An up-converting material can generate one high-energy photon for every two or more incident low-energy photons. This material when placed on the rear side of a traditional bi-facial photovoltaic solar cell has the potential to increase the overall efficiency of the PV device by utilising sub-bandgap photons that would otherwise be transmitted through the device. This paper reviews and examines the application of rare-earth doped inorganic phosphors for achieving external quantum efficiencies for sub-bandgap photons for a bi-facial crystalline silicon photovoltaic device. The mechanisms for up-conversion are discussed and recent experimental results on a bi-facial silicon photovoltaic-upconversion device are analysed. (C) 2007 Elsevier B.V. All rights reserved.