Luminescent solar concentrators. 2: Experimental and theoretical analysis of their possible efficiencies

A comprehensive dataset of photonic features on spectral converters for energy harvesting

Building integrated photovoltaics is a promising strategy for solar technology, in which luminescent solar concentrators (LSCs) stand out. Challenges include the development of materials for sunlight harvesting and conversion, which is an iterative optimization process with several steps: synthesis, processing, and structural and optical characterizations before considering the energy generation figures of merit that requires a prototype fabrication. Thus, simulation models provide a valuable, cost-effective, and time-efficient alternative to experimental implementations, enabling researchers to gain valuable insights for informed decisions. We conducted a literature review on LSCs over the past 47 years from the Web of ScienceTM Core Collection, including published research conducted by our research group, to gather the optical features and identify the material classes that contribute to the performance. The dataset can be further expanded systematically offering a valuable resource for decision-making tools for device design without extensive experimental measurements.

Spatially Resolved Optical Efficiency Measurements of Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are able to concentrate both direct and diffuse solar radiation, and this ability has led to great interest in using them to improve solar energy capture when coupled to traditional photovoltaics (PV). In principle, a large-area LSC could concentrate light onto a much smaller area of PV, thus reducing costs or enabling new architectures. However, LSCs suffer from various optical losses which are hard to quantify using simple measurements of power conversion efficiency. Here, we show that spatially resolved photoluminescence quantum efficiency measurements on large-area LSCs can be used to resolve various loss processes such as out-coupling, self-absorption via emitters, and self-absorption from the LSC matrix. Further, these measurements allow for the extrapolation of device performance to arbitrarily large LSCs. Our results provide insight into the optimization of optical properties and guide the design of future LSCs for improved solar energy capture.

Low-Loss, High-Transparency Luminescent Solar Concentrators with a Bioinspired Self-Cleaning Surface

Luminescent solar concentrators (LSCs) have emerged as a disruptive technology that can potentially enable carbon-neutral buildings. The issues with current LSCs, however, are low optical efficiencies and limited long-term outdoor stability. Here we simultaneously address them by developing an LSC with aggregation-induced-emission (AIE) molecules embedded in a polydimethylsiloxane (PDMS) matrix. The AIE-emitter displayed a near unity emission quantum yield when embedded in the PDMS and the apparent absorption-emission Stokes shift reached 0.59 eV, effectively suppressing the reabsorption loss of waveguided photons inside an LSC. Moreover, the surface texture of the PDMS matrix was engineered using a bioinspired nanolithography method with a natural lotus leaf as the template. This allowed the fabricated AIE-PDMS LSC to inherit the superhydrophobic, self-cleaning properties of the leaf and meanwhile to possess a light-trapping capability. Our 100 cm² LSC, when coupled with commercial Si PVs, delivered efficient solar power conversion, high visible transmittance, and high working stability.

Infrared Efficiency and Ultraviolet Management of Red-Pigmented Polymethylmethacrylate Photoselective Greenhouse Films

Red-pigmented photoselective polymethylmethacrylate (PMMA) films were prepared by casting from polymer/chloroform solution. The films were doped with efficient red fluorescent perylene dyes specialized for plastic coloration, namely KREMER 94720 and KREMER 94739, which have excellent weathering stability and a high fluorescence quantum yield. The effect of the doping concentration was studied using the atomic force microscope (AFM), optical transmission, color measurement, time-resolved fluorescence, and Fourier transform infrared spectroscopy (FTIR). The obtained results suggested the potential usefulness for photoselective greenhouse cladding applications as the lowest doping concentration (10−5 wt%) displaying the UV-open effect, whereas the best UV-blocking and thermic effects were obtained for the highest doping concentration (10−1 wt).

