Investigation Of Self-powered The Performance For CuInS₂/TiO₂ Heterojunction Photodetectors

Photodetectors（PDs）can convert optical signals into electrical signals and are basic components in the fields of optical communication,environmental detection,integrated circuits,and sensing equipment.In recent years,self-powered PDs that can be monitored in real time,unattended and do not require power supply are conducive to the miniaturization,integration and intelligence of devices,and play an important role in future optoelectronic devices.In order to meet the application of self-powered PDs in various fields,the photoresponse characteristics of the device need to be further improved.One-dimensional（1D）TiO₂ nanorod（NRs）arrays and two-dimensional（2D）Cu In S₂（CIS）nanoflakes（NFs）can be constructed type II heterojunction PDs with photovoltaic effects,and the ordered 1D NRs array structure of TiO₂ is conducive to carrier directional transmission.In multivariate CIS semiconductors,the stoichiometric ratios of Cu,In and S can change the band structure of semiconductors,adjust the electron transport behavior and optical properties,affect the generation,separation and transmission process of photogenerated carriers in CIS/TiO₂ devices,and then regulate the self-powered optical response characteristics of heterojunction devices.In this paper,a simple solvothermal method was used to deposit CIS NFs on the surface of TiO₂ NRs to prepare a wide-spectrum,high-performance CIS NFs/TiO₂NRs heterojunction self-powered photodetector.By changing the precursor solvent of CIS NFs,Cu/In ratio and the concentration of the surfactant CTAB,the interface contact between CIS and TiO₂ heterojunction is adjusted and the stoichiometric ratio of CIS,the band gap width and Fermi energy level position of CIS are regulated,the built-in electric field of CIS/TiO₂ heterojunction is improved,the separation and transmission of photo-generating carriers is promoted,and the optical response characteristics of self-powered heterojunction devices are enhanced.The main research contents and conclusions of the paper are as follows:（1）The effect of precursor solvents on the growth process of CIS NFs films was studied by changing the mixed solvents of CIS precursor DMF and glycol and the mono-solvent of ethanol.It was found that thiourea has little solubility in ethanol solvents,the release of S^2-is slow,and S^2-binds with[In（Tu）_n（H₂O）_x]³⁺and[Cu（Tu）_n（H₂O）_x]⁺metal complex ions on the surface of TiO₂ NRs,which favors the heterogeneous nuclei and growth of CIS NFs.The CIS/TiO₂ heterojunction prepared by using ethanol solvents to deposit CIS NFs in TiO₂ NRs has a large interface contact area and a built-in electric field in the heterojunction promotes the generation and separation of heterojunction photo-generating carriers.The CIS/TiO₂ device exhibits a wide spectral photoresponse from UV to visible at 0 V bias,with the enhanced responsivity,detectivity,light sensitivity,linear dynamic range and response time is 44.8 m A/W,1.3×10¹² Jones,2.7×10~4,88.5 d B and 2 ms at 365 nm illumination.The photocurrent of the device under mixed UV and visible light illumination is a linear superposition of monochromatic UV and visible illumination time currents,with UV and visible dual-band photoresponse characteristics,which can be applied to UV-encrypted visible light communication.（2）By changing the Cu_xIn_2-xS₂（0.8≤x≤1.3）precursor Cu/In ratio,the effects of Cu_xIn_2-xS₂ stoichiometric ratios deposited on the surface of TiO₂ NRs on CIS morphology,crystal structure,energy band structure,and optical and electrical properties were studied.The study found that with the increase of x value in the precursor,the proportion of Cu/In in Cu_xIn_2-xS₂ NFs films increases,the radial size and thickness of Cu_xIn_2-xS₂ NFs increase first and then decrease,and The size of Cu_1.1In_0.9S₂ NFs is the largest when the x value is 1.1.Cu_xIn_2-xS₂ is a chalcopyrite phase when the x value in the Cu_xIn_2-xS₂ precursor does not exceed 1.1,and the Cu₂S precipitation phase occurs when the x value increases to 1.3.The Cu_xIn_2-xS₂ film deposited in the precursor when the x value does not exceed 1.1,and as the optical band gap decreases as the x value increases,the closer the Fermi energy level of the NFs film is to the top of the valence band,and the corresponding Cu_xIn_2-xS₂/TiO₂heterojunction devices increase in optical response,detection rate,linear dynamic range and sensitivity,the Cu_1.1In_0.9S₂/TiO₂ devices deposited at x=1.1 exhibit up to2.6×10~4 light sensitivity,788.5 m A/W light responsivity,4.6×10¹² Jones detectivity,and 88 d B linear dynamic range at 0 V bias and 365 nm（2 m W/cm~2）illumination.The built-in electric field with a large heterojunction provides a greater driving force for the interface separation and transmission of photogenerated carriers increasing the photocurrent.（3）By changing the concentration of the surfactant CTAB in the CIS precursor,the effect of CTAB on the CIS morphology,crystal structure,and optical and electrical properties of TiO₂ NRs surface deposition was studied.It is found that CTAB can regulate the nucleation rate of Cu⁺and In³⁺on the surface of TiO₂ NRs,increase the proportion of Cu/In in CIS NFs films,and improve the distribution of CIS NFs.With the increase of CTAB concentration,the light absorption in the visible area of CIS/TiO₂ heterojunction film increased.When the CTAB concentration is 6mmol/L,the stoichiometric ratios of Cu,In,and S are close to 1:1:2,and the light sensitivity,responsivity,detectivity and light sensitivity for the self-powered CIS/TiO₂heterojunction device is 8.3×10~3,315.8 m A/W,2.9×10¹² Jones and 78 d B at 365 nm（2 m W/cm~2）illumination.