Efficient Fullerene-Free Polymer Solar Cells Based on Alkylthio Substituted Conjugated Polymers

A series of medium and wide band gap polymers with different backbones and side-chains were designed and synthesized as electron donors for fullerene-free polymer solar cells (PSCs). The relationship between the molecular structures of the polymer donors and the photovoltaic properties of the fullerene-free PSC devices was fully investigated. The optical band gaps and the energy levels of the polymers were modulated by the copolymerization of the benzodithiophene (BDT) unit with the various comonomers. Moreover, by incorporating alkylthio substituents, the highest occupied molecular orbital (HOMO) levels of the polymer donors were effectively reduced, and thus the open-circuit voltages (V-oc) of the fullerene-free PSC devices were strongly improved. Simultaneously, the extinction coefficients of the polymers were also enhanced by introducing the alkylthio groups, which ensured sufficient light-harvesting capability. The exciton dissociation probability P(E,T) in the blend films of the alkylthio substituted polymers was also higher than that of the alkyl substituted polymers. Among all the six polymers, poly[bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo [1,2-b ':4,5-b] dithiophene]-alt-[1,3-bis (2-ethylhexyl)-5,7-di(thiophen-2-y1)-4H,8H-benzo dithiophene-4,8-dione] (PB1-S), exhibited the most suitable optoelectronic properties for fullerene-free PSCs because of its complementary absorption band with ITIC and the low HOMO level. The fullerene-free PSC based on the PB1-S:ITIC active layer exhibited excellent photovoltaic performance, with a power conversion efficiency (PCE) as high as 10.49%.