Photovoltaic properties of Aurivillius Bi4NdTi3FeO15 ceramics with different orientations

Journal of Alloys and Compounds 800 (2019) 134e139

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Photovoltaic properties of Aurivillius Bi4NdTi3FeO15 ceramics with different orientations Lin Cao, Zhenzhong Ding, Xingzhong Liu, Junchen Ren, Yikang Chen, Meng Ouyang, Xiaoqin Chen*, Fujun Yang** Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Sciences, Hubei University, Wuhan, 430062, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 March 2019 Received in revised form 23 May 2019 Accepted 27 May 2019 Available online 3 June 2019

C-axis preferentially and randomly oriented Bi4NdTi3FeO15 (BNTF) ceramics were prepared by molten salt synthesis and conventional solid-state reaction methods, respectively. The ceramics with different orientations all exhibit Aurivillius structure containing four perovskite layers and the Lotgering factor f of c-axis preferentially oriented BNTF prepared by molten salt synthesis was calculated to be 0.865. BNTF powders show two absorption edges due to the electron excitations not only from valence band (VB) to Ti 3d conduction band (CB) but also to Fe eg CB. The ferroelectric and photovoltaic properties of BNTF ceramic samples were investigated. The ferroelectric performance of randomly oriented ceramic sample is better than that of c-axis preferentially oriented one. Significant photovoltaic effect was observed in both ceramic samples. Compared with the ceramic with c-axis preferred orientation, the randomly oriented ceramic exhibits a larger photocurrent Jsc and a smaller photovoltage Voc, which was discussed from the grain alignment in the two ceramics and the anisotropy of the crystal structure. The present work provides a new way to control photovoltaic properties of lead-free Bi5Ti3FeO15 (BTF) based compounds and accelerates their application in ferroelectric photovoltaic (FEPV) and energy fields. © 2019 Elsevier B.V. All rights reserved.

Keywords: Aurivillius Bi4NdTi3FeO15 Orientation Ferroelectrics Photovoltaics

1. Introduction As we all know, growing energy crisis and environmental issues have become more and more serious, which requires the development and application of renewable green energy, solar energy [1]. Photovoltaic technology thereby has gradually been paid much attention from both academia and industry [2]. Although the traditional silicon-based photovoltaic technology has dominated the solar cell industry for decades due to their mature technology and relatively high conversion efficiency, further improvements in efficiency are hampered because the bandgap of the material limits the maximum photovoltage [3e6]. Therefore, in order to obtain solar cells with higher efficiency and at lower cost, next generation photovoltaic technology is highly desirable. In recent years ferroelectrics have got renewed attention for the breakthrough in photovoltaic application. Firstly, the photo-generated carriers are

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Yang).

(X.

Chen),

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separated by the depolarization field which exists in the entire ferroelectric material therefore the photovoltage is not restricted by the bandgap. Secondly, photovoltaic responses can be generated without forming complex junction structures. Additionally, the photocurrent is proportional to the polarization magnitude and the photovoltaic response can be tuned by controlling the polarization [7,8]. All the above special characteristics make ferroelectric materials attract much attention in the photovoltaic research field. Much work has been done about the FEPV effect in ferroelectric perovskite oxides, such as LiNbO3 [9], BaTiO3 [10] and Pb(Zr,Ti)O3 [11]. Although the depolarization field can effectively maintain the separation of the photo-generated carriers, the wide bandgaps of the FEPV materials (>3.3 eV) result in insufficient light absorption and thereby limit the photocurrent [12,13]. Furthermore, wide bandgaps more than 3 eV make conventional ferroelectric materials absorb only ultraviolet light (less than 700 nm) which contains only 3.5% of solar radiation intensity, while as for visible light (380e700 nm), comprising 40% of the solar irradiation, conventional ferroelectric materials can hardly absorb [14]. A good FEPV material should simultaneously have a large internal electric field and a narrow bandgap, which will improve the absorption of photons and the efficiency of separating photo-generated carriers.

