As a promising technology for the new generation of photovoltaic systems, dye-sensitized solar cell (DSSC) has attracted tremendous attention due to their low manufacturing costs, relatively simple process technology, renewable and high-power conversion efficiency. The main components of a DSSC are a cathode, sensitizer dye, photoanode and an electrolyte solution containing a redox couple. The photosensitizer is the main part of the DSSC structure, which directly affects the absorption of visible light and the production and injection of photoelectron. The photosensitizers used in DSSC are divided into two types, viz., organic dyes and inorganic dyes according to the structure. Inorganic dyes include coordination complexes, such as polypyridyl complexes of ruthenium and osmium, metal porphyrin and phthalocyanine, while organic dyes include natural and synthetic organic dyes. Among the photosensitizers investigated, coordination complexes have been the best so far.
Coordination complexes have been widely used as photosensitizers for dye sensitized solar cells, and a few simple examples are given below.
Among coordination complexes, polypyridyl ruthenium photosensitizers were widely used and studied for their high stability, excellent redox properties, and good response to natural visible sunlight. The photosensitizers anchored on the surface of semiconductor film electrode with carboxylate or phosphonate group enable the electron injection into the conduction band of the semiconductor. A well-known Ru complexes was reported in 1993 by Grätzel and co-workers  with the cis-[RuIIL2(NCS)2] (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid) photosensitizer, also known as N3. Since then, it has been employed as a standard dye in several studies, especially to compare the performance of other dyes or the results of modifications introduced in semiconductor composition/treatment.
Figure 1. The structure of N3
- Other coordination complexes
Research has also been extended to other coordination complexes such as Os (II), Cu (I) and Fe (II). For example, the maximum incident photon to current conversion efficiency of the osmium photosensitizer shown in Figure 2a was found to be 50% lower than that of the Ru complex, but photochemical stability of osmium complex was superior as compared to Ru complex. In addition, Cu(I) complex of 6,6'-disubstituted-2,2'-bipyridines (Figure 2b) have been reported to be effective photosensitizers for TiO2. These dyes showed surprisingly high incident photon to current efficiencies for DSSC. Interestingly, since iron is a common and inexpensive metal, it can also provide a very economical alternative to ruthenium sensitizing complexes.
Figure 2. The structure of (a)osmium complex, (b)copper complex
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- Grätzel, M.; et al. Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate) ruthenium (II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 1993, 115: 6382-6390.