Defect Control And Interface Engineering Of Tantalum Nitride Thin Film Photoanode

The continuous increase in the total global energy consumption and global warming continue to drive scientists around the world to vigorously develop green and renewable energy.Hydrogen production by photoelectrochemical（PEC）water splitting is an important direction in the field of new energy research and development by scientists.Hydrogen production by PEC water splitting,aims to split water molecule into hydrogen and oxygen through the photogenerated carriers by semiconductor materials under the irradiation of sunlight.The oxygen generation reaction on photoanode is a four-charge continuous reaction involving multiple steps,which requires high activation energy and fast carrier separation efficiency.Meanwhile,the photogenerated holes,which generated by semiconductor under the sunlight irradiation,have strong oxidability that liable to cause the corrosion of semiconductor materials to further degenerate the PEC efficiency.Therefore,the oxygen production process on photoanode is the main factor that affecting the efficiency of PEC water splitting.The key to improve the efficiency of PEC water splitting is to find a suitable photoanode semiconductor material,which has good stability,high carrier concentration and appropriated band position.Tantalum nitride（Ta₃N₅）has been paid widely attention by scientists because of its suitable bandgap width（～2.1e V）,ideal band position,thus theoretically having excellent on-set potential,carrier density and carrier mobility.The theoretical maximum photoelectric conversion efficiency（PCE）of Ta₃N₅ photoanode is up to 15.9%.However,the actual reported PCE of Ta₃N₅ photoanode devices is still limited at2%～3%,which is far below from its theoretical value,and the stability of devices cannot meet the requirements of commercial applications.It is mainly because Ta₃N₅has large amount of intrinsic defects.These defects not only cause the composite center of photogenerated carriers,which reduce the photogenerated current density,but also proved to be effective in pinning the surface Fermi levels from band bending.In addition,some researches have shown that Ta₃N₅surface states cause serious carrier recombination and self-oxidation,which lead to the unstable of Ta₃N₅ photoanodes.Therefore,our main research content is to find an appropriate method to fabricate Ta₃N₅photoanode,and improve its PEC water splitting performance and stability.In this dissertation,electron-beam evaporation（EB）is employed as preparation method of Ta₃N₅ light-absorbing layer.The PEC water splitting performance of Ta₃N₅photoanode devices improved by optimizing the experimental conditions,such as nitrided duration,nitrided temperature,NH₃ flow rate and types of oxygen-producing co-catalyst.Finally,we successfully fabricated Ta₃N₅ photoanode device with high efficiency oxygen-producing co-catalyst Co Ni Fe-B_i modification:the highest PCE of Ta₃N₅ photoanode achieved 2.11%,the onset potential of device is 0.48 V_RHE and the current density is 7.3 m A/cm²（1.23 V vs.RHE）.To further investigate the forming mechanisms and operational principles of each defects in Ta₃N₅ film,we adjust the defects concentration by changing the NH₃ flow rate during nitridation process,and test the PCE of Ta₃N₅ photoanodes subsequently.We find that with the increase of NH₃ flow rate,the PCE of Ta₃N₅ photoanode increased first and then decreased.The onset potential,saturation photocurrent density and fill factor also regularly changed obviously.Multiple characterization methods were employed to analyze the types and concentrations of defects inside the Ta₃N₅ film.The defect levels of O_N,V_N and Ta³⁺were experimentally explained,and their effects on the catalytic performance of Ta₃N₅ photoanode were understood.Finally,by establishing a physical model,the previous experimental results were further verified through theoretical calculations.Based on the above defect research,this dissertation designed a pre-treatment process of the precursor film in the preparation process of Ta₃N₅ photoanode:using hydrogen peroxide to increase the O_N shallow level defect concentration in the film,and reduce the Ta³⁺deep level defect concentration.We found that the Ta₃N₅ photoanode after soaking for 2 hours in H₂O₂solution had the best performance:the PCE achieved2.25%with 8.2 m A/cm²photocurrent density（1.23 V vs.RHE）and 0.46 V_RHE onset potentialBy combining the atomic layer deposition method with EB evaporation,and using one-stepnitridationprocesstoobtainthestructureof In:Ga N/Ta₃N₅/Mg:Ga N/Co Ni Fe-B_i photoanodes.Through this interface engineering,both the onset potential and the photogenerated current density of the device are significantly improved.The highest PCE of the device reached 3.46%,various catalytic performance parameters were significantly improved,and the stability of the device was also greatly improved.This optimization strategy can not only successfully construct the built-in electric field,but also improve the hole injection efficiency and carrier separation efficiency.This Ta₃N₅ photoanode device with n-i-p structure can be used for reference in the field of PEC water splitting.