Solar radiation energy (photons) must be absorbed by dyes and converted into electric energy efficiently as possible for power generation. The main task now is to develop organic dye systems that have the ability to absorb photons in high degree from the visible to the NIR range. Hence, the following physical and chemical properties are important:
· The dyes should exhibit a very high value of the extinction coefficient.
· The gap between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the dye should be small.
· The mobility of electrical charges in the molecule should be high.
· The ratio of the incident photons to the current (IPCE) should be almost 100%, i.e. almost every photon inject an electron.
· The dyes should be stable under solar irradiation.
· The dyes should be soluble in common solvents for further application.
· The synthesis of the dyes should be realizable in industrial scale.
The class of polymethine dyes offer almost all of these properties and, therefore, show a high potential for the conversion of solar energy into electrical energy. Based on polymethine dyes, we synthesized symmetrical and unsymmetrical cyanine dyes with different number of methine units and different heterocycles as well as symmetrical and unsymmetrical dyes of squaric acid and croconic acid with different heterocycles and functional moieties. These dyes are applied in dye sensitized solar cells (DSSC) and organic thin film solar cells.