Luminescent solar concentrators: the road to low-cost energy from the sun

Abstract

Photovoltaics (PVs) enable conversion of solar light into electricity without detrimentally affecting the environment. Unfortunately, solar cells require high-cost semiconductors, which makes them expensive. Concentrator systems help to minimize the coverage needed, but normal concentrators only work for a narrow angular range of solar radiation. Moreover, they need tracking devices and are poorly suited to capturing diffuse radiation. Luminescent solar concentrators (LSCs) avoid these disadvantages.1, 2 An LSC consists of a glass or plastic plate containing or coated with luminophores (dyes or phosphors) that absorb sunlight and emit light at longer wavelengths (see Figure 1). A substantial part of the longer-wavelength light is trapped by total internal reflection and guided to the edges of the LSC plate, where it is absorbed by small-area PV cells. The idea of using LSCs already dates back 30 years,3, 4 and much research has been carried out in this area.5–8 However, up to now, the technology has not delivered on its promise. Although theoretically concentration could be huge,1, 9 the best LSC-based PV system to date10 has a conversion efficiency of 7.1% and saves a factor 2.5 of PV-cell area. The reason for these low numbers is primarily losses of various kinds. For example, light is absorbed by the luminophore, but no luminescent light is emitted because of limited quantum efficiency (QE), i.e., the ratio of absorbed to emitted number of photons. Another important means of loss is escaping luminescent light from the concentrator at angles larger than the critical angle for total internal reflection (see Figure 1). One effective way to prevent this is by applying a wavelength-selective filter on top of the light guide,1, 11, 12 which keeps the luminescent light inside. However, Figure 1. (left) Luminescent solar concentrator with luminophores (dots) absorbing sunlight (dashed) and emitting longer-wavelength light (solid line) that is absorbed by a photovoltaic (PV) cell. (right) Luminescent plates with light guided towards the edges. (Photo courtesy of P. P. C. Verbunt.)