Dark states and delocalization: competing effects of quantum coherence on the efficiency of light harvesting systems

TL;DR

This paper investigates how dark states and exciton delocalization influence the efficiency of light harvesting systems, revealing a dominant dark state protection mechanism and the impact of chromophore coupling strength and system size.

Contribution

It identifies the dominant role of dark state protection over delocalization in optimizing light harvesting efficiency and highlights the importance of system size parity and coupling strength.

Findings

Dark state protection enhances energy conversion efficiency.

System size parity affects exciton distribution and efficiency.

Increasing coupling strength boosts power output.

Abstract

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.