Highly Transparent, Dual-Color Emission, Heterophase Cs3Cu2I5/CsCu2I3 Nanolayer for Transparent Luminescent Solar Concentrators

Transparent luminescent solar concentrators (TLSCs) have been attracting wide attentions for their applications in transparent photovoltaic (PV) windows, smart greenhouses, and mobile electronics on account of the simple architecture and low-cost preparation. We report a novel strategy to fabricate TLSCs using the heterophase lead-free perovskites. The heterophase nanolayered films which combined CsCu₂I₃ and Cs₃Cu₂I₅ were prepared in one step using a dual-source coevaporation technique. The CsCu₂I₃/Cs₃Cu₂I₅ films exhibited UV light absorption, a high average visible transmission (AVT) of 86.70%, and dual-color white emission between 350 and 760 nm. Importantly, the TLSCs incorporated with the CsCu₂I₃/Cs₃Cu₂I₅ films exhibited an impressive optical conversion efficiency of 1.15% under keeping a high AVT of 86.70%. Meanwhile, the TLSCs incorporated with the heterophase films showed considerable stability under ambient conditions. The CIE 1960 color coordinates (0.2082, 0.4680) of the TLSCs incorporated with the CsCu₂I₃/Cs₃Cu₂I₅ films showed excellent aesthetic quality as compared with those of the TLSCs incorporated with lead-based perovskites. Our finding offers a strategy to prepare lead-free metal halides toward high-performance TLSCs and future transparent PV windows.

Modelling of polymer optical fiber-based solar concentrators

A comprehensive model for the theoretical simulation of luminescent solar concentrators (LSCs) has been developed and examined. It can simulate the interdependent effects of multiple dopants having two main electronic energy states, which are incorporated simultaneously into the fiber core, as well as the effect of the cladding. The available experimental results appear to confirm the accuracy of the model, which is a valuable tool for gaining insight into the behavior of LSC prototypes, since it may guide the designers at the early stages of optimization processes.

Spectral response of large-area luminescent solar concentrators

Measuring the spectral response (SR) of large-area (>100cm²) luminescent solar concentrators (LSCs) has proven difficult because common laboratory photovoltaic (PV) instruments that offer monochromatic incidence measure devices with limited sizes (typically <50cm²). This report addresses this issue through a method called regional measurements. In this method, large-area LSCs are configured to small surface and edge regions, which are sequentially illuminated and measured, respectively. The measured SRs of large-area LSCs are consistent with those from the conventional method and the Monte Carlo ray-tracing simulation. This method is also applied to analyze scattering effects in the LSCs, showing the relationships of the scattering-induced power gain and power loss to the surface root-mean-squared roughness (R_q) of the devices. The results explain why the PV performance of the LSCs can be improved through proper surface scattering treatment.

Luminescent Solar Concentrators from Waterborne Polymer Coatings

This study reports for the first time the use of waterborne polymers as host matrices for luminescent solar concentrators (LSCs). Notably, three types of waterborne polymer dispersions based either on acrylic acid esters and styrene (Polidisp® 7602), acrylic and methacrylic acid esters (Polidisp® 7788) or aliphatic polyester-based polyurethane (Tecfin P40) were selected as amorphous coatings over glass substrates. Water soluble Basic Yellow 40 (BY40) and Disperse Red 277 (DR277) were utilized as fluorophores and the derived thin polymer films (100 μm) were found homogeneous within the dye range of concentration investigated (0.3–2 wt.%). The optical efficiency determination (ηopt) evidenced LSCs performances close to those collected from benchmark polymethylmethacrylate (PMMA) thin films and Lumogen Red F350 (LR) with the same experimental setup. Noteworthy, maximum ηopt of 9.5 ± 0.2 were recorded for the Polidisp® 7602 matrix containing BY40, thus definitely supporting the waterborne polymer matrices for the development of high performance and cost-effective LSCs.

Geometry effects on luminescence solar concentrator efficiency: analytical treatment

Luminescence solar concentrators act as semitransparent photovoltaic cells of interest for modern urban environments. Here, their efficiencies were analytically derived for different regular unit shapes as simple, integral-free expressions. This allowed analysis of the shape and size effect on the device performance. All regular shapes appear to have a similar efficiency as revealed by optical path distribution formulas, despite differences in the perimeter length. Rectangles of the same area feature higher efficiency due to reduced average optical path. It comes with the cost of a longer perimeter, and the relation between these two is provided. An explicit formula for the critical size of an LSC unit, above which its inner part becomes inactive, has been obtained. For square geometry with matrix absorption coefficient α this critical size is ∼2.7/α, corresponding to 70-90 cm for common polymer materials. Obtained results can be used for treatment of individual units as well as for analysis of tiling for large areas.

