Charge multiplication by impact ionization (II) produces ~1% of the photogenerated charge in silicon photovoltaic (PV) devices under solar illumination and ~30% for photons above the silicon direct band gap. Despite these observations few efforts have been reported to find materials and structures that optimize the rate of II and thereby convert a larger fraction of the solar spectrum to useful electrical energy. In contrast the dramatically increased rate of II near band offsets in reverse-biased heterojunctions has been used extensively to improve the efficiency of avalanche photodiodes. By combining and expanding these existing technologies Prof. Stephen Kevan and Prof. Dave Cohen\'s work will provide both a simple and practical route to improving the conversion efficiency of existing PV devices as well as a broadly applicable approach to overcoming the Shockley-Queisser limit in next-generation PV materials.
HAII promises eventually to improve the power conversion efficiency of PV devices by as much as a factor of two.