L. Cao et al. / Journal of Alloys and Compounds 800 (2019) 134e139

However, the FEPV mechanism and how to further enhance the FEPV performances are still need investigation because the output photovoltage and photocurrent are affected not only by ferroelectric properties but also by other factors including grain size, domain size, the charge carrier mobility, etc [15e17]. Bi5Ti3FeO15, which can be regarded as inserting the famous multiferroic BiFeO3 unit into the typical ferroelectric compound Bi4Ti3O12, exhibits four-layer perovskite structure. It has a high Curie temperature (730
C) [18], and the bandgap is 2.88 eV for its thin film sample [19], lower than that of conventional ferroelectric materials. The large crystalline anisotropy of four-layer structured perovskite phase leads to anisotropic electrical properties. For example, the polarization lies in aob plane, therefore a/b-axis oriented samples exhibit better ferroelectric properties than c-axis oriented ones [20]. The dielectric constant, loss tangent, complex impedance and conductivity were also measured to be very different along different crystallographic orientations [21,22]. Although the photovoltaic properties of BTF-based compounds have been studied, the anisotropy of photovoltaic properties is rarely reported. Therefore, in the present study, ceramic samples of BTF-based compounds with different orientations were prepared and the microstructure, ferroelectric, optic and photovoltaic performances were studied. To meet the requirements of a good FEPV material better, the sample composition we chose is Bi4NdTi3FeO15 because the substitution of Nd for volatile Bi was demonstrated to improve the ferroelectric properties [23] without affecting the bandgap of three-layered Aurivillius compounds [24,25]. 2. Experimental Bi2O3, Nd2O3, Fe2O3 (analytic pure) and TiO2 (spectral pure) were the raw materials. For clarity, randomly oriented and c-axis preferentially oriented samples were denoted as BNTF-1 and BNTF2 respectively. Conventional solid-state reaction method was used to prepare BNTF-1 [26]. For BNTF-2, the molten salt method was employed to prepare BNTF powders firstly. An equimolar mixture of KCl and NaCl was chosen as the salt. Bi2O3, Nd2O3, Fe2O3 and TiO2 in stoichiometric quantities were thoroughly mixed by ball milling with the mixture of KCl and NaCl for 12 h. In order to compensate the volatilization loss, excess Bi2O3 (15 wt%) was added and the mass ratio of oxides/chlorides was set to be 1/1. The synthetic mixture was put into an alumina crucible and heated at 850
C for 4 h. The reaction products were crushed and washed with hot deionized water repeatedly until the Cl ions could not be detected by AgNO3. Secondly, the dried powders were pressed into pellets with a diameter of 10 mm and a thickness of 1 mm and sintered at 900
C for 4 h in air. For measurement of absorption spectrum, BNTF-1 and BNTF-2 powder samples were prepared by the same annealing process to BNTF-1 and BNTF-2 ceramic samples. The crystal structure was examined by an x-ray diffractometer (Bruker D8 ADVANCE) with Cu Ka radiation. A scanning electron microscope (SEM, JEOL, JSM-6510LV) was used to characterize the microstructure. For electrical measurement, the discs were lapped and polished to ~0.2 mm thick. Silver was pasted on both surfaces as electrodes. The ferroelectric hysteresis loops were measured using a precision workstation ferroelectric tester system (Radiant Technologies). The frequency-dependent dielectric properties were measured using a LCR meter (HIOKI, IM3563) at room temperature. Absorption spectrum was obtained using a UV-vis spectrometer (UV-3600). For the measurement of photocurrent density-voltage (J-V) characteristics, a transparent indium tin oxide electrode was deposited on one surface of the sample using radio frequency sputtering and silver was pasted on another surface. The J-V curves were recorded using a Keithley 2450 digital multimeter under AM 1.5 illumination.