Organic semiconductors for visible light communications

Organic semiconductors are an important class of optoelectronic material that are widely studied because of the scope for tuning their properties by tuning their chemical structure, and simple fabrication to make flexible films and devices. Although most effort has focused on developing displays and lighting from these materials, their distinctive properties also make them of interest for visible light communications (VLCs). This article explains how their properties make them suitable for VLC and reviews the main uses that have been explored. On the transmitter side, record white VLC communication has been achieved by using organic semiconductors as colour converters, while direct modulation of organic light-emitting diodes is also possible and could be of interest for display-to-display communication. On the receiver side, organic solar cells can be used to harvest power and data simultaneously, and fluorescent antennas enable fast and sensitive receivers with large field of view. This article is part of the theme issue 'Optical wireless communication'.

Cross talk and optical efficiency of an energy-harvesting color projector utilizing ceramic phosphors

A color projector screen was fabricated by filling three kinds of ceramic phosphor powders in the periodic hollow columns formed in a ${50}\;{{\rm mm}}\; \times \;{50}\;{{\rm mm}}\; \times \;{10}\;{{\rm mm}}$50mm×50mm×10mm acrylic waveguide. When a blue laser beam excited a single spot on the screen, a disk-shaped cross-talk pattern appeared. Its intensity was 5 orders of magnitude lower than that of the excited spot. The solar cells attached to the waveguide edge harvested less than 0.8% of the incident optical power. The photons scattered by the phosphors are responsible for these characteristics, and the use of non-scattering luminescent materials is desired for improving them.

Quantum-Cutting Luminescent Solar Concentrators Using Ytterbium-Doped Perovskite Nanocrystals

We introduce and demonstrate the concept of quantum-cutting luminescent solar concentrators (QC-LSCs) using Yb³⁺-doped perovskite nanocrystals. These NCs feature a photoluminescence quantum yield approaching 200% and virtually zero self-absorption loss of PL photons, defining a new upper limit of 150% for the internal optical efficiency (η_int) of LSCs that is almost independent of LSC sizes. An un-optimized 25 cm² QC-LSC fabricated from Yb³⁺-doped CsPbCl₃ NCs already displayed an η_int of 118.1 ± 6.7% that is 2-fold higher than previous records using Mn²⁺-doped quantum dots (QDs). If using CsPbCl _xBr_{3- x} NCs capable of absorbing ∼7.6% of solar photons, the projected external optical efficiency (η_ext) of QC-LSCs can exceed 10% for >100 cm² devices, which still remains a challenge in the field. The advantage of QC-LSCs over conventional QD-LSCs becomes especially obvious with increasing LSC sizes, which is predicted to exhibit a more than 4-fold efficiency enhancement in the case of window-size (1 m²) devices.

Impact of Stokes Shift on the Performance of Near-Infrared Harvesting Transparent Luminescent Solar Concentrators

Visibly transparent luminescent solar concentrators (TLSC) have the potential to turn existing infrastructures into net-zero-energy buildings. However, the reabsorption loss currently limits the device performance and scalability. This loss is typically defined by the Stokes shift between the absorption and emission spectra of luminophores. In this work, the Stokes shifts (SS) of near-infrared selective-harvesting cyanines are altered by substitution of the central methine carbon with dialkylamines. We demonstrate varying SS with values over 80 nm and ideal infrared-visible absorption cutoffs. The corresponding TLSC with such modification shows a power conversion efficiency (PCE) of 0.4% for a >25 cm² device area with excellent visible transparency >80% and up to 0.6% PCE over smaller areas. However, experiments and simulations show that it is not the Stokes shift that is critical, but the total degree of overlap that depends on the shape of the absorption tails. We show with a series of SS-modulated cyanine dyes that the SS is not necessarily correlated to improvements in performance or scalability. Accordingly, we define a new parameter, the overlap integral, to sensitively correlate reabsorption losses in any LSC. In deriving this parameter, new approaches to improve the scalability and performance are discussed to fully optimize TLSC designs to enhance commercialization efforts.

