Thermodynamic losses occur due to the fact that the light incident on a solar cell originates only from a narrow angular region but the cell can emit into the complete hemisphere. To reduce these losses, we are developing angle-selective filters, which limit the emission and thus can increase the system efficiency. Designing and optimising suitable photonic thin-film filters is a challenge. We have developed programs which make this optimisation feasible and which allow the efficiency of such systems to be estimated on the basis of coupled optical and electrical properties. We are working on implementing such a system to provide experimental proof of the concept.
Photonic thin-film filters can display angle-selective properties. One example is a Bragg filter, which consists of alternating layers of two materials with different refractive indices. At certain wavelengths, such filters have angle-dependent reflection peaks, which correspond very well to the requirements of a solar cell system. By cleverly modifying individual layer thicknesses and adding further layers, we further adapt this region and optimise the filter transmission in the range in which light falls on the cell. We apply a genetic algorithm to do so. The effect of such filters can be included in PC1D simulations, allowing the potential gain to be estimated.
A further-reaching approach is the introduction of a photonic solar cell, in which the cell and filter are combined in a single optoelectronic component. As an example, a thin GaAs solar cell can be introduced as a defect layer in a 1D photonic crystal (Fig. 1). In this case, in contrast to the effect discussed above, the emitted radiation is not reflected but the emission itself is suppressed (Fig. 2). To simulate this effect, we apply scattering matrix formalism. In addition, we couple these simulations with electric simulations (PC1D). Initial results show the desired suppression, so that we are now working on implementing such a system.