Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal

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Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering Hyeon Han, Donghoon Kim, Kanghyun Chu, Jucheol Park, Sang Yeol Nam, Seungyang Heo, Chan-Ho Yang, and Hyun Myung Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16700 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering

Hyeon Han,†,§ Donghoon Kim,†,§ Kanghyun Chu, é Jucheol Park,‡ Sang Yeol Nam,‡,# Seungyang Heo,† Chan-Ho Yang,éS¶ and Hyun Myung Jang*,†

†

Department of Materials Science and Engineering, and Division of Advanced Materials

Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea é

Department of Physics, Korea Advanced Institute of Science and Technology (KAIS T), Daejeon 343411, Republic of Korea

‡

Gyeongbuk Science & Technology Promotion Center, Gumi Electronics & Information

Technology Research Institute, Gumi 39171, Republic of Korea #

Department of Materials Science and Engineering, Kumoh National Institute of Technology,

Gumi 39177, Republic of Korea ¶

KAIST Institute for the NanoCentury, Daejeon 343411, Republic of Korea

KEYWORDS: switchable photovoltaic effect, ferroelectric photovoltaic effect, band gap, hexagonal ferrites, thin film.

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ABSTRACT Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ~104 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (Eg). Here, we present a promising FPV based on hexagonal YbFeO3 (h-YbFO) thin-film heterostructure by exploiting its narrow Eg. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressivestrained h-YbFO/Pt/MgO heterojunction device shows ~3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density-functional theory studies indicate that the compressive strain reduces Eg substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates towards thin-film photovoltaics and optoelectronic devices.


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1. INTRODUCTION Currently, ferroelectric photovoltaics (FPVs) are being extensively studied owing to their switchable photovoltaic responses, along with abnormally high photovoltages of ~104 V.1-3 FPVs mostly belong to metal-oxide (MO) photovoltaics that are known to be chemically stable and environmentally safe.1 They can be manufactured inexpensively under ambient conditions.1 In particular, the FPVs function both as photon absorbers and charge separators. In contrast, other MO-based photovoltaics such as n-Cu2O/p-Cu2O can fulfill only one of these two. Hence, FPVs can be manufactured to a single active-layered structure. However, their extremely low output photocurrent densities (order of nanoamperes to microamperes per cm2) do limit practical applications of FPVs to solar cells. The observed low photocurrent density, which is the main drawback of FPVs, is attributed primarily to wide band-gap (Eg) characteristics of ferroelectric materials: Eg of ë2.7 eV for BiFeO3 (BFO),4 ë3.6 eV for Pb(Zr,Ti)O3 (PZT),5 and ë3.5 eV for BaTiO3.6 Notwithstanding the band-gap problems, the research activity of FPV solar cells has been stimulated by the three recent breakthroughs: (i) attainment of the power conversion efficiency (PCE) exceeding the Shockley-Queisser limit in a BaTiO3 single crystal,7 (ii) achievement of the PCE of 8.1 % by band-gap tuning of Bi2(Fe,Cr)O6 ferroelectric multilayers8 and (iii) observation of pronounced switchable photovoltaic effects in organometal trihalide perovskite devices.9-11 Various studies have been attempted to enhance FPV efficiencies. These include: (i) band-gap tuning,8,12 (ii) domain-direction control,2,13 (iii) adopting nanometer-scale tips,7,14 (iv) electrode engineering,15,16 (v) fabricating multilayer structures,5,17 (vi) finding a ferroelectric having a narrow Eg,18,19 (vii) applying piezoelectric strain,20 and (viii) imposing interfacial strain gradients in polymorphic phases.21 Among various factors influencing the FPV efficiency, narrow Eg and large polarization are known as the two most important factors.3,8 This is because the absorption of sun light by ferroelectric materials is limited by their Eg



