The physical limits of light trapping in thin-films and photonic structures that operate at the limit

Light-trapping is used in photovoltaic cells to increase the power conversion efficiency and lower the cost by reducing the amount of active material required to efficiently absorb sunlight. In the case of thick crystalline silicon solar cells, a well-known approach is to use geometric textures that scatter incident rays into modes that are trapped by total internal reflection in the absorbing layer leading to a maximum possible enhancement in optical path length of 4n², where n is the refractive index of the absorber. This limit is applicable at near bandgap wavelenths for device structures that have an acceptance cone of full sky. If device is designed with an acceptance cone of half angle of e then maximum possible enhancement in optical path length is 4n²/sin²(¿) under low absorption limit. This is the well know geometric optics limit of light trapping. Such textures are substantially less effective for thin-film solar cells and an extension of this approach into the wave domain is needed. Here, using principles of unitary time evolution and information theory, we show that light trapping in the wave domain is subject to the same upper limit that was derived for the geometric optics domain. Furthermore, we show that practical subwavelength structures can be designed with light-trapping performance that approaches the theoretical limit. The enhancement in optical absorption exceeds that of previously proposed structures by an order of magnitude.