Triphenylamine modified bis-diketopyrrolopyrrole molecular donor materials with extended conjugation for bulk heterojunction solar cells

Two new solution-processable enlarged π-conjugated donor–acceptor (D–A) organic small molecules consisting of dialkoxysubstituted benzo1,2-b:4,5-b′]dithiophene (BDT) or dioctyltertthiophene (3T) as the central donor units, diketopyrrolopyrrole (DPP) as the acceptor unit and triphenylamine (TPA) as the terminal conjugated segment, TPA–DPP–BDT and TPA–DPP–3T, were designed and synthesized. Both small molecules possess broad absorption ranging from 300 to 800 nm with an optical band at approximately 1.50 eV and relatively low HOMO energy levels from −5.12 to approximately −5.16 eV. Expectedly, the UV–Vis absorption onset (810 nm) of TPA–DPP–BDT is largely red-shifted (60 nm) relative to that (750 nm) of previously reported BDT(TDPP)₂, which consists of BDT and DPP units. Unlike most of the TPA based molecules, strong molecular aggregation was observed in the solid state for both small molecules. In addition, atomic force microscopy (AFM) and X-ray diffraction (XRD) investigations indicated that TPA–DPP–3T and TPA–DPP–BDT exhibit good miscibility with fullerene derivatives. The organic solar cells based on TPA–DPP–BDT/PC₆₁BM(1:1) demonstrated power conversion efficiencies as high as 4.04% with a short-circuit current density (J_sc) of 11.40 mA cm⁻² and a fill factor (FF) of 53.2% when the active layer of the cell was annealed at 130 °C for 10 min.