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3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of BNTF-1 and BNTF-2 ceramics. There are two things should be paid attention to. Firstly, the Aurivillius structure containing four perovskite layers was identified by indexing all the diffraction peaks on the basis of an orthorhombic cell (black columns, Joint Committee for Powder Diffraction Standard (JCPDS) No. 82-0036). It can be seen that no peak belonging to Nd2O3 was detected, implying that Nd is incorporated into BTF in a way of substitution for Bi successfully. Secondly, (119) peak is the strongest peak in Fig. 1a suggesting BNTF-1 sample is of random orientation. In contrast, (119) reflection is suppressed and (00l) reflections are enhanced largely in Fig. 1b, which indicates a preferential c-axis orientation has been successfully realized through molten salt method. Lotgering factor f is often used to evaluate the degree of texture, which is defined as: f¼(p-p0)/(1-p0), where p ¼ ƩI(00l)/ƩI(hkl), p ¼ p0 for randomly oriented sample and f varies from 0 to 1 [27]. The f for BNTF-2 sample was calculated to be 0.865, close to 1, demonstrating a preferential c-axis orientation. Fig. 2 gives the surface and cross-sectional SEM images of BNTF1 and BNTF-2 ceramics. It can be clearly seen that the plate-like morphology appears in both samples, which is related to the layered perovskite structure of BNTF. For BNTF-1 ceramic, the random arrangement of the plate-like grains can be clearly observed not only in the surface SEM image (Fig. 2a) but in the cross-sectional SEM image (Fig. 2c). While for BNTF-2 ceramic, plate-like grains lie in the surface of the ceramic (Fig. 2b) and laminated distribution of elongated grains can be clearly observed in the cross-sectional SEM image (Fig. 2d). The thickness direction of the plate-like grains corresponds to the c-axis of the four-layer Aurivillius crystal structure (marked in Fig. 2d), and the surface of the plate-like grains is parallel to the aob plane of the crystal structure (marked in Fig. 2b). Combined with the surface and crosssectional SEM images, it can be concluded that BNTF-1 and BNTF-2 ceramics are of random and preferential c-axis orientations respectively which is consistent with the results of XRD analysis as shown in Fig. 1. What's more, the grain size of BNTF-1 is ~1e2 mm, while the grain size of BNTF-2 is much larger (most grains are larger than 2 mm), as shown in Fig. 2 (a) and (b). The thickness of the plate-

Fig. 1. X-ray q  2q diffraction patterns of (a) BNTF-1 and (b) BNTF-2 ceramics.

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Fig. 2. Surface SEM images of BNTF-1 (a), BNTF-2 (b) ceramics and cross-sectional SEM images of BNTF-1 (c), BNTF-2 (d) ceramics.

like grains of BNTF-1 and BNTF-2 is similar, varying from 200 to 300 nm. The much larger grain size parallel to the aob plane of BNTF-2 ceramic might be originated from the following two reasons. One possible reason is that the valid annealing time is longer for BNTF-2 ceramic, and another more important reason is that the growth in the aob plane is considerably faster than along the c axis and the preferential growth of c-axis oriented grains is greatly promoted by molten salt synthesis [28]. Fig. 3 plots the polarization-electrical field (P-E) hysteresis loops for BNTF-1 and BNTF-2 ceramic capacitors. The typical P-E loops although somewhat leaky have been observed. This leakage-related contribution was also demonstrated in earlier reports and seems to

be intrinsic [29,30]. It can be clearly seen that the values of the coercive field 2Ec are identical while the values of remanent polarization 2Pr are very different at the same applied electric field. The measured 2Pr values for BNTF-1 and BNTF-2 capacitors are approximately 6.1 and 2.1 mC/cm2 respectively. It is well-known that major spontaneous polarization (Ps) is along the a-axis and minor Ps is along the c-axis for odd-n-numbered Aurivillius phase materials while for even-n-numbered ones Ps is only along the a/baxis (n represents the number of BO6 octahedra in the pseudoperovskite layer, for BNTF, n ¼ 4) [31]. Actually, Suzuki et al. found that a/b-axis oriented BTF ceramics exhibited enhanced polarization and piezoelectric properties compared with randomly oriented ceramics [32]. Zhang et al. reported strong piezoresponse in aob plane rather than along the c-axis in BTF [33]. As for the present BNTF samples, because Nd-doping does not change the fourlayered Aurivillius structure indicated by Fig. 1, the P-E loops show obvious ferroelectric anisotropy, consistent with previous reports [33,34]. Fig. 4 (a) shows the UV-vis optical absorption spectra of BNTF-1 and BNTF-2 powder samples. It can be seen that although the absorption values are different, two absorption edges can be clearly observed in both powder samples. The band 1 is attributed to the transition from the hybridized O 2p þ Bi 6s þ Fe t2g VB states to Fe eg CB states and the band 2 from the same VB states to Ti 3d CB states [35,36]. The bandgaps can be calculated by Tauc relation [37],

ahnf hn  Eg

Fig. 3. P-E loops of BNTF-1 and BNTF-2 ceramic capacitors.


