer wavelengths.The shift is due to lattice expansion.On the other hand, co-dopant

Figure 1 .
Figure 1.XRD Results for YAG XRD data for YAG is compared with the ICDD data file 88-2048.