Subwavelength nonimaging light concentrators for the harvesting of the solar radiation

Light trapping and the broadband absorption of the solar radiation is of interest to various solar energy harvesting applications. In the current work, we report a new paradigm for light trapping, that is light trapping based on arrays of subwavelength nonimaging light concentrators (NLCs). We numerically show that silicon NLC arrays provide > 75% broadband absorption enhancement of the solar radiation compared with that of optimized nanopillar arrays. The paper focuses on free-floating arrays of subwavelength compound parabolic concentrators (henceforth CPC arrays) as a case study. The calculations reveal that CPC arrays function as anti-transmission layers as only few photons transverse the CPC arrays which is in contrast to nanopillar arrays that function as anti-reflection layers. We show that the absorption enhancement in CPC arrays is due to efficient occupation of Mie modes which is motivated by the unique CPC geometry, and we demonstrate light trapping at the Yablonovitch limit. Finally, we examine the performance of a photovoltaic cell based on CPC arrays with respect to base doping levels and surface recombination. We show that the short-circuit current density of the CPC-based cell is > 75% higher than the short-circuit current density of a photovoltaic cell based on optimized nanopillar arrays. We believe that light trapping based on NLC arrays paves the way to various applications such as ultra-thin photovoltaic cells.