Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

The use of lanthanide ions to convert photons to different, more useful, wavelengths is well-known from a wide range of applications (e.g. fluorescent tubes, lasers, white light LEDs). Recently, a new potential application has emerged: the use of lanthanide ions for spectral conversion in solar cells. The main energy loss in the conversion of solar energy to electricity is related to the so-called spectral mismatch: low energy photons are not absorbed by a solar cell while high energy photons are not used efficiently. To reduce the spectral mismatch losses both upconversion and downconversion are viable options. In the case of upconversion two low energy infrared photons that cannot be absorbed by the solar cell, are added up to give one high energy photon that can be absorbed. In the case of downconversion one high energy photon is split into two lower energy photons that can both be absorbed by the solar cell. The rich and unique energy level structure arising from the 4fn inner shell configuration of the trivalent lanthanide ions gives a variety of options for efficient up- and downconversion. In this perspective an overview will be given of recent work on photon management for solar cells. Three topics can be distinguished: (1) modelling of the potential impact of spectral conversion on the efficiency of solar cells; (2) research on up- and downconversion materials based on lanthanides; and (3) proof-of-principle experiments. Finally, an outlook will be given, including issues that need to be resolved before wide scale application of up- and downconversion materials can be anticipated.

Lanthanide ions as spectral converters for solar cells

Research on the luminescent solar concentrator (LSC) over the past thirty-odd years is reviewed. The LSC is a simple device at its heart, employing a polymeric or glass waveguide and luminescent molecules to generate electricity from sunlight when attached to a photovoltaic cell. The LSC has the potential to find extended use in an area traditionally difficult for effective use of regular photovoltaic panels: the built environment. The LSC is a device very flexible in its design, with a variety of possible shapes and colors. The primary challenge faced by the devices is increasing their photon-to-electron conversion efficiencies. A number of laboratories are working to improve the efficiency and lifetime of the LSC device, with the ultimate goal of commercializing the devices within a few years. The topics covered here relate to the efforts for reducing losses in these devices. These include studies of novel luminophores, including organic fluorescent dyes, inorganic phosphors, and quantum dots. Ways to limit the surface and internal losses are also discussed, including using organic and inorganic-based selective mirrors which allow sunlight in but reflect luminophore-emitted light, plasmonic structures to enhance emissions, novel photovoltaics, alignment of the luminophores to manipulate the path of the emitted light, and patterning of the dye layer to improve emission efficiency. Finally, some possible ‘glimpses of the future’ are offered, with additional research paths that could result in a device that makes solar energy a ubiquitous part of the urban setting, finding use as sound barriers, bus-stop roofs, awnings, windows, paving, or siding tiles.

Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminesce...

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

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), en...

Compound Copper Chalcogenide Nanocrystals

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, such that emitted light is trapped in the sheet and travels to the edges where it can be collected by solar cells. LSCs offer potentially lower cost per Wp. This paper reviews results mainly obtained within the framework of the Fullspectrum project. Two modeling approaches are presented, i.e., a thermodynamic and a ray-trace one, as well as experimental results, with a focus on LSC stability.

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Luminescent Solar Concentrators - A review of recent results

This chapter reviews recent work by the Quantum Photovoltaic Group (QPV) at Imperial College London and their collaborators in the application of nanostructures to efficiently harvest light within the luminescent solar concentrator (LSC). In addition, the work of other groups and their means for optimizing the efficiency of the LSC are reviewed, as it is most likely that a combination of approaches will lead to the increases in system efficiency required for the commercialization of the LSC. Concentrating light with relatively inexpensive devices onto photovoltaic (PV) cells is a promising route to reducing the cost of PV energy generation and may be achieved using LSCs, which were first proposed about thirty years ago. A LSC (see Figure 9.1) generally consists of a transparent plate or slab doped with luminescent centers. Incident light is absorbed by the luminescent centers and reemitted approximately isotropically, ideally with high luminescence quantum efficiency (LQE). While some of the emission is lost through the escape cones, a large fraction of the emission is trapped within the plate by total internal reflection (TIR) and wave guided to the concentrator edges, where it can be converted by PV cells. Concentration is achieved due to the geometrical ratio between the large surface area and the small edge areas. The LSC also converts the broad incident spectrum into a narrow emission spectrum that can be matched to the band gap of the PV cell such that the cell is operating at optimal efficiency and thermalization occurs in the LSC rather than in the more expensive cell, where the heat would reduce cell efficiency.

Luminescent solar concentrators

The invention relates to a fluorescence collector for focusing and converting solar radiation into electricity, said collector being constructed from a substrate (5) and at least one polymer layer (6) or sol-gel layer (6) as carrier structures for at least one type of semiconducting nanoparticles and at least one fluorescent dye. The solar radiation is coupled into the collector and reflected internally, emerging at a defined location, where a photovoltaic cell (1) is disposed. Solar power is thereby converted into electricity.

Fluorescence collector and use thereof

Nanorods are a novel and promising component for luminescent solar concentrators (LSCs). In particular, their spectra suggest reduced re-absorption losses. We report the incorporation of core-shell nanorods in homogeneous and thin film LSCs. The rods in the solid host appear to retain their spectral features compared to their dissolved state. Short-circuit current measurements with a calibrated solar cell have been compared with the computational simulation of the LSCs. Our raytrace model was applied to fit the fundamental emission spectrum and extract the quantum efficiency (QE) of the nanorods in the concentrators. In the case of the homogeneous LSC, the extracted QE was (67±4)%, which is in good agreement with the quoted value of about 70% for rods in solution. The thin film samples showed noticeably worse performance, which was attributed to possible agglomeration of rods and to macroscopic defects in the film. Finally, the raytrace model was applied to compare the self-absorption between a typical quantum dot concentrator and a nanorod concentrator. The result supported the argument that nanorods exhibit a smaller spectral overlap and consequently less re-absorption losses.

Luminescent solar concentrators: Nanorods and raytrace modeling

Luminescent solar concentrators are planar concentrators based on trapping light and wave-guiding it to attached PV cells. They are well suited for building integration, viable in locations with significant indirect sunlight and can be complementary to geometric high-concentration systems. Nanorods have been proposed as novel luminescent centres with promising properties, such as reduced self-absorption. Several nanorod doped concentrators have been fabricated and characterised in order to investigate the dependence of LSC performance on size and dopant concentration. The experimental results illustrate the effect of re-absorption and the associated losses with increasing size and concentration. Computational results using a raytrace model are in good agreement with the experiments. Furthermore, the raytrace model was employed to make realistic projections and extend the parameter range of the experiment.

Jana Quilitz

Papers

Luminescent Solar Concentrators - A review of recent results

Luminescent solar concentrators

Fluorescence collector and use thereof

Luminescent solar concentrators: Nanorods and raytrace modeling

The effect of size and dopant concentration on the performance of nanorod luminescent solar concentrators