Measurement method for photoluminescent quantum yields of fluorescent organic dyes in polymethyl methacrylate for luminescent solar concentrators

High Quantum Yield Shortwave Infrared Luminescent Tracers for Improved Sorting of Plastic Waste

More complete recycling of plastic waste is possible only if new technologies that go beyond state-of-the-art near-infrared (NIR) sorting are developed. For example, tracer-based sorting is a new technology that explores the upconversion or down-shift luminescence of special tracers based on inorganic materials codoped with lanthanide ions. Specifically, down-shift tracers emit in the shortwave infrared (SWIR) spectral range and can be detected using a SWIR camera preinstalled in a state-of-the-art sorting machine for NIR sorting. In this study, we synthesized a very efficient SWIR tracer by codoping Li3Ba2Gd3 (MoO4)8 with Yb3+ and Er3+, where Yb3+ is a synthesizer ion (excited near 976 nm) and Er3+ emits near 1550 nm. Fine-tuning of the doping concentration resulted in a tracer (Li3Ba2Gd(3-x-y)(MoO4)8:xYb3+, yEr3+, where x = 0.2 and y = 0.4) with a high photoluminescence quantum yield for 1550 nm emission of 70% (using 976 nm excitation). This tracer was used to mark plastic objects. When the object was illuminated by a halogen lamp and a 976 nm laser, the three parts could be easily distinguished based on reflectance and luminescence spectra in the SWIR range: a plastic bottle made of polyethylene terephthalate, a bottle cap made of high-density polyethylene, and a label made of the tracer Li3Ba2Gd3(MoO4)8:Yb3+, Er3+. Importantly, the use of the tracer in sorting may require only the installation of a 976 nm laser in a state-of-the-art NIR sorting system.

Design of more efficient luminescent solar concentrators by using peripherally dye-doped stacked optical fibers

Ways of improving the optical efficiency of luminescent solar concentrators based on multiple poly(methyl methacrylate) plastic optical fibers peripherally doped with two promising types of dyes are analyzed by means of a Monte-Carlo computational model developed by us. By comparing the performance of optical fibers doped with lumogen red and lumogen yellow, or combinations of them at several concentrations, this work clarifies how to achieve a better compromise between the trapping efficiency of the sunlight and the reabsorption of the light emitted by the mixture in stacked optical fibers connected to a photovoltaic solar cell.

Enhancing LED spectral output with perylene dye-based remote phosphor

LEDs offer a wide range of spectral output with high efficiencies. However, the efficiencies of solid-state LEDs with green and yellow wavelengths are rather low due to the lack of suitable direct bandgap materials. Here, we introduce and develop perylene-enhanced green LEDs that produce a higher wall-plug efficiency of 48% compared to 38% for a solid-state green LED. While the wall-plug efficiency of the perylene-enhanced red LED is still lower than that of a solid-state red LED, we demonstrate that remote phosphor colour converters are effective solutions for targeted spectral tuning across the visible spectrum for horticultural lighting. In this work, we retrofit existing white LEDs and augment photosynthesis via spectral output tuning to achieve a higher red-to-blue ratio. Our results show a significant improvement in plant growth by up to 39%, after a 4-month growth cycle. We observe no visible degradation of the colour converter even under continuous illumination with a current of 400 mA. This opens up new opportunities for using perylene-based colour converters for tuneable illumination with high brightness.

Leaf-inspired luminescent solar concentrator based on two-stage photoconversion

Geometrical gain of a luminescent solar concentrator is drastically increased by laying out a luminescent fiber in a luminescent plate with air gap around it and attaching a photovoltaic (PV) cell to the tip of the fiber. The plate converts an incident photon to a first photoluminescence (PL) photon, and the fiber converts it to a second PL photon. Thus, the fiber carries the optical power as a leaf vein transports water and nutrients. The probability of the first PL photon resulting in the second PL photon reaching the PV cell can be measured by exciting a single spot on the plate with a laser beam. In experiment, 2 mm-thick, 50 mm-square and 50 mm-diameter circular devices were assembled with off-the-shelf components. For each case, geometrical gain exceeded 1000 and this probability averaged over the incident area was of the order of 0.01. Connecting multiple small-area devices to a single PV cell with a clear fiber would increase geometrical gain further and alleviate the absorption and scattering of PL photons during waveguiding.

