Algae are dependent on photosynthesis, just as plants are. Photons are spent to create organic compounds and oxygen from carbon dioxide, essential for the life of algae. Diatoms, a major group of algae and one of the most common types of phytoplankton, are responsible for nearly a quarter of the oxygen we breathe. Evolution has shaped algae to become optimally efficient at what they do best harnessing the energy in sunlight.
From 100,000 different species a small team of researchers from NTNU and SINTEF chose Coscinodiscus wailesii, an unicellular diatom with extraordinary photosynthetic capabilities, to study how its sunlight conversion capabilities can be harnessed for solar cells, which at best, can convert around 30% of the sunlight they are exposed to. This particular species was one of a number initially selected as having the optimal cell structure for photosynthesis.
With electron microscopes, the team found the algae has a surface designed to let as much light into its structure as possible and keep it there. It was discovered diatoms are endowed with a glass-like multi-layered silicon shell, and that symmetrical patterns and tiny pores create cylinders where sunlight is trapped in the organism.
Nanotechnology is being used to make copies of these algae shells, and a range of different materials is currently being tested to make molds. The flexible characteristic of gold makes it one of the promising candidates: a lump of gold is exposed to an electron beam and single gold atoms evaporate onto the surface of the algae shells, thus producing a golden thin film replicate.
The physics behind how algae absorb sunlight are not yet fully understood, however. This is where computer simulation comes in. The structural elements are varied in size, and complex algorithms are applied to the new models in order to uncover the secrets of algae and pinpoint what characteristics produce high efficiency rates.
Dye-sensitized solar cells
The pivotal question of the research is, how this knowledge can be applied to create the next-generation of solar cells which are more efficient and less expensive. One of the most promising approaches to date is to coat the algae onto dye-sensitized solar cells.
Dye-sensitized solar cells belong to the group of thin film solar cells. They are not based on P-N junction semiconductors, like most conventional solar cells. Molecular dye is attached to a layer of titanium dioxide on a glass or plastic substrate. The dye then absorbs photons when exposed to light, which excites the electrons to flow into the titanium and generate an electrical current.
Although silicon cells have about twice the efficiency, dye-sensitized solar cells are showing promising results in low-light conditions. The materials they are based on are also cheaper and healthier for the environment.
Instead of the outer surface of a dye-sensitized solar cell simply being a flat layer, it could be coated with algae. After the small diatoms are allowed to settle on the transparent conductive glass surface, the living organic material could then be removed, leaving behind the skeleton shells, which have the nanostructure needed to form a template. A thin film of titanium dioxide nanoparticles laid on top would serve as the semiconductor.
The tiny holes in diatom shells traps the photons inside the structure, which increases the interaction with the dye. This promotes the conversion of light to electricity and improves energy production. Although the Norwegian research team has not released any laboratory data describing performance, Engineers at Oregon State University have tripled the electrical output of dye-sensitized solar cells with algae.
It takes vast amounts of seawater to produce the right algae for solar cell production. On top of this, natural seawater is unstable to work with due to varying biological activity. A promising alternative is to add a mixture of nutrients to distilled water.
Carbon dioxide, nitrogen, phosphorus, zinc, vitamins and certain trace elements are put together in a number of different trial mixtures, reviewed one by one to figure out the most potent formula.
The researchers have found that if growth takes place in the presence of nickel sulphate, the pores become much larger. The influence of environmental conditions on the physical properties of the shell structures could be exploited for particular applications.
Being inspired by nature seems to be increasingly relevant in photovoltaics. The research team is confident that it should be able to create solar cells that are just as good as algae at converting sunlight in the future the question is when?