Nanostructure Zinc Oxide: Synthesis, Characterization And Application In Dye-Sensitized Solar Cells (DSSCs)

The energy crisis has forced human being to search and develop new greenrenewable energy. Among them, owing to low price of raw materials, simple productionprocess, only10%-20%cost of the crystalline silicon solar cells, suitable for massproduction, the Dye-Sensitized Solar Cell (DSSC) has great potential applications andattracted much attention. By now, the most studied and reported photoanode materialsfor DSSC is TiO2. Due to the similarity of the energy band gap and the electron-injection process of ZnO to that of TiO2, and its higher electronic mobility and easiersynthetic process, ZnO has been expected to be compatible with TiO2as a workingelectrode in DSSCs. But which kind of ZnO nanostructures is most favorable to obtain ahigher photoelectric conversion efficiency, is still a subject worthy of systematic andprofound study.In this dissertation, a variety of nano-structure ZnO, such as nano-film,nanoparticles, nanospheres, nanorods, nanowires, and composite structure(nanoparticles/nanowires) have been successfully prepared on FTO substrates by thesolution methods, respectively,and the appropriate DSSC have also been fabricated.Thestructure, morphology and optical properties of the nanomaterials were characterizedand analyzed by XRD,SEM,AMBIOS and ultraviolet-visible spectrophotometer,respectively, and the properties of the DSSCs were investigated by current-voltagecharacteristics.The main results are as following:1) ZnO thin films were deposited on glass substrates by using multi-step sol-gelspin coating method. This research investigated the effect of annealing temperature inthe range from350-600°C and the effect of zinc acetate concentration of0.35,0.5and0.65mol/L on the microstructure, surface morphology and optical properties. It showsthat the c-axis orientation of ZnO thin films is improved with the increase of annealingtemperature and the concentration of zinc acetate, while the grain size increases from16.6to19.7nm. With the increasing ofannealing temperature, optical band gap of thefilms decrease. The transmittance increases with the increase in sol concentration. Whenthe concentration of zinc increases from0.35M to0.65M, the optical band gap energyof ZnO thin films attenuates from3.307to3.227ev, and whereas the Urbach energy ofthe films increase from68.2to82.4meV. Meanwhile,it is found that the tensile stress exists in the films, which decreases with the increase in annealing temperature up to500°C, on further increasing the annealing temperature up to600°C, the stress in thefilm changes from tensile to compressive nature. The same trend happened with strainincase of ZnO zinc concentration in the films.2) ZnO nanorod array films were successfully synthesized on seeded FTOsubstrates by Spin-coating and hydrothermal methods. It was found that the ZnOnanorods were denser and more uniform with the addition of PEI inseed solution, whilethe nanorods become smaller in diameter and longer in length with PEI in growthsolution. From the above analysis, these are beneficial to improve the dye absorptionand the photovoltaic performance of the DSSC. The conversion efficiency of DSSCbased on nanorod array films with PEI in growth solution can reach to1.18%, whichincreased by84%than the DSSC without PEI in the growth solution. Meanwhile, thefill factor and current density were increased by18%and52%respectively.3) The experiment is performed to explore the effect of growth time and coupleduse of polyethyleneimine (PEI) and ammonium hydroxide (NH4OH) on the propertiesof nanowires and DSSCs. It is found that with the addition of polyethyleneimine (PEI)and ammonium hydroxide (NH4OH) in growth solution, the NWs are smaller indiameter and longer in length by prolonging the growth time without refreshing thegrowth solution. These beneficial effects ultimately resulted in an improved short circuitdensity and the overall power conversion efficiency. A reasonable overall conversionefficiency of1.25%is achieved with photoanode based on ZnO nanwires containingPEI and NH4OH, which is much higher than ZnO NWs not containing PEI and NH4OH.4) ZnO nanoparticles/nanospheres, nanowires and nanowires/nanoparticlescompound structure are synthesized and successfully used in DSSCs. It is found thatabout63%higher conversion efficiency is achieved in ZnO nanosphers cell comparedwith ZnO nanoparticle cell, and the conversion efficiency of DSSC derived fromcompound ZnO nanowires/nanoparticles photoelectrode is34%higher than that ofsingle nanowires cell. Therefore, the relations of these four-type DSSC’s conversionefficiency can be given as follow: NP+NW> NS>NW> NP, which indicated that thenanosphere aggregates and compound nanostructures are suitable for using asphotoanode in DSSC than nanoparticles and single nanowire photoanode.Overall, a lot of experimental work has been carried out to prepare a variety ofnanostructure ZnO by some inexpensive way and to study the corresponding DSSC inpresent thesis and some interesting results have been achieved. However the properties of the currently available DSSC is still low, means, some core issues，such as theinfluence and mechanism of nanostructrue on DSSCs is still a subject worthy of furtherinvestigation in the future.