A Triphenylamine–Naphthalenediimide–Fullerene Triad: Synthesis, Photoinduced Charge Separation and Solution-Processable Bulk Heterojunction Solar Cells

A new naphthalenediimide (NDI) derivative linked to triphenylamine (TPA) and fullerene (TPA–NDI–C₆₀ triad; S3) was designed, synthesized and characterized. Femtosecond laser transient absorption spectral measurements revealed that fast electron transfer from TPA to the singlet-excited state of NDI occurred to form a charge-separated state (TPA^.+–NDI^.−–C60) with a rate constant of approximately 10¹² s⁻¹, followed by the charge-shift reaction from NDI^.− to C₆₀ to produce the charge-separated state (TPA^.+–NDI–C₆₀ ^.−) that decayed by back electron transfer with a rate constant of 4.4×10⁹ s⁻¹ (with a corresponding lifetime of 230 ps). Organic bulk heterojunction solar cells were fabricated using the triad S3 as an n-type semiconductor along with the conventional donor polymer poly(3-hexylthiophene). The power conversion efficiency reached 3.03 % before annealing and 4.85 % after annealing in the optimized devices. To rationalize the performance of these organic photovoltaic devices, atomic force microscopy was used to study the morphology of poly(3-hexylthiophene):S3 blend surfaces, which were found to be well interlaced and free from projections, lumps, or indentations.