RU2315078C2 - Photoluminophores for short-wave light-emitting diodes (led) - Google Patents

Abstract

FIELD: inorganic chemistry, electronic technique and light technique.

SUBSTANCE: invention relates to photoluminophores based on calcium and strontium silicates activated with rare-earth element ions of the stoichiometric formula: a x Σ[(CaCO)_1-x-y(SrO)_x(TR²⁺O)_y] x b.SiO₂ wherein a = 1, 2 and 3; b = 1, 2 and 3; x = 0.01-0.8 mole part; y = 0.001-0.05 mole part, and TR²⁺ represents Yb²⁺ or Eu²⁺ and Sm²⁺. Luminescence intensity value is up to 170 relative units, maximal post-luminescence period is 2.5 mcs, not above. Proposed photoluminophores show enhanced chromatic index of white color as compared with industrial produced color diode.

EFFECT: improved and valuable properties of photoluminophores.

1 tbl, 2 dwg, 12 ex

Description

Application. The invention relates to the field of materials for electronic engineering and lighting engineering, specifically to light-transforming phosphors used in the production of various types of light-emitting diodes. This area of technology begins to develop rapidly and has even received the special name "semiconductor lighting" after the pioneering work of VS Abramov and S. Nakamura et al. [1]. Violet laser and its use, Springer, Berlin, 1997 [2].

Thanks to the creation of intensely emitting heterojunctions from JnN-GaN, the blue glow of which can excite the yellow-orange light of a phosphor powder from yttrium aluminum garnet, it is possible to synthesize very bright white radiation. In this case, the production of white color is based on the principle of Newton's complementary colors, according to which a part of the blue color not absorbed by the phosphor is mixed in the necessary proportions with the yellow-orange luminescence of crystallophosphorus and creates a total white glow. The color temperature of this total white color is from 12,000 to 2,500 K.

For similar white LEDs, somewhat later, in 1999, it was proposed to use a yellow phosphor based on yttrium aluminum garnet activated by cerium, the chemical formula of which is usually written as (Y _1-x Ce _x ) ₃ Al ₅ O ₁₂ [3 ].

This well-known phosphor - analogue has been repeatedly modified, but its parameters changed insignificantly:

- the position of the spectral maximum of radiation λ from 545 to 560 nm;

- the half-width of the spectral maximum from 120 to 130 nm;

- a material the refractive index η _n-pa = 1.92;

- the color of the grains of the material is intensely yellow.

- the average diameter of the grains of the phosphor d ₅₀ from 2 microns to 10 microns.

Such a phosphor made it possible to obtain a bright white glow with luminous intensity I 1-1.5 cd in nitride indium gallium LEDs (LED excitation mode U = 3.6 V j = 20 mA). Despite the widespread use of the known photoluminophore based on yttrium aluminum garnet, there were significant disadvantages, which are as follows:

- a very wide spectrum of radiation, reaching in the short wavelength region up to x = 420 nm (at the level of 1/10 of the maximum height) and up to λ = 750 nm in the long wavelength region;

- the intense color of the color of the powder Fl, which causes intense loss of exciting blue;

- a very narrow width excitation spectrum of PL, which does not allow the use in the production of the entire set of manufactured crystals for LEDs.

Attempts to eliminate the disadvantages of the known garnet-based photophosphors led to the creation of a new photophosphor based on the strontium orthosilicate Sr ₂ SiO ₄ activated by divalent Eu ⁺² , the formula of which is written in the form [5] in the form Sr _2-x SiO ₄ · Eu _x where 0.001≤x <1. Recently discovered strontium phosphor emits in the region of 520-550 nm, while the main excitation in it falls on wavelengths λ in the subrange from 380 to 420 nm. According to the authors of the patent application [5], their phosphor is not inferior in effectiveness to the known composition based on yttrium-aluminum garnet. One can agree with the opinion of patent applicants, but with a number of significant additions. First of all, the total luminous efficiency of the combination of LED + photoluminophore in integrated light will be significantly lower in comparison with the known combination of InGaN (λ = 450 nm) + Y ₃ Al ₅ О ₁₂ Се (λ = 550 nm). This is explained by the fact that the light efficiency of LEDs emitting in the region of λ = 380-420 nm is 10 times lower in comparison with the light efficiency of these devices at λ = 450-465 nm.

