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 lies 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.