Shellfish polymer promises greener solar cells

Technology News | August 1, 2016

By Rich Pell

Chitosan is a natural polymer derived from chitin that can be found in shrimps, lobsters and crabs and can be used as the host in polymer electrolytes. By improving its solubility, researchers at the University of Malaysia (Kuala Lumpur, Malaysia) have developed a gel polymer electrolyte based on phthaloylchitosan for dye-sensitized solar cells.

These dye-sensitized solar cells (DSSC) have the potential as a low cost greener alternative to the available silicon cells. While single cyrstal solar cells have reached efficiencies of 30% in the lab and 15-16% in the field, they can be expensive. Polycrystalline and other thin film based solar cells are comparatively cheaper but lower in efficiency, and less expensive amorphous silicon solar cells have lower life time of around three to four years.

A DSSC on the other hand can reach efficiencies of around 8% and is signifcantly chaper to make. It consists of a cheaper wide band semiconductor such as titanium dioxide (TiO2) sensitized with a dye to absorb light, and an electrolyte.

The TiO₂ is deposited on a conducting glass substrate (indium tin oxide, ITO or Fluorine doped tin oxide, FTO) and forms a nanoporous network of particles which increases the surface area for the dye coverage. The counter electrode is another conducting glass coated with a thin layer of platinum and the electrolyte is sandwiched between the working electrode containing the dye sensitized TiO₂ and the electrode.

In the DSSC cell, the photoanode consists of a transparent conducting glass (TCG) onto which is coated with the TiO₂ and soaked in a ruthenium dye. The gel phthaloylchitosan electrolyte that has been added with mediators is sandwiched between the photoanode and the counter electrode, usually platinum.

The dye molecules absorb incident light and become energized. This leads to an electron being released from each dye molecule. The electrons enter and percolate the TiO2 semiconductor and leave the cell to the external circuit through the TCG and finally reach the counter electrode.

At the counter electrode, the mediators capture the electrons and transfer them to the dye molecules that have released electrons after absorbing light. The dye is then replenished. The electron flow is completed and current is produced until the light is switched off.

The researchers showed that the DSSC employing N-phthaloylchitosan gel polymer electrolyte exhibited high power conversion efficiency of 7 – 8 %.