The Carnot factor in solar-cell theory

In a two-level system the open circuit voltage or difference between the electro-chemical potentials at the contacts, is related to the energy gap E-g by a Carnot factor; V = (1 - T-s/T-p)E-g. Here, T-s and T-p are the absolute temperatures of the ambient (surroundings) and of the incident blackbody radiation ("pump"), respectively. A simple two-step statistical thermodynamic argument is given here which makes this result plausible. Its standing in a more general context is also discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.