Enhanced Adjustable Photovoltaic Response in Multilayer BiFeO3

Jun 20, 2019 - In this paper, the photovoltaic effect of BiFeO3 (BFO) film prepared by spin coating was investigated. The photovoltaic effect of the m...
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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

Enhanced Adjustable Photovoltaic Response in Multilayer BiFeO3 Films Qifu Yao,† Wei Liu,† Rui Zhang,† Yuhui Ma,† Yile Wang,† Ruiyuan Hu,† Weiwei Mao,*,‡ Yong Pu,*,‡ and Xing’ao Li*,†,‡

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†

Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210023, P. R. China ‡ School of Science & New Energy Technology Engineering Laboratory of Jiangsu Provence, Nanjing University of Posts and Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: In this paper, the photovoltaic effect of BiFeO3 (BFO) film prepared by spin-coating was investigated. The photovoltaic effect of the multilayer BFO film is significantly enhanced compared to the single layer BFO film. The short-circuit current density increases to 62.6 μA/cm2 while the open-circuit voltage is 0.29 V, which is due to the efficient transport of carriers by the electron transport layer and the hole transport layer in the multilayer film structure. In addition, it is also possible to control the photovoltaic effect of BFO film by applying an external electric field, and a possible mechanism model is proposed to explain this phenomenon. This work found a simple way to enhance the photovoltaic effect of BFO and also achieved precise control of the BFO film photovoltaic effect. KEYWORDS: BiFeO3 film, Photovoltaic effect, Transport layer, Multilayer film, External electric field

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INTRODUCTION Since energy and environmental issues have become more and more serious, more researchers have begun to investigate extensive candidate materials for renewable energies in the environment.1−6 As a novel candidate of photovoltaic materials, ferroelectric materials have been extensively investigated, since the anomalous photovoltaic effect in BiFeO3 (BFO) has been reported.7,8 BFO is a multiferroic material that has both ferroelectric and magnetic order at room temperature. Since 2003, due to its potential in magnetoelectric devices, this material has received much attention.9 In recent years, with the development of solar cell research, BFO was widespread concerned again for its narrow band gap (2.7 eV) in the visible light range. The narrow band gap provides potential possibilities for applications in solar cells and memory devices.10−15 More and more researchers have observed the photovoltaic effect of BFO single crystals, films, and ceramics under visible light illumination.16−19 Yang et al. found that BFO films prepared by metal−organic chemical vapor deposition (MOCVD) can obtain a very large photovoltage.7 Ji et al. reported the bulk photovoltaic effect of epitaxially grown BFO under visible light. They found that BFO films grown by magnetron sputtering have a switchable photovoltaic response.20 With the inexhaustible efforts of researchers, they found that, compared with the semiconductor p−n junction, © 2019 American Chemical Society

the photovoltaic effect in ferroelectrics has the following unique characteristics: (i) Abnormal photovoltaic effect: in the epitaxial film, using a well-arranged domain structure to obtain a large open circuit voltage; (ii) Switchable photovoltaic effect: The direction of photovoltage and photocurrent can vary with changes in the external electric field. At present, ferroelectric material with significant polarization-tuned photovoltaic response can be prepared by high-quality film formation methods such as pulsed laser deposition.21−23 However, the above methods are costly and require a specific substrate as a support, and the multilayer structure cannot be utilized to effectively separate electron−hole pairs, and thus its application is greatly limited. In contrast, chemical solution deposition with a variable film structure and low cost is easier to generalize. At present, many researchers have improved the photovoltaic effect of BFO through doping and made some progress.24−29 In addition, there are methods such as forming a heterojunction by the combination of graphene with BFO,30 and modifying BFO with a precious metal to improve the photovoltaic response.31 These methods all improve the photovoltaic effect by modifying the BFO itself, but there are Received: April 9, 2019 Revised: June 17, 2019 Published: June 20, 2019 12439

DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) The X-ray diffraction diagrams of FTO/TiO2 (FTO-TO) films, FTO/BFO (FTO-BFO) films, and FTO/TiO2/BFO (FTO-TOBFO) films. (b) Schematic diagram of device structure of FTO/TiO2/BFO/Spiro/Ag films. (c, d) Surface and cross-sectional SEM images of the FTO/BFO films. (e, f) Surface SEM image of the FTO/TiO2/BFO films and cross-sectional SEM image of the FTO/TiO2/BFO/Spiro films, respectively. solution was spun onto the compact layer at a speed of 2500 r/min, maintained at 30 s, and then calcined at 300 °C for 10 min and sintered at 550 °C for 50 min in a muffle furnace. The prepared hole transport layer (Spiro) was spin-coated onto the BFO and stored in a dry environment for 1 day. All spin-coating processes are carried out in the glovebox. Finally, 150 nm silver layers were evaporated as electrodes (0.1 nm/s), and the effective area of the electrodes is 0.09 cm2. Characterization. X-ray diffraction (XRD) was used to characterize the crystalline structure of FTO/TiO2, FTO/BFO, and FTO/ TiO2/BFO films. The cross section and surface morphology of FTO/ BFO and FTO/TiO2/BFO films were characterized by SEM. Ferroelectric hysteresis loops were measured using a Radiant precision materials analyzer. UPS was used to obtain the band edge of the film (He I, hν = 21.22 eV). Photovoltaic Measurements. The J−V and J−t curves were tested using an electrochemical workstation. The polarization of the BFO film is controlled by applying voltage. In order to reduce the interference caused by the decrease in the performance of hole transport layer, the device was placed in an air environment for 1 week before the BFO film was polarized. The simulated sunlight with an illumination intensity of 100 mW/cm2 (AM 1.5G) was generated by the solar simulator.

few reports on enhancing the separation of electron−hole pairs through a multilayer structure. Electron transport layers (such as TiO2, ZnO) and hole transport layers (such as SpiroOMeTAD, NiO) are widely used in perovskite solar cell devices to improve efficiency. Therefore, the enhancement of the BFO photovoltaic effect can be achieved by a multilayer structure.32−36 In this work, we try to add TiO2 and Spiro-OMeTAD (Spiro) on both sides of BFO to improve the photovoltaic effect of BFO. And the BFO film was also directly spin-coated on the FTO substrate for comparison. In addition, we tried to explain the improvement of photovoltaic effect through the energy level model. Furthermore, we also explored the impact of external electric fields on BFO photovoltaic effects and proposed a simple mechanism model to explain the phenomenon. Finally, a photovoltaic device based on BFO films was designed to realize the reversal of the photovoltaic effect when the external electric field is reversed.

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EXPERIMENTAL DETAILS

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Preparation of TiO2 Dense Layers. For preparing the TiO2 dense layer precursor solution, 369 μL of titanium isopropoxide was slowly dissolved in 2.53 mL of isopropanol and stirred for a while. A 35 μL portion of hydrochloric acid (2 M) was dissolved in another 2.53 mL of isopropanol. The hydrochloric acid solution was added dropwise into the titanium isopropoxide. After being stirred for 1 h, the clarified solution was spin-coated onto the ozone treated FTO glasses at 2500 r/min for 30 s, dried at 125 °C for 10 min, and annealed at 500 °C for 30 min in air condition. BFO Precursor Solution. First, 1.019 g of Bi(NO3)3·5H2O was dissolved in 4 mL of 2-methoxyethanol until the solution was clarified, and then 0.808 g of Fe(NO3)3·9H2O was added to the above solution according to the stoichiometric ratio. A little glacial acetic acid as a dehydrating agent was added to the solution, and the precursor solution of 0.5 mol/L is prepared and stirred until the solution is clarified. Preparation of BFO Thin Films. FTO is first cleaned, dried, and cleaned with ultraviolet ozone for 15 min, and then the prepared TiO2 dense layer is coated on the FTO. After heating at 125 °C for 10 min, the film was calcined in a muffle furnace, heated from 30 to 500 °C in 30 min, and maintained for 30 min. The prepared BFO precursor

RESULTS AND DISCUSSION

First, we used FTO/glass as the substrate to fabricate multilayer thin film structure devices (FTO/TiO2/BFO/ Spiro) by the spin-coating method as shown in Figure 1b. For comparison, we also prepared a device structure with only one layer of BFO film coated on FTO/glass (FTO/BFO). In order to test whether the BFO film was successfully prepared, we performed XRD and SEM characterization of different films, as shown in Figure 1. In the XRD characterization, we also tested the FTO/TiO2 films in order to prevent the diffraction peak interference of the substrate material. No obvious differences between the diffraction peaks of FTO/ BFO and FTO/TiO2/BFO were observed in Figure 1a, because the TiO2 layer is too thin to have obvious characteristic peaks. A distinct BFO phase (marked with asterisks) was detected compared to FTO/TiO2 films, and no second phase was found. Panels (c) and (e) in Figure 1 are the 12440

DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

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Figure 2. (a) The J−V curves of FTO/BFO (BFO) and FTO/TiO2/BFO/Spiro (TO-BFO-SPIRO) films under illumination. The illustration is the J−V curves of FTO/TiO2/BFO/Spiro films under illumination and dark. (b) The J−t curves of FTO/BFO (BFO) and FTO/TiO2/BFO/Spiro (TO-BFO-SPIRO) films.

