  The power conversion efficiencies (PCEs) of organic solar cells (OSCs) using
non-fullerene acceptors (NFAs) have now reached 18%. However, this is still
lower than inorganic solar cells, for which PCEs >20% are commonplace. A key
reason is that OSCs still show low open-circuit voltages (Voc) relative to
their optical band gaps, attributed to non-radiative recombination. For OSCs to
compete with inorganics in efficiency, all non-radiative loss pathways must be
identified and where possible, removed. Here, we show that in most NFA OSCs,
the majority of charge recombination at open-circuit proceeds via formation of
non-emissive NFA triplet excitons (T1); in the benchmark PM6:Y6 blend, this
fraction reaches 90%, contributing 60 mV to the reduction of Voc. We develop a
new design to prevent recombination via this non-radiative channel through the
engineering of significant hybridisation between the NFA T1 and the
spin-triplet charge transfer exciton (3CTE). We model that the rate of the back
charge transfer from 3CTE to T1 can be reduced by an order of magnitude,
allowing re-dissociation of the 3CTE. We then demonstrate NFA systems where T1
formation is suppressed. This work therefore provides a clear design pathway
for improved OSC performance to 20% PCE and beyond.
