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The efficiency of natural light harvesting systems is largely determined bytheir ability to transfer excitations from the antenna to the energy trappingcenter before recombination. Exciton diffusion length similarly limits organicphotovoltaics and demands bulk heterojunction architectures. Dark stateprotection, achieved by coherent coupling between subunits within the antenna,can significantly reduce radiative recombination and enhance the efficiency ofenergy trapping. In this work we extend the dark state concept to thedouble-excitation manifold by studying the dynamical flow of excitations. Weshow the lowest double-excitation state carries minimal oscillator strength butrelaxation to this state from higher lying double excitations can be relativelyrapid such that the lowest double excitation state can act as a dynamical darkstate protecting excitation from radiative recombination. This mechanism issensitive to topology and operates differently for chain and ring structures,while becoming more pronounced in both geometries when the size of the antennaincreases. When the exciton-exciton annihilation (EEA) mechanism is considered,the double-excitation population is quickly depleted and the dynamics changesdramatically. However the efficiency and output power are still significantlydifferent from those calculated using the single-excitation manifold alone,justifying the necessity of considering the double-excitation manifold.Remarkably, in certain scenarios, the EEA can even increase the overall lightharvesting efficiency by bringing population down from the double-excitationdark states to the single-excitation manifold.

Double-excitation manifold's effect on exciton transfer dynamics and the efficiency of coherent light harvesting