Table 1. Comparison of Photovoltaic Parameters in Some Previous Worka film structure

irradiation intensity

Jsc (μA/cm2)

Voc (V)

ref

FTO/BFO/ITO FTO/BLFO/ITO FTO/BLFO/AuNPs/ITO FTO/BFO/Au FTO/Bi0.8K0.2FeO3/Au Nb-STO/BFMO/ITO Pt/Ti/SiO2/Si/BFO/AgNPs/Pt FTO/TiO2/BFO/SPIRO/Ag

AM 1.5G AM 1.5G AM 1.5G AM 1.5G AM 1.5G 85 W/m2

3.6 5.3 18.5 0.3 1.3 1.81 35 62.6

0.2 0.2 0.3 0.6 0.41 0.33

31 31 31 38 38 39 40 this work

AM 1.5G

0.29

a

ITO, BLFO, AuNPs, Nb-STO, BFMO, and AgNPs represent indium-tin-oxide, La-doped BFO, Au nanoparticles, Nb-doped SrTiO3, Mn-doped BFO, and Ag nanoparticles, respectively. The effective areas of the devices in refs 31, 38, and 39 are 0.0625, 0.785, and 0.00785 mm2, respectively.

Figure 3. (a) UV−visible absorption spectra of BFO and TiO2/BFO/Spiro (TO-BFO-SPIRO). (b) Tauc plot (αhυ)2 versus hυ for BFO. (c) UPS spectrum of BFO film. (d) Energy level maps based on UV−visible absorption and UPS results, indicating the energy level of each layer involved in the films.

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DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

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Figure 4. (a) The J−V curves of FTO/TiO2/BFO/Spiro films polarized by different electric fields under sunlight simulator illumination. (b) The J−t curves of FTO/TiO2/BFO/Spiro films after polarization of different electric fields under sunlight simulator illumination. (c) The Voc as a function of applied voltage. (d) Ferroelectric loops of FTO/TiO2/BFO/Spiro films under different electric fields.

surface SEM images of FTO/BFO films and FTO/TiO2/BFO films, respectively. It can be seen that the film prepared by spin-coating BFO on TiO2 is more compact. Panels (d) and (f) in Figure 1 are cross-sectional SEM images of FTO/BFO films and FTO/TiO2/BFO/Spiro films. The thickness of the BFO layers is approximately 160 nm, and the TiO2 and Spiro layers are approximately 50 nm. After the films were successfully prepared, we conducted photovoltaic effect tests on FTO/BFO films and FTO/TiO2/ BFO/Spiro films. Figure 2a shows the J−V curves of two differently structured films illuminated by a solar simulator. Comparing the two curves, it is clear that the FTO/BFO film spin-coating directly on the FTO does not exhibit an opencircuit voltage (Voc) and a short-circuit current density (Jsc), while the FTO/TiO2/BFO/Spiro films exhibit a Voc of 0.29 V and a Jsc of 62.6 μA/cm2. From the inset, the dark J−V curve shows negligible Jsc and Voc. And a distinct photovoltaic response is observed under illumination. The J−t curves of Figure 2b were measured by the solar light simulator switch on and off. When there is no light, the Jsc of the FTO/TiO2/BFO/ Spiro films is close to zero. When exposed to light, its Jsc can be immediately increased to 62.6 μA/cm2. Matsuo et al. have prepared BFO film by pulse laser deposition; although the Voc can reach 0.6 V, the Jsc is only 1.3 μA/cm2. In addition, they also increase the Jsc of the BFO film to 15 μA/cm2 by Mn doping.37 In our work, the BFO films were prepared by spincoating based on FTO/glass. Although the voltage is small, the Jsc can reach 62.6 μA/cm2. In addition, the photovoltaic effect was not observed in the FTO/BFO films prepared by us. This

