Manipulation of Oxygen Vacancy for High Photovoltaic Output in

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23372−23381

Manipulation of Oxygen Vacancy for High Photovoltaic Output in Bismuth Ferrite Films Tiantian Yang,† Jie Wei,*,† Yaxin Guo,† Zhibin Lv,† Zhuo Xu,† and Zhenxiang Cheng‡ †

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Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China ‡ Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia S Supporting Information *

ABSTRACT: Very recently, the ferroelectric photovoltaic property of bismuth ferrite (BiFeO3, BFO) has attracted much attention. However, the physical mechanisms for its anomalous photovoltaic effect and switchable photovoltaic effect are still largely unclear. Herein, a novel design was proposed to realize a high photovoltaic output in BiFeO3 films by manipulating its oxygen vacancy concentration through the alteration of the Bi content. Subsequent results and analysis manifested that the highest photovoltaic output was achieved in Bi1.05FeO3 films, differing 1000 times from that of Bi0.95FeO3 films. Simultaneously, the origin of photovoltaic effect in all BiFeO3 films was suggested as the bulk photovoltaic mechanism instead of the Schottky effect. Moreover, oxygen vacancy migration should be the dominant factor determining the switchable photovoltaic effect rather than the ferroelectric polarization. A switchable Schottky-to-Ohmic interfacial contact model was proposed to illustrate the observed switchable photovoltaic or diodelike effect. Therefore, the present work may open a new way to realize the high power output and controllable photovoltaic switching behavior for the photovoltaic applications of BiFeO3 compounds. KEYWORDS: bismuth ferrite film, ferroelectric, photovoltaic, oxygen vacancy, diodelike effect ferroelectric polarization. Yang et al.2 prepared a specific BFO film with all domain walls parallelly aligned at 71°, whereas the experiments showed that the open-circuit photovoltage (VOC) in BFO is not determined by the width of band gap but is related to the particular interface of domain walls. Gao et al.24,25 changed the distribution of oxygen vacancies by applying the pulse to the heterojunction to study the shortcircuit photocurrent under different oxygen vacancy distributions. Although the ferroelectric photovoltaic effect or property of BFO has been studied for years, the physical mechanisms for the anomalous photovoltaic effect and switchable photovoltaic effect in BFO are still largely unclear. For example, the origin of the anomalous photovoltaic effect observed in BFO epitaxial thin films was first attributed to domain walls, in which electrons and holes could be efficiently separated by the internal electric field.2,26 However, subsequent studies suggested that the bulk photovoltaic effect (BPV) arising from the noncentrosymmetric nature of ferroelectric materials might be the actual mechanism for the anomalous photovoltaic effect.27−30 Alexe et al.17 demonstrated via temperature photovoltaic measurements that BPV should indeed be the origin of the anomalous photovoltaic effect in BFO films. For

1. INTRODUCTION Bismuth ferrite (BiFeO3, BFO), as a prototype multiferroic material, has attracted much attention because of its high ferroelectric Curie temperature (TC ∼ 1100 K) and Neel temperature (TN ∼ 650 K), making it have broad application prospects in optoelectronic devices, spintronic devices, ferroelectric memory, magnetic storage, and other fields.1−7 More recently, the ferroelectric photovoltaic effect in BFO systems is of particular interest mostly due to its large polarization (∼90 μC/cm2), relatively low band gap (Eg = 2.2−2.8 eV), and large light absorption coefficient,1,2,6−10 which make it able to absorb more photons in the visible range unlike the conventional ferroelectric materials such as BaTiO3, LiNbO3, or Pb(Zr, Ti)O3.11−15 Especially, an extremely large open-circuit voltage (VOC) exceeding several times its band gap (anomalous photovoltaic effect) and a unique polarization-dependent switchable behavior of photocurrent (switchable photovoltaic effect) have been successively reported in BFO films and crystals,1,2,6−10,16,17 which make BFO attractive for application in optoelectronic or photovoltaic devices. Thereafter, several studies have shown that polarization,18−21 domain wall,2,22 and oxygen vacancies20,22−24 were the main factors affecting the ferroelectric photovoltaic effect. Yi et al.20 studied the mechanism of photovoltaic effect generated in BFO films, indicating that photovoltaic effect originates from ferroelectric polarization, and the direction of photocurrent is opposite to © 2019 American Chemical Society

