Polymer/fullerene bimolecular crystal formation has been investigated using a variety of conjugated polymers and fullerenes to understand the design rules that influence donor/acceptor interaction. Modifications of the polymer by varying the substitution side chain position, density, and branching have demonstrated the importance of the “pocket” dimensions (free volume between side chains where the fullerene resides) for controlling intercalation. Yet the effect of pocket height has not been systematically explored due to the solubility limitations in polymers. In this report, we present an experimental investigation into the effect of the pocket height by synthesizing poly[2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) dimers with varied side chain lengths and track the morphological changes of the dimer/fullerene blends using grazing-incidence X-ray scattering, thermal measurements, and photoluminescence quenching. We identify two regimes: 1) oligomers with side chains ≥ heptyl (C7) form bimolecular crystals and 2) oligomers with ≤ hexyl (C6) form amorphous blends. This work provides the first observation of an order-to-disorder transition mediated by side chain length in donor-fullerene intercalated blends.