WO2020177907A1

WO2020177907A1 - Light converting arrangement, and a method in relation thereto

Info

Publication number: WO2020177907A1
Application number: PCT/EP2019/082098
Authority: WO
Inventors: Haichun Liu; Hans ÅGREN; Qingyun Liu
Original assignee: Solar Nanoconverter AB
Current assignee: Solar Nanoconverter AB
Priority date: 2019-03-01
Filing date: 2019-11-21
Publication date: 2020-09-10
Anticipated expiration: 2021-09-01

Abstract

A light converting arrangement (2) comprising a transparent polymeric film (4) and upconversion nanoparticles (UCNPs) (6) wherein the UCNPs are incorporated into the film according to a predetermined incorporation procedure resulting in a predefined incorporation structure, the polymeric film has a thickness, a first surface (8) and a second surface (10). The arrangement further comprises an optical microstructure layer (12) comprising a plurality of microstructures arranged at one side of said layer wherein said microstructures are shaped by a microstructure shaping procedure, said plurality of microstructures include one or many types of microstructures defined by predetermined type definition, wherein said optical microstructure layer (12) is structured to be arranged in connection to said transparent polymer film (4) in a predetermined relationship to said UCNP incorporation structure such that said optical microstructure layer (12) faces incident light and such that photon conversion performance of said UCNPs is maximized with regard to incident light.

Description

Light converting arrangement, and a method in relation thereto

Technical field

The present disclosure relates to a light converting arrangement, and a method in relation to such an arrangement. In particular the light converting arrangement may be applied in connection with solar cells.

Background

The infrared (IR) region, which contains roughly half of the irradiated solar energy, has for long constituted the hardest part to be better utilized in the solar irradiation spectrum in solar cells. This is because the involved photosensitizers such as dyes and perovskites typically have very limited response to IR light. Commercially incumbent silicon-based solar cells also underutilize the IR-region, mostly converting visible light into electrical energy. Lanthanide-doped upconversion nanoparticles (UCNPs), which are able to convert the spectral energy in the infrared range into shorter- wavelength IR, visible, or even UV range, can provide a solution to circumvent the transmission loss of photovoltaic devices by converting two or more sub-band-gap photons into one above-band-gap photon, and potentially change the status of IR energy utilization of various type of solar cells.

Generally, photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength.

Recent advances of upconversion nano-chemistry has led to a readily access to high- quality UCNPs with high luminescence quantum efficiency up to 19% and above.

However, such high quantum efficiency of UCNPs is sealed by their relatively high excitation-intensity threshold, typically above 1 W/cm², which is much higher than that of IR light in the solar spectrum. As a result, when incorporated in solar cells, most of the light converting capacity of upconversion nanoparticles is wasted due to the low excitation irradiance, leading to compromised upconversion efficiency. This is fundamentally rooted in the multi-photon nature of the upconversion luminescence (UCL). In view of the nonlinearity of UCNPs, it is herein applied a strategy of modulating the spatial distribution of delivered excitation photons to evoke the prominent intrinsic photon conversion capacity of upconversion nanocrystals, which can be only achieved at high excitation intensity.

Crystalline silicon (c-Si) solar cells typically contain p- and n-doped regions of silicon with an n-type emitter layer on top of a p-wafer with a so-called p-n junction between the two layers. Monocrystalline silicon cells are more expensive than polycrystalline cells but have higher efficiency. The majority of installed silicon-based modules have efficiencies in the range 15-17 %, but the most efficient ones in the market now exceed 22 %. Mono- and polycrystalline silicon cells are the dominant technologies in the world today, each with a market share of 40-50 %. The current industry expectation is that monocrystalline silicon cells (in particular mono PERC-cells) will gain market share at the expense of poly crystalline solar cells in coming years.

A perovskite solar cell (PSC) is a type of solar cell which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide -based material, as the light-harvesting active layer. Perovskite materials, such as methylammonium lead halides and all-inorganic cesium lead halide, are cheap to produce and simple to manufacture.

A dye-sensitized solar cell (DSSC, DSC, DYSC or Gratzel cell) is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system.

The DSSC has a number of attractive features; it is simple to make using conventional roll-printing techniques, is semi-flexible and semi-transparent which offers a variety of uses not applicable to glass-based systems, and most of the materials used are low-cost. In practice it has proven difficult to eliminate a number of expensive materials, notably platinum and ruthenium, and the liquid electrolyte presents a serious challenge to making a cell suitable for use in all weather. Although its conversion efficiency is less than the best thin- film cells, in theory its price/performance ratio should be good enough to allow them to compete with fossil fuel electrical generation by achieving grid parity.

In general, grouped together thin film cells currently have a market share of around 5 %.

Below is listed some patent documents that disclose background technology regarding applying upconversion nanoparticles in solar cells.

CN-107437586 relates to a method of preparing a polymer solar cell with an organic molecule inorganic upconversion nano heterostructure.

CN-105219390 relates to upconversion material capable of being applied to dye-sensitized solar cells and preparation method therefor.

An object of the present invention is to achieve an improved upconversion nanoparticle arrangement which is applicable e.g. for solar cells.

Summary

The above-mentioned objects are achieved by the present invention according to the independent claims.

Preferred embodiments are set forth in the dependent claims.

The dearth of high upconversion luminescence (UCL) intensity at low excitation irradiance hinders the prevalent applications of lanthanide-doped upconversion nanoparticles (UCNPs) in many fields ranging from optical bioimaging to photovoltaics.

In this disclosure, microlens arrays (MLAs) are used as spatial light modulators to manipulate the distribution of excitation light fields in order to increase UCL, taking advantage of its nonlinear response to the excitation irradiance. It is disclosed that multicolored UCL from NaYF₄:Yb³⁺,Er³⁺@NaYF₄:Yb³⁺,Nd³⁺ and NaYF₄:Yb³⁺,Tm³⁺@ NaYF₄:Yb³⁺,Nd³⁺ core/shell UCNPs can be increased by more than one order of magnitude under either 980 or 808-nm excitation, by arranging a polymeric MFA onto the top of these samples. The observed typical green (525/540 nm) and red (654 nm) UCF bands from Er³⁺ and blue (450/475 nm) UCL band from Tm³⁺ exhibit distinct

enhancement factors due to their different multi -photon processes. Importantly, in a ray tracing simulation it is revealed that the MLA is able to spatially confine the excitation light (980 and 808 nm) by orders of magnitude, thus amplifying UCL by more than 225- fold (the 450 nm UCL band of Tm³⁺) at low excitation irradiance. The proposed MLAs method has immediate ramifications for the improved performance of all types of UCNPs- based devices, as for example the here demonstrated UCNP-enhanced dye sensitized solar cells.