Received: April 17, 2019 Accepted: June 5, 2019 Published: June 5, 2019 23372

DOI: 10.1021/acsami.9b06704 ACS Appl. Mater. Interfaces 2019, 11, 23372−23381

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ACS Applied Materials & Interfaces the BFO films with periodic ferroelectric stripe domain patterns,26 high VOC values might be and could only be observed in the specific geometry with the domain walls running parallel to the collecting electrodes. On the contrary, VOC value beyond the band gap could not be observed at the polycrystalline BFO films at all, in which the ferroelectric photovoltaic effect was generally attributed to the Schottky barrier formed in the interface between the metal and the semiconductor.31,32 On the other hand, Choi et al.1 observed the switchable photovoltaic effect in a BFO single crystal, displaying that the photocurrent could be reversibly switchable by applying an external electric field. Later studies displayed that BFO thin films exhibited or did not exhibit the switchable photovoltaic behavior depending on the quality of the films or even the synthesis process. Concerning the physical mechanism for this switchable photovoltaic effect, there is also no unified conclusion. Initially, ferroelectric polarization was considered to be the dominant factor controlling the switchable photovoltaic behavior.20 However, more and more evidence revealed that the electromigration of chemical defects such as oxygen vacancies might play a key role in the switchable photovoltaic effect.23 Moreover, the photocurrent density (short-circuit current density, JSC) of BFO observed in bulk crystals or thin films is very small, usually several μA/cm2 below the magnitude of mA/cm2 for general silicon-based solar cells.18,29,32 It makes the power conversion efficiency of BFO-based photovoltaic devices very poor, usually less than 1%, severely hampering future applications in ferroelectric photovoltaics or photoelectronics.14,33 Thanks to the fruitful investigations on the physical properties of BFO compounds in the past, oxygen vacancy is believed to play a key role on leakage and ferroelectric properties of BFO, which in turn should have great impacts on its photovoltaic properties.23,34,35 On the basis of the above analysis and discussion, we proposed a novel design to realize high photovoltaic output in bismuth ferrite films. In this framework, manipulation of oxygen vacancy by adjusting the content of Bi in the BFO film capacitor finally achieved more than 1000 times improvement on its photovoltaic output (VOC × JSC). Meanwhile, in our case, all Au/BixFeO3/FTO (x = 0.95, 1, and 1.05) film capacitors have the same geometric electrode structure and thereby almost the same Schottky barrier, whereas their ferroelectric photovoltaic performances are quite different because of the different oxygen vacancy concentration in these films. It revealed that the origin of photovoltaic effect in all BixFeO3 (x = 0.95, 1, and 1.05) films should be the bulk photovoltaic effect. Similarly, oxygen vacancy concentration worked on the switchable photovoltaic effect since this effect could be observed in the Bi0.95FeO3 and BiFeO3 films and not in the Bi1.05FeO3 film. Based on these observations, a switchable Schottky-to-Ohmic interfacial contact model was proposed to illustrate the switchable photovoltaic or diodelike effect in our samples. In our case, oxygen vacancy migration should be the dominant factor determining the switchable photovoltaic effect, rather than the ferroelectric polarization.

room temperature. Then, 2-methoxyethanol was added and stirred for another 3 h to form the precursor solution. The depositions were carried out by the spin-coating technique at 600 rpm for 9 s and then at 4000 rpm for an additional 30 s. The wet film was preannealed at 200 °C for 2 min and then annealed at 550 °C for 5 min in air. The above process was repeated several times to obtain the desired thickness. Finally, the sample was annealed at 550 °C for 30 min to obtain a denser film. The phase purity and crystalline structure of the films were investigated by X-ray diffraction (XRD, X’pert PRO) and laser Raman spectroscopy (HR800). The surface morphologies of the films were observed with a scanning electron microscope (SEM, Quanta F250). Ion valences were determined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB Xi+). The surface element contents of as-prepared films were also measured and analyzed by XPS after cleaning and etching the films by a 3 keV Ar-ions gun (etch time: 1 min; etch current: 2.73 μA; spot size: 3.25 mm; source gun type: Al Kα). The leakage current characteristics of the films (J−E) were characterized by the Keithley 4200-SCS meter. The photovoltaic effect was evaluated illuminating the film between the electrodes by a 405 nm laser (hv = 3.06 eV) with a maximum power density of 50 mW/cm2. Simultaneously, the current−voltage curves were recorded by a precise source/measure unit (Keysight B2901a). The absorption spectrum was investigated by UV−visible spectroscopy.

