Research On Organic-inorganic Hybrid Ultraviolet Photodetector Based On TiO₂

Ultraviolet?UV?radiation is electromagnetic waves in the range of 10 nm-400 nm in theelectromagnetic radiation spectrum.In addition to daily sunlight,its sources also include ultraviolet emitting devices,flames,and specific cells.UV photodetectors?PDs?have wild applications in many field such as medicine,military,environment and so on.With the development of technology,the structure of the UVPD and the material used for UVPD fabricating also has been developed continually.However,in the course of research,researchers have discovered such contradictions and needs in the design and preparation of UVPDs.From the perspective of device structure,photoconductive PD has a higher gain and responsivity,but the device also has a slow response speed and high noise single.However,photovoltaic PD is faster in response speed and good at controlling the noise single,but the photocurrent can also be limited.From the perspective of materials,the inorganic semiconductor materials have better performance in photoelectric property,but have less choices and not easy to be as high quality crystals.On the other hand,organic semiconductor materials have more choices and they are easier to be made.But also their photoelectric properties are not good enough.The above problems make it difficult for UVPDs to have breakthrough in photoelectric performance.With the continuous advancement of research,the requirements for the performance of UVPDs in various fields are also increasing,including faster response,higher light and dark suppression ratio and even have more specific detection range etc.Based on the above,the goal this paper wants to realize is to achieve a high-performance ultraviolet detector with high responsiveness and fast response speed by preparing semiconductor materials with excellent photoelectric properties and selecting a device structure that can provide a good foundation for the detector performance.preparation.Therefore,this paper focused on the preparation of materials and the selection of device structure,and proved that the use of hybrid organic and inorganic semiconductor materials to prepare heterojunction photovoltaic UVPDs is a reasonable and efficient method to improve the detection performance of devices.In addition,the optoelectronic performance of the device is further optimized by selecting doped materials with excellent optoelectronic performance.Based on this,the thesis mainly carried out the following research work:At first,a TiO₂ thin film photoconductive UV detector was designed and prepared.The TiO₂ film prepared by the sol-gel method is uniform,flat and has a good quality.The seasonable annealing temperature enables TiO₂ to have an anatase-type crystal structure,which is suitable for preparing an UVPD.Subsequently,the detector was prepared by photolithography and magnetron sputtering method.The light absorption of the detector is only in the UV wavelength range,no filter is required;at a bias of 7 V,the rise and fall times of the device are 1.09 s and 9.43 s;the light current and the dark current are 1.08?A/mm² and 10 nA/mm²,therefore the light and dark suppression ratio is about three orders of magnitude which means the detector has a great gain.The excellent photoelectric properties of TiO₂ materials have laid a foundation for the preparation of heterojunction photovoltaic UV detectors.The detailed preparation process and mechanism analysis are given in Chapter 2.In the chapter 3,a thin film of organic semiconductor material PVK is added on the basis of TiO₂ film.The two materials constitute a heterojunction of organic-inorganic hybrids,thereby preparing a PVK/TiO₂ photovoltaic ultraviolet detector.And by adjusting the thickness of the PVK film,the specific gravity of the two materials in the light absorption process changes.Combined with the difference between the absorption peaks of the heterojunction and the ITO substrate,two sets of devices with different optimal detection wavelengths were obtained.After a series of comparative experiments,two types of films with the optimal thickness were selected according to the different optimal detecting wavelength.Among them,in the thinner group,the dark current density of the optimal device is 0.02?A/cm²,the rise and fall times are 28 ms and 11 ms,respectively,and the peak value of the responsivity appears at 320 nm.In the thicker group,the dark current density of the optimal device is 0.025?A/cm²,and the peak of the responsivity appears at 340 nm.Through data comparison,it can be found that,compared with photoconductive UV detectors,photovoltaic detectors have lower dark current and shorter response/recovery time.This is due to the structural advantages of the heterojunction.Through the existence of the depletion layer and the change of the energy band,the dark current and noise are effectively suppressed,and the response speed of the device is improved.In Chapter 4,based on the optimal device of the thin layer PVK/TiO₂ UVPD mentioned in chapter 3,the method of doping CuInS₂/ZnS quantum dots in PVK thin films further improves the light-dark suppression ratio of photovoltaic detectors.This kind of quantum dot has high electrical conductivity,so it can make up for the disadvantage of PVK organic material in electrical conductivity and transport the carriers more effectively.In addition,the material has good light absorption in the UV wavelength range,so it can improve the utilization of light by the device.Finally,the unique character of quantum dots can attract electrons,make the depletion layer on the TiO₂ side thicker,and enhanced the electric field intensity of the depletion layer and can separate photo-generated carriers more effectively,thereby improving the photoelectric performance of the device.After doping quantum dots,the responsivity of the modified device increased by about 4.2 times,and the rise time was shortened to 24 ms.In Chapter 5,based on the optimal device of the thick layer mentioned in chapter 3,the photoelectric performance of the detector is improved by doping PFTBT polyfluorene material in the PVK layer.Because the solution used to make thick PVK layer is relatively sticky at this time,quantum dots,which are inorganic materials,are easy to be agglomerated dispersed in PVK solution,which affects device performance.Therefore,PFTBT polyfluorene with better dispersibility in organic materials was selected.This material can improve the electrical conductivity and the light absorption of the organic material,optimize the property of the material.In addition,the introduction of PFTBT is used as a trap center which can greatly increasehe conductivity of the material and extend carrier life,so that the optical and electrical properties of the device can be significantly improved.After doping,the responsivity of the device is increased by about 3.7 times,and the rise time is shortened to 22 ms.In this thesis,photovoltaic detectors were prepared using hybridization of organic and inorganic materials.Through the introduction of space charge regions and potential barriers,a significant suppression of the dark current of the device and a significant increase in response speed are achieved.By introducing quantum dot material into the thin film layer of organic materials,the quantum size effect and surface effect are used to attract and trap electrons,thereby enhancing the strength of the space electric field,improving the efficiency of carrier separation,and optimizing the performance of the device.In addition,by doping the PFTBT polyfluorene material,through the trap effect,the carrier recombination is reduced,the life of the carrier is significantly prolonged,and the performance of the photovoltaic UV detector in response and response speed is further improved.This paper provides a valuable reference for the material selection,structural design,and working mechanism analysis of UVPDs.