Enhanced dye-sensitized photoconversion efficiency via reversible production of UV-induced surface states in nanoporous TiO2

Brief UV illumination of dye-sensitized solar cells can result in a remarkable increase in their photoconversion efficiency (Ferrere, S; Greg, B. A. J. Phys. Chem. B 2001. 105, 7602). Further investigation of this phenomenon reveals that a major effect of UV illumination is to reversibly create a high concentration of photoactive surface states continuously distributed below the conduction bandedge in the nanoporous TiO2 films. The ability to create, and then eliminate, surface states allows, for the first time, a clear assessment of the influence of these states on the dye-sensitization process. Positive conduction bandedge (mobility edge) shifts apparently also result from UV illumination, and the difficulties in quantifying such shifts in functioning cells are discussed. We conclude that the major cause of the increased efficiency is the photoproduction of surface states that may improve photoinjection and carrier transport while possibly slowing the recombination rate. The creation of these surface states is strongly inhibited by the presence in the electrolyte solution of the Li+ ion, which is known to specifically adsorb to TiO2 surfaces. We present an in-depth characterization of the UV-induced changes in the dye-sensitized solar cell through comparisons of otherwise identical cells in LiI-containing solution and in tetrabutylammonium iodide-containing solution, before and after UV illumination. The "UV effect" is also observed in hydroquinone/benzoquinone solutions thus, it is not dependent on the presence of the I-/I-2, redox couple. Although surface states are usually deleterious for planar semiconductor electrodes, we show that a high density of surface states may be beneficial for the photoconversion process in nanoporous solar cells.