Dissertation > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Photocell > Solar cells

Study of Three Kinds of Dyes with Dihydroquinoline, Schiff Base,Nitro Group Structures and Sulfur/Iodide-based Hybrid Electrolytes in Dye-sensitized Solar Cells

Author CongJiaYan
Tutor SunLiCheng; YangXiChuan
School Dalian University of Technology
Course Fine Chemicals
Keywords dye-sensitized solar cell photosensitizer nitro group hybrid electrolyte
CLC TM914.4
Type PhD thesis
Year 2012
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As one of main components of dye-sensitized solar cells (DSCs), dye sensitizer takes the function of injecting the photo-excited electrons into conduction band of semiconductor. The oxidized dye is then regenerated by the redox couple in electrolyte. Dye sensitizer is the key factor of DSCs’light-to-electricity efficiency, and also one of the components with highest price. To cut the cost and simplify the synthesis procedures, we designed and synthetized three D-π-A (donor-π conjuction-acceptor) metal-free organic dyes, which was based on dihydroquinoline donor, isophorone conjunction and cyanoacrylic acid acceptor. The structures of the dyes have been characterized by mass spectra (MS) and proton nuclear magnetic resonance (’H-NMR) technology. The results showed that the introduction of alkyl chains prevent the congregation of dyes on TiO2surface. Seen from calculation, there is some distribution of electric cloud in acceptor at ground state. This probably increases the possibility of electron recombination between dyes and TiO2conduction band, and causes the decrease of open circuit voltages of DSCs. The distribution of electons in electron-withdrawing and conjuction parts should be reduced in future dyes design.The HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) of near infrared (near-IR) photosensitizer is very close, which makes the energy levels of dye are difficult to fit the high-gap semicondutioner. This makes the design of near-IR photoshensitizer a challenging field in DSCs. In order to utilize the light in near-IR region more efficiently, two metal-free dyes with Schiff base structure were designed and synthesized. The structures of the dyes have been characterized by MS and1H-NMR spectra. Results showed that the LUMO (Lowest Unoccupied Molecular Orbital) level of dye T1was too positive, causing that the excited electrons cannot efficiently inject to the conduction band of TiO2. This made the DSCs sensitized by dye T1did not work. To lower the LUMO, the carboxyl group of T1was changed to hydroxyl group. However, the cut-off absorption of UV-Vis (ultraviolet-visible) spectrum was red shifted a lot, when the dye sensitized to the surface of TiO2. This caused great decrease of LUMO level. The reason might structure of hydroxyl group is too short. When connected to the TiO2surface, TiO2helped acceptor to withdraw electrons. The discovery of this phenomenon broadened the design mentality of photosensitizer for DSCs. Nitro group is a good electron-withdraw chromophore, which can be easily introduced into the organic compouds. In this paper, pure organic D-π-A photosensitizers with nitro groups were firstly applied in DSCs. Two pure organic D-π-A dyes with nitro group as acceptor were designed and synthesized. The structures of the dyes have been characterized by MS and1H-NMR spectra. Results showed the HOMO and LUMO levels are fit the dye regeneration and electron injection. However, the efficiencies of DSCs sensitized by this kind of dye are not very high. The reason was found to be the nitro group cannot efficiently inject the electrons to the conduction band of TiO2. When a reverse bias was added to the DSCs, the color was changed from red to light yellow, and the efficiencies raised a lot. Efficiencies of DSCs sensitized by JY1had five-fold increase, when a reverse voltage was added to the devices. A reasonable mechanism was concluded according to the UV-Vis, Near-IR, IPCE (Incident Photon-to-Current Conversion Efficiency) spectra and related references. When a bias was added to the device,-NO2received electrons and changed to-NO22-; and then bonding to the HO2surface. The bonding makes the injection more efficient, and makes the device a higher light-to-electricity efficiency.Electrolyte is one of the most important components in DSCs. By now,I/I3-redox is the most efficient redox that ever found. However, the containing of I3-and polyiodide in this kind of electrolyte can absorb some of the visible light causing the energy loss. Moreover, the gap between potential of I-/I3-and HOMO level of efficient dyes (e.g. N719) is large. This also caused energy loss. Therefore, a new kind of sulfur, iodine hybrid electrolyte was designed, synthesized and prepared. Results showed the transparent and colorless hybrid electrolytes obtained higher short-circuit current, open-circuit voltage and efficiency than conventional I-/I3-based electrolyte. The increase of current was found to be the more light absorption of dye, which was ascribed to the colorless of hybrid electrolyte. The higher voltage was ascribed to the more positive redox potential, inhabitation of recombination between dyes/TiO2and the conduction band move of TiO2in different electrolytes. The regeneration experiments showed that DSCs with hybrid electrolytes can work continuously and stably. According to the experimental results, the mechanism of how hybrid electrolyte works was assumed.

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