High Absorption Coefficient Cyclopentadithiophene Donor-Free Dyes for Liquid and Solid-State Dye-Sensitized Solar Cells

We report a series of “donor-free” dyes featuring moieties of oligo(4,4-dihexyl-4 H -cyclopenta[1,2-b:5,4-b′]dithiophene) (CPDT) functionalized with cyanoacrylic end groups for mesoscopic titania solar cells based on I – /I 3 – or Co(II)/Co(III) redox couple and spiro-OMeTAD hole transporter. These were compared with similar cells using an oligo(3-hexylthiophene) dye (5T), which we reported before. Extending the CPDT moiety of the dye molecules from one to three (denoted as CPDT-1, CPDT-2, and CPDT-3) widens the photoresponse overlap with the solar spectrum, increases the molar absorption coefficient up to 75 000 M –1 cm –1, and improves the short-circuit current ( J SC ), open-circuit voltage ( V OC ), and power conversion efficiency (PCE) for all types of DSSCs. Among these sensitizers, CPDT-3 shows the highest PCE of 6.7%, 7.3%, and 3.9% with I – /I 3 –, Co(II)/Co(III) redox couple, and spiro-OMeTAD hole transporter, respectively, compared with 7.6%, 9.0%, and 4.0% for 5T. Benefiting from the high absorption of CPDT-3, we demonstrate 900 nm thick mesoporous TiO 2 film with remarkable J SC of 10.9 mA cm –2 in solid-state DSCs.