Photovoltaic, photo-impedance, and photo-
capacitance effects of the flexible (111) BiFeO 3 film
Cite as: Appl. Phys. Lett.115, 112902 (2019);doi: 10.1063/1.5120484 Submitted: 18 July 2019
.
Accepted: 23 August 2019.
Published Online: 10 September 2019
ZhongshuaiXie,1YuxiYang,1LiangFang,2YaojinWang,1 XifengDing,1,a)GuoliangYuan,1,a)and Jun-MingLiu3 AFFILIATIONS
1School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2Department of Physics, Soochow University, Suzhou 215006, China
3Laboratory of Solid State Microstructure, Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
a)Authors to whom correspondence should be addressed:[email protected]and[email protected]
ABSTRACT
Ferroelectric polarization allows a depolarization electric field to separate electron-hole pairs excited by lights, and thus, the photovoltaic properties of ABO3-type films on a hard SrTiO3or Si substrate have been extensively studied recently. However, there are a few reports on the photocapacitance and photoimpedance of these oxide films, especially on flexible substrates. In this work, the strong photovoltaic, giant photocapacitance, and photoimpedance effects were observed in the flexible (111) BiFeO3films with natural downward polarization. The flexible mica/SrRuO3/BiFeO3/Au cells show a maximum photovoltaic efficiency at 150C, a 95.5% decrease in photoimpedance effects, and a 316% increase in photocapacitance effects under 405-nm-wavelength light illumination at 25C. Most importantly, these properties do not show obvious degradation when the device is bent to 3 mm radius for 104times. This work is of vital importance for us to develop new flexi- ble photoelectronic devices.
Published under license by AIP Publishing.https://doi.org/10.1063/1.5120484
Ferroelectric photoelectric (e.g., photovoltaic, photoimpedance, and photocapacitance) effects have attracted enormous attention because of their superior potentials for optoelectronics, energy conversion, informa- tion storage, etc.1The photoexcited electron-hole pairs can be separated by the electric field of ferroelectric polarization (EFE) which crosses the whole ferroelectric layer, which is different from the inner electric field (Ein) in the depletion region of the semiconductor PN/Schottky junc- tion.1–3As a consequence, it is possible to switch ferroelectric polariza- tion, change EFE, and then tailor the photoelectric effect through an external electric field. Although the abnormal photovoltaic effect has been observed in many ferroelectric films, there are a few reports on the photoimpedance and photocapacitance effects of conventional ABO3- type ferroelectric oxide films. As is known, a large and stableEFEis the key factor of any photoelectric devices based on ferroelectric materials. A macroscopic polarization (Pm) and the corresponding EFE should be introduced by a large external electric field in most cases. However, in such a poling process, the ferroelectric film with a large-size electrode is easy to break. Recently, researchers achieved naturalPmin the as-grown PbZr1–xTixO3,4BaTiO3,5and BiFeO32,6–8epitaxial thin films on single- crystal SrTiO3or Si substrates.
BiFeO3films have already shown excellent photovoltaic effects due to their high polarization (>90 lC/cm2) along the h111i direction,
special domain structure, and relatively narrow bandgap (Eg 2.8 eV).9,10There have been quite a few reports on the abnormal pho- tovoltaic effects of BiFeO3films on a SrTiO3substrate recently.2,3,11–13 However, the single-crystal substrate is rigid and fragile, which is not consistent with the trend to develop flexible photovoltaic cells and other flexible electronics under the background of the “Internet of Things.”
Recently, some ferroelectric oxide films are grown on a flexible inorganic mica substrate,14–18so it is valuable to achievePmin the as-grown flexible ferroelectric film and explore flexible photovoltaic, photoimpedance, and photocapacitance effects simultaneously.
In this paper, we fabricate a flexible photoelectronic device based on the (111) BiFeO3film with natural downwardPm. The separation of electron-hole pairs underEFEnot only introduces a maximum pho- tovoltaic effect at 150C but also allows a 95.5% decrease in the photo- impedance effect and a 316% increase in the photocapacitance effect under 405-nm-wavelength light illumination at 25C.
