1.School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 2.Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
Fund Project:Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. FRF-TP-18-005A2)
Received Date:28 June 2019
Accepted Date:29 August 2019
Available Online:01 November 2019
Published Online:05 November 2019
Abstract:Ferroelectrics materials, as a candidate of materials, have recently received attention, for they possess applications in photovoltaic devices and can couple the light absorption with other functional properties. In these materials, the strong inversion symmetry is broken, which is because the spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the band-gap, thus permitting efficiency beyond the maximum possible value in a conventional p-n junction solar cell. Much effort has been made to study the ferroelectric photovoltaic effect in several families of ferroelectric perovskite oxides, such as Pb(Zr,Ti)O3, LiNbO3, BaTiO3, KNbO3, Na0.5Bi0.5TiO3-BaTiO3, AgNbO3 and BiFeO3. However, their photo-electric conversion efficiency is now still very low though this field is being studied. The observed output photocurrent is very low due to the negative influence of a wide band-gap and small absorption coefficient, which is caused by the wide band-gaps for most of ferroelectric materials such as Pb(Zr,Ti)O3 (~3.5 eV), and BaTiO3 (~3.3 eV), especially. Although the BiFeO3 system with low band-gap (2.7 eV), which can absorb most visible light for electron transition, is considered as a potential photovoltaic material, it is difficult to synthesize pure perovskite structure. The BiFeO3-BaTiO3 (BF-BT) ferroelectric material with excellent piezoelectric and ferroelectric properties has been widely concerned by researchers in recent years. However, it is still unclear whether this system has the same advantages as BiFeO3 materials with excellent photovoltaic properties. In this work, the Bi(Fe0.96Mg0.02–xTi0.02+x)O3-0.3BaTiO3 ferroelectric ceramics are prepared by the conventional synthesis method to uncover the piezoelectric and ferroelectric properties, as well as the photovoltaic performance with different ratios of Mg2+/Ti4. Because of the electronic production caused by replacing Mg2+ ions with Ti4+ ions, the conductivity of sample increases, and thus its piezoelectric and ferroelectric properties deteriorate. The piezoelectric coefficient d33 decreases from 195 pC/N at x = 0 to 27 pC/N at x = 0.02. Conversely, the range of absorption spectrum increases when the Mg2+ ions are replaced by Ti4+ ions. The band gap of sample decreases from 1.954 eV at x = 0 to 1.800 eV at x = 0.02. The photocurrent of sample increases from 3.71 nA/cm2 at x = 0 to 32.45 nA/cm2 at x = 0.02 because of the combined action of reducing the band gap and internal bias field. Keywords:bismuth ferrite/ ferroelectric materials/ photovoltaic/ perovskite structure
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3.1.相结构
图1给出了BFM0.02–xT0.02+x-BT (0 ≤ x ≤ 0.02)基陶瓷的XRD图谱, 所有的样品均为纯的钙钛矿结构. 未发现任何杂相, 说明Mg2+和Ti4+离子进入晶格中, 形成了稳定的固溶体. 当x = 0时, XRD衍射图谱与R相的标准卡片(PDF#73-0548)相对应, 指示了其相结构为R相. 然而, 与R相标准卡片的不同是, 陶瓷样品的XRD图谱在2θ = 20°附近出现了两个衍射峰, 说明陶瓷样品相结构不是单一的R相, 而是R-T两相共存. 该结果与我们先前报道的结果一致[11]. 随着x含量的增加, 位于2θ = 20°的双峰一直存在, 这说明所有样品均为R和T两相共存. 另外, 从图1(b) XRD放大图可知, x = 0时陶瓷样品的(111)与($1{{\bar 1}}1$)的峰强之比要高于R相标准卡片对应的(111)R与($1{{\bar 1}}1$)R峰强之比(约为1/3), 以及位于45°出现了双峰. 这也进一步证实了陶瓷样品的相结构不是单一的R相, 而是R-T两相共存. 随着x含量的增加, (111)R峰强增高, 且在 x ≥ 0.003时, (111)R峰的强度要明显高于${{{(1\bar 11)}}_R}$峰的强度, 这说明R-T两相比例发生了变化, 但是其相结构仍为R-T两相共存. 另外从图1(b)可以看出, 随着x含量的增加, 陶瓷样品衍射峰的峰位几乎保持不变. 众所周知, 离子半径较小的Ti4+ (r = 0.605 ?)取得离子半径较大的Mg2+ (r = 0.72 ?), 将会引起晶胞体积变小. 根据布拉格定理2dsinθ = nλ, 衍射峰的峰位应该向大角度偏移. 然而, 在BFM0.02–xT0.02+x-BT体系中衍射峰峰位的不变, 说明了该材料中存在另外一种能抵消晶胞体积变小的情况. Fe3+的离子半径r为0.645 ?, 而Fe2+的离子半径r为0.78 ?离子. 当发生Fe3+离子向Fe2+离子的转变时, 将会引起晶胞体积变大, 进而可以抵消由Ti4+取代Mg2+所引起晶体体积变小. 因此, 由上述可知随着x含量的增加, 该材料也存在着由Fe3+离子向Fe2+离子的转变. 图 1 BFM0.02–xT0.02+x-BT (0 ≤ x ≤ 0.02)基陶瓷的XRD图谱 (a) 20°—70°; (b) 38°—40°, 43.5°—46.5° Figure1. X-ray diffraction patterns of BFM0.02–xT0.02+x-BT ceramics with different x content (0 ≤ x ≤ 0.02) in a selected 2θ range of 20°—70° (a), 38°—40° and 43.5°—46.5° (b).
