Solvent Effects on the Adsorption Geometry and Electronic Structure of Dye-Sensitized TiO₂: A First-Principles Investigation

Abstract

The performance of dye-sensitized solar cells (DSSCs) depends significantly on the adsorption geometry of the dye on the semiconductor surface. In turn, the stability and geometry of the adsorbed molecules is influenced by the chemical environment at the electrolyte/ dye/TiO 2 interface. To gain insight into the effect of the solvent on the adsorption geometries and electronic properties of dye-sensitized TiO 2 interfaces, we carried out first-principles calculations on organic dyes and solvent (water or acetonitrile) molecules coadsorbed on the (101) surface of anatase TiO 2. Solvent molecules introduce important modifications on the dye adsorption geometry with respect to the geometry calculated in vacuo. In particular, the bonding distance of the dye from the Ti anchoring atoms increases, the adsorption energy decreases, and the two C−O bonds in the carboxylic moieties become more symmetric than in vacuo. Moreover, the adsorbed solvent induces the deprotonation of the dye due to the changing the acid/base properties of the system. Analysis of the electronic structure for the dye-sensitized TiO 2 structures in the presence of coadsorbed solvent molecules shows an upward shift in the TiO 2 conduction band of 0.2 to 0.5 eV (0.5 to 0.8 eV) in water (acetonitrile). A similar shift is calculated for a solvent monolayer on unsensitized TiO 2. The overall picture extracted from our calculations is consistent with an upshift of the conduction band in acetonitrile (2.04 eV vs SCE) relative to water (0.82 eV vs SCE, pH 7), as reported in previous studies on TiO 2 flatband potential (Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 1426−1430) and suggests a relevant role of the solvent in determining the dye− semiconductor interaction and electronic coupling.

Alignment of energy levels in dye/semiconductor interfaces byGWcalculations: Effects due to coadsorption of solvent molecules

“…In this study we have limited our investigation to the first layer of co-adsorbed molecules. Indeed, this is supposed to give most of the effects due to the solvent because of the relatively large adsorption energy per molecule (0.39 eV for acetonitrile and 0.68 eV for water, both at full coverage) [26]. In our simulation we apply periodic boundary conditions simulating a periodic TiO 2 slab with a large density of adsorbed LO dyes.…”

Structural heterogeneity and dynamics of dyes on TiO2: implications for charge transfer across organic–inorganic interfaces

“…It is well known that the adsorption of any charged or dipolar species directly affects the surface dipole, inducing a shift in the surface density of states (DOS) relative to vacuum (i.e., a shift in the work function). [39][40][41] This effect has been noted in several computational studies, mostly concerned with water contamination of dyesemiconductor systems. 24 Specifically, we examine the influence of various conformations with and without solvent, conformational fluctuations, binding motifs, and solvation dynamics on dye-TiO 2 coupling.…”

“…This shift of 1.5 eV is consistent with the range of shifts found previously (0.8-1.5 eV) for monolayer acetonitrile coverage for several dyes. 41 Nevertheless, bulk solvation (i.e., additional layers of solvent) does modestly increase the shift fluctuations yielding significant coupling prior to charge injection. Since the timescale for such a rare fluctuation may be significantly longer than that of the actual electron transfer event itself, one may (in such a case) envision a transition into a solvent-driven Marcus theory picture where the timescales governing electron injection may be much slower than suggested by the electronic coupling strength estimated from calculations without solvent.…”

First-principles calculation of electronic energy level alignment at electrochemical interfaces

“…Choosing this reference energy is a crucial step to compare different systems and to investigate level shifts. The method is a common approach in level-alignment problems 7,11 . In the case of symmetric slabs, the asymptotic potentials are the same in both sides and can be used as a unique energy reference.…”

“…On the other hand, the short-and long-range interactions of the solvent molecules with the semiconductor surface atoms could significantly affect the electronic structure of the semiconductor 7 . Adsorption of solvent molecules on the semiconductor surface results in a net electric dipole (see appendix A), which in turn, causes shifts in the energy positions of the conduction and valance bands at the semiconductor surface 11 . To explore the interrelation between the molecular structure of the solvent and the level alignment at the interface, one has to consider the reasons behind the formation of electric dipole layer at the surface 6 .…”