  Despite many advances towards improving the stability of organic photovoltaic
devices, environmental degradation under ambient conditions remains a
challenging obstacle for future application. Particularly conventional systems
employing fullerene derivatives are prone to oxidise under illumination,
limiting their applicability. Herein, we report on the environmental stability
of the small molecule donor DRCN5T together with the fullerene acceptor PC70BM.
We find that this system exhibits exceptional device stability, mainly due to
almost constant short-circuit current. By employing ultrafast femtosecond
transient absorption spectroscopy we attribute this remarkable stability to two
separate mechanisms: 1) DRCN5T exhibits high intrinsic resistance towards
external factors, showing no signs of deterioration. 2) The highly sensitive
PC70BM is stabilised against degradation by the presence of DRCN5T through
ultrafast long-range energy transfer to the donor, rapidly quenching the
fullerene excited states which are otherwise precursors for chemical oxidation.
We propose that this photoprotective mechanism be utilised to improve the
device stability of other systems, including non-fullerene acceptors and
ternary blends.
