Harrick PlasmaApplicationsDevice FabricationTiO2 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.

Relevant Articles from Harrick Plasma Users

  • Wang J and Lin Z. “Dye-Sensitized TiO2 Nanotube Solar Cells with Markedly Enhanced Performance via Rational Surface Engineering”. Chem. Mater. (2010) 22(2): 579-584.
  • Ye M, Xin X, Lin C and Lin Z. “High Efficiency Dye-Sensitized Solar Cells Based on Hierarchically Structured Nanotubes”. Nano Lett. (2011) 11(8): 3214-3220.
  • Xin X, Scheiner M, Ye M and Lin Z. “Surface-Treated TiO2 Nanoparticles for Dye-Sensitized Solar Cells with Remarkably Enhanced Performance”. Langmuir (2011) 27(23) 14594-14598.

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.

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