n

(1)

where a denotes the absorption coefficient, hn is the photon energy, Eg represents band gap energy and n is equal to 1/2 for three- or four-layered Aurivillius compounds [38,39]. Therefore, the Eg value can be estimated by extrapolating the linear portion from the plot of (ahn)2 ~hn, shown in Fig. 4b. The bands 1 are almost overlapped and the bands 2 are only slightly separated for the two powder samples. Therefore, the bandgaps of Eg1, Eg2 corresponding to the bands 1, 2 were estimated to be extremely close, shown in the inset of Fig. 4 (b). Eg1 equals to 2.54 eV for both powder samples and Eg2 are 2.16 and 2.19 eV for BNTF-1 and BNTF-2 respectively, consistent

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Fig. 4. (a) The UV-vis absorption spectra and (b) the plots of (ahn)2 vs. hn of BNTF-1 and BNTF-2 powder samples. The inset in the middle is a schematic diagram showing interband transitions, and the inset in Fig. 4 (b) is the enlarged curves of (ahn)2 vs. hn.

with La-doped BTF powders [35]. The difference of absorption value between the two powder samples might be induced by different grain morphology (Supplementary Information, Fig. S1). Large platelike grains of BNTF-2 might reflect more light than small irregular ones of BNTF-1 do, therefore the absorption value of BNTF-1 is higher than that of BNTF-2. Notably, two additional absorption peaks appear at 588 nm and 745 nm. In our previous report, the similar absorption peaks were also observed in Nddoped three-layered Aurivillius powders [25]. They are coming from the electron transitions from the 4I9/2 state to 2G7/2 or 4G5/2 and 2H9/2 states of Nd3þ respectively [40e42]. The dark and illuminated J-V curves for BNTF-1 and BNTF-2 capacitors under visible illumination of 77 mW/cm2 are presented in Fig. 5 (a) and (b). The inset (c) shows the device structure. For comparison, the illuminated curves of both capacitors were plotted together in inset (d) and the dark curves were plotted together in inset (e). As shown in Fig. 5 (a) and (b), the two capacitors exhibit nearly zero dark current without illumination, while their photocurrent responses are clearly enhanced under light irradiation. BNTF-1 capacitor with random orientation has an open circuit voltage Voc of 0.09 V with a short circuit photocurrent Jsc of 11.5 mA/ cm2 while for BNTF-2 capacitor with preferential c-axis orientation the Voc and Jsc are 0.16 V and 5.27 mA/cm2 respectively. It is widely accepted that the photocurrent is originated from two factors: one is the remnant polarization which produces a

depolarization electric field existing over the whole ferroelectric volume; the other is the electrode interface Schottky barrier. And the photocurrent has been reported to be dependent on a variety of factors including absorption, bandgap, carrier mobility and light intensity [14]. For the two capacitors, composition, top and bottom electrodes, absorption and light intensity were all kept same. The bandgaps of the two samples are almost same too. Therefore, it is reasonable to infer that the larger remnant polarization of BNTF-1 capacitor with random orientation would strengthen the internal polarization electric field, improve the mobility of photo-generated carriers thereby induce a larger photocurrent, as shown in inset (d) of Fig. 5. In addition, the carrier mobility is also related to the electrical conductivity. Low conductivity i.e. high resistance may impede the drift of photo-generated carriers. Fig. 6 shows the frequency dependence of ac conductivity for BNTF-1 and BNTF-2 ceramics. For dielectrics, the ac conductivity is ably calculated from the measured dielectric data (Supplementary Information, Figs. S2 and S3) using relation:

sac ¼ ε0 ε0 u tan d; where ε0 is real part of dielectric constant, ε0 represents the permittivity of free space, u is the angular frequency and tand is dielectric loss. From Fig. 6, it can be clearly seen that the sac values of BNTF-1 are higher than those of BNTF-2 at all frequencies and the curves can be divided into two parts, frequency-independent

Fig. 5. Measured J-V characteristics of BNTF-1 (a) and BNTF-2 (b) ceramic capacitors. Insets (c), (d) and (e) show the device structure, the comparisons of illuminated and dark J-V curves for BNTF-1 and BNTF-2, respectively.

