This article reports the synthesis, crystallographic structure and OFET and OPV performance of the conjugated oligomer of cyclopentadithiophene (CPDT) with benzothiadiazole (BT). Synthesis of the oligomer composed of the CPDT-BT-CPDT sequence is accomplished using direct arylation reactions. Theoretical and experimental X-ray single crystallography confirms that two CPDT-BT-CPDT molecules are not entirely disordered, but are actually stacking directly across each other at the central BT units with an intermolecular distance of 3.61 Å, providing valuable insight into the polymer bulk structure. The performance of the oligomer in OFET devices is investigated by fabricating bottom gate top contact devices and demonstrates a hole mobility of 5.0 × 10−3 cm2 V−1 s−1. OPV devices of the oligomer blended with PC61BM and PC71BM show power conversion efficiency (PCE) of 1.61%. One potential use for the oligomer could be as a sensitiser in a ternary blend with P3HT–PC61BM or PCPDTBT–PC61BM OPVs; the PCE can be relatively increased by 3–9% depending on concentration, primarily as a result of increased short circuit current density.