Harnessing the properties of colloidal quantum dots in luminescent solar concentrators

This review summarizes the recent progress, challenges and perspectives of luminescent solar concentrators based on colloidal quantum dots via harnessing their properties.

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

The phenomenon of self-absorption is by far the largest influential factor in the efficiency 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 coefficients. 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.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Emissive Molecular Aggregates and Energy Migration in Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are light harvesting devices that are ideally suited to light collection in the urban environment where direct sunlight is often not available. LSCs consist of highly luminescent compounds embedded or coated on a transparent substrate that absorb diffuse or direct solar radiation over a large area. The resulting luminescence is trapped in the waveguide by total internal reflection to the thin edges of the substrate where the concentrated light can be used to improve the performance of photovoltaic devices. The concept of LSCs has been around for several decades, and yet the efficiencies of current devices are still below expectations for commercial viability. There are two primary challenges when designing new chromophores for LSC applications. Reabsorption of dye emission by chromophores within the waveguide is a significant loss mechanism attenuating the light output of LSCs. Concentration quenching, particularly in organic dye systems, restricts the quantity of chromophores that can be incorporated in the waveguide thus limiting the light absorbed by the LSC. Frequently, a compromise between increased light harvesting of the incident light and decreasing emission quantum yield is required for most organic chromophore-based systems due to concentration quenching. The low Stokes shift of common organic dyes used in current LSCs also imposes another optimization problem. Increasing light absorption of LSCs based on organic dyes to achieve efficient light harvesting also enhances reabsorption. Ideally, a design strategy to simultaneously optimize light harvesting, concentration quenching, and reabsorption of LSC chromophores is clearly needed to address the significant losses in LSCs. Over the past few years, research in our group has targeted novel dye structures that address these primary challenges. There is a common perception that dye aggregates are to be avoided in LSCs. It became apparent in our studies that aggregates of chromophores exhibiting aggregation-induced emission (AIE) behavior are attractive candidates for LSC applications. Strategic application of AIE chromophores has led to the development of the first organic-based transparent solar concentrator that harvests UV light as well as the demonstration of reabsorption reduction by taking advantage of energy migration processes between chromophores. Further developments led us to the application of perylene diimides using an energy migration/energy transfer approach. To prevent concentration quenching, a molecularly insulated perylene diimide with bulky substituents attached to the imide positions was designed and synthesized. By combining the insulated perylene diimide with a commercial perylene dye as an energy donor-acceptor emitter pair, detrimental luminescence reabsorption was reduced while achieving a high chromophore concentration for efficient light absorption. This Account reviews and reinspects some of our recent work and the improvements in the field of LSCs.

Photonic microstructures for energy-generating clear glass and net-zero energy buildings

Transparent energy-harvesting windows are emerging as practical building-integrated photovoltaics (BIPV), capable of generating electricity while simultaneously reducing heating and cooling demands. By incorporating spectrally-selective diffraction gratings as light deflecting structures of high visible transparency into lamination interlayers and using improved spectrally-selective thin-film coatings, most of the visible solar radiation can be transmitted through the glass windows with minimum attenuation. At the same time, the ultraviolet (UV) and a part of incident solar infrared (IR) radiation energy are converted and/or deflected geometrically towards the panel edge for collection by CuInSe2 solar cells. Experimental results show power conversion efficiencies in excess of 3.04% in 10 cm × 10 cm vertically-placed clear glass panels facing direct sunlight, and up to 2.08% in 50 cm × 50 cm installation-ready framed window systems. These results confirm the emergence of a new class of solar window system ready for industrial application.

Cylindrical array luminescent solar concentrators: performance boosts by geometric effects

This paper presents an investigation of the geometric effects within a cylindrical array luminescent solar concentrator (LSC). Photon concentration of a cylindrical LSC increases linearly with cylinder length up to 2 metres. Raytrace modelling on the shading effects of circles on their neighbours demonstrates effective incident light trapping in a cylindrical LSC array at angles of incidence between 60-70 degrees. Raytrace modelling with real-world lighting conditions shows optical efficiency boosts when the suns angle of incidence is within this angle range. On certain days, 2 separate times of peak optical efficiency can be attained over the course of sunrise-solar noon.