Ray-Trace Modeling to Characterize Efficiency of Unconventional Luminescent Solar Concentrator Geometries

Luminescent solar concentrators (LSCs) are a promising technology to help integrate solar cells into the built environment, as they are colorful, semitransparent, and can collect diffuse light. While LSCs have traditionally been cuboidal, in recent years, a variety of unconventional geometries have arisen, for example, circular, curved, polygonal, wedged, and leaf-shaped designs. These new designs can help reduce optical losses, facilitate incorporation into the built environment, or unlock new applications. However, as fabrication of complex geometries can be time- and resource-intensive, the ability to simulate the expected LSC performance prior to production would be highly advantageous. While a variety of software exists to model LSCs, it either cannot be applied to unconventional geometries, is not open-source, or is not tractable for most users. Therefore, here we introduce a significant upgrade of the widely used Monte Carlo ray-trace software pvtrace to include: (i) the capability to characterize unconventional geometries and improved relevance to standard measurement configurations; (ii) increased computational efficiency; and (iii) a graphical user interface (GUI) for ease-of-use. We first test these new features against data from the literature as well as experimental results from in-house fabricated LSCs, with agreement within 1% obtained for the simulated versus measured external photon efficiency. We then demonstrate the broad applicability of pvtrace by simulating 20 different unconventional geometries, including a variety of different shapes and manufacturing techniques. We show that pvtrace can be used to predict the optical efficiency of 3D-printed devices. The more versatile and accessible computational workflow afforded by our new features, coupled with 3D-printed prototypes, will enable rapid screening of more intricate LSC architectures, while reducing experimental waste. Our goal is that this accelerates sustainability-driven design in the LSC field, leading to higher optical efficiency or increased utility.

Orange/Red Benzo[1,2-b:4,5-b']dithiophene 1,1,5,5-Tetraoxide-Based Emitters for Luminescent Solar Concentrators: Effect of Structures on Fluorescence Properties and Device Performances

Luminescent solar concentrators (LSCs) are a class of optical devices able to harvest, downshift, and concentrate sunlight, thanks to the presence of emitting materials embedded in a polymer matrix. Use of LSCs in combination with silicon-based photovoltaic (PV) devices has been proposed as a viable strategy to enhance their ability to harvest diffuse light and facilitate their integration in the built environment. LSC performances can be improved by employing organic fluorophores with strong light absorption in the center of the solar spectrum and intense, red-shifted emission. In this work, we present the design, synthesis, characterization, and application in LSCs of a series of orange/red organic emitters featuring a benzo[1,2-b:4,5-b']dithiophene 1,1,5,5-tetraoxide central core as an acceptor (A) unit. The latter was connected to different donor (D) and acceptor (A') moieties by means of Pd-catalyzed direct arylation reactions, yielding compounds with either symmetric (D-A-D) or non-symmetric (D-A-A') structures. We found that upon light absorption, the compounds attained excited states with a strong intramolecular charge-transfer character, whose evolution was greatly influenced by the nature of the substituents. In general, symmetric structures showed better photophysical properties for the application in LSCs than their non-symmetric counterparts, and using a donor group of moderate strength such as triphenylamine was found preferable. The best LSC built with these compounds presented photonic (external quantum efficiency of 8.4 ± 0.1%) and PV (device efficiency of 0.94 ± 0.06%) performances close to the state-of-the-art, coupled with a sufficient stability in accelerated aging tests.

Development of an Active Optical Lens for Arc Flashing Detection

This paper contains the design of active optical lenses used for the detection of arc flashing emissions. The phenomenon of an arc flashing emission and its characteristics were contemplated. Methods of preventing these emissions in electric power systems were discussed as well. The article also includes a comparison of commercially available detectors. An analysis of the material properties of fluorescent optical fiber UV-VIS-detecting sensors constitutes a major part of the paper. The main purpose of the work was to make an active lens using photoluminescent materials, which can convert ultraviolet radiation into visible light. As part of the work, active lenses with materials such as Poly(methyl 2-methylpropenoate) (PMMA) and phosphate glass doped with lanthanides, such as terbium (Tb³⁺) and europium (Eu³⁺) ions, were analyzed. These lenses were used to make optical sensors, which were supported by commercially available sensors in their construction.

