Performance Enhancement And Application Of Self-powered TiO₂ Based Heterojunction Photodetectors

The wide band-gap semiconductor TiO₂has stable physical and chemical properties and high light absorption coefficient in the ultraviolet range.It is easy to construct composite heterojunctions with other materials and is widely used in photocatalysis,solar cells and photodetectors.The heterojunction based on photovoltaic effect can realize photodetection without external voltage driving,and meet the requirements of miniaturization,integration and low power consumption of electronic components.One-dimensional（1D）TiO₂nanorods（NRs）have the characteristics of carrier directional transmission and light trapping.Self-powered PDs based on 1D TiO₂NRs are advantageous for achieving high photocurrent and fast response speed.The generation,separation and transport of photogenerated carriers at different wavelengths will affect the photoresponse characteristics of TiO₂based self-powered PDs.In this paper,the light absorption and carrier transport capacity of TiO₂based self-powered PDs are improved by controlling the orientation of the TiO₂NRs and the local interfacial electric field,and the device performance is enhanced.The application of TiO₂-based self-powered PDs in optical control logic,machine vision,encrypted optical communication and non-destructive testing is explored.The main research contents and conclusions of this thesis are as follows:（1）TiO₂NRs are synthesized by hydrothermal method.The orientation,size and defect state of NRs are changed by adding anhydrous ethanol in the precursor solution,and then the photoelectric properties are regulated.It is found that the length and the cross-sectional area of the TiO₂NRs with anhydrous ethanol are increased,the orientation becomes better,the light absorption is improved and the Fermi level is close to the conduction band.The In/TiO₂/FTO device exhibits good self-powered photoresponse characteristics under ultraviolet illumination.The In/TiO₂PDs prepared when the ratio of anhydrous ethanol:water is 29:1 have a responsivity of4.8×10^-3A/W under 375 nm illumination.The rise time and decay time are 19 ms and86 ms,respectively.Using the linear photoresponse characteristics of In/TiO₂-0.5 ml PDs under monochromatic light（375 nm and 405 nm）and mixed light（375 nm+405nm）,the optically controlled“and”gate logic operation is realised.（2）The core-shell structure can form more heterojunction interfaces,which helps to improve the photocurrent of heterojunction PDs.In this work,TiO₂NRs are used as a template to obtain SrTiO₃（STO）shell layers by in situ conversion on the surface of TiO₂NRs by a secondary hydrothermal process.The band alignment of STO/TiO₂heterojunction conducive to the transport of photogenerated carriers is obtained by regulating the conversion of STO nanocrystals（NCs）.The ferroelectric polarization effect inside STO is used to enhance and suppress the photocurrent by polarization bias.Using the external polarization bias to control the photocurrent of the device,an irradiance adaptive self-powered PDs that can be applied to the field of machine vision is proposed.（3）The combination of wide band-gap and narrow band-gap semiconductors can not only broaden the spectral response band of photodetectors,but also improve the photoresponse characteristics of the device by band engineering.In this chapter,Sb₂Se₃nanoparticles（NPs）are composited on TiO₂NRs films by a two-step potentiostatic electrochemical deposition.The thickness of Sb₂Se₃,Se element content and crystal quality are changed by controlling the annealing temperature.It is found that the Au/Sb₂Se₃/TiO₂/FTO device exhibits a wide spectral self-powered photoresponse in the ultraviolet-visible-near infrared（UV-VIS-NIR）range.The Sb₂Se₃-290℃/TiO₂PD has a responsivity of 13.48 A/W,a detectivity of 7.9×10¹¹Jones,and a rise/decay time of 18/20μs under 405 nm illumination.The light signals of different bands are combined and encoded to realize the encrypted transmission of light information in infrared light communication and visible light communication.This result is of great significance for the improvement of information transmission security in light communication.In addition,the good infrared photoresponse characteristics of the device are used to construct the basic model of the infrared fault detection application.