La modified TiO2 photoanode and its effect on DSSC performance: A comparative study of doping and surface treatment on deep and surface charge trapping

• DSSC performance is investigated using photoanodes of doped and La3+ surface treated TiO2.
• TiO2 and Mg–TiO2 treated with La3+ absorbed less visible light.
• A high concentration of absorbed oxygen on surface treated oxides reduced band bending.
• Increased surface free energy in the modified DSSC anodes is caused more by Mg2+ at Ti4+ than by La3+ at the surfaces.
• Near surface charge traps due to La3+ treatment promotes e–h recombination.

The effect of Lanthanum ions (La3+) on charge trapping in dye-sensitized solar cell (DSSC) photoanodes has been investigated with doped and surface-treated TiO2 nanoparticles. Doped nanoparticles consisting of 0.5 mol.% Mg and La co-doped TiO2, 0.5 mol.% Mg doped TiO2 and pure TiO2 were synthesized by the sol gel method. Surface-treated nanoparticles of Mg doped TiO2 and pure TiO2 were prepared by ball milling in 0.05 M aqueous La3+ solution. All materials were analyzed by XRD, XPS and UV–Vis DRS. Cell performance, surface free energy state changes and electron injection efficiency of DSSCs based on these nanoparticles were evaluated using current –voltage measurements, EIS and Incident photon to current conversion efficiency. Doped materials had La and Mg ions incorporated into the TiO2 lattice, while no lattice changes were observed for the surface-treated materials. Less visible light was absorbed by treated oxides compared with doped oxide samples. The overall power conversion efficiencies (PCE) of DSSC photoanodes based on doped materials were twice those of photoanodes fabricated from treated nanoparticles. Doping establishes deep traps that reduce the recombination of electron–hole (e–h) pairs. Conversely, the presence of absorbed oxygen in treated materials enhances e–h recombination with electrolyte at surface trap sites.

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