Figure 2: Results of PTB7:PC₇₁BM blends prepared without additive.

(a) SEM image of the blend viewed from an angle of 52° with a score made across the film showing large domains and a skin of material on top, the black scale bar is 250 nm. Removing the skin by plasma-ashing the topography of the domains measured with AFM is shown in b, and in phase mode in c, indicating that the large domains consist of smaller spheres of material, where the white scale bar is 50 nm. The time-resolved photoluminescence in the blend is presented on short (d) and long (e) timescales, with the blend decay shown as open circles. In d, a best fit to the decay (red line) with a time constant of 330 fs and a pre-exponential factor of 0.81 is found. The slower part is shown in e, indicating that the decay is faster in the blend when compared with a film of PC₇₁BM (black line). The blue line is the best fit from the model for single 60-nm spheres of pure fullerene. The green dashed line is a simulated decay if the pure fullerene spheres were 150 nm, while the orange dotted line is a simulated decay if the spheres were 60 nm of fullerene with 0.2 wt% of PTB7 mixed inside them; neither fits the experimental data. (f) Schematic of morphology, showing large fullerene-rich domains comprised of small spheres of pure PC₇₁BM surrounded by a PTB7-rich mixed phase. A finely mixed phase of the two materials sits between the large domains and also forms a skin on top of the film.