A Refined Prediction Parameter for Molecular Alignability in Stretched Polymers and a New Light-Harvesting Material for AlGaAs Photovoltaics

Light-harvesting concentrators have a high potential to make highly efficient but precious energy converters, such as multijunction photovoltaics, more affordable for everyday applications. They collect sunlight, including diffusively scattered light, on large areas and redirect it to much smaller areas of the highly efficiency solar cells. Among the best current concepts are pools of randomly oriented light-collecting donor molecules that transfer all excitons to few aligned acceptors reemitting the light in the direction of the photovoltaics. So far, this system has only been realized for the 350–550 nm wavelength range, suitable for AlGaInP photovoltaics. This was achieved by using acceptor molecules that aligned during mechanical stretching of polymers together with donors, that stay random in that very same material and procedure. However, until recently, very little was known about the factors that are responsible for the alignability of molecules in stretched polymers and therefore it was difficult to find suitable donors and acceptors, as well as for other spectral ranges. Recently, a structural parameter was introduced with a high predictivity for the alignability of molecules that contain rigid band-like structures or linear aromatic π-systems. However, for light concentrators in more red spectral ranges, molecular systems often contain larger and extended, planar-like π-systems for which the previously reported parameter is not directly applicable. Here, we present a refined prediction parameter also suitable for larger plane-like structures. The new parameter depends on the number of in-plane atoms divided by out-of-plane atoms as determined by computational geometry optimization and additionally the planar aspect ratio for molecules that contain only in-plane atoms. With the help of this parameter, we found a new system that can efficiently collect and redirect light for the second 500–700 nm AlGaAs layer of current world-record multijunction photovoltaics. Similarly, as the previously reported system for the blue-green layer, it has also overall absorption and re-directioning quantum efficiencies close to 80–100%. Both layers, together, already cover about 75% of the energy in the solar spectrum.

Luminance, color gamut, and energy-harvesting characteristics of luminescent layers placed above a solar cell

One can display images and harvest energy by utilizing ambient light with a stack of an optical shutter array, a luminescent layer and a solar cell. In our experiment, a luminescent layer and a corresponding color filter were attached to a polycrystalline Si solar cell with 13% power conversion efficiency. For each configuration using BBOT, Coumarin 6 and Lumogen F Red 305, the power conversion efficiency was measured to be 6.7%, 8.0% and 8.9%, respectively. The luminance of these configurations was proportional to the illuminance in all cases. Its color gamut was comparable to the National Television System Committee standard.

Spectral study on utilizing ambient light with luminescent materials for display applications

A luminous reflective display can be constructed by placing an electro-optic shutter on the stack of a luminescent layer, a color filter, and a reflector in this order. The luminescent materials convert a part of the incident light to photoluminescence photons. The reflector redirects the downward photon flux toward an observer. The color filters prevent the photons with unwanted wavelengths from being reflected. The upward spectral flux from this multi-layer structure is formulated. Experiments with off-the-shelf components revealed more than three-fold increase in spectral flux and up to 55% color gamut extension, compared with a control device without luminescent materials.

Precise Control of Copper-Localized Surface Plasmon Resonance in the Near Infrared Region for Enhancement of Up-Conversion Luminescence

The surface plasmon resonance of copper in the near infrared region provides a novel method for enhancement of up-conversion luminescence compared to using gold and silver, as the former grants significant cost savings. In this study, we made a flat Cu film covered TiO2 to enhance the up-conversion fluorescence intensity. The results show that the deposition of copper/TiO2 dioxide nanocomposite film prepared via spin-coating has no effect on the structure of NaGdF4:Er3+, Yb3+. The absorption wavelength of the copper film moved from the original visible wavelength (~600 nm) range to the infrared wavelength after covering TiO2, and most obviously, the copper film covered two layers of TiO2 by about 16 nm; the peak of the absorption appeared at 835 nm due to the enhanced excitation field. The behavior of the nanocomposite film with NaGdF4:Er3+ and Yb3+ under 980 nm excitation was investigated; it provides a novel way for studying mental-enhanced fluorescence. Besides, the peaks of the fluorescence spectrum show different emissions at 542 nm and 660 nm, respectively. The copper nanoparticles-covered TiO2 layer can obviously enhance the fluorescence intensity, and the maximum enhancement factors of emission of NaGdF4:Er3+ and Yb3+ nanoparticles are 3.1 and 1.9 on the nanocomposite film, respectively.

