CONVERSION ELEMENT, RADIATION-EMITTING SEMICONDUCTOR DEVICE

AND METHOD FOR PRODUCING A CONVERSION ELEMENT

The invention relates to a conversion element, a radiation- emitting semiconductor device and a method for producing a conversion element.

An object to be solved is to provide an improved conversion element for radiation-emitting semiconductor devices. Another object to be achieved is to specify a method by means of which a conversion element can be produced.

A conversion element is specified. The conversion element is intended to convert electromagnetic primary radiation of a first wavelength range into electromagnetic secondary

radiation of a second wavelength range. The conversion element may in particular be formed as a conversion layer which can be applied onto a transparent carrier or a

semiconductor light-emitting chip.

According to one embodiment, the conversion element contains a matrix material. The matrix material is preferably

permeable or transparent to electromagnetic radiation, in particular visible light.

According to one embodiment the conversion element contains particles of a phosphor. The particles of the phosphor preferably convert electromagnetic primary radiation of a first wavelength range into electromagnetic secondary

radiation of a second wavelength range. Here it is possible that the conversion element contains particles of a single kind of phosphor or that the conversion element contains particles of at least two different kinds of phosphor. The different kinds of phosphor are then intended to convert the primary radiation into secondary radiation of different wavelength ranges, e.g. colors.

According to one embodiment the conversion element contains particles of the phosphor, wherein the particles are embedded in the matrix material. For example, the conversion element consists of the matrix material and the embedded particles of the phosphor or phosphors. The fact that the particles are embedded means, for example, that most of the particles, e.g. at least 90 % of the particles, are completely covered by the matrix material at their outer surface. In this case the matrix material is located between different particles and the matrix material mediates a mechanical connection between the particles.

According to one embodiment, the matrix material comprises a colorless polyimide. Preferably, the matrix material consists of a colorless polyimide. 'Colorless' means that no or hardly any electromagnetic radiation in the visible range is

absorbed by the material. In particular, the colorless polyimide is clear and transparent and does not color the passing electromagnetic radiation.

Polyimide is a polymer of imide monomers. Advantageously, polyimides can be highly heat-resistant and are used in a wide variety of applications where robust organic materials are required, for example applications at high temperatures. The advantages of the polyimides are good thermal stability, good chemical resistance and excellent mechanical properties. According to one embodiment the matrix material is adapted to have a refractive index of at least 1.5. The refractive index describes how light propagates through a medium. Preferably the refractive index determines the extent to which the path of light is bent or refracted when entering the material. The refractive index of at least 1.5 is obtained at a wavelength of 589 nm.

According to one embodiment the matrix material is adapted to have a glass transition temperature of at least 200 °C. If the glass transition temperature is exceeded, a solid glass or polymer turns into a rubbery to viscous state. The glass transition temperature ranges from at least 200 °C to 360 °C. The glass transition temperature can be determined, for example, by differential scanning calorimetry (DSC) . The heat capacity is recorded as a function of the temperature. The heat capacities of liquid and glassy phases differ, with a continuous transition near the glass transition temperature. The high glass transition temperature leads to a high

chemical and thermal resistance.

According to one embodiment the conversion element contains a matrix material and particles of a phosphor, wherein the particles are embedded in the matrix material and the matrix material comprises a colorless polyimide with a refractive index of at least 1.5 and the matrix material has a glass transition temperature of at least 200°C.

According to one embodiment the colorless polyimide has the following general formula:

wherein the substituents Ri to R are each independently selected from the group comprising:

- aromatic hydrocarbons,

- cyclic hydrocarbons,

- aliphatic hydrocarbons,

- heteroatoms .

Here and in the following the symbol * stands for binding sites of the repeating structural unit which are connected to the next repeating structural unit of a polymer chain forming the polyimide. In addition, at the end of the polymer chain the polymer is terminated by end groups which may, for example, be selected from the same group of substituents as the substituents Ri to R . But other end groups are also conceivable. In the formula n indicates the repeating

structural unit, wherein n can e.g. be selected from a range between at least 10 to at most 10000. In the formula the dashed line between R and R can be, for example, a covalent bond or no bond. Preferably, heteroatoms can be part of the aromatic hydrocarbons and/or cyclic/aliphatic hydrocarbons. Moreover, double bonds or triple bonds are also possible in the cyclic or aliphatic hydrocarbons.

