Fluorescent Solar Energy Concentrators: Principle and Present State of Development

Abstract

A review of the history and recent developments of fluorescent concentrators is given. The principle of this innovative device is based on light guiding in a transparent matrix doped with fluorescent centers, mainly organic dyes. The dyes strongly absorb a certain band of the solar spectrum and emit at a red-shifted frequency for which the concentrator is transparent. Since the light is emitted isotropically a large part is guided to the edges of the concentrator and arrives in concentrated form at the smaller side-edge. This concentrated light could be easily guided to an adapted solar cell. Work was done in the late seventies and early eighties, but then interest vanished because of several difficulties like unstable dyes and nonavailability of dyes in the infrared, which would be best adapted to silicon solar cells. Theoretically very high efficiencies can be reached if several plates with different colors are stacked and equipped with adapted solar cells. In recent years new interest has emerged in fluorescent concentrators. Two concepts are being most actively investigated now. The first is a band pass mirror on the front surface of the concentrator that reflects selectively the emitted light but transmits the light to be absorbed. In this manner also rays not within internal reflection are retained. It has been shown that only in this way the ultimate efficiency can be reached for a single plate concentrator. This efficiency is not much lower than for a directly illuminated ideal solar cell. A second new approach consists of replacing organic dyes with quantum dots. They offer the hope of greater stability, they can be tuned to different absorption wavelength by their size and by the spread of sizes their redshift can be tailored. A third approach is trying to put the dye centers in a surface coating, which opens the possibility to apply as a wafe guiding plates also more temperature resistive materials like glass.