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Functional Inorganic Materials and Devices

Photovoltaic effect of ferroelectric-luminescent heterostructure under infrared light illumination Haoxin Mai, Teng Lu, Qian Li, Qingbo Sun, Khu Vu, Hua Chen, Genmiao Wang, Mark G. Humphrey, Felipe Kremer, Li Li, Ray L. Withers, and Yun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09745 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Materials & Interfaces

Photovoltaic effect of ferroelectric-luminescent heterostructure under infrared light illumination Haoxin Mai1, Teng Lu1, Qian Li2,*, Qingbo Sun1, Khu Vu3, Hua Chen4, Genmiao Wang1, Mark G. Humphrey1, Felipe Kremer4, Li Li5, Ray L. Withers1 and Yun Liu1,* 1

Research School of Chemistry, The Australian National University, ACT 2601, Australia Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 3 Research School of Physics & Engineering, The Australian National University, Canberra, ACT 2601, Australia 4 Centre for Advanced Microscopy, The Australian National University, 131 Garran Road, Canberra, 2601, Australia. 5 Australian National Fabrication Facility, Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia 2

KEYWORDS: photovoltaic, ferroelectrics, upconversion, BiFeO3, non-radiative energy transfer, depolarization field ABSTRACT: In this report, a ferroelectric-luminescent heterostructure is designed to convert infrared light into electric power. We use BiFeO3 (BFO) as the ferroelectric layer, and Y2O3:Yb,Tm (YOT) as the upconversion layer. Different from conventional ferroelectric materials, this heterostructure exhibits switchable and stable photovoltaic effects under 980 nm illumination, whose energy is much lower than the bandgap of BFO. The energy transfer mechanism in this heterostructure is therefore studied carefully. It is found that a highly efficient non-radiative energy transfer process from YOT to BFO plays a critical role in achieving the below-bandgap-photon excited photovoltaic effects in this heterostructure. Our results also indicate that by introducing asymmetric electrodes, both the photovoltage and photocurrent are further enhanced when the built-in field and the depolarization field are aligned. The construction of ferroelectricluminescent heterostructure is consequently proposed as a promising route to enhance the photovoltaic effects of ferroelectric materials by extending the absorption of the solar spectrum.

1. INTRODUCTION Ferroelectric (FE) materials, due to their efficient carrier separation through the strong internal field induced by polarization, are capable of achieving a photovoltage much higher than their bandgap, known as the bulk photovoltaic effect1-2. By virtue of their bulk photovoltaic effects, FE materials are considered to be promising materials that can exceed the Shockley–Queisser Limit1-10. Although many efforts have been devoted to promoting the efficiency of photovoltaic (PV) devices based on FE materials, such as domain engineering and defect modification1-2, 4-5, poor sunlight absorption capabilities resulting from their high bandgap energy have restricted the PV application of these devices1, 9. One strategy for increasing solar absorption is to reduce the bandgap of FE materials by modifying their composition without impairing the ferroelectricity, such as doping in perovskite oxides or the use of solid solutions8-14. In particular, one recent study shows that power conversion efficiencies can reach 8% in multi-layer B-site substituted FE thin films, for which the tunable bandgap ranges from 2.6 eV to 1.4 eV9. Despite the utilization of UV light and part of the visible light in the solar spectrum, most FE materials are transparent to near infrared light, which accounts for almost 50% of the solar spectrum. To improve infrared light harvesting, upconversion phosphors are introduced into PV devices15-18. Upconversion is an intriguing process, converting multiple lower energy photons into one higher energy photon15. Based on this unique property, upconverters have over recent years been integrated into many types of materials and widely applied in the areas of bioprobes, imaging, photocatalysis as well as in silicon-based and trihalide perovskite solar cells15-29. In particular, owing to the resultant ability to absorb below bandgap solar photons and transform them into

