Preparation Of Cu₂O Self-powered Visible Photodetector By Thermal Oxidation And Electrochemical Deposition And Performance Optimization

Cu₂O is an intrinsic p-type semiconductor oxide,with a bandgap width of about2.0 e V,and because of superiority like simple and multiple preparation methods,high UV-Visible light absorption coefficient,and great hole diffusion length,it is extensively concerned as a material for the light-absorbing layer of the photodetector.However,the Cu O impurity phase is easily mixed into Cu₂O during the preparation process,and the interface between Cu₂O and Cu O increases carrier scattering,making it difficult to obtain high mobility.In addition,Cu₂O-based visible photodetectors mostly use heterojunction devices.The Cu₂O heterojunction photodetector enhances the recombination rate of photo-generated carriers in the depletion layer due to the conduction band offset,the low lattice matching of the heterointerface and the high density of the interface state that lead to the generation of compound current and greatly reduces the efficiency of photodetector.For that,this paper,taking the improvement of electrical properties of Cu₂O thin films and the optimization of structure and performance of Cu₂O heterojunction and homojunction photodetection as the major objectives,prepared Cu₂O/Cu thin films,microwires,and nanonetworks by employing the thermal oxidation and electrochemical deposition methods,making a variety of self-powered photodetectors such as Cu₂O/Cu graded heterojunction and Cu₂O PN homojunction,and expanding the application of Cu₂O in the flexible,omnidirectional light,and transparent photodetection.The specific research contents are as follows:（1）The preparation of Cu₂O/Cu thin films and core-shell microwires with the thermal oxidation method.First,using the high-temperature thermal oxidation copper foil and copper microwire method,the concentration gradient of the reducing gas and oxygen within the annealing furnace was adjusted,preparing Cu₂O/Cu thin films and core-shell microwires with adjustable thickness,respectively,and combined with the thermal oxidation principle of copper,the regulation mechanism model for the thickness of Cu₂O prepared with the thermal oxidation method was established.Secondly,with the photoluminescence spectroscopy,X-ray photoelectron spectroscopy,electron paramagnetic resonance spectroscopy and other characterization methods,the density distribution of oxygen vacancies in Cu₂O thin films and micro-lines was studied,and it’s proved that there lies a gradient distribution of oxygen vacancy density inside Cu₂O.Based on the energy band theory,this paper proposed the energy band model hypothesis for the gradient junction within Cu₂O prepared with the thermal oxidation method,which lays a material foundation for the construction of visible light detectors.（2）Preparation and properties research of planar and microwire graphene/Cu₂O/Cu graded heterojunction self-powered photodetectors.Taking graphene as a transparent electrode,planar and microwire graphene/Cu₂O/Cu visible light detectors were made.The effect of the Cu₂O layer thickness on the detector’s performance was studied,and it’s found that with the decrease of Cu₂O thickness,the response peak showed blue-shift,and the visible light responsivity first increased and then decreased.When the thickness of Cu₂O is reduced to 650 nm,which matches the penetration depth of visible light within Cu₂O,driven by the spontaneous graded electric field of Cu₂O,the photo-generated carriers are rapidly separated,facilitating the device to own the highest response to visible light under zero bias,reaching 375m A/W（500 nm）,the response recovery time reaches 1μs,and the detection performance index is higher than that of similar Cu₂O-based photodetectors.After thousands of times of bending,the graphene/Cu₂O/Cu planar visible light detector maintains the responsivity to visible light above 95%,showing a stable function in bending detection.What’s more,the microwire detector displays excellent omnidirectional photodetection stability.The research shows that the Cu₂O/Cu structured detector has a fantastic application prospect in the field of flexible optoelectronic devices.（3）The preparation of Cu₂O/Cu thin films with the electrochemical deposition method and performance research of homojunction visible light detectors.n-Cu₂O thin films and p-Cu₂O thin films were prepared using the electrochemical deposition method.The change law of conductivity type and carrier concentration of Cu₂O thin films with the electrolyte’s p H was studied.It’s found that the Cu₂O thin film prepared from acidic electrolyte shows the n-type conductivity,and that from alkaline electrolyte has the p-type conductivity.Moreover,the hole concentration of p-Cu₂O films increases monotonically with the rise of the electrolyte’s p H.The n-Cu₂O/p-Cu₂O homojunction visible light detector was prepared.Due to the advantages of low interface state density and low conduction band deviation at the homojunction interface,under zero bias,the n-Cu₂O/p-Cu₂O homojunction visible light detector has a maximum responsivity of 45 m A/W for the visible light band of 450 nm,with a response time of 100 ms.In addition,with the Ag nanowire network as the template and electrode,the Cu₂O@Ag network was constructed with the electrodeposition method.Based on the change law of morphologies and deposition rates of p-Cu₂O and n-Cu₂O investigated by SEM,it is found that p-Cu₂O grows uniformly and densely along the surface of Ag nanowires,and its thickness increases linearly with the electrodeposition time,while n-Cu₂O grows inhomogeneously,following the Volmer-weber（VW）mode,and its thickness increases nonlinearly as the electrodeposition time extends.With the increase of nucleation sites,the VW structure expands steadily to eventually form a dense n-Cu₂O network.Also,an n-Cu₂O/p-Cu₂O homojunction network transparent visible light detector was designed and prepared,whose visible light transmittance exceeds 55%,and responsivity for visible light reaches up to 25m A/W（490 nm）at zero bias voltage.For the first time,a Cu₂O ultra-soft visible light detector based on PDMS substrate was prepared,whose capability in light detection remains stable when the bending angle reaches 180°.