Prospects for photovoltaic efficiency enhancement using low-dimensional structures

The use of photovoltaic solar cells provides an elegant way of converting sunlight to electricity. The photovoltaic industry is currently growing very rapidly, at a compounded rate of about 30% each year. Energy conversion efficiency is a key parameter with this technology since it directly impacts both material and deployment costs. The performance of the traditional bulk semiconductor solar cell is limited to about 33% while thermodynamic limits on the conversion of sunlight to electricity are much higher, at 93%. Low-dimensional structures appear capable of allowing much of this gap to be bridged. These structures allow increased flexibility with traditional efficiency enhancement approaches such as those based on `stacked' or tandem cells, which double efficiency limits to 68%. Perhaps more interestingly, they offer scope for completely new device concepts such as those relying on excitations between multiple energy bands and improved `hot-carrier' cells, that offer scope for similarly high performance.