suitable wavelengths without varying the structure of PV materials, upconversion materials have been suggested as spectral converters that can significantly reduce the non-absorption and thermalization losses in PV devices16-18, 27-32. It has been reported that the power conversion efficiency of a Si solar cell is increased about 130% when it is integrated with Ag and NaYF4:Yb,Er nanoparticles30, while the power conversion efficiency of perovskite solar cells is also found to be enhanced with the assistance of an upconversion layer27-29. Most such PV devices, however, still rely on built-in asymmetry to generate PV effects, and accordingly the output voltages are constrained by the Shockley–Queisser Limit, regardless of how much solar spectrum is absorbed. In order to expand solar absorption and retain the bulk photovoltaic effect, an appropriate integration of upconverters with polar materials that can provide strong and stable depolarization fields is of critical importance. Therefore, in this report, we construct a ferroelectric-luminescent heterostructure to demonstrate PV effects under infrared light. We choose bismuth ferrite (BiFeO3, BFO) as the FE layer due to its relatively narrow bandgap, large remnant polarization and high Curie temperature33-34. In particular, the bandgap of BFO ranges from 2.2 – 2.7 eV (within the range of visible light), theoretically permitting the separation of electron-hole pairs in BFO generated by using the energy from the energy transfer upconversion (ETU) process1, 7, 26, 35. The ETU process is the most efficient process for most upconverters15-18. Recently, a significant enhancement of the photocatalytic activity of a NaGdF4:Yb,Er@BFO core@shell structure was observed with visible and near infrared irradiation, indicating that energy transfer from the upconverter to BFO is an experimental possibility when BFO particles closely contact with the upconverters26. Y2O3:Yb,Tm (YOT) is selected as the luminescent layer. Owing to the formation of a compact

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heterostructure, PV effects would be induced by 980 nm photons by virtue of the highly efficient energy transfer process between the upconversion layer and the FE layer. We believe that the construction of the ferroelectric-upconversion heterostructure can increase the utilization of the solar spectrum for FE materials without attenuating the depolarization field, and represents a significant contribution to the development of highly efficient PV devices. 2. EXPERIMENTAL SECTION 2.1 BFO-YOT heterostructure fabrication. Pure and doped Y2O3 (with 30% Yb and 1% Tm) pellets were synthesized by conventional solid state methods as targets for thin film deposition36. The BFO-YOT heterostructure was formed on a Si substrate using the pulsed laser deposition (PLD) technique. An excimer laser (KrF, λ = 248 nm) was used to ablate the YOT and BFO targets. The laser repetition rate was set at 5 Hz, and the target-to-substrate distance was kept at 5 cm. The 190 nm Y2O3:Yb,Tm layer was first deposited on a (100) silicon wafer, using a 1.5 J/cm2 laser power density and a 60 min deposition time. The oxygen pressure used was 1 × 10-5 torr, and the substrate temperature was 750 °C. A 70 nm BFO thin film was then deposited on the YOT layer at 750 °C and 50 mTorr of oxygen. The power density of the laser was 2.5 J/cm2, and the deposition time was 60 min. The as-deposited thin films were annealed at 750 °C for 30 minutes under 400 Torr O2. As a comparison, a 190 nm pure Y2O3 layer and a 85 nm BFO layer were subsequently deposited on another (100) silicon substrate under the same conditions. ITO electrodes (~50 nm) were deposited at 500 °C and 10 mTorr of oxygen by PLD. The deposition time was 30 min. Gold electrodes (~300 nm) were deposited by DC sputtering at room temperature. The deposition time was 3 min.

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diffractometer using Cu-Kα radiation. UV-visible absorption spectra were measured on a Varian Cary 5E UV-Vis-NIR Spectrophotometer using the diffuse reflection method. AFM investigations were performed under ambient conditions on a commercial AFM system (Cypher, Asylum Research), which has integrated signal generator and lock-in amplifier (LiA) modules. Olympus AC240TM conductive silicon probes (Tip coating: 5 nm/20 nm Ti/Pt; force constant: calibrated 1.8 N/m) were used. The imaging contact set-points used were around 100 nN. The thickness of the thin film was determined by scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD) images were obtained using a JEOL-2100F TEM. Ferroelectric hysteresis loops were recorded using a ferroelectric analyzer (TF2000). The UC emission spectra were measured by a spectrophotometer (HR4000, Ocean Optics) with an optical fiber, and a 980 nm laser (JDSU 29-7503) was used as the upconversion excitation source. Open-circuit voltage and short-circuit current were measured under the illumination of the 980 nm laser as mentioned above. The fluorescence lifetimes for UCL and BFO-UCL samples were measured using a 5 ns pulsed tunable optical parametric oscillator (Opolette 355 II from Opotek) centered at 980 nm, with detection provided by a R7459 Hamamatsu photomultiplier and display and averaging via a Tektronix DPO3014 digital oscilloscope. 3. RESULTS AND DISCUSSION 3.1 Configuration of the BFO-YOT heterostructure. In this heterostructure, the YOT layer serves as the upconversion layer, transforming the 980 nm photons into a wavelength that can be utilized by the BFO layer (Scheme 1)15. According to the XRD analysis, the YOT layer is phase pure and highly crystalline. It shows strong out-of-plane preferential orientation along the [111] direction (Figure S1a), consistent with literature results36. The thickness of the YOT layer is about 190 nm (Figures 1b and S2),

