Bandgap Tuning And Optimization On The Optical And Electrical Properties For MgTiO₃

Compared with halide perovskite,perovskite oxides and their derivatives have excellent stability,and their band gap width can be regulated by doping and other means.Therefore,perovskite oxides and their derivatives have attracted more and more attention in the photovoltaic field.As a perovskite oxide,Mg Ti O₃ is widely used in microwave dielectric devices due to its low dielectric loss,high dielectric constant and good stability.However,Mg Ti O₃ has a wide band gap,which is not conducive to its absorption of visible light and low utilization efficiency of visible light.Therefore,there are few researches on photovoltaic.In this paper,the changes of crystal structure,resistivity,band gap structure and light response of Mg Ti O₃ doped with Mn and Mg_0.5Mn_0.5Ti O₃ doped with Co were studied by combining theory and experiment.This paper attempts to obtain a new narrow gap oxide perovskite material so as to expand its application in solar energy utilization.First,the thermodynamic software thermo-Calc was used to predict Mn doping sites,and it was found that Mn could replace the position of Mg and form a single phase.Then on the basis of thermodynamic calculation results for Mg_1-xMn_xTi O₃（x=0,0.125,0.25,0.5）first-principles calculation.The results show that with the doping of Mn,the band gap of the material decreases.In addition,the light absorption coefficient of the system doped with Mn reaches 10⁴ cm^-1 in the range of visible light,indicating that this type of material can effectively utilize visible light.Then according to the theoretical calculation results,Mn doped Mg Ti O₃ ceramic block was prepared by solid-phase synthesis.Pure Mn-doped Mg Ti O₃ ceramic blocks were obtained by changing calcination temperature,sintering temperature and the ratio of Mg to Ti.It was found that some grains of the sample were bonded,and grain boundaries were easily distinguished at some positions,indicating that Mg Ti O₃had certain sintering characteristics at 1100℃.With the increase of Mn doping concentration,the resistivity of the material decreases,and the valence state of Mn ions gradually changes from bivalent to trivalent and tetravalent.When the Mn doping concentration is greater than 0.1,it can be seen that there is a significant difference between the bright current and the dark current,indicating that the material generates light response under visible light.Then,Mn-doped Mg Ti O₃ thin film was prepared by sol-gel method,and its optical transmittance was tested.It was found that Mn doping can reduce the band gap of the material.When the Mn doping concentration is x=0.5,the band gap is 1.85 e V.Based on the leakage problem of Mg_1-xMn_xTi O₃ and its generation mechanism,a solution of co-doped Mg_0.5Mn_0.5Ti_1-xCo_xO₃（x=0.05,0.1,0.15）was proposed.In this study,the band structure of Mg_0.5Mn_0.5Ti_1-xCo_xO₃（x=0.05,0.1,0.15）was calculated by first principles,and the results show that Co doping hardly affects the band structure of the system.Then Mg_0.5Mn_0.5Ti_1-xCo_xO₃ ceramic blocks were prepared by solid-phase synthesis.It was found that the resistivity of the materials increased with Co doping.At this time,there are only two forms of Mn ion in the material,namely bivalent and trivalent.It can be considered that Co doping changes the composition of Mn ion valence state.The photocurrent of Mg_1-xMn_xTi O₃ under visible light is significantly higher than that of Mg_1-xMn_xTi O₃ without Co doping,indicating that Co doping inhibits photocarrier recombination and achieves the desired goal.