Photoelectrochemical Properties of J Aggregates of Benzothiazole Merocyanine Dyes on a Nanostructured TiO2Film

Abstract

We studied the photoelectrochemical properties of nanoporous TiO2 electrodes sensitized by a series of benzothiazole merocyanine dyes, 3-carboxyalkyl-5-[2-(3-alkyl-2-benzothiazolinyldene) ethylidene]-2-thioxo-4-thiazolidinone, having a different alkyl chain length. Broadening of absorption spectra and a large red shift of the absorption threshold up to 680 nm were observed by adsorbing dyes on a porous TiO2 electrode, suggesting the formation of various kinds of J aggregates of the dye on the TiO2 surface. The dye was fixed by a chelate-like linkage of the carboxylate anchoring group on the TiO2 surface, rather than by an ester linkage. The solar light-to-power conversion efficiency (ηsun) and the incident photon-to-current conversion efficiency (IPCE) increased with increasing the length of alkyl chain attached to the benzothiazole ring. The maximum ηsun value was 4.5%, which is the second highest value among organic dye systems reported so far (AM-1.5, 100 mW/cm2). The J aggregates of the dyes having long alkyl chains showed an excellent ability for sensitization on the TiO2 electrode, whereas those having short alkyl chains hardly showed the sensitization especially at a light wavelength (λ) of >600 nm. The IPCE decreased significantly with increasing methylene length between the carboxyl anchoring group and the dye chromophore, that is, the conjugated skeleton, suggesting that the distance between the chromophore and the TiO2 surface affected the IPCE. Moreover, a formation of fine packing of the dye chromophores in the J aggregate has proved to be important to obtain a high IPCE. We tested also the stability of the dye in a sealed solar cell under simulated solar light with an UV cut filter. The ηsun and the color of the dye remained unchanged for more than 750 h, and the total turnover number of the dye for the photon-to-electron conversion exceeded 1.6 × 106 cycles, suggesting that the merocyanine dye itself is fundamentally stable during the photoreaction in this system without UV irradiation.