Study On The Surface Microstructure Optimization And Photoelectric Properties Of TiO₂ And ZnO Photoanodes

In today’s society,people’s demand for energy is increasing,and fossil energy is becoming more and more scarce.What’s more,the environmental pollution caused by this is more and more serious.Faced with this situation,people can only focus on the development of renewable energy sources,among which solar energy is undoubtedly a good choice.For solar cells,high efficiency and low cost are key indicators for solar energy conversion and application.In recent years,it has been shown that quantum dots（QDs）have relatively high extinction coefficient and can absorb most of the sunlight.In addition,it also has the advantages of quantum size effect and multi-exciton effect.Therefore,quantum dots sensitized solar cells（QDSSCs）have attracted great research interest.However,in general,the loading rate of QDs attached to the photoanode material of QDSSCs is low,and the carrier recombination rate is also high,which are not conducive to the further improvement of photoelectric conversion efficiency of QDSSCs.TiO₂,ZnO are a kind of semiconductor materials with low preparation cost,good stability and non-toxicity,so they are often selected as photoanode materials for solar cells.This paper is based on TiO₂,ZnO materials by constructing 3D layered heterostructures and surface modification to improve the loading rate of QDs on the photoanode surface and carrier transfer rate,so that the photovoltaic performance of solar cells has been further improved.The main work of this paper is as follows:（1）The TiO₂nanosheet array films（NSs）with large area exposure of{001}high-energy crystal surface were prepared by hydrothermal method.Then,a ZnO seed layer was attached to the TiO₂NSs by dip-coating method,and ZnO nanorods（NRs）were grown on the surface of TiO₂NSs by a hydrothermal method to prepare a 3D ZnO/TiO₂hierarchical heterojunction films.In order to further improve the load quality of QDs（quantity and density）,we have carried out simultaneous etching and doping treatment on ZnO/TiO₂composite films.By studying the structure,morphology and photoelectric properties of etching and doping samples with different reaction times,it is found that when the etching time is 40 min,the ZnO nanorods become hollow nanotube structure.This hollow nanotube structure not only provides a direct path for charge transfer,but also increases the loading rate of QDs.Finally,CdS QDs were used to sensitize the photoanode material,and the effect of simultaneous etching and doping treatment on the quality of QDs was studied.The results show that after CdS QDs sensitization,the light absorption intensity of CdS/ZnO/TiO₂photoanode increases,and the light absorption range is extended to the visible region.More importantly,after simultaneous etching and doping treatment,the loading quality of CdS QDs is significantly improved compared with untreated ZnO/TiO₂.Finally,the photocurrent density of CdS/ZnO/TiO₂after etching and doping treatment reaches 5.6 m A cm^-2,which is respectively TiO₂and ZnO/TiO₂photoelectrode 41 and 17 times.（2）ZnO NRs were grown on the surface of FTO substrate by a simple hydrothermal method,and CdS NSs were grown on the surface of ZnO NRs by a secondary hydrothermal method to construct 3D ZnO/CdS heterostructure.By adjusting the amount of triethanolamine（TEA）during the growth of CdS NSs,ZnO/CdS composites with different morphologies were obtained,and the prepared materials were subjected to the morphology,structure and photoelectric properties test respectively to explore the effects of triethanolamine（TEA）on the growth and photoelectric properties of CdS NSs.It’s shown that the 3D ZnO/CdS composite material is better than the single-phase ZnO in light absorption and photoelectric properties,mainly because the band gap of CdS is relatively narrower band gap（2.4 e V）and the absorption range of sunlight can be extended to the visible region.In addition,the energy levels of ZnO and CdS are well matched,which can form a good heterostructure,thereby promoting the separation and transport of charge between them.It was also found that with the increase in the amount of the modifier triethanolamine（TEA）,the morphology of the CdS changed from a mixture of nanosheets and small particles to smooth nanosheets and then to aggregated block structures.When the amount of TEA added is 2.5 ml,CdS forms a smooth nanosheet structure.The photocurrent density of this 3D ZnO/CdS composite films is about 9 m A cm^-2at 0 V under the illumination of 100 m A cm^-2,which is 3.6 times that of pure ZnO NRs.The improvement in photoelectric performance is attributed to the following three aspects:（i）This unique three-dimensional structure has multiple reflection and scattering effects,which enhances light absorption;（ii）CdS NSs shorten the transport distance of photogenerated carriers from the photoanode to the electrolyte,so the transport efficiency of photogenerated carriers is high;（iii）The three-dimensional ZnO/CdS heterogeneous film provides a larger surface area,and forms a bulit-in electric field between ZnO and CdS,which promotes the separation of photogenerated carriers at the interface.