The (111) BiFeO3/SrRuO3 films were epitaxially grown on a (001) single-crystal mica substrate by a pulsed laser deposition tech- nique (supplementary material).Figure 1(a)shows the structural sche- matic image of the (111) BiFeO3/SrRuO3 films on the flexible monoclinic (001) mica substrate (a¼5.25 A˚ , b¼9.02 A˚ , c¼10.14 A˚ , a¼90,b100, andc¼90),14,15where the pseudocubic structure
is used to index the crystal planes of the rhombohedral BiFeO3film (a¼b¼c3.96 A˚ anda¼b¼c89.5) and the orthorhombic SrRuO3film (a¼5.55 A˚ , b¼5.55 A˚ , c¼7.83 A˚ , anda¼b¼c¼90) for easy comparison. There are only diffraction peaks of the (111) BiFeO3/SrRuO3films and of the (001) mica crystal in the X-ray dif- fraction (XRD) patterns shown inFig. 1(b). Furthermore,Fig. 1(c) shows the cross-sectional images of transmission electron microscopy (TEM) taken along the zone axis of [010] mica, confirming the 300- nm-BiFeO3/75-nm-SrRuO3/mica structure. There are the sharp BiFeO3/SrRuO3 [Fig. 1(d)] and SrRuO3/mica [Fig. 1(f)] interfaces without observable interdiffusion of ions, indicating the high quality of the heterostructure.Figures 1(e)and1(g)show the electron dif- fractions in the selected area of these two interfaces where the reciprocal lattices are also clearly indexed. Obviously, both XRD and TEM results indicate the [111]BFO//[111]SRO//[001]micaepitax- ial relationship.17The SrRuO3film was grown on mica through van der Waals heteroepitaxy, while the BiFeO3film was grown on SrRuO3through common heteroepitaxy.
The ratio of the area with downwardPmto the total area (i.e., the ratio of downwardPm) is as high as97% in the BiFeO3film grown at 680C. The 1st, 2nd, 3rd, and 4th BiFeO3films were grown on SrRuO3/mica at 600C, 640C, 660C, and 680C, respectively. The crystal grains increase gradually with the growth temperature rising according to the morphologies of these BiFeO3films. The spherical crystal grains suggest an island growth mode at 600–660C [Figs. 2(a), 2(c), and2(e)], while the flattened grains mean the combination of island growth and in-plane growth modes at 680C [Fig. 2(g)].
Besides, the ferroelectric polarization of these four films can be ana- lyzed according to their 55lm2phase images where the downward polarization (brown color) was polarized by 18 V in a 33lm2area and then the upward polarization (yellow color) was polarized by 18 V in the central 11lm2area with a piezoelectric force micro- scope (PFM,supplementary material). The ratio of downwardPmis
calculated in the as-grown BiFeO3 films outside of the 33lm2 polarization area, and such a ratio is 67% [Fig. 2(b)], 74% [Fig. 2(d)], 93% [Fig. 2(f)], and 97% [Figs. 2(h)and2(i)] in the 1st, 2nd, 3rd, and 4th BiFeO3films. Obviously, the ratio of downwardPmincreases with the growth temperature rising from 600C to 680C. Such an97%
ratio of downwardPmis stable in a large-size 4th BiFeO3film (supple- mentary material, Fig. S1).6,19Such a ratio is also consistent with the asymmetric polarization vs electric field loops, which are unsaturated due to a mass of oxygen vacancies (VOS) (supplementary material, Fig.
S2). Only the 4th BiFeO3film and its cells are discussed in the follow- ing paragraphs.
