Solar cell as a heat engine: energy-entropy analysis of photovoltaic conversion

A thermodynamic theory for the voltage or free energy generated by a quantum solar energy converter which has recently been proposed, is developed here in a more direct and simpler way. We consider separately the luminescene and conversion of a single photon of the incident radiation. The energy/entroypy balance for the conversion process yields an expression for the voltage in a form familiar from the classical thermodynamics of the work carried out by the heat engine. A similar balance for the absorption and emission of light gives an expression for the irreversible entropy generation which reduces the open circuit voltage generated by the solar cell. Detailed expressions for losses due to individual mechanisms, including non-radiative recombinations, are obtained with the use of an approximation where photons in the incident and emitted beams are modelled as an ideal two-dimensional gas. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim