The Study Of The Semiconducting And Photovoltaic Properties Of Ion-doped Modified KNbO₃ Ferroelectric Ceramics

Due to their unique crystal structure,ferroelectric materials possess a variety of physical properties such as ferroelectricity,dielectricity and piezoelectricity simultaneously,and have long been a hot topic in the research of new functional materials.Recent results have shown that ferroelectric materials have anomalous photovoltaic effects and thus exhibit potentially significant advantages in terms of photovoltaic conversion efficiency,mainly due to their open-circuit voltage beyond the bandgap limit under illumination and carrier separation mechanisms associated with polarization.Thus,ferroelectric materials not only show promising applications in the fields of electronic devices and information storage,but also show potential value for applications such as photovoltaics,photocatalysis,photoelectric sensors and multifunctional energy harvesters and sensors.However,most conventional oxide ferroelectric materials do not absorb enough light and have a low electrical conductivity,which results in relatively small photocurrents in ferroelectric photovoltaic devices,therefore new types of ferroelectric photovoltaic materials need to be developed.In this paper,the effects of ionic doping modification of different elements at the A/B site on the crystal structure,electrical and optical properties of KN are systematically investigated using the classical ferroelectric KNbO₃ as a matrix,in particular the evolution of the band gap structure,the effect of polarization on the structure and the potential mechanism of the polarization-enhanced ferroelectric photovoltage effect.A series of semiconducting（1-x）KNbO₃-xSrNi_0.5Hf_0.5O_3-δ（KNSNH100x,x=0.02-0.08）ferroelectric ceramics was prepared using the solid-phase method,which simultaneously exhibit narrow bandgap and good ferroelectricity.The results of the analysis of the evolution of the band gap structure based on first principles calculations show that the3d split states e_g and t_2g of Ni play a crucial role in the reduction of the bandgap of KN,as demonstrated by experimental observations and first-principles calculations.Under simulated solar irradiation,the KNSNH2 in the device-based form tested show a J_scmaximum of 245.1 nA/cm² and its enhanced photovoltaic performance can be attributed to a narrow bandgap（0.783,1.44 and 2.25 e V）and good ferroelectric polarization（P_s～18.92μC/cm²）.The J_sc value of KNSNH2 is enhanced to 511.9 nA/cm² after 60 k V/cm polarization,which is twofold higher than before polarization.The enhancement of J_sc in KNSNH2 after polarization is due to polarization-induced lattice distortion and increased oxygen defects,and the pre-polarization optimizes its band gap structure resulting in enhanced light absorption in the visible region.In addition,the expanded E_dp after high-field polarization facilitates the transport of photogenerated carriers,and these together contribute to the enhanced photovoltaic response of KNSNH2.For the（1-x）KNbO₃-xSrCo_0.5Hf_0.5O_3-δ（KNSCH100x,x=0.02-0.08）series of semi-conducting ceramics synthesized by solid solution of Sr/（Co,Hf）in the A/B position,both narrow band gap and high-field polarization capability.The Co ions possess a relatively small electronegativity difference from the O ions and enter the lattice of KN causing a contraction of its bandgap structure.In-depth analysis of the bandgap structure of KNSCH shows that the splitting state of the 3d energy state of Co mainly contributes to the reduction of its band gap,and the absorption peaks near 400,700 and 1400 nm in the absorbance spectrum of KNSCH originate from the electronic absorption transitions of the 3d splitting states ¹A_1g→¹T_2g、¹A_1g→¹T₁ and ⁵E→⁵T₂ of the Co³⁺ion,respectively.In addition,oxygen vacancy defects,induced by the composition design,also reduce some of the band gap values of the KNSCH.All of the KNSCH system show a two-level bandgap,with KNSCH2showing a smaller bandgap value of～1.23 e V（sub-bandgap value of 0.53 e V）.The enhancement of the J_sc of KNSCH2 from 155.4 nA/cm² to 356.8 nA/cm² after 60 k V/cm polarization can be attributed to the fact that polarization optimizes its band gap structure to facilitate the separation and transport of photogenerated carriers.The structural,electrical and optical properties of the semiconducting（1-x）KNbO₃-xBaNi_0.5Hf_0.5O_3-δ（KNBNH100x,x=0.02-0.08）series of ceramics have been investigated.In these ceramics,the incorporation of Ba,Ni and Hf into the lattice of KN causes lattice distortion leading to a reduction in the asymmetry of the cell,which simultaneously succeeds in reducing the band gap of KN.The d-d electronic absorption transitions ³A_2g（³F）→³T_1g（³P）、³A_2g（³F）→³T_1g（³F）and ³A_2g（³F）→³T_2g（³F）of the 3d split state of Ni dominate the band gap reduction in KNBNH,and additionally the oxygen vacancy defect also contributes to part of the bandgap reduction.The incorporation of Ba,Ni and Hf into the lattice of KN reduces the bandgap and enhances the absorption of light,thus improving the photovoltaic performance,with KNBNH6 achieving a maximum J_sc of 138.7 nA/cm² and further enhancing the photovoltaic performance to 702.9 nA/cm² after 30 k V/cm polarization.The analysis of XRD,Raman spectra and XPS patterns after polarization shows that the increase in lattice distortion of KNBNH6 after polarization leads to an increase in its polarity.Furthermore,polarization causes an increase in the concentration of oxygen defects in KNBNH6 resulting in an upward shift of the valence band,thus it can be deduced that polarization optimizes the bandgap structure,which affects its light absorption and enhances the photovoltaic performance of KNBNH6;the polarization-induced E_dp and the increase in polar microregions also contribute to the increase in J_sc.The structural,electrical and optical properties of（1-x）KNbO₃-xBaCo_0.5Hf_0.5O_3-δ（KNBCH100x,x=0.03-0.12）series of ferroelectric semiconductor ceramics have been investigated.The incorporation of Ba,Co and Hf ions into the lattice of KN reduces the asymmetry of its cells,leading to changes in its ferroelectric and dielectric properties.The tetragonal phase appears when the doping content increases to x=0.09,and the content of the tetragonal phase increases as the doping content increases.The XRD structure refinement results show that the percentage of orthorhombic and tetragonal phases in KNBCH9 and KNBCH12 are 76.93%and 23.07%,52.63%and 47.37%,respectively.The electron absorption transitions ¹A_1g→¹T_2g、¹A_1g→¹T₁ and ⁵E→⁵T₂ of the 3d split state of the Co ion play a crucial role in reducing the band gap of the KNBCH,where the band gap value of KNBCH9 is reduced from 3.33 e V to 1.19 e V（sub-bandgap～0.67 e V）.The reduced band gap in the KNBCH enhances its absorption of light and thus its photovoltaic performance,with KNBCH9 showing a J_sc maximum of 172.5 nA/cm².The J_sc of KNBCH9is further enhanced to 894.6 nA/cm² after polarization,and its enhanced photovoltaic performance can be attributed to the increased polar microregion or increased polarity of the electric domains within its body after polarization and the formation of E_dp that accelerate the separation and transport of its photogenerated carriers.In addition,XRD and Raman spectra results of KNBCH9 after polarization suggest that polarization increased its cell asymmetry.