Technically, it is proposed to utilize polymer-based microlens arrays (MLAs), in combination with UNCPs, to spatially modulate the excitation light. It turns out that the light concentration caused by the MLA can lead to significant UCL enhancement, readily higher than one order of magnitude. It is performed ray tracing simulations, linking to the excitation light intensity response of nanocrystals, to seek optimal microlens

configurations for the upconversion enhancement. The proposed strategy can conquer the high excitation-intensity threshold of upconversion nanomaterials and bring about future breakthroughs in their energy applications to enhance the performance of solar cells in the IR range. It should be noted that the approach is general and can be combined with other strategies, chemical or photonic, of enhancing UCL.

Without altering the already existing designs and merits of solar cells, upconversion nanotechnology mediated by lanthanide upconversion nanoparticles (UCNPs) will provide a solution to circumvent the above-mentioned transmission loss by converting two or more sub-band-gap photons into one above-band-gap photon.

An important feature of the present invention is the use of colloidal UCNPs as energy- relay materials to make use of the otherwise useless IR light in dye-sensitized solar cells (DSSCs). Incorporation of UCNPs remarkably improved the performance of solar cells in the IR range. Recent advances of upconversion nanochemistry has led to readily access to high-quality upconversion nanomaterials with high luminescence quantum efficiency up to 19%. However, such exceptional upconversion performance is not always at hand, and the practical efficacy is often severely compromised by the excitation light level for most UCNPs, with low excitation irradiance leading to low upconversion efficiency. This is fundamentally rooted in the multi-photon or nonlinear nature of the upconversion emission.

An important aspect of the present invention is combining UCNPs with optical microstructures, e.g., microlens arrays (MLAs), to maximize the photon conversion performance of these nanoparticles.

Taking advantage of the nonlinearity of UCNPs, the role of the MLAs is to control the spatial excitation light pattern to evoke the exceptional light conversion capacity of UCNPs even at low excitation irradiance.

Incorporation of UCNPs and MLAs into polymeric films can be easily integrated into photovoltaic devices to improve their performance in the IR range. In addition, the same UCNP -incorporated MLA films can be easily translated to laser detection field as IR detection cards. Current IR detection cards suffer from either low sensitivity or material fatigue issue when used for visualizing, profiling and aligning IR laser beams, leading to the loss of beam-profile information in the spatial and time domains.

Below some important aspects and features of the present invention are listed:

-High-quality UCNP-incorporated polymeric films are provided.

-A microlens array structure is arranged on the surface of UCNP-incorporated polymeric films.

-UCNP-incorporated MLA films enhanced solar cells.

-UCNP-incorporated MLA films used as detection cards to visualize IR laser beams.

By applying the arrangement and method according to the present invention it is achieved to increase the response of solar cells to IR solar radiation and the performance of IR detection cards. Among various resulting applications of the present invention may be mentioned highly efficient light-converting (NIR-to-VIS) MLA films that can be easily integrated into photovoltaic and IR detection devices. According to one resulting application of the present invention is high-quality UCNP- incorporated polymeric films. According to one aspect of the present invention, it involves incorporation of the UCNPs (being e.g. high-quality pure UCNPs or dye-sensitized UCNPs with various compositions) into low-cost polymer matrices to obtain transparent polymer films. In situ polymerization of nanoparticles (NPs) in polymer matrixes to obtain bulk polymer-NP composites often results in the loss of transparency in the final product. In large bulk polymerization of nanocomposites a phase separation of the NPs during the polymerization process regularly occurs which leads to agglomeration of the NPs and a turbid final product because of scatter. The inventors have developed procedures to incorporate UCNPs in transparent polymer composite materials which prevent

agglomeration of the NPs during the polymerization process. These procedures revolve around either ligand exchange of the NPs’ original stabilizing ligands or the selection of appropriate polymeric hosts that inhibit agglomeration of the NPs during polymerization. Thereby, the loading of NPs is maximized without sacrificing the transparency of the films and generate large-area films with uniform and well controllable thickness.

More particularly, the upconversion nanoparticles after synthesis are coated with oleates, so they can be well dispersed in non-polar solvents such as cyclohexane, hexane, toluene, chloroform. Since these nanoparticles are incorporated into a polymer matrix, they are required to be well dispersible in the matrix without agglomeration. Then the ligands on the nanoparticle surfaces should be miscible with the polymer matrix. Then, for a selected matrix, we may need to use ligand exchange to replace the oleates with others in order to make the nanoparticles compatible with the matrix. If we want to keep the oleates on the nanoparticle surfaces, we have to choose appropriate polymer matrix. Note that this is a general strategy, which is not limited to concrete ligands and polymers.

In addition, we developed procedures to control the axial distribution of UCNPs in the film. The desired NP distribution is the confinement of NPs within a thin layer near the surface of the film with micrometer precision, in order to make sure all NPs are localized near the focusing plane of the MLA that to be generated subsequently. For the polymer materials, we use cost-effective polymethyl methacrylate (PMMA) and polycarbonate (PC), which enables large-scale production with low cost, but not limited to these two. This may be achieved by using a spin coating method, layer-by-layer, to control the axial distribution of nanoparticles in the films. Spin-coating method can produce films with micrometer precision (thickness). By using nanoparticle+polymer mixture for a specific layer or layers (but not other layers), the axial distribution can be controlled.

