Preparation And Optical Band-gap Regulation Of Sodium Potassium Niobate Based Ceramics

With the continuous development of economy,the world is facing the problem of energy shortage.As a clean and renewable energy source,solar energy is an effective way to solve the energy problems.Photoelectric conversion efficiency of the traditional semiconductor photovoltaic devices is a limit to further improvement.However,ferroelectric materials offer a new opportunity for the development of photovoltaic devices.Unlike the photovoltaic effect of p-n junctions,ferroelectric photovoltaic uses a built-in electric field caused by the spontaneous polarization of ferroelectric materials to separate the photo-generated carriers,so that the output voltage is not limited by the bandgap of the materials.Among various ferroelectric materials,sodium potassium niobate(K_0.5Na_0.5NbO₃,KNN)is one of the most promising candidates to replace lead-based ceramics due to its high Curie temperature and high dielectric properties.Regrettably,similar to other ferroelectric materials,KNN has a wide optical bandgap,thus leading to relatively low sunlight absorption efficiency and output photocurrent density.To address these problems,we chemically modify KNN through chemical doping to reduce the optical bandgap width and improve the absorption efficiency in the visible light range,so that it can be used for photovoltaic applications in the near future.The main research contents are as follows.?1?New solid solutions,KNN-xFe₂O₃,were formed with Fe₂O₃ doping,whose occupancy mechanism,microstructure,electrical properties and optical bandgap were systematically studied.XRD results indicated that all the ceramic samples are of single perovskite orthorhombic structure at room temperature.Fe³⁺preferentially entered the A site and then entered the B site with further increase in concentration.The substitution resulted in a change in the lattice structure,resulting in an enhancement in both dielectric and ferroelectric properties of the materials.Optical bandgap width of the ceramic samples is decreased significantly with increasing doping concentration,from 3.25 eV for pure KNN to 2.35 eV for the solid solution with x=0.04.In this case,the enhanced light absorption is attributed to the oxygen vacancies.In addition,the strain effect after the poling treatment further reduced the bandgap width of the ceramics.?2??1-x?KNN-xBiFeO₃ ceramics were prepared by using the conventional solid-state reaction method.Their microstructure,electrical properties and optical bandgap were systematically studied.It is found that the phase structure undergoes orthorhombic-tetragonal-cubic evolution with increasing doping concentration.An orthorhombic-tetragonal composite phase was obtained at x=0.01,which exhibited the highest ferroelectric properties,with Pr=15.8 C/cm² and Ec=8.44 kV/cm.The introduction of BiFeO₃ led to grain refinement,which further enhanced ferroelectric properties of the materials.Meanwhile,optical bandgap width of the ceramics is reduced from 3.25 eV to 2.01 eV,with a nonlinear relationship to the content of Fe.In addition,the bandgap is further narrowed by about 0.05 eV after poling treatment.As a result,the light absorption efficiency was greatly enhanced.