TiO2 sensitization

Liu, J. Kuo, Y.-T. Klabunde, K.). Rochford, C. Wu, J. Li,)., Novel dye-sensitized solar cell architecture using Tio2-coated vertically aligned carbon nanofiber arrays. Acs. Appl. Mater. Interfaces 2009,1,1645-1649. 

Briefly, we describe here various steps involved in the preparation of dye-sensitized solar cell. The nanocrystalline TiO2 prepared by depositing TiO2... 

Based on extensive screening of hundreds of ruthenium complexes, we discovered that the sensitizer excited-state oxidation potential should be negative and at least - 0.9 V versus saturated calomel electrode (SCE), in order to inject electrons efficiently onto the TiO2 conduction band. The ground-state oxidation potential should be about 0.5 V versus SCE, in order to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or an hole conductor. A significant decrease in electron-injection efficiencies will occur if the excited-and ground-state redox potentials are lower than these values. 

In search of new sensitizers that absorb strongly in the visible region of the spectrum, Arakawa et al. have developed several sensitizers based on 1,10-phenanthroline ligands. Among these new classes of compounds, complexes 29 and 30 are noteworthy they show an intense and a broad MLCT absorption band at 525 nm in ethanol. The energy levels of the LUMO and HOMO for 29 were estimated to be —1.02 and 0.89 V versus SCE, respectively, which are slightly more positive than the sensitizer 22. When anchored onto the TiO2 surface, these sensitizers yield more than 75% photon-to-electron-injection efficiencies [15,47,48]. 

Scheme 1 illustration of the interfacial charge-transfer processes in nanocrystalline dye sensitized solar cell. S, S, and S represents the sensitizer in the ground, oxidized, and excited states, respectively. Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of TiO2. The oxidized sensitizer (3) is reduced by the I /I redox couple (4). The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with oxidized dye molecule (6). 

Scheme 2 Pictorial representation of blocking of the oxidized redox couple reaching onto surface of TiO2 for conduction band electrons using hydrophobic sensitizers, which forms an aliphatic net work.

Figure 13 Photocurrent action spectra of nanocrystalline TiO2 films sensitized by complexes 22. 56. and 8. The incident photon-to-current conversion efficiency is plotted as a function of wavelength.

The performance of the three sensitizers 22,8 and 56, which contain different degrees of protonation were studied on nanocrystalline TiO2 electrodes [80]. Figure 13 show the photocurrent action spectra obtained with a monolayer of these complexes coated on TiO2 films. 

Reisner, E., Powell, D.J., Cavazza, C., Fontecilla-Camps,, C., and Armstrong, F.A. (2009) Visible light-driven H2 production by hydrogenases attached to dye-sensitized TiO2 nanoparticles. Journal of the American Chemical Society, 131 (51), 18457-18466. 

Fig. S Schematic energy band diagram of suitable and unsuitable metal oxides for organic dyes as sensitizers. Election injection from the dye-excited state to the energy level of conduction band (CB) is energetically favorable for wide bandgap oxides such as TIO2 and ZnO [261, 264], but forbidden for large bandgap oxides such as ZrOz [461].

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