Toward an Impurity Band PV: Dynamics of Carriers Generated via Sub-band gap Photons

Intermediate band solar cells are a pathway to cells that surpass the Shockley-Queisser limit by enabling the utilization of sub-band gap photons. A proposed method for fabricating an intermediate band material is to use impurities that introduce electronic levels within the band gap. At sufficiently high dopant concentrations, band formation may lead to a suppression of Shockley-Reed-Hall recombination, an idea known as ``lifetime recovery'' [1]. We investigate a proposed intermediate band material, silicon hyper-doped with sulfur. This material system exhibits strong sub-band gap optical absorption and metallic conductivity at sufficiently high sulfur concentrations [2], which makes it a strong candidate for an impurity-band material. We employ low-temperature photoconductivity using sub-band gap light to estimate the trapping rate of electrons in the conduction band. We vary the sulfur concentration near the critical value for the metal-insulator transition to test the idea of ``lifetime recovery'' in the S:Si system.\\[4pt] [1] A. Luque and A. Mart\'i, Adv. Mater. 22, 160 (2010).\\[0pt] [2] M. T. Winkler et.al. Phys. Rev. Lett. 106, 178701 (2011)