Dye-sensitized solar cell (DSSC) is one of the major development trends of solar cells due to the possibility of low-cost conversion of photovoltaic energy. The DSSCs using ruthenium complexes as sensitizers have achieved the highest photo-to-electric conversion efficiencies over 11% with very good stability, implying potential practical applications. It is very important to study the structure-performance-relationships between the structures and the photo-to-electric conversion performances. In this paper, we review the recent research progress of the ruthenium sensitizers. The sensitizers are divided into several groups according to the various substituent groups attached to the bipyridine ring and the numbers of the —NCS ligand. The relationships between the structures, the spectroscopic properties, the electron injection efficiency, the electron transfer and recombination are discussed. The structure characters of the high efficiency sensitizers are summarized, which provides valuable information for design and screening of better sensitizers. Furthermore, special attention has been paid to the design principles of these dyes. Co-sensitization, an emerging technique to extend the absorption range, is also discussed as a choice to improve the performance of the solar cell devices. The working principle of the DSSC is also discussed in detail.

Contents
1 Introduction
1.1 The structure and working principle of DSSC
1.2 Sensitizers
2 Ruthenium sensitizer
2.1 N3, N719, Black dye and HIS-2
2.2 Amphiphilic ruthenium photosensitizers
2.3 Highly molar extinction and stability bipyridyl ruthenium dye
2.4 The ruthenium sensitizers containing a -NCS
2.5 The ruthenium sensitizers modifying the carboxyl bipyridyl side
2.6 The ruthenium sensitizers without -NCS
2.7 The ruthenium sensitizers containing hole transporting units
2.8 The ruthenium sensitizers containing phenanthroline or quinoline ligands
2.9 Sensitizers with novel anchoring groups
3 Conclusion