E.P.J. Merkx | TU Delft Repository

The potential of transparent sputtered NaI:Tm²⁺, CaBr₂:Tm²⁺, and CaI₂:Tm²⁺ thin films as luminescent solar concentrators

Journal article (2021) - Evert P.J. Merkx, Maarten P. Plokker, Erik van der Kolk

The parameters governing the performance of a luminescent solar concentrator (LSC) are determined for sputtered thin-films of NaI:Tm²⁺, CaBr₂:Tm²⁺, and CaI₂:Tm²⁺. These parameters are determined by using six gradient thin film material libraries, combinatorially sputtered from metallic and pressed powder targets. These films show strong 4f¹³→4f¹²d¹ absorption of maximally 752 cm⁻¹ at.%⁻¹ for NaI:Tm²⁺, 31 cm⁻¹ at.%⁻¹ for CaBr₂:Tm²⁺, and 473 cm⁻¹ at.%⁻¹ for CaI₂:Tm²⁺. This absorption covers the entire visible spectrum and does not overlap with the infrared 4f-4f emission at 1140 nm. Decay measurements are used to estimate the quantum yields of the thin-films. These quantum yields can be as high as 44 % for NaI:Tm²⁺, when doped with 0.3 at.% Tm. Even at doping percentages as low as 0.3 at.%, the films appear to show luminescence quenching. The concentration-dependent absorption and quantum yield are combined with the index of refraction, resolved from transmission measurements, to simulate the optical efficiency of a thin film Tm²⁺-doped halide LSC. These simulations show that LSCs based on Tm²⁺ can display excellent color rendering indices of up to 99 %, and neutral color temperatures, between 4500K and 6000K. Under optimal conditions, thin-films constrained to a thickness of 10μm and 80 % transmission of the visible spectrum, would be able to display optical efficiencies of 0.71 %. This optical efficiency compares favorably to the maximally achievable 3.5 % under these constraints. This efficiency is largely independent of the size of LSC itself. ...

Luminous Glass

A Study on the Optics Governing Luminescent Solar Concentrators and Optimization of Luminescent Materials through Combinatorial Gradient Sputter Deposition

