A film, a method for producing a film, and an optoelectronic device

Abstract

There is provided a film, comprising: a layer of packed semiconductor nanocrystals, and a matrix material comprising at least one metal oxide, wherein the matrix material encapsulates the semiconductor nanocrystals. Further provided are a method for producing a film, and an optoelectronic device.

Description

[0001] Description

[0002] A FILM, A METHOD FOR PRODUCING A FILM, AND AN OPTOELECTRONIC

[0003] DEVICE

[0004] A film, a method for producing a film, and an optoelectronic device are provided .

[0005] Embodiments provide a film with improved performance . Further embodiments provide a method for producing a film with improved performance and an optoelectronic component with improved performance .

[0006] According to at least one embodiment , a film is provided . According to at least one embodiment , the film comprises a layer of packed semiconductor nanocrystals .

[0007] "Packed" means here that the film comprises a plurality of semiconductor nanocrystals that are present within the film in a compressed manner . That is , the semiconductor nanocrystals comprise a low separation distance , i . e . , an average distance between each two semiconductor nanocrystals is low . An average distance between two semiconductor nanocrystals may be chosen from the range of 0 . 5 nm inclusive to 250 nm inclusive . Accordingly, an average center-to-center distance may be chosen from the range 0 . 5 nm + the particle diameter to 250 nm + the particle diameter . In particular, the semiconductor nanocrystals are packed in an ordered manner, such that , for example , an average distance between each two semiconductor nanocrystals is the same . A semiconductor nanocrystal is a particle with a diameter having a size of several nanometers. Typical diameters of semiconductor nanocrystals may comprise a size of 1.5 nm to 20 nm diameter, in particular 3 nm to 10 nm, for example 5 nm.

[0008] The three-dimensional form, i.e. the geometry, of such a semiconductor nanocrystal is not restricted. For example, possible geometries of semiconductor nanocrystals comprise spheres, rods, tetrapods, or cuboids. The plurality of semiconductor nanocrystals may be monodisperse, i.e., it comprises semiconductor nanocrystals of the same form and size, or polydisperse, i.e., it comprises semiconductor nanocrystals of different sizes and / or different forms.

[0009] A semiconductor nanocrystal comprises or is formed of a semiconductor material. For example, a semiconductor nanocrystal comprises at least a core and possibly a shell, wherein the shell fully or partially coats the core. The core and / or the shell comprise at least one semiconductor material. For example, the core comprises a semiconductor material and the shell comprises a different semiconductor material. Semiconductor materials may be selected, for example, from I I-VI-compound semiconductor materials or III- V-compound semiconductor materials. In some embodiments, the semiconductor nanocrystal may also comprise a thin metal oxide coating, in particular a thin silica coating, on its outer surface. In other embodiments, the semiconductor nanocrystals are free of any inorganic coating.

[0010] A semiconductor nanocrystal is, due to its confined dimension, principally configured to absorb or to convert electromagnetic radiation, in particular in the UV and / or visible and / or IR wavelength range .

[0011] Converting electromagnetic radiation means here , that the semiconductor nanocrystal is configured to absorb incident electromagnetic radiation of a first wavelength range (primary radiation) , convert the primary radiation into electromagnetic radiation of a second wavelength range ( secondary radiation) and emit the secondary radiation . I f primary radiation is fully converted into secondary radiation, it is called full conversion or total conversion, otherwise partial conversion . In particular, the secondary radiation comprises higher wavelengths than the primary radiation . In this case , the conversion is called a downconversion .

[0012] Thus , a semiconductor nanocrystal may also be called a downconverter or a fluorophore . Another term, which may be used here for the semiconductor nanocrystals as described here , is quantum dot .

[0013] According to at least one embodiment , the film further comprises a matrix material comprising at least one metal oxide .

[0014] A matrix material is to be understood to be a material that embeds the semiconductor nanocrystals . "Comprising a metal oxide" means that the matrix material comprises a metal oxide or a combination of di f ferent metal oxides , or consists of a metal oxide or of a combination of di f ferent metal oxides .

[0015] According to at least one embodiment , the matrix material encapsulates the semiconductor nanocrystals . The encapsulation may be individually, this means , each of the semiconductor nanocrystals is fully covered by matrix material . However, also an aggregate of a few semiconductor nanocrystals may be encapsulated . Then the aggregate is fully covered by matrix material .

[0016] According to at least one embodiment a film is provided, the film comprising : a layer of packed semiconductor nanocrystals , and a matrix material comprising at least one metal oxide , wherein the matrix material encapsulates the semiconductor nanocrystals .

[0017] The inventors found out that with the combination of semiconductor nanocrystals and matrix materials , films having an enhanced reliability, brightness , long-term stability, in particular chemical stability and luminescence stability may be provided .

[0018] Commercial quantum dot downconverters have a protective silica shell . These commercial QD materials exist as aggregates of individually silica encapsulated QDs surrounded by additional silica forming QD-silica aggregates , where the aggregated QD silica clusters have an average particle si ze in the range of 5 pm to 15 pm, which makes them adaptable into traditional on-chip phosphor processing methods .

[0019] However, with a particle si ze of approximately 10 pm they are too big for applications in devices in the pm scale , like , for example , p-LEDs ( LED : light emitting diode ) . On the other hand, without their protective coating they cannot withstand the harsh conditions of on-chip operation and they degrade rapidly even under mild operating conditions . In contrast , a film as described here may be used, for example , as a conversion element in, for example LEDs or p- LEDs , having long-term stability and, thus , reliability .

[0020] Additionally, due to the matrix material in the film, physical and optical film properties may be tuned . For example , a variety of film thicknesses with the same optical properties may be reali zed . On the other hand, films of the same physical thickness with varying optical properties , like , for example , absorption behavior, may be provided .

