  Natural light harvesting systems exploit electronic coupling of identical
chromophores to generate efficient and robust excitation transfer and
conversion. Dark states created by strong coupling between chromophores in the
antenna structure can significantly reduce radiative recombination and enhance
energy conversion efficiency. Increasing the number of the chromophores
increases the number of dark states and the associated enhanced energy
conversion efficiency, yet also delocalizes excitations away from the trapping
center and reduces the energy conversion rate. Therefore, a competition between
dark state protection and delocalization must be considered when designing the
optimal size of a light harvesting system. In this study, we explore the two
competing mechanisms in a chain-structured antenna and show that dark state
protection is the dominant mechanism, with an intriguing dependence on the
parity of the number of chromophores. This dependence is linked to the exciton
distribution among eigenstates which is strongly affected by the coupling
strength between chromophores and the temperature. Combining these findings, we
propose that increasing the coupling strength between the chromophores can
significantly increase the power output of the light harvesting system.
