Dye-Anchoring Modes at the Dye···TiO₂ Interface of N3- and N749-Sensitized Solar Cells Revealed by Glancing-Angle Pair Distribution Function Analysis

Abstract

The efficient transport of electrons from the sunlight-harvesting dye molecules into the electrical circuit of a dye-sensitized solar cell (DSSC) is imperative to its effective operation. A dye···semiconductor interface comprises the working electrode of a DSSC. Dye molecules adsorb onto the semiconductor surface, whereupon they transfer electronic charge into the conduction band of the semiconductor; this process initiates the electrical circuit. It is therefore important to characterize this interfacial structure in order to understand how efficiently the dye binds, or anchors, onto the semiconductor surface and imparts charge transfer to it. Armed with such knowledge, the performance of DSSCs may then be improved systematically. The structural determination of a thin-film interface is nonetheless a challenging task. We herein report the results of a glancing-angle pair distribution function (gaPDF) experiment that generated synchrotron X-ray diffraction patterns of DSSC working electrodes sensitized by the archetypal ruthenium-based DSSC dye complexes N3 and N749. This gaPDF experimental approach represents the first diffraction-based strategy for the characterization of intact DSSC working electrodes. The gaPDF structural signatures were compared with PDFs simulated from possible interfacial structures that were computed using density functional theory. The differences between the experimental observation and these simulated structures revealed a preference for each dye, N3 and N749, to adopt a bidentate-bridging dye anchoring mode when sensitized onto TiO2. Our results also suggest that this anchoring mode is sometimes supported by an auxiliary anchor, in the form of a monodentate carboxylic acid. This work not only demonstrates the successful application of a gaPDF method to DSSC research, but it also advocates the applicability of a gaPDF to many types of thin-film samples.

Electrolyte/Dye/TiO2 Interfacial Structures of Dye-Sensitized Solar Cells Revealed by In Situ Neutron Reflectometry with Contrast Matching

“…Furthermore, we can be confident of the results since the use of contrast-matched and non-contrast-matched data makes the co-realized by DFT studies. 24,46 The increase in dye-layer thickness for N749, from 10.9 to 12.5 Å upon I2 addition, suggests that the molecular orientation of this dye likewise reorients considerably from an anchoring mode akin to that shown in Figure 1b(ii)(2) toward the configuration shown in Figure 1b(ii)(3); however, the tilting of the dye toward the TiO2 is perhaps not quite sufficient to impart more than one carboxylate anchoring group onto the TiO2 surface: cf. the thickness observed is 0.3 Å larger than that of the lateral height displayed in Figure 1b(ii) (3).…”

“…A myriad of ex situ materials characterization methods has been employed to determine the interfacial structure of DSC working electrodes outside of the device, including imaging (atomic force microscopy, − scanning tunneling microscopy (STM) , ), X-ray scattering (grazing-incidence X-ray scattering, X-ray reflectometry (XRR) ,− ), X-ray diffraction, − and optical, vibrational (infrared (IR) and Raman), − X-ray absorption, − and photoelectron spectroscopy. , These studies have managed to quantify the essence of dye/TiO 2 interfacial structures that represent an exposed DSC working electrode. However, none of them have been able to directly probe this dye/TiO 2 interfacial structure while it is held within its DSC device assembly.…”

Extracting interface correlations from the pair distribution function of composite materials

“…[19][20][21][22] By convention, the PDFs of complex mixtures are usually interpreted as linear sums of the individual component PDFs; 23 such is the basis of the widely-used 'differential PDF' approach, for example. [24][25][26][27][28][29] Yet this interpretation remains an approximation because it assumes the interface contribution is weak and incoherent-and so can be neglected.…”

Extracting Interface Correlations from the Pair Distribution Function of Composite Materials

“…[19][20][21][22] By convention, the PDFs of complex mixtures are usually interpreted as linear sums of the individual component PDFs; 23 such is the basis of the widely-used 'differential PDF' approach, for example. [24][25][26][27][28] Yet this interpretation remains an approximation because it assumes the interface contribution is weak and incoherent-and so can be neglected.…”