Self absorption in luminescent solar concentrators

Luminescent solar concentrators are photovoltaic devices made of thin transparent material, in which luminescent particles are dispersed. The incident light enters the device through its large facets and is subsequently absorbed by the luminescent particles, which re-emit it whilst changing its direction of propagation. Most of the re-emitted light hits the surfaces of the transparent plate in the regime of total internal reflection, which results in wave-guiding towards a facet with an attached solar cell, where the power conversion takes place. Loss mechanisms and their amplification by means of self-absorption prevent as yet the device prototypes from reaching their theoretical performance parameters (10% power conversion efficiency and concentration factor 4.5x ). In this thesis we investigate the impact of self-absorption in luminescent solar concentrators based on various luminescent species. This information is used to build a luminescent solar concentrator prototype which is capable of circumventing the loss amplification by self-absorption. The results are well understood in the light of validated ray-tracing/Monte-Carlo simulations, which predict that luminescent species whose absorption- and emission-spectra overlap are more prone to self-absorption effects. These simulations permit furthermore the separate investigation of the direct effects of losses, without the amplification effects. Based on this information the actual liquid-phase prototype of an LSC with the organic dye Lumogen Red 305 was built. A consequence of increased luminophore concentration is increased absorption. Increase of absorption in its turn increases self-absorption and thus increases the losses. On the other hand increased absorption contributes to more photons in the device, which means that more photons can be converted into electricity. This competition of losses and gains at increasing luminophore concentration leads to an expectation of the existence of an unknown optimal concentration. To find it, the luminophore concentration was increased stepwise and the device efficiency was determined at every step. It turned out, that upon addition of luminophores a saturation of the device efficiency is observed, which means that increase of luminophore concentration compensates the increased self-absorption losses. Furthermore the performance of CdTe/CdSe semiconductor nanocrystals – with highly reduces self-absorption but a weak luminescence quantum efficiency was compared with that of highly luminescent and highly self-absorbing CdSe-Multishell quantum dots. Both achieve approximately the same low efficiency of 1.2-1.3%, which indicates the pathway for further improvement: the increase of luminescence efficiency of CdTe/CdSe nanocrystals. Simulations predict that an otherwise identical CdTe/CdSe luminescent solar concentrator with an increased luminescence quantum efficiency (95%) could surpass the currently achieved performance with a device efficiency 3.7% with a concentration factor of 2.5. This is significant as so far high performance values could be reported for either high concentration factors (1.8) with low device efficiencies (2.7%) or high device efficiencies (7.1%) Finally, we describe the construction of a luminescent solar concentrator using Lumogen Red 305 with both performance quantifiers, concentration factor and optical efficiency, in the top of its category of concentrators

Luminescent solar concentrators: the route to 10% efficiency

Luminescent solar concentrators consist of a highly transparent plastic plate, in which luminescent species are dispersed, which absorb incident light and emit light at a red-shifted wavelength, with high quantum efficiency. Fundamental issues have hampered efficiency improvements, in particular re-absorption of light emitted by luminescent species due to small Stokes' shifts and stability of these species. In this contribution, we will address recent advances to minimize re-absorption, which would allow surpassing the 10% luminescent solar concentrator efficienc

Compensation of self-absorption losses in luminescent solar concentrators by increasing luminophore concentration

Self-absorption in luminophores is considered a major obstacle on the way towards efficient luminescent solar concentrators (LSCs). It is commonly expected that upon increasing luminophore concentration in an LSC the absorption of the luminophores increases as well and therefore self-absorption losses will have higher impact on the performance of the device. In this work we construct a fully functioning liquid phase LSC where the luminophore concentration can be altered without changing other conditions in the experimental set-up. We step-wise enlarge the concentration of the luminophores Lumogen Red 305 and Lumogen Orange 240, while monitoring the electrical output and self-absorption effects. Contrary to common belief, self-absorption does not increasingly limit the performance of LSCs when the luminophore concentration increases

Compensation of self-absorption losses in luminescent solar concentrators by increasing luminophore concentration

\u3cp\u3eSelf-absorption in luminophores is considered a major obstacle on the way towards efficient luminescent solar concentrators (LSCs). It is commonly expected that upon increasing luminophore concentration in an LSC the absorption of the luminophores increases as well and therefore self-absorption losses will have higher impact on the performance of the device. In this work we construct a fully functioning liquid phase LSC where the luminophore concentration can be altered without changing other conditions in the experimental set-up. We step-wise enlarge the concentration of the luminophores Lumogen Red 305 and Lumogen Orange 240, while monitoring the electrical output and self-absorption effects. Contrary to common belief, self-absorption does not increasingly limit the performance of LSCs when the luminophore concentration increases.\u3c/p\u3