Secondly, the emission band of silicate Фl is not wider than 90-100 nm, which leads at λ _max = 550 nm to the practical absence of the red and dark red radiation necessary for white balance.

Thirdly, as follows from the data of the well-known prototype patent, for the silicate photoluminophore, the dependence of the position of the spectral maximum on the concentration of the introduced activator is very strong, i.e. Eu ⁺² . With an increase in the concentration of Eu ⁺² from x = 0.001 to x = 0.1, the shift in the position of the maximum is Δ = 550 nm - 520 nm = 30 nm. In this case, the patent applicants are silent about the fact that with the same change in Eu ^{+ 2} concentrations, a very sharp decrease in the luminosity of the phosphor occurs, which is well known from the theory of luminescence as a phenomenon of concentration quenching.

In addition, while working on the invention, we showed that the very narrow extremum of excitation for Sr ₂ SiO ₄ Eu does not allow it to be used in combination with longer wavelength LEDs, for example, based on InGaN with λ _radiation = 440-460 nm.

The purpose of the invention.

The basis of the present invention is the creation of a new family of silicate photophosphors having high light efficiency in a wide spectral range of excitation by LEDs not only from wide-gap gallium nitride, but also more convenient for producing blue-colored indium gallium nitride.

Disclosure of the invention.

New photoluminophors for short-wavelength emitting diodes are proposed, characterized in that they are based on complex silicates of Ca and Sr activated by doubly charged ions from the series of lanthanides [TR ²⁺ ], having the general stoichiometric formula

a · Σ [(CaO) _1-xy (Sr ₂ O) _x (TR ²⁺ O) _y ] · b (SiO ₂ ),

wherein

a = 1, 2, 3,

A = 1, 2, 3,

x = 0.01-0.8 molar fractions,

y = 0.001-0.05 molar fractions,

TR ²⁺ = Yb ²⁺ or Eu ²⁺ + Sm ²⁺ .

In accordance with the invention, a whole homologous series of silicate bases for photoluminophores is proposed, the cationic sublattice of which is composed of oxides of divalent elements of calcium, strontium, europium, samarium, ytterbium.

In accordance with the invention, the following materials are included in this series of photophosphors:

ΣMe ⁺² О · SiO ₂ а = 1, b = 1 or MeSiO ₃

Σ (Me ^{+ 2} O) ₂ · SiO ₂ a = 2, b = 1 or Me ₂ SiO ₄

Σ (Me ^{+ 2} O) ₃ · SiO ₂ a = 3, b = 1 or Me ₃ SiO ₅

Σ (Me ^{+ 2} O) ₂ · (SiO ₂ ) ₂ a = 2, b = 2 or Me ₂ Si ₂ O ₆

S (Me ^{+ 2} O) ₃ · (SiO ₂ ) ₂ a = 3, b = 2 or Me ₃ Si ₂ O ₇

Σ (Me ^{+ 2} O) ₃ · (SiO ₂ ) ₃ a = 3, b = 3 or Me ₃ Si ₃ O ₉

Each of the compounds of the proposed series has its own emission characteristics according to the excitation and emission spectra, according to the dependence of the luminescence intensity on the concentration of activating impurities, however, common to all the proposed photoluminophores is a single mechanism of luminescence excitation through a charge transfer band formed by a divalent rare-earth ion, such as Eu ^{+ 2} , Sm ⁺² , Yb ⁺² . Common for all photoluminophors is also the observed long-wavelength shift of the maxima in the emission and excitation spectra of the phosphors when some of the calcium ions are replaced by large ions of strontium, europium (+2), samarium (+2). ytterbium (+2). Also common is the very short afterglow of each of the proposed photoluminophores, which is τ _e = 1.10 ^-6 sec and decreases with the additional introduction of samarium and / or ytterbium ions into the photoluminophore.

This series of photoluminophors also has the same optimal content of the main activator, europium ion, which is in the range from 0.01 molar fraction to 0.03 molar fraction of Eu with respect to the sum of divalent metals that form the basis of the cationic sublattice of the photoluminophore composition.

