Naphthalene Substituent via β-Position Linkage: An Effective Extended Conjugated Moiety Can Make a Decent Trade-Off between Optical Band Gap and Open Circuit Voltage in Symmetry-Breaking Benzodithiophene Based Polymer Solar Cells
Bare naphthalene rings via α- and β-position linkage as side chain substituents applicable in asymmetric benzodithiophene (BDT) building block were first employed to design new light-harvesting polymers. Accordingly, two D-A type polymers based on naphthyl substituted BDT as D-building block and the well-known 4,7-di(thiophen-2-ethylhexyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) as acceptor unit, were synthesized. The polymer PBDTβNPFBT with naphthalene via the β-position linkage shows appropriate π-π distance, and efficiently broadens the absorption and narrows the optical band gap (Egopt) by extending the π-conjugated degree, which is beneficial to capture more photons and thus improve the short-circuit current density (JSC). Meanwhile, PBDTβNPFBT based devices also exhibit the desirable high open circuit voltage (VOC) due to the low saturation dark current density (JS) arising from the exactly appropriate π-π distance. As a result, the power conversion efficiency (PCE) of 9.80% for the PBDTβNPFBT/PC71BM based PSCs is the highest efficiency among the reported famous BDT and DTBT backbone photovoltaic polymers. In addition, the PCE of 7.33% based on PBDTβNPFBT/ITIC without any treatment is also impressing for the non-fullerene PSCs. Thus, it can be concluded that naphthyl unit via β-position linkage can make a better trade-off between Egopt and VOC, and finally dramatically increase PCE.