Theoretical Study of N749 Dyes Anchoring on the (TiO₂)₂₈ Surface in DSSCs and Their Electronic Absorption Properties

Abstract

We have performed calculations on the panchromatic N749 dyes adsorbed on the (TiO₂)₂₈ surface. N749 is a prototypical form of Ru­(II) complexes for dye sensitized solar cells (DSSCs), which possesses a terpyridine tridentate ligand bearing four different protonation states (0, 1, 2, or 3 carboxylic protons). Depending on the type of proton bonding interaction (protonated and deprotonated), seven N749/(TiO₂)₂₈ surface models (N749-0H/(TiO₂)₂₈, N749-1H-P/(TiO₂)₂₈, N749-1H-DP/(TiO₂)₂₈, N749-2H-P/(TiO₂)₂₈, N749-2H-DP/(TiO₂)₂₈, N749-3H-P/(TiO₂)₂₈, and N749-3H-DP/(TiO₂)₂₈) have been applied in this study for the geometry optimization, frontier molecular orbital level diagrams, and calculated absorption spectra. The moderate surface area of the (TiO₂)₂₈ cluster is suitable for N749 dyes adsorbing behaviors so that all calculations can be performed using the Gaussian 09 program package. We have carefully examined these seven N749/(TiO₂)₂₈ assemblies that could influence the DSSC device performance. The calculated absorption spectra of these seven various N749/(TiO₂)₂₈ models are in good agreement with the experimental results by Hagfeldt et al. (<i>J</i>. <i>Phys</i>. <i>Chem</i>. <i>B</i> <b>2002</b>, <i>106</i>, 12693–12704) with onset ranging from the visible to near-IR region. The combination of the adsorption energy onto TiO₂ and calculated absorption spectra (cf. the experimental results) concludes that the deprotonated dyes constitute the most favorable and dominant structure in the DSSC devices. The frontier molecular orbital graphs indicate that the electron charge distributions of all HOMOs are located at the N749 dyes, while LUMOs are localized at the (TiO₂)₂₈ surface or delocalized at the interfacial regions of N749/(TiO₂)₂₈. The corresponding transitions are thus more like a type of optical electron transfer, injecting the electron to the interfacial TiO₂