3. RESULTS AND DISCUSSION 3.1. Crystalline Structure. The phase purity and crystalline structure of BixFeO3 (x = 0.95, 1, and 1.05) films were measured by X-ray diffraction (XRD), as shown in Figure 1. XRD patterns show that all detectable diffraction peaks of

Figure 1. XRD patterns of BixFeO3 (x = 0.95, 1, and 1.05) films.

the films can be easily indexed into a rhombohedral distorted perovskite structure with a space group of R3c (PDF#861518), indicating a single phase structure without any secondary phase. Meanwhile, the sharpness of the main peaks indicates a highly crystallized quality of all films. It notes that the change in Bi content did not alter the crystalline structure of BiFeO3, simultaneously confirmed by the observation from the Raman spectra as shown in Figure S1. Surface morphology SEM images and cross-section structure of BixFeO3 (x = 0.95, 1, and 1.05) films are shown in Figure S2, and the thickness of the BiFeO3 film is about 500 nm. The other films have almost the same thickness because of the deposition in the same condition. Details on film growth, morphology, and structure are given in Figures S1−S3 of the Supporting Information. 3.2. Leakage Properties. Figure 2 shows the leakage current density (J) as a function of electric field (E) of BixFeO3 (x = 0.95, 1, and 1.05) films measured at room temperature. Leakage current densities of all of the films increase markedly with the increase of electric field. The leakage current density of the Bi0.95FeO3 film is 2 orders of magnitude higher than that of the Bi1.05FeO3 film. Evidently, Bi content has a great impact on the leakage properties of BFO films, which should be

2. EXPERIMENTAL SECTION BixFeO3 (x = 0.95, 1, and 1.05, corresponding to Bi0.95FeO3, BiFeO3, and Bi1.05FeO3) films were deposited on the F-doped SnO2 (FTO)/ glass substrate using a chemical solution deposition process. In a typical run, bismuth nitrate (Bi(NO3)3·5H2O) and iron nitrate (Fe(NO3)3·9H2O) were mixed and stirred in acetic acid for 30 min at 23373


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and the semiconductor that charges must overcome. The current density across the barrier is ÄÅ É 1/2 Ñ ÅÅ ÑÑ 3 i y Å q V Φ 1 j z jj zz ÑÑÑÑ − J = AT 2 exp −ÅÅÅÅ j z ÅÅ kBT kBT jk 4πε0εrL z{ ÑÑÑÑ (2) ÅÅÇ ÑÖ where A is the Richardson constant and Φ is the height of the Schottky barrier. PF emission, being similar to Schottky emission, involves a process of charge carrier transmission, in which charge carriers trapped in defect centers emit into the conduction band under an applied electric field and thereby contribute to the conduction process. PF emission is hence called the “internal Schottky emission”. The current density due to the bulk limited PF emission is given by36,38 ÅÄÅ ÑÉ ÅÅ −(φt − e eE /πεrε0 ) ÑÑÑ Å ÑÑ JPF = AE expÅÅ ÑÑ ÅÅ k T ÑÑÑ B (3) ÅÇÅ Ö

Figure 2. Leakage current density of BixFeO3 (x = 0.95, 1, and 1.05) films measured at a varied direct-current electric field from −200 to 200 kV/cm at room temperature.

primarily attributed to the change in the oxygen vacancy.23,36 As requirement of the charge balance in BFO, Bi vacancies lead to an increase in the oxygen vacancy concentration, whereas excessive Bi should be bound to a decrease in the oxygen vacancy concentration. Combined with the SEM analysis (as shown in Figure S2), one can deduce that excessive Bi leads to a smoother surface, larger grain size, less grain boundaries, and lower oxygen vacancy concentration, which should be the main reason for the lower leakage current density at the high electric field region in the Bi1.05FeO3 film. It should be noted that the plots of leakage current density vs electric field corresponding to BixFeO3 (x = 0.95, 1) in Figure 2 show a slight offset with respect to the 0 point, instead of x = 1.05. Such a slight offset may be relevant to the measuring process. The measurement of leakage current was obtained by applying the voltage from −10 to +10 V (or the electric field from −200 to 200 kV/cm) to the samples. When the negative bias was continuously applied to BFO, its internal spontaneous polarization shifted into one direction. It would form an internal electric field and affect oxygen vacancy migration. Once the applied voltage was reversed, the internal partial carriers were not fully inverted to the new direction, eventually resulting in a slight offset. The offset did not apparently present in the Bi1.05FeO3 film due to the lowest oxygen vacancy concentration in this film. To find why excessive Bi can improve the electrical properties of the film, the leakage-conduction mechanisms for the films were analyzed and discussed. Previously, many reports proposed several leakage-conduction mechanisms for BFO, including space charge-limited conductance (SCLC), Poole−Frenkel emission (PF), Fowler−Nordheim tunneling (FN), and Schottky barrier model (SE).37−39 Among them, SCLC and PF belong to the bulk limited conduction emission mechanism, whereas FN and SE belong to the interface limited mechanism. The conduction mechanism of SCLC is formed by the current impeding the formation of space charge when the speed of charges injected from the electrodes into the films is faster than they can penetrate the film, which can be expressed as38

where A is a constant, φt is the trap ionization energy, and T is the temperature. Another type of leakage current conduction mechanism is the FN leakage-conduction mechanism. The carrier at the electrode passes through the electrode−film interface barrier, entering the film through a tunneling pattern, and its corresponding expression is36 ij −Cϕ 3/2 yz zz i J = BE2 expjjjj z j E zz k {

