The final step in the preparation of ppp-DTS(PTTh222)222 involves end capping of the PT-DTS-PT core with 2-hexylbithiophene units via a microwave assisted Stille cross coupling reaction. Methyl transfer (instead of 2-hexylbithiophene transfer) can occur leading to the formation of (MePT)DTS(PTTh22). Although (MePT)DTS(PTTh22) is difficult to separate from the ppp-DTS(PTTh222)222 product via column chromatography, it is readily extracted using hexanes solvent to give absolute ppp-DTS(PTTh222)222. Trace impurities of (MePT)DTS(PTTh22) in BHJ solar cells fabricated from synthesis batches of ppp-DTS(PTTh222)222 significantly influence the photovoltaic properties, causing a ∼50% reduction in efficiency and affecting all of the relevant device parameters (Jsc, Voc and FF). From a broader perspective, despite molecular design, the suitability of a material for efficient devices is often only determined by trial and error in the device processing laboratory. As shown by the data presented in this publication, promising materials found to be unsuitable for device applications may suffer from highly dilute impurities that act to increase carrier recombination.