Development of Dithienosilole-Pyridalthiadiazole-Based Copolymer as an Electron Donor in Organic Photovoltaic Cells

The effect of nitrogen insertion in a benzothiadiazole (BT) moiety was investigated via the synthesis of a novel copolymer based on dithienosilole (DTS) and pyridalthiadiazole (PT), the heterocyclic counterpart. Stille cross coupling of the electron rich and electron poor units yields poly[(4, 4’-didodecyldithieno[3, 2-b:2’, 3’-d]silole-2, 6-diyl)- alt -(2, 1, 3-pyridalthiadiazole-4, 7-diyl), a nonsoluble polymer. Dense π–π stacking interactions that inhibited polymer solubility were cured by the introduction of two solubilizing hexylthiophene spacers, and a new soluble low-bandgap copolymer, namely poly[(4, 4’-didodecyldithieno[3, 2-b:2’, 3’-d]silole-2, 6-diyl)- alt -{4, 7-bis[2-(3-hexyl)thienyl]-2, 1, 3-pyridalthiadiazole-5, 5’-diyl}] (PDTSDTPT), was alternatively obtained. For reasons of comparison, the corresponding oligomer, i.e., ODTSDTPT, was also synthesized. Their chemical, thermal, optical, electrical, and electrochemical properties were investigated. UV–visible spectroscopy revealed that PDTSDTPT alternating copolymer is more absorbing than its BT analog. It has a lower optical bandgap, higher absorptivity, and red-shifted absorption spectrum into the near-infrared region up to 900 nm. Photovoltaic applicability was investigated for PDTSDTPT and its derived oligomer in bulk heterojunction solar cells employing PC 60 BM, [6,6]-phenyl-C 60 butyric acid methyl ester, as the n-type semiconductor. Significant improvement of open-circuit voltage was evidenced with PT-based materials. The short-circuit current ( J SC ) and fill factor ( FF ), however, remain a material of continuous improvement by morphology control.