is perhaps because the surface of the FTO is rough (Figure S1), and the BFO precursor solution we prepared has a higher concentration and a higher viscosity. Therefore, the solution after spin-coating is unevenly distributed, and aggregation occurs during calcination and crystallization, resulting in more pores and poor uniformity. However, previous studies have successfully observed photovoltaic effects.31,38 In these works, the FTO/BFO films exhibited a Jsc much lower than 5 μA/cm2. Li et al. modified the La-doped BFO by Au nanoparticles, and the Jsc increased from 3.6 to 18.5 μA/cm2.31 In our work, the Jsc of FTO/TiO2/BFO/Spiro films reaches to 62.6 μA/cm2, which is higher than that of most methods for improving photovoltaic effects, such as BFO doping and precious metal modification.39−41 A brief summary of experimental photovoltaic parameters is given in Table 1. As we all know, the increase in Jsc is closely related to carrier mobility, and the migration of carriers at the material interface is closely related to the energy levels of the material. In order to study the reasons for the increase in Jsc, we have studied the energy level positions of BFO. The UV−visible absorption of BFO and TiO2/BFO/Spiro films is presented in Figure 3a; the addition of TiO2 and Spiro has little effect on the UV−visible absorption of the BFO layer. And the Tauc plot of (αhυ)2 versus hυ is plotted according to Figure 3a as shown in Figure 3b. The calculated BFO direct band gap is 2.65 eV. This is consistent with the results reported in other literature.42,43 Then we used the UPS spectrum of Figure 3c to calculate the valence band maximum (VBM) of the BFO film is 6.03 eV. Combined with the band gap obtained in Figure 3b, the 12442

DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

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Figure 5. (a) The J−V curve of a thin film with a FTO/TiO2/Spiro structure polarized by different electric fields under sunlight simulator illumination. (b) The J−t curve of the thin film of the FTO/TiO2/Spiro structure after polarization of different electric fields under sunlight simulator illumination.

Figure 6. Diagram showing the regulation mechanism of the photovoltaic response. Einf represents the internal electric field formed at the interface by the contact of the transport layer and BFO layer; Einp is the built-in electric field induced by the ferroelectric polarization and oxygen vacancies in BFO. (a) No voltage is applied across the film. (b) After applying a forward voltage to the film. (c) After applying a reverse voltage to the film. The ±V in the figure only represents the direction in which the voltage was previously applied.

polarized films shows that the Voc increased from 0.211 to 0.276 V and the Jsc increased from 36.1 to 53.2 μA/cm2. After applying a reverse voltage on both sides of the film, the Voc and Jsc are reduced. And the reverse voltage is larger, and the Voc and Jsc are smaller. Figure. 4b is the J−t curves after polarization of different applied voltage. It can be seen from this figure that the Jsc is steadily increasing or decreasing. In addition, the current density increases as a whole compared to other documents due to the addition of TiO2 and Spiro on both sides of BFO. Figure 4c is a graph of Voc versus applied voltage. The Voc in Figure 4a can be seen more clearly from this figure. When the applied voltage increases, the Voc is significantly enhanced. Studies have shown that the photovoltaic response of BFO is closely related to its polarization.20,44,45 So we tested the ferroelectric loops of FTO/ TiO2/BFO/Spiro films. It can be seen from Figure 4d that the FTO/TiO2/BFO/Spiro films show significant residual polarization and coercive field. In addition, the curves exhibit significant lossy loop behavior due to leakage currents caused by oxygen vacancies in the BFO film. For comparison, we prepared thin films without the BFO layer (FTO/TiO2/Spiro). Figure 5 is the J−V curves and the J−t curves of FTO/TiO2/Spiro films. It can be seen from Figure 5a that all curves have no Voc. Comparing with Figure 4, it can be concluded that the BFO layer plays an important role in the photovoltaic effect of the films. And the applied voltage can regulate the photovoltaic response of the BFO layer. Figure

conduction band minimum (CBM) of the BFO film is calculated to be 3.38 eV. In addition, the VBM and CBM of TiO2 were found in the literature as 7.2 and 4.0 eV, respectively.35 And the VBM and CBM of Spiro were found in the literature as 5.22 and 2.05 eV, respectively.34,36 As sketched in Figure 3d, the energy level positions of TiO2, BFO, and Spiro are well matched for the effective separation of electron−hole pairs. Therefore, the large Jsc in the multilayer film structure can be attributed to the addition of the TiO2 layer and Spiro layer. As a typical multiferroic material, BFO exhibits ferroelectricity at room temperature, so we further study the effect of an external electric field on the photovoltaic effect. Figure 4 is a series of data for the regulation of the photovoltaic effect on FTO/TiO2/BFO/Spiro films by an external electric field. Figure 4a is the J−V curves of FTO/TiO2/BFO/Spiro films after polarization in different electric fields. The black curve (DARK) is the J−V curve of these films tested in the dark, with almost no Voc and Jsc. The J−V curve (0 V) under illumination conditions showed significant Voc and Jsc, and the film had a significant photovoltaic response compared with the curve under no-light conditions. Compared with the Voc and Jsc shown in Figure 2a, the Voc and Jsc of the 0 V curve in Figure 4a are reduced. This is due to the performance degradation of the Spiro after the device was stored in the air for 1 week. A forward voltage of 1 V was then applied across the films to polarize the BFO layer. Measuring the J−V curve of the 12443

DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

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Figure 7. Photovoltaic response data of two films connected in parallel. (a) The J−V curves after polarization of different electric fields. (b) The enlarged view of J−V curves at the origin of the coordinates. (c) The J−t curves after polarization of different electric fields. (d) Forward voltage is applied across the films. (e) Reverse voltage is applied across the films.

the depolarization field will remain after the external bias is removed. This will create a built-in electric field opposite to the Einf direction, thereby Ein = Einf − Einp, and the photovoltaic effect is decreased. Furthermore, we added TiO2 and Spiro on both sides of BFO to enhance the current density. Although the regulation of the photovoltaic response is achieved, the TiO2 layer and Spiro layer cannot be repositioned, so that the direction reversal of the Voc and Jsc cannot be achieved. So we designed a parallel connection structure. The principle can be explained by the simple structure of Figure 7d,e. As shown in Figure 7d, after a forward voltage is applied across the films, the left device will generate a larger direction-up built-in electric field. The device on the right will produce a smaller direction-down built-in electric field. Therefore, current I1 and current I2 will be formed on the left and right sides of the device under illumination, respectively. And the devices as a whole will generate an upward current. As shown in Figure 7e, after a reverse voltage is applied across the films, the left device will produce a smaller direction-up built-in electric field. The right device will produce a larger direction-down built-in electric field. Therefore, the devices as a whole will generate a downward current under illumination to achieve the reversal of the photovoltaic response. Figure 7a,b shows the J−V curves of the device. After applying voltage polarization in different directions, the Voc and Jsc are reversed. The device’s J−t curves are shown in Figure 7c, which produces a stable current density under illumination.

5b shows the change in Jsc of FTO/TiO2/Spiro films with time. Obviously, applied voltage has little effect on Jsc. Figure 6 is the diagram showing the regulation mechanism of the photovoltaic response. The band structure at the interface and the ferroelectric polarization in FTO/TiO2/ BFO/Spiro play an important role in the separation of electron−hole pairs. Under illumination, the excited carriers from the BFO layer will be separated by the net electric field (Ein). The Ein can be described as Ein = Einf ± Einp

where Einf represents the internal electric field formed at the interface by the contact of the transport layer and BFO layer.44 The Einf is irreversible (for specific analysis, refer to the Support Information, Figure S2).46 Einp is the built-in electric field induced by ferroelectric polarization and oxygen vacancies (Vo) in BFO. Figure 6a is a schematic diagram of polarization when no voltage is applied. At this time, Vo is evenly distributed and there is no residual polarization inside the BFO, so Einp = 0, Ein = Einf. As shown in Figure 6b, after applying a forward voltage, a positive bias on the Spiro will orient the electric dipoles inside the BFO and make Vo migrate to the BFO/TiO2 interface, thereby forming heavy n-doping of BFO at the TiO2/BFO interface region and bending the BFO energy band downward. The BFO energy bands in the Spiro/ BFO interface area will bend upward.47−49 When the external bias is removed, the internal depolarization field of the BFO still exists and generates an additional built-in electric field (Einp), sharing the same direction with Einf, so Ein = Einf + Einp and the photovoltaic effect is enhanced. As shown in Figure 6c, after a reverse voltage is applied, a positive bias on the TiO2 will make Vo migrate to the BFO/Spiro interface, the electric dipoles inside BFO will be oriented in opposite directions, and

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CONCLUSION To sum up, the FTO/TiO2/BFO/Spiro films prepared by spin-coating have a Jsc of 62.6 μA/cm2, which is much higher than the Jsc of FTO/BFO films in previous work. This indicates 12444

