Infrared reflecting coverglass for multijunction cells in a terrestial high-concentrating photovoltaic system

Multijunction cells are composed of multiple layers of semiconductors, with different bandgaps. Among them, GaInP/GaAs/Ge cells are the most popular ones, due to their high performances [1]. These cells have higher efficiencies that silicon ones and should be really convenient in a concentrating photovoltaic plants. Photons are absorbed by a layer if their energy is equal or higher to the semiconductor bandgap. Otherwise they are transmitted to the lower semiconductor. All the photons with less energy than the lowest bandgap are converted into waste heat. They represent about the 5% of the total solar spectrum [2]. Moreover, the Germanium layer usually produces a higher current that the other subcells and even the excess current is converted into heat. Cooling is an important task in designing CPV: the higher the concentration the higher the temperature. Thus, a passive cooling effort can be obtained reducing the infrared radiation absorbed by the cell. An infrared reflecting layer placed on the top of the cell encapsulation should achieve this goal. This solution should reduce the quote of low-energy photons absorbed by the cell, decreasing both the sunlight directly converted into heat and the sunlight converted into excess current by the Germanium layer. Infrared reflecting coverglasses have been successfully tested on space applications. This research focuses on PMMA application for IR reflecting purposes in a terrestrial high CPV system.