Can up- and down-conversion and multi-exciton generation improve photovoltaics?

Photon up-conversion (UC) and photon-induced multiple-exciton generation (MEG) are proposed directions that are of increasing interest for improving photovoltaic (PV) conversion efficiencies via photon (or light) management. Straightforward analysis of these approaches for non-concentrated single-junction cells in the detailed balance limit yields a theoretical PV conversion limit of 49%, instead of 31% without UC and MEG. With what we estimate to be optimistic, maximal realistic efficiencies (25% for UC; 70% for MEG) this limit becomes <40%, i.e., similar to 1.25 times the theoretical efficiency of conventional single-band gap cells. While this result does not detract from the fascinating fundamental scientific challenge to make UC and MEG simple and cheap ways to improve PV, such reality checks should be considered when evaluating the short-term promises of these and other options, such as spectral splitting and tandem arrangements. (C) 2008 Elsevier B.V. All rights reserved.