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high electrocatalytic activity for hydrogen evolution. Nanoscale 2015, 7 (23), 10391−10397. (7) Yang, S. Y.; Seidel, J.; Byrnes, S. J.; Shafer, P.; Yang, C.-H.; Rossell, M. D.; Yu, P.; Chu, Y.-H.; Scott, J. F.; Ager, J. W., III; Martin, L. W.; Ramesh, R. Above-bandgap voltages from ferroelectric photovoltaic devices. Nat. Nanotechnol. 2010, 5, 143. (8) Bhatnagar, A.; Roy Chaudhuri, A.; Heon Kim, Y.; Hesse, D.; Alexe, M. Role of domain walls in the abnormal photovoltaic effect in BiFeO3. Nat. Commun. 2013, 4, 2835. (9) Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures. Science 2003, 299 (5613), 1719−1722. (10) Guo, R.; You, L.; Zhou, Y.; Shiuh Lim, Z.; Zou, X.; Chen, L.; Ramesh, R.; Wang, J. Non-volatile memory based on the ferroelectric photovoltaic effect. Nat. Commun. 2013, 4, 1990. (11) Puli, V. S.; Pradhan, D. K.; Katiyar, R. K.; Coondoo, I.; Panwar, N.; Misra, P.; Chrisey, D. B.; Scott, J. F.; Katiyar, R. S. Photovoltaic effect in transition metal modified polycrystalline BiFeO3thin films. J. Phys. D: Appl. Phys. 2014, 47 (7), 075502. (12) Wu, F.; Guo, Y.; Guo, B.; Zhang, Y.; Li, H.; Liu, H. Photovoltaic effect of a bilayer thin film with (Na0.5Bi0.5)1−xBaxTiO3/ BiFeO3 heterostructure. J. Phys. D: Appl. Phys. 2013, 46 (36), 365304. (13) Prashanthi, K.; Gaikwad, R.; Thundat, T. Surface dominant photoresponse of multiferroic BiFeO3nanowires under sub-bandgap illumination. Nanotechnology 2013, 24 (50), 505710. (14) Zhou, Y.-E; Tan, X.-Y.; Yu, B.-F.; Liu, L.; Yuan, S.-L.; Jiao, W.H. Effects of Thickness and Polarization Field on the Photovoltaic Properties of BiFeO3 Thin Films. Chin. Phys. Lett. 2014, 31 (03), 037304. (15) Nakashima, S.; Uchida, T.; Nakayama, D.; Fujisawa, H.; Kobune, M.; Shimizu, M. Bulk photovoltaic effect in a BiFeO3thin film on a SrTiO3substrate. Jpn. J. Appl. Phys. 2014, 53 (9S), 09PA16. (16) Zhou, Y. e.; Yu, B.; Zhu, X.; Tan, X.; Qian, L.; Liu, L.; Yu, J.; Yuan, S. Photovoltaic properties of Pt/BiFeO3 thin film/fluorinedoped tin oxide capacitor. J. Sol-Gel Sci. Technol. 2014, 72 (1), 74−79. (17) Hung, C.-M.; Jiang, M.-D.; Anthoninappen, J.; Tu, C.-S. Photoinduced electric phenomena in antiferromagnetic BiFeO3 ceramics. J. Appl. Phys. 2013, 113 (17), 17D905. (18) Spaldin, N. A.; Cheong, S.-W.; Ramesh, R. Multiferroics: Past, present, and future. Phys. Today 2010, 63 (10), 38−43. (19) Choi, T.; Lee, S.; Choi, Y. J.; Kiryukhin, V.; Cheong, S. W. Switchable Ferroelectric Diode and Photovoltaic Effect in BiFeO3. Science 2009, 324 (5923), 63. (20) Ji, W.; Yao, K.; Liang, Y. C. Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films. Adv. Mater. 2010, 22 (15), 1763−1766. (21) Zhang, W.; Yang, M.-M.; Liang, X.; Zheng, H.-W.; Wang, Y.; Gao, W.-X.; Yuan, G.-L.; Zhang, W.-F.; Li, X.-G.; Luo, H.-S.; Zheng, R.-K. Piezostrain-enhanced photovoltaic effects in BiFeO 3 / La0.7Sr0.3MnO3/PMN−PT heterostructures. Nano Energy 2015, 18, 315−324. (22) Mix, C.; Jakob, G. Multiferroic and structural properties of BiFeO3 close to the strain induced phase transition on different substrates. J. Appl. Phys. 2013, 113 (17), 17D907. (23) Fan, H.; Fan, Z.; Li, P.; Zhang, F.; Tian, G.; Yao, J.; Li, Z.; Song, X.; Chen, D.; Han, B.; Zeng, M.; Wu, S.; Zhang, Z.; Qin, M.; Lu, X.; Gao, J.; Lu, Z.; Zhang, Z.; Dai, J.; Gao, X.; Liu, J.-M. Large electroresistance and tunable photovoltaic properties of ferroelectric nanoscale capacitors based on ultrathin super-tetragonal BiFeO3 films. J. Mater. Chem. C 2017, 5 (13), 3323−3329. (24) Tu, C. S.; Hung, C.-M.; Xu, Z.-R.; Schmidt, V. H.; Ting, Y.; Chien, R. R.; Peng, Y.-T.; Anthoninappen, J. Calcium-doping effects on photovoltaic response and structure in multiferroic BiFeO3 ceramics. J. Appl. Phys. 2013, 114 (12), 124105. (25) Anthonyraj, C.; Muneeswaran, M.; Gokul Raj, S.; Giridharan, N. V.; Sivakumar, V.; Senguttuvan, G. Effect of samarium doping on

that the addition of an electron transport layer (TiO2) and a hole transport layer (Spiro) on both sides of the BFO film can greatly enhance the current density of the photovoltaic response. In addition, the work also shows that the Voc and Jsc of the BFO photovoltaic response can be regulated by polarization, and a possible theoretical model is drawn to illustrate the mechanism of photovoltaic regulation. Finally, a device that can achieve the reversal of the photovoltaic effect was designed. This work provides a reliable and simple method for developing photovoltaic devices based on ferroelectric thin films.