Doctoral thesis (2020) - E.P.J. Merkx

A luminescent solar concentrator (LSC) is a concept from the 1970s that can find novel application as an electricity-generating window. An LSC converts sunlight to light of a different color by a process called luminescence. This light is transported to the edges of the LSC, where photovoltaic cells convert this incoming light to electricity. Since only a small part of the incoming sunlight is absorbed, most sunlight will still illuminate the rooms behind the LSC-window. Turning buildings and offices into nearly zero-energy buildings (nZEBs) is unlikely to happen by using electricity from rooftop photovoltaics (PVs) alone. Turning the envelope of a building, especially the large amount of glass used as windowpanes or facades, into a source of electricity by using LSCs can go a long way towards making these nZEBs a reality. Why then is not every window already an LSC? As will be explained in Chapter 2 and Chapter 3, current LSCs can be efficient at converting sunlight, but suffer from strong coloring, or are not compatible with large-scale industrial processes. To solve the issue of coloring, one solution is to dope halides, such as table salt, with rare-earth elements, specifically divalent Thulium (Tm2+). This combination absorbs the entire visible spectrum. Another strategy is to dope insulating nitride or oxynitride materials with divalent or trivalent Europium (Eu2+ or Eu3+). Eu2+ or Eu3+ are strong absorbers of ultraviolet light. In this thesis, optimizing the luminescent properties of these rare-earth-doped materials is researched using combinatorial synthesis methodology and a novel, fast but detailed characterization setup. The combinatorial synthesis methodology implies that a continuum of rare-earth-doped compositions is deposited on a single 5 × 5 cm2 piece of glass. This composition spread is equivalent to many hundreds of individual samples. The novel characterization setup can characterize the luminescence and other optical properties of these compositions in a matter of minutes. In Chapter 4, this technique is used to form and analyze solid solutions of Eu2+-doped halides. The broad-band Eu2+-emission is sensitively susceptible to its local environment, unlike the infrared line-emitter Tm2+. Researching such solid solutions is of great importance for Tm2+-doped halide LSCs. A solid solution can combine the luminescent properties of its constituents, potentially yielding uniform absorption of the entire visible spectrum, which would make an LSC-window only dimming, without coloring the incident light. Unfortunately, while these halide materials solve the problem of coloring, they are very sensitive to water and are not used in large scale industrial production. This is why the focus is shifted in Chapter 5 to materials composed of silicon (Si), aluminum (Al), oxygen (O) and nitrogen (N): the SiAlON material family. These SiAlONs are chemically stable, scratch-resistant and, because of their likeness to amorphous glass, do not scatter light. These SiAlONs are sputtered on a large-scale by industrial glass manufacturers. Next to fabricating all these materials and characterizing their luminescence, it is also important to predict how they would behave if they were applied as large-scale LSCs. This is done through modeling all optical processes that occur within an LSC, presented in Chapter 3 and Chapter 6. In Chapter 3, a new way of modeling the optical processes within an LSC is presented. The industry-standard is ray-tracing, which can get slow when an LSC absorbs more light, or becomes larger in size. The model presented in Chapter 3 calculates all efficiency steps in an LSC in the same amount of time, regardless of the LSC’s size or transparency. In Chapter 6, we use all these methodologies—fast synthesis and characterization of luminescent thin-films, and modeling of light transport through an LSC—to simulate how efficient an LSC based on AlN:Eu3+,O2– would be. Such an LSC would be transparent in the visible spectrum, as it only absorbs ultraviolet light. AlN:Eu3+,O2– emits red luminescence. Therefore, AlN:Eu3+,O2– will not parasitically absorb the emission that makes its way to the LSC’s edges. The methodology to predict the performance of an LSC used in Chapter 6 is not specific to AlN:Eu3+,O2–, but applicable to all combinatorially synthesized luminescent thin-films. As mentioned before, halide-type materials doped with Tm2+ have been often suggested as promising materials for LSCs. In the final chapter, Chapter 7, sputtered thin-films of NaI, CaBr2, and CaI2 doped with Tm2+ have therefore been evaluated on their performance as LSC; both in terms of simulated optical efficiency, as well as in terms of aesthetic appeal. Our Tm2+-based thin-film LSCs absorb the entire visible spectrum and emit a line of near-infrared radiation centered at 1140 nm. Chapter 7 demonstrates the universality of the techniques presented in Chapters 5 and 6. These techniques are adapted to take hygroscopic nature of the halides into account. The chapter does forgo on fully taking industrial compatibility into account: halides are not often sputtered, and the water-sensitivity will be a hurdle for implementation on window glass. By combining theory and modeling, we see that 10 μm thick films which transmit 80 % of the visible spectrum would be able to achieve optical efficiencies of 0.71 %. This efficiency already compares favorably to the maximally achievable optical efficiency of 3.5 % at those transmission constraints. Further research will have to show whether the photoluminescent quantum yield of the sputtered thin-films can be increased to achieve unity photon conversion. ...

