Poor ultraviolet (UV) quantum conversion efficiency contributes to a reduction in the efficiency of silicon based photovoltaic cells. In the UV, the main loss mechanism is through surface recombination of photo-generated carriers due to the shallow absorption depth of high energy photons. One method for greater utilisation of the UV region is by down-shifting UV photons to lower energies where the quantum efficiency of silicon is higher. This work determines the potential enhancement in efficiency that can be obtained by a luminescent down-shifting layer applied to silicon based solar cells. The efficiency is determined through detailed balance arguments. The maximum calculated efficiency enhancement due to an ideal down shifting process is 0.6% absolute using the AM1.5G standard spectra. Applying a similar analysis to a multicrystalline silicon solar cell results in an efficiency enhancement due to the down-shifting process of 0.17% absolute. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.