The downwardPmis introduced by the downward electric field that mainly originates from the inhomogeneous distribution of bismuth vacancies (VBi3) and the SrRuO3/BiFeO3interface effect. Here, the Bi ions volatilize more seriously, and the correspondingVBi3ions increase with the increasing temperature or its time expanding during the BiFeO3film growth. The bottom BiFeO3layer was first grown and then annealed at 680C for over 2 h, while the top BiFeO3layer was finally grown and annealed for a short time. It is noted thatVBi3ions hardly move due to their large effective mass, whileVOScan redistribute above 300C or under a large strain or at a high electric field. As a result, the VBi3
density of the bottom BiFeO3layer is the highest and the Vos density of the top BiFeO3layer is the highest,6,20,21which introduces a downward electric field toward the SrRuO3bottom. Besides, this interfa- cial valence mismatch can introduce a downward electric field too when the SrRuO3-(SrO)0/(FeO2)-BiFeO3interface is dominant.8On the con- trary, the epitaxial compressive strain from the (001) SrTiO3substrate and/or its flexoelectric effect commonly induce an upward polarization rather than downward polarization in BaTiO3films,5,22BiFeO3films,6 PbZr1-xTixO3 films,4,23,24 and 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 films.25,26Since the epitaxial strain from the substrate is commonly the largest at the SrRuO3/BiFeO3interface and relaxes gradually toward the BiFeO3surface, the strain gradient produces an upward electric field FIG. 1.Structure information of (001)-mica/(111)-SrRuO3/(111)-BiFeO3. (a)
Schematic image, (b) XRD pattern, and (c) cross-sectional TEM image. The HRTEM images of interfaces and electron diffractions of the SrRuO3/BiFeO3(d) and (e) and mica/SrRuO3(f) and (g) interfaces, respectively, in the corresponding selected area are shown.
FIG. 2.Downward polarization of the as-grown BiFeO3film. The topography and out-of-plane PFM images of the BiFeO3film grown on mica/SrRuO3at (a) and (b) 600C, (c) and (d) 640C, (e) and (f) 660C, and (g)–(i) 680C are shown.
through the flexoelectric effect which induces an upward polarization in the BiFeO3 film. Recently, Byung Chul Jeon et al. successfully manipulated both upward and downward polarizations of the epitax- ial (001) BiFeO3film through changing its growth temperature to precisely control the SrRuO3/BiFeO3interface and the epitaxial strain of the BiFeO3film.6
The 10-lm-thick mica/SrRuO3/BiFeO3/Au sample not only shows strong photovoltaic effects but also is flexible enough to paste on most bending surfaces by the roll-to-roll process. As is shown in Fig. 3(a), the downwardPmgives an upwardEFEacross the BiFeO3 film and the BiFeO3/Au Schottky junction allows an upwardEinnear the surface.2,12Because the photogenerated electron-hole pairs are sep- arated byEFEandEinand the holes are accumulated at the top surface of the BiFeO3film under light illumination, its surface potential is higher than that of the film in the dark, which is shown in the image of Kelvin Probe Force Microscopy (KPFM,supplementary material Fig. S3). The observed surface potentials have been separated into four different levels with opposite polarization and different light condi- tions.27Furthermore,EFEandEinalso separate electron-hole pairs and introduce a strong photovoltaic effect. As is shown inFig. 3(b), the as- grown sample shows the short-circuit photocurrent density (Jsc) of 1.02 mA/cm2and the open-circuit voltage (Voc) of0.52 V when the laser with the 405 nm wavelength illuminates the sample surface.28 On the contrary, 532-nm-wavelength lights only introduce a tinyJsc since their photon energy (2.33 eV) is smaller than the bandgap of 2.8 eV of the BiFeO3film. Most importantly, the 10-lm thick photo- voltaic cells are flexible and theirJscandVocdo not show serious atten- uation at a 2.2 mm bending radius [Figs. 3(b) and 3(c)]. Even the
photovoltaic cells were bent to 3 mm radius for 10 000 times, there are small changes in JscandVoc, which is consistent with the negligible change in the surface morphology of the BFO film before and after bending (supplementary material, Fig. S4). Moreover, the flexible pho- tovoltaic cells can be kept in a reel type and then fixed on a large smooth surface whether it is curved or not. For example, the photovol- taic cells with 10lm thickness and 35 m2area can be curved and then kept in a 350 ml bottle. Furthermore,Jscbecomes zero in the dark and then recover to a high value under light illumination immediately [Fig.
3(d)], and thus, the flexible photovoltaic cells can be used as a purple light detector.29When photovoltaic cells are illuminated for over 10 s, their temperature and the correspondingJscincrease gradually to sta- ble values for the heat effect. In a word, the flexible photovoltaic cells are not only produced, saved, and installed in a cost-effective way but also can be easily used in most objects with curved surfaces.
