TiO2 Nanostructures for Solar Cells

Harrick Plasma → Applications → Device Fabrication → TiO2 Nanostructures for Solar Cells

Dye-sensitized solar cells (DSSC) have been heavily investigated as a promising low-cost alternative to silicon-based solar cells. A combination of TiCl₄ chemical and O₂ plasma treatment has been studied to improve the performance of DSSCs based on TiO₂ nanostructures.

In a previous study, Wang et al. assembled a dye-sensitized (ruthenium N-719 organic dye) solar cell using highly ordered, vertically oriented TiO₂ nanotubes as the photoanode. They varied the TiO₂ nanotube film thickness and subjected the nanotubes to a two-step surface modification process to increase dye loading and improve the electronic interaction between the dye and TiO₂ surface, thereby improving the power conversion efficiency (PCE). The ability to promote fast electron generation within the dye along with fast recovery and diffusion of the redox couple in the electrolyte is a key aspect to achieving high PCE.

Following synthesis, the TiO₂ nanotubes were air-annealed at 500ºC to crystallize the TiO₂, immersed in a TiCl₄ aqueous solution, and then treated with O₂ plasma (HIGH RF 30W, 1-20 min). TiCl₄ immersion led to the formation of a thin TiO₂ blocking layer, which reduced the number of exposed structural defects formed during high-temperature annealing and thereby reduced the chance of charge recombination. Furthermore, O₂ plasma increased the hydrophilicity of the nanotube surface, promoting further dye loading and absorption. Plasma exposure also enhanced the bond between dye and TiO₂, which facilitated charge transfer across the dye/electrode interface.

Prolonged plasma exposure (>10 min) did not further improve device performance but instead resulted in lower PCE. While the reason for poorer performance was unclear, the researchers suggested that the surface became less hydrophilic with longer plasma exposure, leading to less dye absorption.

Interestingly, O₂ plasma treatment alone (without TiCl₄ immersion) resulted in lower PCE below that of untreated TiO₂ films for all thicknesses. It is possible that O₂ plasma damaged the TiO₂ surface, creating cracks and defects that can increase electron trap density. This suggests that the TiCl₄ solution step was essential for forming the blocking layer to protect the nanotubes from plasma-induced structural damage.

Once the optimal TiO₂ nanotube film thickness and plasma conditions were found (14 micron and 10 min treatment, respectively), researchers were able to fabricate DSSCs with PCE of 7.37% (backside illumination mode).

In a subsequent paper, Lin’s group further developed the TiO₂ nanotube surface by applying a hydrothermal treatment to induce the formation of TiO₂ nanoparticles on the nanotube surface, thereby creating additional surface roughness and surface area for dye loading. With an additional O₂ plasma treatment (10 min), the resulting device performance improved to yield PCE of 7.75%.  

Finally, DSSCs based on TiO₂ nanoparticles were also fabricated with TiCl₄ immersion followed by O₂ plasma treatment. The large surface area of the nanoparticles (optimized with 21 µm film thickness), combined with the two-step surface modification process, yielded additional improvement of the PCE to 8.35%. These studies demonstrate the importance of structural morphology and surface characteristics in optimizing solar cell performance.

Supplemental References (Do not report using Harrick Plasma instruments)

[1] Grätzel M. “Dye-Sensitized Solar Cells.” J. Photochem. Photobiol. (2003) 4(2): 145-153.

[2] Hagfeldt A, Boschloo G, Sun L, Kloo L and Pettersson H. “Dye-Sensitized Solar Cells.” Chem. Rev. (2010) 110(11): 6595-6663.