23.2.微观结构 -->
3.2.微观结构
图2给出了BFM0.02–xT0.02+x-BT铁电陶瓷的SEM形貌图, x = 0时, 晶粒尺寸为5 μm左右, 随着x值的增大, 即Mg2+离子浓度的降低和Ti4+离子浓度的增加, 陶瓷样品的晶粒尺寸逐渐减小, 说明通过Ti4+取代Mg2+过程中抑制了晶粒的长大. 陶瓷样品的平均晶粒尺寸由x = 0的8.37 μm减少至x = 0.02时的0.79 μm. 尽管随着x含量的增加, 晶粒尺寸呈现快速减少, 但是其密度几乎保持不变, 其相对密度均为93%左右. 图 2 BFM0.02–xT0.02+x-BT陶瓷样品的SEM形貌图 Figure2. SEM images of the fracture surface for BFM0.02–xT0.02+x-BT ceramics with different x content (0 ≤ x ≤ 0.02).
23.3.电学性能 -->
3.3.电学性能
图3(a)为BFM0.02–xT0.02+x-BT陶瓷样品的电滞回线, 所有样品均呈现出相对饱和的电滞回线, 且随着x含量的增加, 陶瓷样品的剩余极化Pr呈现了明显的下降. 从图3(b)可看出, Pr从x = 0 mol的14.89 μC/cm2, 降低至了x = 0.02 mol时的6.37 μC/cm2. 这里可能的原因是由于Ti4+取代Mg2+使陶瓷样品中产生大量的电子, 如缺陷方程(1)所示, 以及Fe3+的变价产生大量的空穴, 如方程(2)所示: 图 3 BFM0.02–xT0.02+x-BT陶瓷样品的(a)电滞回线, (b)剩余极化Pr和矫顽场EC随含量x的变化, (c)漏电流I-E曲线, 以及(d)压电性能d33和kp随含量x的变化 Figure3. Ferroelectric hysteresis loops (a), the variation of polarization (Pr) and electric field (EC) (b), leakage current density (I)-electric field (E) (c), as well as the piezoelectric coefficient d33, planar mode eletromechanical coupling coefficient kp (d) as a function of x for BFM0.02–xT0.02+x-BT (0 ≤ x ≤ 0.02).
图4为BFM0.02-xT0.02+x-BT陶瓷样品的紫外-可见光吸收光图谱. 所有样品拥有相似的吸收谱线, 从紫外至红外区均具有明显的吸收峰, 且其带隙吸收边为500—600 nm. 此外, 随着x含量的增加, 带隙吸收边出现红移. 该结果表明Ti4+取代Mg2+扩宽了其光吸收范围. 图 4 BFM0.02–xT0.02+x-BT铁电陶瓷的光吸收谱图 Figure4. UV-vis-NIR absorption spectra of the BFM0.02–xT0.02+x-BT ceramics with different x content (0 ≤ x ≤ 0.02).
其中, m为表征半导体类型的常数(当m = 2时为间接带隙半导体, 当m = 1/2时为直接带隙半导体); A, Eg和hν分别为常数项、禁带宽度和入射光子能量. 当m = 1/2时计算的Eg几乎均为约1.5 eV, 远低于其实际值. 图5所示为(αhν)1/2-hν图, 可见当m = 2时, 由线性部分切线与横轴交点可得各样品的禁带宽度: x = 0, 0.005, 0.01, 0.02 mol时, 禁带宽度分别为1.954, 1.919, 1.848, 1.800 eV. 该结果表明随着x含量的增加, 陶瓷样品的禁带宽度下降, 可能的原因是Ti4+与氧离子的电负性差值(为1.9)低于Mg2+与氧离子的电负性差值(为2.13). 当Ti4+取代Mg2+时将在其禁带中增加电子态, 从而有利于扩宽光的吸收范围和降低材料的禁带宽度[24]. 图 5 BFM0.02–xT0.02+x-BT铁电陶瓷的(αhν)1/2-(hν)曲线 (a) x = 0; (b) x = 0.005; (c) x = 0.01; (d) x = 0.02 Figure5. Plots of (αhν)1/2 versus hν for the BFM0.02–xT0.02+x-BT ceramics with different x content: (a) x = 0; (b) x = 0.005; (c) x = 0.01; (d) x = 0.02.
