Molecular Scale Characterization of the Titania−Dye−Solvent Interface in Dye-Sensitized Solar Cells

Abstract

Charge separation at the dye/titania interface in dye sensitized solar cells is strongly influenced by the thickness and homogeneity of the sensitizing dye layer, as this controls the potential drop across the interface, and the probability of an excited electron being transferred from the dye to the titania. In this study we use atomic force microscopy and the depth profiling method neutral impact collision ion scattering spectroscopy (NICISS) to investigate the thickness and homogeneity of N719 dye adsorbed to titania before and after rinsing with pure acetonitrile. Both experimental methods show that the dye layers are closed but inhomogeneous. Inhomogeneity is more pronounced for unrinsed samples.

In Situ Mapping of the Molecular Arrangement of Amphiphilic Dye Molecules at the TiO2 Surface of Dye-Sensitized Solar Cells

“…On both, flat and mesoporous TiO 2 , we consistently observe monolayers of Z907, not agglomerations, as earlier AFM reports of lower resolution suggest. 19 We observed dye agglomerations only upon special sample preparation procedures, e.g. when the rinsing of the sample was omitted.…”

“…17,18 Ex-situ atomic force microscopy (AFM) results indicated the existence of large dye aggregates on the flat TiO 2 substrate for standard device preparation procedure, but molecular resolution was not achieved. 19 Recently developments in the field of AFM have made it possible to achieve sub-nanometer mapping of soft and hard surfaces in solution paving the way for in-situ local observations of a functional DSC's surface. [20][21][22] Here we report in situ molecular-level AFM images of adsorbed dye molecules at mesoporous and flat TiO 2 surfaces in a device-relevant liquid.…”

Nanoparticle/Dye Interface Optimization in Dye‐Sensitized Solar Cells

“…Theoretical values for anatase TiO 2 range from 0.77 to 1.16 molecules nm −2 depending upon the particular anchoring mechanism assumed for the dye 38. For the N719 dye on anatase TiO 2 , experimental surface coverage values of 0.31 molecules nm −2 and 0.26 molecule nm −2 have been reported,39, 40 while density functional theory yields an optimized packing density of 0.744 molecules nm −2 for the dye bound with two carboxylate groups on the oxide surface 41. In both cases, the experimental values are found to be significantly lower than the theoretical maximum which supports the notion that higher packing densities can be achieved.…”

Nano-TiO2 for Solar Cells and Photocatalytic Water Splitting: Scientific and Technological Challenges for Commercialization

“…While electrons are injected into the conduction band of the semiconductor, holes are left behind in the dye (oxidized state) which is rapidly regenerated (reduced) by the electrolyte, thus preventing back transfer of electrons. Electrons travel through the semiconductor to the transparent electrode and the external load to finally reach the counter electrode where they regenerate the electrolyte [20,[42][43][44][45].…”

“…Obviously, the dye molecule should bear suitable anchoring groups to strongly attach to the TiO 2 surface [66,81] while forming a monolayer to maximize the injection of the photoelectrons in the semiconductor. Indeed, the efficiency of the DSSC is strongly influenced by the thickness, the morphology and the homogeneity of the dye layer [43] and it has been proved that aggregates, even as small as dimers, dramatically reduce the photocurrent efficiency [83]. Therefore, the anchoring groups play an important role not only in the overall performance of the cell but also in its long-term stability [84].…”