(4)

where B and C are constants and φi is the potential barrier height. First, we could exclude the FN leakage-conduction mechanism because the thickness of as-prepared films in this case is about 500 nm, much larger than 50 nm for the possible tunneling depth.40 Second, we can determine if it is the SCLC, PF, or SE conduction mechanism by, respectively, plotting the log E − log J, E1/2 − ln J/E, and E1/2 − ln J curves according to expressions 1−3. After fitting the corresponding curves, we observed the linear parts in log E − log J curves under the low electric field regions, and their corresponding slopes are around 1 for all films, whereas the linear fits in the high electric field regions were found in E1/2 − ln J curves, as shown in Figure 3. It clearly shows that the leakage mechanism for asprepared films is a typical Ohmic conduction behavior under the low electric field regions and the SE mode in the high electric field regions. Meanwhile, the PF mechanism can be ruled out in all films since no linear behavior is found in the E1/2 − ln J/E curves, as shown in Figure S4.

9 V2 μεrε0 (1) 8 L where μ is the charge carrier mobility, εr is the relative dielectric constant, ε0 is permittivity of free space, V is voltage, and L is the film thickness. Schottky barrier results from the Fermi-level difference between the metal electrode and the semiconductor film. The energy difference creates a potential barrier between the metal J=

Figure 3. Leakage current behaviors presented as the (a) SCLC model and (b) SE model for BixFeO3 (x = 0.95, 1, and 1.05) films. 23374


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Figure 4. (a, e), (b, f), (c, g), and (d, h) corresponding out-of-plane PFM phase images of Bi1.05FeO3 films with 5, 10, 12, and 15 V tip bias on and after the tip bias domain writing, respectively.

Figure 5. (a) Bi 4f, (b) Fe 2p, and (c) O 1s core-level XPS spectra for BixFeO3 (x = 0.95, 1, and 1.05) films.

Table 1. Binding Energy (eV) of Bi, Fe, and O Elements for Bi0.95FeO3, BiFeO3, and Bi1.05FeO3 elements binding energy (eV)

Bi Fe

O

Bi0.95FeO3 Bi 4f7/2 Bi 4f5/2 Fe 2p3/2 satellite Fe 2p1/2 oxygen−metal bond dangling bond surface adsorbed oxygen RIR

3.3. Piezo Force Microscopy (PFM). Although BFO has been well known as a ferroelectric for a long time,41−43 the saturated ferroelectric hysteresis loop is very difficult to achieve in pure BFO polycrystalline samples (BFO polycrystalline films or ceramics) mainly due to the problem of large leakage currents.41,44−47 Doping or substitution by other metal ions in A- or B-site of BFO is believed to be an effective method to possibly observe the saturated ferroelectric hysteresis loop. In our case, pure BFO polycrystalline films (BixFeO3 (x = 0.95, 1, 1.05)) prepared by the spin-coating process presented severe leakage behaviors (as shown in Figure 2), and hence we could not observe the saturated loops in these films. To confirm the

158.5 163.8 709.7 717.9 723.3 528.9 530.9 532.1 1.57

± ± ± ± ± ± ± ± ±

0.1 0.1 0.2 0.2 0.2 0.1 0.1 0.3 0.01

BiFeO3 158.4 163.7 709.0 717.7 724.1 529.0 530.6 531.7 2.65

± ± ± ± ± ± ± ± ±

0.2 0.1 0.3 0.2 0.3 0.3 0.1 0.1 0.02

Bi1.05FeO3 158.2 163.5 709.4 717.6 723.2 528.6 530.7 531.4 3.03

± ± ± ± ± ± ± ± ±

0.1 0.2 0.2 0.3 0.2 0.1 0.2 0.1 0.02

ferroelectric nature of our samples, we investigated the ferroelectricity of the Bi1.05FeO3 film by a supplementary piezo force microscopy (PFM) measurement as shown in Figure 4. Domain writing was performed by applying 5, 10, 12, and 15 V direct current bias in the PFM mode. An obvious and clear domain inversion can be seen in Figure 4h, corresponding to an applied domain writing bias of 15 V, demonstrating the ferroelectric nature of BixFeO3 films. 3.4. X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS was employed to determine the chemical states of Bi, Fe, and O elements in as-prepared films. Figure 5 shows Bi 4f, Fe 2p, and O 1s XPS spectra for BixFeO3 (x = 0.95, 1, and 23375


