Towards nanowire tandem junction solar cells on silicon
The development of photovoltaics as a serious means of producing renewable energy has accelerated greatly in the last ten years, with prices for silicon-based solar cell systems dropping dramatically in the last few years. The next great opportunity for photovoltaics following this competitiveness in prices will be to enhance the cell and panel efficiencies. It is quite generally seen that the most viable platform on which this should be realized will be as augmented silicon solar cells, in which a top cell will be combined with the silicon bottom cell in a tandem configuration, by which the efficiency can be enhanced by a factor from 20% to 50%, depending on details of the approach. In this paper, we report on the status of one such approach, namely, with a top cell comprising III-V nanowires, connected to the bottom silicon cell in a two-terminal or four-terminal configuration. Among the most important opportunities, we show that a substrate-free growth, called Aerotaxy, offers a radical reduction in the total price picture. Besides the description of the key technical approaches, we also discuss the environmental issues.