Solar cells fabricated from organic semiconductors possess several benefits over traditional inorganic technologies. Organic solar cells are lightweight and flexible, whilst also offering the possibility of semi-transparent modules and superior performance in low-light intensity applications. However, whilst the power conversion efficiencies of organic solar cells have exceeded 18% under sunlight, this is still lower than inorganic technologies, where efficiencies of >20% are commonplace. Of relevance to solar cell operation, organic semiconductors have a reduced ability to screen the Coulomb interaction between two charge carriers of opposite signs compared to their inorganic counterparts. Consequently, organic semiconductors are excitonic materials, where the strong Coulombic interactions between electrons and holes results in a large exciton binding energy (~0.2-0.5 eV) and localised excited states. In these excitonic states, the spins of the paired electrons interact strongly, resulting in the formation of distinct spin-singlet (electron spins anti-parallel) and spin-triplet (electron spins parallel) states. As singlet and triplet excitons possess dramatically different optical and electronic properties, exciton spin becomes a key consideration in the operation of optoelectronic devices fabricated from organic semiconductors, such as solar cells and light emitting diodes.

In this talk, I will present our recent work on spin management in organic solar cells[1]. We find that in benchmark organic solar cells fabricated using non-fullerene electron acceptors, up to 90% of the charge carrier recombination proceeds via low energy triplet exciton states. As the radiative decay of triplet excitons back to the spin-singlet ground state is spin-forbidden, recombination via dark triplet excitons constitutes a significant voltage loss pathway in organic solar cells. Through the identification of systems where recombination via triplet excitons is suppressed, we demonstrate a novel tactic to turn off this loss pathway that involves the strong electronic coupling between the electron donor and acceptor components in the organic solar cell. Consequently, we propose molecular design rules to engineer-out recombination via triplet excitons in future systems. Therefore, our findings provide a framework to further reduce the performance gap between organic and inorganic solar cell technologies.