Here (Fig. 7.14) solar cells are mounted vertically on the edges of the light collecting plates, which have dissolved dyes. The upper plate collects blue photons, the dye reemits (fluoresces) and that light is sent to the cells on the edge. The fluorescence quantum efficiency of isolated dye molecules approaches unity. (Total internal reflection helps keep the reemitted light inside the plate, until absorbed in a solar cell).
The redder light passes through and is caught in the second plate that has a red dye. The effective collecting area is multiplied by this scheme, and the scheme also effectively involves two separate bandgaps, leading to higher efficiency in principle.
In the collection of the reemitted light from the dye molecules, two methods, fluorescence (prompt response) and phosphorescence (delayed response), are used. A dye molecule’s characteristic color comes from the wavelength of its emission band. In the system shown in Figures 7.14 and 7.15, dyes absorbing blue and red, respectively, are located in upper and lower plates. The emission bands of these dyes match the absorption properties of the solar cells mounted around the edges of the light-collecting plates. A larger index of refraction in the glass promotes high efficiency in collection of the fluorescent and phosphorescent light by the solar cells, relative to its escape out of the glass into the vacuum. An effective area
Figure 7.15 Area multiplication on the orderof50 by emission offluorescent light and its capture in solar cells at the periphery . (Sketch by M. Medikonda).