The hydrocarbon substituents may be, for example, saturated, unsaturated, normal, branched or cyclic hydrocarbon

substituents, in particular alkyl substituents. But also aromatic substituents are conceivable. The colorless

polyimide preferably contains two imide groups.

The surprising advantage of using the colorless polyimide as the matrix material is its good UV-radiation stability, which prevents browning of the conversion element during use e.g. in connection with a blue-light-emitting device like a light- emitting diode or a laser diode. Furthermore, the colorless polyimide being used as the matrix material leads to a long lifetime of the conversion element, and a high moisture resistance can be obtained. In addition, the properties of the matrix material are easily tailored by selecting various types of monomers. This leads to a higher flexibility and hardness of the matrix material.

As an example of the colorless polyimide the following polyimide with the formula can be used as the matrix

material :

According to one embodiment the colorless polyimide is synthesizable or synthesized from a polyamic (acid) precursor which has the following general formula:

wherein the substituents Ri to R are each independently selected from the group comprising:

- aromatic hydrocarbons,

- cyclic hydrocarbons,

- aliphatic hydrocarbons,

- heteroatoms .

In the formula n indicates the repeating structural unit, wherein n can e.g. be selected from a range between at least 10 to at most 10000. In the formula the dashed line between R and R can be, for example, a covalent bond or no bond. Preferably, heteroatoms can be part of the aromatic

hydrocarbons and/or cyclic/aliphatic hydrocarbons. Moreover, double bonds or triple bonds are also possible in the cyclic or aliphatic hydrocarbons.

The hydrocarbon substituents may be, for example, saturated, unsaturated, normal, branched or cyclic hydrocarbon

substituents, in particular alkyl substituents. But also aromatic substituents are conceivable.

Starting from the polyamic (acid) precursor, the colorless polyimide can be synthesized through a polycondensation. In the polycondensation of the polyamic (acid) precursor, the free electron pair of the nitrogen atom reacts with the carbonyl atom to loose small molecules, preferably water, to obtain the colorless polyimide.

The properties of the polyamic (acid) precursor are easily tailored by selecting various types of monomers with

different substituents. This results in a higher flexibility and hardness of the synthesized matrix material. As an example of a polyamic (acid) precursor the following polyamic (acid) with the formula can be used:

Preferably monomers used for the polyamic (acid) precursor are alicyclic dianhydrides and fluorine-containing groups.

One example of the monomer for forming the polyamic (acid) precursor is represented by the following formula:

The polyamic (acid) precursor is synthesized from the monomers by polycondensation. During polycondensation monomers join together, losing small molecules as byproducts, preferably water, to obtain the polyamic (acid) precursor.

Especially preferred, the monomer has at least two functional groups that are particularly reactive. The functional groups may be for example -OH, -COOH, N¾, CHO. The synthesis of the polyimide via the polyamic (acid) precursor is a two-step synthesis. However, a synthesis route starting from the monomer to obtain the colorless polyimide in one reaction without any intermediate states, i.e. a one-step synthesis, is also possible. According to one embodiment of the conversion element the phosphor is selected from the group comprising ceramic phosphor and/or quantum dots.

For example, one of the following materials is suitable for the particles of phosphor: rare earth doped garnets, rare earth doped alkaline earth sulfides, rare earth doped

thiogallates , rare earth doped aluminates, rare earth doped silicates, rare earth doped orthosilicates, rare earth doped chlorosilicates , rare earth doped alkaline earth silicon nitrides, rare earth doped oxynitrides, rare earth doped aluminum oxynitrides, rare earth doped silicon nitrides, rare earth doped sialons, or quantum dots.

Possible materials for the phosphors are in particular, but not exclusively the following aluminum-containing and/or silicon-containing particles of phosphor:

(Bai- _x-y Sr _x Ca _y ) Si0 ₄ :Eu ²⁺ (0 < x < 1, 0 < y < 1), (Ba _4-X _

ySr _x Ca _y ) ₃ Si0 ₅ : Eu ²⁺ (0 < x < 1, 0 < y < 1), Li ₂ SrSi0 ₄ : Eu ²⁺ ,

Ca ₈ Mg (Si0 ₄ ) ₄ C1 ₂ :Eu ²⁺ , Oxo-nitrides like (Bai- _x-y Sr _x Ca _y ) Si ₂ 0 ₂ N ₂ : Eu ²⁺ (0 < x < 1; 0 < y < 1), SrSiAl ₂ 0 ₃ N ₂ : Eu ²⁺ , Ba _4-x Ca _x Si ₆ ON ₁₀ : Eu ²⁺ (0 < x < 1), (Ba _4-x Sr _x ) Y ₂ Si ₂ Al ₂ 0 ₂ N ₅ :Eu ²⁺ (0 < x < 1), Sr _x Si ₍₆ - _y) Al _y O _y N _(8-y) : Eu ²⁺ (0.05 < x < 0.5; 0.001 < y < 0.5),

Ba ₃ Si ₆ 0 ₁₂ N ₂ :Eu ²⁺ , Si _6-Z A1 _Z 0 _Z N _8-Z : Eu ²⁺ (0 < z < 0.42), M _x Si _12-m _ _n Al _m+n O _n Ni _6-n : Eu ²⁺ (M = Li, Mg, Ca, Y; x = m/v; v = Valence of M, x < 2), M _x Sii _2-m-n Al _m+n O _n Ni _6-n : Ce ³⁺ , AE _2-x-a RE _x Eu _a Sii- _y 0 _4-x-2y N _x (AE = Sr, Ba, Ca, Mg; RE = rare earth metal element) , AE _2-X _ _a RE _x Eu _a Sii- _y 0 _4-x-2y N _x (AE = Sr, Ba, Ca, Mg; RE = rare earth metal element) Ba ₃ Si ₆ 0 ₄₂ N ₂ : Eu ²⁺ or nitrides like La ₃ Si ₆ Nn : Ce ³⁺ , (Ba _4-X - _y Sr _x Ca _y ) ₂ Si ₅ N ₈ :Eu ²⁺ , (Ca _4-x-y Sr _x Ba _y ) AlSiN ₃ : Eu ²⁺ (0 < x < 1; 0 < y < 1), Sr (Sr _4-x Ca _x ) Al ₂ Si ₂ N ₆ :Eu ²⁺ (0 < x < 0.2), Sr(Sr ₄ _ _x Ca _x ) Al ₂ Si ₂ N ₆ :Ce ³⁺ (0 < x < 0.2) SrAlSi ₄ N ₇ : Eu ²⁺ , (Ba _4-X _

ySr _x Ca _y ) SiN ₂ :Eu ²⁺ (0 < x < 1; 0 < y £ l), (Ba _4-x-y Sr _x Ca _y ) SiN ₂ : Ce ³⁺ (0 < x < 1; 0 < y < 1), ( Sri_ _x Ca _x ) LiAl ₃ N ₄ : Eu ²⁺ (0 < x < 1),

(Bai- _x-y Sr _x Ca _y )Mg ₂ Al ₂ N ₄ :Eu ²⁺ (0 < x < 1; 0 < y < 1), (Ba _4-X _ _y Sr _x Ca _y ) Mg ₃ SiN ₄ : Eu ²⁺ (0 < x < 1; 0 < y < 1).

Quantum dots may comprise a core and a shell ("core-shell quantum dot"), wherein both the core and the shell comprise or are formed of a semiconductor material. The bandgap of the shell is usually adjusted by the material and the size so that the shell absorbs the electromagnetic radiation of the excitation wavelength. The core of the quantum dot is usually adjusted via the material and the size so that it emits at least part of the energy absorbed by the electromagnetic primary radiation of the first wavelength range as

electromagnetic secondary radiation of the second wavelength range .

