Detailed balance limit for solar cell efficiency

The principle of detailed balance was used in 1960 to derive a thermodynamic limit for energy conversion efficiency of semiconductor junction photovoltaic cells. Absorption and emission of photons must be balanced, the cell being a black body. Non-radiative recombinations of solar-generated electron-hole pairs are thus particularly deleterious, affecting silicon junctions. Gallium arsenide, however, is inherently more efficient because of its direct band gap with predominant radiative transitions. The maximal efficiency shows a broad maximum as a function of the semiconductor energy gap. Silicon ties within this maximum. Detailed balance and its applications are reviewed. Current efforts to overcome this limit are discussed. Concentrated illumination enhances efficiencies; multijunction cells - not covered in the detailed balance limit - deliver higher output. Attempts are made to more fully utilize the blue solar spectrum. (C) 2008 Elsevier B.V. All rights reserved.