Extended oligo-cyclopentadithiophene dyes for liquid and solid-state dye-sensitised solar cells

Abstract

A new series of dyes called T-CPDT, without a strong electron-donor moiety, was designed, based on a combination of previously-reported CPDT and 5T dyes, and synthesised. By addition of a thiophene unit between alkylated CPDT unit(s) and the cyanoacrylic acid acceptor group, their optical, electrochemical and photovoltaic properties were tuned and studied. Upon extending CPDT units from 1 to 3, hence, T-CPDT-1, T-CPDT-2 and T-CPDT-3, the UV-Vis absorption undergoes a bathochromic shift together with a larger extinction coefficient, while the first oxidation potential is less positive. These dyes were applied in DSSC devices with I⁻/I₃⁻ electrolyte and with a Spiro-OMeTAD hole transporter. Among the T-CPDT series, T-CPDT-3 shows the highest PCE of 5.88% with I⁻/I₃⁻ electrolyte and 4.38% for a solid-state device, mainly due to larger J_sc and V_oc. Their photovoltaic results with I⁻/I₃⁻ electrolyte were compared to those of CPDT-3 and 5T under the same conditions. We also show that the addition of the extra thiophene to CPDT-3, forming T-CPDT-3, leads to slower electron recombination kinetics, compared to those of CPDT-3 and 5T. Interestingly, the solid-state device with T-CPDT-3 achieves a very high J_sc of 11.27 mA cm⁻² with a less than 1 micron thick TiO₂ film which is unusually thin for solid-state devices.