Thermoresponsive scattering coating for smart white LEDs.

White light emitting diode (LED) systems, capable of lowering the color temperature of emitted light on dimming, have been reported in the literature. These systems all use multiple color LEDs and complex control circuitry. Here we present a novel responsive lighting system based on a single white light emitting LED and a thermoresponsive scattering coating. The coated LED automatically emits light of lower correlated color temperature (CCT) when the power is reduced. We also present results on the use of multiple phosphors in the white light LED allowing for the emission of warm white light in the range between 2900 K and 4150 K, and with a chromaticity complying with the ANSI standards (C78.377). This responsive warm white light LED-system with close-to-ideal emission characteristics is highly interesting for the lighting industry.

Anisotropic light emissions in luminescent solar concentrators-isotropic systems.

In this paper we develop a model to describe the emission profile from randomly oriented dichroic dye molecules in a luminescent solar concentrator (LSC) waveguide as a function of incoming light direction. The resulting emission is non-isotropic, in contradiction to what is used in almost all previous simulations on the performance of LSCs, and helps explain the large surface losses measured in these devices. To achieve more precise LSC performance simulations we suggest that the dichroic nature of the dyes must be included in the future modeling efforts.

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.

Progress in phosphors and filters for luminescent solar concentrators.

Luminescent solar concentrators would allow for high concentration if losses by reabsorption and escape could be minimized. We introduce a phosphor with close-to-optimal luminescent properties and hardly any reabsorption. A problem for use in a luminescent concentrator is the 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 are also used to predict the ultimate performance of luminescent concentrators.

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.

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.

Measured surface loss from luminescent solar concentrator waveguides.

The surface and edge emissions from dye-filled and dye-topped polycarbonate and polymethyl methacrylate luminescent solar concentrators were measured. We demonstrate that about 40-50% of the absorbed light energy (and 50-70% of the photons) is lost through the top and bottom surfaces of the filled waveguide. In most cases the escape cone losses are greater at the top than the bottom surface.