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Electromagnetic simulations of a photonic luminescent solar concentrator

Johannes Gutmann, Marius Peters, Benedikt Bläsi, Martin Hermle, Andreas Gombert, Hans Zappe, and Jan Christoph Goldschmidt

Open Access Open Access

Abstract

Luminescent solar concentrators (LSC) are used in photovoltaic applications to concentrate direct and diffuse sunlight without tracking. We employed 2D FDTD simulations to investigate the concept of a photonic LSC (PLSC), where the luminescent material is embedded in a photonic crystal to mitigate the primary losses in LSCs: the escape cone and reabsorption. We obtain suppressed emission inside the photonic band gap, which can be utilized to reduce reabsorption. Furthermore, the efficiency of light guiding is strongly enhanced in a broad spectral range, reaching up to 99.7%. Our optimization of design parameters suggests emitting layers of sub-wavelength thickness.

©2012 Optical Society of America

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Figures (10)
Fig. 1
Fig. 1 (a) Cross-section of a conventional luminescent solar concentrator (LSC) that consists of a macroscopic plate doped with luminescent material that absorbs incoming light and emits at longer wavelengths. Most of the emitted light is trapped inside the plate and guided to solar cells at the edges. (b) The concept of the photonic luminescent solar concentrator (PLSC), where the luminescent material is embedded in a photonic structure to improve light guiding to the edges by mitigating escape cone and reabsorption losses.

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Fig. 2
Fig. 2 Sketch of the three simulation setups (not to scale): (a) the reference setup with homogeneous medium (n = 1.5), (b) the slab setup corresponding to a microscopic LSC and (c) the PLSC setup, comprising an emitting layer sandwiched between Bragg stacks. Detector planes at the edges and top and bottom surfaces keep track of the energy fluxes to obtain the total emitted flux and the relative amount of flux guided to the edges. The position of the point-dipole source was varied in the y-direction to study the position dependent emission.

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Fig. 3
Fig. 3 Relative emission Erel of slab and PLSC (a) averaged over sy and (c) as a function of source position sy. The small variation in the slab case is caused by waveguide modes due to the wavelength-sized thickness. Similar effects are seen for the PLSC, however, the relative emission in this case is dominated by suppression inside the photonic band gap (PBG). The band structure of an ideal (i.e. infinite) Bragg stack along its density of states, that is zero inside the PBG, is shown in (b) (calculated with the MPB Package [22]).

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Fig. 4
Fig. 4 (a) Light guiding efficiency LGE of slab and PLSC simulation setup averaged over different source positions sy. The LGE of the slab varies little around the expected value for 2D TIR due to coherence effects. For the PLSC, strongly enhanced light guiding is obtained for frequencies slightly larger than the design frequency f0 due to the angular reflection characteristic of the Bragg stack shown in (b).

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Fig. 5
Fig. 5 Energy density pattern obtained by monochromatic emission in the PLSC (ts = 2λ0) with (a) f = f0, (b) f = 1.075 f0, and (c) f = 0.75 f0. While for f = f0 perfect suppression is obtained in directions normal to the surface, the angle of guided light is larger for f = 1.075 f0, satisfying the TIR condition and thus resulting in optimum LGE. For f = 1.075 f0, light can also propagate in the escape cone, which results in reduced LGE in the range of 2D TIR.

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Fig. 6
Fig. 6 Light guiding efficiency LGE of slab and PLSC setup as a function of source position sy. Modes inside the slab cause deviations from the 2D TIR limit. For the PLSC, the LGE is dominated by the strong enhancement of the Bragg stack that overlays the mode pattern.

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Fig. 7
Fig. 7 The effect of the number of Bragg bi-layers on (a) the relative emission Erel, (b) the light guiding efficiency LGE and (c) the mean LGE inside the PBG. Saturation is observed for more than 20 bi-layers.

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Fig. 8
Fig. 8 Investigation of (a) relative emission, (b) light guiding efficiency and (c) mean LGE inside the PBG as a function of the active layer thickness ts. This design parameter significantly influences the relative emission, whereas no strong impact on the light guiding is observed.

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Fig. 9
Fig. 9 Combined qualitative evaluation of Erel and LGE by calculating the ratio of the integral LGE in range A to Erel integrated over range B. (a) shows example spectra of Erel and LGE for ts = ¼ λ0/ns with the integral ranges A and B. (b) plots the ratio of the integrals vs. the active layer thickness ts. Thus, thin ts are beneficial for the PLSC application.

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Fig. 10
Fig. 10 (a) The relative emission is plotted together with the absorption (Abs) and emission (Em) spectra of the organic dye Lumogen® Red (using λ0 = 650nm). (b) shows the transmission T of the investigated Bragg stack from the outside to the luminescent layer along with the Lumogen® Red spectra. The reflection sidelobes in the absorption range cause severe losses which shows the need for photonic structures optimized for high transmission in the absorption range.

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Equations (2)

Equations on this page are rendered with MathJax. Learn more.

E rel = total flux emitted in investigated setup total flux emitted in reference setup .
LGE= flux through edge detector planes total emission flux .
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