Uniaxial Freezing, Freeze‐Drying, and Anodization for Aligned Pore Structure in Dye‐Sensitized Solar Cells

A variety of methods are available for creating the titanium dioxide (TiO2) semiconductor surface layer of dye-sensitized solar cells (DSSCs); however, many of them are used independently to create surface morphologies that are influenced by only one process. A series of experimental techniques are utilized, some not originally used for thin film preparation, to create a semiconductor surface that exhibits variations in morphology on the macro-, micro-, and nanoscales. The techniques used to create the micro- and nanostructures are uniaxial freezing, freeze-drying, and anodization or etching, combined with the macrostructural techniques of the doctor blade method, screen printing, and/or electrophoretic deposition. When several of these techniques are used together to create, and modify, a thin film for DSSC, these techniques can produce a TiO2 semiconductor layer for DSSC that has very high current and voltage characteristics, and a surface morphology more complex than can be created by using any one of the techniques alone. YE-SENSITIZED solar cells (DSSCs) are comprised of three major and two minor components: the semiconductor base layer, the dye, the electrolyte, and the two electrodes, respectively. Titanium dioxide (TiO2) plays a crucial role as the semiconductor base layer material, and its main structural function is to provide a substrate for organic dye attachment, and also to provide a barrier between the charge transfer electrolyte and the conducting glass anode (to prevent charge recombination— where electrons return to the electrolyte instead of traveling to the electrode). Electrochemically, the TiO2 semiconductor accepts electrons injected by the organic dye, and transfers them to the conducting glass anode, with a negligible loss of energy. 1 Other semiconductors of similar nature (ZnO, etc.) can be used, but TiO2 is the focus of most current research. This is due to the close match of the band gap of TiO2 to energy of the electrons emitted by many of the metallo-organic dyes that are readily available on the market, and the ability of these dyes to attach with chelation. 2 Many methods are used to produce the TiO2 surface for DSSCs, including electrophoretic deposition (EPD), screen printing, and doctor blade deposition. The goal of each of these methods is to produce a semiconductor layer whose surface maximizes the amount of dye that can attach, while minimizing the recombination losses. The doctor blade technique was the first technique 3 used to create the semiconductor band gap layer of DSSCs, and is the least sophisticated. The technique consists of using a flat, sharp object (like a razor blade) or a round, thin object (like a glass stirrer) to lay a thin layer of slurry, with a thickness determined by a spacer. The simplest spacer is adhesive tape, which is between 8 and 10 mm thick on average, 4 and it is placed on opposite sides of the area where the film is to be laid, and the doctor blade is dragged across. The surface structure produced by this method is flat, with variations based on rolling technique, flatness of the spacing material, and position of the blade during the process. One of the drawbacks to this method is that there is limited control over the properties of the final surface and its microstructure, and there is no direct way to manipulate the morphology at the nanoscale. Another drawback is that the thickness of the film is directly related to the thickness of the spacers, and it is difficult to find appropriate spacing material with the desired thickness of 1–7 mm. 5 Overall, the doctor blade method is rather limited to produce the complex surface needed for DSSC to maximize dye attachment.