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-4H-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 a oligo(3-hexylthiophene) dye (ST), 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 TiO2, film with remarkable J(SC) of 10.9 mA cm(-2) in solid-state DSCs.