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ACS Applied Materials & Interfaces 1.05) films. Figure 5a shows the high-resolution spectra of Bi 4f for three BFO samples. The binding energies of Bi 4f7/2 and Bi 4f5/2 for all films are listed in Table 1. Compared with the XPS data illustrative map, there is no remarkable difference in the two peaks’ profile, implying that Bi elements in all films are in the Bi3+ valence state.48 However, the two peaks in the Bi 4f spectra of three samples inappreciably deviated from the standard position, which may be attributed to the difference in the chemical environment, electric charge effect, instrument state, etc. Figure 5b shows the high-resolution Fe 2p spectra of three samples. The binding energies of Fe corresponding to Fe 2p3/2 and Fe 2p1/2 are shown in Table 1, which should be attributed to the interaction of the spin−orbital effect. The satellite peak around 717.7 eV (about 8.0 eV above the Fe 2p3/2 peak) was found in all BFO samples, indicating that the valence state of the Fe element is Fe3+.49,50 For BFO films with different Bi contents, however, some peaks observed in the Bi and Fe spectra transferred to a high binding energy. The displacement of these XPS peaks could generally be ascribed to the formation of oxygen vacancies, which increase the equilibrium electron density and hence increase the binding energy.51 Figure 5c shows O 1s core-level XPS spectra of all samples. The three peaks of O 1s XPS spectra corresponding to binding energy from low to high are responsible for the oxygen−metal bond, the dangling bond, and surface adsorbed oxygen.52 In general, the emergence of these two peaks of the dangling bond and adsorbed oxygen in the O 1s spectrum was considered representative of the formation of oxygen vacancies in perovskite oxides.53 Based on the relative peak’s intensity ratio (RIR) between the oxygen−metal bond and the sum of the dangling bond and adsorbed oxygen [RIR = IO−M /(IDL + IADS), where IO−M is the relative peak’s intensity of the oxygen−metal bond, IDL is the relative peak’s intensity of the dangling bond, and IADS is the relative peak’s intensity of the adsorbed oxygen], the content of oxygen vacancies can be roughly estimated.52,54 In other words, the lower value of RIR manifests the higher concentration of oxygen vacancies. The calculated RIR values for Bi0.95FeO3, BiFeO3, and Bi1.05FeO3 are about 1.57, 2.65, and 3.03, respectively. It implies that the lower the Bi content, the higher the oxygen vacancy concentration in BFO films. In the meanwhile, the surface element composition of BFO films was also measured and analyzed by XPS after ion cleaning the films by a 3 keV Ar-ion gun. As shown in Table 2, the results displayed that the element composition or content of BFO films is the same as what we expected to be prepared. For example, the measured element ratio of Bi/Fe/O in the

Bi0.95FeO3 film is 0.95:1:3.69, which is almost the same as that of its molecular formula. However, it is obvious that the content of O element is slightly higher than the ratio set by the precursor, which may be attributed to the small amount of adsorbed oxygen residually attached to the surface of the film although the films were etched by Ar ions. Furthermore, it should be noted that the element ratio between the oxygen and the metal in BFO films gradually increased accompanied with the increased content of Bi, which clearly shows that the lower the Bi content, the higher the oxygen vacancy concentration in BFO films. Therefore, XPS confirmed that the Bi1.05FeO3 film had the least oxygen vacancy concentration; conversely, the highest one was for the Bi0.95FeO3 film. In brief, more oxygen vacancies in the Bi0.95FeO3 film due to the small content of Bi result in smaller particles, more defects, and poor uniformity, which directly affect the film’s electric and photovoltaic properties. 3.5. Photovoltaic Effect. To study the photovoltaic properties of BixFeO3 (x = 0.95, 1, and 1.05) films, we measured the photocurrent and photovoltage at zero bias by controlling the light to be in the on or off condition, called short-circuit current density (JSC) and open-circuit voltage (VOC), respectively. JSC and VOC were measured under light and dark conditions, in which the light source was alternately turned on and off for 15 s, and repeated four times. As shown in Figure 6, all films presented apparent optical response

Figure 6. (a) Photocurrent density (JSC) and (b) open-circuit voltage (VOC) for BixFeO3 (x = 0.95, 1, and 1.05) films at zero bias by controlling the light to be in on and off conditions.

behavior when the light was turned on. JSC of Bi1.05FeO3 is the largest, and its value is 13 times higher than that of Bi0.95FeO3 and about 6 times higher than that of BiFeO3. Similarly, VOC of Bi1.05FeO3 is also significantly higher than that of Bi0.95FeO3 and BiFeO3; detail values are shown in Table 3. Accordingly, Table 3. Values of Photocurrent Density (JSC) and OpenCircuit Voltage (VOC) for BFO Films under Zero Bias and Light On Condition

Table 2. Surface Element Contents for Bi0.95FeO3, BiFeO3, and Bi1.05FeO3 sample

elements

Bi0.95FeO3

Bi Fe O Bi Fe O Bi Fe O

BiFeO3

Bi1.05FeO3

JSC/VOC

Bi0.95FeO3

BiFeO3

Bi1.05FeO3

JSC (μA/cm2) VOC (mV)