With high-quality pure UCNP- and dye-sensitized-UCNP-incorporated polymeric films, we fabricated microlens array structure on the surface of the film on the opposite side to the UCNP layer. The MLA structure is manufactured with but not limited to the LIGA- like (Lithography, Electroplating, and Molding) and roll-to-roll (R2R) nano -imprinting technologies. Structuring of aperture arrays of microlenses can be within 1-1000 pm on thin films. The microlens molds are also manufactured using but not limited to integrated circuit (IC) technologies like photolithography, photo-resist processing and reactive ion etching. These manufacturing technologies allow a very accurate shaping of the lens profile and a precise positioning of the lenses within an array. Different parameters of MLA can be designed such as numerical aperture (NA), radius of curvature (ROC), field of view (FOV), etc. Prior to the microstructure manufacturing, ray tracing simulation are performed to investigate the spatial light modulation effect of the MLA. The UCNP layer should be ideally located on the focusing plane of the MLA structure. The thickness of the film and the NA, ROC and FOV are optimized for the MLA in order to achieve a maximum UC emission enhancement. Various light irradiation conditions, normal incidence and others, are investigated, and the results are put together in order to identify a group of parameters which ensure considerable UC emission enhancement for all conditions. The optimal parameters obtained from the ray tracing simulations are fed back to the manufacturing of the MLA film.

After obtaining UCNP-incorporated MLA films with optimized MLA parameters, their utility in solar cells were tested. The goal is to reach high value increases for state-of-the- art solar cells working in the visible region. Importantly, the selected polymer materials have high transparency in the visible and NIR range, more than 95%, so the polymeric MLA films would not disturb the utility of visible light in these solar cells. In addition, the function of the developed UCNPs does not depend on the type and structure of the solar cells, and can be implemented rather straightforwardly to various types, such as DSSCs, silicon-based solar cells, and perovskite solar cells with current top performance but which have poor response in the NIR range. Our product can easily step into the photovoltaic industry, e.g., as an accessory for any type of solar cells. The technical maturity of nanoparticle synthesis, polymer film production and MLA structure fabrication ensures the potential of mass production.

According to another embodiment, UCNP-incorporated MLA films are applied as detection cards to visualize IR laser beams.

Current IR laser detection cards are made of organic or inorganic IR sensitive materials spread on certain planar substrates. Those organic formats have high sensitivity so can be used to detect low power sources.

However, they have low fatigue threshold and the visualized laser beam pattern can thus easily become smeared under the irradiation of high laser power, losing spatial information of the laser beam pattern. The inorganic formats, typically also lanthanide elements-based, can visualize laser beams with a high spatial resolution, but cannot work for low power sources due to their poor sensitivity. The present invention regarding UCNP-incorporated MLA films will improve this and potentially change the state of the art of IR laser detection.

Brief description of the drawings

Figure 1 is a schematic illustration of microlens array-enhanced quantum yield of upconversion luminescence due to their nonlinear response to excitation intensity

Figure 2 shows scanning electron microscope (SEM) images of (a) the MLA composed of polycarbonate material, side view, and (b) surface structure, top view (c) Optical microscopy image of the MLA. (d) Schematic of the optical setup for the MLA

enhancement effect study: D - Detector; S - Source. UCL spectra of

NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under (e) 980 nm and (f) 808 nm CW excitation with and without MLA light modulation (Average excitation intensity: 1.3 W/cm²). UCL spectra of NaYF₄:20%Yb³⁺,0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under (g) 980 nm and (h) 808 nm CW excitation with and without MLA light modulation (Average excitation intensity: 1.3 W/cm²). Figure 3 illustrates excitation intensity dependent enhancement factors induced by the addition of MLA of different emission bands of (a)

NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ and (b)

NaYF₄:20%Yb³⁺,0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under 808 nm CW excitation. UCL spectra of NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under (c) 980 nm and (d) 808 nm CW excitation with and without MLA light modulation (average excitation intensity: 0.1 W/cm²). UCL spectra of

NaYF₄:20%Yb³⁺,0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under (e) 980 nm and (f) 808 nm CW excitation with and without MLA light modulation (average excitation intensity: 0.1 W/cm²).

Figure 4 shows: (a) Geometry used in the ray-tracing simulations. Simulated excitation light intensity distribution (b) before and (c) at the right edge of the MLA. (d) Line profile analyses of excitation light intensity along the selected lines indicated by white lines in (b) and (c). (e) Calculated enhancement factors for upconversion emission bands with different slope factors.

Figure 5 shows: (a) Schematic illustration of the configuration of UCNP- and/or MLA- enhanced DSSC. S - Source (b) The current density-voltage (J-V) characteristics of DSSC, DSSC+UCNP, DSSC+MLA and DSSC+UCNP+MLA under AMI .5 G light irradiation (0.1 W/cm²).

Figure 6 shows: TEM images of (a) core NaYF₄:20%Yb³⁺,2%Er³⁺, (b) core-shell

NaYF₄:20%Yb³⁺, 2%Er³⁺@20%Yb³⁺,30%Nd³⁺, (c) core NaYF₄:20%Yb³⁺,0.5%Tm³⁺, (d) core-shell NaYF₄:20%Yb³⁺, 0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles. Scale bars: 50 nm.

Figure 7 shows: (a) Upconversion emission spectra of NaYF₄:20%Yb³⁺,

2%Er³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under CW 980 nm and 808 nm excitation. Excitation power density for both lasers was 1.3 W/cm². Excitation power-density response of NaYF₄:20%Yb³⁺, 2%Er³⁺@20%Yb³⁺,30%Nd³⁺ under (b) CW 980 nm excitation and (c) CW 808 nm excitation (d) Upconversion emission spectra of

NaYF₄:20%Yb³⁺, 0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under CW 980 nm and 808 nm excitation. Excitation power density for both lasers was 14.5 W/cm2. Excitation power-density response of NaYF₄:20%Yb³⁺, 0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ under (e) CW 980 nm excitation and (f) CW 808 nm excitation. Laser beam diameter for both lasers: ~1.0 mm.

Figure 8 illustrates a transmission spectrum of the microlens array.

Figure 9 shows upconversion luminescence spectra of

NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ nanoparticles under 980 nm CW excitation (Average excitation intensity: 3.9 W/cm²) without and with the addition of MLA, using different solvents as the interface medium (ethanol, water, methanol).

Figure 10 is a schematic side view illustrating the light converting arrangement according to the present invention.

Figure 11 is a flow diagram of the method according to the present invention.

Figure 12 is a schematic structure of the dye-sensitized upconversion materials featuring energy-cascade upconversion.

