Tunability Of Ferroelectric And Bandgap In Ba/ni Co-doped KNbO₃

Currently, the energy issue has become a major bottleneck in national sustainable development. Thus, the effective use of renewable energy has become a pressing problem. Solar energy, for its use of clean, lead-free and non-toxic characteristics, becomes the ideal of new energy to solve the problem of the current energy and environmental. Ferroelectric material, is a great potential for development of new solar photovoltaic materials. The spontaneous polarization of the ferroelectrics, was used to separate the light generated electron- hole pairs, which is similar with the bulid-in electric field of the p-n semiconductor photovoltaic effect. Relative to the bulid-in electric field within the barrier layer, its internal overall the polarization electric field can produce more efficient carrier separation ability. However, a larger bandgap and a smaller photovoltaic output current density, limits the application of ferroelectric photovoltaic effect. Based on the nature of the problem, the chemical doping will be used to reduce bandgap of a classic ferroelectric material KNb O3, to increase the photoelectric conversion efficiency of the KNb O3-based materials in the photovoltaic applications. The main results are listed as follows.Firstly, KNb O3 ceramics were prepared by a solid-state reaction method, then mixes the perovskite ferroelectric oxides KNb O3 with Ba and Ni to introduce Ni2+ to the B-site and oxygen vacancies, which designes and synthesizes a series of different doping ratio of Ba/Ni co-doped KNb O3 polycrystalline ceramic——(K1-x Bax)(Nb1-x/2Nix/2) O3-δ. The structure analysis means that(K1-x Bax)(Nb1-x/2Nix/2) O3-δ crystals undergo a series of structural phase transitions at room temperature: from orthorhombic of KNb O3 to tetragonal phase of low doping of KNb O3, tetragonal to cubic phase of(K0.5Ba0.5)(Nb0.75Ni0.25) O3-δ. At the same time, the element Ni in the B-site doped into KNb O3 presentes +2 valences, indicating that the introduction of doping oxygen vacancies in the crystal lattice of KNb O3.Secondly, the Ba/Ni-codoping brings great impact on KNb O3 ferroelectricity. Before KNb O3-based materials have been used in the photovoltaic effect, it must ensure that it has ferroelectricity: Raman spectroscopy shows the structure and the phase evolution of(K0.5Ba0.5)(Nb0.75Ni0.25) O3-δ samples, also shows that with the increase of the Ba/Ni co-doping, the ferroelectricity of(K1-x Bax)(Nb1-x/2Nix/2) O3-δ at room temperature weakens rapidly. Electric hysteresis loop tests show that the maximum polarization intensity is 4.61 μC/cm2 of(K0.9Ba0.1)(Nb0.95Ni0.05) at room temperature, the remnant polarization is 1.47 μC/cm2, and the coercive field is 11.3 k V/cm. Until x = 0.5,(K1-x Bax)(Nb1-x/2Nix/2) O3-δ lost ferroelectricity at room temperature. The measurements of dielectric constant vis temperature show further that the structure of(K1-x Bax)(Nb1-x/2Nix/2)O3-δ changes abruptly: from orthorhombic to tetragonal phase, tetragonal to cubic.Finally, the transmission spectrum showes that the bandgap of KNb O3 is 3.3 e V, and the bandgap of(K1-x Bax)(Nb1-x/2Nix/2) O3-δ changes from 1.1 e V to 2.0 e V. Also, KNb O3 and(K1-x Bax)(Nb1-x/2Nix/2) O3-δ are the direct bandgap semiconductor materials. The structure analysis shows that, the origin of the bandgap change is mainly structural contribution due to cation–anion bond length relaxations and charge exchange relative to its constituent binary subsystems due to the differences in the electronegativity between the constituents. Density functional theory calculations show that at low valences ion Ni2+ displaces over perovskite B-site Nb5+, introducting to oxygen vacancies in the crystal lattice, there is a impurity band above the valence band of KNb O3, which the valence band is made of O:2p states, the conduction band of Nb:4d states and the impurity level Ni:3d electron states. Oxygen vacancies and impurity band work together to reduce the bandgap of KNb O3. Because of non-equivalent distribution of dopant atoms Ni into KNb O3, there is a nonlinear variation of the bandgap.