We report the synthesis of three fully π-conjugated diblock copolymers containing selenophene- and thiophene-based repeating units. All of these diblock copolymers undergo phase separation, and by systematically changing the compatibility of the two blocks through side chain modification, we are able to access different thin film morphologies. Introducing a bulky 2-ethylhexyl side chain increases solubility while retaining crystallinity of the selenophene block. While poly(3-hexylselenophene)-b-poly(3-hexylthiophene) and poly(3-(2-ethylhexyl)selenophene-b-poly(3-(2-ethylhexyl)thiophene) form more disordered fibrillar structures, poly(3-hexylthiophene)-b-poly(3-(2-ethylhexyl)selenophene) forms long (1–2 μm) solid state fibrillar structures that are reminiscent of the lamellae that are formed by nonconjugated block copolymers. We further use electron energy loss spectroscopy to visualize thiophene- and selenophene-rich domains at the nanometer scale in each of these examples. By studying new polymer compositions and relating them to solid state structure, we further our understanding of heterocycle induced phase separation and phase separation in general.