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 60BM, [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.