The Fabrication And Characterization Of Semiconductor Nanostructure Solar Cells

With the modern synthesis techniques and characterization technology of micro/nano-structure (length scale:<10-/m), many physical and chemical properties of these nanostructures have been studied, developed and utilized, which do not exist in the bulk materials. For example, to the classical semiconductor photoelectronic devices, devices with better performance can be realized through the use of new structural nanomaterials or new concepts (such as lower threshold current requirements and more narrow spectrum width by quantum well lasers), and new applications can be developed (such as the field effect light emitting diode by the carbon nanotube which can adjust the emission spectrum). This dissertation focuses on one special kind of photoelectronic devices, which emphasises on the study about the radiation energy converting into the electric energy:solar cells. It also meets the requirement of the energy crisis and environmental issues in the current widespread attention. Through a series of synthesis, characterization, modification and application forward to the photoanode (TiO2porous film, TiO2nanotube arrays) and the sensitizer (dye, PbS quantum dots) of the new type solar cells, our research study has been playing a major role in the improvement of sensitized cells and promoting the synthesis techniques of other nanomaterials than TiO2or PbS QDs.Currently, the clean and sustainable energy, solar energy, is mainly utilized by the crystalline silicon cells. But the large-scale photovoltaic power generation is constrained due to the high cost. Therefore, a lot of academic study targeting low cost and high efficiency cells has been developed. Among the new concept cells, the sensitized solar cell is very valuable and potential. Its structure and carrier transport mechanism are very different from the silicon ones. Relying on the energy level difference, the carriers are transferred spontaneously, while the light harvesting material and the carrier transport material are independently of each other. The traditional photoanode is nanocrystalline TiO2porous film, which is easy to be fabricated but leads to a disordered structure, that can increase the light scattering and the dark current and decreases the quantum efficiency. If the vertical direction ordered arrays was chosen as the photoanode to replace the TiO2porous film, it may be able to achieve the optimized balance of the optical property and the electrical property.Among the lots of oriented TiO2nanostructures, NTAs (nanotube arrays) are one special type, which is highly ordered on a vertical direction, periodically distributed one the other two-dimensional plane directions. Due to the bizarre morphology, TiO2NTAs have been applied to many devices such as photocatalytic devices, sensors and ultracapacitors. Nowadays, the synthesis technique of ordered TiO2NTAs by electrochemical system has already been very developed, but unfortunately, the understanding about the basic principle of self-organization process still lack a final word of conclusion, which also significantly affects the more flexible and creative regulation for TiO2NTAs. In the aspect of tube-based sensitized solar cells, their convert efficiency, whether dye-sensitized or quantum dots-sensitized, have not exceeded the efficiency of porous film based cells.In this dissertation, we first studied some basic research and mechanism analysis of the nanotube controllability. The influencing factors included the oxidation time, oxidation voltage, electrolyte composition, etc. These basic researches lay the foundation for sensitized solar cells. More in-depth, we also captured the correspondence between the ion concentration changing and the current oscillation during the oxidation-decomposition process. A new mechanism of ion drift and diffusion of titanium tube growth theory has been established. Furthermore, some modulation methods have been designed to solve out improvement and novel structures of TiO2NTAs, and especially smooth tubes with high orientation have been achieved. These modulation methods also have a significant impact on the growth speed. Under some particular conditions, the nanotube growth rate would increase from20μm/h to122μm/h, over five times range.Secondly, we carried on performance improvement research of TiO2NTAs based DSSCs (dye-sensitized solar cells). To create a larger surface area, new space secondary structures were constructed by HF method. The most appropriate HF solution sample has a35%higher efficiency compared to the control untreated sample; further integration with the TiCl4modification and physical modulation could offset the adversely affected on tube walls brought by HF treatment and increase the electrical performance. Overall, an efficiency of4.35%can be reached in the back-side illuminated DSSC, which is114%higher than the basic value. Based on that, greater efficiency improvements of nanotube based DSSCs could be hopefully realized with the development of other key parts of the cell.Finally, we also carried on performance improvement research of TiO2NTAs based PbS QDSSCs (quantum dots-sensitized solar cells). And for the first time, its efficiency is higher than that of TiO2porous film based ones. The realization of high efficient PbS QDSSCs is mainly due to a novel in-situ method called EACBD (electric-field assisted chemical bath deposition), which was enlighten by traditional hydrothermal method. We use an electric field to attract Pb ions gathered in TiO2NTAs surface, and then injected into the S ions, guiding PbS QDs synthesized in large quantities around the TiO2NTAs and forming a cohesive in-situ growth. In this way, the increase of sensitizer absorption and the decrease of carrier recombination were achieved at the same time.All the works in this dissertation were supported by National Major Basic Research Project (2012CB934302), and Natural Science Foundation of China (11074169,11174202,11204176and61234005).