  We present a neutron spectroscopy based method to study quantitatively the
partial miscibility and phase behaviour of photovoltaic active layer made of
conjugated polymer:small molecule blends, presently illustrated with the
regio-random poly(3-hexylthiophene-2,5-diyl) and fullerene [6,6]-Phenyl
C$_{61}$ butyric acid methyl ester (RRa-P3HT:PCBM) system. We perform both
inelastic neutron scattering and quasi-elastic neutron scattering measurements
to study the structural dynamics of blends of different compositions enabling
us to resolve the phase behaviour. The difference of neutron cross sections
between RRa-P3HT and PCBM, and the use of deuteration technique, offer a unique
opportunity to probe the miscibility limit of fullerene in the amorphous
polymer-rich phase and to tune the contrast between the polymer and the
fullerene phases, respectively. Therefore, the proposed approach should be
universal and relevant to study new non-fullerene acceptors that are closely
related - in terms of chemical structures - to the polymer, where other
conventional imaging and spectroscopic techniques present a poor contrast
between the blend components.
