Designing Photonic Materials for Effective Bandgap Modification and Optical Concentration in Photovoltaics

The limiting efficiency for photovoltaic energy conversion based on a semiconductor p-n junction is typically determined using the method of detailed balance put forth by Shockley and Queisser. Here, we describe how this theory is altered in the presence of a photonic structure that is capable of modifying the absorption and emission of photons and optimize a device with optical loss. By incorporating specifically designed photonic structures, higher maximum efficiencies can be achieved for low bandgap materials by restricting the absorption and emission of above bandgap photons. Similarly, restriction of the emission angle leads to increased optical concentration. We consider how both of these effects are modified in the presence of a nonideal photonic structure. Further, we find that the energy of the photonic bandgap that is needed for maximum efficiency depends critically on the reflectivity of the photonic crystal.