  Recent developments in nanoscale experimental techniques made it possible to
utilize single molecule junctions as devices for electronics and energy
transfer with quantum coherence playing an important role in their
thermoelectric characteristics. Theoretical studies on the efficiency of
nanoscale devices usually employ rate (Pauli) equations, which do not account
for quantum coherence. Therefore, the question whether quantum coherence could
improve the efficiency of a molecular device cannot be fully addressed within
such considerations. Here, we employ a nonequilibrium Green function approach
to study the effects of quantum coherence and dephasing on the thermoelectric
performance of molecular heat engines. Within a generic bichromophoric
donor-bridge-acceptor junction model, we show that quantum coherence may
increase efficiency compared to quasi-classical (rate equation) predictions and
that pure dephasing and dissipation destroy this effect.
