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Recent research has made dye-sensitized solar cells more efficient. Image credit: ROLAND HERZOG, EPFL
Imagine never having to charge your phone, e-reader or tablet. The researchers reported that the solar cells they developed could use low-intensity diffuse light that is present inside the building and outdoors on the cloudy days, and the work efficiency has set a record. These batteries may one day give birth to a device housing that can continue to charge some gadgets without plugging in.
Diffuse light solar cells are not new, but the best batteries depend on expensive semiconductors. In 1991, Michael Graetzel, a chemist at the Swiss Federal Institute of Technology, invented the so-called dye-sensitized solar cell (DSSC). It performs best in dim light and is less expensive than standard semiconductor components. However, in sunny conditions, the best DSSC can only convert 14% of sunlight's energy into electricity, and standard solar cells can reach about 24%. This is mainly because the energy comes so fast that the DSSC cannot handle it. When energy comes at a slower rate, such as in low-intensity room lighting, Graetzel's DSSC can convert 28% of its absorbed light energy into electricity.
The DSSC still has two electrodes that collect negative and positive charges. But in the middle, they possess a different electronic conductor, usually a collection of titanium dioxide (TiO2) particles, not just silicon. However, TiO2 is a very weak light absorber. To this end, the researchers coated the surface of these particles with organic dye molecules that act as superabsorbents. The absorbed photons excite the electrons and holes on these dye molecules just as they do in silicon. The dye immediately "transfers" the excited electrons to the TiO2 particles, and the electrons move quickly along them to the positive electrode. At the same time, holes are dumped into a conductive liquid called an electrolyte. There, they continue to penetrate and enter negatively charged electrodes.
The problem with DSSC is that holes cannot pass through the electrolyte very quickly. Therefore, they often accumulate near the dye and TiO2 particles. If the excited electrons eventually crash into the hole, they will merge, producing heat instead of electricity.
To solve this problem, the researchers tried to make the electrolyte thin so that the holes could travel to the destination without going too far. However, any defects in these thin layers can cause a fatal blow to the equipment and destroy the entire solar cell. Graetzel and colleagues now propose a possible solution. They designed a combination of dye and hole-conducting molecules. It can wrap tightly around the TiO2 particles to create a tight layer without any defects. This means that the distance traveled by slowly moving cavities before reaching the negative electrode becomes smaller. The researchers reported in the Joule magazine that the tight layer increased the diffuse light efficiency of the DSSC to 32%—close to the theoretical maximum.
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