WO2019025392A1

WO2019025392A1 - Quantum yield recovery

Info

Publication number: WO2019025392A1
Application number: PCT/EP2018/070644
Authority: WO
Inventors: Arjan Meijer; Christian MATUSCHEK; Itai Lieberman
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2017-08-03
Filing date: 2018-07-31
Publication date: 2019-02-07
Anticipated expiration: 2020-02-03
Also published as: TW201925417A

Abstract

The present invention relates to a process for quantum yield recovery of an optical medium.

Description

Quantum yield recovery

Field of the Invention

The present invention relates to a process for quantum yield recovery of

5

an optical medium. The present invention further relates to an optical medium obtained or obtainable by the process for quantum yield recovery and to the use of such an optical medium in an optical device.

10 Background Art

Semiconducting nanocrystals such as quantum dots, quantum rods, etc. are of great interest as color converter materials in optical devices such as light emitting diodes (LEDs) and liquid crystal displays (LCDs) due to their

^ narrow fluorescence emission.

Their widespread use has been restricted however by their sensitivity to oxygen and water, especially under high temperature and high light intensity condition, for example exposure to Vacuum Ultra-Violet (VUV)

20

light. Luminescent nanocrystals undergo (oxidative) damage when exposed to oxygen, moisture, temperature and/or high intensity light resulting in a significant loss or drop of their quantum yield (QY).

25 Several approaches to better protect nanocrystals have been described.

For example, encapsulation of the semiconducting nanocrystals in solid polymer beads as described, for example, in WO 2014/140936 A1 or WO 201 1/036447 A1 , or incorporation of the semiconducting nanocrystals into

2Q a thin layer using, in particular, polysilazanes as described, for example, in US 4,678,688, US 2009/0186171 A1 , US 2012/0269989 A1 , US

2015/0331 153 A1 and JPH10-14674 A.

Summary of the invention

35 Recently, encapsulating the semiconducting nanocrystals in capsules or incorporating the semiconducting nanocrystals into a thin layer using organo-polysilazanes (modified organic polysilazane) has been

suggested.

However, even though the stability and performance of the luminescent nanocrystals could be improved by these approaches, the quantum yield of the encapsulated nanocrystals still degrades after a certain period of time, in particular due to the exposure to high temperature and/or high light intensity condition. Moreover, during the manufacturing process of an optical medium, exposure to these quantum yield degrading conditions very often cannot be avoided.

Therefore, a process for recovering the quantum yield of an optical medium, in particular an optical medium applicable in optical devices, which comprises a nanosized fluorescent material encapsulated in an organic material and which does not attain its initial or usual quantum yield (anymore) is highly desirable. Such a process would make it possible to prolong the lifetime of an optical medium and, thus, also the lifetime of an electronic devices applying such an optical medium.

Then, the present inventors have surprisingly found that the quantum yield of an optical medium comprising a light luminescent part which comprises at least one nanosized fluorescent material and an organic material, the quantum yield of which has dropped due to exposure to high temperature and/or high light intensity condition, can be recovered by treating said optical medium simultaneously with heat and moisture.

Moreover, the present inventors have found that such heat and moisture treatment can easily and effectively be carried out by heating such an optical medium in a humid environment.

Based on the above, it is thus an object of the present invention to provide a practical method capable of recovering the quantum yield of an optical medium, which comprises a light luminescent part comprising at least one nanosized fluorescent material and an organic material, easily and effectively.

Moreover, it is an object of the present invention to provide an optical medium, in particular an optical medium applicable in optical devices, the quantum yield of which has been recovered using said process.

To solve these problems, the present invention provides a process for quantum yield recovery of an optical medium, said process comprising at least the steps of: a) providing an optical medium comprising a light luminescent part that comprises at least one nanosized fluorescent material and an organic material; and b) heat treating and simultaneously moisture treating the optical medium by heating the optical medium in a humid environment.

This problem is further solved by an optical medium obtained or obtainable by the process for quantum yield recovery according to the present invention and using such an optical medium in an optical device.

In another aspect, the invention further relates to a composition

comprising at least the optical medium in the form of a capsule (200) obtained or obtainable by the process of the invention and a matrix material.

In another aspect, the invention relates to a formulation comprising at least the optical medium in the form of a capsule (200) obtained or obtainable by the process of the invention and at least one solvent. In another aspect, the invention further relates to an optical device comprising at least the optical medium obtained or obtainable by the process of the invention.

Description of drawings

Fig. 1 shows a cross sectional view of an optical medium having a film-like structure that can be treated by the process of the invention.

Fig. 2 shows a cross sectional view of an optical medium in the form of a light luminescent capsule that can be treated by the process of the invention.

Fig. 3 shows a cross sectional view of one embodiment of an optical device (300).

Fig. 4 shows the QY measurement results of the optical films obtained from Examples 1 to 6.

Fig. 5 shows the QY measurement results of the optical film obtained from Example 7.

Fig.6 shows the QY measurement results of the capsules-containing optical film obtained from Example 10. Detailed Description of the invention

According to the present invention, a process for quantum yield recovery of an optical medium is provided, wherein said process comprises at least the steps of: a) providing an optical medium comprising at least a light luminescent part that comprises at least one nanosized fluorescent material and an organic material; and b) heat treating and simultaneously moisture treating the optical medium by heating the optical medium in a humid environment.

As used herein, the term "quantum yield recovery" is understood to mean that the quantum yield of an optical medium, which for any of the above- explained reasons has dropped to or is at a value lower than an initial or

0 usual value, is recovered back by treating said optical medium with the process of the invention to approximately the initial or usual value or even a higher value, i.e. to a value higher than before the treatment. This means, for example, when the quantum yield of an optical medium, which

5 usually has a QY of 70 to 75% (typical value of quantum rods in solution), has dropped for example due to exposure to high temperature and/or high light intensity condition to below 60%, or even below 40%, the QY can be recovered back to values of 70 to 75% or even more by subjecting said optical medium to the heat and moisture treatment of the present invention

^ However, the process of the invention is not limited by any QY values of the optical medium to be treated before or after the heat and moisture treatment. This means, within the framework of the present invention it does not matter which initial QY value the optical medium to be treated

5 currently has or to which value the initial QY of the optical medium to be treated has dropped.

Moreover, the process of the invention is not limited in any way by the time Q period the optical medium to be treated has already been used. This

means, an optical medium having a reduced QY can be subjected to the process of the invention for QY recovery no matter whether it has already been in use for some time or whether it has just been prepared.

Accordingly, the process of the invention may also be part of the

5

manufacturing process of an optical medium, especially towards the end of the manufacturing process and in such manufacturing processes, which require processing steps in which quantum yield degrading conditions cannot be avoided.

The optical medium of the present invention is characterized in that it comprises a light luminescent part which comprises at least one nanosized fluorescent material and an organic material.

In some embodiments of the present invention, the optical medium can be in the form of or processed in the form of a sheet or film, a lens or a capsule. In particular, the optical medium can be an optical sheet or film, a filter or a lens, for example a color filter, color conversion sheet or remote phosphor tape. As used herein, the term "sheet" includes "layer" and "film" like structures.

The expression "light luminescent part" is used herein to indicate that part of an optical medium that comprises the semiconductor light emitting nanocrystals, preferably said semiconductor light emitting nanocrystals are semiconductor fluorescent materials.

