Self-Powered Photodetectors Based On ZnO Nanostructures

The ultraviolet light of earth is mainly from radiation of the sun.Ultraviolet light has been used in medical science,sterilization,identification and other fields.But it can react with the human skin.When it is serious,it can cause skin cancer.Therefore,the detection and prevention of ultraviolet light have aroused worldwide concerns.Since the 1950s,UV detection technology has developed very rapidly.It has played a very important role in the remote control,space security communications and so on.The development of semiconductors has experienced the first generation of semiconductors which is represented by silicon,and the second generation of semiconductors which is represented by gallium arsenide.Since the twenty-first century,wide bandgap semiconductor materials have become the third generation of semiconductor materials.Wide bandgap semiconductors can only absorb UV light and are resistant to high temperatures.The UV photodetectors based on the third generation of semiconductor materials can save the cost of filters,and have good photoelectric properties.The UV photodetectors based on GaN,SiC and other wide bandgap semiconductors have become a hotspot.However,the growth process of such semiconductor material's single crystal is complex and the growth substrate is expensive.These conditions are not conducive to the large-scale production of these devices.Therefore,looking for a wide bandgap semiconductor material with simple preparation process,low cost,good performance is particularly important.The lattice structure of ZnO is similar to GaN.At room temperature,ZnO has a bandgap of 3.37 eV.ZnO can well absorb UV light shorter than 385nm,and has a light blindness with visible light.In addition,ZnO has the advantages of large exciton binding energy(about 60 me?),higher electron mobility,lower recombination rate of electron-hole pairs and longer carrier lifetime.ZnO is easier to crystallize,and has a high growth anisotropy.ZnO can form a variety of nanostructures,such as nanorods,nanoneedles,nanosheets,nanoflowers and so on.The preparation process of ZnO is also diverse.People have prepared a variety of ZnO structures with different morphologies through the hydrothermal method,thermal evaporation,plasma sputtering,chemical vapor deposition and other methods.The preparation process of ZnO is relatively simple,so it can be easily mass production.The traditional silicon-based detectors have been used in many practical applications,but they need extra bias to work effectively,which will limit their applications in some special circumstances,and their production process is also much complex.Photochemical battery type UV photodetectors with sandwich structure can operate well without any additional bias because they have a built-in electric field formed between the semiconductor and the electrolyte.The advantages of ZnO and the photochemical battery type self-powered UV photodetector have attracted our interests.In this paper,we have prepared ZnO semiconductor materials by the hydrothermal method,and fabricated a photochemical battery self-powered UV photodetector based on ZnO nanostructures,and then study its photoelectric properties.Firstly,we prepared the ZnO nanorods by the hydrothermal method as the photoelectric anode,and the maximum response of the resulted UV photodetector was 0.02 A/W.We notice that ZnS has a bandgap of 3.7 eV,and belongs to the group II-VI compound,which is same to ZnO.They have matching energy levels,which can form a type ? heterojunction.And ZnS particles adhere to the surface of ZnO nanorods,can increase the scattering of light,which can increase the UV light absorption.Therefore,we have modified the surface of ZnO nanorods and deposited ZnS nanoparticles on the surface of ZnO nanorods to form ZnO/ZnS nanorod core-shell structure.A self-powered photoelectrochemical cell-type UV detector was fabricated using the ZnO/ZnS core-shell nanorod array as the active photoanode and deionized water as the electrolyte.Without any external bias,the maximum response occurs at 340nm,about 0.056A/W.Compared with the UV detector based on pure ZnO nanorod structure,the responsivity is increased by 180%.When irradiated with UV light at 365 nm,the response time was measured to be about 0.04 s.Secondly,as we know,ZnO can grow into a variety of nanostructures,and the specific surface area of different nanostructures is different.The porous ZnO nanosheets has a large specific surface area than nanorods,which is beneficial to increase the contact area between the material and the electrolyte,thus it can improve the responsivity of the UV detector.We prepared a porous ZnO nanosheet structure by the hydrothermal method.Then we fabricated a self-powered photoelectrochemical cell-type UV detector using the porous ZnO nanosheet structure as the active photoanode and deionized water as the electrolyte.The maximum response is higher than 0.12 A/W.In the case of zero bias,the device can run normally,and has a faster response time.The rise time and fall time of this device are both about 0.04s.Under the irradiation of 0.0471 mW/cm2 ultraviolet light at a wavelength of 365 nm,the open circuit voltage was 0.4 V and the short circuit current was 5.64 uA/cm2.