  The great progress in organic photovoltaics (OPV) over the past few years has
been largely achieved by the development of non-fullerene acceptors (NFAs),
with power conversion efficiencies now approaching 20%. To further improve
device performance, loss mechanisms must be identified and minimized. Triplet
states are known to adversely affect device performance, since they can form
energetically trapped excitons on low-lying states that are responsible for
non-radiative losses or even device degradation. Halogenation of OPV materials
has long been employed to tailor energy levels and to enhance open circuit
voltage. Yet, the influence on recombination to triplet excitons has been
largely unexplored. Using the complementary spin-sensitive methods of
photoluminescence detected magnetic resonance (PLDMR) and transient electron
paramagnetic resonance (trEPR) corroborated by transient absorption and
quantum-chemical calculations, we unravel exciton pathways in OPV blends
employing the polymer donors PBDB-T, PM6 and PM7 together with NFAs Y6 and Y7.
All blends reveal triplet excitons on the NFA populated via non-geminate hole
back transfer and, in blends with halogenated donors, also by spin-orbit
coupling driven intersystem crossing. Identifying these triplet formation
pathways in all tested solar cell absorber films highlights the untapped
potential for improved charge generation to further increase plateauing OPV
efficiencies.
