This paper discusses ways of reducing the two major losses encountered in a single-junction photovoltaic (PV) device-that of sub-band gap transmission and lattice thermalisation losses-via the application of passive luminescent devices called up- and down-converters, respectively. Down-conversion (DC) results in the generation of more than one lower energy photon being generated per incident high-energy photon, while up-conversion (UC) generates one photon with energy for every two or more sub-band gap photons absorbed. A related process of wavelength down-shifting (DS) is similar to DC except that the external quantum efficiency of the DS process is less than unity. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.