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Fig. 6. Frequency dependence of ac conductivity of BNTF-1 and BNTF-2 ceramic capacitors.

part in low-frequency regions and frequency-dependent part in high frequency regions. The frequency dependence of sac in ceramics is generally described using Jonscher's power law: sac ¼ sdc þ Aun, where A is the dispersion parameter that represents the intensity of polarizability and n is the dimensionless frequency exponent that denotes the interaction between mobile ions with the lattice around them [43]. From Jonscher's power law, the frequency independent values of sac at low frequencies are corresponding to the dc conductivity [21]. At 20 Hz, BNTF-1 and BNTF-2 show ac conductivity of 8.72  106 S/m and 8.77  107 S/ m, respectively. The difference of dc conductivity between BNTF-1 and BNTF-2 can also be obtained by calculating the slope of the J-V curve as shown in the insets (d) and (e) of Fig. 5 [44]. It can be clearly seen that the dc conductivity of randomly oriented BNTF-1 is higher than that of c-axis preferentially oriented BNTF-2 whether with light irradiation or not and the difference of dark conductivity between BNTF-1 and BNTF-2 is consistent with the result of Fig. 6. The lower conductivity of BNTF-2 is due to the presence of more grain boundaries per unit thickness resulting from the orderly arrangement of plate-like grains and the particular (Bi2O2)2þ layers acting as the insulating layers along the c-axis as shown in Fig. 7 (a) and (b) [22,45], although the actual BNTF-2 capacitor structure is not as ideal as that shown in Fig. 7 (b). As a

result, higher conductivity i.e. low resistance might be beneficial for the drift of photo-generated carriers thereby lead to larger photocurrent in BNTF-1. As for the photovoltage, larger Voc was obtained in BNTF-2 capacitor with c-axis preferential orientation. It can also be inferred that the intrinsic factors such as the different polarization and conductivity might mainly contribute to the larger Voc of BNTF-2 owing to the identical extrinsic factors such as the measuring electric field, the light intensity and the device structure. Seidel's group had ever reported that the Voc was derived to be inversely proportional to the overall conductivity which includes dark and light conductivity and they found that either decreasing the dark or light conductivity would really increase the Voc experimentally [16]. Based on the calculation results of ac conductivity as shown in Fig. 6, the dc conductivity of BNTF-1 is almost ten times larger than that of BNTF-2. Therefore it is reasonable to infer that although the polarization is twice larger, the much larger conductivity of BNTF-1 might lead to the smaller Voc. Certainly, the different sintering conditions of the two samples induce different defect states thereby might also contribute to different electrical properties. The factors influencing the FEPV performances are so complicated that more work needs to be done to understand the mechanism further. Based on the ferroelectric and photovoltaic responses of BNTF with different orientations, it might be expected that a/b-axis preferentially oriented BTF-based ferroelectric compounds with smaller grain size will exhibit improved photovoltaic properties.

4. Conclusions Randomly and c-axis preferentially oriented BNTF ceramics were prepared by conventional solid-state reaction method and molten salt synthesis, respectively. BNTF-2 ceramic prepared by molten salt synthesis has a high degree of c-axis texture fraction (Lotgering factor is 0.865), which was confirmed by XRD and SEM characterizations. Two absorption edges, which were ascribed to electron excitations from VB to Fe eg CB and to Ti 3d CB, were observed in both powders. The ferroelectric performance of randomly oriented BNTF-1 is better than that of c-axis preferentially oriented BNTF-2. Significant photovoltaic effects were all observed in both samples. Compared with the ceramic with c-axis preferred orientation, the randomly oriented ceramic exhibits a larger photocurrent Jsc and a smaller photovoltage Voc. The different photovoltaic performance was preliminarily discussed from the different grain alignment in the two samples and the anisotropy of the unit cell.

