Theoretical Study And Modification On Novel Ru(â…¡) Terpyridyl Sensitizers For Dye- Sensitized Solar Cells

As a promising new device for exploiting solar energy, dys-sensitized solar cells(DSSCs) have attracted a lot of people’s attention as a result of their flexible fabrication, low cost, low pollution and higher conversion efficiency. However, the efficiency of DSSCs is still lower than that of traditional silicon-based solar cells up to now, which has limited the large scale application in market. DSSCs is composed of different elements and as a key element, dyes has a great impact on the efficiency of DSSCs. Among various sensitizers, Ru(II)-polypyridyl complexes have exhibited good photovoltaic properties and higher efficiency. But the lower light harvesting ability in near-infrared region(NIR) compared with those of organic dyes has retarded further development of Ru dyes. To solve the problem, researchers need not only experimental efforts but also computational protocol which can save substantial experimental time and resources and give a deep understanding in working mechanism of DSSCs. Herein, we designed novel Ru(II) terpyridyl sensitizers and systematically discussed the key factors influencing short-circuit photocurrent density(Jsc) and open-circuit photovoltage(Voc) in order to give theoretical guidelines for synthesis of efficient Ru dyes. The detailed works are listed as follows:(1) To overcome the shortcomings of traditional Ru dyes in low molar extinction coefficient(ε) of the metal-to-ligand charge transfer band in the near-infrared region, we designed and studied two Ru dyes, HIS-2a and HIS-2b derived from HIS-2. The intrinsic causes for varying short-circuit photocurrent density(Jsc) and open-circuit photovoltage(Voc) have been systematically discussed on the dyes and dye/(Ti O2)38 systems including the conduction band energy shift, light harvesting efficiency, electron injection driving force and so on. Those results suggest that, compared with HIS-2 and HIS-2b, the absorption spectrum of HIS-2a is red-shifted and the transition intensity in NIR region is increased leading to a larger Jsc. At the same time, the conduction band energy of HIS-2a shifts little compared with other dyes, which will not have much effect on Voc. Hence, we could speculate that HIS-2a is a promising candidate as a more efficient Ru(II)-terpyridyl dye with increased Jsc.(2) A series of Ruthenium sensitizers(denoted as DX2-DX5) on the basis of a phosphine-coordinated ruthenium sensitizer(coded as DX1) for dye sensitized solar cells(DSSCs) have been designed with the aim of enhancing the light harvesting ability in NIR region and further increasing the short-circuit photocurrent density(Jsc). Density functional theory(DFT) and relativistic time-dependent DFT calculations have been performed to evaluate the optical and photovoltaic properties of Ru dyes, taking the effect of spin-orbit coupling(SOC) into consideration. The causes for varied Jsc and open-circuit photovoltage(Voc) have been systematically investigated through investigating the light harvesting efficiency, electron injection driving force, dye regeneration driving force, electronic coupling and conduction band energy shift. The calculated results reveal that the light harvesting efficiency of DX5 in NIR region has been increased compared with other sensitizers, together with the enhanced conduction band energy shift, which may improve both Jsc and Voc. These results suggest that DX5 can serve as a good sensitizer for future DSSC applications.