Luminescent solar concentrators utilizing stimulated emission

Luminescent solar concentrators (LSCs) are an emerging technology that aims primarily to reduce the cost of solar energy, with great potential for building integrated photovoltaic (PV) structures. However, realizing LSCs with commercially viable efficiency is currently hindered by reabsorption losses. Here, we introduce an approach to reducing reabsorption as well as improving directional emission in LSCs by using stimulated emission. Light from a seed laser (potentially an inexpensive laser diode) passes through the entire area of the LSC panel, modifying the emission spectrum of excited dye molecules such that it is spectrally narrower, at wavelengths that minimize reabsorption to allow net gain in the system, and directed towards a small PV cell. A mathematical model, taking into account thermodynamic considerations, of such a system is presented which identifies key parameters and allows evaluation in terms of net effective output power.

A Low Reabsorbing Luminescent Solar Concentrator Employing π-Conjugated Polymers

A highly efficient thin-film luminescent solar concentrator (LSC) utilizing two π-conjugated polymers as antennae for small amounts of the valued perylene bisimide Lumogen F Red 305 is presented. The LSC exhibits high photoluminescence quantum yield, low reabsorption, and relatively low refractive indices for waveguide matching. A Monte Carlo simulation predicts the LSC to possess exceptionally high optical efficiencies on large scales.

Large Stokes Shift and High Efficiency Luminescent Solar Concentrator Incorporated with CuInS2/ZnS Quantum Dots

Luminescent solar concentrator (LSC) incorporated with quantum dots (QDs) have been widely regarded as one of the most important development trends of cost-effective solar energy. In this study, for the first time we report a new QDs-LSC integrated with heavy metal free CuInS₂/ZnS core/shell QDs with large Stokes shift and high optical efficiency. The as-prepared CuInS₂/ZnS QDs possess advantages of high photoluminescence quantum yield of 81% and large Stocks shift more than 150 nm. The optical efficiency of CuInS₂/ZnS QDs-LSC reaches as high as 26.5%. Moreover, the power conversion efficiency of the QDs-LSC-PV device reaches more than 3 folds to that of pure PMMA-PV device. Furthermore, the PV device is able to harvest 4.91 folds solar energy with the assistance of this new CuInS₂/ZnS QDs-LSC for the same size c-Si PV cell. The results demonstrate that this new CuInS₂/ZnS QDs-LSC provides a promising way for the high efficiency, nonhazardous and low cost solar energy.

A silicon nanocrystal/polymer nanocomposite as a down-conversion layer in organic and hybrid solar cells

Silicon nanocrystal (Si-nc) down-conversion is demonstrated to enhance organic and hybrid organic/inorganic bulk heterojunction solar cells based on PTB7:[70]PCBM bulk heterojunction devices. Surfactant free surface-engineered Si-ncs can be integrated into the device architecture to be optically active and provide a means of effective down-conversion of blue photons (high energy photons below ∼450 nm) into red photons (above ∼680 nm) leading to 24% enhancement of the photocurrent under concentrated sunlight. We also demonstrate that the down-conversion effect under 1-sun is enhanced in the case of hybrid solar cells where engineered Si-ncs are also included in the active layer.

Spectrally-selective all-inorganic scattering luminophores for solar energy-harvesting clear glass windows

All-inorganic visibly-transparent energy-harvesting clear laminated glass windows are the most practical solution to boosting building-integrated photovoltaics (BIPV) energy outputs significantly while reducing cooling- and heating-related energy consumption in buildings. By incorporating luminophore materials into lamination interlayers and using spectrally-selective thin-film coatings in conjunction with CuInSe2 solar cells, most of the visible solar radiation can be transmitted through the glass window with minimum attenuation while ultraviolet (UV) radiation is down-converted and routed together with a significant part of infrared radiation to the edges for collection by solar cells. Experimental results demonstrate a 10 cm × 10 cm vertically-placed energy-harvesting clear glass panel of transparency exceeding 60%, invisible solar energy attenuation greater than 90% and electrical power output near 30 Wp/m(2) mainly generated by infrared (IR) and UV radiations. These results open the way for the realization of large-area visibly-transparent energy-harvesting clear glass windows for BIPV systems.

