DSSC anchoring groups: a surface dependent decision

Abstract

Electrodes in dye sensitised solar cells are typically nanocrystalline anatase TiO2 with a majority (1 0 1) surface exposed. Generally the sensitising dye employs a carboxylic anchoring moiety through which it adheres to the TiO₂ surface. Recent interest in exploiting the properties of differing TiO₂ electrode morphologies, such as rutile nanorods exposing the (1 1 0) surface and anatase electrodes with high percentages of the (0 0 1) surface exposed, begs the question of whether this anchoring strategy is best, irrespective of the majority surface exposed. Here we address this question by presenting density functional theory calculations contrasting the binding properties of two promising anchoring groups, phosphonic acid and boronic acid, to that of carboxylic acid. Anchor-electrode interactions are studied for the prototypical anatase (1 0 1) surface, along with the anatase (0 0 1) and rutile (1 1 0) surfaces. Finally the effect of using these alternative anchoring groups to bind a typical coumarin dye (NKX-2311) to these TiO₂ substrates is examined. Significant differences in the binding properties are found depending on both the anchor and surface, illustrating that the choice of anchor is necessarily dependent upon the surface exposed in the electrode. In particular the boronic acid is found to show the potential to be an excellent anchor choice for electrodes exposing the anatase (0 0 1) surface.

What Makes Hydroxamate a Promising Anchoring Group in Dye-Sensitized Solar Cells? Insights from Theoretical Investigation

“…Furthermore, we find that the formed hydroxyl group involves hydrogen-bonded to surface O 2c with distance d(O 2C -H) of 1.69 Å, which produces the largest adsorption energy of -1.52 eV among the three cases, in reasonable agreement with the other work 21 . Note that the position of the proton does not affect the stability very much, nevertheless the proton prefers to bind to an under-coordinated surface O 2c atom, as previously suggested by Nilsing et al 43 As discussed above, CH bearing hydroxamate has the smallest adsorption energy, compared with CC and CP, but the hydroxamate is more stable to water attack under highly oxidizing conditions from the experimental and applied viewpoints, which makes it preferable for DSSC application.…”

“…2. The relevant bond lengths and adsorption energies are listed in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 adsorption energy is -1.04 eV for this case, in close agreement with the works by O'Rourke et al, 21 Jiao et al, 4 and Nilsing et al 42 . As for CH, its adsorption structure is completely deprotonated with the doubly bound O atom and the deprotonated O atom coordinated to two different Ti 5c atoms, that leads to an adsorption energy of -0.50 eV, which is smaller than CC.…”

“…13,[24][25][26]45 In order to make the comparison of different anchors more direct and consistent, only the bidentate bridging mode is adopted in this study, although some differences may arise for other adsorption modes. 21 Below we present details for the adsorption of these anchoring groups, the optimized adsorption geometries are shown in Fig. 2.…”

What Makes the Photocatalytic CO2 Reduction on N-Doped Ta2O5 Efficient: Insights from Nonadiabatic Molecular Dynamics

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 in contrast to that of COOH is also suggested by the direct calculations of the binding energy, although the effect may depend on the choice of the surface plane. 95 …”

Adsorption of Methyl Acetoacetate at Ni{111}: Experiment and Theory

“…Only the two uppermost Ni layers were fully relaxed, while the two bottom layers were fixed in the bulk positions; this procedure is often found to accelerate the convergence of calculated surface properties with respect to the thickness of the simulation slab. 34,35 The positions of the MAA molecule were also allowed to fully relax. In order to compensate for the use of an asymmetric slab, all simulations included a dipole correction as implemented in VASP, based on a method proposed by Makov and Payne.…”