2.2 Characterization. The structures of the thin-film samples were investigated with a PANalytical X’pert Empyrean

Scheme 1. Layout of the BFO-YOT heterostructure and a schematic illustration of the upconversion luminescent process and the energy transfer mechanism. The region between the two electrodes is polarized. CB and VB denote the conduction band and valence band, while the blue e and hollow circle denote the photo-generated electrons and holes, respectively.


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and the surface of the YOT layer is smooth and compact, consisting of small particles (root mean square roughness: ~0.25 nm, Figure S1b). Under the excitation of a 980 nm laser, two intense blue emission bands centered at 465 nm and 475 nm are seen, ascribed to 1D2→3F4 and 1G4→3H6 transitions, respectively, while the weak red emission band at ca. 650 nm is assigned to 1G4 →3F4 and 3H4→3H6 transitions (Scheme 1 and Figure S1c)17. The corresponding energies of these blue emissions are 2.67 eV and 2.61 eV, respectively. The BFO layer (the FE layer) was then deposited on the YOT layer by PLD (Scheme 1). We find that the YOT layer not only serves as an upconversion layer, but also acts as a buffer layer improving the crystallinity and compactness of the BFO layer. The as-prepared BFO thin film is found to be highly crystalline with a preferential orientation of [110]C. No minor phases or evidence for other orientations are identified (Figure 1a). These results are in agreement with our previous studies37. The crosssection TEM and SEM images show that the interface between the BFO layer and the YOT layer is sharp and well-defined, and that the thickness of the BFO layer is ~85 nm (Figures 1b and S3). The contact between BFO and YOT is compact. The particle size at the surface of the BFO layer is ~50 nm, and its roughness is ~1.7 nm (Figure S3, inset). The cross-sectional HRTEM image also reveals the high crystalline quality of the BFO-YOT heterostructure (Figure S4). The [110]C preferential orientation of the BFO layer along the out-of-plane direction on the [111] YOT

layers is confirmed by the d110 of the BFO layer (0.279 nm) and the SAD pattern (Figure S4). The bandgap of this thin film calculated from the UV-vis absorption spectrum is ~2.6 eV (Figure 1c), in agreement with previous results35, 37. It is worth noting that there is a large spectral overlap between the blue emission of the YOT layer and the absorption of BFO, implying that BFO can absorb part of the blue emission of the YOT layer (Figure 1c, inset). This is confirmed by the photoluminescence spectra shown in Figure S2. After depositing the BFO layer, the intensity of the blue emission from the YOT layer is reduced by 75%, while the red emission, which is not covered by the absorption region of BFO, shows almost no reduction compared with the pure YOT. To investigate the surface FE and PV properties of this heterostructure, 1 mm × 1 mm ITO transparent electrodes were deposited on the BFO layer, as shown in Scheme 1. The space between the two electrodes is ~10 µm. Here, we define positive polarization as the sample is polarized when a positive voltage is applied to the left electrode. The Charge-Voltage hysteresis loop and the subsequent I-V curve displayed in Figure 2d indicates that the sample can be polarized laterally, and that the intensity of the coercive voltage is ~±6.3 V. As a depolarization field is established between the two electrodes after the lateral polarization, it is proposed that once excitons are generated by the above bandgap luminescence, the electron-hole pairs can be separated and PV effects can be observed (Scheme 1).

