  Driven by the rapid development of non-fullerene electron acceptors (NFAs),
the power conversion efficiencies of organic solar cells (OSCs) have reached
levels suitable for commercial applications. However, the poor operational
stability of high-performance NFA OSCs is a remaining fundamental challenge
that must be addressed. Whilst previous studies have primarily focused on the
NFA component, we consider here the degradation pathways of both the donor and
acceptor materials in the benchmark PM6:Y6 blend. Here, we show that light
soaking greatly increases the energetic disorder and trap state density in PM6,
with little effect on Y6. This is corroborated by electron paramagnetic
resonance spectroscopy, which reveals increased recombination via trapped
polarons on PM6 after light soaking. In addition, ultrafast optical
spectroscopy studies on light-soaked samples show that PM6 singlet excitons are
rapidly converted into interchain polaron pairs on sub-100 fs timescales; this
process outcompetes electron transfer to Y6, significantly reducing the charge
generation yield of the blend. We make similar observations in the parent
polymer, PBDB-T, indicating that this class of donor materials, used in most
high-performance OSCs to date, are intrinsically unstable to light soaking.
Thus, we reveal that the donor polymer can be a further critical weak link in
efficient OSC systems, whose degradation mechanism needs to be addressed
collectively with NFAs.
