Synthesis Of Porphyrins For Dye-Sensitized Solar Cells Applications

Dye-sensitized solar cells (DSCs) have drawn much attention as an alternative for Si-based solar cells because of their low-cost production and environmental friendly. Conventional DSCs consist of working electrode, electrolyte, and counter electrode. Working electrode includes conductive substrate, mesoporous semiconductor films, and dyes. When the cell is exposed to sunlight, electrons in the ground state of dye molecules will be excited to the excited state, then the electrons are inject into the conduction band of semiconductor with a ultrafast rate, completing the conversion of solar energy to electric energy. Thus, the properties of dyes determine the light-harvesting and electron injection efficiency of the cell. Porphyrins are a kind of dyes raise rapidly in recent years because of their advantages such as broad and strong absorption and stability in structure. Porphyrins have achieved conversion efficiency up to13%, which is the highest efficiency so far. Thus, this work chose porphyrin as framework, and series of D-π-A porphyrin dyes have designed and synthesized. The main contents of this thesis are listed as following:Bis(9,9-dihexyl-9Hfluorene-7-yl)amine and4-ethynylbenzoic acid were chosed as electron donor and electron acceptor, respectively. By changing the substitution at the10-and20-of porphyrin ring, ZZX-N1(substituted by3,5-di-tert-butylbenzene) and ZZX-N2(substituted by2,6-bis(octyloxy)benzene) were designed and synthesized. Under the optimized cell fabrication conditions, ZZX-N1exhibited higher conversion efficiency than ZZX-N2(5.78%vs3.61%). The EIS and transient photovoltage decay measurements revealed that the higher efficiency of ZZX-N1-sensitized cells was due to the retarded charge recombination. DFT calculations suggested that this could be due to small voids among ZZX-N1dye molecules on the TiO2surface, preventing the charge recombination with I3-.Bis(9,9-dihexyl-9Hfluorene-7-yl)amine and bis(4-hexylphenyl)amine were chosed as electron donor. To elongation the π-conjugation,9,9-dihexyl-9H-fluorene was inset between donor and porphyrin. Thus, ZZX-N3and ZZX-N4was designed and synthesized, and ZZX-N5without electron donor was synthesized, too. Comparing to ZZX-N5, the existence of electron donor in ZZX-N3and ZZX-N4broadened their absorption, and higher conversion efficiencies were achieved. But the EIS suggested that the charge recombination dynamics in ZZX-N3-ZZX-N5-sensitized cells were nearly the same, indicating that the extra alkyl chains in bis(9,9-dihexyl-9Hfluorene-7-yl)amine and bis(4-hexylphenyl)amine didn’t offer the extra shielding effect between TiO2surface and electrolyte.Based on the framework of ZZX-N3,2,6-bis(octyloxy)benzene groups were replaced by end substituted alkynyl groups to extend the π-conjugation in the direction perpendicular to D-A axis. Thus, ZZX-N6and ZZX-N6C12were synthesized. Comparing to YD2-o-C8, the Q bands of ZZX-N6and ZZX-N6C12were red-shifted to686nm and702nm, respectively. The absorption onsets were red-shifted40-60nm. Under the optimized conditions, ZZX-N6-sensitized solar cell achieved conversion efficiency of7.21%, which is comparable with YD2-o-C8-sensitized cells under the same conditions.Thienyl groups were incorporated into three porphyrin dyes, ZZX-N7-ZZX-N9, and bis(4-hexylphenyl)amine was chose as electron donor. In consistent with their trend in π-conjugation, the absorption was red-shifted from ZZX-N7to ZZX-N8and ZZX-N9. But broader absorption didn’t lead to higher conversion efficiency:under low dye-loading density, the conversion efficiency increased from ZZX-N7<ZZX-N8<ZZX-N9; while under high dye-loading density, the trend was ZZX-N7> ZZX-N8> ZZX-N9. Under optimized conditions, ZZX-N8achieved the highest conversion efficiency of7.78%.The adsorption kinetics and mechanisms were studied under different uptaking solvents, initial concentration, TiO2films, and dyes. Fittings of adsorption data at pseudo-first order model and pseudo-second order model revealed that the chemisorption of dye molecules (second order reaction) was the rate determining step. Besides, the intraparticle diffusion model suggested that the adsorption in THF consisted of only boundary layer diffusion; while adsorptions in alcohols consisted of bulk diffusion, boundary layer diffusion, and intraparticle diffusion. The uptaking solvent seemed to be the key factor determining the adsorption kinetics and adsorption mechanisms.