New simple panchromatic dyes based on thiadiazolo[3,4-c]pyridine unit for dye-sensitized solar cells

Three new organic D-A-π-A photosensitizers with thiadiazolo[3,4-c] pyridine moiety incorporated between non-planar triarylamine and cyanoacrylic acid, have been developed and applied in dye-sensitized solar cells. It has been demonstrated that the incorporation of electron-withdrawing thiadiazolo[3,4-c]pyridine unit can effectively tune the HOMO and LUMO energy levels, extending the absorption spectra into the deep-red region and covering the whole visible region. The photoresponse of the cells based on the three dyes reached above 800 nm, a value which is comparable to the incident photon-to-current conversion efficiency onset of the established dye N719. Typically, three different bulky electron-donating units such as triphenylamine, N,N-bis(9,9-dimethylfluoren-2-yl)aniline and 4-(hexyloxy)-N-(4-(hexyloxy) phenyl)-N-phenylaniline were utilized to investigate their influences on the photovoltaic performances of DSSCs. The results indicate that the introduction of the strongest electron-donating 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-N- phenylaniline unit can further red-shift the intramolecular charge transfer band and enhance the light-harvesting properties, as well as retard the electron recombination between electrons at the TiO₂ and oxidized species of dye or I₃^- in the electrolyte. Under simulated AM 1.5G irradiation, the cell based on the dye derived from 4-(hexyloxy)-N-(4-(hexyloxy) phenyl)-N-phenylaniline produced a short-circuit photocurrent of 14.19 mA cm^-2, an open-circuit photovoltage of 0.462 V, a fill factor of 0.64, corresponding to a power conversion efficiency of 4.20%. Though the power conversion efficiency is still low, the methodology of tuning the HOMO and LUMO energy levels by incorporation of an electron-withdrawing thiadiazolo[3,4-c] pyridine unit in such D-A-π-A dyes could be generally applied to design more efficient panchromatic sensitizers in DSSCs.