Effect of the triazole ring in zinc porphyrin-fullerene dyads on the charge transfer processes in NiO-based devices

Abstract

Herein, the synthesis of three covalently linked donor–acceptor zinc porphyrin–fullerene (ZnP–C₆₀) dyads (C₆₀trZnPCOOH, C₆₀trZnPtrCOOH and C₆₀ZnPCOOH) is described, and their application as sensitizers in NiO-based dye-sensitized solar cells (DSCs) is discussed. To the best of our knowledge, this is the first example where covalently linked ZnP–C₆₀ dyads have been used as chromophores in NiO-based DSCs. In an effort to examine whether the distance of the chromophore from the electron acceptor entity and/or the NiO surface affects the performance of the cells, a triazole ring was introduced as a spacer between ZnP and the two peripheral units C₆₀ and –COOH. The triazole ring was inserted between ZnP and C₆₀ in dyad C₆₀trZnPCOOH, whereas both the anchoring group and C₆₀ were connected to ZnP through triazole spacers in C₆₀trZnPtrCOOH, and dyad C₆₀ZnPCOOH did not contain any triazole linker. Photophysical investigation performed by ultrafast transient absorption spectroscopy in solution and on the NiO surface demonstrated that all the porphyrin–fullerene dyads exhibited long-lived charge-separated states due to electron shifts from the reduced porphyrin core to C₆₀. The transient experiments performed in solution showed that the presence of triazole ring influenced the photophysical properties of the dyads C₆₀trZnPCOOH and C₆₀trZnPtrCOOH and in particular, increased the lifetime of the charge-separated states compared to that of the C₆₀ZnPCOOH dyad. On the other hand, the corresponding studies on the NiO surface proved that the triazole spacer has a rather moderate impact on the charge separation (NiO–ZnP˙⁺–C₆₀˙⁻) and charge recombination (NiO–³*ZnP–C₆₀) rate constants. All three dyads exhibited enhanced performance in terms of photovoltaic measurements with more than threefold increase compared to the reference compound PhtrZnPCOOH in which the C₆₀ acceptor is absent. Two different electrolytes were examined (I₃⁻/I⁻ and Co^III/II) and in most cases, the presence of the triazole ring enhanced their photovoltaic performance. The best performing dyad in I₃⁻/I⁻ was C₆₀trZnPCOOH (PCE = 0.076%); in Co^III/II, the best performing dyad was C₆₀trZnPtrCOOH (PCE = 0.074%).