5.31 ± 0.02 1.0 ± 0.1

10.97 ± 0.01 22.8 ± 0.1

66.42 ± 0.03 89.3 ± 0.2

atomic content 14.3(9)% 15.0(4)% 55.8(6)% 9.9(2)% 9.9(9)% 38.5(5)% 15.6(3)% 14.3(7)% 55.8(9)%

the short-circuit current density and open-circuit voltage increased with the increase of Bi content in films. That is, the lesser the oxygen defects, the better are the photovoltaic properties of the BiFeO3 film. In addition, if the photovoltaic output could be simply estimated as the arithmetic product of JSC and VOC (VOC × JSC), one can find that the photovoltaic output of Bi1.05FeO3 increases more than 1000 times compared with that of Bi0.95FeO3. It means that we can greatly tune or improve the photovoltaic output of the BFO compound just by

Bi/Fe ∼ 0.95 O/Fe ∼ 3.69 Bi/Fe ∼ 0.99 O/Fe ∼ 3.85 Bi/Fe ∼ 1.08 O/Fe ∼ 3.88 23376


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Figure 7. Time-dependent photocurrent densities for (a) Bi0.95FeO3, (b) BiFeO3, and (c) Bi1.05FeO3 films under different biases.

Figure 8. JSC−V curves for (a) Bi0.95FeO3, (b) BiFeO3, and (c) Bi1.05FeO3 films at rves were observed in the Bopposite bias voltages. Inset illustrates the device structure under the measurement.

should be present in the Bi0.95FeO3 film and less in the Bi1.05FeO3 film. Based on the above discussion, it is deduced that the photocurrent reduced greatly in the Bi0.95FeO3 thin film because many photogenerated electrons are trapped by the positive oxygen vacancies and consequently result in severe recombination with photogenerated holes. Therefore, the minimum short-circuit current was obtained from the Bi0.95FeO3 film compared with the others because the highest oxygen vacancy concentration was found in this film, which was also confirmed by XPS analysis. Blom et al. believed that the origin of photovoltaic effect in the metal-BFO polycrystalline film heterojunction was generally attributed to the Schottky barrier formed in the interface between the metal and the semiconductor.31,55 In our case, all Au/BixFeO3/FTO film capacitors have the same geometric electrode structure and thereby almost the same Schottky barrier. Therefore, they should present similar photovoltaic properties if the Schottky barrier was the origin of photovoltaic effect in the BFO films. Apparently, it is not the case in our samples. JSC and VOC of Bi1.05FeO3 are quite higher than that of Bi0.95FeO3 as observed in Figure 6 and Table 3. Therefore, the Schottky barrier induced the photovoltaic effect is not dominant in our samples. As discussed and analyzed above, since all BixFeO3 films have the same crystalline structure and almost the same band gap, they should have similar bulk photovoltaic effects. The difference in the photocurrent and photovoltage of BixFeO3 films was indeed affected by the existence and difference of oxygen vacancies since oxygen vacancies as the trap centers greatly influenced the recombination of photogenerated carriers. Therefore, the origin of photovoltaic effect in all BixFeO3 films should be attributed to the bulk photovoltaic effect, as observed in the BFO single crystal.56 3.6. Switchable Photovoltaic Property. To investigate the switchable photovoltaic property, we measured the timedependent photocurrent for the three films under different biases, as shown in Figure 7. It can be seen that all three samples presented an enhanced photovoltaic response after applying a negative bias voltage, relative to their virgin states, as shown in Figure 6 and Table 3. For example, photocurrent for the Bi1.05FeO3 film increased from 66.4 up to 139.5 μA/cm2 after applying a bias voltage of −5 V. More interestingly, as