The nanosized fluorescent material is not specifically limited. A wide variety of publically known nanosized light emitting materials can be used as desired. It is also possible to use a mixture of more than one nanosized light emitting materials.

According to the present invention, the term "nanosized" means the size in between 1 nm and 999 nm.

The type of shape of the nanosized light emitting material is not particularly limited. For examples, spherical shaped, elongated shaped, star shaped, polyhedron shaped, banana shaped, star shaped, pyramidal shaped, tetrapod shaped, tetrahedron shaped, platelet shaped, cone shaped, and irregular shaped semiconducting nanocrystals can be used. Thus, according to the present invention, the term "a nanosized fluorescent material" is understood to mean that the light emitting material which size of the overall diameter is in the range from 1 nm to 999 nm.

5

And in case of the material has elongated shape, the length of the overall structures of the fluorescent material is in the range from 1 nm to 999 nm.

Preferably, the nanosized fluorescent material is selected from the group 10 consisting of nanosized inorganic phosphor materials, quantum sized materials such as quantum dots and or quantum rods, and a combination of any of these, and quantum sized materials are especially preferred.

^ As used herein, the term "quantum sized" means the size of the

semiconductor material itself without ligands or any surface modifications, which can show the quantum size effect. Generally, quantum sized materials, such as quantum dot materials and quantum rod materials, can emit tunable, sharp and vivid colored light due to "quantum confinement"

^ effect.

Therefore, the nanosized fluorescent material according to the present invention is more preferably a quantum sized material comprising ll-VI, III- 25 V, or IV-VI semiconductors, or a combination of any of these.

For example, the semiconductor nanocrystal may be selected from InP, CdS, CdSe, CdTe, ZnS, ZnSe, ZnSeS, ZnTe, ZnO, InPZnS, InPZn, ~_n Cu₂(ZnSn)S .

According to the present invention, a nanosized fluorescent material also encompasses such materials having a core/shell structure.

35

As used herein, the term" core/shell structure" means the structure having a core part and at least one shell part covering said core. For example, said core/shell structure can be core/one shell layer structure, core/double shells structure or core/multishell structure, wherein core/multishell structures stand for the stacked shell layers consisting of three or more shell layers and wherein each stacked shell layers of double shells and/or multishell can be made from the same or different materials.

It is further preferable that the size of the overall structures of the quantum sized material, is from 1 nm to 100 nm, more preferably, it is from 1 nm to 30 nm, even more preferably, it is from 5 nm to 15 nm.

For example, Cds, CdSe, CdTe, ZnS, ZnSe, ZnSeS, ZnTe, ZnO, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgSe, HgTe, InAs, InP, InPZnS, InPZn, InSb, AIAs, AIP, AlSb, Cu₂S, Cu₂Se, CulnS2, CulnSe₂, Cu₂(ZnSn)S , Cu₂(lnGa)S₄, TiO₂ alloys and combination of any of these may be preferably used as a core of the nanosized light emitting material (120).

The shell of the nanosized light emitting material may preferably be selected from the group consisting of ll-VI, lll-V, or IV-VI semiconductors.

For example, for red emission use CdSe/CdS, CdSeS/CdZnS,

CdSeS/CdS/ZnS, ZnSe/CdS, CdSe/ZnS, InP/ZnS, InP/ZnSe,

InP/ZnSe/ZnS, InPZn/ZnS, InPZn/ZnSe/ZnS dots or rods, ZnSe/CdS, ZnSe/ZnS or combination of any of these, can be used preferably. For example, for green emission use CdSe/CdS, CdSeS/CdZnS,

CdSeS/CdS/ZnS, ZnSe/CdS, CdSe/ZnS, InP/ZnS, InP/ZnSe,

InP/ZnSe/ZnS, InPZn/ZnS, InPZn/ZnSe/ZnS, ZnSe/CdS, ZnSe/ZnS or combination of any of these can be used preferably. And for blue emission use, such as ZnSe, ZnS, ZnSe/ZnS, or combination of any of these, can be used preferably.

As a quantum dot, publically available quantum dots, for examples, CdSeS/ZnS alloyed quantum dots product number 753793, 753777, 753785, 753807, 753750, 753742, 753769, 753866, InP/ZnS quantum dots product number 776769, 776750, 776793, 776777, 776785, PbS core-type quantum dots product number 747017, 747025, 747076, 747084, or CdSe/ZnS alloyed quantum dots product number 754226, ⁵ 748021 , 694592, 694657, 694649, 694630, 694622 from Sigma-Aldrich, can be used preferably as desired.

Examples of quantum rod materials have been described, for example, in 10 WO 2010/095140 A1 .

Further, according to the present invention, the surface of the nanosized fluorescent material may be covered or coated with one or more kinds of ^ surface ligands.

Without wishing to be bound by theory it is believed that such surface ligands may lead to disperse the nanosized fluorescent material in a solvent more easily.

20

The surface ligands in common use include phosphines and phosphine oxides such as Tnoctylphosphine oxide (TOPO), Tnoctylphosphine (TOP), and Tributylphosphine (TBP); phosphonic acids such as

25 Dodecylphosphonic acid (DDPA), Tridecylphosphonic acid (TDPA),

Octadecylphosphonic acid (ODPA), and Hexylphosphonic acid (HPA); amines such as Dedecyl amine (DDA), Tetradecyl amine (TDA),

Hexadecyl amine (HDA), and Octadecyl amine (ODA), thiols such as

2Q hexadecane thiol and hexane thiol; mercapto carboxylic acids such as mercapto propionic acid and mercaptoundecanoicacid; and a combination of any of these. Polyethylenimine (PEI) also may be used.

Examples of surface ligands have been described, for example, in WO

35

2012/059931 A1 . In the process of the present invention, the quantum yield of an optical medium can be recovered after the quantum yield has dropped, for example because of temperature quenching, by exposing the optical medium simultaneously to a heat and moisture treatment.

According to the present invention, the heat treating is preferably carried out at a temperature in the range from about 20°C to about 95°C. In general, the higher the temperature is set during the heat treatment, the faster the recovering process of the optical medium progresses. However, the temperature may not be set too high, as high temperatures may damage the optical medium depending on the materials of which it consists, thereby resulting in a performance decrease.

According to preferred embodiments of the method of the invention, the heat treating is carried out at temperatures in the range from 70°C to 90°C, more preferably in the range from 80°C to 88°C. Most preferably, the temperature is set at 85°C.

Further, according to method of the present invention, the moisture treating is preferably carried out in a humid environment having a humidity in the range from 35% relative humidity (hereinafter "%-rh") to 95%-rh. According to preferred embodiments of the present invention, the humid environment has a humidity from 70%-rh to 90%-rh, more preferably from 80%-rh to 88%-rh, even more preferably it is from 83%-rh to 87%-rh Most preferably, the humidity is set at 85%-rh.

In general, the higher the humidity is set during the moisture treatment, the faster the recovering process of the optical medium proceeds. The same applies to the temperature set during the heat treatment. In the quantum yield recovery process of the present invention, if the heat and moisture treating is carried out at lower temperatures and humidity, for example at ambient conditions (about 23°C/50%-rh), the recovery progress is very slow, but full recovery performance can still be achieved. However, best results concerning recovery time and recovery performance were obtained if the heat and moisture treatment were carried out simultaneously at 85°C and 85%-rh.

