Structure and Principle

A dye-sensitized solar cell consists of an electrolyte sandwiched between a photoelectrode and a catalytic-electrode (counter electrode). The photoelectrode in this case, is a conductive glass plate coated with porous titanium dioxide to create a layer which can adsorb photosensitive dye molecules. Light energy absorbed in the dye is converted to electricity via solar cell electrochemical properties. In nature, light absorbed by pigment molecules is converted into a different form of energy through photosynthesis. Photosynthesis occurs when chloroplast pigments in leaves convert light energy into chemical energy, which is then used to produce carbohydrates. Because dye-sensitized solar cells also extract electrical energy from light absorbed by pigment molecules (dye), they are also known as "pseudo-photosynthetic solar cells."

Sony's Unique "Concerto Effect" Realizes Highest Standard for Photovoltaic Performance

The following four illustrate some of the methods currently used to enhance photovoltaic performance of dye-sensitized solar cells.

• 1. Dye molecules can be modified to increase the range of wavelengths across which light can be absorbed.
• 2. Light absorption can also be enhanced by increasing the amount of dye molecules adsorbed to the titanium dioxide surface.
• 3. Internal resistance can be reduced by modifying the electrode structure and electrolyte.
• 4. The efficiency of the solar cell module itself can also be enhanced by increasing the aperture ratio.

Sony uses all of these approaches to enhance photovoltaic performance. It has also developed its own unique approach, known as the "Concerto Effect." As described below, this method achieves two goals: 1. It expands the light absorption wavelength range and 2. It ensures effective dye molecule adsorption.

Different dyes absorb light at different wavelengths. By mixing two dyes with different absorption wavelengths, Sony was able to expand the light absorption wavelength range. At the same time, it improved the efficiency by which absorbed light is converted into electricity by increasing the dye molecule adsorption density on the titanium dioxide surface. The combined performance of the two dyes was greater than the sum of their individual performance levels. Because the dyes seemed to resonate together to produce an enhanced effect, Sony dubbed this method the "Concerto Effect."

The dye mixture used to create the Concerto Effect consists of two dyes: black dye and D131. When black dye is used alone, its molecules tend to cluster together to form aggregates. This prevents efficient photoelectric conversion by slowing the passage of electrons from the dye molecules to the electrode, and by hindering the flow of electrons from the dye molecules into the titanium dioxide. However, when black dye and D131 are mixed together, the D131 molecules prevent the black dye from forming aggregates and both dyes are adsorbed effectively while avoiding aggregate formation.

In many cases, dye mixtures reduce rather than improve photovoltaic performance because they fail to provide effective routes for carrying electrons to the electrode. However, the mixture used in creating Sony's Concerto Effect provides highly-efficient routes for transporting energy from the light absorbed by each dye to the electrode. In addition to increasing the light absorption wavelength range, this approach also improved the overall efficiency with which absorbed light was converted into electricity. As a result, Sony's prototype dye-sensitized solar cell module set a new world record* with a photovoltaic conversion efficiency of 8.4%.