A thermodynamic cycle for the solar cell*

A solar cell is a heat engine, but textbook treatments are not quite satisfactory from a thermodynamic standpoint, since they present solar cells as directly converting the energy of light into electricity, and the current in the circuit as maintained by an electrostatic (or, in some cases, a chemical) potential. We propose a dynamic picture of a thermodynamic cycle in which the gas of electrons in the p phase serves as the working substance. The interface between the p and n phases acts as a self-oscillating piston, modulating the absorption of heat from the photons so that it may do net work during a complete cycle of its motion. We draw a simple hydrodynamical analogy between this model and the "putt-putt" engine of toy boats, in which the interface between the water’s liquid and gas phases serves as the piston. We point out some testable consequences of this model.