The quantum dots may have a core of CdSe, InP, InGaAs, GalnP, CuInSe ₂ with a diameter of e.g. at least 2 to at most 10 nm. This allows the emission spectra to be defined. The core can be surrounded by a CdS or ZnS shell, which defines the optical absorption and protects the core.

Further, a radiation-emitting semiconductor device is

specified. The radiation-emitting semiconductor device in particular comprises a herein described conversion element. Hence all features disclosed for the conversion element are also disclosed for the radiation-emitting device and vice versa . According to one embodiment the radiation-emitting semiconductor device comprises a radiation-emitting

semiconductor element. The radiation-emitting semiconductor element, such as a light-emitting diode chip or a laser diode chip, has an epitaxially grown semiconductor layer sequence with an active zone, which is suitable for generating

electromagnetic radiation.

According to one embodiment the radiation-emitting

semiconductor device comprises a conversion element. The conversion element is arranged to emit secondary radiation of the second wavelength range, which is different from the first wavelength range. The conversion element is preferably arranged downstream of the radiation-emitting semiconductor element. The conversion element is set up to generate a partial conversion or a full conversion. This is particularly dependent on the phosphor material used and the thickness of the conversion element. "Downstream" means that at least 50 %, in particular at least 85 % of the radiation emitted by the radiation-emitting semiconductor element enters the conversion element.

The conversion element can be designed as a layer, which for example is in direct contact with the radiation-emitting semiconductor element. In addition, the conversion element may be in the form of a cladding in which the radiation- emitting semiconductor element is at least partially or completely embedded. It is also possible that the conversion element is arranged at a distance from the radiation-emitting semiconductor element.

The conversion element is in particular a herein described conversion element. The here described radiation-emitting semiconductor device is particularly suitable for the use in LED applications, in particular in LED Display applications.

Furthermore, a method for producing a conversion element is provided. Preferably by means of the method described herein, the described conversion element can be produced. This means that all features disclosed for the conversion element are also disclosed for the method for producing the conversion element and vice versa.

According to one embodiment of the method for producing a conversion element a polyamic (acid) precursor in a solvent is provided. The polyamic (acid) precursor is an organic polymer which is dissolved in the solvent. The solvent is preferably a liquid. The solvent is used to solve reactants to react together. The addition of the polyamic (acid) precursor into the solvent preferably takes place under an inert atmosphere, for example nitrogen atmosphere.

According to one embodiment of the method, particles of a phosphor are introduced in the solvent. For example, the particles of the phosphor are added in one step or are added stepwise to the solvent. The addition of the particles of the phosphor into the solvent preferably takes place under the inert atmosphere, for example nitrogen atmosphere.

According to one embodiment of the method, a mixture

comprising the polyamic (acid) precursor and the particles of the phosphor in the solvent is obtained. Preferably the mixture consists of the polyamic (acid) precursor and the particles of the phosphor in the solvent. The concentration range of the particles of the phosphor to the polyamic (acid) precursor is preferably between 5 % and 80 %.

According to one embodiment of the method, the mixture is cured to form the conversion element. The curing comprises a partial or complete removal of the solvent, in the course of which the imidization to a colorless polyimide occurs.

Thereby, the cured conversion element can be obtained. The curing can occur in one method step or in at least two method steps. This means that the mixture can be pre-cured at a first temperature in a first step and e.g. finally cured at a higher second temperature in a following method step.

According to one embodiment the method for producing a conversion element comprises providing a polyamic (acid) precursor in a solvent, introducing particles of a phosphor in the solvent, wherein a mixture comprising the

polyamic (acid) precursor and the particles of the phosphor in the solvent are obtained and the mixture is cured to form the conversion element. The whole production of the conversion element preferably takes place under an inert atmosphere, for example nitrogen atmosphere. Thereby it is possible, but not necessary, to conduct the method for producing a conversion element in the given order.

According to one embodiment of the method, the polyamic (acid) precursor has the following general formula:

wherein the substituents Ri to R ₃ are each independently selected from the group comprising:

- aromatic hydrocarbons,

- cyclic hydrocarbons,

- aliphatic hydrocarbons,

- heteroatoms .