Important in the invention is that a number of photoluminophores for short-wavelength light emitting diodes are obtained in one way, which consists in heat treatment of a mixture of oxygen-containing salts of calcium and strontium, such as, for example, carbonates or oxalates, or formates, nitrate salts of europium, samarium and ytterbium with fine silica in a reducing atmosphere with a controlled content of water vapor. Our proposed method is common for the reagents used, for the temperature range used (T = 1200-1400 ° C), for the value of the recovery potential of the gaseous medium (5% H ₂ + 95% N ₂ ), for the content of H ₂ O vapor in the calcining atmosphere . This method also allows the formation of uniform dispersion grains of the proposed photophosphors, for example, in the size range from 4 to 12 microns. The high uniformity of the particle size distribution of the entire series of photophosphors allows one to compose two- or three-component mixtures of silicate photophosphors, which, depending on their particular composition, have special spectral characteristics of radiation and excitation. The uniformity of the grains makes it possible to form polymer suspensions with a high filling of photoluminophores from them.

A brief description of the drawings.

On figa shows the excitation spectrum of the phosphor (Ca, Sr) ₂ SiO ₄ · Eu · Sm

Figure 1c shows the excitation spectrum of the phosphor (Ca, Sr) ₂ SiO ₄ · Eu · Yb

On figa, in the emission spectra of the phosphors (Ca, Sr) ₂ SiO ₄ · EuSm and (Ca, Sr) ₂ SiO ₄ · Eu · Yb.

The best options for implementing the invention.

In the process of working on the invention, we synthesized specific luminescent materials.

Example 1

Mix 0.6 M CaCO ₃ , 0.38 M SrCO ₃ , 0.002 M Eu (NO ₃ ) ₃ · 6H ₂ O; 0.018 M Sm (NO ₃ ) ₃ · 6H ₂ O and 1.0 M SiO ₂ (fine silica with a specific surface area S = 6 m ² / gram). Chda grade acetone is added to the mixture for better homogenization. After drying at 80 ° C for 2 hours, the mixture is transferred to a 1 liter quartz crucible and loaded into a controlled atmosphere furnace. At the first stage of the heat treatment process, a nitrogen stream N ₂ with a controlled amount of 5% H _{2 is} fed into the furnace. The temperature of the first stage is 950 ° C, the exposure time is 2 hours. Then the temperature is raised to 1250 ° C (rise rate 10 ° / minute) and the mixture is held for 3 hours, bringing the gas mixture to 2 liters / minute. The dew point of the nitrogen-hydrogen mixture is controlled at the outlet of the furnace and is + 10 ° C. The flow of the mixture is reduced to 1 liter / minute at a temperature of 900 ° C and completely stopped at T = 450 ° C; cooled to 25 ° C, the product is washed with hot water, dried at T = 120 ° C for 2 hours and sieved through a 400 mesh sieve.

The resulting photoluminophore powder of the composition (CaO) _0.6 (SrO) _0.38 (EuO) _0.02 · SiO ₂ has a white color with a slightly bluish tint. When excited by ultraviolet light with λ = 365 nm, the color of the phosphor is violet-blue with λ _rad = 450 nm. Preliminary X-ray diffraction studies indicate the formation of a compound with a monoclinic crystal lattice. The luminous intensity of the obtained product is 85% of the known reference sample based on BaMgAl ₁₀ O ₁₇ Eu.

Example 2. Mix 0.84 M CaCO ₃ (purity 99.95%) 0.14 M SrCo ₃ (purity 99.9%) 0.015 M Sm (NO ₃ ) ₃ · 6H ₂ (in the form of a 5% solution). 0.005 M Eu (NO ₃ ) ₃ · 6Н ₂ О (as a 5% solution). 0.5 M fine silica (99.99% purity) is introduced into the charge and ground in a planetary mill with the addition of 200 ml of isopropyl alcohol (2000 rpm, 10 minutes). The suspension is dried in a quartz cuvette at T = 90 ° C for 2 hours, after which it is loaded into a quartz open cuvette with a volume of 2 liters. Heat treatment of the charge is carried out at T = 1000 ° C for 2 hours (at the first stage of the process) and at T = 1300 ° C for 4 hours. The flow rate of the nitrogen-hydrogen mixture is 4 l / min. The product is cooled together with the furnace to 100 ° C, reducing the rate of entry of the hydrogen mixture to 1 l / min. The product is washed with hot water, dried at 110 ° C for 3 hours and sieved through a 500 mesh sieve.

