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

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.