Here, n indicates the repeating structural unit, wherein n can be selected from a range between at least 10 and at most 10000. In the formula, the dashed line between R ₂ and R ₃ can be, for example, a covalent bond or no bond. Preferably, heteroatoms can be part of the aromatic hydrocarbons and/or cyclic/aliphatic hydrocarbons. Moreover, double bonds or triple bonds are also possible in the cyclic or aliphatic hydrocarbons .

The hydrocarbon substituents may be, for example, saturated, unsaturated, normal, branched or cyclic hydrocarbon

substituents, in particular alkyl substituents. But also aromatic substituents are conceivable.

According to one embodiment of the method, a viscosity of the mixture is at most 1000 cP. Preferably, the viscosity is at most 150 cP. As an example the viscosity of the mixture is between 50 cP and 150 cP. The low viscosity can be achieved preferably by solving the mixture in the solvent.

The advantage of this low viscosity is the good mixing opportunities of the reactants. In addition, the low

viscosity leads to a better coating of the radiation-emitting semiconductor elements. Further, the low viscosity allows for a spray coating of the mixture e.g. directly onto the

radiation-emitting semiconductor element. According to one embodiment of the method, after adding the particles of the phosphor the mixture is continuously stirred to homogenization. Due to the continuous mechanical stirring, the particles of the phosphor can be well homogenized with the matrix material. For homogenization a magnetic stirrer is used. The low viscosity leads to a better and cheaper

homogenization in comparison to a material with high

viscosity as the low viscosity allows for the use of the magnetic stirrer. Preferably, the particles of the phosphor are added before the curing process begins. The reason for this is that the homogenization is enhanced due to the lower viscosity before curing.

According to one embodiment of the method, the solvent has a boiling point in the range between 90 °C to 200 °C.

Preferably more than one solvent can be used. Especially preferably a solvent mixture can be used. Preferably, the solvent is selected from a group of polar solvents. Polar solvents have as substituents at least one functional group. The solvent molecule shows a dipole moment, which leads to an increase of the boiling point. For example, y-butyrolactone, pyridines, acetic anhydride and I\/-methyl-2-pyrrolidone or mixtures thereof can be used as solvents.

According to one embodiment of the method, the mixture is pre-cured at a first temperature ranging between 90 °C and 130 °C. Preferably, by the pre-curing, part of the solvent is evaporated and the imidization of the polyamic (acid)

precursor starts to take place. The pre-curing can occur for example before the mixture is applied on the radiation- emitting semiconductor element or when it is applied on the radiation-emitting semiconductor element. The pre-curing is performed because it is gentle on the mixture. Another advantage is that by the pre-curing a part of the solvent is gently evaporated. Only at elevated temperatures does the mixture cure or imidize. Thus, no pot life limitation occurs.

According to one embodiment of the method the mixture is cured at a second temperature ranging between 130 °C and 200 °C. The curing can be done stepwise, starting at a temperature as low as 130 °C and reaching up to a temperature as high as 200 °C to complete the imidization and formation of a colorless polyimide phosphor layer without any residues of solvents. For example the curing occurs on the radiation- emitting semiconductor element to form preferably the cured conversion element. An advantage is that only at elevated temperatures the mixture cures or imidizes. Thus, no pot life limitation occurs. Furthermore, when the imidization takes place before the mixture is attached on the radiation- emitting semiconductor device, the adhesion between the conversion element and the radiation-emitting semiconductor device gets poorer.

According to one embodiment the mixture is spray-coated using a spray-coating station. The mixture is spray-coated, for example, on the radiation-emitting semiconductor element or a, e.g. transparent, carrier to obtain preferably an

enveloping conversion element or a foil. The spray-coating station is a device with which the mixture can be spray- coated in a controlled manner on the radiation-emitting semiconductor element.