Concentric re-emission pattern from a planar waveguide with a thin uniform luminescent layer

When a beam of light excites a single spot on a thin luminescent layer embedded in a planar waveguide, a concentric re-emission pattern is observed. An analytical expression is formulated by following the series of events in the waveguide: generation of angle-dependent photoluminescence spectra, reflection at the waveguide-air boundary, absorption by the luminescent layer, and generation of next-generation photoluminescence. The formula reproduces the peak radii observed in the experiments with some organic dyes. It provides insights for the re-emission events in a luminescent solar concentrator and the cross talk in an energy-harvesting display based on photoluminescence.

Simulation of a Novel Configuration for Luminescent Solar Concentrator Photovoltaic Devices Using Bifacial Silicon Solar Cells

In this study, a novel configuration for luminescent solar concentrator photovoltaic (LSC PV) devices is presented, with vertically placed bifacial PV solar cells made of mono-crystalline silicon (mono c-Si). This LSC PV device comprises multiple rectangular cuboid lightguides, made of poly (methyl methacrylate) (PMMA), containing Lumogen dyes, in particular, either Lumogen red 305 or orange 240. The bifacial solar cells are located in between these lightguide cubes and can, therefore, receive irradiance at both of their surfaces. The main aim of this study is to theoretically determine the power conversion efficiency (PCE) of five differently configured LSC PV devices. For this purpose, Monte Carlo ray tracing simulations were executed to analyze the irradiance at receiving PV cell surfaces, as well as the optical performance of these LSC PV devices. Five different LSC PV devices, with different geometries and varying dye concentrations, were modeled. To maximize the device efficiency, the bifacial cells were also attached to the back side of the lightguides. The ray tracing simulations resulted in a maximum efficiency of 16.9% under standard test conditions (STC) for a 15 × 15 cm2 LSC PV device, consisting of nine rectangular cuboid 5 × 5 × 1 cm3 PMMA lightguides with 5 ppm orange 240 dye, with 12 vertically positioned 5 × 1 cm2 bifacial cells in between the lightguides and nine 5 × 5 cm2 PV cells attached to the back of the device. If the cells are not applied to the back of this LSC PV device configuration, the maximum PCE will be 2.9% (under STC), where the LSC PV device consists of 25 cubical 1 × 1 × 1 cm3 PMMA lightguides with 110 ppm red 305 dye and 40 vertically oriented bifacial PV cells of 1 × 1 cm2 in between the lightguides. These results show the vast future potential for LSC PV technologies, with a higher performance and efficiency than the common threshold PCE for LSC PV devices of 10%.

Alkyl Conformation and π-π Interaction Dependent on Polymorphism in the 1,8-Naphthalimide (NI) Derivative

The 1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits different photoluminescence (PL) and aggregation-caused quenching properties due to its crystal polymorphism, which depends on the solvent evaporation process in tetrahydrofuran solution. In the slow drying process, molecules aggregated into an energetically more stable form (time-dependent density functional theory calculation), of which the PL peak maximum was 453 nm, corresponding to blue emission at the 365 nm excitation. However, the fast evaporation process induces an energetically less stable form, with a PL peak maximum of 508 nm, corresponding to green emission. The main difference between the two crystal structures is the alkyl conformation, as confirmed by X-ray single-crystal analysis. Due to the different alkyl conformations, NI groups aggregated into more obliquely aligned structures that emit blue PL, which plays a role in weakening the π-π interactions between molecules relative to green PL crystals. We found that the conformational stable molecular stacking induced instability in the electronic energy levels of the blue crystal compared to the green crystal.

Ecofriendly and Efficient Luminescent Solar Concentrators Based on Fluorescent Proteins

In recent years, luminescent solar concentrators (LSCs) have received renewed attention as a versatile platform for large-area, high-efficiency, and low-cost solar energy harvesting. So far, artificial or engineered optical materials, such as rare-earth ions, organic dyes, and colloidal quantum dots (QDs) have been incorporated into LSCs. Incorporation of nontoxic materials into efficient device architectures is critical for environmental sustainability and clean energy production. Here, we demonstrated LSCs based on fluorescent proteins, which are biologically produced, ecofriendly, and edible luminescent biomaterials along with exceptional optical properties. We synthesized mScarlet fluorescent proteins in Escherichia coli expression system, which is the brightest protein with a quantum yield of 61% in red spectral region that matches well with the spectral response of silicon solar cells. Moreover, we integrated fluorescent proteins in an aqueous medium into solar concentrators, which preserved their quantum efficiency in LSCs and separated luminescence and wave-guiding regions due to refractive index contrast for efficient energy harvesting. Solar concentrators based on mScarlet fluorescent proteins achieved an external LSC efficiency of 2.58%, and the integration at high concentrations increased their efficiency approaching to 5%, which may facilitate their use as "luminescent solar curtains" for in-house applications. The liquid-state integration of proteins paves a way toward efficient and "green" solar energy harvesting.

