Silicon photovoltaics using conducting photonic crystal back-reflectors

Currently, research is being directed towards thinning conventional 200-300 mu m thick silicon photovoltaic cells by an order of magnitude or more. The benefits of reducing the cell thickness include decreased material costs, enhanced cell flexibility, and reduced effects of light-induced degradation. However, one of the major challenges associated with reducing the active region to this extent is the corresponding reduction of light absorption. To mitigate this effect it has been proposed that the cell should incorporate enhanced light-trapping strategies. One potential approach to enhance light trapping in thin photovoltaic cells is to structure the back-reflector in the form of a photonic crystal (PC). It has recently been shown that two fundamental attributes of PC back-reflectors optically coupled to thin semiconductor films contribute to enhanced absorption in the semiconductor: (i) the PC back-reflector behaves as a perfect mirror, exhibiting complete reflection over stop-gap frequencies; and (ii) the PC-semiconductor film interface couples incident light into resonant states that propagate along the plane of the film, thereby further enhancing the absorption. Although the ability of PC back-reflectors to enhance absorption is encouraging, significant challenges arise when attempting to incorporate this light trapping technique in photovoltaic devices. Herein, we describe the underlying physical mechanisms that give rise to absorption enhancements in thin Si wafers featuring PC back-reflectors, and describe hurdles that will have to be surmounted in order to reduce-to-practice a PC back-reflector into an actual PV device.