Synthesis And Application Of Novel Dithienopyrrole Based-dyes For Dye-sensitized Solar Cells

Rapid economic development is nesscessary based on industrialization, which hascreated energy crisis and environmental crisis that need to be resolved urgently. The clean andrenewable energy produced from the solar system is the best choice. Third generation solarcells, dye sensitized solar cells （DSSCs） have drawn intensive interests due to low costmanufacturing, easily and cheaply available raw materials and moderate to good photovoltaicefficiencies. Sensitizer, including metal complexes and metal-free organic dyes, plays a vitalrole in the light harvesting efficiency providing electron injection into the conduction band ofan oxide semiconductor （e.g TiO2） upon light excitation. As the research progress, it wasfound that organic dyes could be an effective alternative to the noble Ru complexes sensitizersdue to several advantages:1) The molecular structures of organic dyes are in diverse form andcan be easily designed and synthesized.2) Concerning the cost and environment issues,organic dyes are superior to noble metal complexes.3) The molar extinction coefficients oforganic dyes are usually higher than those of Ru complexes, making them attractive for thinfilm and solid-state DSSCs.Thus, four series of dyes were designed and synthesized. To study the relationship of thestructure and the photoelectric conversion efficiency, their photophysical,photoelectro-chemical and photovoltaic characteristics were investigated in detail. On thebasis of these advantages, the research work was designed and divided into the following fourareas:First, three organic sensitizers with planar dithienopyrrole linkers were designed andsynthesized which have delivered high molar extinction coefficients. The introduction of DTPas π-bridge in the molecular structure not only enhances the charge carrier mobility but alsosignificantly improves the light-harvesting ability of dyes. Among the three dyes studied, DTPwith a branched alkyl chain exhibits an extremely high molar extinction coefficient （ε） of14.510⁴M^-1cm^-1and a maximum power conversion efficiency of6.36%under simulated AM1.5solar irradiation (100mW cm^-2). The high conversion efficiency reveals that these DTPbased dyes are promising candidates for the performance enhancement of DSSCs.Second, the asymmetric “H” type dyes were first time synthesized. DTP, carbazole andtriphenylamine units, respectively, were employed as π-bridge to construct organic dyes.Contrast to the corresponding “U” type molecules （H1, H2） and “H” type dyes （H3–H5）significantly increase the HOMO level of the dye molecules, reduce the energy band gap,effectively enhance the light-harvesting capacity and prevent the aggregation of the dyes on the TiO₂surface. H-type molecule H3containing DTP unit shows better photovoltaicperformance, for DSSCs as compared to H4and H5.Third, three new dendritic triphenylamine-based D–D–π–A sensitizers were designedand synthesized for DSSCs. As a result, compared to the reported organic dyes, high open-circuit voltages were obtained, the highest open-circuit photovoltage is853mV. Byintroducing the thiophene unit as π-bridge, Vocobviously decreased. However, theopen-circuit voltage slightly increased from822to836mV with the introduction of a TPAunit at the4-position of thiophene. The results of this work indicate that the extendedπ-conjugation and good planarity can enhance the charge recombination, which results in alower Voc. Introduction of bulky groups into side chain of π-bridge may be the effectivemethod for increasing the open-circuit voltage of DSSCs.Fourth, two novel triphenylamine polymer dyes were designed and synthesized. Bycomparison, we found that the introduction of thiophene unit can better display thelight-harvesting ability of the molecule. The two polymer dyes based cells exhibit high opencircuit voltage, which can be attributed to the increase of the anti-aggregated ability of themolecules by the introduction of triphenylamine units. The overall light-to-electricity powerconversion efficiency of the DSSCs sensitized with two polymer dyes （P1, P2） were3.69%and4.27%, respectively.