Abstract
Here we report the first theoretical characterization of the interface between the CuGaO2 delafossite oxide and the carboxylic (–COOH) and phosphonic acid (–PO3H2) anchoring groups. The promising use of delafossites as effective alternative to nickel oxide in p-type DSSC is still limited by practical difficulties in sensitizing the delafossite surface. Thus, this work provides atomistic insights on the structure and energetics of all the possible interactions between the anchoring functional groups and the CuGaO2 surface species, including the effects of the Mg doping and of the solvent medium. Our results highlight the presence of a strong selectivity toward the monodentate binding mode on surface Ga atoms for both the carboxylic and phosphonic acid groups. Since the binding modes have a strong influence on the hole injection thermodynamics, these findings have direct implications for further development of delafossite based p-type DSSCs.
Highlights
The increasing world energy demands have boosted research toward the development of technologies that can exploit renewable sources in an efficient way (Hagfeldt et al, 2010)
An open issue is whether the dye-anchoring groups that are used for binding the dye to the rocksalt NiO surfaces are good for a stable and irreversible binding to the Cu-based delafossite oxide most exposed surfaces. To address this specific issue related to the dye-electrode interface in delafossitebased p-dye-sensitized solar cells (DSSC), we present here a first-principles study on two anchoring groups on CuGaO2 (001) surface: carboxylic acid (– COOH), which is the most common anchoring group used in both n- and p-type DSSCs, and phosphonic acid (–PO3H2), which is one of the anchoring groups explored in NiO-based p-type DSSCs in search for higher efficiencies (Pellegrin et al, 2011; Klein et al, 2018) and guarantees more stability in aqueous environments where COOH tends to desorb from oxide surfaces (De Angelis et al, 2011; Galliano et al, 2017)
In p-type DSSCs, a first key parameter to assess the suitability of a p-semiconductor is the absolute position of its VB edge with respect to the electrolyte redox potential: the difference between these two values determines the open circuit potential VOC of the cell (Qin et al, 2008; Preat et al, 2011)
Summary
The increasing world energy demands have boosted research toward the development of technologies that can exploit renewable sources in an efficient way (Hagfeldt et al, 2010). We perform state-of-the-art DFT-based periodic calculations on anchoring groups –COOH and –PO3H2 on delafossite CuGaO2, which has been proposed as the most convenient for DSSC applications thanks to its wide band gap and easier p-type doping with respect to other delafossites such as CuAlO2 or CuCrO2 (Schiavo et al, 2018).
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