Detailed description

The light converting arrangement, and the method in relation to the arrangement, will now be described in detail with references to the appended figures. Throughout the figures the same, or similar, items have the same reference signs. Moreover, the items and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

First, the light converting arrangement and the method, will be described with references to figures 10 and 11, respectively. Thereafter will follow a detailed disclosure of the arrangement and method with references to figures 1-9, where, for example, various diagrams illustrating various aspects of the arrangement are shown. Note that some of the drawings are colour drawing in order to be able to fully disclose all aspects of the present invention.

With references to figure 10, a light converting arrangement 2 is provided, that comprises a transparent polymeric film 4 and upconversion nanoparticles (UCNPs) 6, wherein the UCNPs are incorporated into the film according to a predetermined incorporation procedure resulting in a predefined incorporation structure. The polymeric film has a thickness, a first surface 8 and a second surface 10. The arrangement further comprises an optical microstructure layer 12 comprising a plurality of microstructures arranged at one side of the layer 12, wherein the

microstructures are shaped by a microstructure shaping procedure. The plurality of microstructures include one or many types of microstructures defined by predetermined type definition. The optical microstructure layer 12 is structured to be arranged in connection to the transparent polymer film 4 in a predetermined relationship to the UCNP incorporation structure such that the optical microstructure layer 12 faces incident light and such that photon conversion performance of the UCNPs is maximized with regard to the incident light.

According to one embodiment the optical microstructures comprise a microlens array comprising a plurality of microlenses. Preferably, the optical microstructures are shaped on the first surface 8 of the transparent polymeric film.

The microstructure shaping procedure applied to shape the microstructures comprises using LIGA-like (Lithography, Electroplating, and Molding) and roll-to-roll (R2R) nano imprinting technologies, or using integrated circuit (IC) technologies like

photolithography, photo-resist processing and reactive ion etching, but not limited to these technologies.

In a further embodiment the microstructure shaping procedure comprises applying different parameters of the optical microstructure such as one or many of numerical aperture (NA), radius of curvature (ROC), field of view (FOV), and also the thickness of the transparent film.

The shape of one microstructure may be essentially half spherical and has a radius (ROC) of 50 pm (see figure 2). The shape may also be a smaller part of a sphere which is illustrated in figure 4a, i.e. the part cut off along a circle of the sphere, where the circle is less than the great circle of the sphere.

Below is included some examples of values/ranges of NA ROC FOV:

NA: 0.08 ~ 0.62 ROC: 8 - 2000 mm

FOV: The MLA field of view increases with the number of lenses. Can be as large as 180°.

According to one embodiment, the incorporation structure of UCNPs comprises confining UCNPs within a thin layer near the second surface of the polymer film with micrometre precision, in order to make sure all nanoparticles are localized near the focusing plane of the optical microstructure layer. See figure 4a where D2 designates the position of the UCNPs.

According to a further embodiment the UCNPs comprises lanthanide UCNPs. Preferably, the UCNPs comprises the core NaYF₄:20%Yb³⁺,2%Er³⁺ and NaYF₄:20%Yb³⁺,0.5%Tm³⁺ nanoparticles and corresponding core-shell structured

NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ and

NaYF₄:20%Yb³⁺,0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺, respectively. This embodiment will be further discussed below.

Generally, the photon conversion performance of the UCNPs comprises conversion from infrared light to visible light. Also this aspect will be further discussed below, and in particular with references to the figures, e.g. the diagrams presented in figures 2e-2h, figures 3a-3f, and figures 7a-7f.

Preferably, the transparent polymer film comprises polymethylmethacrylate (PMMA) or polycarbonate (PC).

The present invention also relates to a method in relation to a light converting arrangement which has been described above and with reference to the flow diagram of figure 11.

The method comprises incorporating upconversion nanoparticles (UCNPs) into a transparent polymeric film according to a predetermined incorporation procedure resulting in a predefined incorporation structure, the polymeric film has a thickness, a first surface and a second surface. The method further comprises:

-shaping a plurality of microstructures, according to a microstructure shaping procedure, on one side of an optical microstructure layer, wherein said plurality of microstructures include one or many types of microstructures defined by predetermined type definition, and

-arranging said optical microstructure layer in connection to said transparent polymer in a predetermined relationship to said UCNP incorporation structure such that said optical microstructure layer faces incident light and such that photon conversion performance of said UCNPs is maximized with regard to incident light.

The method further comprises shaping the optical microstructures on the first surface of the transparent polymeric film. Preferably, the microstructure shaping procedure comprises applying different parameters of the optical microstructure such as one or many of numerical aperture (NA), radius of curvature (ROC), field of view (FOV), and also the thickness of the transparent film.

According to a further embodiment of the method, the incorporation structure of UCNPs comprises confining UCNPs within a thin layer near the second surface of the polymer film with micrometre precision, in order to make sure all nanoparticles are localized near the focusing plane of the optical microstructure layer.

The present invention also comprises a solar cell provided with a light converting arrangement as described above, and also below. The solar cell may be e.g. a perovskite solar cell (PSC), or a dye-sensitized solar cell (DSSC, DSC, DYSC or Gratzel cell).

Furthermore, the present invention comprises a detection card to visualize infrared radiation, comprising a light converting arrangement as described above, and also below.

In the following is included further disclosure and detailed discussion of various aspects of the light converting arrangement, and method in connection with the arrangement, according to the present invention. Experiment

Materials

Yttrium(III) chloride hexahydrate (YC1₃·6H₂O, 99.99%), neodymium (III) chloride hexahydrate (NdC1₃·6H₂O, 99.99%), erbium (III) chloride hexahydrate (TmC1₃·6H₂O, 99.99%), thulium (III) chloride hexahydrate (TmC1_3·6H₂0, 99.99%), sodium hydroxide (NaOH), ammonium fluoride (NH₄F), 1-octadecene (ODE), oleic acid (OA), ethanol, methanol, and cyclohexane were purchased from the company Sigma-Aldrich. All the chemicals were used without further purification.

