Dye-sensitized solar cells

Research on the Grätzel cell - the dye-sensitized solar cell (DSC)

Great expectations are set on the third generation solar cells – they have to be relatively cheap, simple to mass-produce, and have high efficiency.

– The solar cell technology we employ is the one devised by a Swiss group about fifteen years ago, when they discovered the Grätzel cell. The method used for making the cells themselves is a very simple one – it is only the chemistry part of it that is high-tech!

The Grätzel cell, named after its inventor Michael Grätzel, is also referred to as the "nanostructured" or "dye-sensitized" solar cell. Its particular feature is that it imitates the same photosynthesis process as green plants employ. A film of inter-connected nano-size particles of titanium oxide (the white colour agent in paint) is dipped into a solution of dye molecules; these are the agents that catch the solar energy and transform it into electricity.

– This discovery in fact occurred by sheer accident; it did work, yet nobody could explain why. When you look at the components of this, it is not easy to understand just how it all works – but it does result in an efficient and reliable solar cell. We do know a little more today, yes, but some of the detail questions still remain unsolved.

Solar cells with a potential

Certain forecasts exist which predict that the global energy consumption will be doubled by 2050. Electricity directly generated by solar energy may well become an important factor in the future, especially so in countries with plenty of sunshine and high current price levels on energy. Satellites, lighthouses and weather observation posts are some of the places where solar cells are found today, i e locations that lack connection with a distribution grid. In developing countries they may also be employed to power TV and radio sets. In addition, they may be used in toys and wireless electronics, thus replacing battery power. Today the global production of solar cells is growing by some 50% a year.

Three generations of cells may be distinguished: Today´s solar cell is of the silicon technology type, representing a reliable power source. These are supplied mainly in the form of solar panels, and the makers certify that they will last for twenty years. Their efficiency is some 13%. However, the production cost of these is far too high.

The second-generation cell is of the so-called thin-film solar cell type. Technically they are just as good as the silicon ones, yet their cost is expected to be just one-fifth of the silicon cells.

Nanostructured solar cells are what we refer to as the third generation. These have not yet left the design/research stage, thus many makers want to be first when it comes to commercial manufacture of them. The present top figure for efficiency in these, so far, is 11%.

The Boschloo research team (previously lead by Anders Hagfeldt) is one of the leaders in the field, internationally speaking. They are co-operating with both Grätzel in Switzerland and different units at KTH, and also with Swerea IVF AB, under the auspices of the Center for Molecular Devices (CMD), subordinate to KTH Chemistry. Here Swerea IVF's responsibility is process development and design of test cells and modules, whereas KTH handles development of components, cell characterisation and metrologies.

Unique future possibilities

In principle, solar cells are working like a battery, with a positive terminal/pole and a negative one. To obtain electricity from sunshine, transparent contact with a layer that can conduct electric current is required. In this case the negative pole consists of a piece of glass covered by a conductive layer, a porous film of particles of titanium oxide as well as a dye. The positive pole consists of the same type of glass, but this is instead covered by a porous layer of carbon or platinum. The two glass pieces are then put together, with an electrolyte in between; this carries the current and creates a charge between the two poles.

- We still have some way to go, but we believe nanostructured and molecular solar cells will offer some unique features in the future, features that the other types do not have.

One of the advantages of the nanostructured solar cell is that both different designs and colours may be incorporated in it. Thus, instead of using glass, flexible plastics may be covered. Due to its low investment cost, and thanks to simple production methods, it may offer some very great opportunities as far as efficient mass-production is concerned.