图6为BFMT-BT陶瓷样品在不同电压下极化的陶瓷光电流-时间(J-t)曲线. 当样品未极化时, 陶瓷样品在光照射下显示出非常弱的光电流信号, 对应图6中(I)区的J-t曲线. 然而当样品在1 kV下极化后, 其光电流密度达到2.64 nA/cm2, 对应图6中(II)区的J-t曲线. 可能的原因是其内部的偶极子部分发生了偏转, 形成了内偏电场, 有利于促进光生电子和空穴的分离. 当样品在3 kV下极化时, 其光电流密度达到6.06 nA/cm2, 对应图6中(IV)区的J-t曲线. 主要原因是高的极化电压进一步促进材料内部中偶极子的偏转, 使材料的内偏电场升高, 进而促进光生电子和空穴的分离. 图 6 BFMT-BT铁电陶瓷的光电流密度-时间(J-t)曲线图, 其中(I)区为未极化; (II)区为1 kV下极化; (III)区为2 kV下极化; (IV)区为3 kV下极化 Figure6.J-t characteristics of BFMT-BT ceramics, which are polarized at different voltage: (I) V = 0 kV; (II) V = 1 kV; (III) V = 2 kV; (IV) V = 3 kV.
图7为BFM0.02-xT0.02+x-BT铁电陶瓷的光电流密度-电压(J-V)曲线图. 当x = 0时, 未极化的样品在光照下, 表现出极其微弱的光电流J = 0.34 nA/cm2; 然而, 当样品极化后测试短路电流为3.71 nA/cm2, 如图7(a)所示. 该结果表明了极化有利于增强其光电性能. 当x增加至0.01时, 未极化的样品在光照下的短路电流增加至J = 7.10 nA/cm2, 如图7(c)所示, 主要原因是Ti4+取代Mg2+扩宽陶瓷样品的光吸收范围, 降低了其禁带宽度. 由于采用的光源是模拟太阳光照的氙灯(AM1.5), 所以大部分的光是位于红外区域. 因此, x含量的增加更有利于了光生载流子的生成, 从而导致光电流的提高. 当x含量增加至0.02时, 未极化样品的短路电流增加至12.86 nA/cm2, 如图6(d)所示. 然而将该陶瓷极化后, 其短路电流高达32.45 nA/cm2, 主要原因是极化使陶瓷内部偶极子的翻转, 形成内偏电场, 促进光生电子和空穴的分离, 从而更有利于陶瓷样品短路电流的升高. 图 7 BFM0.02–xT0.02+x-BT铁电陶瓷的光电流密度-电压(J-V)曲线图 (a) x = 0; (b) x = 0.005; (c) x = 0.01; (d) x = 0.02 Figure7.J-V characteristics of BFM0.02–xT0.02+x-BT ceramics with different x content: (a) x = 0; (b) x = 0.005; (c) x = 0.01; (d) x = 0.02
图8为铁电陶瓷光伏效应原理图. 未极化陶瓷样品的电偶极子是随机分布, 其内偏电场为0, 如图8(a)所示. 在光照下, 部分电子受到激发跃迁到导带, 从而产生电子和空穴分离, 进而形成光电流, 如图8(b)所示. 这里受光激发产生电子跃迁的程度与材料的禁带宽度息息相关, 禁带宽度越低, 受光激发的电子数也就越多. 因此, 随着x含量的增加, 陶瓷样品的短路电流也就越大. 当陶瓷样品在高电压下极化, 使其内部电偶极子发生转向、重排, 其内建电场不为零, 如图8(c)所示. 在光照下, 受光激发产生电子和空穴受到内建电场的作用, 发生分离, 从而降低电子和空穴的复合概率, 如图8(d)所示. 因此极化后的陶瓷样品具有更高短路电流, 如图6和图7所示. 由此可知, 为了提高光伏响应, 一方面需要降低材料的带隙宽度, 使得价带中的电子较易发生跃迁; 另一方面, 材料本身应具有较大的极化强度, 从而为电子的定向移动提供有效的内建电场. 图 8 BFM0.02–xT0.02+x-BT铁电陶瓷光伏效应原理图 (a)未极化、无光条件; (b)未极化、有光条件; (c)极化、无光条件; (d)极化、光照条件 Figure8. Schematic diagram of photovoltaic effect for BFM0.02–xT0.02+x-BT ceramics, non-polarization and dark condition (a), non-polarization and light condition (b), polarization and dark condition (c), as well as polarization and light condition (d), respectively.