[0021] Thus , the film as described here is appropriate to be used as a conversion element with adaptable properties in dependence of the intendent application of the conversion element .

[0022] Additionally, as the semiconductor nanocrystals are free of a common silica shell the film can be produced in a thickness that enables the film to be applied in p-LEDs .

[0023] As a broad definition, a p-LED could be seen as any light emitting diode ( LED) - generally not a laser - with a particularly small si ze . Typical heights of such p-LEDs are in the range of 1 . 5 pm to 10 pm, for example . Additionally or alternatively, typical lateral dimensions of a p-LED are in the range of 0 . 5 pm to 50 pm .

[0024] Furthermore , the film as described here comprises - due to the presence of the matrix material - in particular regular and dense packed semiconductor nanocrystals , even in case of polydisperse semiconductor nanocrystals . Thus , also semiconductor nanocrystals with geometries that do not pack nicely by themselves , like , for example tetrapods , are suitable in the film as described here as they are individually encapsulated by the matrix material . Thus , an enhanced film quality with, for example, a consistent thickness, may be realized enabled by a uniform or nearly uniform size of encapsulated semiconductor nanocrystals.

[0025] According to at least one embodiment, the matrix material comprises a plurality of individual encapsulation layers, each encapsulation layer encapsulating at least one semiconductor nanocrystal. In particular, the individual encapsulation layers are in mechanical contact to each other. Thus, the presence of voids within the film is possible. This matrix material is able to realize a well-ordered film, even in the case of a polydisperse plurality of semiconductor nanocrystals or with semiconductor nanocrystals having an irregular geometry like, for example, tetrapods. Due to the presence of the individual encapsulation layers particles of the semiconductor nanocrystals with a uniform size and shape, and thus, a uniform packing of the semiconductor nanocrystals may be realized. Furthermore, the thickness of the individual encapsulation layers may be controlled, so that, for example, physically thick but not optically dense films, i.e. films with a low concentration of semiconductor nanocrystals, may be realized. Exemplary thicknesses of the encapsulation layers are in the range of 0.25 nm inclusive, or 0.5 nm inclusive, and 150 nm inclusive. Additionally, a regular spacing between the semiconductor nanocrystals and a good protection of the semiconductor nanocrystals against humidity and oxygen may be realized.

[0026] According to at least one embodiment, the matrix material consists of individual encapsulation layers or primarily comprises individual encapsulation layers. According to at least one embodiment , the matrix material comprises a first barrier layer on the semiconductor nanocrystals . The first barrier layer can be present additionally or alternatively to the individual encapsulation layers . Further, according to at least one embodiment , the first barrier layer infills gaps between the semiconductor nanocrystals , i . e . , it is present also in-between the semiconductor nanocrystals . Thus , the matrix material may consist of the first barrier layer, which encapsulates the semiconductor nanocrystals . On the other hand, the matrix material may comprise individual encapsulation layers and a first barrier layer, wherein the encapsulation layers and the first barrier layer may comprise the same or di f ferent metal oxides . In both cases , the matrix material is free of voids . In other words , the film is homogeneously formed of the matrix material , in which the semiconductor nanocrystals are embedded . The matrix material comprising encapsulation layers and / or the first barrier layer supports an enhanced stability of the film and a high protection of the semiconductor nanocrystals against humidity and oxygen . Additionally, by infilling the gaps between the semiconductor nanocrystals , the scattering behavior of the film may be adapted, and thus , for example , the absorption enhanced .

[0027] According to at least one embodiment , the matrix material comprises at least one additional barrier layer on the layer of packed semiconductor nanocrystals . At least one additional barrier layer may be present directly on the plurality of individual encapsulation layers , each encapsulating a semiconductor nanocrystal , or on a first barrier layer . With at least on additional barrier layer, the physical thickness and the optical density of the film may be controlled . According to at least one embodiment , at least two adj acent barrier layers comprise di f ferent metal oxides . In particular, all barrier layers comprise di f ferent metal oxides . Thus , the matrix material is formed of a plurality of barrier layers , for example the first barrier layer and at least one additional barrier layer, wherein each barrier layer comprises another metal oxide . The semiconductor nanocrystals can be present in the first barrier layer or in the first and at least one additional barrier layer .

[0028] According to at least one embodiment , the film comprises multiple layers of packed semiconductor nanocrystals and a first barrier layer in-between, the layers of packed semiconductor nanocrystals and a first barrier layer inbetween being separated by at least one additional barrier layer . That is , the film comprises a stack of barrier layers , wherein every second barrier layer comprises semiconductor nanocrystals - being individually encapsulated by encapsulation layers of matrix material or not - embedded in a first barrier layer, and every other second layer comprises at least one additional barrier layer but is free of semiconductor nanocrystals . The metal oxides of the barrier layers and - i f present- the encapsulation layers can be the same or di f ferent .

[0029] Stacking barrier layers comprising the matrix material and at least partially comprising semiconductor nanocrystals enhances the protection of the semiconductor nanocrystals against environmental influences . Additionally, the physical thickness and optical density of the film can be accurately adj usted, and, thus , the film can be adapted to its intended application . According to at least one embodiment , the at least one metal oxide comprises a metal chosen from Al , Ti , Zr, Hf , Si , and mixtures thereof . Thus , the metal oxide used for the matrix material may be alumina, titania, zirconia, hafnia or silica, for example . All these metal oxides have a transparency for electromagnetic radiation, in particular in the UV, visible and / or IR range , such that the film may be used as an absorption and / or a conversion element in an optoelectronic device , for example . Additionally, the metal oxide can be chosen such that the film has an enhanced chemical stability even under illumination .

