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Luminescent concentrators would allow for high concentration if losses by reabsorption and escape could be minimized. We present new phosphors and filters that facilitate this. Another type of lightguide-based concentrators, diffraction-based, is discussed as well.

This report is about an internship which was part of a project that aims to develop a Luminescent Solar Concentrator (LSC) for commercial use. In the design of the LSC a wavelength selective mirror is used, in this case in the form of a Cholesteric Liquid Crystal (CLC) layer. The usage of this filter increases the efficiency of the system. To optimize the design of the LSC, a computer program has been adapted to simulate the behavior of the CLC. The calculations match the experiment and can be used to predict the Stokes parameters of there flected and the transmitted light. The results have been used tomake design decisions for the CLC.

In a Luminescent Solar Concentrator (LSC), short-wavelength light isconverted by a luminescent material into long-wavelength light, which is guided towards a photovoltaic cell. In principle, an LSC allows for high concentration, but in practice this is prevented by lossmechanisms like limited sunlight absorption, limited quantum efficiency and high self absorption. To tackle these problems, a suitable luminescent material is needed. Another important loss mechanism is the escape of luminescent radiation into directions that do not stayinside the light guide. To reduce this amount, wavelength-selectivefilters can be applied that reflect the luminescent radiation back into the light guide while transmitting the incident sunlight. In this paper, we discuss experiments and simulations of new luminescent and filter materials. We will introduce a phosphor with close-to-optimal luminescent properties. A problem for use in an LSC is the largescattering of this material; we will discuss possible solutions forthis. Furthermore, we will discuss the use of broad-band cholesteric filters in combination with this phosphor.

We report conversion efficiencies of experimental single and dual lightguide luminescent solar concentrators. We have built several 5x5cm2 and 10x10 cm2 LSC demonstrators, consisting of c-Si photovoltaiccells attached to luminescent lightguides of Lumogen F Red 305 dyeand perylene perinone dye. The highest overall efficiency obtained was 4.2% on a 5x5 cm2 stacked dual lightguide using both luminescentmaterials. To our knowledge this is the highest reported experimentally determined efficiency for c-Si PVs based LSCs. Furthermore, we also produced a 5x5 cm2 LSC specimen based on an inorganic phosphor layer with an overall efficiency of 2.5%.

Transmission gratings that combine a large diffraction angle with ahigh diffraction efficiency and low angular and wavelength dispersion could be used to collect sunlight in a light guide. In this paperwe determine what characteristics a grating should have in order tobe useful for such a solar concentrator. To this end we compare thediffractive properties of polarization gratings and classical surface relief gratings. It is found that polarization gratings and classical surface relief gratings have qualitatively comparable diffractive properties as long as their thickness-parameters are within the same regime. The diffraction efficiency of these gratings can be closeto 100% for a broad range of incoming angles when the period is large compared to the wavelength of the incoming light. This no longerholds for small-period gratings. For solar concentrators the more easily producible surface relief gratings are preferred over polarization gratings.

The efficiency of Luminescent Solar Concentrators could be greatly enhanced by the use of wavelength-selective filters, since they reduce the amount of luminescent light lost. To accomplish this, polarization-independent filters have been made by combining layers of cholesteric liquid crystals, either a right- with a left-handed layer, ortwo right-handed layers with a half-lambda waveplate. Normal cholesteric filters have a reflection bandwidth which is narrower than thespectral and angular range of the luminescent emission. To broadenthe reflection band, we employed a pitch gradient in the cholestericlayer. The measured transmission bands compare well with calculations.

Luminescent solar concentrators would allow for high concentration if losses by reabsorption and escape could be minimized. We introducea phosphor with close-to-optimal luminescent properties and hardlyany reabsorption. A problem for use in a luminescent concentrator isthe large scattering of this material; we discuss possible solutions for this. Furthermore, the use of broad-band cholesteric filters to prevent escape of luminescent radiation from this phosphor is investigated both experimentally and using simulations. Simulations arealso used to predict the ultimate performance of luminescent concentrators.