Dye Sensitized Solar Cells (DSSC)

by Satoshi Ookubo, Nikkei Electronics, Feb 27, 2008
Source: TechOn
http://techon.nikkeibp.co.jp/english/NEWS_EN/20080227/148136/

Tsutomu Miyasaka, president of Peccell and a professor at Toin University of Yokohama holding the new dye-sensitized solar cell module. The module was obtained by connecting eight pieces of 10cm square panels with six embedded cells. The 2.1 x 0.8m solar cell module is reportedly composed of an array of these 10cm square panels.

Peccell Technologies Inc prototyped a large-size solar cell module measuring 2.1 x 0.8m. The company developed a transparent conductive dye-sensitized solar cell using a plastic substrate in collaboration with Fujimori Kogyo Co Ltd and Showa Denko KK.

With the use of plastic substrate, the 0.5mm thick module is flexible and weighs only 800g/m2. According to Peccell, it is the lightest solar cell and the largest in the world for a dye-sensitized cell. The module conversion efficiency is expected to be approximately 3%. When installed indoors, the output voltage of the module exceeds 100V.

The module is on display at the 1st International Photovoltaic Power Generation Expo (PV EXPO 2008) from Feb 27-28, 2008. Peccell is planning to mass-produce dye-sensitized solar cells based on the latest technology from fiscal 2009. The company hopes to "expand the annual production scale to 1MW" by 2010, according to Tsutomu Miyasaka, president of Peccell and a professor at Toin University of Yokohama.

The cell lasts for 5,000-6,000 hours with the sealing treatment applied on the plastic substrate surface (when operated continuously at +50 to +60°C). If it is used normally and not operated continuously, the cell can last for five years, the company said. The company reportedly aims to develop a product with a module conversion efficiency of 8% and a generation cost of ¥120/W at a price of ¥10,000/m2.

The heart of the cell made by a printing process
The dye-sensitized solar cell has the following structure. First, a transparent conductive film is formed on the surface of the substrate. Next, a porous film consisting of TiO2 particles is formed on the conductive film. Then, another substrate, which serves as a common electrode, with a conductive film formed on the surface is placed on the porous film.

A dye is attached to the porous film surface and the gap between the porous film and the substrate with the transparent conductive film is filled with an electrolyte. When the dye is excited by light entering the solar cell, electrons come out of the dye and migrate into the TiO2 film and then to the conductive film.

The electron-depleted dye receives, in turn, electrons from the electrolyte. In this way, the electromotive force of approximately 0.7V per cell is generated. The cells are connected in series to obtain a higher output voltage.

The company improved the porous film formation technology and the transparent conductive film to increase the module size. Using a new paste that includes fine TiO2 particles with a diameter of only 60nm, the company succeeded in forming a porous film by coating based on a printing process at room temperature and drying.

According to Peccell, the process eliminates the need for a vacuum apparatus and the baking process at high temperatures, which are necessary for the pervasive method, thereby facilitating the production of solar cells.

Moreover, an aqueous sol containing colloidal TiO2 with a size of 10-35nm is dispersed in the paste for the bonding between the particles. After the paste dries, the sol improves the bonding between the TiO2 particles constituting the porous film to enhance the flow of electric charge.

This improves the adhesion between the TiO2 particles and the plastic film, thereby preventing the porous film from being detached. The porous film is about 15?m thick.

Resistance at electrodes reduced to 1/40 for larger module

To improve the transparent conductive film, the company formed a metal sheet with the resistance of 0.3? per square on the plastic film. Thus far, ITO has been used for the transparent conductive film of dye-sensitized solar cells. In this case, the sheet resistance reaches 13? per square. Thus, the resistance of the latest conductive film is reduced to about 1/40 of that of the existing film.

With the reduced sheet resistance, the new solar cell can alleviate the degradation of output due to a resistance component in the transparent conductive film. In other words, the output of the cell is less likely to decrease even if the module is enlarged.

The plastic film with the transparent conductive film was jointly developed by Peccell and Fujimori. It was manufactured by the following process. First, a wiring pattern made of a thin Ag film was formed on a plastic film by a photomechanical process with the use of Ag halide. Then, a Ni or Cu film with a thickness of 1-2?m was formed on the Ag film by plating.

The wiring was designed to have a mesh pattern with a wire width of 30?m, for example and a distance of 125?m between the adjacent wires. The film has a light transmittance of 75% or higher. The film can be manufactured by a high-speed production method such as a roll-to-roll process because no vacuum processing is required during the entire procedure.