Enhanced photovoltaic performance of polymer solar cells through design of a fused dithienosilolodithiophene structure with an enlarged π-conjugated system

Polymer solar cells (PSCs) have garnered considerable attention in recent years. However, improving the fill factor (FF) remains a big challenge. The FF is closely related to the mobilities of the donor. Thus, improving the mobilites of the donor by constructing an enlarged π-conjugated fused ring in polymeric backnones could improve FF and power conversion efficiency (PCE). Here, a fused-ring dithienosilolodithiophene (DTTS)-based low-bandgap polymer of PDTTS with an enlarged π-conjugated system was created and its thermal stability, absorption range, molecular energy, and the corresponding photovoltaic properties were investigated. UV-vis absorption spectra showed that PDTTS with enlarged conjugation on polymeric backbones exhibited a more obvious absorption shoulder peak compared with a similar polymer of PDTS containing a dithienosilole (DTS) unit. Hence, PDTTS had a more planar structure than PDTS and resulted in stronger intermolecular π–π interactions. X-ray diffraction revealed PDTTS to exhibit greater crystallinity than PDTS, which suggested more ordered π–π stacking in PDTTS backbones. The mobility of PDTTS was more than one order of magnitude higher than that of PDTS, and benefited from the more planar conjugated structure of PDTTS. Photovoltaic devices based upon PDTTS/PC71BM blends exhibited efficiencies up to 6.33% with a higher FF of 65% for as-cast films without processing additives. Under optimized processing conditions, PDTS/PC71BM blends exhibited an efficiency of 3.22% with a FF of 53%. These preliminary results suggest that polymers with an enlarged π-conjugated system possess higher mobility and facilitate effective carrier transportation, and boost FF and PCE values.