values and large polarization is crucial to efficiently separate the photo-generated exciton (electron-hole) pairs. Here, we present an easy and simple method of simultaneously achieving the above noted two important goals of FPVs, i.e., reduced Eg and enhanced polarization, by applying a film strain to epitaxially grown hexagonal YbFeO3 (h-YbFO) thin-film heterostructures. We further show the switchable FPV effect of these h-YbFO thin-film devices. The crystal structure of hYbFO is featured by an alternative stacking of two distinct layers: one layer of corner-linked FeO5 bipyramids and the other layer of trivalent Yb3+ cations (Figure 1a). We demonstrate enhanced FPV efficiency by suitably exploiting the substrate-induced film strain. More explicitly, a compressive-strained h-YbFO/Pt/MgO heterojunction device shows ~3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device or multiferroic BiFeO3 which is known as the prototypic FPV. The origin of this enhanced PCE is investigated by examining the substrate-dependent band gap (Eg) and polarization (Pr), in conjunction with first-principles calculations. The present study will offer a new standard for selecting substrates towards optimal design of ferroelectric optoelectronics and photovoltaic devices.

2. RESULTS AND DISCUSSION 2.1. Epitaxial Growth and Substrate-Induced Film Strain. For the implementation of ferrite-based heterojunction solar cells, we have grown (0001)oriented hexagonal YbFeO3 (h-YbFO) thin films on two different substrates, Pt (15 nm)/MgO (111) and Pt (15 nm)/Al2O3 (0001), by using pulsed laser deposition (PLD). Figure 1b shows the thickness-dependent out-of-plane lattice parameter of h-YbFO layers as obtained by x-ray diffraction (XRD) measurements. The h-YbFO films grown on Pt (111)/MgO (111) substrates (hereafter, h-YbFO//MgO) show a decrease in the out-of-plane lattice parameter with the film thickness, which indicates the presence of in-plane bi-axial compressive strain. In contrast, the


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h-YbFO films grown on Pt (111)/Al2O3 (0001) substrates (h-YbFO//Al2O3) show the presence of in-plane tensile strain and gradual relaxation of the in-plane strain with increasing film thickness. The well-formed strain in the h-YbFO thin films is caused by the Pt-buffered layers, which distinctly shows a compressive strain on the MgO (111) substrate and a tensile strain on the Al2O3 (0001) substrate (Figure S1c). Filtered high-resolution transmission electron microscopy (HR-TEM) images at the interface of h-YbFO/MgO heterostructure (Figure 1c) reveals periodically located misfit dislocations (denoted by a vertical red line) in the substrate side of interface, which demonstrates the existence of in-plane compressive strain in the hYbFO film. In contrast, h-YbFO/Al2O3 interface shows a misfit dislocation in the film side (Figure 1d), which suggests the existence of in-plane tensile strain in the film. Two selected area electron diffraction (SAED) patterns corresponding to the h-YbFO/MgO and hYbFO/Al2O3 heterostructures are shown in Figure 1e and 1f, respectively. In-plane XRD Phiscan spectra (Figure S2a, S2b) show that both h-YbFO and Pt-buffered layers are characterized by six distinct peaks that are 60o apart from each other, demonstrating a six-fold hexagonal symmetry as well as a heteroepitaxial growth.22,23 The film-strain value can be obtained by evaluating the effective lattice mismatch between the h-YbFO film and the substrate. For doing this, we have first conducted XRD theta-2theta measurements for both the fully strain-relaxed h-YbFO film layers and the two substrates, MgO and Al2O3 (Figure S3). To obtain both the c-axis and in-plane (a or b) lattice parameters, we have performed XRD scans by tilting the sample stage to such a degree of the inter-planar angles relative to the surface planes: tilt angle ( ) of 54.7o for MgO (200) reflection, 57.6o for Al2O3 (102) reflection, and 62.8o for h-YbFO (112) reflection. The inplane lattice constants obtained from the XRD data (Figure S3) are 4.211 6.056