A luminescent solar concentrator (LSC) is a concept from the 1970s that can find novel application as an electricity-generating window. An LSC converts sunlight to light of a different color by a process called luminescence. This light is transported to the edges of the LSC, where photovoltaic cells convert this incoming light to electricity. Since only a small part of the incoming sunlight is absorbed, most sunlight will still illuminate the rooms behind the LSC-window. Turning buildings and offices into nearly zero-energy buildings (nZEBs) is unlikely to happen by using electricity from rooftop photovoltaics (PVs) alone. Turning the envelope of a building, especially the large amount of glass used as windowpanes or facades, into a source of electricity by using LSCs can go a long way towards making these nZEBs a reality. Why then is not every window already an LSC? As will be explained in Chapter 2 and Chapter 3, current LSCs can be efficient at converting sunlight, but suffer from strong coloring, or are not compatible with large-scale industrial processes. To solve the issue of coloring, one solution is to dope halides, such as table salt, with rare-earth elements, specifically divalent Thulium (Tm2+). This combination absorbs the entire visible spectrum. Another strategy is to dope insulating nitride or oxynitride materials with divalent or trivalent Europium (Eu2+ or Eu3+). Eu2+ or Eu3+ are strong absorbers of ultraviolet light. In this thesis, optimizing the luminescent properties of these rare-earth-doped materials is researched using combinatorial synthesis methodology and a novel, fast but detailed characterization setup. The combinatorial synthesis methodology implies that a continuum of rare-earth-doped compositions is deposited on a single 5 × 5 cm2 piece of glass. This composition spread is equivalent to many hundreds of individual samples. The novel characterization setup can characterize the luminescence and other optical properties of these compositions in a matter of minutes. In Chapter 4, this technique is used to form and analyze solid solutions of Eu2+-doped halides. The broad-band Eu2+-emission is sensitively susceptible to its local environment, unlike the infrared line-emitter Tm2+. Researching such solid solutions is of great importance for Tm2+-doped halide LSCs. A solid solution can combine the luminescent properties of its constituents, potentially yielding uniform absorption of the entire visible spectrum, which would make an LSC-window only dimming, without coloring the incident light. Unfortunately, while these halide materials solve the problem of coloring, they are very sensitive to water and are not used in large scale industrial production. This is why the focus is shifted in Chapter 5 to materials composed of silicon (Si), aluminum (Al), oxygen (O) and nitrogen (N): the SiAlON material family. These SiAlONs are chemically stable, scratch-resistant and, because of their likeness to amorphous glass, do not scatter light. These SiAlONs are sputtered on a large-scale by industrial glass manufacturers. Next to fabricating all these materials and characterizing their luminescence, it is also important to predict how they would behave if they were applied as large-scale LSCs. This is done through modeling all optical processes that occur within an LSC, presented in Chapter 3 and Chapter 6. In Chapter 3, a new way of modeling the optical processes within an LSC is presented. The industry-standard is ray-tracing, which can get slow when an LSC absorbs more light, or becomes larger in size. The model presented in Chapter 3 calculates all efficiency steps in an LSC in the same amount of time, regardless of the LSC’s size or transparency. In Chapter 6, we use all these methodologies—fast synthesis and characterization of luminescent thin-films, and modeling of light transport through an LSC—to simulate how efficient an LSC based on AlN:Eu3+,O2– would be. Such an LSC would be transparent in the visible spectrum, as it only absorbs ultraviolet light. AlN:Eu3+,O2– emits red luminescence. Therefore, AlN:Eu3+,O2– will not parasitically absorb the emission that makes its way to the LSC’s edges. The methodology to predict the performance of an LSC used in Chapter 6 is not specific to AlN:Eu3+,O2–, but applicable to all combinatorially synthesized luminescent thin-films. As mentioned before, halide-type materials doped with Tm2+ have been often suggested as promising materials for LSCs. In the final chapter, Chapter 7, sputtered thin-films of NaI, CaBr2, and CaI2 doped with Tm2+ have therefore been evaluated on their performance as LSC; both in terms of simulated optical efficiency, as well as in terms of aesthetic appeal. Our Tm2+-based thin-film LSCs absorb the entire visible spectrum and emit a line of near-infrared radiation centered at 1140 nm. Chapter 7 demonstrates the universality of the techniques presented in Chapters 5 and 6. These techniques are adapted to take hygroscopic nature of the halides into account. The chapter does forgo on fully taking industrial compatibility into account: halides are not often sputtered, and the water-sensitivity will be a hurdle for implementation on window glass. By combining theory and modeling, we see that 10 μm thick films which transmit 80 % of the visible spectrum would be able to achieve optical efficiencies of 0.71 %. This efficiency already compares favorably to the maximally achievable optical efficiency of 3.5 % at those transmission constraints. Further research will have to show whether the photoluminescent quantum yield of the sputtered thin-films can be increased to achieve unity photon conversion.

Sm ²⁺ ^-doped SiAlO sputter deposited coatings for self-absorption and scatter-free luminescent solar concentrator applications

Journal article (2020) - David de Vries, Sadiq van Overbeek, E.P.J. Merkx, Erik van der Kolk