[0030] According to at least one embodiment , at least one metal oxide comprises silica . That is , the matrix material may consist of silica . According to another alternative , the matrix material comprises at least two of a first barrier layer, at least one additional barrier layer, and encapsulation layers , wherein at least one of the first barrier layer, the at least one additional barrier layer, and the encapsulation layers contains silica . In particular, the first barrier layer as defined above contains silica as matrix material . Further, the plurality of encapsulation layers comprises according to at least one embodiment silica . Thus , the semiconductor nanocrystals are coated at least partially by silica, i . e . are in mechanical contact with silica . Indirectly, they may be also coated by additional barrier layers of the matrix material containing di f ferent metal oxides .

[0031] According to at least one embodiment , the film comprises a thickness in the range of 100 nm inclusive to 50 pm inclusive . Such a film thickness , is high enough to make the film appropriate for absorbing and / or converting electromagnetic radiation, in particular fully converting a primary radiation, for example UV and / or blue light . On the other hand, such a thickness is small enough to implement the film in a p-LED as , for example , a conversion element .

[0032] According to at least one embodiment , the film comprises a concentration of semiconductor nanocrystals in the range of 0 . 5 wt% inclusive to 90 wt% inclusive . The concentration of the semiconductor nanocrystals may be valid for the whole film, or only with respect to , i f present , the barrier layer that comprises the packed semiconductor nanocrystals . Additionally or alternatively, the concentration of semiconductor nanocrystals in the film may have a gradient .

[0033] Generally, a concentration of semiconductor nanocrystals in the film corresponds to the optical density of the film, which has an influence on its optical properties like , for example , the degree of absorption or conversion of a primary radiation . For example , the higher the concentration the higher is the optical density and the more of a primary radiation is absorbed or converted .

[0034] As the physical thickness of the film and the optical density of the film can be independent from each other and thus adj usted independently when applying the matrix material on the semiconductor nanocrystals , the si ze of the film and its optical properties can independently from each other be tuned .

[0035] According to at least one embodiment , the semiconductor nanocrystals and / or the matrix material comprise inhomogeneities with a si ze in a range of 0 . 1 pm inclusive to 1 pm inclusive . Such inhomogeneities may act as scatterers in the film, and, thus, increase the path-length of incoming light, i.e. the primary radiation.

[0036] Further, a method for producing a film is provided. The method is configured to produce a film as described here. Hence, all features and embodiments disclosed with respect to the film are also valid for the method and vice versa.

[0037] According to at least one embodiment, the method for producing a film comprises the step: providing semiconductor nanocrystals. The semiconductor nanocrystals may have the same properties as defined above with respect to the semiconductor nanocrystals of the film.

[0038] According to at least one embodiment, the method further comprises the step: mixing the semiconductor nanocrystals with at least one solvent to form a first solution. The first solution comprises, thus, colloidally dispersed semiconductor nanocrystals that are not aggregated. The solvent may be chosen of, for example, toluene, cyclohexane, decane, octane, or other organic solvents.

[0039] According to at least one embodiment, the method further comprises the step: processing the first solution to form a layer of packed semiconductor nanocrystals. Processing the first solution may include one or more method steps being performed in order to produce the layer of packed semiconductor nanocrystals, in particular a layer of densely packed semiconductor nanocrystals.

[0040] Prior to processing the first solution, the first solution is according to at least one embodiment applied on a substrate. The substrate may be a layer of an optical device , for example , the surface of a semiconductor chip .

[0041] According to at least one embodiment semiconductor nanocrystals are provided that are individually encapsulated by a matrix material comprising at least one metal oxide . The semiconductor nanocrystals may be individually encapsulated by a method like the reverse micelle method, for example . Thus , with a method as described here , a film comprising a layer of packed semiconductor nanocrystals individually encapsulated by encapsulation layers of matrix material may be provided .

[0042] According to at least one embodiment , the first solution is treated with a stabili zer . Exemplary stabili zers are terpinol or titanium acetylacetonates ( acac ) or aluminum acetylacetonates ( acac ) . When treating the first solution with a stabili zer, a dispersed solution may be produced wherein the individual semiconductor nanocrystals do not aggregate .

[0043] Additionally or alternatively, according to at least one embodiment , the method for producing a film further comprises the step forming a matrix material on the layer of packed semiconductor nanocrystals by applying at least one metal oxide precursor . With this method step, a film comprising a layer of packed semiconductor nanocrystals , and a matrix material comprising at least one metal oxide , wherein the matrix material encapsulates the semiconductor nanocrystals , is formed . During the step of forming a matrix material , the at least one metal oxide precursor infills the layer of packed semiconductor nanocrystals and, thus , may individually cover the semiconductor nanocrystals , and finally forms the matrix material comprising the metal oxide . In particular, a first barrier layer as described above may be formed by this method step . A metal oxide precursor means here a material that is able to chemically react in order to form the corresponding metal oxide .

[0044] According to at least one embodiment , the method for producing a film comprises the steps : providing semiconductor nanocrystals , mixing the semiconductor nanocrystals with at least one solvent to form a first solution, processing the first solution to form a layer of packed semiconductor nanocrystals , wherein semiconductor nanocrystals are provided that are individually encapsulated by a matrix material comprising at least one metal oxide and / or wherein a matrix material is formed on the layer of packed semiconductor nanocrystals by applying at least one metal oxide precursor .