Nanoparticle Syntheses

Synthesis of core NaYF ₄:20%Yb³⁺ , 2%Er³⁺ nanoparticles

Upconversion nanoparticles were synthesized with NaYF₄ as the host material using a method as previously reported. YbC1₃·6H₂O, (0.2 mmol), YC1₃·6H₂O, (0.78 mmol) and ErC13·6H₂O (0.02 mmol) were mixed with 15 mL 1-octadecene (ODE) and 6 mL oleic acid (OA) and heated to 160 °C and maintained for 30 min under argon atmosphere to form a homogeneous solution. After the precursor solution cooled down to room temperature, a methanol solution of 0.1 g NaOH and 0.148 g NH₄F was slowly added and the mixture was stirred for 5 min. The mixture was heated to 80 °C and maintained for 30 min to evaporate methanol. Subsequently the solution was degassed, quickly heated to 300 °C, and kept for 60 min, under protection of argon atmosphere. The solution was then cooled and the nanoparticles were purified through centrifugation using ethanol and water. The supernatant was discarded, and the precipitate was finally suspended in cyclohexane.

Synthesis of core-shell NaYF₄: 20%Yb³⁺, 2%Er³⁺@NaYF₄:20%Yb³⁺, 30%Nd³⁺

nanoparticles

NaYF₄:20%Yb³⁺, 2%Er³⁺@NaYF₄:20%Yb³⁺, 30%Nd³⁺ core-shell nanoparticles were synthesized by epitaxially growing a NaYF₄: 20%Yb³⁺, 30%Nd³⁺ shell onto the as- prepared NaYF₄:20%Yb³⁺, 2%Er³⁺ core nanoparticles following a previously reported protocol.

In a typical synthesis, YbCl_3*6H₂0 (0.1 mmol), YCb*6H₂0 (0.25 mmol), and

NdCh_*6H₂0 (0.15 mmol) were mixed with 15 mL ODE and 6 mL OA in a 100 mL three- neck flask. The solution was heated to 150 °C and kept for 30 min to form a homogeneous solution, and then cooled down to room temperature. A suspension of the NaYF₄:20%Yb³⁺, 2%Er³⁺ core nanoparticles dispersed in cyclohexane was added to the flask. The solution was maintained at 110 °C to remove the cyclohexane solvent and then subsequently cooled down to room temperature. A methanol solution of 0.05 g NaOH and 0.075 g NH₄F in 10 mL methanol was added into the flask and stirred for 5 min. Then the solution was heated to 80 °C to remove methanol. After methanol was evaporated, the solution was heated to 300 °C and incubated for 60 min under an argon atmosphere. The mixture was then cooled down to room temperature. The nanoparticles were precipitated with acetone, collected after centrifugation, then washed thrice with ethanol/water (1 :1 v/v) and finally dispersed in cyclohexane for subsequent use.

Synthesis of core NaYF ₄:20%Yb³⁺ , 0.5%Tm³⁺ nanoparticles

The synthesis of core NaYF₄:20%Yb³⁺, 0.5%Tm³⁺ was similar to that of core

NaYF₄:20%Yb³⁺, 2%Er³⁺, but the types of lanthanide chlorides and their amount were adjusted accordingly.

Synthesis of core-shell NaYF4: 20%Yb³⁺, 0.5%oTm³⁺ (afNaYFr. 20%Yb³⁺, 30%Nd³⁺ nanoparticles

The synthesis process of core-shell NaYF₄:20%Yb³⁺, 0.5%Tm³⁺@NaYF₄: 20%Yb³⁺, 30%Nd³⁺ was similar to that of core-shell NaYF₄:20%Yb³⁺, 2%Er³⁺@NaYF₄:20%Yb³⁺, 30%Nd³⁺, but the types of lanthanide chlorides and their amount were adjusted accordingly.

Characterization

The structure and morphological characterization were performed on a transmission electron microscope (JEOF, JEM- 1400). Luminescence spectra were recorded on an Edinburgh FS5 spectrophotometer equipped with 808 nm and 980 nm diode lasers. An optical microscope (Olympus CX23) and a surface profiler (Veeco, Dektek 150) were used to characterize the surface structure of MLA. The solar simulator (Newport Oriel, LSC-100) and computerized Keithley 2400 source meter were used for current density- voltage (IV) measurement. Investigation of upconversion luminescence enhancement by addition of MLA

A suspension of UCNPs in cyclohexane (500 mL) was dropped onto the conducting side of a piece of FTO glass and then dried in mild air under room temperature. A collimated beam of NIR excitation light was shined on the nanoparticles, and generated emission light was collected by a fiber, placed at the rear side of the FTO glass slide, and then detected by a connected spectrometer. When MLA was applied, the MLA was attached to the UCNP surface with assistance of a thin layer of solvent (water, ethanol, methanol) between the MLA and the UCNP layer.

Current density-voltage (IV) measurement and characteristics

All the measurements below were performed on dye sensitized solar cells (DSSC) purchased from the 3GSolar company. The current density- voltage measurements were performed with a solar simulator (Newport, AM 1.5G, illumination at 0.1 W/cm²) and a computerized Keithley 2400 source meter. UCNPs in cyclohexane (250 pL, 10 mg/mL) were first dropped onto a thin cover slide (1.5 cm x 1.5 cm, thickness of 0.17 mm) and then dried in the air to form a UCNP layer. Current density-voltage (IV) measurements were performed on a reference DSSC, a DSSC with a dry UCNP layer or MLA placed on top, and a DSSC with both a dry UCNP layer and MLA placed on top to compare the efficiency differences. For all IV measurements, a black mask was placed on top of the whole DSSC setup to create a 1.0 cm x 0.7 cm exposure area.

Results and discussion

Motivation of using microlens arrays as spatial light modulators to enhance upconversion luminescence

Due to the nonlinearity of UCL, its quantum yield (QY) is generally not constant, but increases with the excitation intensity (see figure 1). For instance, for a standard two- photon UCL band, the QY, F_{2 -r h}, is scaled with the excitation intensity (I_ex) according to:

where F_s is the maximum QY that the two-photon UCL can achieve, and I_b is the balancing excitation intensity, at which the QY reaches the half of the maximum QY.

Thus, given the same dose of excitation photons, a higher excitation intensity would lead to a larger number of emission photons.