Figure 1. Characterization of as-prepared BFO-YOT heterostructure. (a) XRD pattern of the BFO-YOT heterostructure. (b) Crosssection TEM image of the BFO-YOT heterostructure. (c) Calculated band gap of the BFO-YOT heterostructure with the tangent of the linear part shown as a dotted line. The inset shows the overlap of the BFO-YOT heterostructure UV–vis absorption spectrum with the emission spectrum of the YOT layer. Both the BFO-YOT heterostructure absorption and the YOT emission are normalized. (d) Lateral Charge–Voltage hysteresis loop and the corresponding I-V curve of the BFO-YOT heterostructure.



3.2 PV effects under the excitation of a 980 nm laser. To measure the PV effects of the BFO-YOT heterostructure, the asprepared sample is poled negatively with a -10 V bias and then exposed to radiation from the 980 nm laser. The thickness of the BFO layer is optimized. We suggest that an 85 nm BFO layer will have the most intense PV effects in this system for the present experiment conditions, and thus the thickness of the BFO layer will be kept as 85 nm in this report. As a default setting, the laser power we used in the PV detection is maintained at 200 mW unless otherwise specified. Figure 2a depicts the corresponding IV curves employing the 980 nm laser as well as in the dark. A typical negative diode behavior is observed both in the dark and under illumination38, while the conductance increased significantly along the negative direction when the 980 nm laser was switched on. More importantly, after switching on the 980 nm laser, this heterostructure exhibited an approx. +25 nA short circuit current (Isc) and an ca. -0.4 V open circuit voltage (Voc), while the Isc direction is opposite to the forward bias direction, consistent with the typical PV phenomena reported in the literature3, 7. When the 980 nm laser is switched off, both Isc and Voc reduce to 0. As a result, it is clear that the current and voltage we detect are induced by the 980 nm laser. In addition, we find that the observed PV effects are switchable with variation of the polarization direction. After switching the polarization direction (positive poling), Isc displayed the opposite direction whereas the intensity remains at ca. 25 nA. Again, the direction of the corresponding Voc is also switched (ca. +0.4 V) (Figure 2b). No PV effects will be observed when the sample is unpoled (Figure S5a). These results show that the detected PV effects are related

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to the depolarization field on the BFO surface, which is switchable along with the polarization. More experiments were carried out to detect the PV properties of the BFO-YOT heterostructure under 980 nm illumination. To examine the repeatability of switching, the switching of the photocurrent direction was performed 10 times. The variation in the absolute value of Isc was within 4 nA, while the photocurrent direction and the polarization direction were always opposed (Figure S5b). The time dependence of Isc with regards to turning the 980 nm laser on or off after negative or positive poling are shown in Figure 2c and Figure S5c, respectively. Isc remained at ca. +25 nA (negative poling) or -25 nA (positive poling) when the laser was on, and dropped to 0 as soon as the laser was switched off, again confirming that the photocurrent direction relies on the polarization direction. No degradation of Isc was observed when the sample was exposed to 980 nm laser radiation for more than 10 min (Figure 2d). These results indicate that the photocurrent of the BFO layer induced by the 980 nm laser is stable. According to the results above, the energy of the YOT blue emissions is 2.61 eV – 2.67 eV, higher than the bandgap of BFO (Figure 1c), and these emissions can be strongly absorbed by the ferroelectric BFO layer (Figure 1c, inset). We therefore suggest that these near infrared-induced PV effects may stem from the utilization of the above bandgap energy at the 1D2 and 1G4 states of the Tm3+ ions in the YOT layer by the BFO layer, and thus the energy transfer mechanisms between the YOT layer and BFO layer were studied carefully.

Figure 2. PV properties of the BFO-YOT heterostructure. Current−voltage (I−V) characteristics for the BFO-YOT heterostructure in the dark and under a 200 mW, 980 nm laser, with the polarization towards (a) left and (b) right. Insets are the I−V curves plotted on log scales. Time dependence of zero bias photocurrent for the BFO-YOT heterostructure after negative poling with (c) the 980 nm laser switching ON and OFF and (d) in a 10 min period following the laser being switched off. The poling voltage is 10 V, and the poling time is 10 min.


