Dye Anchoring on CuCrO₂ Surfaces for p-Type Dye-Sensitized Solar Cell Applications: An Ab Initio Study

Abstract

The possibility of stably anchoring dye molecules on the exposed surface of a p-type semiconductor is crucial to have efficient dye-sensitized photoelectrodes. Here, we theoretically characterize the adsorption mechanism of carboxylic and phosphonic anchoring groups onto the (012) surface of stoichiometric and reduced CuCrO 2 delafossite. Density Functional Theory is employed to accurately predict the preferred adsorption modes and their energies, both in gas phase and solution (water and acetonitrile). On the stoichiometric (012) surface, we found a strong selectivity toward the molecular monodentate binding modes at the highly active Cr-sites, stabilized by strong Hydrogen-bonds with the surface oxygens, for both anchoring groups; deprotonated bidentate bridging anchoring is only identified when the proton transferred to the surface is kept far away from the molecule during the structural relaxation process. On the other hand, the bidentate anchoring becomes the preferred adsorption mode when Cu + vacancies are considered at the topmost layer of the surface slab. The identification of stable bidentate bridging anchoring modes on the CuCrO 2 surface might have important implications for the device stability, as well as for the efficiency of the interfacial hole injection and suggest it as an alternative material to NiO for p-type DSSCs.

Defining the Role of Cr3+ as a Reductant in the Hydrothermal Synthesis of CuCrO2 Delafossite

“…1−6 The examples of p-type TOSs include NiO and the family of CuMO 2 delafossites, of which CuCrO 2 has been applied as a hole transport layer in various solar cells in addition to solar fuel devices such as dye-sensitized photoelectrosynthesis cells. 7,8 Previous studies have pointed to the inferior performance for p-type TOSs in comparison to their n-type counterparts, exhibiting short hole diffusion lengths which impede charge extraction. 9,10 It has been proposed that this is due to the electronic defects which result in poor charge separation at the metal oxide interface.…”

“…Transparent oxide semiconductors (TOSs), named for both their semiconducting properties and their transparency to visible light due to having a wide band gap, are often used in heterojunction solar cells as charge transport layers. TiO 2 , SnO 2 , and ZnO are primary examples of n-type TOSs which have been used as electron transport layers in dye-sensitized solar cells, quantum dot solar cells, organic photovoltaics, and perovskite solar cells. − The examples of p-type TOSs include NiO and the family of CuMO 2 delafossites, of which CuCrO 2 has been applied as a hole transport layer in various solar cells in addition to solar fuel devices such as dye-sensitized photoelectrosynthesis cells. , …”

First-Principles Modeling of Dye Anchoring on (001) γ-Monoclinic WO3 Surfaces: The Role of Oxygen Vacancies

“…We note that similar stabilization of bidentate coordination on defective surfaces was also recently reported for delafossite CuCrO 2 . 77 The corresponding structural features are illustrated in Figure S4b. Finally, when the molecule is fully deprotonated, contrary to the TiO 2 and NiO case, no tridentate coordination is possible due to unfavorable disposition of coordinative W atoms on the surface, and only a much less stable bidentate anchoring was found (BB 2D ′ a ), with an adsorption energy of −0.79 eV.…”

“…Based on the present results, both BA and PA anchoring groups are preferably adsorbed in the molecular monodentate mode on clean surfaces, while the mono-deprotonated bidentate bridging binding mode becomes the preferred configuration on the defective surface, in line with the findings reported by some of us for p-type (012) CuCrO 2 surfaces. 77 In the case of the Cat molecule, favoring in all cases a flat orientation with respect to the surface plane, two configurations (BH−flat and M 1D −flat) were found to be very close in energy in solution on clean surfaces, while it coordinates in the mono-deprotonated monodentate mode on defective surfaces. A summary of the most stable adsorption energies, in the implicit solvent, of the considered anchoring groups, on both clean and defective surfaces, is displayed in Figure 6.…”

“…This brief overview, clearly, reveals a lack of fundamental understanding of the interaction mechanism of traditional anchoring groups with clean and defective WO 3 substrates, that would definitely require a more systematic investigation. To this respect, quantum mechanical calculations have been widely applied to study the interface phenomena in molecule-functionalized semiconductors − ,− and are a valuable tool to provide atomistic insights on dyes’ anchoring group interactions with the metal oxide surface. However, again, while a large number of theoretical studies on the dye adsorption modes on the TiO 2 surface have been reported (see, for instance, refs and and references therein), to the best of our knowledge, the theoretical characterization of the adsorption mechanism of common anchoring groups on WO 3 surfaces has not been afforded yet.…”