Combinatorial reactive co-sputtering using Al, Si and Sm targets in an Ar + O ₂ atmosphere, resulted in Sm doped SiAlO thin films with a wide Sm concentration- and Si:Al composition gradient. By combining position dependent EDX spectra and laser excited emission spectra, ternary phase diagrams were constructed that directly show the relation between Sm emission intensity, index of refraction, thickness and composition. Using this approach, the Sm ²⁺ and Sm ³⁺ emission intensity ratio was controlled towards films with predominantly Sm ²⁺ emission, which is most favorable for luminescent solar concentrator (LSC) applications. The optimum Sm ²⁺ efficiency was reached when the Al content was about equal to the Sm content. When the Si:Al ratio decreases, the Sm ²⁺ emission intensity strongly drops to almost zero. However, sputtering without Al resulted in no Sm ²⁺ emission intensity at all. The excitation and emission properties of Sm ²⁺ in the optimized thin films, especially the ratio between the 4f→4f and 5d→4f emission that is sensitively susceptible to the co-ordination polyhedron, closely resembles that of Sm ²⁺ doped crystalline powders with the same composition. This strongly suggest that the Sm ²⁺ ions in our amorphous films are coordinated in the same way. A homogeneous thin film on float glass clearly shows the light concentration effect of the red Sm ²⁺ emission. Due to an unexplained low Sm ²⁺ absorption of our films, even the optimized thin films do not luminesce brightly. ...

Luminescent Solar Concentrators

Book chapter (2020) - Evert P.J. Merkx, Erik Van der Kolk

Intended as an electricity generating replacement for windows, luminescent solar concentrators (LSCs) are at the borderline of indoor photovoltaics. In urban environments, space to install outdoor PV is limited to just the roof of a building, directly limiting the amount of energy that can be generated. LSCs aim to overcome this limitation by seamlessly integrating PV into the building envelope. This chapter explicates the luminescent phenomena and the working principle behind LSCs. Analytical and discretized techniques for simulating LSCs are presented and provided as downloadable examples. These same techniques show that an LSC can increase the performance of its PV frame with negligible dimming, even at nonperfect conversion efficiencies. This enhancement increases with window size. An overview of the optical properties of 28 state-of-the-art LSCs is presented. These LSCs are evaluated on performance for building-integrated purposes. Simulations show that LSCs with an optical efficiency of more than 2.8% are already possible for nontoxic quantum dot-based LSCs, without compromising on color rendering properties. Next to these state-of-the-art LSCs, a new development in the form of thulium-doped halides is highlighted. These halides are able to absorb the entire visible spectrum without coloration, and could be scaled to efficient LSCs of arbitrary sizes thanks to their lack of self-absorption. As IPV is an emerging application for LSCs, most of the chapter focuses on LSCs for window applications in order to outline the theoretical foundations. In addition to that, developments in LSCs for usage as indoor-only photovoltaic are illustrated with a fully worked-out example of an LSC as tabletop. Next to this IPV simulation, colorful designs that apply LSCs in other ways than just windows are highlighted. ...

Modelling and optimization of UV absorbing photovoltaic windows using a thin film AlN:Eu³⁺ luminescence library

Journal article (2019) - Evert P.J. Merkx, Thomas G. Lensvelt, Erik van der Kolk

Building-integrated photovoltaics are drawing much attention to make the built environment self-sufficient in terms of electricity. Luminescent solar concentrators (LSCs) can provide this electricity generation when used as photovoltaic windows. Paramount to an efficient LSC are non-overlapping absorption and emission spectra, to avoid self-absorption. This non-overlap can be achieved by absorbing incoming UV light and emitting in the red to infrared. In this article, we present a technique for optimizing LSCs without self-absorption, using Eu³⁺-doped AlN as a model system. The parameters affecting light absorption, emission and transport are extracted from a combinatorially sputtered gradient material library. This library results from a single deposition, with a gradient in thickness and Eu concentration. AlN:Eu³⁺ absorbs strongly until 450 nm, with a peak solar absorption of 499 cm⁻¹ at%⁻¹ at 350 nm due to a charge transfer band. The strongest emission is at 622 nm, thereby exhibiting no self-absorption. The presented optimization model strikes a balance between concentration quenching and absorptivity of Eu dopants by using the parameters extracted from the material library. For thicker films, concentration quenching can be avoided by using a lower dopant concentration, while still outperforming thinner films due to fast increasing absorption. The results demonstrate that, while AlN:Eu³⁺ itself should only be viewed as a model system, thin films doped with rare earths can yield industry-compatible, high efficiency LSCs because of their high absorption coefficients and lack of self-absorption. ...

