Solar cells directly convert light energy from the sun into electrical energy and are an increasingly important source of non-fossil-fuel energy. Jaejung Ko at Korea University and co-workers1 now describe a new type of organic dye-sensitized solar cell (DSSC) that offers improved efficiency through encapsulation of the dye.

In a typical solar cell, irradiation excites electrons, causing charge separation. The electrons are transferred into an electron-deficient ‘capture layer’ and ultimately to electrodes. The efficiency of the solar cell is dependent on how easily these charges can be transferred to the electrodes to produce an electric current. In traditional solar cells, both the source of electrons and the electron capture material are provided by a semiconductor such as silicon.

DSSCs separate these two functions: the electrons are provided by a photosensitive dye molecule and the electron capture material is a nanocrystalline metal oxide such as titania (TiO2).

“Our goal is to increase the efficiency of DSSCs — how much light energy is converted into electrical energy — so that eventually they might compete with more traditional inorganic solar cells,” says Ko. “The most important factor is the ability of separated charges to reach the electrodes, which can be hampered by charge recombination and by aggregation of dye molecules, preventing transfer of the charge to the oxide material.”

Fig. 1: (Upper) The cyclodextrin adsorbed to titania (TiO2) encapsulates an organic dye molecule (shown in red). (Lower) Titania nanocrystals coated with cyclodextrin (lower).

His group’s approach to this problem (Fig. 1) is to coat the titania nanocrystals with cyclodextrin molecules — rings of sugar molecules containing between six and eight sugars. These molecules adsorb strongly to the surface of the titania. The cyclodextrin rings have an apolar central cavity that can accommodate a dye molecule, which is threaded through the cyclodextrin to make contact with the titania surface.

The coating of the titania nanocrystals helps to prevent interfacial charge recombination, and the encapsulation of the dye in the cyclodextrin prevents aggregation of the dye. ”There is still much to do, but in the immediate future, we plan to study the stability of the encapsulated dye and the lifetime of the solar cell devices,” says Ko.