  The transport of electrons through serially coupled quantum dot molecules
(SCQDM) is investigated theoretically for application as an energy harvesting
engine (EHE), which converts thermal heat to electrical power. We demonstrate
that the charge current driven by a temperature bias shows bipolar oscillatory
behavior with respect to gate voltage due to the unbalance between electrons
and holes, which is different from the charge current driven by an applied
bias. In addition, we reveal a Lenz's law between the charge current and the
thermal induced voltage. The efficiency of EHE is higher for SCQDM in the
orbital depletion situation rather than the orbital filling situation, owing to
the many-body effect. The EHE efficiency is enhanced with increasing
temperature bias, but suppressed as the electron hopping strength reduces. The
fluctuation of QD energy levels at different sites also leads to a reduction of
EHE efficiency. Finally, we demonstrate direction-dependent charge currents
driven by the temperature bias for application as a novel charge diode.