For providing the humid environment required for the heat and moisture treatment of an optical medium, any apparatus or device capable of providing the desired heat and moisture conditions can be used.

Preferably, the optical medium to be treated is placed in a climate chamber or oven set to hold the desired temperature and humidity over a desired time period.

According to a preferred embodiment of the process of the present invention, the heat and moisture treating is performed for 10 min or more, preferably from 20 min to 10 days, more preferably from 25 min to 5 days, and most preferably from 30 min to 2 days. It is particularly preferred that the heat and moisture treating is performed at 85°C and 85%-rh for 24 hours.

As outlined before, the recovery duration time strongly depends on the conditions set during heat and moisture treatment. In general, however, at certain fixed recovery conditions (i.e., fixed temperature and humidity), the recovery performance increases the longer the heat and moisture treating is performed. After a certain treatment time, however, no further increase in recovery performance is observed (i.e. a function "QY" versus

"treatment time" reaches a plateau), and if the heat and moisture treatment is maintained too long, even a decrease in recovery

performance may result (i.e. the QY vs. treatment time function decreases from a maximum). In the following, two specific embodiments of an optical medium which are especially suitable for quantum yield recovery according to the process of the invention, but which are not to be considered as limiting for the scope of the invention, are described.

With reference to Fig. 1 , according to a first embodiment the optical medium (100) comprises a light luminescent part that comprises at least 0 one nanosized fluorescent material (1 10) (for example, red and/or green) and an organic material, which is a matrix material (120) encapsulating the nanocrystals (1 10). Optionally, a barrier layer (130) can be placed over the light luminescent part.

5

According to this embodiment, the optical medium (100) preferably has a layered or film-like structure, such as an optical film, and the

semiconducting crystals (1 10) are incorporated into the matrix material (120) in order to protect the nanocrystals (1 10) from external influences, ^ as mentioned before.

The nanosized fluorescent material (1 10) is not specifically limited. A wide variety of publically known nanosized light emitting materials as presented 5 before can be used as desired.

The matrix material according to this embodiment of the optical medium (120) is selected from polysilazanes, water soluble polymers and

Q combinations of any these.

More preferably, the matrix material (120) is selected from polysilazanes such as organo-polysilazanes, water soluble polymers such as substituted or unsubstituted polyvinyl alcohols and combinations of any of these.

According to the present invention, the term "organo-polysilazane" means a polysilazane comprising at least one of organic substituent in a repeating unit of said polysilazane.

Even more preferably, as substituted and/or unsubstituted polyvinyl

5

alcohols, polyvinyl alcohol (unsubstituted), cation-substituted polyvinyl alcohols, anion-substituted polyvinyl alcohols, acryl-substituted polyvinyl alcohols, acetoacetyl substituted polyvinyl alcohols (such as Gohsefimer™ Z from Nippon Gohsei ), vinyl acetates (such as Exceval™ from Kuraray, 10 Nichigo G-Polymer™ from Nippon Gohsei), silanol substituted polyvinyl alcohols (such as R-1 130 series from Kuraray), or a combination of any of these can be used.

^ Examples of cation-substituted polyvinyl alcohols, anion-substituted

polyvinyl alcohols, acryl-substituted polyvinyl alcohols are described, for example, in JPS61 -10483 A, JPH01 -206088 A, JPS61 -237681 A, JPS63- 307979 A, JPH07-285265 A, JPH07-009758 A, and JPH08-025795 A.

20

Particularly preferably, organo-polysilazanes are used as the matrix material (120) according to this embodiment of the optical medium, which have a repeating unit represented by following general formula (I):

25 -[SiR¹R²-NR³]- (I) wherein R¹, R² and R³ are, independently of each other, identically or differently selected from a hydrogen atom, an alkyl group, an alkenyl 2Q group, a cycloalkyl group, an aryl group, an alkoxy group, an amino group, a silyl group, an alkylsilyl group or an alkylamino group, and at least one of Ri, R2, R3 is a hydrogen atom, with the proviso that at least one of Ri, R2, R3 is not a hydrogen atom.

35 In some embodiments, the at least one of R¹, R² and R³ which is not a hydrogen atom, can be substituted by one or more of halogen atoms, alkyl groups, alkoxy groups, amino groups, silyl groups, and / or alkyl silyl groups. For example, fluoro alkyl group, perfluoro alkyl group, silyl alkyl group, trisilyl alkyl group, alkylsilylalkyl group, trialkyl silyl group, alkoxy silyl alkyl group, fluoro alkoxy group, silyl alkoxy group, alkyl amino group,

5

dialkyl amino group, alkyl amino alkyl group, alkyl silyl group, dialkyl silyl group, alkoxy silyl group, dialkoxy silyl group, trialkoxy silyl group can be used.

10 In some embodiments, as said combination, an alkyl aryl group is suitable.

According to the present invention, said alkyl group, or said alkenyl group can be straight chain or branched chain, with preferably being of straight ^ chain.

The term "aryl" denotes an aromatic carbon group or a group derived there from.

20

Aryl groups may be monocyclic or polycydic, i.e. they may contain one ring (such as, for example, phenyl) or two or more rings, which may also be fused (such as, for example, naphthyl) or covalently bonded (such as, for example, biphenyl), or contain a combination of fused and bonded rings. 25 Heteroaryl groups contain one or more heteroatoms, preferably selected from O, N, S and Se.

Particular preference is given to mono-, bi- or tricyclic aryl groups having 6 2Q to 25 carbon atoms, which optionally contain fused rings and are optionally substituted. Preference is furthermore given to 5-, 6- or 7-membered aryl groups, in which, in addition, one or more CH groups may be replaced by N, S or O in such a way that O atoms and/or S atoms are not linked directly to one another. Preferred aryl groups are, for example, phenyl, biphenyl, terphenyl,

[1 ,1 ':3',1 "] terphenyl-2'-yl, naphthyl, anthracene, binaphthyl, phenanthrene, pyrene, dihydropyrene, chrysene, perylene, tetracene, pentacene, benzo- pyrene, fluorene, indene, indenofluorene, and spirobifluorene.

5

More preferably, R3 of the chemical formula (I) is a hydrogen atom.

In a preferred embodiment of the present invention, wherein the organod polysilazane comprises repeating units of formulae (I) and (II),

[-SiR¹R²-NR³-]x

15

wherein the formula (I), R¹, R² and R³ are at each occurrence,

dependency or independently of each other, an alkyl group, an alkenyl group, a cydoalkyi group, an aryl group, an alkylsilyl group, an alkylamino group, or an alkoxygroup; in addition one or two of Ri, R2, and Rs can be hydrogen; wherein the formula (II) R⁴ and R⁵ are at each occurrence, dependency or independently of each other, an alkyl group, an alkenyl group, a cydoalkyi group, an aryl group, an alkylsilyl group, an alkylamino group, an alkoxygroup, or a combination of these; with the proviso that one of R₄, and Rs can be hydrogen, and 0< x+y <1 .