Reflectance matrix approach to absolute photoluminescence measurements with integrating spheres

Absolute measurements of photoluminescence are commonly performed using an integrating sphere setup, as this allows the collection of all emitted photons independent of the spatial characteristics of the emission. However, such measurements are plagued by multiple reflection effects occurring within the integrating sphere that make the sample illumination and sphere throughput sample dependent. To address this problem, we developed a matrix theory for integrating spheres with photoluminescent surfaces. In conjunction with a bispectral luminescence data set, this model allows for multiple reflection effects to be fully accounted for. The bispectral data is obtained by mounting both the sample and a non-luminescent reference on the sphere and permuting their positions in order to compare direct and diffuse sample illumination conditions. Experimental measurements of a photoluminescent standard confirm the validity of the method.

Enhancing the photoluminescence of surface anchored metal-organic frameworks: mixed linkers and efficient acceptors

We present two approaches to enhance the photoluminescence quantum yield (PLQY) of surface-anchored metal-organic frameworks (SURMOFs). In the first approach we fabricate SURMOFs from a mix of an emissive linker with an optically-inert linker of equivalent length, diluting the emissive linker while maintaining the SURMOF structure. This approach enhances the internal PLQY. However, the increase in internal PLQY is achieved at the expense of a drastic reduction in optical absorption, thus the external PLQY remains low. To overcome this limitation, a second approach is explored wherein energy-accepting guest chromophores are infiltrated into the framework of the active linker. At the correct acceptor concentration, an internal PLQY of 52% - three times higher than the previous approach - is achieved. Additionally, the absorption remains strong leading to an external PLQY of 8%, an order of magnitude better than the previous approach. Using this strategy, we demonstrate that SURMOFs can achieve PLQYs similar to their precursor chromophores in solution. This is of relevance to SURMOFs as emitter layers in general, and we examine the optimized emitter layer as part of a photon upconversion (UC) SURMOF heterostructure. Surprisingly, the same PLQY is not observed after triplet-triplet annihilation in the UC heterostructure as after its normal photoexcitation (although the UC layers exhibit low thresholds consistent with those reported in our previous work). We discuss the potential bottlenecks in energy transport that could lead to this unexpected reduction in PLQY after excitation via triplet-triplet annihilation, and how future design of SURMOF UC multilayers could overcome these limitations.

Gain investigation of Perylene-Red-doped PMMA for stimulated luminescent solar concentrators

Luminescent solar concentrators (LSCs) utilizing stimulated emission by a seed laser are a promising approach to overcome the limitations of conventional LSCs, with a significant reduction of the photovoltaic material. In our previous work, we demonstrated the principle of a stimulated LSC (s-LSC) and correspondingly developed a model for quantifying the output power of such a system, taking into account different important physical parameters. The model suggested Perylene Red (PR) dye as a potential candidate for s-LSCs. Here, we experimentally investigate the gain of PR-doped polymethyl methacrylate (PMMA) required for s-LSCs using a single pump wavelength (instead of the solar spectrum) as a proof of principle. The results found from the experiment are well matched with the previously developed numerical model except for gain saturation, which occurs at a comparatively small seed laser signal power. To investigate the gain saturation, two approaches were taken: investigating (i) spectral hole burning and (ii) triplet state absorption. Experimental investigation of spectral hole burning with PR dyes showed a small effect on the gain saturation. We developed a general state model considering triplet state absorption of the PR dyes for the second approach. The state model suggests that the PR dyes suffer from significant triplet state absorption loss, which obstructs the normal operation of the PR-based s-LSC system.

Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial

Upper-excited state emission is not usually observed from molecules owing to competition with much faster nonradiative relaxation pathways; however, it can be made more efficient by modifying the photonic density of states to enhance the radiative decay rate. Here, we show that embedding the small molecule zinc tetraphenylporphyrin (ZnTPP) in a hyperbolic metamaterial enables an ∼18-fold increase in fluorescence intensity from the second singlet excited state ( S₂) relative to that from the lowest singlet excited state ( S₁). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from S₁) to blue (dominated by emission from S₂) with an instrument-limited decay lifetime <10 ps. Our results are consistent with a broadband Purcell enhancement in the radiative rate of both transitions predicted via transfer matrix modeling and point to a general opportunity to harness upper-excited states for spectrally tunable, ultrafast fluorescence via radiative decay engineering.

Effect of polysiloxane encapsulation material compositions on emission behaviour and stabilities of perylene dyes

Influence of phenyl and methyl group containing polysiloxane encapsulation materials on the fluorescence properties of two perylene diimides.

Cellulose nanocrystals as host matrix and waveguide materials for recyclable luminescent solar concentrators

We report the use of an ecofriendly and recyclable CNC material as the host matrix for luminescent solar concentrator applications.

Modeling of stimulated emission based luminescent solar concentrators

The efficiency improvement of luminescent solar concentrators (LSCs) necessary for practical realization is currently hindered by one major loss mechanism: reabsorption of emitted photons by the luminophores. Recently, we explored a promising technique for reducing reabsorption and also improving directional emission in LSCs utilizing stimulated emission, rather than only spontaneous emission, with an inexpensive seed laser. In this work, a model is developed to quantify the gain (i.e. the amount of amplification of a low power seed laser propagating through the solar-pumped concentrator) of stimulated-LSCs (s-LSCs) considering the effects of different important physical parameters. The net optical output power, available for a small PV cell, from the concentrator can also be determined from the model, which indicates the performance of s-LSCs. Finally, the performance of different existing material systems is investigated using literature values of the parameters required for the model, and a set of optimal parameters is suggested for practical realization of such a device.

Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers

Organometal halide perovskite solar cells have demonstrated high conversion efficiency but poor long-term stability against ultraviolet irradiation and water. We show that rapid light-induced free-radical polymerization at ambient temperature produces multifunctional fluorinated photopolymer coatings that confer luminescent and easy-cleaning features on the front side of the devices, while concurrently forming a strongly hydrophobic barrier toward environmental moisture on the back contact side. The luminescent photopolymers re-emit ultraviolet light in the visible range, boosting perovskite solar cells efficiency to nearly 19% under standard illumination. Coated devices reproducibly retain their full functional performance during prolonged operation, even after a series of severe aging tests carried out for more than 6 months.

External Quantum Efficiency Improvement with Luminescent Downshifting Layers: Experimental and Modelling

Core-shell quantum dots CdSe/ZnS and lumogen yellow organic dye are characterized by their inclusion in luminescent downshifting (LDS) layers. Layers were deposited on top of crystalline silicon cell (c-Si), dye synthesized solar cell (DSSC), and cadmium telluride (CdTe) minimodules. External quantum efficiency measurements for the solar cell/LDS devices are discussed. Experimental results were compared with an optical model developed by Rothemund, 2014.

Eu³⁺-based bridged silsesquioxanes for transparent luminescent solar concentrators

The sol-gel preparation of a bridged silsesquioxane containing europium(III) salts and 2-thenoyltrifluoroacetone has been achieved from a new ethane tetracarboxamide-based organosilane. Free-standing films with thicknesses up to 440 μm and maximum absolute quantum yield (q) of 0.34 ± 0.03 (excitation at 320 nm) were prepared by the drop cast method, while thin films (∼200-400 nm) spin-coated on glass substrates led to highly luminescent coatings with q = 0.60 ± 0.02 (excitation at 345 nm). The thin films were tested as planar luminescent solar concentrators and the optimized device displays an optical conversion efficiency of 12.3% in the absorbing spectral region of the active layer (300-380 nm).

Quantifying self-absorption losses in luminescent solar concentrators

Analytical equations quantifying self-absorption losses in circular luminescent solar concentrators (LSCs) are presented that can easily be solved numerically by commercial math software packages. With the quantum efficiency, the absorption and emission spectra of a luminescent material, the LSC dimensions, and the refractive index as the only input parameters, the model gives an accurate account of the decrease of LSC efficiency due to self-absorption as a function of LSC radius, thickness, and luminescence quantum efficiency. Results give insight into how many times light is reabsorbed and reemitted, the red shift of the emission spectrum, and on how multiple reabsorptions and reemissions are distributed over the LSC. As an example case the equations were solved for a circular LSC containing a Lumogen F Red 305 dye with 80% luminescence quantum efficiency, and it follows that for an LSC with a 50 cm radius the self-absorption reduces the number of photons reaching the LSC edge by a factor of four compared to the case when there would be no self-absorption. The equations can just as well be solved for any material for which the optical properties are known like type I and type II quantum dots.