[0045] With a method as described here semiconductor nanocrystals may be regularly packed and form with the matrix material a highly ordered and regular film . Additionally, in particular when providing already individually encapsulated semiconductor nanocrystals , this method allows separately tuning the physical thickness and the optical density of the resulting film by si ze-tuning the matrix material , i . e . the thickness of the individual encapsulations . Furthermore , with this method a film may be reali zed which reveals enhanced stability, as the semiconductor nanocrystals are well protected against environmental humidity and oxygen by the matrix material . Additionally, with this method a matrix material can be produced, which is composed of a plurality of individual encapsulation layers , each encapsulation layer encapsulating at least one semiconductor nanocrystal . Alternatively or additionally, a matrix material can be produced that is free of voids , i . e . homogenously embeds the semiconductor nanocrystals , i . e . completely infills the gaps between the semiconductor nanocrystals .

[0046] According to at least one embodiment , the step processing the first solution comprises at least one of adding a second solution comprising a ligand composition and applying energy . In particular, the step processing the first solution comprises adding a second solution comprising a ligand composition and applying energy, for example heat .

[0047] The ligand composition in the second solution comprises according to at least one embodiment a molecule or salt capable of acting as a ligand or linker for the semiconductor nanocrystals in order to stabili ze the layer of packed semiconductor nanocrystals . This allows for increasing the thickness of the layer of packed semiconductor nanocrystals and, thus , increasing the thickness of the finally produced film or of the concentration of semiconductor nanocrystals in the finally produced film . The second solution may also act as a crosslinker or aggregator for semiconductor nanocrystals that are encapsulated by individual encapsulation layers and leads to a stronger aggregation between the semiconductor nanocrystals , which also serves for increasing the final physical thickness and / or optical density of the film . In particular, the ligand composition is di f ferent from the first solution and the mixture of first and second solution . In case of semiconductor nanocrystals that are already individually encapsulated with encapsulation layers of matrix material the ligand composition may comprise an acid, a base or a salt, like, for example, HC1, KOH, or KOI . Other examples for a ligand composition are tetraethyl ortho silicate or a bifunctional molecule like 1,4- bis ( trimethoxysilyl ) benzene .

[0048] Applying energy, which is optional, may comprise heating, applying light, or sonication, for example. Heating may be performed in a temperature range of room temperature to 150 degrees C. When heating, low-boiling point components, if present, may be removed in order to form a dense packed layer of semiconductor nanocrystals.

[0049] According to at least one embodiment, the at least one metal oxide precursor is chosen from tetraethyl orthosilicate, tetramethyl orthosilicate, sodium silicates, aluminum chloride, aluminum nitrate, aluminum tri-sec-butoxide, titanium chloride, titanium nitrate, titanium n-butoxide, hafnium chloride, hafnium nitrate, zirconium propoxide, and tetrakis-ethylmethylaminohafnium. With these metal oxide precursors, matrix materials containing silica, alumina, titania, zirconia, or hafnia may be produced.

[0050] According to at least one embodiment, applying a metal oxide precursor comprises applying a third solution comprising the at least one metal oxide precursor on the layer of packed semiconductor nanocrystals. Additionally, the third solution may comprise at least one of a solvent, and an acid or a base. An example for a solvent is ethanol, examples for acids are HC1 (aq) , acetic acid, sulfuric acid, and Lewis acids such as trifluoroborane, and for bases NH3(aq) , KOH, LiOH, NaOH, phosphate, carbonate, and acetate. If the semiconductor nanocrystal is encapsulated by encapsulation layers comprising a metal oxide , the addition of water in the third solution may be not necessary as it can be drawn from the already existing metal oxide coating .

[0051] According to at least one embodiment , the steps providing semiconductor nanocrystals , mixing the semiconductor nanocrystals with at least one solvent to form a first solution, and processing the first solution to form a layer of packed semiconductor nanocrystals , are repeated at least once . In particular, the steps are repeated while using the previously produced packed layer of semiconductor nanocrystals as a substrate . Thus , in sum a thicker packed layer of semiconductor nanocrystals can be reali zed, which af fects the final physical thickness and / or optical density of the film .

[0052] According to at least one embodiment , forming the matrix material comprises forming at least one additional barrier layer on the layer of packed semiconductor nanocrystals . In particular, the at least one additional barrier layer is formed on the layer of packed semiconductor nanocrystals that are individually encapsulated . Alternatively or additionally, the at least one additional barrier layer is formed on the layer of packed semiconductor nanocrystals with a first barrier layer thereon and / or in-between . With the at least one additional barrier layer the physical thickness of the film may be increased and / or the optical density decreased .

[0053] According to at least one embodiment multiple layers of packed semiconductor nanocrystals may be formed, wherein the layers of packed semiconductor nanocrystals are separated by at least one additional barrier layer . Thus , the method steps may be repeated in a suitable sequence , in order to produce a film comprising alternating barrier layers comprising semiconductor nanocrystals and being free of semiconductor nanocrystals , wherein the metal oxides in the barrier layers may be the same or di f ferent from each other .

[0054] According to at least one embodiment , the metal oxide precursor is applied by a method chosen from atomic layer deposition, doctor blading, spin coating, spray coating, immersion, and electro spraying . In particular, the metal oxide is applied by atomic layer deposition .

[0055] These methods are suitable for fully covering and / or individually encapsulating the semiconductor nanocrystals with the metal oxide . The metal oxide precursor completely or partially infills the gaps between the semiconductor nanocrystals in order to fully cover the individual semiconductor nanocrystals and embed them in the matrix material . Additionally, with these methods mechanical stable films may be produced that show reduced film cracking upon drying . Furthermore , these methods allow for tuning the final film thickness according to the intended application .