Furthermore preferably, the matrix material (120) comprises an organo- polysilazane selected from one or more members of the group consisting of organo-polysilazanes represented by following chemical formula (III) and organo-polysilazanes represented by following chemical formula (IV),

[SiR⁶R⁷-NH]_a - [SiHR⁸-NH]_b

[Si R⁹R¹⁰-NH]c - [SiHR¹¹-NH]_d - [SiR¹²R¹³NH]_e (IV)

35

wherein the formula (III), R⁶, R⁷ R⁸ are at each occurrence, dependency or independently of each other, an alkyl group having 1 to 15 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a cycloalkyi group having 3 to 10 carbon atoms, or an aryl group having 3 to 10 carbon atoms; the ratio of a and b is in the range from 1 :3 to 3:1 and a+b=1 ; wherein the formula (IV) R⁹, R¹⁰ R¹¹ are at each occurrence, dependency or

independently of each other, an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a cycloalkyi group having 3 to 10 carbon atoms, or an aryl group having 3 to 10 carbon atoms; R¹² is an alkenyl group having 2 to 10 carbon atoms; R¹³ is an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a cycloalkyi group having 3 to 10 carbon atoms, or an aryl group having 3 to 10 carbon atoms; and c+d+e =1 .

In some embodiments of this optical medium, the matrix material (120) can further comprise perhydropolysilazane. The mixing ratio of

perhydropolysilazane to organopolysilazane is in the range from 0:100 to 90:10 by weight. Preferably, it is in the range from 0:100 to 40:60 by weight. More preferably, from 0:100 to 30:70 by weight.

Examples of organo-polysilazanes and perhydropolysilazane are described, for example, in WO 2015/007778 A1 , JP 2015-1 15369 A and JP 2014-77082 A.

The average molecular weight M_w of the polysilazanes is not particularly limited according to this embodiment of an optical medium. Preferably, it is in the range from 1 ,000 to 20,000; with being more preferably in the range from 1 ,000 to 10,000.

In some embodiments of this optical medium, optionally, the matrix material (120) can further comprises one or more of transparent polymers.

As the transparent polymer, publically known transparent polymers which are suitable for optical mediums such as optical devices can be used, in particular to adjust the optical transparency of the matrix material (120) in a specified visible light wavelength, and the refractive index of the matrix material (120), and to control the oxygen absorption and/or moisture absorption of the matrix material (120) in a suitable range.

The term "transparent" means within this embodiment of an optical medium at least around 60 % of incident light transmit at the thickness used in an optical medium and at a wavelength or a range of wavelength used during operation of an optical medium. Preferably, it is over 70 %, more preferably, over 75%, the most preferably, it is over 80 %.

Further, within this embodiment of an optical medium the term "polymer" means a material having a repeating unit and having the weight average molecular weight (Mw) 1000 or more. In preferred embodiments, the weight average molecular weight (Mw) of the transparent polymer is in the range from 1 ,000 to 250,000. More preferably it is from 5,000 to 200,000 with more preferably being from 10,000 to 150,000.

The transparent polymer is preferably selected from one or more members of the group consisting of poly (meth)acrylates, polystyrene methyl (meth)acrylates, polystyrene, polyvinyl acetate, and polydivinylbenzene from the view point of better optical transparency, lower oxide absorption and high resistivity in high humidity condition.

According to a further embodiment of this optical medium, it can be preferred that the optical medium further comprises a barrier layer (130) placed over the light luminescent part.

Preferably, polysilazanes, in particular perhydropolysilazane (hereafter "PHPS") may be used to prepare the barrier layer (130). Without wishing to be bound by theory, it is believed that

perhydropolzsilazanes may realize wet fabrication process instead of vapor deposition process and can reduce fabrication damage of nanosized fluorescent material in the process, and a barrier layer made from PHPS has less defects in the layer.

Thus, in one embodiment of the present invention, the barrier layer (130) is a layer obtained from PHPS.

The barrier layer (130) according to the present invention may comprise a gradient structure comprised of an outermost part and subsequent part in the layer, wherein the outermost part consists of silicon nitride. Preferably, the gradient may be a hydrogen content.

More preferably, the outermost part of the gradient structure to the matrix material (120) may comprise a higher amount of hydrogen than the opposite side of the gradient structure to the barrier layer (130).

Without wishing to be bound by theory, it is believed that the barrier layer fabricated by using PHPS solution may have lower refractive index than the refractive index of a barrier layer fabricated by any vapor deposition method (such as CVD), and may lead to a better refractive index matching to the matrix materials of the present invention.

The barrier layer (130) according to the present invention may have a refractive index in the range from 1 .38 to 1 .85, preferably in the range from 1 .45 to 1 .60. More preferably, the barrier layer (130) is fabricated from PHPS and has the refractive index in the range from 1 .38 to 1 .85, preferably in the range from 1 .45 to 1 .60. By changing the drying condition of the PHPS layer and by controlling vacuum ultraviolet (hereafter "VUV") light irradiation condition, the refractive index value of the barrier layer (130) can be controlled.

As used herein, the term "vacuum ultraviolet" means an ultraviolet light having a peak wavelength in the region from 190 nm to 80nm.

The optical medium (100) can be prepared, and thus be provided for the quantum yield recovery method of the invention, by any method known to a person of ordinary skill in the art. Without being limited thereto, the optical medium (100) may for example be prepared by a method

comprising at least the following steps (a) and (b) and optionally (c) and <^d>^:

(a) providing at least one nanosized fluorescent material (1 10) and a matrix material (120) onto a substrate, and

(b) applying a steam process at a temperature in the range from 35°C to 180°C, and optionally

(c) preparing a barrier layer (130) by providing perhydropolysilazane solution onto the surface of the matrix material, and

(d) exposing the perhydropolysilazane to vacuum ultraviolet light. Depending on the intended application, the optical film (100) prepared by the above-described method, which can be subjected to the QY recovery method of the invention, can have a thickness that ranges between 0.5 μιτι to up to 1 mm, without being limited thereto. Typically, it has a thickness that ranges between 10 m to up to several 100 mm. The thickness of the barrier layer (130) can be in the range from 0.1 μιτι to 1 .0 μιτι. With reference to Fig. 2, according to a second embodiment of an optical medium that is especially suitable for quantum yield recovery according to the process of the present invention, the optical medium (100) is in the form of a capsule (200) comprising an inner core (210) and a shell encapsulating the inner core (210), in which the inner core (210) comprises the light luminescent part comprising the at least one nanosized fluorescent material (220) and the organic material (230) and the shell comprises a polymer layer (240).

Thus, in this embodiment of the optical medium the semiconductor nanocrystals (220) are encapsulated within capsules in order to protect the nanocrystals from external influences, as mentioned before. In accordance with this embodiment, the inner core (210) comprises the at least one nanosized fluorescent material (220) and the organic material (230).

According to embodiments of this optical medium, the core (210) can further comprises one or more organic solvents to adjust refractive index value of the inner core (210) to the polymer layer (240) comprised in the shell and to increase out coupling efficiency of the light luminescent capsule (200).

More preferably, the inner core (210) essentially consists of a plurality of nanosized light emitting materials (220) and an organic material (230).

The nanosized fluorescent material (220) is not specifically limited. A wide variety of publically known nanosized light emitting materials as presented before can be used as desired.