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Figure 3. Luminescence decay curves of upconversion emissions for the YOT (blue) and the BFO-YOT heterostructure (red). The emission intensities of the YOT and BFO-YOT heterostructures are both normalized. The solid lines represent the fitting curves to an exponential decay function, / , in which t is the estimated lifetime. 3.3 Mechanisms of the energy transfer in BFO-YOT heterostructure. As the bandgap of the BFO layer (2.6 eV) is much higher than the 980 nm photon energy (1.27 eV), electronhole pairs cannot be generated directly following absorbance of the 980 nm photons. Therefore, thermal effects must be taken into account before discussing the energy transfer between the BFO layer and the YOT layer. In principle, temperature gradients across different semiconductors result from non-uniform heating under illumination and possibly give rise to a thermal current39-40. Nevertheless, the currents stemming from the photothermoelectric effects are proportional to the temperature differences, and should not be reversed upon varying the polarization direction. In our case, the photocurrent shows well repeatable switching over 10 poling cycles (Figure S5b), and thus the photo-thermoelectric effect can be excluded. Pyroelectric current, generated from the change of polarization because of the fluctuation of temperature induced by illumination, may also affect the observation of the photocurrent in our system41. As the pyroelectric current is also derived from the variation of temperature, it is not expected to be stable. Our results, however, show that the photocurrent is maintained at ca. +25 nA for 10 min under the illumination of the 980 nm laser (Figure 2d), while the photocurrent increases or vanishes abruptly when the laser is switched on or off, respectively (Figure 2c). A pyroelectric current is therefore not the major contribution to the current observed in this system since otherwise it would produce current spikes at the switching events. More importantly, no photocurrent and photovoltage was detected once the YOT layer was replaced with a pure Y2O3 layer (Figure S6), in which the ETU process cannot occur because of the absence of Yb3+ and Tm3+ ions. As a result, we suggest that the PV effects induced by the 980 nm laser are based on energy transfer from the YOT layer to the BFO layer. The mechanisms of energy transfer in the YOT-BFO heterostructure thin film are depicted in Scheme 1. The entire process can be divided into 3 steps: upconversion in the YOT layer, energy transfer from the YOT to BFO, and PV effects generated at the BFO layer.

1. Upconversion in the YOT layer. When the YOT layer is exposed to the 980 nm laser, the expected ETU process takes place, in which the Yb3+ ions in the ground-state (2F7/2) absorb a 980 nm photon and transit to the excited-state 2F5/2, after which the harvested energy from the excited Yb3+ ions is transferred to the nearby Tm3+ ions15-17. According to the power dependent upconversion emission intensity plot (Figure S7), the ETU in our YOT layer is a three-photon process, in which the Tm3+ ions in the ground-state (3H6) and the first excited-state (3F4) are pumped to the emitting states 1G4 and 1D2, respectively. Without the combination with the BFO layer, these excited Tm3+ ions will drop back to the 3H6 and 3F4 states by radiative decay, producing the blue emissions at 465 nm and 475 nm (Figure S2, blue line), whereas in our BFO-YOT heterostructure, about 75% of the radiation energy is transferred into the BFO layer (Figure S2, red line). It is worth noting that the saturation energy of this YOT layer is around 300 mW, beyond which the upconversion emission intensity becomes insensitive to the laser power (Figure S7b). 2. Energy transfer from the YOT to BFO. Since the blue emissions of the YOT and the BFO absorption partially overlap, energy transfer from the YOT layer to the BFO layer may occur. Generally, there are two approaches for energy transfer between donor and acceptor, namely radiative and non-radiative processes42-43. Radiative energy transfer (RET) corresponds to emission from the donor and reabsorption by the acceptor. As energy loss in the emission and absorption steps is inevitable, the efficiency of RET is fairly low (~10%)44-45. On the other hand, the non-radiative energy transfer (NRET) process corresponds to a nonradiative dipole–dipole interaction between donor and acceptor, and thus the transfer efficiency is significantly enhanced (typically >50%) by greatly suppressing the energy loss23-24. Since the BFO layer closely contacts with the YOT layer, and both the YOT layer and the BFO layer exhibit highly preferred out-ofplane orientations (Figures 1a and S4), which are in line with the NRET direction, we suggest that an NRET process may well take place in this system. Figure 3 demonstrates the blue emission decay curves of the YOT and the BFO-YOT. The lifetime of the



