Triplet Excitons in Organic Solar Cells

Driven by the development of non-fullerene electron acceptor materials, the performance of organic solar cells (OSCs) has recently shown a remarkable improvement, with power conversion efficiencies nearly doubling from 11% to 19% in less than 10 years. However, the efficiency of OSCs is still lower than inorganic technologies, where efficiencies of &gt;20% are commonplace. This is primarily due to excessive non-radiative recombination in OSCs, which reduces the open circuit voltage from the radiative limit.<br/><br/>In our work, we have identified recombination via low energy triplet excitons as a key factor responsible for the large non-radiative losses in OSCs. In state-of-the-art systems, such as ‘PM6:Y6’, up to 90% of the charge carrier recombination proceeds via triplet exciton states; this reduces the open circuit voltage by up to 60 mV. To address this issue, we propose two viable strategies. First, through the identification of systems where recombination via triplet excitons is suppressed, we demonstrate that significant hybridisation of the molecular triplet exciton and triplet charge transfer state can disfavour terminal recombination into the former. Second, we show that triplet-triplet annihilation has the potential to mitigate against triplet exciton losses by recycling up to half of these states formed back into singlet excitons, providing an opportunity for radiative recombination to occur. Therefore, our findings provide a framework to alleviate non-radiative losses via triplet excitons in OSCs, which could push efficiences towards and beyond the 20% milestone.