manipulating its oxygen vacancy concentration. Therefore, our work may open a way to the design of photovoltaic devices with high power output by simply manipulating oxygen vacancies in the perovskites. It should be mentioned that the open-circuit voltage (VOC) values of our films are relatively low compared with those in some other reports in the literature.2,7,17 Yang et al.2 observed a high VOC of more than 15 V in high-quality epitaxial BFO films with stripe 71° domains. Alexe et al.17 also reported a high VOC of 13 V in the BFO single crystal. However, relatively low VOC was generally observed in polycrystalline BFO films.8−10,19,23,32,33 Especially, VOC of polycrystalline BFO films prepared by the chemical solution deposition process is usually less than 1 V. For example, Dong et al.19 reported a low VOC of 0.63 V in polycrystalline BFO films. Even a very low VOC of 0.08 V was also observed in polycrystalline BFO films prepared by a sol−gel/spin-coating method.8 Therefore, it is very difficult to obtain a high VOC in polycrystalline BFO films prepared by the chemical solution deposition process. The possible reason for low VOC in our films could be explained as follows. Many defects or grain boundaries exist in polycrystalline BFO films, which act as recombination centers to trap the photon-generated carriers. The photon-generated electrons and holes cannot be efficiently separated and reach the electrodes, which eventually lead to a relatively low VOC. Based on the above results, a possible mechanism was proposed to illuminate the effect of oxygen vacancies on the photovoltaic properties and the origin of ferroelectric photovoltaic effect in BFO films. In this case, since all BixFeO3 films exhibited the same crystalline structure as shown before, they have almost the same band gap (shown in Figure S3; the optical band gap of the film can be estimated by Tauc’s plot from the UV−visible spectra), indicating that the number of electron−hole pairs generated in the films under illumination is almost the same. Therefore, all Au/BixFeO3/FTO film capacitors are expected to exhibit similar photovoltaic performance. In our case, however, quite different photovoltaic properties were observed in Au/BixFeO3/FTO film capacitors, which implies that oxygen vacancy has a great influence on the electric and photovoltaic performances of the BixFeO3 films. As mentioned above, Bi defects induced more oxygen vacancies in the BixFeO3 films. In other words, more oxygen vacancies 23377


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proposed to illustrate the switchable photovoltaic or diodelike effects in our samples, as shown in Figure 9. When a bias

seen in Figure 7a,b, the apparent reversible switching photovoltaic effect was found in both Bi0.95FeO3 and BiFeO3 films since the photocurrent was flipped at +3 and +5 V bias, respectively. However, it was difficult to observe this effect for Bi1.05FeO3 even at a bias higher than +5 V until it was broken down, indicating that the direction of polarization is not necessary for the reversal of photocurrent. Therefore, it is not always possible to control the direction of photocurrent by changing the polarization direction as reported previously.1,20,57 Figure 8 shows the J−V curves of the three films at opposite bias voltages under the light on or off condition. By applying the bias voltage onto the samples, it is expected to observe a pronounced diodelike behavior in both Bi0.95FeO3 and BiFeO3 films since the asymmetric J−V curves were observed in these two films. In this case, applying the positive and negative bias voltages on the top electrode is depicted as downward and upward poling, respectively. For example, it could be described that the diode forward direction is from bottom to top when a negative direct-current bias voltage was applied on the top electrode of the Bi0.95FeO3 film (i.e., upward poling) (as shown in the inset of Figure 8a). When positive bias voltage was applied to the Bi0.95FeO3 film, the poling switched to the downward direction, and the diode forward direction also changed. In this condition (applying a positive bias voltage), however, the diodelike behavior is not so distinct since the currents along two directions (upward and downward) are not quite different. This result is well in accordance with the photocurrent switching behavior as shown in Figure 7, which should be linked to the presence of a depletion layer of the Schottky junction at the Au−BFO and BFO−FTO interfaces. A similar one-side diode effect was also observed in the BiFeO3 film, in spite of the requirement of higher bias voltage. In our case, however, the symmetric J−V curves were observed in the Bi1.05FeO3 film even at bias voltage higher than ±5 V, although it could not completely exclude the diodelike nature of the sample.32,58−60 It should be noted that the oxygen vacancy concentration in these films is quite different, as confirmed by XPS analysis. The difference in J−V curves might be intensely related to the different oxygen vacancy concentration in the three samples, which is also discussed later. Concerning the mechanism for this diodelike behavior or photocurrent switchable character, either the ferroelectric polarization or the electromigration of chemical defects was considered as the dominating factor.23 In this case, the photocurrents in Bi0.95FeO3 and BiFeO3 films were flipped at ±3 and ±5 V bias, respectively. This means that the maximum value of the reversal electric field (E) for photocurrent is only 100 kV/cm, which is far lower than the coercive field of the polycrystalline BFO film as EC ∼ 250 kV/cm.29,61 That is, the reversal of ferroelectric polarization does not really occur at this low bias voltage. In our case, therefore, ferroelectric polarization is not the dominating factor controlling the photocurrent switching in Bi0.95FeO3 and BiFeO3 films. In fact, the most significant difference in BixFeO3 films is the concentration of oxygen vacancy. As discussed above, the oxygen vacancy intensely affected the leakage current, photocurrent, and photovoltage in BixFeO3 films. In this case, meanwhile, the electromigration of oxygen vacancies is also considered to play a key role in this kind of switchable photovoltaic effect. Based on the above analysis and discussion, a switchable Schottky-to-Ohmic interfacial contact model was

Figure 9. Distribution diagram of oxygen vacancies and the corresponding barrier height for three BFO films in the original state (a−c) and the bias state (d−f).

