Pathways towards a 50% efficiency spectrum-splitting photovoltaic system: Application of built-in filters and generalization of concept

Abstract

A UNSW team led by Martin Green achieved a landmark result in photovoltaics in 2014 when it demonstrated the first conversion of sunlight to electricity with efficiency above 40%. Besides this being one of the highest efficiencies ever reported, the work used a method readily accessible to the industry in that an external filter directed a portion of light normally wasted by commercial triple-junction solar cells to an additional silicon cell. Here, we propose two further developments that can make 50% conversion efficiency a realistic target. One potential development is to build filters into the rear of the cells themselves. Compared with the external filter, such internal filters can lead to an even higher efficiency because these internal filters can not only divide sunlight properly for individual cells as an external filter does, but also reflect the cells' rear-emitted light back for reabsorption additionally. The other potential development is a generalisation of concept, where, for instance, the detached silicon cell may be replaced by another tandem cell that can better utilise the directed light unabsorbed by the first triple junction cell. This may also offer some flexibility in the filter design, since the spectrum shape of the directed light will no longer need to be rectangular. Detailed design principles will be discussed as is the potential for further efficiency improvement.