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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/acssuschemeng.9b01984.

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The 2D AFM topography images of FTO and FTO/ TiO2. Analysis of the built-in electric field Einf direction at different applied voltages (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.M.). *E-mail: [email protected] (Y.P.). *E-mail: [email protected] (X.L.). ORCID

Xing’ao Li: 0000-0002-0412-0443 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (51872145, U1732126, 51602161), the Open Project of National Lab of Solid State Microstructures, Nanjing University (M30044), the College Postgraduate Research and Innovation Project of Jiangsu Province (KYCX18_0842), the Key Project of National High Technology Research of China (2011AA050526), the Ministry of Education of China (No. IRT1148), the Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY219115), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001).

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REFERENCES

(1) Chu, S.; Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294−303. (2) Yang, Y.; Wang, Z. L. Hybrid energy cells for simultaneously harvesting multi-types of energies. Nano Energy 2015, 14, 245−256. (3) Wang, X.; Wang, Z. L.; Yang, Y. Hybridized nanogenerator for simultaneously scavenging mechanical and thermal energies by electromagnetic-triboelectric-thermoelectric effects. Nano Energy 2016, 26, 164−171. (4) Wu, Y.; Zhong, X.; Wang, X.; Yang, Y.; Wang, Z. L. Hybrid energy cell for simultaneously harvesting wind, solar, and chemical energies. Nano Res. 2014, 7 (11), 1631−1639. (5) Wang, S.; Wang, X.; Wang, Z. L.; Yang, Y. Efficient Scavenging of Solar and Wind Energies in a Smart City. ACS Nano 2016, 10 (6), 5696−5700. (6) Zhang, J.; Wang, Q.; Wang, L.; Li, X. a.; Huang, W. Layercontrollable WS2-reduced graphene oxide hybrid nanosheets with 12445

DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446

Research Article

ACS Sustainable Chemistry & Engineering

Derived from Epitaxial Tetragonal BiFeO3 Films. ACS Appl. Mater. Interfaces 2017, 9 (32), 27284−27289. (45) Li, D.; Zheng, D.; Jin, C.; Zheng, W.; Bai, H. High-Performance Photovoltaic Readable Ferroelectric Nonvolatile Memory Based on La-Doped BiFeO3 Films. ACS Appl. Mater. Interfaces 2018, 10 (23), 19836−19843. (46) Fan, Z.; Yao, K.; Wang, J. Photovoltaic effect in an indium-tinoxide/ZnO/BiFeO3/Pt heterostructure. Appl. Phys. Lett. 2014, 105 (16), 162903. (47) Guo, Y.; Guo, B.; Dong, W.; Li, H.; Liu, H. Evidence for oxygen vacancy or ferroelectric polarization induced switchable diode and photovoltaic effects in BiFeO3 based thin films. Nanotechnology 2013, 24 (27), 275201. (48) Fang, L.; You, L.; Zhou, Y.; Ren, P.; Shiuh Lim, Z.; Wang, J. Switchable photovoltaic response from polarization modulated interfaces in BiFeO3 thin films. Appl. Phys. Lett. 2014, 104 (14), 142903. (49) Zhou, Y.; Fang, L.; You, L.; Ren, P.; Wang, L.; Wang, J. Photovoltaic property of domain engineered epitaxial BiFeO3 films. Appl. Phys. Lett. 2014, 105 (25), 252903.

