Organic photovoltaics (OPVs) have recently surpassed 10% power conversion efficiency (PCE), a key milestone towards commercialisation. Achieving validated lifetimes beyond 10 years is equally important, yet the physical degradation of active organic layers during device operation is not yet well understood. Efficient OPVs rely on solid-state light absorbing mixtures of electron donating and accepting molecules with nanometre-scale domains. As the initial microstructure in OPVs is kinetically trapped during fabrication, thermal- and photo-induced demixing arising from prolonged operation can severely degrade PCE. We propose to use neutron reflectometry at OFFSPEC on pristine and aged OPVs using a new series of high-performance donor polymers to probe microstructural evolution and layer demixing, helping reveal design strategies for significant OPV lifetime improvement.