Fig. 7. (a) The configurations of BNTF-1 and BNTF-2 ceramic capacitors, (b) Schematic diagrams for the structure of BNTF capacitors in c-axis and a, b-axis orientations.

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Acknowledgements The authors are grateful for financial support from the National Nature Science Foundations of China under Grant Nos. 51002047 and 11274101. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.05.307. References [1] A. Polman, M.W. Knight, E.C. Garnett, B. Ehrler, W.C. Sinke, Photovoltaic materials: present efficiencies and future challenges, Science 352 (2016) aad4424. [2] P.K. Nayak, S. Mahesh, H.J. Snaith, D. Cahen, Photovoltaic solar cell technologies: analysing the state of the art, Nat. Rev. Mater. 4 (2019) 269e285. [3] R. Smith, W. Duan, J. Quarterman, A. Morris, C. Collie, M. Black, F. Toor, A.K. Salem, Surface modifying doped silicon nanowire based solar cells for applications in biosensing, Adv. Mater. Technol. 4 (2019) 1800349. [4] Y.F. Zhuang, S.H. Zhong, X.J. Liang, H.J. Kang, Z.P. Li, W.Z. Shen, Application of SiO2 passivation technique in mass production of silicon solar cells, Sol. Energ. Mat. Sol. C. 193 (2019) 379e386. [5] P. Kowalczewski, L.C. Andreani, Towards the efficiency limits of silicon solar cells: how thin is too thin? Sol. Energ. Mat. Sol. C. 143 (2015) 260e268. [6] L.C. Andreani, A. Bozzola, P. Kowalczewski, M. Liscidini, L. Redorici, Silicon solar cells: toward the efficiency limits, Adv. Phys. X 4 (2019), 1548305. [7] Y. Li, X.X. Cui, N.N. Sun, J.H. Du, X.W. Li, G.X. Jia, X.H. Hao, Region-dependence and stable ferroelectric photovoltaic effect driven by novel in-plane self-polarization in narrow-bandgap Bi2FeMo0.7Ni0.3O6 thin film, Adv. Opt. Mater. 7 (2018) 1801105. [8] H. Liu, J. Chen, Y. Ren, L. Zhang, Z. Pan, L. Fan, X. Xing, Large photovoltage and controllable photovoltaic effect in PbTiO3-Bi(Ni2/3þxNb1/3-x)O3-d ferroelectrics, Adv. Electron. Mater. 1 (2015), 1400051. [9] R. Inoue, S. Takahashi, Y. Kitanaka, T. Oguchi, Y. Noguchi, M. Miyayama, Enhanced photovoltaic currents in strained Fe-doped LiNbO3 films, Phys. Status Solidi A 212 (2015) 2968e2974. [10] L. Kola, D. Murali, S. Pal, B.R.K. Nanda, P. Murugavel, Enhanced bulk photovoltaic response in Sn doped BaTiO3 through composition dependent structural transformation, Appl. Phys. Lett. 114 (2019), 183901. [11] Y. Zhou, J. Zhu, X. Liu, Z. Wu, Photovoltaic effect of ferroelectric Pb(Zr0.52Ti0.48) O3 deposited on SrTiO3 buffered n-GaAs by laser molecular beam epitaxy, Funct. Mater. Lett. 10 (2017), 1750036. [12] S. Kumari, N. Ortega, A. Kumar, J.F. Scott, R.S. Katiyar, Ferroelectric and photovoltaic properties of transition metal doped Pb(Zr0.14Ti0.56Ni0.30)O3d thin films, AIP Adv. 4 (2014), 037101. [13] T. Chen, J. Meng, S. Wu, J. Pei, Q. Lin, X. Wei, J. Li, Z. Zhang, Room temperature synthesized BaTiO3 for photocatalytic hydrogen evolution, J. Alloy. Comp. 754 (2018) 184e189. [14] Y. Yuan, Z. Xiao, B. Yang, J. Huang, Arising applications of ferroelectric materials in photovoltaic devices, J. Mater. Chem. A 2 (2014) 6027e6041. [15] M.M. Yang, Z.D. Luo, D.J. Kim, M. Alexe, Bulk photovoltaic effect in monodomain BiFeO3 thin films, Appl. Phys. Lett. 110 (2017) 183902. [16] J. Seidel, D. Fu, S.Y. Yang, E. Alarcon-Llado, J. Wu, R. Ramesh, J.W. Ager 3rd, Efficient photovoltaic current generation at ferroelectric domain walls, Phys. Rev. Lett. 107 (2011) 126805. [17] X. Cui, Y. Li, N. Sun, J. Du, X. Li, H. Yang, X. Hao, Double perovskite Bi2FeMoxNi1-xO6thin films: novel ferroelectric photovoltaic materials with narrow bandgap and enhanced photovoltaic performance, Sol. Energ. Mat. Sol. C. 187 (2018) 9e14. [18] J.B. Li, Y.P. Huang, G.H. Rao, G.Y. Liu, J. Luo, J.R. Chen, J.K. Liang, Ferroelectric transition of Aurivillius compounds Bi5Ti3FeO15 and Bi6Ti3Fe2O18, Appl. Phys. Lett. 96 (2010) 222903. [19] Y. Bai, J. Chen, X. Wu, S. Zhao, Photovoltaic behaviors regulated by band-gap and bipolar electrical cycling in holmium-doped Bi5Ti3FeO15 ferroelectric films, J. Phys. Chem. C 120 (2016) 24637e24645. [20] M. Miyayamaa, I.S. Yib, Electrical anisotropy in single crystals of Bi-layer

