Highly efficient GaAs solar cells by limiting light emission angle

In a conventional flat plate solar cell under direct sunlight, light is received from the solar disk, but is re-emitted isotropically. This isotropic emission corresponds to a significant entropy increase in the solar cell, with a corresponding drop in efficiency. Here, using a detailed balance model, we show that limiting the emission angle of a high-quality GaAs solar cell is a feasible route to achieving power conversion efficiencies above 38% with a single junction. The highest efficiencies are predicted for a thin, light trapping cell with an ideal back reflector, though the scheme is robust to a non-ideal back reflector. Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light trapping geometry not only allows for much thinner cells, but also for significantly higher overall efficiencies with an excellent rear reflector. Finally, we present ray-tracing and detailed balance analysis of two angular coupler designs, show that significant efficiency improvements are possible with these couplers, and demonstrate initial fabrication of one coupler design. A new light-management design could allow single-junction GaAs solar cells to reach power-conversion efficiencies as high as 38%. This is the finding of Emily Kosten and co-workers from the California Institute of Technology in the USA and FOM Institute AMOLF in The Netherlands, who studied the impact of limiting the angles of light emission in very thin light trapping GaAs solar cells. Reducing the angles of light emission allows for cells as thin as 50 nm due to extreme light trapping, and recycling of photons within the cells gives high efficiencies. In addition, the researchers modeled designs for limiting the angles of light emission and fabricated the necessary optical microstructures for one such design.