[0056] According to at least one embodiment , forming the matrix material comprises applying at least two di f ferent metal oxide precursors . Thus , a film may be produced comprising at least two barrier layers each comprising a di f ferent metal oxide . The metal oxides of the barrier layers may be the same or di f ferent from the metal oxide of the individual encapsulation layers , i f present .

[0057] According to at least one embodiment , at least one of the first solution comprising the semiconductor nanocrystals , the packed layer of semiconductor nanocrystals and the layer comprising the semiconductor nanocrystals and the matrix material is subj ected to a further treatment . A further treatment may be chosen from applying energy, adding a solvent , adding a base , and adding an acid and / or adding a metal salt like a Zn salt or aluminum salt , e . g . , ZnC12 or AICI3. By subj ecting the first solution, the packed layer of semiconductor nanocrystals and / or the layer comprising the semiconductor nanocrystals and the matrix material to a further treatment , inhomogeneities with a length scale of about 0 . 1 to 1 pm may be induced . Such inhomogeneities may act as scatterers , thus increasing the path-length of incoming light . By treating the first solution, the packed layer of semiconductor nanocrystals and / or the layer comprising the semiconductor nanocrystals and the matrix material with bases , new ligands for the semiconductor nanocrystals may be induced and / or the matrix material may be densi fied and / or the matrix material may be partially dissolved and re- formed to improve homogeneity . Exemplary bases are alkali hydroxide or alkaline earth hydroxide . Thus , the film properties may be adj usted according to the intended application of the film .

[0058] Furthermore , according to at least one embodiment an optoelectronic device is provided . The optoelectronic device comprises a film as described herein . Thus , all features and embodiments related to the film and the method for producing a film are also valid for the optoelectronic device and vice versa .

[0059] According to at least one embodiment , the film is formed as a conversion element or as a color filter . While a conversion element may absorb and emit electromagnetic radiation, a color filter is configured to only absorb electromagnetic radiation .

[0060] According to at least one embodiment , the optoelectronic component further comprises a semiconductor chip configured to emit a primary radiation; and a conversion element being configured to partially or fully convert the primary radiation into secondary radiation, wherein the conversion element comprises the film . In particular, the conversion element consists of the film according to embodiments described herein .

[0061] The electromagnetic radiation of the first wavelength may also be called primary radiation and corresponds to the emission spectrum of the semiconductor chip . According to at least one embodiment , the primary radiation comprises wavelengths from the UV range and / or from the visible range , in particular from the blue range . For example , the primary radiation comprises wavelengths in the range of 385 nm to 500 nm .

[0062] The semiconductor chip may be a light emitting diode chip or a laser diode chip . According to at least one embodiment , the light emitting device is a light emitting diode ( LED) . In particular, the semiconductor chip comprises an epitaxial grown semiconductor layer sequence with an active region being able to generate electromagnetic radiation . For example , the active region comprises a pn-j unction or a quantum well structure .

[0063] The conversion element is , in particular, applied on the radiation exit surface of the semiconductor chip such that light emitted from the semiconductor chip reaches at least partially the conversion element . The semiconductor nanocrystals in the conversion element convert the electromagnetic radiation of the first wavelength range into electromagnetic radiation of a second wavelength range , also called secondary radiation . The electromagnetic radiation of the second wavelength range corresponds to the emission spectrum of the semiconductor nanocrystals . In particular, the first wavelength range is di f ferent from the second wavelength range . The second wavelength range is , for example , chosen from wavelengths from the green or red range of the electromagnetic spectrum . In particular, the semiconductor nanocrystals fully convert the primary radiation into secondary radiation, such that the secondary radiation corresponds to the emission spectrum of the optoelectronic device .

[0064] Due to the properties of the film as described above , the conversion element , and, thus , the optoelectronic device have an enhanced luminescence stability . Moreover, due to the possibility of separately tuning physical and optical si ze of the film, the conversion element may be adj usted for its intended application .

[0065] According to at least one embodiment , the optoelectronic device is a p-LED . Due to the thickness of the film, being in a range of 100 nm to 50 pm, for example , 300 nm to 50 pm, in particular 2 pm to 30 pm, it is suitable to be applied in a p-LED .

[0066] As a broad definition, a p-LED could be seen as any light emitting diode ( LED) - generally not a laser - with a particularly small si ze . As a rule , a growth substrate is removed from p-LEDs , so that typical heights of such p-LEDs are in the range of 1 . 5 gm to 10 gm, for example . Additionally or alternatively, typical lateral dimensions of a g-LED are in the range of up to 100 gm, in particular of up to 70 gm, for example in the range of 0 . 5 gm to 50 gm, for example .

[0067] In principle , a g-LED does not necessarily have to have a rectangular radiation emission surface .

[0068] Mostly, such g-LEDs are provided on wafers with - for the g- LED non-destructively - detachable holding structures .

[0069] At present , g-LEDs are mainly used in displays . The g-LEDs form pixels or subpixels and emit light of a defined colour . Small pixel si ze and a high density with close distances make g-LEDs suitable , among others , for small monolithic displays for AR applications , especially data glasses . In addition, other applications are being developed, in particular regarding the use in data communication or pixelated lighting applications .

[0070] Advantageous embodiments and developments of the film, the method for producing a film and an optoelectronic device will become apparent from the exemplary embodiments described below in conj unction with the figures .

[0071] Fig . l shows a cross sectional side view of a film according to an exemplary embodiment .

[0072] Fig . 2 schematically shows a cross sectional side view of the packing behavior of semiconductor nanocrystals according to a reference example and according to an exemplary embodiment . Fig . 3a, b schematically show a cross sectional side view of method steps for producing a film according to an exemplary embodiment .

[0073] Fig . 4 schematically shows a cross sectional side view of method steps for producing a film according to an exemplary embodiment .