Noticing that UCNPs demand high excitation intensity to trigger their light-conversion capacity, it is devoted the efforts to seeking suitable light concentrators. It is realized that micrometer-sized lens arrays, among others, can be ideal candidates to improve the performance of UCNP-sensitized devices (see figure 1), e.g., solar cells. MLAs can not only efficiently concentrate excitation light due to their small radius of curvature, but also be easily integrated into photonic devices. In addition, the availability of relatively low- cost raw materials (e.g. polymer) and fabricating techniques potentially makes the cost affordable for large-scale production. In this disclosure, it is investigated the excitation- light modulation effect of MLAs on the luminescence intensity of UCNPs.

Morphologies and optical properties of upconversion nanoparticles

Core NaYF₄:20%Yb³⁺,2%Er³⁺ and NaYF₄:20%Yb³⁺,0.5%Tm³⁺ nanoparticles and corresponding core-shell structured NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ (denoted as YbEr@YbNd) and NaYF₄:20%Yb³⁺,0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺ (denoted as YbTm@YbNd) were synthesized following above reported protocols.

The morphologies of the synthesized nanoparticles were characterized on a transmission electron microscope. The core NaYF₄:20%Yb³⁺, 2%Er³⁺ nanoparticles have an average diameter of ~30 nm (Fig. 6(a)), and the core-shell YbEr@YbNd nanoparticles ~40 nm (Fig. 6(b)). The Tm³⁺-doped core and core-shell have similar average diameter to their Er³⁺-doped counterparts, ~30 nm and ~41 nm, respectively (Fig. 6(c) and Fig. 6(d)).

The UCF properties of the core-shell YbEr@YbNd and YbTm@YbNd nanoparticles were subsequently studied under continuous-wave (CW) 980 nm and 808 nm excitation. The YbEr@YbNd nanoparticles emitted relatively strong emission bands at 525/540 nm and 654 nm under both 980 nm and 808 nm excitation (Fig. 7(a)), originating from the ²Hii/2/⁴S3/2®⁴Ii5/2 and the ⁴Fc>/2®⁴Ii5/2 transition of Er³⁺ ions, respectively. The UCL intensity under 808 nm excitation was weaker than that under 980 nm excitation. A much weaker emission band at 409 nm, originating from the Er³⁺

²H_9/2®⁴II 5/2 transition, was also detected under both excitation approaches. The excitation intensity response of the nanoparticles was then quantified for both 980 and 808 nm excitation, by varying the excitation power and recording corresponding UCL spectra. The two-photon green emission bands at 525/540 nm exhibit near quadratic dependence on the excitation intensity at low excitation intensity (0.70 W/cm² - 8.67 W/cm², 23-285 mW with beam diameter of ~1.0 mm) upon CW 980 nm excitation, featuring a slope efficiency of 1.5 (Fig. 7(b)). Under the same condition, the red band at 654 nm shows a steeper dependence on the excitation intensity, with a slope efficiency of 1.8. The three-photon blue band at 409 nm shows a slope efficiency of 2.2. The excitation intensity (980 nm) dependence of all these upconversion bands become relented with increasing excitation power, which can be ascribed to a saturation effect. These UCL bands of YbEr@YbNd nanoparticles exhibit similar response to the intensity of CW 808 nm excitation light, but with a faster saturation trend (Fig. 7(c)).

The YbTm@YbNd nanoparticles emitted relatively strong emission bands at 650 nm, 475 nm, and 450 nm (Fig. 7(d)), originating from the transitions ¹G4 ³F4, ¹G4 ³H6, and ¹D2®³ 4 of Tm³⁺, respectively. These emission bands all show nonlinear dependence on the excitation intensity under both 980 nm and 808 nm excitations (Fig. 7(e) and 7(f)).

Upconversion luminescence enhancement by using a microlens arrays as excitation light spatial modulator

The MLA used in the experiments was made of polycarbonate (PC). The PC material has high transparency in the NIR range, up to 95% (Fig. 8). Therefore, there is less than 5% energy loss through the MLA structure. Scanning electron microscope (SEM) and optical microscope were used to characterize the surface structure and profile of the MLA. As shown in Fig. 2(a), the side-view SEM image illustrates a well-defined MLA with a period 51.02 pm. The height of the microlens is determined to be 16.47 mm and the width 47.08 pm, with a gap of 3.50 pm between neighbouring microlenses. The thickness of the slab part next to the curved layer is 75.00 pm. The top-view SEM image (Fig. 2(b)) and the optical microscopic image (Fig. 2(c)) illustrate well the periodic structure of the MLA. The UCL spectra of the nanoparticle samples were recorded with and without the addition of the MLA into the optical path of the excitation beam (Fig. 2(d)). During the

measurements, the MLA was attached to the nanoparticle layer with help of a thin-layer ethanol solvent in between.

Figure 2(e) presents the result for the YbEr@YbNd nanoparticles under 980 nm

excitation. Notably, the intensity of the green (525/540 nm) and red (654 nm) emission bands of Er³⁺ were enhanced by a factor of -3.6 and -14.0, respectively, after adding the MLA. The Er³⁺ blue emission band (409 nm), which was not detectable without the MLA at the applied excitation intensity (1.3 W/cm²), could still be well detected after the addition of the MLA to modulate the excitation light. After adding the MLA there was noticeable change in the emission colour even to the naked eye during the experiments (inset of Lig. 2(e)). The modulation effect of the MLA when using the 808 nm excitation light was also studied. As shown in Lig. 2(f), under 808 nm excitation, the green and red emission bands of Er³⁺ of YbEr@YbNd nanoparticles were enhanced by a factor of -3.7 and -16.0, respectively, after adding the MLA. Similar comparative studies were performed on the YbTm@YbNd nanoparticles. After the MLA was added into the optical path of the excitation beam (either 980 or 808 nm), right in front of the nanoparticle layer, the intensities of the different UCL bands of Tm³⁺ were remarkably enhanced (Lig. 2(g) and 2(h)). The strongest two-photon UCL band at 800 nm under 980 nm excitation was enhanced by a factor of 4.5 (Lig. 2(g)). The emission bands at 450 nm, 475 nm and 650 nm of Tm³⁺ become very significant after adding the MLA, with an enhancement factor of 108, 45, and 45, respectively, compared to without the MLA (Lig. 2(g)).

