Design And Synthesis Of Double D-π-A Dye Molecule And Study On Dye-sensitized Solar Cells

As a promising alternative to silicon based photovoltaic devices, dye-sensitized solar cells (DSSCs) have attracted wild attention due to their low cost and easy production. In the last a few decades a lot of efforts have been devoted to the synthesis and investigation of materials for DSSCs. As one of the key components, dye sensitizer fulfills the functions of light-harvesting and electron transfer. We focused on design and synthesis of series of novel double D-Ï€-A dye sensitizers and their applications in DSSCs. The main studies are summarized as follows:1. The coadsorption effect of deoxycholic acid (DCA) and sodium deoxycholate (DCNa) on the conduction band (CB) shift of TiO2and charge recombination in dye-sensitized solar cells (DSSCs) was first studied quantitatively. The effect degree varied as the cation differed from H+to Na+. DCA shifted the CB positively by26mV, but DCNa shifted the CB negatively by33mV. As compared with the dye-alone based DSSC, coadsorption of dye NKX-2677with DCA retarded the charge recombination rate by10times, corresponding to70mV enhancement of open-circuit photovoltage (Voc), and coadsorption of NKX-2677with DCNa slowed down the charge recombination rate by3times, corresponding to37mV enhancement of Voc. Suppression of charge recom-bination in combination with the CB movement, arising from coadsorption, resulted in Voc gain of44mV(=70mV-26mV) for the DSSC with NKX-2677/DCA and70mV(=37mV+33mV) for the DSSC with NKX-2677/DCNa. The experimentally observed Voc enhancements of41mV by DCA coadsorption and65mV by DCNa coadsorption were in good agreement with the calculated improvements.2. An organic dye FNE92containing two D-Ï€-A branches linked with a thiophene unit has been designed and synthesized. As compared with the rod-shape of the single D-Ï€-A analogue dye FNE91, the cross shape of the double D-Ï€-A branched dye FNE92is favorable for reducing intermolecular interaction and retarding charge recombination. Controlled intensity modulated photovoltage spectroscopy reveals that electron lifetime for FNE92based DSSC is14-fold longer than that for FNE91based DSSC. Linking two D-Ï€-A branches with a thiophene unit increases Voc by90mV and short-circuit photocurrent (Jsc) by4.85mA cm-2. As a consequence, power conversion efficiency is improved by about2-fold. This work presents a new route to designing sensitizers with high suppression ability of charge recombination towards high performance DSSCs.3. A series of organic dyes containing two D-Ï€-A branches linked with varied number of thiophene units have been designed and synthesized. With increasing the number of thiophene unit the electron diffusion length was increased. As a consequence, Voc and Jsc were enhanced simultaneously, and power conversion efficiencies were thus improved. This work indicates that solar cell performance can be tuned by varying the number of linking thiophene unit.4. Three novel double D-Ï€-A dyes, FNE95, FNE96and FNE97, comprising indolo[2,3-b]carbazole as an electron donor, were designed and synthesized. As compared with a single D-Ï€-A dye FNE98based on a carbazole unit as an electron donor, FNE95exhibited a broader monochromatic incident photon-to-current conversion efficiency (IPCE) spectra, and hence a larger Jsc, when hexyloxy groups were transferred from para-positions to ortho-positions, photovoltage increased while photocurrent decreased. This work indicates that employing conjugated and planar indolo[2,3-b]carbazole as an electron donor can induce a larger Jsc.5. Two anthraquinone-based double D-Ï€-A dyes were designed and synthesized. These dyes showed an intense absorption band only in the short-wavelength region and gave poor photovoltaic performance. Density functional theory (DFT) calculations revealed that the strong electron-withdrawing character of anthraquinone framework suppresses the efficient electron injection from the dye to the CB of TiO2. This work emphasizes the importance of choosing appropriate electron-withdrawing building blocks for double D-A-Ï€-A dyes.