Rare-earth Doped Phosphor For LED With Broader Emission Band

White light emitting diode(WLED) are proposed to take the place of conventional light sources for general illumination, due to their promises of huge energy saving, reducing carbon emission, high efficiency, and long lifetime. The most widespread commercial strategy to achieve(WLED) is using a blue LED chip to excite yellow emitting YAG:Ce3+phosphor. The color rendering index(CRI) of such a WLED is low(<80) due to deficiency of red fluorescent component in YAG:Ce3+. To achieve high CRIs(> 80), a red phosphor is blended with a yellow or green one. However, some drawbacks such as fluorescence reabsorption,non-uniformity of luminescence, and time dependent shift of color point arise simultaneously.Higher CRI requires both green, yellow, and red component in the spectrum of a phosphor. This indicates broad-band emission property of the phosphor. A broader band means a balance between green and red component in the spectrum. Thus to achieve high CRIs, An ideal solution is to broaden the emission band of single yellow phosphor so as to enrich the red component. In turn, the color rendering is improved without the drawbacks of phosphor blend. Thus, to find a yellow phosphor with an emission band broader than YAG:Ce3+ is our interested topic.In order to achieve broad-band emission phosphor, two schemes are tried in this dissertation: one method is substituting the rare-earth ion of an already knownphosphor; another method is introducing new optical center into an already known phosphor. Here is the results:1. Synthesis and luminescent properties of a new yellow phosphor Sr Al Si4N7:Ce3+ are reported. In comparison with YAG: Ce3+, the phosphor exhibits smaller thermal quenching and a broader emission band centering at 555 nm with a bandwidth as large as 115 nm. CRI of the w LED fabricated by such a phosphor is 81,CCT is 5624 K, showing much improvement compared with w LED fabricated by YAG: Ce3+. There are two Sr sites in Sr Al Si4N7 lattice. Coordination numbers of both Sr sites are 9. Average Sr1-N and Sr2-N distances are 2.89 ? and 2.91 ?respectively. Sr Al Si4N7:Ce3+appears a broad emission band due to a spectral overlap of the two luminescence centers.Sr Al Si4N7 contains spatial infinite chains of edge-sharing [Al N4] tetrahedra running along [0 0 1] in the lattice. Such an edge-sharing type is really unreasonable.Bound valance sum(BVS) calculation is introduced to show the rationality of the infinite chains of edge-sharing [Al N4] tetrahedra. Results show that [Al N4]tetrahedral shorten the Al-N bond length and realized edge-sharing. However, such a connection type is energy metastability and reduces the stability of Sr Al Si4N7 lattice.Thus edge-sharing [Al N4] tetrahedra is responsible for the preferred orientation.With increasing excessive Al N in raw materials, Sr Al Si4N7:Ce3+ shows better luminescence property. Al3+substituting for Si4+is provided with charge compensation by O2-substituting for N3-, as result of which, blue-shift of PL occurs.Since Al N is proved to be oxidated by residual O2, covering method is introduced to decrease the least excessive Al N required in raw materials from 8 mol% to 4 mol%.2. Eu2+is introduced into Sr Al Si4N7:Ce3+yellow phosphor to broaden the emission band. CRI of w LED fabricated by Sr Al Si4N7:0.05Ce3+,0.01Eu2+increases into 88 without luminous efficiency loss compared with w LED by Sr Al Si4N7:Ce3+.The Ce3+, Eu2+ co-doped Sr Al Si4N7 endows a highly color tunable emission from550 nm(yellow) to 610 nm(red). These properties allow Sr Al Si4N7 with a widespread application prospect. There are three subbands that can be ascribed toCe1, Ce2, and Eu2 in the emission spectrum. Their emission color is green-yellow,orange-yellow, and red respectively. The decrease and increase progress of the three subbands give rise to the red shift of emission. Energy transfer progress between Ce1-Ce2 pair, Ce1-Eu2 pair are observed and proved to be both dipole-dipole interaction. The energy transfer rate of Ce1-Eu2 is proved to be two times of Ce1-Ce2. Thus y=0.035 is calculated to be an important concentration to determine the influence of Ce1-Eu2 energy transfer on Ce1-Ce2 energy transfer. The co-doped Eu2+ ion adjusts the relative quantity of green and red component in emission spectra by Ce1-Eu2 energy transfer progress. Co-doped Eu2+ increases both bandwidth and absorption of the phosphor. Thus w LEDs with Sr Al Si4N7:0.05Ce3+,y Eu2+show increase CRI without luminous efficiency loss. Phosphors with low concentration doped Eu2+ are favorable to application in w LEDs requiring high CRI while heavier doped samples are in favor of w LEDs in need of low CCT.3. Luminescence of Pr3+ in Li2 Sr Si O4 shows a much broader f-f emission band compared with YAG:Pr3+. Eccentric distance and sphericity are introduced to quantify the distortion polyhedron. Results show that [Sr O8] polyhedron in Li2 Sr Si O4 suffers an off-center of Sr and a larger distorted non-spherical-shape distribution of O compared with [YO8] in YAG. Then, Pr3+ is co-doped with Eu2+ in Li2 Sr Si O4 to provide a single phosphor for w LEDs. Energy transfer processes between Eu-Pr is observed. There are two routines for the Energy transfer processes:from 5d of Eu2+to both3P2 or1D2 of Pr3+. Such a progress increase the transition between 1D2®3H4and change the emission shape of Pr3+. Inokuti-Hirayama model is introduced to study the dynamics of energy transfer. Results show that he electric interaction type of Eu-Pr is dipole-quadrupole and Critical energy transfer distance R0 of Eu-Pr transfer is 3.6 ?, much smaller than that of Ce-Pr transfer in YAG.