Optimizing Tm²⁺-doped dihalide thin film luminescent solar concentrators

Conference paper (2019) - Evert P.J. Merkx, Erik Van Der Kolk

Local property-mapping of gradient thin films of MI2:x% Tm2+ (M=Ca,Sr), where x and film thickness vary, is used to optimize for LSC use. Tm2+ absorbs the entire visible spectrum and emits at 1140 nm. ...

Functionalizing window coatings with luminescence centers by combinatorial sputtering of scatter-free amorphous SiAlON

Eu²⁺ thin film composition libraries

Journal article (2019) - Evert P.J. Merkx, Sadiq van Overbeek, Erik van der Kolk

SiAlON window coatings are applied on an industrial scale to achieve e.g. scratch-resistance and anti-reflection. Doping these SiAlONs with rare-earths adds luminescent functionality, which could be applied in photovoltaics. By using a combinatorial reactive sputtering approach, an amorphous thin film composition library with a Si:Al ratio from 0.062:1 to 3.375:1 and a Eu doping from 4.8 at% to 26 at% is created. This library uniquely combines high absorption, strong emission and absence of light scattering. By combining position-dependent EDX measurements with transmission and emission spectra, properties like the index of refraction, absorption strength, emission wavelength and decay times of the library can directly be related to the composition. Throughout the library, an index of refraction of 1.63±0.03 is observed, typical for a film with low nitrogen content. The library also shows a large absorption coefficient of 1294±8cm⁻¹at.%⁻¹. Laser-excited emission spectra show that the library has a strong redshift from 500 nm to 550 nm with increasing Al concentration. An increase in Eu concentration also causes a shift of the emission to red. Decay spectra show that a high degree of Si greatly improves the luminescence intensity. These functionalized SiAlON coatings can be of great interest for transparent and scatter-free luminescent solar concentrators applied as windows. ...

Method for the Detailed Characterization of Cosputtered Inorganic Luminescent Material Libraries

Journal article (2018) - Evert P.J. Merkx, Erik Van Der Kolk

Understanding the behavior of combinatorially developed luminescent materials requires detailed characterization methods that have been lacking thus far. We developed a device for directly surveying the luminescent properties of thin-film libraries created through combinatorial gradient sputter deposition. Step-scan recorded excitation-, emission- and luminescence decay spectra of a thin-film library were resolved and combined with EDX measurements on the same film, relating composition to luminescent properties. This technique was applied to a single-substrate gradient thin-film library of NaBr_0.73I_0.27 to NaBr_0.09I_0.91, doped with 6.5% to 16.5% Eu²⁺. This gradient film closely followed Vegard's law, with emission fluently shifting from 428 to 439 nm. In comparison, pure NaBr:Eu²⁺ showed emission at 428 nm and NaI:Eu²⁺ at 441 nm. Luminescence decay measurements demonstrated a great degree of concentration quenching in the gradient film. From these measurements we could conclude that an optimized phosphor would most efficiently luminesce when close to NaI:Eu²⁺. This gradient film confirmed that the method presented in this work allows to both study and optimize luminescent behavior in a broad range of host- and dopant systems. ...

Rapid optimization of large-scale luminescent solar concentrators: evaluation for adoption in the built environment

Journal article (2017) - E. P.J. Merkx, O. M. Ten Kate, E. Van Der Kolk

The phenomenon of self-absorption is by far the largest influential factor in the eficiency of luminescent solar concentrators (LSCs), but also the most challenging one to capture computationally. In this work we present a model using a multiple-generation light transport (MGLT) approach to quantify light transport through single-layer luminescent solar concentrators of arbitrary shape and size. We demonstrate that MGLT offers a significant speed increase over Monte Carlo (raytracing) when optimizing the luminophore concentration in large LSCs and more insight into light transport processes. Our results show that optimizing luminophore concentration in a lab-scale device does not yield an optimal optical efficiency after scaling up to realistically sized windows. Each differently sized LSC therefore has to be optimized individually to obtain maximal efficiency. We show that, for strongly self-absorbing LSCs with a high quantum yield, parasitic self-absorption can turn into a positive effect at very high absorption coeficients. This is due to a combination of increased light trapping and stronger absorption of the incoming sunlight. We conclude that, except for scattering losses, MGLT can compute all aspects in light transport through an LSC accurately and can be used as a design tool for building-integrated photovoltaic elements. This design tool is therefore used to calculate many building-integrated LSC power conversion efficiencies. ...