Understanding the origin of inefficient photocurrent generation in organic
solar cells with low energy offset remains key to realizing high performance
donor-acceptor systems. Here, we probe the origin of field-dependent free
charge generation and photoluminescence in non-fullerene acceptor (NFA) based
organic solar cells using the polymer PM6 and NFA Y5 - a non-halogenated
sibling to Y6, with a smaller energetic offset to PM6. By performing
time-delayed collection field (TDCF) measurements on a variety of samples with
different electron transport layers and active layer thickness, we show that
the fill factor and photocurrent are limited by field-dependent free charge
generation in the bulk of the blend. We also introduce a new method of TDCF
called m-TDCF to prove the absence of artefacts from non-geminate recombination
of photogenerated- and dark charge carriers near the electrodes. We then
correlate free charge generation with steady state photoluminescence intensity,
and find perfect anticorrelation between these two properties. Through this, we
conclude that photocurrent generation in this low offset system is entirely
controlled by the field dependent exciton dissociation into charge transfer
states