The organic material (230) according to this embodiment is preferably selected from Cs to C₄2 alkanes, Cs to C₄2 alkenes, Cs to C₄2 alcohols and combinations of any of these. Still more preferably, the organic material according is selected from Cs to C₄2 alkanes and Cs to C₄2 alkenes, and combinations of any of these.

As used herein, "Cs to C₄2 alkanes" mean saturated hydrocarbons having only single covalent bonds between their carbons and include straight- chain alkanes having a chain length of 5 to 42 carbon atoms, i.e. linear alkanes wherein the carbon atoms of the backbone are joined in a snakelike structure, as well as branched alkanes, wherein the carbon backbone splits off in one or more directions, overall having 5 to 42 carbon atoms.

As used herein, "Cs to C₄2 alkenes" mean unsaturated hydrocarbons that contain at least one carbon-carbon double bond and include straight- chain alkenes having a chain length of 5 to 42 carbon atoms and branched alkenes overall having 5 to 42 carbon atoms.

As used herein, "Cs to C₄2 alcohols" include Cs to C₄2 alkanes wherein at least one hydrogen atom of the carbon backbone is substituted by a hydroxyl group.

Preferably, the Cs to C₄2 alkanes, Cs to C₄2 alkenes and Cs to C₄2 alcohols are of the straight-chain type.

According to the present invention, the Cs to C₄2 alkanes and Cs to C₄2 alkenes may be unsubstituted, mono- or polysubstituted by halogen or CN. Also, it may be possible according to the present invention that one or more non-adjacent Ch groups are replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH₃)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡ C- in such a manner that oxygen atoms are not linked directly to one another. Preferably, said Cs to C₄2 alkanes and Cs to C₄2 alkenes are unsubstituted.

Non-limiting examples of compounds particularly suitable as organic material (230) according to this embodiment of the optical medium are nonan, decan, undecan, dodecan, tridecan, tetradecan, pentadecan, hexadecan, heptadecan, octadecan, nonadecan, eicosan, heneicosan, docosan, tricosan, tetracosan, pentacosan, hexacosan, heptacosan, octacosan, nonacosan, triacontan, hentriacontan, dotriacontan,

tritriacontan, tetratriacontan, pentatriacontan, hexatriacontan,

heptatriacontan, octatriacontan, nonatriacontan, tetracontan,

hentetracontan, dotetracontan, 1 -hexene, 1 - heptene, 1 -octene,1 -decene, 1 -undecene, 1 -dodecene, 1 -tridecene, 1 -tetradecene, 1 -pentadecene, 1 - hexadecene, 1 -heptadecene, 1 -octadecene,1 -nonadecene, 1 -eicocene, 1 - heneicosene, 1 -dococene, 1 -tricocene, 1 -tetracocene, 1 -pentacocene, 1 - hexacocene, 1 -heptacocene, 1 -octacocene, 1 -nonacocene, ethyl-decene, and methyl-hexene.

More preferably, the organic material (230) according to this embodiment of the optical medium is selected from one or more of C6 to C30 alkanes and C6 to C30 alkenes. Even more preferably, the organic material (230) is selected from one or more of C16 to C30 alkanes and C16 to C30 alkenes.

Still more preferably, the organic material (230) is selected from one or more members of the group consisting of octadecane, tetracosane, octacosan, octadecene.

The ratio of the nanosized light emitting material (220) and the organic material (230) in the inner core (210) according to this embodiment of the optical medium is in the range from 0.1 :100 to 100:1 . Preferably, the ratio of the nanosized light emitting material (220) and the organic material (230) in the inner core (210) is in the range from 1 :30 to 100:1 , more preferably in the range from 1 :10 to 99:1 .

The optical medium according to this embodiment comprises a shell encapsulating the inner core (210) in order to protect the semiconductor nanocrystals (220) from external influences, as mentioned before. The polymer layer (240) comprised in the shell preferably comprises a transparent polymer selected from one or more of the group consisting of poly(meth)acrylates, polystyrene methyl (meth)acrylates, polystyrene, polyvinyl acetate, and polydivinylbenzene.

As used in connection with this embodiment of the optical medium, the term "transparent" means at least around 60% of incident light transmit at the thickness used in an optical medium and at a wavelength or a range of wavelength used during operation of an optical medium. Preferably, it is over 70%, more preferably, over 75%, and most preferably, over 80%.

Within this embodiment of an optical medium, the term "polymer" means a material having a repeating unit and having a weight average molecular weight (Mw) of 1000 or more. Preferably, the weight average molecular weight (Mw) of the transparent polymer is in the range from 1 ,000 to 250,000, more preferably in the range from 5,000 to 200,000 and most preferably in the range from 10,000 to 150,000.

More preferably, the polymer layer (240) is a transparent polymer selected from one or more of the group consisting of poly (meth)acrylates, polystyrene methyl (meth)acrylates, polystyrene, polyvinyl acetate, and polydivinylbenzene. Still more preferably, the polymer layer (240) comprises a transparent polymer selected from one or more members of the group consisting of polydivinylbenzene, poly methyl (meth)acrylates, and polystyrene methyl (meth)acrylates.

Even more preferably, the polymer layer (240) is a transparent polymer selected from one or more members of the group consisting of

polydivinylbenzene, poly methyl (meth)acrylates, and polystyrene methyl (meth)acrylates.

According to embodiments of this optical medium, the polymer layer (240) may be at least partly covered with a ligand and/or a protection layer (250).

The surface ligands include phosphines and phosphine oxides such as Trioctylphosphine oxide (TOPO), Trioctylphosphine (TOP), and

Tributylphosphine (TBP); phosphonic acids such as Dodecylphosphonic acid (DDPA), Tridecylphosphonic acid (TDPA), Octadecylphosphonic acid (ODPA), and Hexylphosphonic acid (HPA); amines such as Dedecyl amine (DDA), Tetradecyl amine (TDA), Hexadecyl amine (HDA), and Octadecyl amine (ODA), thiols such as hexadecane thiol and hexane thiol mercapto carboxylic acids such as mercapto propionic acid and

mercaptoundecanoicacid; and a combination of any of these.

Examples of suitable surface ligands have been described, for example, in WO 2012/059931 A.

For the protection layer (250), any type of optically transparent material can be used. Preferred examples are polymers selected from polyvinyl alcohols, polyethyl imides, polydivinylbenzene, polymethyl

(meth)acrylates, polystyrene methyl (meth)acrylates, polysiloxanes, and polysilazanes. The protection layer (250) itself may additionally be coated with one or more of the above-described surface ligands.