According to one embodiment the pre-curing occurs at the spray-coating station. Preferably, the mixture is heated to the pre-cure temperature and at the same time the mixture is spray-coated on the radiation-emitting semiconductor element. Thereby part of the solvent can be removed and the

imidization to the polyimide starts.

An advantage of the present conversion element is a high UV- stability, which helps avoid browning of the conversion element. Moreover, one advantage is that the properties of the conversion element can be chosen through different types of monomers to form the colorless polyimide.

In addition, the conversion element shows low thermal

expansion at elevated temperatures, which is helpful during the curing of the conversion element. Furthermore, the conversion element is extremely moisture-resistant and no pot life limitation occurs as the precursor will only cure or imidize, when subjected to the elevated temperature.

Further advantageous embodiments and developments of the conversion element, the radiation-emitting semiconductor device and the method for producing the conversion element will become apparent from the embodiments described below in connection with the figures.

In the figures:

Figure 1 shows a schematic sectional view of a conversion element according to an exemplary embodiment;

Figures 2 and 3 show a schematic sectional view of a

radiation-emitting semiconductor device according to an exemplary embodiment; and Figures 4, 5, 6 and 7 show schematic sectional views of various process stages of a method for producing a conversion element and applying the conversion element to a radiation- emitting semiconductor device according to an embodiment.

In the exemplary embodiments and figures identical or identically acting elements can each be provided with the same references. The illustrated elements and their

proportions to each other are not to be regarded as true to scale but individual elements such as layers, components and areas may be oversized for better representability and/or better understanding.

Figure 1 shows a schematic sectional view of a conversion element 1 according to an exemplary embodiment comprising a matrix material 2 and particles of a phosphor 3, wherein the particles are embedded in the matrix material 2. The matrix material 2 comprises or consists of a colorless polyimide.

The matrix material 2 has a refractive index of at least 1.5 obtained by a wavelength of 589 nm. Furthermore, the matrix material 2 has a glass transition temperature of at least 200 °C. The phosphor 3 is selected from the group comprising ceramic phosphor or quantum dots. The phosphor 3 is adapted to convert electromagnetic primary radiation of a first wavelength range into electromagnetic secondary radiation of a second wavelength range.

The exemplary embodiment illustrated in Figure 2 shows a radiation-emitting semiconductor device 5. The radiation- emitting semiconductor device 5 comprises a radiation- emitting semiconductor element 6 and a conversion element 1. The radiation-emitting semiconductor element 6 can be a light-emitting diode chip or a laser diode chip having an epitaxially grown semiconductor layer sequence with an active zone, which is suitable for generating electromagnetic radiation .

The conversion element 1 is attached in the shape of a foil or a layer downstream of the radiation-emitting semiconductor element 6. By way of example, the conversion element 1 is arranged in direct contact with the radiation-emitting semiconductor element 6. The thickness of the conversion element 1 is dependent on the application of the device. The radiation-emitting semiconductor element 6 emits in operation electromagnetic primary radiation of a first wavelength range. The conversion element 1 converts electromagnetic primary radiation of the first wavelength range into

electromagnetic secondary radiation of a second wavelength range. Depending on the phosphor 3, the density of the phosphor 3 in the conversion element 1 and the thickness of the conversion element 1, the conversion element 1 is adapted to partly or completely convert the electromagnetic primary radiation of the first wavelength range into electromagnetic secondary radiation of the second wavelength range.

Figure 3 differs from figure 2 only in the arrangement of the conversion element 1 on the radiation-emitting semiconductor element 6. The conversion element 1 of this exemplary

embodiment surrounds the radiation-emitting semiconductor element 6. In this exemplary embodiment the radiation- emitting semiconductor element 6 is embedded into the

conversion element 1.

In the following formulas the preparation of two exemplary monomers is shown. The used solvents are AC2O = acetic anhydride, dioxane, ethyl acetate, toluene and NaOH = sodium hydroxide .

