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Conjugated polymers and small molecules based on alternating electron donating (D) and electron accepting (A) building blocks have led to state-of-the-art organic solar cell materials governing efficiencies beyond 10%. Unfortunately, the connection of D and A building blocks via cross-coupling reactions does not always proceed as planned, which can result in the generation of side products containing D-D or A-A homocoupling motifs. Previous studies have reported a reduced performance in polymer and small molecule solar cells when such defect structures are present. A general consensus on the impact of homocouplings on device performance is, however, still lacking, as well as a profound understanding of the underlying causes of the device deterioration. To differentiate the combined effect of molecular weight and homocouplings in polymer solar cells, a systematic study on a small molecule system (DTS(FBBTh2)2) is presented here. The impact of homocouplings on the blend nanomorphology, thermal, and electrooptical properties is investigated. It is demonstrated that small quantities of homocouplings (<10%) already lead to sub-optimal device performance, as this strongly impacts the molecular packing and electronic properties of the photoactive layer. These results highlight the importance of material purity and pinpoint homocoupling defects as one of the most probable reasons for batch-to-batch variations.

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Experimental determination of eutectic state diagrams for organic photovoltaics based on advanced calorimetry highlighting the importance of the amorphous phase.