The light luminescent capsules (200) can be prepared, and thus be provided for the method of the invention, by any method known to a person of ordinary skill in the art. Without being limited thereto, capsules (200) may be prepared for example by a method comprising at least the following steps (a), (b) and (c):

(a) preparing a composition comprising at least the nanosized light emitting material (220), the organic material (230), a precursor for the polymer layer (240), a polymerization initiator, a polar solvent, and a polymer solved in said polar solvent, such as polyvinyl alcohol,

(b) stirring the composition obtained in step (a) at a temperature in the range from the melting point of the organic material (230) to 99°C,

(c) polymerizing the precursor by heat treatment, by irradiating a ray of light, or a combination of any of these. For preparing light luminescent capsules (200) any type of polar solvent can be used singly or in mixture. In particular, the polar solvent can be selected from the group consisting of purified / deionized water; ethylene glycol monoalkyl ethers, such as, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, and ethylene glycol monobutyl ether; diethylene glycol dialkyi ethers, such as, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dipropyl ether, and diethylene glycol dibutyl ether; ethylene glycol alkyl ether acetates, such as, methyl cellosolve acetate and ethyl cellosolve acetate; propylene glycol alkyl ether acetates, such as, propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, and propylene glycol monopropyl ether acetate; ketones, such as, methyl ethyl ketone, acetone, methyl amyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohols, such as, ethanol, propanol, butanol, hexanol, cyclo hexanol, ethylene glycol, and glycerin; esters, such as, ethyl 3-ethoxypropionate, methyl 3-methoxypropionate and ethyl lactate; and cyclic asters, such as, γ-butyrolactone; chlorinated hydrocarbons, such as chloroform, dichloromethane, chlorobenzene, dichlorobenzene.

As the polymerization initiator, any polymerization initiator generating an acid, base or radical when being exposed to radiation or heat can be used.

^ 5 Non-limiting examples of the photo radical-generator include azo

compounds, peroxides, acyl phosphine oxides, alkyl phenons, oxime esters, and titanocenes. Examples of the heat acid-generator include p- toluene sulfonates, benzenesulfonates, p-dodecyl benzenesulfonates, 1 ,4- naphthalenedisulfonat.es. Examples of the heat radical-generators include

20

2,2' azobis(2-methylvaleronitrile), 2,2'-azobis(dimethylvaleronitrile) or a combination of any of these. Examples of the photo base-generator include multi-substituted amide compounds having amide groups, lactams, imide compounds, and compounds having those structures. Examples of

25 the heat base-generator include: imidazole derivatives, such as, N-(2- nitrobenzyloxycarbonyl)imidazole, N-(3-nitrobenzyloxycarbonyl)imidazole, N-(4-nitrobenzyloxycarbonyl)imidazole, N-(5-methyl-2-nitro- benzyloxycarbonyl)imidazole, and N-(4-chloro-2-nitrobenzyloxycarbonyl)-

2Q imidazole; 1 ,8-diazabicyclo(5,4,0)undecene-7, tertiary amines, quaternary ammonium salts, and mixture thereof.

Preferably, untrasonification such as ultrasonic probe (from Hielscher UP200Ht) is used in step (b) to control average particle size and ensure smaller particle size and a better size distribution of particles at the same time. Microcapsulation methods that can be used for preparing the above- described light luminescent capsules have been described in, for example, A. Chaiyasat et. al., eXPRESS polymer Letters Vol.6, No.1 , (2012) 70-77.

Depending on the intended application, the size of the capsules prepared according to the above-described method, which can be subjected to the quantum yield recovery method of the invention, typically ranges from 50 nm to 20 μιτι, without being limited thereto. Depending on the size of the capsule, the total thickness of the shell is usually between 1/4 and 1/10 of the total capsule diameter.

As outlined above, the process of the invention enables recovery or even enhancement of the quantum yield of an optical medium, the QY of which has dropped compared to a certain initial QY value, by treating said optical medium simultaneously with heat and moisture.

A further aspect of the present invention therefore relates to optical medium obtained or obtainable by the process for quantum yield recovery according to the invention.

Yet a further aspect of the present invention relates to the use of an optical medium obtained or obtainable by the process for quantum yield recovery according to the invention in an optical device, in particular as an optical film or filter, such as a color filter, color conversion sheet or remote phosphor tape, or a lens.

For processability reasons, in particular when a layer or film like structure comprising light luminescent capsules is to be applied onto a substrate for preparing an optical device, the optical medium in the form of light luminescent capsules (200) can be provided in a composition that comprises said capsules (200) and a matrix material, into which said capsules are incorporated. For the same or similar processability reasons, the light luminescent capsules (200) can be provided in a formulation that comprises at least said capsules and at least one solvent.

Therefore, the present invention also relates to a composition comprising at least an optical medium in the form of a capsule (200) obtained or obtainable by the process for quantum yield recovery according to the present invention and a matrix material.

As the matrix material for use in a composition together with light luminescent capsules (200), any type of transparent polymers known to the skilled person can be used.

Preferred examples are methyl-acrylate, methyl-methacrylate, ethyl- acrylate, ethyl-methacrylate, butyl-acrylate, butyl-methacrylate, 2- ethylhexyl-acrylate, 2-ethylhexyl-methacrylate; substituted alkyl- (meth)acrylates, for examples, hydroxyl-group, epoxy group, or halogen substituted alkyl-(meth)acrylates; cyclopentenyl(meth)acrylate, tetra-hydro furfuryl-(meth)acrylate, benzyl (meth)acrylate, polyethylene-glycol di- (meth)acrylates, polysiloxanes, polysilazanes, polystyrenes, polyvinyl acetate, polydivinylbenzene, or a combination of any of these.

In view of better coating performance of the composition, film strength and good handling, the matrix material has a weight average molecular weight in the range from 5,000 to 50,000, more preferably from 10,000 to 30,000.

According to the present invention, the molecular weight M_w is determined by means of GPC (= gel permeation chromatography) against an internal polystyrene standard.

Further, the present invention also relates to a formulation comprising at least an optical medium in the form of a capsule (200) obtained or obtainable by the process for quantum yield recovery according to the present invention and at least one solvent.

The type of solvent for use in the formulation is not particularly limited.

5

Preferably, the solvent is selected from the group consisting of purified water; ethylene glycol monoalkyl ethers, such as, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, and ethylene glycol monobutyl ether; diethylene glycol

10 dialkyl ethers, such as, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dipropyl ether, propylene glycol methyl ether and diethylene glycol dibutyl ether; ethylene glycol alkyl ether acetates, such as, methyl cellosolve acetate and ethyl cellosolve acetate;

^ propylene glycol alkyl ether acetates, such as, propylene glycol

monomethyl ether acetate (PGMEA), propylene glycol monoethyl ether acetate, and propylene glycol monopropyl ether acetate; ketones, such as, methyl ethyl ketone, acetone, methyl amyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohols, such as, ethanol, propanol, butanol,

20

hexanol, cyclo hexanol, ethylene glycol, and glycerin; esters, such as, ethyl 3-ethoxypropionate, methyl 3-methoxypropionate and ethyl lactate; and cyclic asters, such as, γ-butyrolactone; chlorinated hydrocarbons, such as chloroform, dichloromethane, chlorobenzene, dichlorobenzene. 25 Those solvents are used singly or in combination of two or more, and the amount thereof depends on the coating method and the thickness of the coating.

2Q More preferably, propylene glycol alkyl ether acetates, such as, propylene glycol monomethyl ether acetate (hereafter "PGMEA"), propylene glycol monoethyl ether acetate, propylene glycol monopropyl ether acetate, purified water or alcohols can be used.

35

The amount of the solvent in the composition can be freely controlled according to the method of coating the composition. For example, if the formulation is to be spray-coated, it can contain the solvent in an amount of 90 wt.% or more. Further, if a slit-coating method is to be carried out, which is often adopted in coating a large substrate, the content of the solvent is normally 60 wt.% or more, preferably 70 wt.% or more.