Similar upconversion enhancement effect of the MLA was also observed when using the 808 nm excitation (Lig. 2(h)). Particularly, the 450 nm UCL band was amplified by up to 135-fold (Lig. 2(h)) by the addition of the MLA. These results prove the excitation light modulation effect of MLA. The observed enhancement factors for different UCL bands under excitation of different wavelength correlate well with the excitation power response of the upconversion materials shown in Lig. 7. Since the UCL intensity ( I_f) is proportional to the nth power of the excitation intensity (/_ex), i.e., I_fxl_ex ⁿ, the emission enhancement factor would be related with the slope factor n, with a larger n leading to a larger enhancement. The bigger enhancement factors of the green (525/540 nm) emission band relative to the red (654 nm) band for the YbEr@YbNd nanoparticles under either 980 or 808 nm excitation (Fig. 2(e)-(f)) are consistent with its steeper excitation power dependence compared to the latter (Fig. 7(b)-(c)).

Similarly, for the YbTm@YbNd nanoparticles, the higher-order multiphoton UC emission band with a larger slope factor n (Fig. 7(e)-(f)) is associated with a bigger emission enhancement factor (Fig. 2(g)-(h)). In addition, the slightly bigger enhancement factors under 808 nm excitation compared to under 980 nm excitation of different emission bands can also be explained by the corresponding steeper excitation power dependence.

Due to the same reason, UCF bands should show a decreasing enhancement factor with increasing the incident excitation intensity, since they would exhibit gradual saturation, featuring elevated excitation power dependence, as evident in Fig. 7. We quantified the enhancement factors of different emission bands of the YbEr@YbNd and YbTm@YbNd nanoparticles under 808 nm excitation with different intensities, and the results verified our prediction (Fig. 3(a)-(b)). It is worthwhile mentioning that the enhancement factor for the 450 nm band under 808 nm excitation reached ~225 at a low excitation intensity of 0.2 W/cm² and dropped to 4.5 at an excitation intensity of 22.3 W/cm² (Fig. 3(b)). An indication of the excitation intensity-dependent enhancement factor is that the use of MFAs would remarkably boost the UCF intensity of UCNPs at excitation levels relevant to natural solar irradiation. We then measured UC emission spectra of the YbEr@YbNd and YbTm@YbNd nanoparticles at low excitation irradiance (0.1 W/cm²) with and without addition of the MFA. As shown in Fig. 3(c)-(f), the UCF enhancement is even more prominent than those shown in Fig. 3(a)-(b). Due to the weakness of the UCF without addition of the MFA, we could not quantify concrete enhancement factors for most emission bands.

Due to instrument limitation, we could not accurately control of the distance between the MFA and the nanoparticle layer. We tried to vary this distance by replacing ethanol with other solvents as the interface medium, with an intention of making use of their different viscosities and surface tensions. The MFA modulation experiment on the YbEr@YbNd nanoparticles under 980 nm excitation was repeated. As shown in Fig. 9, the UCF was significantly enhanced for all the tested solvents, including ethanol, water and methanol. With water, the emission was much less enhanced, while without using any solvent, it was difficult to achieve emission enhancement (data not shown).

Ray-tracing simulations of the excitation light modulation effect of the MLA

To better understand the excitation light spatial modulation effect of the MLA, ray tracing simulation was performed using a commercially available software Zemax. Physical dimensions obtained from the SEM characterization, as schematically depicted in Fig.

4(a), and optical properties of PC were used in the simulations. The simulated excitation beam (at 980 nm), which was collimated with a top-hat intensity distribution before the MLA, had a very small incident angle onto the MLA surface. Figure 4(b) and 4(c) present the simulated excitation light intensity distributions before the MLA (plane D1 in Fig.

4(a)) and at the right edge of the MLA (plane D2 in Fig. 4(a)). As seen, the incident beam was periodically re-distributed by the units of the MLA, with the transmitted light concentrated at the focus of each unit (within the MLA). A line profile crossing the central row of the MLA at the right edge was extracted and compared with that on the incident plane prior to the MLA, as shown in Fig. 4(d). With the intensity for the incident top-hat beam set to 1.3 W/cm², the peak intensity of the transmitted light after the very central unit was as high as 153 W/cm². The excitation light modulation effect of the MLA on the resulting UCL was then estimated. The emission bands at 525/540 nm and 654 nm with the excitation-intensity response shown in Fig. 7(b) were selected as the bands of interest. The UCL intensities integrated over the illuminated area when a nanoparticle layer placed on the incidence plane (if,) and the MLA focusing plane (If,) were calculated by:

Where a is a scaling factor, and ( /_exc ) denotes the excitation-intensity response of the band of interest depicted in Fig. 7(b). With the intensity of incident beam set to 1.3 W/cm², the calculated enhancement factors (If,o/If,i ) for the 525/540 nm and 654 nm bands are 6 and 44, respectively (Fig. 4(e)), which are in qualitative agreement with the obtained experimental results shown in Fig. 2(e). It should be noted that the calculated focusing plane of the MLA is 67.00 pm after the curved part of the MLA (6.50 pm away from plane D2), located within the MLA, and thus maximum concentrated excitation light was not utilized in the experiments. This indicates a room to further increase of the

luminescence from the nanoparticle layer by optimizing the MLA structure to tune the position of the focusing plane, e.g., adjusting the aperture, height, radius of curvature, and refractive index of the MLA.

Improved performance of UCNP-enhanced dye-sensitized solar cell by addition of a microlens array

To investigate the influence of the addition of MLAs on UCNP-enhanced dye-sensitized solar cells (DSSCs), four DSSCs were prepared, one reference cell (without UCNPs), the second and third incorporated with UCNPs or MLA respectively, and the fourth incorporated with both UCNPs and MLA. The device layout is depicted in Fig. 5a. The prepared four cells were tested under direct AM 1.5 G simulated sunlight irradiation (0.1 W/cm²). As shown in Fig. 5b, the control DSSC shows a short-circuit current density ( J_sc ) of 13.20 mA/cm2, Voc=0.698 V, FF=58.19 and h=5.36%. With the addition of UCNPs onto the photoanode, a J_sc of 13.50 mA/cm², and h of 5.53% were achieved, featuring a 3.17% efficiency enhancement over the control cell. By adding a piece of MLA onto the photoanode, a J_sc of 13.94 mA/cm² and h of 5.75% were achieved, featuring a 7.28% efficiency enhancement over the control cell. After incorporation of both UCNPs and the MLA into the device, J_Sc and h were increased to 14.15 mA/cm² and 5.87%, respectively, yielding a 6.15% overall efficiency enhancement over the solely UCNP-enhanced cell.