[0074] Fig . 5 schematically shows a cross sectional side view of a film according to an exemplary embodiment .

[0075] Fig . 6a shows quantum yields of films according to a reference example and according to exemplary embodiments .

[0076] Fig . 6b shows pictures of films according to a reference example and according to an exemplary embodiment .

[0077] Fig . 7a, b, c schematically shows a cross sectional side view of an optoelectronic device according to an exemplary embodiment .

[0078] In the exemplary embodiments and figures , similar or similarly acting constituent parts are provided with the same reference symbols . The elements illustrated in the figures and their si ze relationships among one another should not be regarded as being true to scale . Rather, individual elements may be represented with an exaggerated si ze for the sake of better representability and / or for the sake of better understanding . Figure 1 shows a schematic cross-sectional side view of a film 10 according to an exemplary embodiment. The film 10 comprises semiconductor nanoparticles 11, for example II-VI or III-V compound semiconductor materials, and a matrix material 12 comprising at least one metal oxide, for example silica. The thickness t of the film 10 is in a range of 100 nm to 50 pm, for example 300 nm to 50 pm. The thickness t describes the thickness perpendicular to the plane area of the film 10. With a low thickness t, the film can be used as a (partial) color filter, for example, with a high thickness t, the film can be used as a conversion element in an LED, for example.

[0079] Figure 2 schematically shows a cross sectional side view of the packing behavior of semiconductor nanocrystals 11 according to a reference example and according to an exemplary embodiment. If a polydisperse solution A of semiconductor nanocrystals 11, i.e. semiconductor nanocrystals 11 of different sizes and / or geometries, is processed in order to form a layer of packed semiconductor nanocrystals, a reference film B with poorly packed semiconductor nanocrystals 11 is the result (figure 2, top) . Such an irregular film B could not be applied in optoelectronic devices, due to poor film quality and poor stability with respect to environmental influences.

[0080] However, if a solution Al of polydisperse semiconductor nanocrystals 11 is provided with a matrix material 12 forming a plurality of individual encapsulation layers 12-1, each comprising a metal oxide, monodisperse, i.e. uniform spheres will result, which may be processed to a well-ordered, homogeneous film 10. Thus, a regular packing of irregular semiconductor nanocrystals 11 can principally be realized with the film 10 comprising matrix material 12 , in particular encapsulation layers 12- 1 , and the method for producing a film 10 as described herein . The result is an enhanced film quality . Additionally, due to the individual encapsulation layers 12- 1 , the final si ze of the semiconductor nanocrystals 11 is independent on the original si ze and morphology, but can be controlled by the thickness of the encapsulation layers 12- 1 .

[0081] Figure 3 shows in more detail the method for producing a film 10 according to exemplary embodiments , including the decoupled control of physical thickness and optical density of the film 10 . In Figure 3a, semiconductor nanocrystals 11 are coated with encapsulation layers 12- 1 , in this example with silica as a metal oxide , to get silica-coated semiconductor nanocrystals 11 having a si ze of about 10 to 40 nm ( step I ) . The encapsulation of the semiconductor nanocrystals 11 takes place by a reverse micelle system, comprising a non-polar solvent as a continuous phase , surfactants such as AOT or Igepal , and a continuous phase comprising water, a metal oxide source , and a catalyst . The product of step I is treated with a stabili zer such as terpinol or titanium acac or aluminum acac to give a dispersed solution where the individual particles , i . e . the encapsulated semiconductor nanocrystals 11 , do not aggregate . This is a paste that is applied, in particular screen printed, spin coated or spray coated, onto a substrate 30 and allowed to dry ( step I I ) . The result is a layer of densely packed semiconductor nanocrystals 11 .

[0082] Next , a ligand composition in a second solution comprising a molecule or salt capable of acting as a cross linker or aggregator is deposited onto the layer of densely packed semiconductor nanocrystals 11 to get silica-coated semiconductor nanocrystals 11 that are more strongly aggregated. For example, if adhesion and homogeneity of this film 10 should further be increased it can be additionally treated with acidic or basic solutions, like HC1 or KOH, or a salt like KOI . Other examples for a ligand composition are tetraethyl ortho silicate or a bifunctional molecule like 1 , 4 -bis ( trimethoxy silyl ) benzene .

[0083] The resulting material may be heated to remove low-boiling point components. The resulting layer of packed semiconductor nanocrystals 11 may be repeatedly used as a substrate for further depositions of semiconductor nanocrystals 11, if a higher thickness of the layer of packed semiconductor nanocrystals 11 is desired (not shown here) .

[0084] Next, as shown in figure 3a (step III) , a solution comprising a metal oxide precursor is deposited onto the layer of packed semiconductor nanocrystals 11 to create a film 10, which is homogenously infilled with metal oxide, i.e. with matrix material 12 in form a first barrier layer 12-2. For example, during step III only the metal oxide precursor is applied, while the required water is drawn from the already existing silica shells 12-1. According to another example, the layer of packed semiconductor nanocrystals 11 is in step III immersed in or spin-coated with a silica sol comprising tetraethylorthosilicate (TEOS) , ethanol, and HC1. According to another example, the metal oxide precursor is applied in step III via atomic layer deposition.

[0085] In this example, the film 10 is physically thin and optically dense due to a high concentration of semiconductor nanocrystals 11 in a thin layer of matrix material 12. By tuning the thickness of the encapsulation layers 12-1, the physical thickness as well as the optical density may be changed independently from each other. As shown in figure 3b, which basically corresponds to figure 3a, the encapsulation layers 12-1 have a higher thickness than the encapsulation layers 12-1 of figure 3a. Accordingly, after method step III, a film 10 results, that is physically thick, but not optically dense due to a low concentration of semiconductor nanocrystals 11 in a thick layer of matrix material 12, i.e. the barrier layer 12-2.