The photophosphor powder has a yellowish-cream color on reflection, the color of the glow of its excitation is UV with λ = 365 nm, yellow-orange with a wavelength of spectral maximum λ = 560 nm. When excited in a collapsible cathode ray tube, the light output of the sample is 40 lm / W at chromaticity coordinates x = 0.415 y = 0.500. The average grain diameter d _cp = 6 μm (as measured by the SKC-2000 photosedimentograph). The radiographically determined structure refers to the α _n rhombic form of alkali metal orthosilicates with parameters a = 5.75 Å, b = 9.6 Å, and c = 6.6 Å.

We have prepared a suspension of photoluminophore in an optical epoxy compound (phosphor content of 55%). The suspension was applied to the planar surfaces of a GaN LED (λ _rad = 450 nm) with a load of up to 2 mg / cm ² . After polymerization for 1 hour at T = 120 ° C, the device was filled with an organosilicon compound and lighting parameters were measured on it. The luminous intensity was I _av = 380 millikandel for an exit angle of 20 = 106 °, color coordinates x = 0.345; y = 0.38, supply voltage U = 3.28 V; I = 20 μA. After 100 hours of continuous operation, the power of the emitted light remained virtually unchanged. The standard white LED made in Japan was under these conditions I = 355 mcd under equal excitation conditions.

Example 3. Mix 0.2 M CaCO ₃ , 0.76 M SrCo ₃ 0.04 M Eu (NO ₃ ) ₃ · 6H ₂ O, 0.001 M Yb (NO ₃ ) ₃ · 6H ₂ O, adding 100 ml to the mixture H ₂ O, after which 0.34 M finely divided silica is introduced into the resulting paste (S _beats = 2 m ² / g). Drying is carried out at 130 ° C for 3 hours. The mixture in a quartz crucible is loaded into the furnace, the temperature of which at the first stage is 1050 ° С - 1 hour, then 1200 ° С - 4 hours. The feed rate of the mixture of N ₂ / N ₂ is 2 liters / minute. The product is cooled, washed with hot water, dried and sieved through a 500 micron sieve. The reflection color of the powder is pink-yellow, when excited by UV light with λ = 365 nm, orange-yellow. Color coordinates when excited by an electron beam x = 0.44; y = 0.55, the wavelength of the spectral maximum λ = 575 nm, the light output was 27 lm / W. A definite crystalline structure is trigonal. When filled with a suspension from a phosphor, samples of LED crystals from InGa-N created a luminous intensity I = 420 μd for 20 = 106 ° at the chromaticity coordinates of white radiation x = 0.316 y = 0.328. The Fresnel lens cover above the LED provided a uniform white glow without the blue central “gallo” visible on imported LEDs.

Examples 4-12. The synthesis of photophosphors was carried out in the modes given for 1-3 compositions. The compositions of the synthesized materials are shown in the table.

Comparison of the data on the integral white luminescence indicates a significantly higher value of the total color index of white in comparison with industrially produced white LEDs, in which the use of a standard photoluminophore from garnet provides white with a color index of Rn = 75 units.

Industrial application.

The claimed invention can find great application in electronic and lighting technology. The synthesized photophosphors can be used to produce LEDs of white and warm-white glow colors. Similar LEDs are beginning to be widely used for special lighting in systems of traffic lights, bulbs, etc.

Information sources

1. Abramov B.C. LEDs and Lasers, collection No. 1-2, 2002, pp. 64-68.

2. S. Nakamura. Violet laser and its application. Berlin, 1997.

3. S. Schimizu US. Pat 599892.1999.

4. PCT WO 2004/056940 A1.

Claims (1)

Photophosphors for short-wavelength light-emitting diodes based on calcium and strontium silicates activated by rare-earth ions, characterized in that they have a stoichiometric formula a · Σ [(CaO) _1-xy (SrO) _x (TR ²⁺ O _y ] · b · SiO ₂ , in which a = 1, 2 and 3, b = 1, 2 and 3, x = 0.01-0.8 molar fractions, y = 0.001-0.05 molar fractions, and TR ⁺² represents Yb ⁺² or Eu ⁺² and Sm ⁺² .

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