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.

Zero-reabsorption doped-nanocrystal luminescent solar concentrators

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn(2+)-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.

Luminescent solar concentration with semiconductor nanorods and transfer-printed micro-silicon solar cells

We utilize CdSe/CdS seeded nanorods as a tunable lumophore for luminescent concentration. Transfer-printed, ultrathin crystalline Si solar cells are embedded directly into the luminescent concentrator, allowing the study of luminescent concentrators with an area over 5000 times the area of the solar cell. By increasing the size of the CdS rod with respect to the luminescent CdSe seed, the reabsorption of propagating photons is dramatically reduced. At long luminescence propagation distances, this reduced reabsorption can overcome the diminished quantum yield inherent to the larger semiconductor structures, which is studied with lifetime spectroscopy. A Monte Carlo ray tracing model is developed to explain the performance of the luminescent concentrator and is then used as a design tool to determine the effect of luminescence trapping on the concentration of light using both CdSe/CdS nanorods and a model organic dye. We design an efficient luminescence trapping structure that should allow the luminescent concentrator based on CdSe/CdS nanorods to operate in the high-concentration regime.

Monte Carlo study of PbSe quantum dots as the fluorescent material in luminescent solar concentrators

In this paper, Monte Carlo simulations were performed to determine the potential efficiencies of luminescent solar concentrator (LSC) systems using PbSe quantum dots (QDs) as the active fluorescent material. The simulation results suggest that PbSe QD LSCs display good absorption characteristics, but yield limited LSC power conversion efficiency due to self-absorption and down-conversion loss. It is proposed that the self-absorption loss can be reduced by utilizing Förster resonance energy transfer between two different sizes of PbSe QDs, yielding pronounced improvement in the optical efficiency of LSCs.

Efficiency and loss mechanisms of plasmonic Luminescent Solar Concentrators

Using a hybrid nanoscale/macroscale model, we simulate the efficiency of a luminescent solar concentrator (LSC) which employs silver nanoparticles to enhance the dye absorption and scatter the incoming light. We show that the normalized optical efficiency can be increased from 10.4% for a single dye LSC to 32.6% for a plasmonic LSC with silver spheres immersed inside a thin dye layer. Most of the efficiency enhancement is due to scattering of the particles and not due to dye absorption/re-emission.

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.

Artificial light-harvesting arrays: electronic energy migration and trapping on a sphere and between spheres

A sophisticated model of the natural light-harvesting antenna has been devised by decorating a C(60) hexa-adduct with ten yellow and two blue boron dipyrromethene (Bodipy) dyes in such a way that the dyes retain their individuality and assist solubility of the fullerene. Unusually, the fullerene core is a poor electron acceptor and does not enter into light-induced electron-transfer reactions with the appended dyes, but ineffective electronic energy transfer from the excited-state dye to the C(60) residue competes with fluorescence from the yellow dye. Intraparticle electronic energy transfer from yellow to blue dyes can be followed by steady-state and time-resolved fluorescence spectroscopy and by excitation spectra for isolated C(60) nanoparticles dissolved in dioxane at 293 K and at 77 K. The decorated particles can be loaded into polymer films by spin coating from solution. In the dried film, efficient energy transfer occurs such that photons absorbed by the yellow dye are emitted by the blue dye. Films can also be prepared to contain C(60) nanoparticles loaded with the yellow Bodipy dye but lacking the blue dye and, under these circumstances, electronic energy migration occurs between yellow dyes appended to the same nanoparticle and, at higher loading, to dye molecules on nearby particles. Doping these latter polymer films with the mixed-dye nanoparticle coalesces these multifarious processes in a single system. Thus, long-range energy migration occurs among yellow dyes attached to different particles before trapping at a blue dye. In this respect, the film resembles the natural photosynthetic light-harvesting complexes, albeit at much reduced efficacy. The decorated nanoparticles sensitize amorphous silicon photocells.