Jscincreases four times, andVocremains stable with temperature increasing from 25C to 150C, so the maximum photovoltaic effi- ciency increases by 660% at150C.Figure 4(a)shows the current vs voltage (I-V) curves of the cells under the 405-nm-wavelength light illu- mination at 25–238C, andFig. 4(b)shows the dependence ofJsc,Voc, and photovoltaic efficiencies on characteristic temperature.Jscincreases from 1.02 mA/cm2 at 25C to 4.42 mA/cm2 at 150C,29–31 while it decreases fast to 3.18 mA/cm2 at 160C and then becomes stable at 160–238C.Vocis about 0.52 V at 25–120C, whereas it decreases to 0.08 V with temperature increasing to 238C. As a result, the photovol- taic efficiency increases gradually from 0.031% at 25C to the maximum value of 0.206% at150C.Jscswitches immediately between zero and a high value when the laser switches on and off at 25–238C, suggesting that the photovoltaic effect remains stable even at 238C [Fig. 4(c)].
The maximum photovoltaic efficiency at 150C can be roughly explained according to the dependence ofPmand conductivity (r) of
FIG. 3.Photovoltaic effect of the flexible SrRuO3/BiFeO3/Au cells. (a) Schematic diagram of the energy band and operational principle. (b) Current density vs voltage curves of flat cells under light illumination with the powder density of 455 mW/cm2. (c) Current density vs voltage curves and (d) current density vs time curves of the origin flat cells, 2.2-mm-bending cells, and flat cells after 3 mm bending for 10 000 cycles in the dark or under 405-nm-wavelength light illumination, where the sketch of the flexible photovoltaic cells is shown in the inset of (c).
FIG. 4.Dependence of photovoltaic properties on temperature. (a) Current density vs voltage curves of cells under 405-nm-wavelength light illumination. (b) Dependences ofVoc,Jsc, and efficiency on temperature. (c)Jscvs time curves in the dark and under illumination alternatively and (d) current density vs voltage curves in the dark.
the BiFeO3film on the characteristic temperature. The exact photovol- taic efficiency of the BiFeO3ferroelectric film should come from the complex interplay among many physical parameters, such asPm,EFE, Ein, charge screening at interfaces, carrier density, mobility, r, and energy band structure,29,32which are more complex than those of the silicon-based p-n junction. In this case, ferroelectric polarization rather than Au/BiFeO3Schottky mainly decides the value ofVoc. The ferroelec- tric Curie temperature (TC) of BiFeO3is about 810C, and thus,Pmjust decreases slightly with temperature increasing from 20 to 238C according to the Curie-Weiss law. As a consequence,EFEacross the BiFeO3 film and the corresponding Voc remain relatively stable at 25–150C. On the contrary, the BiFeO3film changes from an insulator to a semiconductor with the temperature increasing from 25C to 150C, which fast increases bothrandIsc. The energy bands of trapped electrons ofVOSare0.3 eV below the conduction band, and thus, more and more free electrons are excited to the conduction band and ther of BiFeO3film increases fast with the increasing temperature.29 For example, the current density of the BiFeO3 film at 0.4 V is 7.5105mA/cm2 at 25C, 1.4102mA/cm2 at 100C, and 27.2 mA/cm2at 238C [Fig. 4(d)]. Therefore, the photovoltaic efficiency (/ JscVoc or rVoc2) increases with the increasing r at 20–150C.
Nevertheless, more and more mobile charges neutralize the immobile polarization carriers at both SrRuO3/BiFeO3and BiFeO3/Au interfaces and then lowerEEFandVocwith the increasing temperature to 238C, so the photovoltaic efficiency decreases gradually at>150C.