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structured ferroelectrics, Ceram. Int. 26 (2000) 529e533. [21] X. Gao, L. Zhang, C. Wang, Q. Shen, Anisotropic electrical and magnetic properties in textured Bi5Ti3FeO15 ceramics, J. Eur. Ceram. Soc. 37 (2017) 2399e2405. [22] S.K. Rout, P.K. Barhai, E. Sinha, A. Hussain, W. Kim, Anisotropic dielectric and electrical properties of hot-forged SrBi4Ti4O15ceramics, Int. J. App. Ceram. Tec. 7 (2009) E114eE123. [23] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Layered perovskites with giant spontaneous polarizations for nonvolatile memories, Phys. Rev. Lett. 89 (2002), 087601. [24] Y.H. Wang, B. Gu, G.D. Xu, Y.Y. Zhu, Nonlinear optical properties of neodymium-doped bismuth titanate thin films using Z-scan technique, Appl. Phys. Lett. 84 (2004) 1686e1688. [25] X.Q. Chen, F. Huang, Z.W. Lu, Y. Xue, J.J. Min, J.H. Li, J. Xiao, F.J. Yang, X.B. Zeng, Influence of transition metal doping (X¼Mn, Fe, Co, Ni) on the structure and bandgap of ferroelectric Bi3.15Nd0.85Ti2X1O12, J. Phys. D Appl. Phys. 50 (2017) 105104. [26] F.J. Yang, P. Su, C. Wei, X.Q. Chen, C.P. Yang, W.Q. Cao, Large magnetic response in (Bi4Nd)Ti3(Fe0.5Co0.5)O15 ceramic at room-temperature, J. Appl. Phys. 110 (2011) 126102. [27] F.K. Lotgering, Topotactical reactions with ferromagnetic oxides having hexagonal crystal structures-I, J. Inorg. Nucl. Chem. 9 (1959) 113e123. [28] S. Priya, S. Nahm, Lead-free Piezoelectrics, Springer, New York, 2012. [29] X.Q. Chen, Y. Xue, Z.W. Lu, J. Xiao, J. Yao, Z.W. Kang, P. Su, F.J. Yang, X.B. Zeng, H.Z. Sun, Magnetodielectric properties of Bi4NdTi3Fe0.7Co0.3O15 multiferroic system, J. Alloy. Comp. 622 (2015) 288e291. [30] H. Zhao, H. Kimura, Z. Cheng, M. Osada, J. Wang, X. Wang, S. Dou, Y. Liu, J. Yu, T. Matsumoto, T. Tohei, N. Shibata, Y. Ikuhara, Large magnetoelectric coupling in magnetically short-range ordered Bi5Ti3FeO15 film, Sci. Rep. 4 (2014) 5255. [31] H. Funakubo, Degradation-free dielectric property using bismuth layerstructured dielectrics having natural superlattice structure, J. Ceram. Soc. Jpn. 116 (2008) 1249e1254. [32] M. Suzuki, Y. Noguchi, T. Uchikoshi, M. Miyayama, Polarization and piezoelectric properties of grain-oriented ferroelectric Bi5FeTi3O15 ceramics prepared by magnetic-field-assisted electrophoretic deposition method, J. Electroceram. 