  Solar cells are engines converting energy supplied by the photon flux into
work. All known types of macroscopic engines and turbines are also
self-oscillating systems which yield a periodic motion at the expense of a
usually non-periodic source of energy. The very definition of work in the
formalism of quantum open systems suggests the hypothesis that the oscillating
"piston" is a necessary ingredient of the work extraction process. This aspect
of solar cell operation is absent in the existing descriptions and the main
goal of this paper is to show that plasma oscillations provide the physical
implementation of a piston.
