The effect of heteroalkyl (-XR, X = Se, S, O) substitution on a series of molecular semiconductors having a 3,3′-diheteroalkyl-2,2′-bithiophene (XBT) central core is studied. Thus, the selenotetradecyl (-SeC14H29) SeBT core is investigated by end-functionalization with two dithienothiophene (DTT), thienothiophene (TT), and thiophene (T) units to give SeBTs 1–3, respectively, for molecular π-conjugation effect examination. Furthermore, the selenodecyl (-SeC10H21) and selenohexyl (-SeC6H13) SeBT cores end-capped with DTTs to give SeBTs 1B and 1C, respectively, are synthesized for understanding -SeR length effects. To address systematically the impact of the chalcogen heteroatom, the newly developed selenoalkyl SeBTs are compared with the previously reported thiotetradecyl (-SC14H29) DDTT-SBT (4) and the new tetradecyloxy (-OC14H29) DDTT-OBT (5). When fabricating organic field effect transistors by the solution-shearing method, the devices based on the tetradecylated DDTT-SeBT (1) exhibit the highest mobility up to 4.01 cm2 V−1 s−1, which is larger than those of the other SeBT compounds and both DDTT-SBT (4) (1.70 cm2 V−1 s−1) and DDTT-OBT (5) (9.32 × 10−4 cm2 V−1 s−1). These results are rationalized by a combination of crystallographic, morphological, and microstructural analysis.