Light trapping, a new approach to spectrum splitting

In this paper, a novel type of light trap is introduced. It enables photovoltaic conversion with separate solar cells optimized for different frequency bands, which are covered by spectrally selective mirrors. Sunlight is coupled into a transparent medium and converted to diffuse radiation by randomization. It is well known that under these conditions light intensity is enhanced by a factor of 2n(2) inside the medium. The unidirectional radiation at the aperture is however only n(2). Two types of light trap are presented. The first one employs concentrated radiation incident on a small volume light trap. It is shown that efficiency depends on the ratio (solar cell area)/(input area). A more detailed analysis shows that under regular conditions part of the light is absorbed directly after entering the trap which results in higher efficiencies. We show that a trapping efficiency above 90% is obtainable. The second type of trap is based on a photonic structure covering the surface of the medium. It has high angular selectivity and broad spectral transmission. An analysis of efficiency and loss mechanisms is presented. If both types of trap are combined even higher efficiency is possible, Consequences for solar cell technology are investigated. It is further shown that the light trapping principle can also be applied to large area stationary modules.