  Organic heterojunction solar cells are analyzed within a minimal model that
includes the essential physical features of such systems. The dynamical
properties of this model, calculated using a master equation approach, account
for the qualitative behavior of such systems. The model yields explicit results
for current-voltage behavior as well as performance characteristics expressed
in terms of the thermodynamic efficiency as well as the power conversion
efficiency at maximum power, making it possible to evaluate the optimal setup
for this device model.
