By Kim Tong-hyung
A team of South Korean researchers suggested a new approach that enables dramatic improvement in the low-cost, thin-film solar cells now being developed in laboratories around the world.
The new technique could improve the power conversion rates of dye-sensitized solar cells by more than 50 percent of the current level once commercialization is made.
Dye-sensitized solar cells, a relatively new class of thin-film solar cell, are considered extremely promising as they are easier to manufacture and use cheaper materials than conventional solar technologies. They can also be engineered into flexible sheets, and are mechanically robust, requiring no protection from minor events like hail or tree strikes.
However, improving efficiency has been a major issue. Current lab prototypes of dye-sensitized solar cells convert about one tenth of the incoming sun's energy into electricity, which is about half as efficient as the commercial, silicon-based cells used in electronics devices and rooftop panels.
Thus, extending the range of light absorption has been a key issue, and scientists have been coming up with a variety of surfaces and structures to generate a bigger jolt.
Park Nam-gyu, a researcher from the Korea Institute of Science and Technology (KIST), said his team has discovered a breakthrough method, which enables a sequential bonding of dyes in the thin, titanium dioxide (TiO2) layers used in dye-sensitized solar cells.
``In the existing prototypes, the TiO2 films bond with just one kind of dye, and developing techniques to allow the films to absorb dyes of different colors, thus allowing the solar cell to absorb and use a broader spectrum of daylight, has been a key issue,'' Park said.
``We were the first to achieve this by developing materials in both the mobile and stationary phase that enables the selective position of dye molecules with different absorption ranges. This could significantly improve power conversion rates that currently max at 11 percent.''
Although there are differences between prototypes, the basic design of dye-sensitized solar cells is based on a semiconductor formed between a photosensitized anode and an electrolyte. The cells are made of a porous film of tiny, nanometer-sized TiO2 particles, which are covered with a layer of dye that is in contact with the electrolyte.
When contacted by sunlight, the dye injects a negative charge in the nano-particles and a positive charge into the electrolyte, thus converting the light into electrical energy.
Selective positioning of the dye molecules is critical in boosting power conversion rates, as it enables different absorption ranges in the TiO2 films. To achieve this, Park's team mimicked the concept of the ``stationary phase'' and ``mobile phase'' in chromatography.
In chemistry, chromatography is a method used to separate individual chemical compounds from mixtures. The components to be separated are distributed between two phases, one of which is stationary, while the other, the mobile phase, moves in a definite direction. Column chromatography is a separation technique in which the stationary bed is a tube. The solid particles of the stationary phase fill the inside wall of the tube, leaving an uninterrupted path in the middle for the gas and liquid of the mobile phase.
In using the method to improve dye-sensitized solar cells, Park's team used the porous TiO2 film, filled with polystyrene, as a stationary phase, while developing a Bronsted-based-containing polymer as a mobile phase. By controlling the release and accumulation of the substances, the researchers managed to vertically align yellow, red and green dyes within the TiO2 film, which was confirmed by an electron probe micro-analyzer.