  A solar cell is a heat engine, but textbook treatments are not wholly
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 potential. We propose 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 that modulates the absorption of heat from the photons
so that it may perform a net positive work during a complete cycle of its
motion, in accordance with the laws of thermodynamics. 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.
