CdS/CdSe Nanocrystals Sensitized Porous ZnO Nanosheet Arrays Solar Cells:Preparation And Investigation Of The Photo-electric Conversion Properties

Semiconductor quantum dot nanocrystals have unique optical and electrical properties due to the low dimensional effects such as quantum size effect, quantum confinement effect, impact ionization effect and miniband effect. In comparison with organic dyes, semiconductor quantum dots have higher quantum efficiency, tunable band gaps, longer hot carriers’ life-time and better thermostability in the field of dye sensitized solar cells. Owing to these properties, the ultimate conversion efficiency of quantum-dot-sensitized solar cells is much higher than that of the cells using single band gap materials as absorber.As one of the most potential distinguished alternative to traditional TiO2nanocrystals, ZnO nanostructures have already been widely used as the working electrode in the dye sensitized solar cells due to the widely variable of morphologies and higher electronic mobility, and obtained acceptable conversion efficiencies. Especially, the synthesis of ZnO nano-arrays with connect to the conducting substrate is hopefully to provide direct pathways for electrons’ transportation, thus to reduce the recombination loss.In this dissertation, we successfully fabricated porous ZnO nanosheet arrays with high surface area on the FTO substrates. Then the working electrodes were prepared with the deposition of CdS and CdSe nanocrystals. After that, we assembled the solar cells and investigated the photo-electric properties of the devices. The summary are as follows:1. Fabrication of porous ZnO nanosheet arrays via hydrothermal method combined with a calsination process afterward.Using a hydrothermal method, we fabricated zinc carbonate hydroxide nanosheet arrays on the FTO substrates. After a high temperature calsination process in the atmosphere, the smooth zinc carbonate hydroxide nanosheets lost H2O and CO2molecules and translated into porous ZnO nanosheets, and we thus successfully obtained porous ZnO nanosheet arrays.2. Deposition of CdS and CdSe quantum dot nanocrystals on the ZnO nanosheets via SILAR and CBD methods.Using simple successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) methods, we deposited CdS and CdSe quantum dots on the surface of ZnO nanosheets respectively, and obtained CdS/ZnO and CdSe/ZnO heterostructures. We also prepared CdSe, CdS co-sensitized ZnO (CdSe/CdS/ZnO) structures with the combination of these two methods.3. Assembly of the cells and test and investigation of the phto-electric conversion properties of different types of working electrode structures.We test the photo-electric properties of the three kinds of sensitized structures via I-V output curves and IPCE spectrum curves, and explained the remarkably improvement of the conversion efficiency in the co-sensitized structure with a band realignment effect.4. Discussion of the main factors that limit the efficiencies of the cells.In comparison with different electrodes and electrolyte systems, we found the main cause of the low efficiencies of the cells is the poor properties of S2-/Sn2-electrolyte system. With the analysis of I-V curves and AC impedance spectrums, we concluded that the low shunt resistance caused by the direct contact at S2-/Sn2-electrolyte/FTO interface is one of the key factors which limit the overall performances of the cells.