THERMODYNAMICS OF SOLAR-CELL EFFICIENCY

A thermodynamic model of a solar cell is created using the internal energy and entropy transport equations. The energy and entropy transfers involved in the interaction of radiation with the cell are analysed and the entropy generation rate arising from this process is calculated. This enables the I-V characteristic, the power output and heat transfers to be determined and it is shown that, for an ideal cell with no non-radiative recombination, the local entropy generation considered as a function of the terminal voltage has a minimum at the open-circuit point. When the ideal cell is operated as a light emitting diode, it absorbs heat from a reservoir and thus acts as a refrigerator, but a very small amount of non-radiative recombination annuls this effect. Finally, a clear relationship between the device efficiency and the Carnot efficiency is obtained in terms of global entropy generation. The general case where the cell is not at ambient temperature is dealt with in an appendix.