Measurement procedure for absolute broadband infrared up-conversion photoluminescent quantum yields: correcting for absorption/re-emission

The internal photoluminescent quantum yield (iPLQY)--defined as the ratio of emitted photons to those absorbed--is an important parameter in the evaluation and application of luminescent materials. The iPLQY is rarely reported due to the complexities in the calibration of such a measurement. Herein, an experimental method is proposed to correct for re-emission, which leads to an underestimation of the absorption under broadband excitation. Although traditionally the iPLQY is measured using monochromatic sources for linear materials, this advancement is necessary for nonlinear materials with wavelength dependent iPLQY, such as the application of up-conversion to solar energy harvesting. The method requires an additional measurement of the emission line shape that overlaps with the excitation and absorption spectra. Through scaling of the emission spectrum, at the long wavelength edge where an overlap of excitation does not occur, it is possible to better estimate the value of iPLQY. The method has been evaluated for a range of nonlinear material concentrations and under various irradiances to analyze the necessity and boundary conditions that favor the proposed method. Use of this refined method is important for a reliable measurement of iPLQY under a broad illumination source such as the Sun.

Luminescent solar concentrators with fiber geometry

The potential of a fibre luminescent solar concentrator has been explored by means of both analytical and ray-tracing techniques. Coated fibres have been found to be more efficient than homogeneously doped fibres, at low absorption. For practical fibres concentration is predicted to be linear with fibre length. A 1 m long, radius 1 mm, fibre LSC doped with Lumogen Red 305 is predicted to concentrate the AM1.5 g spectrum up to 1100 nm at normal incidence by ~35 x. The collection efficiency under diffuse and direct irradiance in London has been analysed showing that, even under clear sky conditions, in winter the diffuse contribution equals the direct.

Comprehensive analysis of escape-cone losses from luminescent waveguides

Luminescent waveguides (LWs) occur in a wide range of applications, from solar concentrators to doped fiber amplifiers. Here we report a comprehensive analysis of escape-cone losses in LWs, which are losses associated with internal rays making an angle less than the critical angle with a waveguide surface. For applications such as luminescent solar concentrators, escape-cone losses often dominate all others. A statistical treatment of escape-cone losses is given accounting for photoselection, photon polarization, and the Fresnel relations, and the model is used to analyze light absorption and propagation in waveguides with isotropic and orientationally aligned luminophores. The results are then compared to experimental measurements performed on a fluorescent dye-doped poly(methyl methacrylate) waveguide.

Nonimaging optics in luminescent solar concentration

Light trapped within luminescent solar concentrators (LSCs) is naturally limited in angular extent by the total internal reflection critical angle, θcrit, and hence the principles of nonimaging optics can be leveraged to increase LSC concentration ratio by appropriately reshaping the edges. Here, we use rigorous ray-tracing simulations to explore the potential of this concept for realistic LSCs with compound parabolic concentrator (CPC)-tapered edges and show that, when applied to a single edge, the concentration ratio is increased by 23% while maintaining >90% of the original LSC optical efficiency. Importantly, we find that CPC-tapering all of the edges enables a significantly greater intensity enhancement up to 35% at >90% of the original optical efficiency, effectively enabling two-dimensional concentration through a cooperative, ray-recycling effect in which rays rejected by one CPC are accepted by another. These results open up a significant opportunity to improve LSC performance at virtually no added manufacturing cost by incorporating nonimaging optics into their design.

Increased efficiency of luminescent solar concentrators after application of organic wavelength selective mirrors

Organic wavelength-selective mirrors are used to reduce the loss of emitted photons through the surface of a luminescent solar concentrator (LSC). A theoretical calculation suggests that application of a 400 nm broad reflector on top of an LSC containing BASF Lumogen Red 305 as a luminophore can reflect 91% of all surface emitted photons back into the device. Used in this way, such broad reflectors could increase the edge-emission efficiency of the LSC by up to 66%. Similarly, 175 nm broad reflectors could increase efficiency up to 45%. Measurements demonstrate more limited effectiveness and dependency on the peak absorbance of the LSC. At higher absorbance, the increased number of internal re-absorption events reduces the effectiveness of the reflectors, leading to a maximum increase in LSC efficiency of ~5% for an LSC with a peak absorbance of 1. Reducing re-absorption by reducing dye concentration or the coverage of the luminophore coating results in an increase in LSC efficiency of up to 30% and 27%, respectively.