The composition as well as the formulation containing light luminescent capsules (200) can also directly be subjected to the heat and moisture treating for quantum yield recovery according to the process of this invention.

Accordingly, the present invention also envisions a process for optical medium quantum yield recovery, wherein the optical medium in the form of capsule (200) is provided in a composition comprising at least said capsules (200) and a matrix material.

Further, the present invention envisions a process for optical medium quantum yield recovery, wherein the optical medium in the form of a capsule (200) is provided in a formulation comprising said capsules (200) and a solvent.

In a further aspect, the present invention relates to an optical device comprising at least an optical medium obtained or obtainable by the process for quantum yield recovery according to the invention, or a composition comprising an optical medium in the form of a capsule obtained or obtainable by the process of the invention and a matrix material, as described above.

Preferably the optical device comprises at least the optical medium or the composition treated by the process of the invention in a layered or film like structure, such as an optical sheet or film, color conversion sheet or remote phosphor tape. The optical device in accordance with the present invention is preferably selected from a liquid crystal display, an Organic Light Emitting Diode (OLED), a backlight unit for display, a Light Emitting Diode (LED), a Micro Electro Mechanical Systems (MEMS), an electro wetting display, or an electrophoretic display, a lighting device, and/or a solar cell.

Fig. 3 shows an exemplary optical device (300) in accordance with the present invention, including a film like structured optical medium (100), at 0 least one nanosized fluorescent material (1 10) (for example, red and/or green), a matrix material (120), a barrier layer (130), and light source (310). Optionally, the optical device can further include a substrate (320).

5 In case the optical device (300) comprises an optical film comprising light luminescent capsules (200), which film is prepared using a composition or formulation as described above, then, in Fig. 3, the nanosized fluorescent material (1 10) represents the light luminescent capsules (200).

^ The type of light source (310) in the optical device (300) is not particularly limited. For example, a light emitting diode (LED), a cold cathode fluorescent lamp (CCFL), an electroluminescent device (EL), an organic light emitting device (OLED) or a combination of any of these, can be

5 used.

Optionally, the optical device (300) can include a substrate (320). In general, transparent substrates are used, which can be flexible, semi-rigid Q or rigid. Publically known transparent substrate suitable for optical devices can be used as desired.

Preferred examples of transparent substrates are selected from a transparent polymer substrate, glass substrate, thin glass substrate

5

stacked on a transparent polymer film, transparent metal oxides (for example, oxide silicone, oxide aluminum, oxide titanium). Of course, it is also envisioned by the present invention that the optical device comprising the layered or film-like structured optical medium (100) according to the first embodiment of the optical medium or the light luminescent capsules (200) according to the second embodiment of the optical medium, for example in an optical film or sheet, is directly

subjected to the process for optical medium quantum yield recovery of the invention.

Definition of Terms

The term "semiconductor" means a material which has electrical conductivity to a degree between that of a conductor (such as copper) and that of an insulator (such as glass) at room temperature.

The term "inorganic" means any material not containing carbon atoms or any compound that containing carbon atoms ionically bound to other atoms such as carbon monoxide, carbon dioxide, carbonates, cyanides, cyanates, carbides, and thiocyanates.

The term "emission" means the emission of electromagnetic waves by electron transitions in atoms and molecules.

The invention is described in more detail below with reference to examples which are not to be considered as limiting for the scope of the invention. Examples

According to the present invention, the average molecular weight M_w is determined by means of GPC (= gel permeation chromatography) against an internal polystyrene standard.

Example 1 Preparation of an optical medium (100) - Organo- polysilazane+Q-rod/PHPS optical film

A 3 cm by 3 cm glass substrate is cleaned by a tissue containing isopropanol, and additionally the substrate is cleaned by spincoating for 30 second at 1000 rpm with isopropanol.

1 g of organo-polysilazane solution (25 wt.% of the organo-polysilazane in toluene) including 1 wt.% of Luperox^® 531 M80 is mixed with 1 g of quantum sized material solution (3 wt.% of the quantum sized materials in toluene). The organo-polysilazane has the repeating unit represented by the chemical formula of [Si(CH₃)₂-NH] - [SiH(CH₃)-NH].

The obtained solution is spin coated onto the cleaned glass substrate at 1000 rpm for 30 seconds, followed by drying at 130°C for 5 minutes. Then, the coated and dried glass substrate is put into a climate chamber and cured at 85°C / 85 %-rh for 16 hours.

The dried and cured sample is cleaned again with isopropanol by spincoating at 2500 rpm for 30 seconds.

Afterwards, perhydropolysilazane (hereafter "PHPS") solution (20 wt.-% of PHPS in dibutylether; from Merck) is printed by syringe with 0.2 μιτι filter until the grass substrate is completely flooded. Then it is spin coated at

2500 rpm for 30 seconds, and dried at 130°C for 5 minutes. After a PHPS drying process, the PHPS layer is exposed to vacuum ultraviolet (hereafter "VUV") light having a peak wavelength of 172 nm at 25 mW/cm² for 30 minutes with a VUV device (from IOT) under nitrogen atmosphere to accelerate nitriding reaction of the PHPS layer.

The finally obtained film has a PHPS barrier layer of around 0.3 μιτι thickness coated on the organo-polysilazane/Q-rod layer. All processes are carried out under nitrogen atmosphere. And except for the VUV light irradiation, all processes are carried out under filtered yellow light condition. Example 2: Preparation of an optical medium (100) - Organo- polysilazane+Q-rod/PHPS optical film

The optical film of Example 2 is prepared in the same manner as described in Example 1 , except that the coated glass substrate is dried at

130°C for 5 hours and no curing of the coated and dried glass substrate in the climate chamber is conducted. Then, the dried sample is cleaned again with isopropanol by spincoating at 2500 rpm for 30 seconds.

The finally obtained film has a PHPS barrier layer of around 0.3 μιτι thickness coated on the organo-polysilazane/Q-rod layer.

Example 3: Preparation of an optical medium (100) - Organo- polysilazane+Q-rod/PHPS optical film

The optical film of Example 3 is prepared in the same manner as described in Example 1 , except that 0.2 g of PHPS solution (20 wt.-% of PHPS in dibutylether) is added into the solution containing 1 g of organo- polysilazane (25 wt.-% of the organo-polysilazane in toluene) and 1 wt.-% of Luperox^® 531 M80. The finally obtained film has a PHPS barrier layer of around 0.3 μιτι thickness coated on the organo-polysilazane/Q-rod layer.

Example 4: Preparation of an optical medium (100) - Organo- polysilazane+Q-rod/PHPS optical film

The optical film of Example 4 is prepared in the same manner as described in Example 3, except that the coated glass substrate is dried at 130°C for 5 minutes and then cured at 85°C / 85 %-rh for 1 hour.

Example 5: Preparation of an optical medium (100) - Organo- polysilazane+Q-rod/PHPS optical film

The optical film of Example 5 is prepared in the same manner as described in Example 2, except that the solution containing 1 g of organo- polysilazane (25 wt.-% of the organo-polysilazane in toluene) does not include any Luperox® 531 M80.

