Design And Synthesis Of Novel Porphyrin Sensitizers For Dye-sensitized Solar Cells

Dye-sensitized solar cells (DSC) is a new type of photovoltaic cell, which simulates the natural photosynthesis principle. Recently, DSC has attracted considerable attention due to its simple fabrication technique, rich raw materials and pollution-free. They are also considered as a credible alternative to conventional silicon-based solar cells because of its high theoretical energy conversion efficiency (33%-36%). However, the reported efficiency of DSC has reached11-13%. Extensive efforts have been carried out on the further improvement of the energy conversion efficiency of DSC. One of the effective methods is to broaden the absorption spectra and to improve the light harvest of devices.Dye sensitizer plays a key role in utilizing the light in DSC. In the past20years, the ruthenium metal-based sensitizers and metal-free organic sensitizers showed the best performance. However, these dyes suffer from the simple structure, the narrow andweak absorption which limit the further increasing efficiency of the DSC.Porphyrin dyes have been studied due to its strong ability of light harvesting and the easy-handling porphyrin rings. Porphyrin dyes have two characteristic absorption bands, namely, Soret band and Q band. The Soret band, located between400and500nm, has a high molar extinction coefficient up to105. The Q band is always located between400to700nm. Therefore, the porphyrin sensitizers can almost utilize the whole visible light region. The DSC based on porphyrin sensitizers with D-A configuration give a world record efficiency up to12.3%in a variety of porphyrin dyes, which demonstrates the promising application of porphyrin dyes for DSC. So far, the commonly used electron donors (D) in the porphyrin dyes are benzene, diphenylamine, triphenylamine and their derivatives. Fluoreneamine and its derivatives have been used in organic sensitizers as the electron donors, giving a satisfied energy conversion efficiency. However, they have not been used in porphyrin sensitizers till now. Therefore, this research focused on the design and synthesis of the novel porphyrin dye with D-A configuration using fluoreneamine as electron donor for DSC.We firstly introduced fluoreneamine as electron donors into porphyrin dye. We designed and synthesized porphyrin dyes WP-1and WP-2using bis(9,9-dihexyl-9H-fluoren-2-yl)amine as electron donor. The solubility of the two dyes are very good because of the four hexyl-chains in the amine donor. The absorption spectra show that WP-1dye has three characteristic absorption peaks, corresponding to one Soret band and two Q band. The molar extinction coefficient of the Soret is up to2*105, and the λmax of Q band reaches670nm. Meanwhile, WP-2dye has one Soret band and two Q bands. The molar extinction coefficient of the Soret is also up to2*105, and the λmax of Q band reaches720nm, which shows the best spectral response among the porphyrin sensitizers. We used WP-1and WP-2dyes as sensitizers to fabricate DSC devices, giving a high energy conversion efficiency of4.01%and6.82%, respectively. Especially, in the ageing stability test, the highest efficiency reached7.07%obtained by the WP-2dye based DSC. After light soaking under nature light for500h, the efficiency of WP-2DSC remained98%of the initial value. The results revealed that the bis(9,9-dihexyl-9H-fluoren-2-yl)amine is an excellent electron donor for push-pull porphyrin sensitizers. In summary, fluoreneamine and its derivatives are great candidates of electron donor in porphyrin sensitizers for DSC.