Monocrystalline silicon photovoltaic luminescent solar concentrator with 4.2% power conversion efficiency

We report conversion efficiencies of experimental single and dual light guide luminescent solar concentrators. We have built several 5 cm × 5 cm and 10× cm × 10 cm luminescent solar concentrator (LSC) demonstrators consisting of c-Si photovoltaic cells attached to luminescent light guides of Lumogen F Red 305 dye and perylene perinone dye. The highest overall efficiency obtained was 4.2% on a 5 cm × 5 cm stacked dual light guide using both luminescent materials. To our knowledge, this is the highest reported experimentally determined efficiency for c-Si photovoltaic-based LSCs. Furthermore, we also produced a 5 cm × 5 cm LSC specimen based on an inorganic phosphor layer with an overall efficiency of 2.5%.

Polymer-and glass-based fluorescence standards for the near infrared (NIR) spectral region

The widespread use and acceptance of fluorescence techniques especially in regulated areas like medical diagnostics is closely linked to standardization concepts that guarantee and improve the comparability and reliability of fluorescence measurements. At the core of such concepts are dependable fluorescence standards that are preferably certified. The ever rising interest in fluorescence measurements in the near-infrared (NIR) spectral region renders the availability of spectral and intensity standards for this wavelength region increasingly important. This encouraged us to develop approaches to solid NIR standards based upon dye-doped polymers and assess their application-relevant properties in comparison to metal ion-doped glasses. The overall goal is here to provide inexpensive, easily fabricated, and robust internal and external calibration tools for a broad variety of fluorescence instruments ranging e.g. from spectrofluorometers over fluorescence microscopes to miniaturized fluorescence sensors.

Promising fluorescent dye for solar energy conversion based on a perylene perinone

We describe the synthesis of a dye based on a perylene perinone and evaluate its potential as the functional material for use in the luminescent solar concentrator (LSC). The dye extends the absorption wavelength of LSCs using the perylene-based dye Lumogen Red 305 by more than ~50 nm, translating into the collection of potentially 25% more photons at a reasonable fluorescent quantum yield and photostability. When the new perinone is used in a two-waveguide LSC in conjunction with Red 305, the integrated edge emission of the total LSC system may be increased more than 24% when compared to the Red 305 dye alone.

Patterned dye structures limit reabsorption in luminescent solar concentrators

This work describes a method for limiting internal losses of a luminescent solar concentrator (LSC) due to reabsorption through patterning the fluorescent dye doped coating of the LSC. By engineering the dye coating into regular line patterns with fill factors ranging from 20 - 80%, the surface coverage of the dye molecules were reduced, thereby decreasing the probability of the re-emitted light encountering another dye molecule and the probability of reabsorption. Two types of fluorescent dyes with different quantum yields were used to examine the effects of patterning on LSC performance. The effect of various dimension and geometry of the patterns on the efficiency and edge emission of LSC are presented and analyzed.

Characterization and reduction of reabsorption losses in luminescent solar concentrators

The effects of excitation wavelength on the optical properties (emission spectrum and quantum yield) of a luminescent solar concentrator (LSC) containing a fluorescent organic dye (Lumogen F Rot 305) are studied. Excitation at wavelengths on the long-wavelength edge of the absorption spectrum of the dye results in redshifted emission, but the quantum yield remains constant at 100%. The origin of this effect and its consequences are discussed. The extent of the long-wavelength tail of the absorption spectrum of the dye is determined and the importance in reabsorption losses is shown. The optical efficiencies and photon transport probabilities of LSCs containing either an organic dye or a rare-earth lanthanide complex are compared using ray-tracing simulations and experiment. The optical efficiency is shown to depend strongly on the Stokes shift of the fluorophore. The lanthanide complex, which has a very large Stokes shift, exhibits a higher optical efficiency than the dye (64% cf. 50%), despite its lower quantum yield (86% cf. 100%).