A range of optical probes are used to study the nanoscale-structure and electronic-functionality of a photovoltaic-applicable blend of the carbazole co-polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) and the electronic accepting fullerene derivative (6,6)-phenyl C70-butyric acid methyl ester (PC70BM). In particular, it is shown that the glass transition temperature of a PCDTBT:PC70BM blend thin-film is not sensitive to the relative blend-ratio or film thickness (at 1:4 blending ratio), but is sensitive to casting solvent and the type of substrate on which it is deposited. It is found that the glass transition temperature of the blend reduces on annealing; an observation consistent with disruption of π–π stacking between PCDTBT molecules. Reduced π–π stacking is correlated with reduced hole-mobility in thermally annealed films. It is suggested that this explains the failure of such annealing protocols to substantially improve device-efficiency. The annealing studies demonstrate that the blend only undergoes coarse phase-separation when annealed at or above 155 °C, suggesting a promising degree of morphological stability of PCDTBT:PC70BM blends.