YOT is estimated as 220 µs from exponential decay function fitting (Figure 3, blue line), whereas the lifetime drops to 64 µs when the YOT layer is integrated with the BFO layer (Figure 3, red line). These results unambiguously show that the energy transfer in the BFO-YOT heterostructure largely takes place via an NRET process, with the NRET efficiency ca. 71% (calculated from the formula E=1- τ’/τ where E is the NRET efficiency, and τ’ and τ denote the lifetime of the donor with and without the acceptor, respectively)43. Considering that the intensity of the blue emission decreases about 75% in the BFO-YOT thin film by compared with that of the pure YOT layer, we conclude that a highly efficient NRET process dominates the energy transfer from the 1D2 and 1G4 states of the Tm3+ ions in the YOT layer to the BFO layer. Note that NRET plays a critical role in the infrared light induced PV effects, and PV effects will be significantly impaired when NRET is obstructed. To illustrate the effects of NRET, a 50 nm Y2O3 layer is inserted between the BFO layer and the YOT layer. As NRET strongly depends on the distance between donor and acceptor and will be tremendously attenuated when the distance between the two materials is larger than 10 nm26, 42-43, 46-47, we assume that NRET is prevented in this structure. When we expose the BFO-Y2O3-YOT structure to the 980 nm laser, it is found that although the middle Y2O3 layer shows little influence on the upconversion emission (Figure S8a), the photocurrent drops to almost 0 (Figure S8b), indicating that most of the energy at the excited Tm3+ ions in the YOT layer is unable to obtain access to the BFO layer without NRET. 3. PV effects generated at the BFO layer. Because the energies of the blue emissions (2.61 eV - 2.67 eV) are higher than the bandgap of BFO (~2.6 eV), we suggest that electrons in the BFO layer will be excited to the conduction band by the energy transferred from the emitting states of the Tm3+ ions through NRET, while holes will be left in the valence band. Due to the lateral depolarization field generated after poling, the energy band of the polarized regions is bent down along with the polarization direction37. The excited electrons are therefore driven by the depolarization field and move in the same direction as the polarization, while the movement of the holes left in the valence band is in the opposite direction, and thus photocurrent is produced. In order to gain further insight into the generation of photocurrent, the laser power dependence of the Isc was examined (Figure 4a). The Isc increases with increasing laser power from 100 mW to 300 mW, and remains almost constant when the laser power is above the upconversion saturation power (~ 300 mW).

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Since the laser power dependence of Isc correlates with the variation in upconversion blue emission intensity with laser power (Figure S7), we propose that the Isc of BFO and the intensity of the 465 nm and 475 nm emission of the YOT layer are connected. A direct correlation is obtained on plotting the intensity of Isc against the peak areas of the 465 nm and 475 nm emissions (Figure 4b). The plot shows that there is indeed a strong linear relationship between the photocurrent and the intensity of the upconversion blue emissions, indicating that the photocurrent in the BFO layer surface is generated by the energy of the abovebandgap photons originating from the emitting states of the Tm3+ ions, rather than by the 980 nm photons directly. From this mechanism, several intriguing results can be inferred and explained. First, as electrons move along the polarization direction, the photocurrent direction will always be opposite to the polarization direction, and thus the photocurrent can be switched simply by changing the polarization direction (Figure 2). Second, the polarized region in the BFO layer is effective in the generation of the photocurrent and photovoltage. As the polarization of BFO presents random orientation without poling, no macroscopic photocurrent is expected owing to a lack of driving force that can separate the excited electrons and hole in long-range scale. This inference is confirmed by the results shown in Figure S6a, in which the I-V curves both in the dark and under 980 nm illumination with various output powers are linear and pass through the coordinate origin, indicating that no PV effects can be detected without poling even when the 980 nm laser power is up to 300 mW (saturation power). In addition, the PV effects will severely decline with increasing the BFO layer thickness (Figure S5b), as enlarging the distance between the BFO/YOT interface and the polarized region will lead to more losses during the energy transfer process. Third, as the photocurrent is generated by the upconversion above bandgap photons rather than the 980 nm illumination, the intensity of the photocurrent is proportional to the areas underneath the upconversion blue emission peaks. As a result, there will be a plateau of the photocurrent generated from the ETU-NRET process because of the existence of upconversion saturation power (Figure 4). To acquire more intense upconversion photocurrent, it is necessary to promote the efficiency of the ETU process as well as that of the NRET process. Host materials with large bandgaps and low phonon energies for the upconversion layer are therefore required (e.g. NaYF4)15. A better connection between the FE layer and the upconversion layer will also contribute to the observed NRET efficiency26. Further studies improving the energy transfer efficiency will be carried out in the future.