[0086] Figure 4 shows the method for producing a film 10 according to additional exemplary embodiments. A first solution comprising colloidally dispersed semiconductor nanocrystals 11 (left) is deposited onto a substrate 30 (step II, top) . A second solution comprising a molecule or salt capable of acting as a ligand composition or linker for the semiconductor nanocrystals 11 is deposited onto the semiconductor nanocrystals 11 on the substrate 30 to form a material where the ligand composition is different from the first solution and the mixture of the first and second solution. The resulting material may be heated to remove low- boiling point components to create a densely packed layer of semiconductor nanocrystals 11. This layer can be used once or repeatedly as a substrate for further semiconductor nanocrystals 11 (after step II, bottom of figure 4) .

[0087] Next, a solution comprising a metal oxide precursor is deposited onto the layer to create a metal-oxide infilled semiconductor nanocrystal 11 film 10, i.e. a film 10 comprising the semiconductor nanocrystals 11 embedded in a matrix material 12 in form of a barrier layer 12-2 (step III, top and bottom) . For example, in step III the layer of packed semiconductor nanocrystals 11 can be immersed in or spin- coated with a silica sol comprising TEOS, ethanol, and an acid (e.g. HC1 (aq) ) or a base (e.g. NH3(aq) ) in order to create the metal-oxide infilled film 10.

[0088] Thus, a film 10 being physically thin but not optically dense can be realized (right of figure 4, top) as well as a film 10 being physically thick and optically thick (right of figure 4, bottom) . In the film 10, the semiconductor nanoparticles 11 are protected against environmental influences like oxygen and humidity by the matrix material 12, i.e. the metal oxide.

[0089] In the methods as shown in figures 3 and 4, it is also possible to apply one or more additional metal-oxide layers, i.e. additional barrier layers 12-2, for example, via atomic layer deposition. This means, in step III alternately different metal oxide precursors are applied on the packed semiconductor nanocrystals 11, wherein the firstly applied metal oxide precursor completely covers the layer of packed semiconductor nanocrystals 11, or wherein the layer of packed semiconductor nanocrystals 11 is covered by at least two barrier layers 12-2 of metal oxide precursors. In this way, a stack of barrier layers 12-2 may be produced. In particular, the first barrier layer 12-2 of metal oxide comprises silica.

[0090] Even if figure 4 shows methods for producing a film 10 with semiconductor nanocrystals 11 that are free of individual encapsulation layers 12-1, the use of a previous encapsulation as shown with respect to figures 3a and 3b

[0091] (method step I) is also conceivable. Figure 5 shows schematically a side-view of a cross section of a film 10 according to another exemplary embodiment . Here , the film 10 comprises barrier layers 12-2 , which alternately comprise semiconductor nanocrystals 11 and are free of semiconductor nanocrystals 11 . The matrix material 12 may be in all barrier layers 12-2 the same or di f ferent from each other . For a better understanding, the barrier layers 12-2 being free of semiconductor nanocrystals 11 are explicitly shown in figure 5 . This exemplary embodiment is shown with semiconductor nanocrystals 11 being free of encapsulation layers 12- 1 , however, the presence of additional encapsulation layers 12- 1 is possible as well .

[0092] Figure 6a shows measurements of quantum yields QY in % over a time span of 15 days ( d) . The reference film Ref , as well as films 10 comprising examples El , E2 and E3 are applied in optoelectronic devices 100 as conversion elements . The photoluminescent quantum yield has been measured at days 0 , 7 , 12 and 15 ( except for example El ) .

[0093] The reference film Ref comprises I I-VI core shell quantum dots without any encapsulation . The example El comprises a film 10 containing I I-VI semiconductor nanocrystals 11 and a ZnS matrix material 12 . Example E2 comprises I I-VI semiconductor nanocrystals 11 and a matrix material 12 comprising barrier layers 12-2 of AI2O3 and TiCy . The example E3 comprises I I-VI semiconductor nanocrystals 11 and a matrix material 12 comprising barrier layers of AI2O3, TiCy , and ZnS in film 10 . The reference example and the examples have been exposed to high humidity and a high temperature of about 85 ° C to 100 ° C during the 15 days . It can be seen that the semiconductor nanocrystals 11 of the examples are well protected by the various matrix materials 12 , such that the films 10 comprising the examples show enhanced optical stability, in particular luminescence stability .

[0094] Additionally, the high QY at the beginning ( day 0 ) prove a good chemical compatibility of the semiconductor nanocrystals 11 and the matrix material 12 .

[0095] Figure 6b shows photographs of the films according to reference example Ref ( top ) and according to example E3 (bottom) at day 15 . It can be seen that the exemplary film (E3 ) has an enhanced film stability under illumination, while reference example Ref has not . The enhanced chemical stability under illumination of the film 10 as described here is supported by the precise tuning of the metal oxides .

[0096] Figure 7 shows schematic side views of a cross section of an optoelectronic device 100 according to exemplary embodiments . The optoelectronic device 100 comprises a semiconductor chip 20 and a conversion element comprising the film 10 as described here . The semiconductor chip 20 is configured to emit electromagnetic radiation of a first wavelength range (primary radiation) . The conversion element comprising the film 10 is arranged on the semiconductor chip 20 . It is able to absorb the primary radiation or to convert it into electromagnetic radiation of a second wavelength range ( secondary radiation) . The thickness t of the film 10 is in a range of 100 nm to 50 pm, for example .