Investigation of 1H NMR chemical shifts of organic dye with hydrogen bonds and ring currents

The (1)H NMR chemical shifts were theoretically computed for the organic dyes 2-(2,6-dimethyl-4H-pyran-4-ylidene)-malononitrile (1), cyano-(2,6-dimethyl-4H-pyran-4-ylidene)-acetic acid methyl ester (2), 2-(2,6-bis(4-(dimethylamino)styryl)-4H-pyran-4-ylidene)-malononitrile (3), and methyl 2-(2,6-bis(4-(dimethylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate (4) at the GIAO/B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory. Moreover, the intramolecular rotational barriers of the molecules were calculated to evaluate the internal flexibility with respect to the torsional degrees of freedom, and the nuclear-independent chemical shifts (NICS) were employed to analyze the ring currents. The difference was explained in terms of intramolecular hydrogen bonds and ring currents of the molecules. The (1)H NMR spectra were reproduced by experiments for the comparison with computationally constructed data. Our results suggest a good guideline in interpreting (1)H NMR chemical shifts using computational methods and furthermore a reliable perspective for designing molecular structures.

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.

Towards an aligned luminophore solar concentrator

Luminescent solar concentrators promise to reduce the cost of solar energy, but are hindered by a number of losses. Escape of luminescence through the large waveguide-air interfaces can be attenuated through alignment of the optical transition dipole of the luminophore along the waveguide surface normal, directing the maximum possible proportion of luminescence into waveguide modes. We demonstrate such alignment using a guest-host dye-doped liquid crystal sandwiched between conductive glass slides. Application of a potential while illuminating through a narrow edge caused a drop in the intensity of luminescence escaping the large surfaces, and an increase in the intensity of light escaping the narrow edges of the system. This is explained in terms of alignment of the transition dipoles of the dye. We discuss implementation in a luminescent solar concentrator.

Dye alignment in luminescent solar concentrators: I. Vertical alignment for improved waveguide coupling

Luminescent solar concentrators (LSCs) use dye molecules embedded in a flat-plate waveguide to absorb solar radiation. Ideally, the dyes re-emit the absorbed light into waveguide modes that are coupled to solar cells. But some photons are always lost, re-emitted through the face of the LSC and coupled out of the waveguide. In this work, we improve the fundamental efficiency limit of an LSC by controlling the orientation of dye molecules using a liquid crystalline host. First, we present a theoretical model for the waveguide trapping efficiency as a function of dipole orientation. Next, we demonstrate an increase in the trapping efficiency from 66% for LSCs with no dye alignment to 81% for a LSC with vertical dye alignment. Finally, we show that the enhanced trapping efficiency is preserved for geometric gains up to 30, and demonstrate that an external diffuser can alleviate weak absorption in LSCs with vertically-aligned dyes.

Dye alignment in luminescent solar concentrators: II. Horizontal alignment for energy harvesting in linear polarizers

We describe Linearly Polarized Luminescent Solar Concentrators (LP-LSCs) to replace conventional, purely absorptive, linear polarizers in energy harvesting applications. As a proof of concept, we align 3-(2-Benzothiazolyl)-N,N-diethylumbelliferylamine (Coumarin 6) and 4-dicyanomethyl-6-dimethylaminostiryl-4H-pyran (DCM) dye molecules linearly in the plane of the substrate using a polymerizable liquid crystal host. We show that up to 38% of the photons polarized on the long axis of the dye molecules can be coupled to the edge of the device for an LP-LSC based on Coumarin 6 with an order parameter of 0.52.

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%).

Effect on the output of a luminescent solar concentrator on application of organic wavelength-selective mirrors

To reduce surface loss in luminescent solar concentrators (LSCs), we systematically apply organic wavelength-selective mirrors, chiral nematic (cholesteric) liquid crystals, onto the LSCs with an air gap and determine their effect on waveguide output. The highest output is achieved using a scattering background and cholesteric mirror with a reflection band significantly redshifted (approximately 150 nm) from the emission peak of the fluorescent dye. The use of an air gap results in light bending away from the waveguide surface normal and, consequently, a redshift of the cholesteric mirrors is required. Up to 35% more dye-emitted light energy exits the waveguide edge after application of the cholesteric, and an increase in absolute edge power of 12% was found for a waveguide using a separate scatterer.