Figure 4. (a) Power dependent I−V curves for the BFO-YOT heterostructure under 980 nm laser radiation. The polarization direction is towards the left. (b) A linear fit to the intensity of the short circuit current and area under the 465 nm and 475 nm upconversion emission peaks, as shown in Figure S7a (r = 0.99837).


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Table 1. Open circuit voltage and short circuit current with different 980 nm laser power Laser Power (mW)

Open Circuit Voltage (Voc, V)

Short Circuit Current (Isc, nA)

100

0.39

13

150

0.41

20

200

0.38

28

250

0.42

38

300

0.38

47

350

0.40

49

The power dependent photovoltage has also been studied in this report. Interestingly, we find that increasing the 980 nm laser power has little effect on the photovoltage. Despite the increase of photocurrent (from ~13 nA to ~ 47 nA) with enhancing the 980 nm laser power until the saturation power, Voc almost remains constant in this process (Figure 4a and Table 1). These results imply that the photovoltage is not strongly affected by the amount of photoelectrons. In this regard, we suggest that in this system, the photovoltage should mainly depend on the internal field made up of that from the depolarization field and that from the external asymmetry, rather than on the amount of photoelectrons generated. 3.4 The effects of asymmetric electrodes. To verify the effect of the internal field on the photovoltage, a built-in field was constructed by introducing asymmetric electrodes. Here, we replaced the left ITO electrode with Au, and measured the I-V curves of the Au/BFO/ITO (from left to right) under 980 nm laser radiation. Figure 5 shows the PV results of the Au/BFO/ITO. The Voc and Isc obtained are ~+1.0 V and ~-27 nA when the sample was poled positively, ~+0.3 V and ~-16 nA when the polarization direction was negative, and ~+0.6 V and -21 nA for the sample without polarization, respectively. These results indicate a significant effect of the asymmetric electrodes on the PV effects of the BFO-YOT heterostructure. To explain these experimental results, energy band diagrams for Au/BFO/ITO in different situations should be examined (Figures 5b – 5d). The work functions of Au and ITO we use here are about 5.1 eV and 4.5 eV, respectively48-49, the electron affinity of BFO used is about 3.3 eV50, the bandgap of BFO in this report is 2.6 eV, and we suppose that the BFO Fermi level is around the middle of the bandgap. The work function of the BFO thin film is therefore about 4.6 eV. Due to the difference in work functions of Au/BFO and BFO/ITO, built-in electric fields (Ebi) would be generated at the Au/BFO and BFO/ITO interfaces, with the value of Ebi= -0.6 eV, because the Ebi of both Au/BFO and BFO/ITO are along the same direction (Figure 5c). This is consistent with the Voc of the sample observed without poling (Figure 5a, black line). When the sample is poled positively (Figure 5a, red line), the energy band declines in the polarization direction, and a depolarization field (Ep) in the same direction as the built-in field is introduced (Figure 5b). As the internal field (Ei) equals Ebi + Ep, and the Ep is estimated to be 0.4 eV according to the Voc for the ITO/BFO/ITO structure (Figure 2), Ei would be 1.0 eV ideally, quite close to the value shown in Figure 5 (~1.0 V), and the Isc would also increase as the Schottky barrier is reduced (Figure 5b). When the polarization direction is

