Research On Preparation, Spectrum Tuning And Photovoltaic Properties Of Zinc Oxide Based Photoelectrical Films

The rapid development of nanoscience and technology is leading people into a new era. Specifically, the semiconductor nanomaterials have been wildly used in many fields such as new energy resources due to their unique properties of optics, electronics and photoelectronics. Among them, ZnO nanomaterials, with merits of high electron mobility, wide band gap and easy manipulation of morphology, have attracted worldwide interest as electrode materials in the3rd-generation solar cell i.e. dye-sensitized solar cell (DSC). However, the energy conversion efficiencies of ZnO-based DSCs still remain low, which greatly limits the development of this field. In this study, we designed and prepared several ZnO photoelectrodes with highly efficient light harvesting capability and photo-to-electric conversion property, and the details are as follows:1. A tunable aqueous method for preparing polydispersed ZnO nanocrystalline aggregates (NAs) as photovoltaic electrode material in DSCs was reported. The results indicated that the ZnO sample prepared with triethanolamine concentration of25%in the aqueous solution (average diameter of700±150nm with primary particles around50nm) possessed the relatively higher dye-loading amount, stronger light scattering capability, lower electron transfer resistant and much prelonged electron lifetime when used as photoanode in DSCs because of its intact morphology and sub-micron scale, leading to the best photovoltaic performance i.e. highest short-circuit current density and energy conversion efficiency of11.35mA/cm2and3.91%, respectively. Further, the stability test of the cell showed its excellent longtime stability, with decay of only4.3%of the efficiency after180days; meanwhile, such ZnO material exhibited well adhesion with the flexible ITO/PET conductive substrate and thus a reasonable efficiency of2.22%, which is the firsttime application of ZnO nanomaterials with aggregate morphology on the flexible substrate ever reported. As such, the ZnO aggregates prepared via such aqueous method show great prospect and meaning in practical research and application.2. A three-dimensional (3D) micron-scale shuttle-like up-conversion material Y2O3:Er3+(denoted as SUC) and its application in ZnO NA electrode was reported. The function mechanism and effect law of the spectra properties and morphology of SUC on the photovoltaic performance of ZnO NA electrode were investigated intensively and systematically. It was found that:1) SUC is able to convert the near-infrared light into visible light owning to the up conversion of Er3+, the fluorescence spectra (i.e. strong green light band of525-560nm and weak red light band centered at660nm) of which well coincide with the strong absorption spectrum of the D205dye. As a consequence, SUC enabled the ZnO photoanode sensitized by D205dye exhibite obvious photoelectrical reponse to infrared light and thus improved the light absorption and utilizing efficiency additionally.2) In view of the particularly3D micron-scale morphology, SUC, meanwhile, could also enhance the light scattering capability to the incident light of the ZnO photoanode effectively, leading to the synchronous improvement of the light utilizing efficiency. Consequently, the ZnO/SUC composite photoanode exhibited much better light harvesting property than ZnO photoanode and the DSC based on the ZnO/SUC composite photoanode with optimized11wt%SUC attained the largest current density of15.52mA/cm2and highest efficiency of5.63%, with enhancement of28.5%and38.0%in comparison with those of ZnO electrode with no SUC. Through optimizing the active area of the cell, the current density and efficiency further increased to17.08mA/cm2and6.26%, respectively. This is the first report on the application of the spectra conversion materials in ZnO-DSCs, and such result exhibits the rationality and effectiveness of SUC in improving the light harvesting and thus light-to-current conversion efficiency of the DSCs.3. An innovative multidimensional (MD) ZnO composite photoanode with combination of3D ZnO NAs and2D ZnO nanosheets (NSs) was developed. The microstructure characterizations revealed that in such NA/NS photoanode, the ZnO NAs serve as the main backbones, while the ZnO NSs act not only as dense building blocks atop the NA film, but also as bridges connecting the ZnO NA-framework. Through finely tuning the microscale morphology of the MD ZnO NA/NS photoanode and investigating the structure-function relationship intensively, it was found that beside the strong light scattering capabity inherited from the ZnO NAs, the MD ZnO NA/NS photoanode also possesses a much higher overall surface area (from36.9m2/g to56.3m/g) and larger dye-loading amount (from1.58×10-7mol/cm2to3.87×10-7mol/cm2) due to the porosity of ZnO NSs. Meanwhile, the electron transport resistance of the MD ZnO NA/NS photoanode dramatically decreases from22.7ohm to12.8ohm and the electron lifetime also prolongs from2.38ms to6.32ms, owing to the single-crystalline NS bridges serving as favorable electron transport highways within the NA film. Consequently, the MD ZnO NA/NS photoanode attained large current density of17.90mA/cm2and open-circuit voltage of0.631V, leading to a remarkable efficiency of6.66%, which is64.0%higher than that of ZnO NA photoanode without NSs. With incorporation of a ZnO nanocrystallite compact layer to suppress the potential electron recombination, the final efficiency was further enhanced to7.35%(the reported world record of the ZnO based DSCs is7.5%). Such result proves the rationality and effectiveness of constructing MD architectures composed of different mono-dimensional nanostructures of specific features with synergistic effects to offer a better approach to high PCEs.