Investigation Of Self-powered UV Photodetector Based On BiOCl/TiO₂ Heterojunctions

Photodetectors can convert optical signals into electrical signals,which are widely used in many fields such as flame,ozone and environmental monitoring,optical communications,vision and biomedical imaging and motion detection.With the rapid development of intelligent manufacturing and the internet of things technology,the photodetector demand for wireless,sustainable,multi-functional and independent sensor networks is increasing.The traditional photodetector with external power supply to achieve photoelectric signal conversion cannot meet the growing new demand.Self-powered photodetectors that extract energy from the surrounding environment without the need for batteries or supply external power have the advantages of maintenance-free,wireless operation,long service life and low power consumption and can work for a long time in some special environments.The P-N junction self-powered photodetector based on the photovoltaic effect can use the optical signal to drive the device to work and simultaneously realize the detection of the optical signal.At present,the photoresponse and response time of this type of device need to be further improved to meet the needs of applications in various fields.The crystalline quality of P-type and N-type semiconductors and the contact of the P-N junction interface affect the generation,separation,and transport of photo-generated carriers,which affects the performance of self-powered photodetectors.In this article,a Bi OCl/Ti O₂heterojunction self-powered photodetector with ultraviolet(UV)response characteristics is constructed.The energy band structure of type II heterojunction is used to realize the separation and transmission of photo-generated carriers without external bias,and realize the rapid response to UV under the condition of self-powered.By adjusting the crystal structure,energy band structure and interfacial contact of Ti O₂and Bi OCl,the separation and transport efficiency of photo-generated carriers in the heterojunction are improved and the photoresponse characteristic of the Bi OCl/Ti O₂heterojunction self-powered UV photodetector is improved.The main research contents and conclusions of this article are as follows:(1)The built-in electric field and interface transfer resistance of the heterojunction affect the separation and transport of photo-generated carriers in the heterojunction.By adjusting the thickness and Fermi level of the anatase phase Ti O₂film,the built-in electric field in the Bi OCl/Ti O₂heterojunction and the transport path of photo-generated electrons in the heterojunction is optimized,the separation of photo-generated electron-hole pairs at the heterojunction interface is improved,the recombination probability of carriers in the Ti O₂film is reduced and the UV light response characteristics of Bi OCl/Ti O₂devices is improved under self-powered conditions.As the thickness of the Ti O₂film is 1?m and the built-in electric field of the heterojunction is 0.66 V,the Bi OCl/Ti O₂device achieves an on/off ratio of 10~3,a photoresponse of 0.17 A/W,a photo detection rate of 1.3×10¹²Jones and a light response speed of 23 ms under 350 nm light(0.18 m W/cm~2).(2)Heterojunction with ordered array structure helps to improve light absorption and carrier directional transport capability.The Bi OCl/Ti O₂heterojunction self-powered UV photodetector is constructed by depositing two-dimensional Bi OCl nanosheets on the surface of one-dimensional rutile phase Ti O₂nanorod arrays.By changing the concentration of Bi and Cl ions in the Bi OCl precursor solution,the deposition of Bi OCl on the surface of Ti O₂nanorod array is regulated,the heterojunction interface contact is optimized,more interfaces for photo-generated carrier separation is provided,the heterojunction interface transfer resistance is reduced and the carrier transmission capacity is improved.The Bi OCl/Ti O₂device prepared with the precursor solution Bi and Cl ion concentration of 25 mmol/L showed better self-powered UV light response characteristics,reaching a on/off ratio of 10~3,a photoresponse of 0.14 A/W,a photo detection rate of 4.8×10¹¹Jones and a light response speed of 7 ms under 380 nm light.In heterojunctions of constructed semiconductors with ferroelectric polarization properties,the depolarization field inside the ferroelectric material affects the separation and transport properties of photo-generated carriers.(3)The depolarization field inside BiOCl is regulated by changing the direction and magnitude of the applied electric field.When the direction of the depolarization field and the built-in electric field of the heterojunction are opposite,the photocurrent value of the device decreases and when the two directions are the same,the photocurrent value of the Bi OCl/Ti O₂heterojunction device increases with the increase of the applied electric field.The depolarization field promotes the separation and transport of photo-generated carriers in Bi OCl and improves the photoelectric response characteristics of Bi OCl/Ti O₂heterojunction photodetectors.After the device is subjected to a polarization voltage of 5 V,the photoresponse is 0.28 A/W,the photosensitivity is 2×10~3and the photo detection rate is 1.8×10¹²Jones under self-powered conditions and 380 nm.In addition,using the polarized light response characteristics of Bi OCl/Ti O₂heterojunction devices,the polarized light of different polarization directions can be encoded,which can realize the encrypted information transmission of ultraviolet polarized light and improve the reliability and security of data transmission.