the structural, optical and magnetic properties of sol−gel processed BiFeO3 thin films. J. Mater. Sci.: Mater. Electron. 2015, 26 (1), 49−58. (26) Gao, H.; Tian, J.; Zheng, H.; Tan, F.; Zhang, W. Magnetic and optical properties of La-doped BiFeO3 films prepared by sol−gel route. J. Mater. Sci.: Mater. Electron. 2015, 26 (2), 700−704. (27) Blessington Selvadurai, A. P.; Pazhanivelu, V.; Murugaraj, R. Structural, Magnetic, Optical and Electrical Properties of Ba Substituted BiFeO3. J. Supercond. Novel Magn. 2014, 27 (3), 839− 844. (28) Kumar, M.; Yadav, K. L. Rapid liquid phase sintered Mn doped BiFeO3 ceramics with enhanced polarization and weak magnetization. Appl. Phys. Lett. 2007, 91 (24), 242901. (29) Lee, C.-C.; Wu, J.-M. Studies on Leakage Mechanisms and Electrical Properties of Doped BiFeO3 Films. Electrochem. Solid-State Lett. 2007, 10 (8), G58−G61. (30) Ren, Y.; Nan, F.; You, L.; Zhou, Y.; Wang, Y.; Wang, J.; Su, X.; Shen, M.; Fang, L. Enhanced Photoelectrochemical Performance in Reduced Graphene Oxide/BiFeO3 Heterostructures. Small 2017, 13 (16), 1603457. (31) Li, F. Z.; Zheng, H. W.; Zhu, M. S.; Zhang, X. A.; Yuan, G. L.; Xie, Z. S.; Li, X. H.; Yue, G. T.; Zhang, W. F. Photovoltaic enhancement by Au surface-plasmon effect for La doped BiFeO3 films. J. Mater. Chem. C 2017, 5 (40), 10615−10623. (32) Ye, N.; Qi, J.; Qi, Z.; Zhang, X.; Yang, Y.; Liu, J.; Zhang, Y. Improvement of the performance of dye-sensitized solar cells using Sn-doped ZnO nanoparticles. J. Power Sources 2010, 195 (17), 5806− 5809. (33) Zhang, K.; Wang, Z. L.; Yang, Y. Enhanced P3HT/ZnO Nanowire Array Solar Cells by Pyro-phototronic Effect. ACS Nano 2016, 10 (11), 10331−10338. (34) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (35) Wang, Y.; Yue, Y.; Yang, X.; Han, L. Toward Long-Term Stable and Highly Efficient Perovskite Solar Cells via Effective Charge Transporting Materials. Adv. Energy Mater. 2018, 8 (22), 1800249. (36) Yang, Z.; Dou, J.; Wang, M. Interface Engineering in n-i-p Metal Halide Perovskite Solar Cells. Solar RRL 2018, 2 (12), 1800177. (37) Matsuo, H.; Noguchi, Y.; Miyayama, M. Gap-state engineering of visible-light-active ferroelectrics for photovoltaic applications. Nat. Commun. 2017, 8 (1), 207. (38) Li, Y.; Liu, X.; Sun, Y.; Sheng, S.; Liu, H.; Yang, P.; Yang, S. Enhanced photovoltaic effect in K substituted BiFeO3 films. J. Alloys Compd. 2015, 644, 602−606. (39) Yan, T.-L.; Chen, B.; Liu, G.; Niu, R.-P.; Shang, J.; Gao, S.; Xue, W.-H.; Jin, J.; Yang, J.-R.; Li, R.-W. Improved photovoltaic effects in Mn-doped BiFeO3ferroelectric thin films through band gap engineering. Chin. Phys. B 2017, 26 (6), 067702. (40) Liu, Q.; Zhou, Y.; You, L.; Wang, J.; Shen, M.; Fang, L. Enhanced ferroelectric photoelectrochemical properties of polycrystalline BiFeO3 film by decorating with Ag nanoparticles. Appl. Phys. Lett. 2016, 108 (2), 022902. (41) Zhu, M.; Zheng, H.; Zhang, J.; Yuan, G.; Wang, K.; Yue, G.; Li, F.; Chen, Y.; Wu, M.; Zhang, W. Polarization dependent ferroelectric photovoltaic effects in BFTO/CuO thin films. Appl. Phys. Lett. 2017, 111 (3), 032901. (42) Sharma, S.; Tomar, M.; Kumar, A.; Puri, N. K.; Gupta, V. Enhanced ferroelectric photovoltaic response of BiFeO3/BaTiO3 multilayered structure. J. Appl. Phys. 2015, 118 (7), 074103. (43) Mai, H.; Lu, T.; Li, Q.; Liu, Z.; Li, Y.; Kremer, F.; Li, L.; Withers, R. L.; Wen, H.; Liu, Y. Above-Band Gap Photoinduced Stabilization of Engineered Ferroelectric Domains. ACS Appl. Mater. Interfaces 2018, 10 (15), 12781−12789. (44) Lu, Z.; Li, P.; Wan, J.-g.; Huang, Z.; Tian, G.; Pan, D.; Fan, Z.; Gao, X.; Liu, J.-m. Controllable Photovoltaic Effect of Microarray 12446

DOI: 10.1021/acssuschemeng.9b01984 ACS Sustainable Chem. Eng. 2019, 7, 12439−12446