Example 6: Preparation of an optical medium (100) - Organo- polysilazane+Q-rod/PHPS optical film

The optical film of Example 6 is prepared in the same manner as described in Example 1 , except that the solution containing 1 g of organo- polysilazane (25 wt.-% of the organo-polysilazane in toluene) does not include any Luperox® 531 M80. Moreover, after the PHPS drying process the PHPS layer is not exposed to VUV light, but placed under 85°C / 85%-rh in a clinnate chamber for 16 hours to perform curing.

Example 7: Preparation of an optical medium (100) - Organo- polysilazane+Q-rod optical film

An optical film is prepared in the same manner as described in Example 1 except of coating the PHPS barrier layer onto the cured organo- polysilazane layer.

Example 8: QY evaluation

First, the absolute quantum yield (QY) of each optical film obtained from Examples 1 to 7 is measured directly after curing by VUV light.

The absolute Quantum Yield (QY) values are measured directly by using an absolute photoluminescence QY spectrometer (Hamamatsu model: Quantaurus C1 1347)

Then, each optical film obtained from Examples 1 to 7 is placed in a climate chamber set to 85°C and 85%-rh in air.

Fig. 4 shows the normalized quantum yield of the optical films obtained from Examples 1 to 6, each of which comprises a PHPS barrier layer, as function of time. As can be seen from Fig. 4, after curing by VUV light, i.e. before heat and moisture treating in the climate chamber (corresponding to time = 0), the QY of the quantum rods has dropped significantly to below 60%, even below 40% (quantum rods in solution usually have a QY of 70% to 75%). Treating the quantum rods-containing optical films at 85°C / 85%-rh for 1 day recovers the QY back to values of 60% to more than 80%.

Fig. 5 shows the normalized quantum yield of the optical film obtained from Example 7, which does not comprise a PHPS barrier layer, as function of time. As can be seen from Fig. 5, after curing by VUV light, i.e. before heat and moisture treating in the climate chamber (corresponding to time = 0), the QY of the quantum rods has dropped significantly to 58% (quantum rods in solution usually have a QY of around 70% to 75%). Treating the quantum rods-containing optical film at 85°C / 85%-rh for 1 day recovers the QY to 70%. After 5 days of heat and moisture treating, the QY even increases to 73%.

Comparative Example 1 :

Each optical film obtained from Examples 1 to 7 is placed in an oven at

85°C (%-rh practically zero) in air.

No recovery of QY is observed when the optical films are just heated. Comparative Example 2:

Each optical film obtained from Examples 1 to 7 is placed in an oven at ambient conditions (about 23°C/50%-rh).

A very slow, but complete recovery of QY is observed for each optical film under ambient conditions, which takes however several weeks.

Example 9: Preparation of an optical medium (200) in form of a capsule 0.27g of polyvinyl alcohol (Mowiol^® 8 - 88, Mw: 67,000; from Sigma Aldrich, hereafter "PVA") is dissolved in 30.0 ml of deionized water. Then, 1 .5 g of divinyl benzene (hereafter "DVB") is mixed with 225 mg of quantum sized materials (from Merck, hereafter "QM") in octadecane solution (15 wt.-% of quantum sized material in octadecane) and 0.16 g of benzoylperoxide (hereafter "BPO").

Then, the obtained DVB/octadecane//BPO/QM solution is mixed with the PVA/water solution and emulsified with T18 digital ULTRA-TURRAX^® at 5000 rpm for 5 min. All steps above are conducted at 40°C to prevent octadecane from solidifying. Afterwards, the obtained emulsion is transferred to a glass flask and polymerized at 70°C for 24 hours under argon atmosphere.

Example 10: Preparation of an optical film comprising light

luminescent capsules (200)

The light luminescent capsules obtained in Example 9 are dispersed in a PVA-purified water mixture (PVA: purified water = 1 : 20). Then, the mixture is dispensed on a glass substrate and cured on a hotplate at 80°C for 30 min to finally obtain an optical film.

Example 11 : QY evaluation

First, the capsules-containing optical film obtained from Example 10 is stored at 85°C for 7 days. The absolute quantum yield (QY) of the optical film is measured directly after this temperature treating.

The absolute QY value is measured directly by using an absolute photoluminescence QY spectrometer (Hamamatsu model: Quantaurus C1 1347). Then, the optical film obtained from Example 10 is placed in a climate chamber set to 85°C and 85%-rh in air.

Fig. 6 shows the normalized quantum yield of the capsules-containing optical film obtained from Example 10 as function of time. As can be seen from Fig. 6, the QY of the light luminescent capsules comprised in the optical film drops from 68% to 1 1 % after storing at 85°C because of temperature quenching. When being subjected to 85°C / 85%-rh subsequently, the QY recovers back to up to 55% after 1 day (day 8) and even to 64% after 5 days (day 14).

Claims

Process for quantum yield recovery of an optical medium, wherein said process comprises at least the steps of: a) providing an optical medium comprising a light luminescent part that comprises at least one nanosized fluorescent material and an organic material; and b) heat treating and simultaneously moisture treating the optical medium by heating the optical medium in a humid environment.

Process according to claim 1 , wherein the heat treating is carried out at a temperature in the range from 20°C to 95°C.

Process according to claim 1 or 2, wherein the humid environment has a humidity in the range from 35%-rh to 95%-rh.

Process according to any one of claims 1 to 3, wherein the heat and moisture treating is performed for 10 minutes or more.

Process according to any one of claims 1 to 4, wherein the organic material is a matrix material selected from polysilazanes, water soluble polymers and combinations of any these.

6. Process according to claim 5, wherein the matrix material is selected from organo polysilazanes, substituted or unsubstituted polyvinyl alcohols and combinations of any of these.

7. Process according to any one of claims 1 to 6, wherein the optical medium further comprises a barrier layer placed over the light luminescent part.

8. Process according to claim 7, wherein the barrier layer is a layer obtained from perhydropolysilazane.

9. Process according to any one of claims 1 to 4, wherein the optical medium is in the form of a capsule comprising an inner core and a shell encapsulating the inner core, in which the inner core comprises the light luminescent part comprising the at least one nanosized fluorescent material and the organic material, and the shell0 comprises a polymer layer.

10. Process according to claim 9, wherein the organic material is

selected from Cs to C₄2 alkanes, Cs to C₄2 alkenes, Cs to C₄2 alcohols and combinations of any of these.

5

1 1 . Process according to claim 9 or 10, wherein the polymer layer

comprises a transparent polymer selected from one or more of the group consisting of poly(meth)acrylates, polystyrene methyl

Q (meth)acrylates, polystyrene, polyvinyl acetate, and

polydivinylbenzene.

12. Process according to any one of claims 9 to 1 1 , wherein the polymer layer is at least partly covered with a ligand and/or covered with one5 or more additional layers.

13. Optical medium obtained or obtainable by the process for quantum yield recovery according to any one of claims 1 to 12. 0 14. Use of an optical medium according to claim 13 or obtained or

obtainable by the process according to any one of claims 1 to 12 in an optical device.

15. Composition comprising at least an optical medium in the form of a5

capsule obtained or obtainable by the process according to any one of claims 9 to 12 and a matrix material.

16. Formulation comprising at least an optical medium in the form of a capsule obtained or obtainable by the process according to any one of claims 9 to 12 and at least one solvent.

17. Optical device comprising at least an optical medium obtained or obtainable by the process according to any one of claims 1 to 12, or a composition according to claim 15.