Recycling of spin-triplet excitons in organic photovoltaics

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Abstract
Non-geminate recombination in organic photovoltaics (OPVs) forms low-energy spin-triplet excitons (T1) that are known to result in irreversible, non-radiative relaxations1,2,3,4,5. Here we experimentally show in an OPV system incorporating a non-fullerene acceptor with a narrowed singlet–triplet gap that T1 excitons can be redissociated through the interfacial charge-transfer state to form free carriers. We corroborate this by identifying the increased population of free carriers following triplet sensitization of the acceptor in an OPV blend, and illustrate the way in which this mechanism alters the evolution of T1 and free carrier populations. We reveal how the distribution of orbitals in the molecule and exciton delocalization in aggregates affect the singlet–triplet energetics of the acceptor in the condensed phase, rendering the traffic between T1 and the spin-triplet charge-transfer state controllable. By introducing this acceptor as a ternary component into other host OPV systems, we manage to recover the triplet-mediated losses and improve OPV efficiencies by maximizing the number of extractable photocarriers. This study deepens our understanding of the fundamentals of OPVs, and shows how to develop future organic optoelectronics by demonstratating the recovery of low-energy T1 excitons into usable charges for electricity or light generation instead of heat.

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