Pentafluorosulfanyl-functionalised BODIPY push-pull dyes for p-type dye-sensitized solar cells

Abstract

We report a push-pull BODIPY-based dye functionalised with an electronegative SF₅ group at the meso position for applications in photocathodes in tandem dye-sensitized solar cells (DSSCs). The push-pull character enhances charge-transfer from the mesoporous NiO cathode surface towards the redox mediator. A Knoevenagel condensation reaction was used to introduce the carboxylic acid to anchor the dye to the oxide surface, via a styryl linker which increases the conjugation in the molecule and shifts the absorption to the red. The room-temperature synthesis and high yields, make the dye promising for manufacture on a large scale. The dye was applied in p-DSSCs giving a power conversion efficiency (0.066%), a short circuit photocurrent (J _SC) of 3.84 mA cm^-2, open circuit voltage (V _OC) of 58 mV and fill factor of 30%.

Fig. 1. Relative energy level diagram and the components in a tandem dye-sensitized solar cell. The maximum V_OC of the tandem device should be the sum of the V_OC of the individual single junction cells (V_OC 1 and V_OC 2). A–F are electron transfer processes following absorption by light: A and D are photoinduced excitation of sensitizers 2 and 1, B and D are dye-regeneration processes at the dye-electrolyte interfaces, and E and F are charge-injection processes at the dye-semiconductor interfaces.

Fig. 2. Chemical structures of the dyes SF5-1–3 synthesised and tested in this work.

Fig. 3. Crystal structure of SF5-2. Thermal ellipsoids drawn at 50% probability.

Fig. 4. Optimised geometry and orbital distribution (left = HOMO, right = LUMO) for SF5-1 (top) and SF5-3 (bottom) in MeCN using cam-B3LYP 6-311G++ (d,p).

Fig. 5. Left: UV-visible absorption (line) and emission spectra (λ_ex = 620 nm, dashed line) of SF5-3 in THF. Right: UV-visible absorption spectra of P1|NiO (red) and SF5-3|NiO (blue).

Fig. 6. Left: transient absorption spectra (λ_ex = 640 nm) of SF5-3 in THF (top) and SF5-3|NiO (bottom). Right: evolution associated spectra of SF5-3 in THF where 1 = 260 ps and 2 = 4.0 ns (top) and SF5-3|NiO where 1 = 0.87 ps, 2 = 8.3 ps, 3 = 230 ns (bottom).

Fig. 7. Plot of current density against voltage for illuminated devices containing dye SF5-3 and the benchmark dye P1. Electrolyte composition: 0.1 M LiI and 1 M I₂ in MeCN.