Half-filled intermediate bands Si material formed by energetically   metastable interstitial sulfur atom
  Hyperdoped metastable sulfur atoms endow crystalline silicon with a strong sub-bandgap light absorption. In order to explore such metastable states, we develop a new high-throughput first-principles calculation method to search for all of the energetically metastable states for an interstitial sulfur atom inside crystalline silicon. Finally, we obtain sixty-three metastable interstitial states and they can be classified into ten types. Interestingly, twenty-eight (44% in total) of lower-energy metastable states can produce a well-isolated and half-filled intermediate band (IB) inside silicon forbidden gap, which makes sulfur hyperdoped silicon to be a desirable material for IB solar cells. 
Ke-Fan Wang, Mingguo Liu, Yaping Ma, Zhenxiang Cheng, Yuanxu Wang,   Xudong Xiao
cond-mat.mtrl-sci