The production and separation of electron-hole pairs decrease the impedance and increase the conductance and the capacitance of the BiFeO3film simultaneously especially at low characteristic frequen- cies. Once a mass of electron-hole pairs of the BiFeO3film with the bandgap of2.8 eV are excited by either lights with the 405/450 nm wavelength or thermal energy and then these pairs are separated by EFEandEin, the impedance decreases greatly and the conductance increases significantly [Figs. 5(a)and5(b)]. For example, the imped- ance of the BiFeO3film decreases from 21.2 MXto 1.05 MX(i.e., 95.5%) and the conductance increases from 3.9 nS to 493 nS (i.e., 125 times) at 113 Hz when it was illuminated by a laser with the 405 nm wavelength and 455 mW/cm2. The impedance at 1 kHz fast switches between2.8 MXand0.5 MX[inset ofFig. 5(a)], and the corresponding conductance at 1 kHz quickly switches between 36 nS and 1lS [inset ofFig. 5(b)] when light switches on and off. Besides, the capacitance increases from 63.6 pF to 265 pF (i.e., 316%) at 113 Hz. However, it just increases5% at 1 MHz under 405-nm- wavelength light illumination [Fig. 5(c)]. The capacitance at 1 kHz immediately changes between 57.3 pF and 150 pF when light switches on and off [inset ofFig. 5(c)].1It is noted that the impedance angle (supplementary material, Fig. S5), the susceptance (supplementary material, Fig. S6), and the dielectric loss (supplementary material, Fig. S7) increase under the illumination of lights with the 405 nm wavelength, and they also switch between high and low values when the lights switch on and off.
Photoimpedance, photoconductance, and photocapacitance effects are strong at low frequencies but weak at high frequency because of some special physical factors. It is known that the capaci- tance depends on the polarization of the material, i.e., (1) dipolar polarization, (2) ionic polarization, (3) interfacial or space charge polarization, and (4) electronic polarization.33 Impedance, conduc- tance, and capacitance seriously depend on light illumination at low
frequencies, but they just change slightly at high frequencies, and thus, (4) is not the main factor for these huge photoimpedance, photocon- ductance, and photocapacitance effects at low frequencies. These effects are mainly attributed to three factors. First,VBi3andVOStrap release photogenerated carriers at low frequencies,33which increases (1) and (2). Second, a number of light-excited carriers accumulate at BiFeO3interfaces and then increase (3). Third, the decrease in direct- current (DC) resistance under light illumination can also enhance the measured capacitance at low frequencies according to the equivalent circuit with bulk resistance in series with parallel DC resistance and capacitance.34 Finally, both photoimpedance and photocapacitance effects are strong even when the characteristic temperature increases to 130C [Fig. 5(d)andsupplementary material, Fig. S8], and thus, the flexible SrRuO3/BiFeO3/Au solar cell device is a good candidate for photoimpedance and photocapacitive sensors in modern optoelec- tronic devices.33,35,36
In a word, the flexible mica/SrRuO3/BiFeO3/Au cells show the strong photovoltaic effect and the giant photoimpedance, photocon- ductance, and photocapacitance effects at low frequencies under 405/
450-nm-wavelentgh light illumination. The cells show a Voc of 0.52 V, aJscof 1.02 mA/cm2, and a maximum photovoltaic efficiency at 150C. Since a large number of electron-hole pairs were excited by 405/450-nm light and then were separated byEFEandEin, the photo- impedance decreases 95.5%, the photoconductance increases 125 times, and the photocapacitance increases 316% compared with those in the dark at 113 Hz. Furthermore, these properties do not show obvi- ous degradation when the cells are bent to 3 mm radius for 10 000 times. This work is of vital importance for us to develop flexible and large-size ferroelectric photoelectronic devices.
FIG. 5.Photoimpedance, photoconductance, and photocapacitance effects.
Dependence of (a) impedance, (b) conductance, and (c) capacitance on the fre- quency of SrRuO3/BiFeO3/Au cells with and without light illumination. (d) Relative permittivity and impedance at 10 kHz as a function of the characteristic temperature with and without 405-nm-wavelength light illumination.
See thesupplementary materialfor further details about the prep- aration and characterization of samples, the surface morphologies and PFM/KPFM images of the BiFeO3film, impedance and its angle, con- ductance and susceptance, and capacitance and its dielectric loss of SrRuO3/BiFeO3/Au cells, respectively.
This work was supported by the National Natural Science Foundation of China (Nos. 51790492, 51431006, 61874055, and 11774249), the National Key Research Program of China (No.
2016YFA0300101), the Natural Science Foundation of Jiangsu Province (No. BK20171209), and the Key University Science Research Project of Jiangsu Province (No. 18KJA140004).
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