switched, Ei will be attenuated as Ebi and Ep oppose each other (Figure 5d). The value of Ei would be 0.4 eV ideally, very close to the Voc we obtained (~0.3 eV), and the Isc also drops over 10 nA because of the increase of the Schottky barrier under the negative polarization (Figure 5a, blue line)51. 4. CONCLUSION A novel ferroelectric-luminescent heterostructure consisting of an upconversion layer, Y2O3:Yb,Tm, and a ferroelectric BFO layer, is fabricated in this study. The BFO layer shows significant preferential orientation along the [110] direction. Lateral PV effects at the surface of this BFO-YOT heterostructure were observed when it was exposed to 980 nm illumination (Voc: ~0.4 V, Isc: ~25 nA). Both Isc and Voc are switchable by changing the poling direction, and exhibit good stability and retention properties. As the photon energy of the 980 nm laser (~1.3 eV) is not able to induce any PV effects in BFO itself (bandgap: ~2.6 eV), we suggest that the mechanism of these PV effects consists of three steps: (i) an ETU process in the YOT layer converting the 980 nm photons into above-bandgap photons, (ii) a highly efficient NRET transferring the above-bandgap energy from the YOT layer to the BFO layer, and (iii) excitation of electron-hole pairs by the above-bandgap energy and separation by the lateral depolarization field, resulting in a stable and switchable photocurrent being generated. We also confirmed that the compact contact between YOT and BFO in this heterostructure ensures the effective energy transfer from the upconversion layer to the FE layer, and thus plays a vital role in the PV effects induced by the infrared light. The effects of asymmetric electrodes were also studied in this report. It was found that both Voc and Isc can be increased when the built-in field and the depolarized field are in the same direction. As a result, we believe that this research will not only enrich but also lead to further investigations of energy transfer between different materials. Moreover, it is worth noting that BFO was recently combined with TiO2 and used as a dye in PV devices, showing high stability and good power conversion efficiency52. Nevertheless, due to the limitation of the bandgap of BFO, only a small part of the solar spectrum can be utilized by this novel PV device. Therefore, this research also provides a new route for enhancing the PV effects of FE materials and FE PV devices by improving the absorption of the solar spectrum without modifying the structure of the materials. More experiments to promote the PV effects of this system by optimizing the composition and thickness of the YOT and BFO layers will be carried out in the future.



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Figure 5. Effects of asymmetric electrodes on PV properties. (a) Current−voltage (I−V) characteristics for the BFO-YOT heterostructure under 200 mW, 980 nm laser radiation, measured with asymmetric surface electrodes (Left: Au, Right: ITO). The poling bias is 10 V, and the poling time is 10 min. Illustrations of the energy band graph of Au/BFO/ITO: (b) the polarization direction is towards the right; (c) no poling; and (d) towards the left. ΦAu and ΦITO denote the Schottky barriers between Au / BFO and BFO / ITO, respectively, and the Ui denotes the depolarization field. AUTHOR INFORMATION ACKNOWLEDGMENT

Corresponding Author

HM, TL and YL acknowledge the support of the Australian Research Council (ARC) in the form of Discovery Projects (DP160104780). The authors acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility (AMMRF) at the Centre of Advanced Microscopy, The Australian National University.

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

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx/acsami.xxxxx. XRD, AFM, and PL images of the YOT layer; crosssectional SEM and HRTEM images of the BFO-YOT heterostructure; current-voltage characteristics for the BFOYOT heterostructure with different BFO thickness; currentvoltage characteristics for the BFO-YOT heterostructure without poling; short circuit current with repetition of the switching polarization direction; time dependence of zero bias photocurrent for the BFO-YOT heterostructure after positive poling; current-voltage characteristics for the BFOY2O3 heterostructure; power dependence upconversion spectra of the YOT layer; upconversion spectra of the YOTY2O3 structure; current−voltage characteristics for the BFO/Y2O3/YOT thin film (PDF).

Author Contributions HM fabricated the thin films and did the FE and PV characterization together with TL and YL. GW and MH did the measurement of upconversion decay. HC did the SEM. LL prepared the cross-section sample. FK did the HRTEM and SAD. All authors contributed to discussion, result analysis and manuscript preparation.

Notes The authors declare no competing financial interest.

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