Investigation On ZnO/ZnS Nanostructure Solar Cells Sensitized By CdS Quantum Dots

The energy crisis has become one of the main challenges that human faced.Energy sources that can be used by human are fossil energy such as oil，gas and coal.These energy reserves decrease year by year and the price is rising.The shortage ofenergy seriously inhibits the development of the society in the future. Thedevelopment of new energy and renewable clean energy plays an important role inthe world economic development. Development and utilization of solar energy is theenergy strategy for the sustainable development of the world’s governments. Solarenergy resources are inexhaustible, therefore, the conversion from the solar energyinto electricity and heat energy for the human service has been always the goal ofscientists. In solar energy, solar power is the most notable. The solar cell has manyadvantages such as safe and reliable, noiseless, non-pollution, widely available andso on. Quantum dots sensitized solar cell occupies an important position in the thirdgeneration solar cells. The theoretical research results show that quantum dots havetunable absorption band, multiple exciton effect and the two-photon excitation effectso that up to66%of the photoelectric conversion efficiency can be obtained.Therefore quantum dots sensitized solar cell received widespread attention fromresearch workers.In this thesis, ZnO was used as photoanode materials and bonded with aplatinum-coated ITO)(20nm thick) counter electrodes to assemble QDSSCs. A mixture of0.5M LiI and0.05M I2aqueous solution was used as electrolyte. ZnS andZnS:Mn2+nanoparticles as compact layer were investigated for the first time toassess their effects on the charge transport. And the function of cosensitization withquantum dots and organic dye on the improvement of conversion efficiency of solarcells was investigated. Main contents are as follows:1. We successfully synthesized ZnO nails and ZnO rods by a simple hydrothermalmethod. Both time-resolved PL and temperature-dependent PL spectra reveal aless nonradiative contribution in ZnO nails, which induces that the relativeemission intensity ratio between UV and deep level emission inroom-temperature PL spectra for nails is2.3times higher than that of rods. Ourresults will stimulate more investigation on ZnO nails and finally pave a way forZnO nails in the application of laser, gas sensors, and light emitting devices.2. The ZnS NPs layers were deposited on ZnO NRs and ZnO/ZnS compositenanostructure was used as photoanode to fabricate QDSSCs. The effects of ZnSon the properties of QDSSCs were discussed. When ZnS NPs layer was spincoated on ZnO NRs, the performance of QDSSC did not change a lot due to theaggregation of ZnS NPs. However, when ZnS NPs layer was formed by(Successive ionic layer adsorption and reaction)SILAR method, it can inhibit theopposite direction transfer of electrons and recombination of electro-hole pairsmore efficiently. The open-circuit voltage of QDSSCs was improved obviously.3. To further improve the energy alignment of ZnO/ZnS/CdS solar cells. The CdSquantum dots sensitized ZnO nanorods solar cells with ZnS:Mn2+nanoparticlesas compact layer were fabricated. The ZnS:Mn2+nanoparticles were depositedon the surface of ZnO nanorods by spin coating. The results indicated that withthe increase of Mn2+doping concentration, the energy level of conduction bandof ZnS:Mn2+nanoparticles turned lower, which is beneficial for the chargetransfer from the conduction band of CdS, resulting in an enhancement ofconversion efficiency of solar cells.4. The ZnO/ZnS core–shell hierarchical structure prepared by SILAR method as photoanode was used in the QDSSCs with N719/QDs cosensitizing. TheQDSSC based on ZnO/ZnS/CdS(12)architecture exhibited a maximum powerconversion efficiency of1.16%due to the better light harvesting. Depositing athin layer of N719dye onto the ZnO/ZnS/CdS(12) structure can further improvethe photovoltaic performance of the solar cells. When ZnO/ZnS/CdS(12)electrode is sensitized with N719for17h, the device shows a37%enhancementin energy conversion efficiency compared with the device only sensitized withCdS QDs.