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 the small-molecule (SM) donors, 2-octylthio-thiophene (TS) unit, 2-octyl-thiazole (TZ) unit and 2-octylthiothiazole (TZS) are strategically selected as end-capping units combining with a fluorinated 2,2'-bithiophene (FBT) core and thiophene-flanked diketopyrrolopyrrole (TDPP) units as bridged acceptor 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, for systematically evaluating the structure-property relationship provoked by slight changes in the terminal groups. Especially, TZS end-units make FBT(TDPP-TZS)2 possess relatively better molecular stacking, higher mobility and better miscibility with 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 (Voc), short circuit current density (Jsc) and fill factor (FF) simultaneously. Under the optimized processing conditions, an outstanding power conversion efficiency (PCE) of 8.91% with an open-circuit voltage (Voc) of 0.80 V, a short current density (Jsc) of 16.75 mA cm-2 and a fill factor (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 strategically hetero-substitution of end-capping units in D(A-Ar)2 type SMs is an efficient strategy to improve photovoltaic performance.