Efficient nanocoax-based solar cells

Despite requiring thick layers of rela-tively costly material, the vast majority of today’s solar pho-tovoltaic (PV) cells employ crystalline media, due to their superior energy conversion efficiency compared to non-crystalline, “thin film” cells [1]. The dominant material, crystalline silicon (c-Si), has weak optical absorption, and so must be relatively thick (~200 μm) to efficiently collect light. However, with charge carrier (electron and hole) mean free paths comparable to this distance, high power conver-sion efficiency η can still be achieved (η ~ 25% for single-junction cells) [2]. Noncrystalline materials such as amor-phous silicon (a-Si), on the other hand, are strongly absorb-ing, such that thin films (under 1 μm) suffice for efficient light collection. However, mean free paths in a-Si are sig-nificantly shorter (~100 nm) than in c-Si, such that thin film efficiency (η < 10%) severely lags its crystalline counterpart [3, 4]. Both types of solar cells are therefore compromised by a coupling of the optical and electronic length scales: crystalline in terms of cost, thin film in terms of efficiency. This “thick–thin” paradox is difficult to resolve in the con-ventional, planar solar cell configuration, where photons and electrons travel essentially in the same direction, i.e. normal to the cell surface. Here we propose to resolve this problem by employing a cell structure based on a coaxial cable.