Improving the photovoltaic performance of fluorinated 2,2′-bithiophene core-based D(A–Ar)2 type small molecules via strategically end-capped heteroaromatic substitution

Abstract

To reveal the influences of introducing various end-capping units onto small-molecule (SM) donors, in this article, 2-octylthio-thiophene (TS), 2-octyl-thiazole (TZ) and 2-octylthiothiazole (TZS) are strategically selected as end-capping units to combine with a fluorinated 2,2′-bithiophene (FBT) core and thiophene-flanked diketopyrrolopyrrole (TDPP) units as electron acceptors to afford three novel D(A–Ar)₂ type SM donors, namely FBT(TDPP-TS)₂, FBT(TDPP-TZ)₂ and FBT(TDPP-TZS)₂. The structure-property relationships provoked by slight changes in the terminal groups were systematically evaluated. In particular, TZS end-units make FBT(TDPP-TZS)₂ possess better molecular stacking, higher mobility and better miscibility in the blend with PC₇₁BM in comparison with the other two SMs. As a result, the introduction of TZS end-groups has been found to enhance the open-circuit voltage (V_oc), short circuit current density (J_sc) and fill factor (FF) simultaneously. After optimization, an outstanding power conversion efficiency (PCE) of 8.91% with a V_oc of 0.80 V, a J_sc of 16.75 mA cm⁻² and a FF of 66.93% was obtained for the FBT(TDPP-TZS)₂ based device, which is among the top values of the D(A–Ar)₂ type single junction small molecular BHJ-OSCs. Our results obtained here indicate that strategic hetero-substitution of end-capping units in D(A–Ar)₂ type SMs is an efficient strategy to improve photovoltaic performance.