Study On Optical Absorption And Carrier Transfer Characteristics Of Planar Metasurface Photocathode

Photoelectrochemical CO₂reduction driven by solar energy is a cutting-edge technology that uses solar energy and uses greenhouse gases CO₂ and H₂O as reactants to produce hydrocarbon fuels.Its research will play an important role in our country’s 2030 carbon peak and 2060 carbon neutral tasks.However,there are many obstacles to the reduction of CO₂photoelectrochemical include:thermodynamic obstacles（CO₂ is a non-polar molecule with stable properties and high bond energy）,kinetic obstacles（the mechanism of CO₂ adsorption and product desorption is unclear）and product selectivity obstacles（CO₂ diversified reduction products）.In order to overcome the above obstacles,it is necessary to have a deeper understanding of the specific process of photoelectrochemical CO₂reduction.Therefore,an in-depth understanding of the mechanism of photoelectric catalytic CO₂ reduction is of great significance.In this thesis,starting from the problems of semiconductor light absorption,carrier migration,and fine characterization of intermediate products,the planar metasurface semiconductor photoelectrode structure is proposed.First of all,this thesis proposes a planar metasurface light-absorbing structure with reference to the interference cavity structure,which can obtain a stronger interference resonance cancellation that further reduces the reflectivity and increases absorption rate in semiconductors.Secondly,use ultrathin planar semiconductors as the absorption layer to make the semiconductor thickness closer to the carrier diffusion length,which is more conducive to the diffusion and transfer of carriers.In addition,the use of a planar structure can be considered that the catalytically active sites on the semiconductor surface are uniformly distributed on the semiconductor surface,which facilitates the subsequent study of the photoelectric catalytic reaction mechanism.Since the surface enhanced Raman spectroscopy technology requires the reaction substrate to be gold,silver,and copper,it is not suitable for semiconductor electrodes,so this thesis considers the use of tip-enhanced Raman spectroscopy technology that has no requirement on the type of reaction substrate.In this thesis,some metal nanoparticles are uniformly dispersed on the electrode as plasmon nanoparticles,acting as probes and Raman signal amplifiers,using the strong resonance of the coupling surface plasmon nanoparticles and metal layers to enhance the Raman signal and improve the measurement accuracy of the intermediate product.The planar metasurface electrode structure designed and optimized by our institute can not only overcome the problem that surface-enhanced Raman spectroscopy technology cannot be applied to semiconductor surfaces,but also avoid the susceptibility shortcoming that the signal of the tip-enhanced Raman spectroscopy technology is vulnerable to interference,and avoid the use of core-shell structured nanoparticles that are challenging to prepare.The energy transfer process of photoelectrochemical CO₂ reduction includes:1)absorption of solar photons to generate carriers,2)the migration of carriers to the electrode-electrolyte interface,and 3)the participation of carriers in the interface reaction.This thesis takes the GaN/Sn O₂/Ag（absorption layer/charge selective transport layer/reflective layer）structure as an example,and focuses on the optical absorption and carrier migration characteristics of the planar metasurface photocathode,so as to lay the foundation for the subsequent research on the reaction mechanism.In order to understand in detail the optical absorption characteristics and carrier transfer characteristics of planar metasurface photoelectrodes,and design a more efficient semiconductor photoelectrode,this thesis establishes the reflection characteristic model and energy absorption distribution model of the planar metasurface absorber through the transfer matrix method,used for the study of the reflectivity of the planar metasurface photoelectrodes and the light absorption of semiconductor films.The physical model of the semiconductor photoelectrode is established through the Poisson equation,carrier drift-diffusion equation and carrier continuity equation to study the carrier migration and transfer inside the semiconductor and the solid-liquid interface.After verifying the correctness of the above model through the experiments data in this thesis and the experimental data in other literatures,this thesis uses the reflectance model and the energy distribution model to accurately optimize the thickness of the GaN planar metasurface photoelectrode,and finally determines the thickness of the structure as 73～80 nm GaN and 0～2 nm Sn O₂.Then use the physical model of the semiconductor photoelectrode to analyze the parameters of the electrode in this thesis.The specific effects of important parameters such as doping concentration,flat band potential,carrier mobility,interface transfer rate constant and interface carrier lifetime on the current density of the planar metasurface semiconductor photocathodes are obtained.The research results of this paper also provide a comprehensive and detailed theoretical basis for the design of high-efficiency photoelectrodes.