  Dark state protection has been proposed as a mechanism to increase the power
output of light harvesting devices by reducing the rate of radiative
recombination. Indeed many theoretical studies have reported increased power
outputs in dimer systems which use quantum interference to generate dark
states. These models have typically been restricted to particular geometries
and to weakly coupled vibrational baths. Here we consider the
experimentally-relevant strong vibrational coupling regime with no geometric
restrictions on the dimer. We analyze how dark states can be formed in the
dimer by numerically minimizing the emission rate of the lowest energy excited
eigenstate, and then calculate the power output of the molecules with these
dark states. We find that there are two distinct types of dark states depending
on whether the monomers form homodimers, where energy splittings and dipole
strengths are identical, or heterodimers, where there is some difference.
Homodimers, which exploit destructive quantum interference, produce high power
outputs but strong phonon couplings and perturbations from ideal geometries are
extremely detrimental. Heterodimers, which are closer to the classical picture
of a distinct donor and acceptor molecule, produce an intermediate power output
that is relatively stable to these changes. The strong vibrational couplings
typically found in organic molecules will suppress destructive interference and
thus favour the dark-state enhancement offered by heterodimers.