electric field lower than its coercive field is applied onto the sample, the positively charged oxygen vacancies migrate toward the electrode interface. In the p-type BFO film, the barrier height can be reduced by introducing a highconcentration oxygen vacancy layer to the interface between the BFO film and the electrodes. In this case, specifically, when a positive bias is applied onto the BFO devicean electric field along the FTO to BFO directionthe positive oxygen vacancies will migrate to the FTO side, and then the barrier height at the FTO−BFO interface will decrease. In this condition, the heightened difference of the Schottky barrier height between FTO/BFO and Au/BFO means that the photocurrent along the electric field E direction is only permitted and the reversed photocurrent is forbidden, namely, generating a diodelike effect.23 The more the oxygen defects exist, the more pronounced is this asymmetry in the barrier height and the lower is the requirement of the bias voltage in the diodelike effect. That is why photocurrent in the Bi0.95FeO3 film can reverse at only 3 V, whereas a higher value of 5 V is necessary for the BiFeO3 film. Moreover, the photocurrent reversal in Bi1.05FeO3 is impossible even at bias voltage higher than 5 V until it was broken down since the concentration of oxygen vacancies in this film is too low to generate enough asymmetry in the barrier height. It can be used to explain why the symmetric J−V curves were observed in the Bi1.05FeO3 film even at bias voltage higher than ±5 V. Note that the asymmetric electrodes of Au and FTO in our samples will also bring about a Schottky barrier imbalance. In fact, we could simply estimate the Schottky barrier height of BFO with different metal electrodes on the basis of the metal-inducedgap-state model just calculating the difference between the work function of the metal and the electron affinity of BFO. The work functions of Au and FTO (ΦAu, ΦFTO) are about 5.1 and 4.4 eV, respectively.62 The electron affinity of BFO (χBFO) is estimated as 3.3 eV. The contacted energy band diagram of metal electrodes and the p-type semiconductor BFO film is shown in Figure S6. Schottky barrier height of the Au−BFO interface can be estimated as ΦAu/BFO = Eg‑BFO−(ΦAu − χBFO), and then the barrier height of the BFO−FTO interface can be estimated similarly as ΦBFO/FTO = Eg‑BFO−(ΦFTO − χBFO). The 23378


ACS Applied Materials & Interfaces imbalance of the Schottky barrier can be obtained by |ΦAu/BFO − ΦBFO/FTO|. Obviously, |ΦAu/BFO − ΦBFO/FTO| = |ΦAu − ΦFTO| ∼ 0.7 eV, which is far lower than the required reversal voltages of 3 V for the Bi0.95FeO3 film or 5 V for the BiFeO3 film. Apparently, the effect of this Schottky barrier imbalance can be ruled out in our samples. In our case, the electromigration of oxygen vacancies should be the dominating factor to determine the switchable photovoltaic effect or diodelike behavior, rather than ferroelectric polarization or the inappreciable Schottky barrier imbalance.63,64

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06704.

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Financial support by the National Natural Science Foundation of China (Grant no. 51272204) is gratefully acknowledged. Thanks largely to Jiamei Liu for the help of XPS measurement. The authors also thank Dai and Ma for their help in using FESEM at the International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, China.

4. CONCLUSIONS We prepared bismuth ferrite films with different Bi contents on FTO/glass substrates by a sol−gel spin-coating method. We concluded by measuring the surface morphology and electric and photovoltaic properties of the films that the lesser the Bi content, the poorer is the morphology of the films, and the more are the chemical defects (oxygen vacancies), resulting in the larger leakage current and degraded photovoltaic performance. On the contrary, photocurrent reversal easily occurs in BFO films with high oxygen vacancies under the application of a small bias voltage. Apparently, the effect of oxygen vacancies on the surface morphology and electric and photovoltaic properties of BFO films should not be underestimated. Better understanding of the effect and inherent mechanism of oxygen vacancies on the photovoltaic characteristics of BFO films will be helpful to expand its application in the field of ferroelectric photovoltaics. Furthermore, the present work may open a way to tune or realize the high power output and controllable photovoltaic switching behavior for photovoltaic applications of BFO compounds.

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Research Article

Details on structure analysis, morphology, and optical band-gap estimation (Figures S1−S3); E1/2 − ln J/E curves of the PF mechanism (Figure S4); electronic band structures for BFO films (Figure S5); band analysis data of BixFeO3 (x = 0.95, 1, and 1.05) films (Table S1); contacted energy band diagram of the metal electrode and the BFO film (Figure S6) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Jie Wei: 0000-0003-0612-8606 Author Contributions

T.Y. conceived and carried out the experiments. T.Y. and J.W. analyzed the data and wrote the paper. All authors read and approved the final manuscript. Notes

The authors declare no competing financial interest. Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China. 23379


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Manipulation of Oxygen Vacancy for High Photovoltaic Output in

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