24 (2008) 91e96. [33] P.F. Zhang, N. Deepak, L. Keeney, M.E. Pemble, R.W. Whatmore, The structural and piezoresponse properties of c-axis-oriented Aurivillius phase Bi5Ti3FeO15 thin films deposited by atomic vapor deposition, Appl. Phys. Lett. 101 (2012) 112903. [34] H. Sun, X. Mao, H. Wang, X. Chen, Multiferroic behavior and orientation dependence of Bi5Fe0.5Co0.5Ti3O15 thin film, Ferroelectrics 452 (2013) 63e68. [35] G. Naresh, T.K. Mandal, Excellent sun-light-driven photocatalytic activity by Aurivillius layered perovskites, Bi5-xLaxTi3FeO15 (x ¼ 1, 2), ACS Appl. Mater. Interfaces 6 (2014) 21000e21010. [36] S. Sun, W. Wang, H. Xu, L. Zhou, M. Shang, L. Zhang, Bi5FeTi3O15 hierarchical microflowers: hydrothermal synthesis, growth mechanism, and associated visible-light-driven photocatalysis, J. Phys. Chem. C 112 (2008) 17835e17843. [37] D.J. Wood, J. Tauc, Weak absorption tails in amorphous semiconductors, Phys. Rev. B 5 (1972) 3144e3151. [38] J. Chen, C. Nie, Y. Bai, S. Zhao, The photovoltaic spectral response regulated by band gap in Zr doped Bi4Ti3O12 thin films, J. Mater. Sci. Mater. Electron. 26 (2015) 5917e5922. [39] S. Kooriyattil, R.K. Katiyar, S.P. Pavunny, G. Morell, R.S. Katiyar, Photovoltaic properties of Aurivillius phase Bi5FeTi3O15 thin films grown by pulsed laser deposition, Appl. Phys. Lett. 105 (2014), 072908. [40] X. Lu, Z. You, J. Li, Z. Zhu, G. Jia, Y. Wang, B. Wu, C. Tu, Growth and properties of pure and rare earth-doped Ca3(BO3)2 single crystal, J. Cryst. Growth 281 (2005) 416e425. [41] F. Lahoz, I.R. Martín, U.R. Rodríguez-Mendoza, I. Iparraguirre, J. Azkargorta, ndez, V. Lavín, Rare earths in nanocrystalline A. Mendioroz, R. Balda, J. Ferna glass-ceramics, Opt. Mater. 27 (2005) 1762e1770. [42] S.M. Lima, J.A. Sampaio, T. Catunda, A.S.S. de Camargo, L.A.O. Nunes, M.L. Baesso, D.W. Hewak, Spectroscopy, thermal and optical properties of Nd3þ-doped chalcogenide glasses, J. Non-Cryst. Solids 284 (2001) 274e281. [43] A.K. Jonscher, The ‘Universal’ dielectric response, Nature 267 (1977) 673e679. [44] L. Chen, B.C. Luo, N.Y. Chan, J.Y. Dai, M. Hoffman, S. Li, D.Y. Wang, Enhancement of photovoltaic properties with Nb modified (Bi,Na)TiO3-BaTiO3 ferroelectric ceramics, J. Alloy. Comp. 587 (2014) 339e343. [45] S.K. Rout, A. Hussain, E. Sinha, C.W. Ahn, I.W. Kim, Electrical anisotropy in the hot-forged CaBi4Ti4O15 ceramics, Solid State Sci. 11 (2009) 1144e1149.