[0097] In one example , the film 10 is applied only on the radiation exit surface of the semiconductor chip 20 ( figure 7a ) . In another example , the film 10 covers also the side surfaces of the semiconductor chip 20 ( figure 7b ) . Further, the semiconductor chip 20 may be applied on a carrier 40 ( figure 7c ) . The optoelectronic device 100 is a LED, in particular a p- LED . The film 10 may be in direct mechanical contact with the semiconductor chip 20 or attached there , for example , with an adhesive (not shown here ) .

[0098] Due to the film 10 as described here , the optoelectronic device 100 has , for example , a high luminescence stability . Additionally, with the tunable physical thickness and optical density of film 10 the optoelectronic device 100 can be adj usted according to its intended application . For example , the film 10 may have variable physical thicknesses t with constant absorption behavior . On the other hand, a film 10 may be provided having a constant thickness t but an adj ustable optical density, i . e . degree of absorption . I f desired, the optical density can even be tuned such that the film absorbs all of the primary radiation .

[0099] The features and exemplary embodiments described in connection with the figures can be combined with each other according to further exemplary embodiments , even i f not all combinations are explicitly described . Furthermore , the exemplary embodiments described in connection with the figures may have alternative or additional features as described in the general part .

[0100] The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments . Rather, the invention encompasses any new feature and also any combination of features , which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments , even i f this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

[0101] This patent application claims the priority of US patent application 63 / 514,123, the disclosure content of which is hereby incorporated by reference.

[0102] References

[0103] 10 film

[0104] 11 semiconductor nanocrystal

[0105] 12 matrix material

[0106] 12- 1 encapsulation layer

[0107] 12-2 barrier layer

[0108] 20 semiconductor chip

[0109] 30 substrate

[0110] 100 optoelectronic device t thickness

[0111] I method step

[0112] I I method step

[0113] I I I method step d days

[0114] QY quantum yield

[0115] Ref reference example

[0116] El exemplary embodiment

[0117] E2 exemplary embodiment

[0118] E3 exemplary embodiment

[0119] A solution of semiconductor nanocrystals

[0120] Al solution of semiconductor nanocrystals

[0121] B reference film

Claims

Claims1. A film (10) , comprising: a layer of packed semiconductor nanocrystals (11) , and a matrix material (12) comprising at least one metal oxide, wherein the matrix material (12) encapsulates the semiconductor nanocrystals (11) .

2. The film (10) according to claim 1, wherein the matrix material (12) comprises a plurality of individual encapsulation layers (12-1) , each encapsulation layer (12-1) encapsulating at least one semiconductor nanocrystal (11) .

3. The film (10) according to any of claims 1 or 2, wherein the matrix material (12) comprises a first barrier layer (12— 2) on the semiconductor nanocrystals (11) .

4. The film (10) according to any of claims 1 to 3, wherein the matrix material comprises at least one additional barrier layer (12-2) on the layer of packed semiconductor nanocrystals (11) .

5. The film (10) according to claim 4, wherein at least two adjacent barrier layers (12-2) comprise different metal oxides .

6. The film (10) according to any of claims 1 to 5, comprising multiple layers of packed semiconductor nanocrystals (11) and a first barrier layer (12-2) inbetween, the layers of packed semiconductor nanocrystals (11) and a first barrier layer (12-2) in-between being separated by at least one additional barrier layer (12-2) .

7. The film (10) according to any of claims 1 to 6, wherein the at least one metal oxide comprises a metal chosen from Al, Ti, Zr, Hf, Si, and mixtures thereof.

8. The film (10) according to any of claims 1 to 7, wherein at least one metal oxide comprises silica.

9. The film (10) according to any of claims 1 to 8, comprising a thickness in the range of 100 nm inclusive to 50 pm inclusive.

10. The film (10) according to any of claims 1 to 9, comprising a concentration of semiconductor nanocrystals (11) in the range of 0.5 wt% inclusive to 90 wt% inclusive.

11. The film (10) according to any of claims 1 to 10, wherein the semiconductor nanocrystals (11) and / or the matrix material (12) comprise inhomogeneities with a size in a range of 0.1 pm inclusive to 1 pm inclusive.

12. A method for producing a film (10) , the method comprising the steps: providing semiconductor nanocrystals (11) , mixing the semiconductor nanocrystals (11) with at least one solvent to form a first solution, processing the first solution to form a layer of packed semiconductor nanocrystals (11) , wherein semiconductor nanocrystals (11) are provided that are individually encapsulated by a matrix material (12) comprising at least one metal oxide and / or wherein a matrix material (12) is formed on the layer of packed semiconductor nanocrystals (11) by applying at least one metal oxide precursor .

13. The method according to claim 12, wherein processing the first solution comprises at least one of adding a second solution comprising a ligand composition and applying energy.

14. The method according to any of claims 12 or 13, wherein the metal oxide precursor is applied by a method chosen from atomic layer deposition, doctor blading, spin coating, spray coating, immersion, and electro spraying.

15. The method according to any of claims 12 to 14, wherein forming the matrix material (12) comprises applying at least two different metal oxide precursors.

16. The method according to any of claims 12 to 15, wherein forming the matrix material (12) comprises forming at least one additional barrier layer on the layer of packed semiconductor nanocrystals (11) .

17. An optoelectronic device, comprising a film (10) according to any of claims 1 to 11.

18. The optoelectronic device (100) according to claim 17, further comprising: a semiconductor chip (20) configured to emit an electromagnetic radiation of a first wavelength range; and a conversion element being configured to partially or fully convert the electromagnetic radiation of a first wavelength range into an electromagnetic radiation of a second wavelength range, wherein the conversion element comprises the film (10) .

19. The optoelectronic device (100) according to claim 18, being a p-LED.

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