Exploration of parameters influencing the self-absorption losses in luminescent solar concentrators with an experimentally validated combined ray-tracing/Monte-Carlo model

Luminescent solar concentrators (LSCs) are low cost photovoltaic devices, which reduce the amount of necessary semiconductor material per unit area of a photovoltaic solar energy converter by means of concentration. The device is comprised of a thin plastic plate in which luminescent species (fluorophores) have been incorporated. The fluorophores absorb the solar light and radiatively re-emit a part of the energy. Total internal reflection traps most of the emitted light inside the plate and wave-guides it to a narrow side facet with a solar cell attached, where conversion into electricity occurs. The eficiency of such devices is as yet rather low, due to several loss mechanisms, of which self-absorption is of high importance. Combined ray-tracing and Monte-Carlo simulations is a widely used tool for eficiency estimations of LSC-devices prior to manufacturing. We have applied this method to a model experiment, in which we analysed the impact of self-absorption onto LSC- eficiency of fluorophores with different absorption/emission-spectral overlap (Stokes-shift): several organic dyes and semiconductor quantum dots (single compound and core/shell of type-II). These results are compared with the ones obtained experimentally demonstrating a good agreement. The validated model is used to investigate systematically the influence of spectral separation and luminescence quantum eficiency on the intensity loss in consequence of increased self-absorption. The results are used to adopt a quantity called the self-absorption cross-section and establish it as reliable criterion for self-absorption properties of materials that can be obtained from fundamental data and has a more universal scope of application, than the currently used Stokes-shift

Spectral conversion for thin film solar cells and luminescent solar concentrators

Full spectrum absorption combined with effective generation and collection of charge carriers is a prerequisite for attaining high efficiency solar cells. Two examples of spectral conversion are treated in this chapter, i.e., up-conversion and down-shifting. Up-conversion is applied to thin film silicon solar cells and efficiency improvements using lanthanides as up-converter material under monochromatic as well as broadband light are presented. Down-shifting is demonstrated in luminescent solar concentrators, and material issues hampering efficiency improvements are discussed, in particular re-absorption of light emitted by luminescent species. A new class of semiconductor hetero-nanocrystals is shown to be an excellent candidate for surpassing the 10% luminescent solar concentrator efficiency barrier

A new design for luminescent solar concentrating PV roof tiles

In our paper we explore the opportunity of combining luminescent solar concentrating (LSC) materials and crystalline PV solar cells in a new design for a roof tile by design-driven research on the energy performance of various configurations of the LSC PV device and on the aesthetic appeal in a roof construction. We present the roof tile in a system and executed optical modeling of the solar roof tile by MonteCarlo/ray-tracing simulations by PVtrace. We determined the range of appropriate values for thickness and dye concentration for the conceptual design of roof tile LSCs. It can be concluded that thickness of PMMA sheet material could best be in the range of 4 to 6 mm and the concentration of BASF Lumogen Red dye in between 80 and 1000 ppm. Because of aesthetic considerations however various concentration values may be used. In follow-up activities include a.o. parameter studies for different BASF Lumogen dyes and a pilot setup for testing the prototypes outdoors in the Netherlands

Leaf Roof – designing luminescent solar concentrating PV roof tiles

The Leaf Roof project on the design features of PV roof tiles using Luminescent Solar Concentrator (LSC) technology has resulted in a functional prototype . The results are presented in the context of industrial product design with a focus on the aesthetic aspects of LSCs. This paper outlines the design of Leaf Roof tiles under consideration of simulation results, experimental measurements on dyes’ absorption spectra, and the energy performance of Leaf Roof elements in the context of their geometry and colors

Spectral conversion for thin film solar cells and luminescent solar concentrators

Full spectrum absorption combined with effective generation and collection of charge carriers is a prerequisite for attaining high efficiency solar cells. Two examples of spectral conversion are treated in this chapter, i.e., up-conversion and down-shifting. Up-conversion is applied to thin film silicon solar cells and efficiency improvements using lanthanides as up-converter material under monochromatic as well as broadband light are presented. Down-shifting is demonstrated in luminescent solar concentrators, and material issues hampering efficiency improvements are discussed, in particular re-absorption of light emitted by luminescent species. A new class of semiconductor hetero-nanocrystals is shown to be an excellent candidate for surpassing the 10% luminescent solar concentrator efficiency barrier