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Implications of Supramolecular Templating of Thick Mesoporous Titania Films for Dye-Sensitized Solar Cells

Written by: 
Jeff Morse, PhD.
Zhang, reported on an approach to synthesize thick mesoporous inorganic oxide films in order to harvest more  incident solar radiation, thereby enhancing the efficiency of the solar cell.

Reviewed by Jeff Morse, PhD., National Nanomanufacturing Network

Mesoporous metal oxide thin films have been created by exploiting block-copolymer self assembly approaches to create well-ordered templates. The metal oxide mesopore structure is formed by additive solution chemistry wherein the polymer hydrophilic-hydrophobic blocks self-assemble into a liquid crystal mesophase surrounded by the inorganic oxide network. The mesoporous inorganic oxide structure is formed after subsequent evaporation of solvents, and removal of the organic template by a calcination step. Using this evaporative induced self-assembly (EISA) method, it is possible to fabricate individual mesoporous metal oxide films with  thicknesses on the order of 300nm and pore domain size on the order of 10-15 nm. These resulting structures have device implications for highly ordered, thin film nanocomposites by design, although many applications require further versatility in the resulting structure of the mesoporous inorganic oxide.

Zhang Figure 1a
High resolution SEM image of body-centered orthorhombic mesostructure of the supramolecular templated titania thin film deposited on glass substrate with a projection along the [001] zone axis, upon thermal annealing at 350 æC; inset is the TEM image in cross-sectional view.
In the case of dye sensitized solar cells (DSSC), for example, the mesoporous titania with dye infused within the pore structures would provide the necessary dimensional requirements providing the interface between exciton transport and electron transport medium in order to enhance the effective collection efficiency of photogenerated charge, thereby increasing the internal quantum efficiency. On the other hand, the photon absorption distance is much greater than the mesoporous titania film thicknesses that have been previously demonstrated, severely limiting the overall quantum efficiency.

Recently, Zhang, reported on an approach to synthesize much thicker mesoporous titania films in order to harvest more of the incident solar radiation, thereby enhancing the efficiency of the solar cell. In their study, the authors investigated the combination of EISA with layer-by-layer deposition in order to build the mesoporous titania structure up to thicknesses on the order of several microns, better matching the photon absorption length. The challenge in this approach is to maintain the accessibility of the dye to the titania pore structure to maintain a continuous interface between the absorbing dye material and the electron conducting titania. Additional challenges include maintaining the pore domain characteristics and minimizing the stress formed in the film with increasing thickness in order to prevent cracking or delamination from occurring.

Zhang Figure 2
(a) Variation of the film thickness with the spin coating cycles. (b) Cross-sectional SEM micrograph of the multilayered mesoporous titania films of 2.56 µm thick. Scale bar: 2 µm.
The authors described a modified EISA pathway to synthesize the thick mesoporous films. Each layer was spin coated to provide highly uniform and repeatable pore structures. Additionally, after each spin coating step the film was cured on a hot plate at 300°C for 15 min, which proved crucial for maintaining the desired pore structure and accessibility. The cure step resulted in the incomplete removal of the organic templating molecules that effectively support the porous structure and maintain pore accessibility, further ensuring the required uptake of the dye within the mesoporous structure. DSSC devices were fabricated by depositing thick mesoporous titania films on a transparent ITO electrode, and the dye was adsorbed into the pores by a simple immersion process. The final cell was assembled employing a Pt counter electrode and electrolyte solution sandwiched between the electrodes. Standard solar illumination testing demonstrated a nearly constant open circuit voltage for cells having different titania film thickness, with increasing short circuit current, hence conversion efficiency, correlated with the increased film thickness. The efficiency increase with film thickness began to roll off for films >3.5 µm as cracking became evident in the mesoporous films, resulting in degradation of charge transport and collection.

The authors describe an innovative method to form thick mesoporous inorganic oxide films. The approach yields uniform and repeatable pore structures via a layer-by-layer deposition process. Further reduction in film stress with increased thickness could potentially mitigate cracking and delamination effects, thereby providing a unique approach to create mesoporous superstructures for a wider range of applications.

Images reproduced with permission from Zhang Y, et. al. 2009. Highly Efficient Dye-Sensitized Solar Cells of Thick Mesoporous Titania Films Derived from Supramolecular Templating. Nanotechnology 20 (505602). DOI: 10.1088/0957-4484/20/50/505602.

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