Enhancing the performance of a thieno[3-4-b]pyrazine based polymer solar cell by introducing ethynylene linkages

Alternating low bandgap π-conjugated polymers consisting of 5,7-dithien-2-yl-thieno[3-4-b]pyrazine and phenylene linked directly (PP-DTTP) and by ethynylene linkages (PPE-DTTP) have been prepared in high yields by Suzuki coupling polymerization and Sonogashira condensation, respectively. Thin films of PPE-DTTP and PP-DTTP exhibit an optical bandgap of 1.55 and 1.36 eV, respectively, and the devices provide estimated power conversion efficiencies (PCE) of 0.82% and 0.30%, respectively, in bulk heterojunction solar cells with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the acceptor. Preliminary optimizations of the devices based on PPE-DTTP gave 1.20% PCE, which ranks in one of the best PCEs of thieno[3-4-b]pyrazines based solar cells. After the introduction of ethynylene linkages in PPE-DTTP, a higher open circuit voltage and a higher short circuit current were obtained with no loss of the fill factor though PPE-DTTP has a higher bandgap, therefore PPE-DTTP shows better performance, which was contributed by the more efficient intramolecular electron transport in PPE-DTTP demonstrated by the hole mobility results and quantum chemistry calculations.

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► Two D–A polymers linked directly and by ethynylene linkages have been prepared.
► The OPV devices based on the polymer with ethynylene linkages showed a PCE to 1.2%.
► The OPV devices based on the polymer linked directly exhibited a PCE of only 0.3%.
► The molecular planarity was enhanced after the introduction of ethynylene linkages.