IMITATING MOTHER NATURE

Photovoltaic cells are made of semiconductor materials such as silicon. When light strikes the cell, a portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. This flow of electrons is a current. By placing conductive plates on the top and bottom of the photovoltaic cell, the current can be drawn off for external use, say, to power a pocket calculator.

In traditional photovoltaic cells silicon acts as both the source of electrons, as well as conductor of the charge carriers. DSC cells separate light harvesting from charge carrier transport, mimicking the principles of solar energy conversion that natural photosynthesis has successfully adopted over the last 3.5 billion years. We can think plant leaves as tiny factories in which sunlight absorbed in the leaf by chlorophyll converts carbon dioxide and water into oxygen and glucose, providing energy for the plant. In DCS cells’ 'artificial photosynthesis', the leaf structure is replaced by a porous titanium oxide nanostructure, and the chlorophyll is replaced by dye molecules.

Dye-sensitized solar cells consist of titanium oxide nanocrystals that are coated with light-absorbing dye molecules and immersed in an electrolyte solution.  Only 10 micrometers thick, the mixture is sandwiched between two glass plates or embedded in plastic. Light striking the dye frees electrons and creates "holes" – the sites of positive charge that result when electrons are lost. The semiconducting titanium dioxide particles collect the electrons and transfer them to an external circuit, producing an electric current.