Investigation Of Photo-transistor Based On Quantum Dots/Nanowires/Graphene

As an important method to obtain optical information,photoelectric detection has been widely used in weapons and equipment,security protection,information communication,medical diagnosis and entertainment.However,most of the existing commercial photodetectors use inorganic semiconductors as the detection target.Meanwhile,the silicon-based circuit is utilized to read out the signal.Due to the high temperature preparation and the detection target complicated with the readout circuit in soldering process,the detector cannot be applied in the flexible electronic devices.In recent years,the preparation of large-scale photodetection arrays on flexible substrates with low-dimensional nanomaterials has become a research hotspot.Low-dimensional nanomaterials,such as zero-dimensional material quantum dots,one-dimensional material nanowires and two-dimensional material graphene,etc..Based on its high photoelectric conversion efficiency and high-frequency response,it has the potential to replace the traditional photoelectric conversion target.Phototransistors are used to replace traditional photodiode structures due to their gate modulation amplification characteristics.However,for the interface defects and band structure mismatch,the optical responsivity and detection noise of low-dimensional material phototransistors are bound.In this thesis,based on the phototransistors with low-dimensional nanomaterials,we focus on improving the photoelectric conversion performance,such as photoelectric responsivity and optical sensitivity of phototransistors.By modifying the interfacial properties between materials and optimizing the energy band structure of the phototransistors,the performance of the phototransistor has been improved.Besides,the detection noise is reduced by printing the photodetection circuit,and photo signal is read out.The research results are as follows:Firstly,with the optical localization effect,the zinc oxide nanowire is utilized to form an ultraviolet phototransistor,combined with a zinc oxide based thin-film transistor.The laser annealing of the zinc oxide nanowire/zinc oxide-based thin film composite channel improves the crystallinity of the zinc oxide-based film and improves the interface characteristics between the zinc oxide nanowire and the film.The photo-sensitivity,responsivity and detectivity are effectively enhanced.The results show that after high energy density laser annealing?350 mJ/cm²?,the film grain size increases,and the defect density in the nanowire and film interface decreases.Compared with the unannealed device,the photocurrent signal is increased by an order.And the optical photo-responsivity of the ultraviolet phototransistor?detection wavelength 280 nm?is as high as 1.9×10⁵/(2,the external quantum efficiency is 8.7×10⁷%,the photo sensitivity is 9.5×10⁵,and the photo-detectivity is 8.12×10¹⁶9)0).The work was published on Materials Letters.Secondly,it is proposed to use reduced graphene oxide?RGO?fragments to modify the energy level between cadmium selenide quantum dots and zinc oxide nanowires for improving the injection efficiency of photogenerated carriers into the transistor channel.And the photo-responsivity of the heterojunction phototransistor is improved.It is found that RGO is easy to form multi-layer graphene structure and defects during the preparation process.Therefore,includes the intrinsic energy level of graphene,the RGO fragments have continuous defect energy levels.The photo-generated carriers generated by the CdSe quantum dots with incident light are influenced by the built-in electric field,and the continuous defect level of the graphene fragments is utilized to efficiently transfer the photo-generated carriers to the transistor channel.Since the absorption peak of CdSe quantum dots is at 580 nm,the phototransistor exhibits high photoelectric responsivity in the ultraviolet to visible light bands.When the incident light wavelength is 215 nm,the photo responsivity is as high as 10⁵ A/W;when the incident light wavelength is 580 nm,the photo responsivity is as high as 2000 A/W.Compared to the photo-transistor without graphene fragments,the responsivity in the visible band is increased by a factor of 100.The work was published on Nano-Micro Letters.Thirdly,for solving the low signal-to-noise ratio of quantum dots photo-transistors,an inverter circuit composed of phototransistors is proposed to improve the detection signal-to-noise ratio.The inkjet printing process is optimized,and a near-infrared detecting inverter based on PbS quantum dots is prepared and the photodetection performance is improved.It is studied that by adjusting the annealing process of the printing transistor,the correlation between grain size,roughness and defect density is analyzed,and the electrical characteristics of the printing transistor are improved.By printing a phototransistor-based inverter circuit,the photo-generated voltage can reach 2.1 V,the photoelectric response is as high as 1.4×10⁶ V/W,and the signal-to-noise ratio is increased to 52 dB.Part of the work was published on Colloid and Polymer Science and Solid-State Electronics.