Unspecified Kohei Okada Profile

Kohei Okada

at Ritsumeikan Univ

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SPIE Journal Paper | 19 July 2024 Open Access

Enhanced photon collection in leaf-inspired luminescent solar concentrators

Hiroto Nishimura, Kohei Okada, Atsuya Suzuki, Yuta Mizuno, Ichiro Fujieda

JPE, Vol. 14, Issue 03, 035501, (July 2024) https://doi.org/10.1117/12.10.1117/1.JPE.14.035501

KEYWORDS: Mirrors, Mirror surfaces, Light emitting diodes, Solar cells, Reflection, Waveguides, Solar concentrators, Photography, Structured optical fibers, Spectroscopy

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A leaf-like structure is formed by placing luminescent plates in the vicinity of a luminescent fiber with their side surfaces facing the fiber. Its optical efficiency is expressed in terms of collection efficiency and absorptance of each luminescent component. This model allows one to calculate its optical efficiency under an arbitrary incident spectral photon flux. A clear lightguide can connect multiple fibers to transfer the photons generated by each leaf-like structure to a single photovoltaic cell. Thus, one can divide the incident area of a luminescent solar concentrator (LSC) into small regions without sacrificing its geometric gain. Lateral size reduction enhances its optical efficiency by alleviating the losses during the photon collection process. Other techniques reported for the conventional planar LSCs, such as edge mirrors and tandem structures, can be combined for further enhancement. In experiments, off-the-shelf luminescent components (2.0 mm-thick plates and 1.5 mm-diameter fibers) were used to assemble samples with various shapes. For example, the efficiency of collecting photons generated in the plates of a square device increased from 0.0052 to 0.024 by decreasing its side length from 50 to 10 mm and attaching mirrors on their outer edges. In contrast, optical efficiency is weakly dependent on leaf geometry, increasing design freedom on leaf shapes. The use of advanced luminescent materials as well as optimizing the leaf-like structure design would increase its optical efficiency further.

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Proceedings Article | 4 October 2023 Poster + Paper

Design considerations for leaf-inspired luminescent solar concentrators based on two-stage photoconversion

Hiroto Nishimura, Kohei Okada, Atsuya Suzuki, Yuta Mizuno, Ichiro Fujieda

Proceedings Volume 12671, 126710F (2023) https://doi.org/10.1117/12.2676298

KEYWORDS: Solar concentrators, Waveguides, Design and modelling, Ray tracing, Photoluminescence

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Concentration factor of an optical concentrator is defined as the ratio of photon fluxes at its incident and exiting surfaces. It is equal to the product of geometric gain and optical efficiency. Scaling up a luminescent solar concentrator (LSC) increases its geometric gain, but its optical efficiency decreases due to the loss of photoluminescence (PL) photons during the concentration process. In a leaf LSC, a luminescent fiber is coupled to the side surface of a luminescent plate such that an incident photon goes through two-stage photoconversion. Its geometric gain increases drastically but the second photoconversion process decreases its optical efficiency. In experiment, a 1.5 mm-diameter fiber was placed between two 2 mm-thick luminescent plates. The plates emitted green PL photons and the fiber converted them to red PL photons. Position-dependent optical efficiency was measured by exciting a single spot at various positions on the plate with a 1 mm-diameter laser light at 450 nm. The optical efficiency averaged over the incident area increased from 0.004 to 0.007 by decreasing the lateral size of the plate from 50 mm to 10 mm. Ray tracing simulations reproduced the measurement. A clear lightguide with arc-bend couplers can guide the PL photons from multiple luminescent fibers to a photovoltaic cell. By connecting N devices, N-fold increase in geometric gain is expected while the optical efficiency remains the same. Hence, this configuration provides a solution to the trade-off between geometric gain and optical efficiency.

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