In the following formulas an exemplary two-step synthesis of a colorless polyimide is shown. The monomers, synthesized above, are used for the production of the polyamic (acid) precursor 4, first step. The monomers condense with a

bridging molecule under a nitrogen atmosphere in a polar solvent 7 to obtain after several hours the polyamic (acid) precursor 4. In a second step the polyamic (acid) precursor 4 reacts via an intramolecular polycondensation reaction to the colorless polyimide. The second step occurs, as well as the first step, under a nitrogen atmosphere and is finished after several hours. The reaction time depends on the reaction temperature. The used solvents 7 are NMP = N-methyl-2- pyrrolidine, GBL = g-butyrolactone, AC2O and pyridine.

Furthermore, an exemplary synthesis route starting from the monomer, synthesized as shown above, to obtain the colorless polyimide in one reaction without any intermediate states, is shown in the following formulas. The monomers condense with a bridging molecule under a nitrogen atmosphere in a polar solvent 7 to obtain after six to eight hours at temperatures between 150 °C to 180 °C the colorless polyimide. The used solvents 7 are NMP = iV-methyl-2-pyrrolidine, n-cresol and isochinoline .

According to the described method firstly a solvent 7 is provided, figure 4.

The solvent 7 is selected from a group of polar solvents 7. Polar solvents 7 have as substituents at least one functional group. For example, g-butyrolactone, pyridines, acetic anhydride and N-methyl-2-pyrrolidone or mixtures thereof can be used as solvents 7.

In a next method step the particles of the phosphor 3 and the polyamic (acid) precursor 4 are provided. A mixture 8

comprising the polyamic (acid) precursor 4 and the particles of the phosphor 3 in the solvent 7 is obtained. The

polyamic (acid) precursor 4 shows a low viscosity of at most 1000 cP. Due to the low viscosity of the mixture 8,

homogenization with a magnetic stirrer is improved, figure 5. The mixture 8 is continuously stirred to homogenize the particles of the phosphor 3 in the polyamic (acid) precursor 4 in the solvent 7. The polyamic (acid) precursor 4 is the starting material for the matrix material 2 of the colorless polyimides .

According to figure 6 the mixture 8, comprising the

polyamic (acid) precursor 4 and the particles of the phosphor 3, is spray-coated directly on the radiation-emitting

semiconductor element 6. For the spray-coating a spray coating station 9 is used. The spray-coating station 9 is a device with which the mixture 8 can be spray-coated in a controlled manner on the radiation-emitting semiconductor element 6. A pre-curing of the mixture 8 occurs at the same time as the mixture 8 is spray-coated on the radiation- emitting semiconductor element 6. The temperature for pre curing of the mixture 8 is in a range between 90 °C and

130 °C. Part of the solvent 7 is evaporated and this leads to a beginning imidization of the polyamic (acid) precursor 4.

According to a following method step the curing of the mixture 8 is performed, figure 7. The mixture 8 is cured at a temperature ranging between 130 °C and 200 °C to evaporate all of the solvent 7, and the imidization to the colorless polyimide structure will be completed. The conversion element 1, comprising the matrix material 2 and the particles of the phosphor 3, is in direct contact to the radiation-emitting semiconductor element 6.

The product resulting from the method for producing a

conversion element 1 and subsequent application to a

radiation-emitting semiconductor element 6 of Figure 7 corresponds, for example, to the exemplary embodiment

according to figure 2 and figure 3. In both examples a radiation-emitting semiconductor element 6 is shown which has the conversion element 1 downstream. The conversion element 1 comprises the particles of the phosphor 3 and the matrix material 2. The features and embodiments described in connection with the figures can be combined with each other according to further embodiments, even if not all combinations are explicitly described. Furthermore, the embodiments described in

connection with the figures may alternatively or additionally comprise further features as described in the general part.

The invention is not limited by the description based on the embodiments of this, rather the invention encompasses any novel features as well as any combination of features, which includes in particular any combination of features in the patent claims, even if this feature or combination itself is not explicitly stated in the patent as an exemplary

embodiment .

References

1 conversion element

2 matrix material

3 particles of a phosphor

4 polyamic (acid) precursor

5 radiation-emitting semiconductor device

6 radiation-emitting semiconductor element 7 solvent

8 mixture

9 spray-coating station