Conclusions

Lanthanide photon upconversion nanoparticles generally exhibit nonlinear response to excitation light, featuring higher quantum efficiency at higher excitation intensity. Thus, effective excitation light concentrators, whenever feasible, are preferred, to make better use of the photon-up-converting capacity of UCNPs. Here, we explored polymer MLAs as light concentrators for irradiating UCNPs and investigated their spatial light modulation effect on the resulting UCL. It is found that an MLA will concentrate excitation light by orders of magnitude, subject to its structure and optical properties, and lead to a very significant enhancement of the UCL. MLA can be easily incorporated into different types of UCNP-enhanced photonic devices, such as dye-sensitized solar cells and silicon-based solar cells, and bring further performance improvement in the NIR range. A test on a dye- sensitized solar cell proved this contention, however, there is much room for optimizing a variety of parameters both for the solar cells and for the light concentrating up-converting layers to make the combined effects even more significant.

Besides pure lanthanide -based inorganic upconversion materials, their derivatives, such as the form of dye -modified upconversion materials (see figure 12) can also be used. The dye molecules, having large absorption cross section, can harvest more IR light in a broad spectrum and transfer the energy to lanthanide ions, which is highly in favor of

upconversion emission from the latter.

The use of the present invention is not limited to dye- and perovskite-sensitized solar cells, but works for various types of solar cells including silicon-based ones. In the present disclosure it is demonstrated upconversion emission enhancement by microlens arrays for 808 and 980 excitable nanoparticles. However, the disclosed technique should also work for other wavelength excited upconversion emission, e.g. Er³⁺ excited at 1523 nm and Ho³⁺ excited at 1157 nm. Thus, the disclosed technique will also increase the PCE of silicon solar cells.

The present invention is not limited to the above-described preferred embodiments.

Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. A light converting arrangement (2) comprising a transparent polymeric film (4) and upconversion nanoparticles (UCNPs) (6), wherein the UCNPs are incorporated into the film according to a predetermined incorporation procedure resulting in a predefined incorporation structure, the polymeric film has a thickness, a first surface (8) and a second surface (10), c h a r a c t e r i z e d i n that the arrangement further comprises an optical microstructure layer (12) comprising a plurality of microstructures arranged at one side of said layer wherein said microstructures are shaped by a

microstructure shaping procedure, said plurality of microstructures include one or many types of microstructures defined by predetermined type definition, wherein said optical microstructure layer (12) is structured to be arranged in connection to said transparent polymer film (4) in a predetermined relationship to said UCNP incorporation structure such that said optical microstructure layer (12) faces incident light and such that photon conversion performance of said UCNPs is maximized with regard to incident light.

2. The arrangement (2) according to claim 1, wherein said optical

microstructures comprise a microlens array comprising a plurality of microlenses.

3. The arrangement (2) according to claim 1 or 2, wherein said optical microstructures are shaped on the first surface (8) of said transparent polymeric film.

4. The arrangement (2) according to any of claims 1-3, wherein said microstructure shaping procedure comprises using LIGA-like (Lithography,

Electroplating, and Molding) and roh-to-roh (R2R) nano-imprinting technologies, or using integrated circuit (IC) technologies like photolithography, photo-resist processing and reactive ion etching.

5. The arrangement (2) according to any of claims 1-4, wherein said microstructure shaping procedure comprises applying different parameters of said optical microstructure such as one or many of numerical aperture (NA), radius of curvature (ROC), field of view (FOV), and also the thickness of the transparent film.

6. The arrangement (2) according to any of claims 1-5, wherein said incorporation structure of UCNPs comprises confining UCNPs within a thin layer near the second surface of the polymer film with micrometre precision, in order to make sure all nanoparticles are localized near the focusing plane of the optical microstructure layer.

7. The arrangement (2) according to claim any of 1-6, wherein said UCNPs comprises lanthanide UCNPs, preferably core NaYF₄:20%Yb³⁺,2%Er³⁺ and

NaYF₄:20%Yb³⁺,0.5%Tm³⁺ nanoparticles and corresponding core-shell structured NaYF₄:20%Yb³⁺,2%Er³⁺@20%Yb³⁺,30%Nd³⁺ and

NaYF₄:20%Yb³⁺,0.5%Tm³⁺@20%Yb³⁺,30%Nd³⁺, respectively.

8. The arrangement (2) according to any of claims 1-7, wherein photon conversion performance of said UCNPs comprises conversion from infrared light to visible light.

9. The arrangement (2) according to any of claims 1-8, wherein the transparent polymer film comprises polymethylmethacrylate (PMMA) or polycarbonate (PC).

10. A method in relation to a light converting arrangement, comprising:

-incorporating upconversion nanoparticles (UCNPs) (6) into a transparent polymeric film (4) according to a predetermined incorporation procedure resulting in a predefined incorporation structure, the polymeric film has a thickness, a first surface (8) and a second surface (10), c h a r a c t e r i z e d i n that the method further comprises

11. The method according to claim 10, comprising shaping said optical microstructures on the first surface of said transparent polymeric film.

12. The method according to claim 10 or 11, wherein said microstructure shaping procedure comprises applying different parameters of said optical microstructure such as one or many of numerical aperture (NA), radius of curvature (ROC), field of view (FOV), and also the thickness of the transparent film.

13. The method according to any of claims 10-12, wherein said incorporation structure of UCNPs comprises confining UCNPs within a thin layer near the second surface of the polymer film with micrometre precision, in order to make sure all nanoparticles are localized near the focusing plane of the optical microstructure layer.

14. A solar cell provided with an arrangement according to any of claims 1-9.

15. A detection card to visualize infrared radiation, comprising an arrangement according to any of claims 1-9.