Naphthalene substituents bonded via the β-position: an extended conjugated moiety can achieve a decent trade-off between optical band gap and open circuit voltage in symmetry-breaking benzodithiophene-based polymer solar cells

Abstract

Bare naphthalene units bonded via the α- and β-positions as side chain substituents on an asymmetric benzodithiophene (BDT) building block were employed in the design of new light-harvesting polymers for the first time. Accordingly, two D–A type polymers, based on naphthyl-substituted BDT as a D-building block and the well-known 4,7-di(thiophen-2-ethylhexyl)-5,6-difluoro-2,1,3-benzothiadiazole (DT_ffBT) as an acceptor unit, were synthesized. The polymer PBDTβNPFBT with naphthalene bonded via the β-position exhibits an appropriate π–π distance. The naphthalene rings efficiently broaden the absorption and narrow the optical band gap (E^opt_g), by extending the degree of π-conjugation, which is beneficial for capturing more photons and thus improving the short-circuit current density (J_SC). Meanwhile, PBDTβNPFBT-based devices also exhibit a desirable high open circuit voltage (V_OC), due to the low saturation dark current density (J_S) arising from the exactly appropriate π–π distance. As a result, the power conversion efficiency (PCE) of 9.80% for the PBDTβNPFBT/PC₇₁BM-based PSC is the highest efficiency ever obtained among the reported BDT and DTBT backbone photovoltaic polymers. In addition, a PCE of 7.33% was obtained for PBDTβNPFBT/ITIC without any treatment, which is also impressive for non-fullerene PSCs. Thus, it can be concluded that a naphthyl unit bonded via the β-position can lead to a better trade-off between E^opt_g and V_OC, ultimately increasing the PCE dramatically.