Direct arylation polymerization approach for the synthesis of narrow band gap cyclopentadithiophene based conjugated polymer and its application in solar cell devices

•D-A type conjugated polymer based on CPDT and benzothiadiazole was synthesized.•Direct arylation approach was used to attain high molecular weight.•PCPDTTBT was found to have low band gap and exhibited broad absorption spectra.•BHJ solar cells displayed efficiency upto 1.86% in preliminary device structure.

Direct arylation polymerization is a simple, cost-effective and efficient method for synthesizing conjugated polymers. The synthesis of a narrow band gap conjugated polymer, PCPDTTBT, comprising of alternating cyclopentadithiophene (CPDT) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole (TBT) units was carefully optimized by direct arylation polycondensation. The synthesized polymer has a number average molecular weight of 36.3 kg mol−1 with a low PDI of 1.25 as measured by GPC against polystyrene standards. PCPDTTBT exhibits an absorption maximum at 617 nm in THF solution whereas in thin film, the absorption spectrum was broader and a maximum was observed at 650 nm. The HOMO, LUMO and band gap calculations were estimated using cyclic voltammetry and were found to be −5.20 eV, −3.57 and 1.63 eV, respectively. Bulk heterojunction polymer solar cells were fabricated by blending PCPDTTBT and PC71BM in three different ratios with six cells for each ratio to optimize the efficiency. It was found that the blend ratio of 1:2 exhibited a high short-circuit current of 10.03 mA cm−2, open-circuit voltage of 0.567 V and a power conversion efficiency of 1.86%. The morphology of the active layer was investigated by employing atomic force microscopy (AFM) in tapping mode and the film of active layer with 1:2 ratio of PCPDTTBT: PC71BM showed most favorable morphology with less phase separation.

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