  We study how much the efficiency of a solar cell as a quantum heat engine
could be enhanced by quantum coherence. In contrast to the conventional
approach that a quantum heat engine is in thermal equilibrium with both hot and
cold reservoirs, we propose a new description that the quantum heat engine is
in the cold reservoir and the thermal radiation from the hot reservoir is
described by the pumping term in the master equation. This pumping term solves
the problem of the incorrect mean photon number of the hot reservoir assumed by
the previous studies. By solving the master equation, we obtain the
current-voltage and the power-voltage curves of the photocell for different
pumping rates. We find that, as the photon flux increases, the power output of
the photocell increases linearly at first and then becomes saturated, but the
efficiency decreases rapidly. It is demonstrated that while the power output is
enhanced significantly by the quantum coherence via the dark state of the
coupled donors, the improvement of the efficiency is not significant.
