How do transition-metal-substituted POMs modify the photoanode of dye-sensitized solar cells? A DFT study

Abstract

Herein, the electronic structures and absorption spectra of a series of transition-metal-substituted polyoxometalates (TMSPs) were systematically investigated to screen promising candidates for the POM/TiO₂ nanocomposite film used in DSSCs using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) gaps (H–L gaps) of the studied TMSPs were remarkably influenced by the substituted transition metals in different periods. Specifically, the H–L gaps increase as the atomic number of the substituted transition metals increases in the same period such as Mn⁴⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ (the fourth row) and Re⁶⁺ < Os⁶⁺ (the sixth row). Considering that the LUMO energy levels of all the studied TMSPs were higher than the conduction band (CB) of TiO₂ (−4.00 eV), the TMSPs could quickly transport the photoinduced electrons as long as the LUMO energy levels of the used dyes were higher than those of the TMSPs. SiW₁₁X⁴⁻ (X = Mn⁶⁺, Ru⁶⁺ and Os⁶⁺) may perform well in DSSCs using the metal-free organic dye D149 due to the proper LUMO energy levels between the CB of TiO₂ and the LUMO energy of D149 (−2.41 eV). The absorption spectra of SiW₁₁X⁴⁻ (X = Mn⁶⁺, Ru⁶⁺ and Os⁶⁺) exhibit broad absorptions in the UV–vis region that are more conducive to extend the range of light captured; this contributes to enhancing the photo-to-electric conversion efficiency. Moreover, the absorption of SiW₁₁Mn⁴⁻ red-shifts when compared with that of SiW₁₁X⁴⁻ (X = Ru⁶⁺ and Os⁶⁺), which results from the smallest H–L gap; this indicates that SiW₁₁Mn⁴⁻ will be a promising high-performance material-modified photoanode. This study opens new opportunities for the development of potential TiO₂-modified materials for DSSCs.