Photoelectrochemical Properties Of The Dye-sensitized Composite Oxide Nanocrystalline Electrode

The utilization of the solar energy is a permanent subject. Dye-sensitized nanocrystalline photoelectrochemical solar cell is a strong competitor for the p-n junction Si solar cell due to its low-cost and high-efficiency. In 2003, the Gratzel group in EPFL (Switzerland) reported 10.6% light-to-electrical energy conversion efficiency, which is a new record for the liquid-junction photoelectrochemical solar cell. This arose more interests in the world for the research of this kind of solar cell. Dye-sensitized nanocrystal TiO₂ porous film electrode is the key part of this kind of solar cell, which is mesoscopic network structure formed by the interconnected nano-sized TiO₂ particles. The interconnection of the particles can allow for the electronic conduction to take place. Due to the small size of the individual particles in the nanoporous electrode, the TiO₂ particles cannot support a high space charge layer. That is to say, an effective energy barrier cannot form on the interface bewteen the TiO₂ and electrolyte, which results in the recombination between the photo-generated electrons on the conduction band of the TiO₂ and the holes in the electrolyte or the oxide dye molecules. So, the interfacial engineering design of the TiO₂ electrode to suppress the recombination of the charges is an important project for the fundamental studies of the dye-sensitized nanocrystalline solar cell. In our dissertation, firstly, we have fabricated successfully the model of the dye-sensitized nanocrystal TiO₂ photoelectrochemical cell. And then, we performed a series of interfacial engineering design on the TiO₂ electrode and prepared the composite oxide nanocrystalline electrodes. The charges transfer and recombination on the interface of the composite electrodes and their influence on the performance of the solar cells were studied. The main contents are following: (1) Titanium oxide is safety, non-poisonous and cheap semiconductor materials, which has extensive application in the field of photocatalysis and photoelectric conversion. But it has photoelectric activity only in the UV region (< 400nm) because the band gap of the TiO2 is about 3.2 eV. So, extending the photoresponse of TiO2 to visible region is the common aim of the researchers. Nanocrystal TiO2 porous film electrode was prepared by Doctor-blade method. Mercurochrome was used as light-harvest antenna to sensitize TiO2 electrode. The spectra of the aggregated dye molecules, the transient photocurrent response and photocurrent/potential properties of the dye-sensitized electrode at different applied bias and different illuminated direction were studied in detail. Combination absorption spectra, electrochemistry, photoelectrochemistry and the theory of the charge transfer, we discussed the generation mechanism of the photocurrent. These works provided experimental foundation for the fabrication of the dye-sensitized solar cell. (2) High-quality nanocrystal TiO2 porous film electrodes have bee prepared. Using cis-di(isothiocyanato)-bis(4,4'-dicarboxy-2,2'-bipyridine)ruthenium(II) (N3) as sensitizer, we fabricated the model device of the dye-sensitized nano-crystal TiO2 photoelectrochemical cell. At 50 mW/cm2 white light, the device generates a 11.3 mA/cm2 short-circuit photocurrent, a 620 mV open-circuit photovoltage, a 38% fill factor and a 2.34 mW/cm2 maximal power output. The efficiency of the device is 5.0%. This result has approached the efficiency (7 %) reported by Gr?tzel group in 1991. At the same time, we used surface photovoltage spectra and field-induced surface photovoltage to study the impurity states generated by the binder on the charge transport of the photo-generated electrons in the dye-sensitized TiO2 electrode. These impurity states acted as the intermediate for the reduction of the electrolyte, which increased the recombination of the photo-generated electrons and holes. These results provided theoretical foundation for the optimization of the TiO2 electrode and improvement of the photoelectric properties of the solar cell. (3) The suppression of the charge recombination is an important project for the fundamental studies of the dye-sensitized solar cell. We used the mixture of the TiO2 and SnO2 to prepare the SnO2/TiO2 composite electrode, which was sensitized by mercurochrome. The photoelectrochemical properties of the sensitized composite electrode were studied. When the weight ratio of the TiO2to SnO2, the IPCE of the composite electrode at 500 nm was optimal. We think there existed a large number of heterojunctions in the SnO2/TiO2 composite electrode. At different mixed ratio, the heterojunctions may show different functions. On the one hand, they could reduce the recombination between the photo-induced electrons and the holes. On the other hand, they could also block the transport of the photo-induced electrons, which increased the recombination. Hence, the appropriate mixed ratio in the composite electrode was so vital that the heterojunctions did not increase the charge recombination. At the same time, we used spectro-electrochemistry to investigate the structure of the energy band in the composite electrode. Based on the difference in the bleaching width of the SnO2 and SnO2/TiO2 composite electrode, we calculated that the effective energy barrier in the composite electrode is about 117 meV. (4) After we studied the photoelectrochemical properties and photogene-rated charges transfer mechanism of the SnO2/TiO2 solar cell, we performed interfacial engineering design on the SnO2/TiO2 composite electrode. A ultra-thin Al2O3 layer (1² μm) was deposited on the surface of the SnO2/TiO2 electrode, which could suppress further the charge recombination and improved the cell's efficiency by 37%. After the deposition of the Al2O3 layer on the composite electrode, there existed SnO2/TiO2 heterojunction and TiO2/Al2O3 insulating energy barriers. The photo-generated electrons can tunnel the Al2O3 layer to the conduction band of TiO2 from the excited dye. The Al2O3 layer slows the recombination of the photo-generated electrons on the TiO2 conduction band and holes in the electrolyte or the oxidized dye. At the same time, the energy barriers between TiO2 and SnO2 accelerate the transfer of the photo-generated electrons to the SnO2 with lower conduction band and further suppress the recombination process. These results indicated that the reasonable interfacial design of the nanocrystalline electrode could improve the performance of the solar cell effectively. (5) An outstanding merit of the dye-sensitized nanocrystalline solar cell is the diversity of the electrode materials. Various oxide semiconductor materials such as TiO2, SnO2, ZnO, Nb2O5 and WO3 could be used as electrode materials. As far as today, TiO2 is the most excellent matiral. But the studies on the other materials can make us understand clearly the properties of the oxide semiconductor electrode and find out a more effective system for the solar cell. We have fabricated the mercurochrome-sensitized ZnO photo-...