, 4.749

, and

for MgO, Al2O3, and h-YbFO, respectively (Table S1). Our next task to do is the

evaluation of the effective misfit strain. It is known that the hexagonal films occasionally show an in-plane 30o rotation or a super cell match with respect to the substrate orientation to



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minimize the lattice mismatch.23 According to the Phi scan results (Figure S2b and S2d), the h-YbFO film grown on the Pt (111)/Al2O3 (0001) substrate shows an in-plane 30o rotation. Moreover, the present h-YbFO//Al2O3 heterojunction structure should exhibit a super-cell match at the interface. More explicitly, 2 unit cells of h-YbFO are matched on 3 unit cells of Al2O3 (0001). Thus, the effective in-plane lattice constant of Al2O3 is √ = 6.170

4.749

heterojunction

!

As a result, the effective misfit strain (f) of the h-YbFO//Al2O3

is

given

by

Thus, the h-YbFO layer on Al2O3 (0001) is tensile strained. In case of the h-

YbFO//MgO (111) heterostructure, the effective in-plane lattice constant of MgO is given by √

"

#

since the MgO (111) is represented by a triangular surface.

Accordingly, the effective misfit strain of the h-YbFO//MgO (111) heterojunction is given by #

⁄

(%) and the corresponding h-YbFO film layer is

compressive-strained.

2.2. Strain Effects on Ferroelectric Photovoltaic Responses. We have investigated the effect of this film strain on the photovoltaic responses. For this, we have fabricated solar cells having an ITO/h-YbFO(250-nm-thick)/Pt heterojunction structure (Figure 2a), where ITO denotes a transparent indium tin oxide top-electrode layer as deposited using PLD. Two opposite poling directions were used to investigate the switchable photovoltaic effect: ‘upward (downward) poling‘ signifies the application of a positive (negative) voltage to the bottom electrode. To secure a complete switching, we applied an electric field of ë1 MV cm−1, which is much stronger than the coercive field (Ec) of ¥0.25 MV cm−1 (Figure 3c). The J-V characteristics of ITO/h-YbFO/Pt/MgO and ITO/hYbFO/Pt/Al2O3 devices for up and down polarization states are displayed in Figure 2b and 2c, respectively, under AM 1.5G illumination. One prominent feature of these J-V curves is


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‘switchable photovoltaic response‘ in which the polarization direction is reversibly switchable with the aid of a bias electric field. This is known as a unique feature of FPV devices.2-5,18 In addition, the observed asymmetric photovoltaic responses (Figure 2b and 2c) can be attributed to a non-zero net built-in potential in the ITO/h-YbFO/Pt heterojunction (See Section 2 in Supporting Information). Hereafter, the ITO/h-YbFO/Pt/MgO heterojunction structure is denoted by ITO/h-YbFO//MgO for simplicity. For the up-polarization state (i.e., for upward poling), the J-V characteristics of three relevant

heterojunction

devices,

ITO/h-YbFO//MgO,

ITO/h-YbFO//Al2O3,

and

ITO/BFO/SRO/ STO, are compared in Figure 2d, and the corresponding photovoltaic parameters such as Jsc, Voc and PCE are summarized in Table 1. Herein, the ITO/BFO/SRO/STO is adopted as the most widely used prototypic FPV device (also abbreviated as ITO/BFO//STO hereafter; with the corresponding P-E and J-V curves in Figure S4). Interestingly, the compressive-strained h-YbFO//MgO cell exhibits ~2.6 times enhanced PCE than that of the tensile-strained h-YbFO //Al2O3 cell or the prototypic BFO//STO cell (Table 1). The observed remarkably enhanced photocurrent density of the h-YbFO//MgO cell (Figure 2c) clearly supports the enhanced PCE under a compressive strain. Figure 2e further suggests that Jsc is more susceptible to the film strain than Voc. The time-dependent photocurrent responses (Figure 2f) reveal repeatable and stable photocurrent responses for both h-YbFO//MgO and h-YbFO//Al2O3 solar cells.

2.3. Possible Causes of Enhanced Photovoltaic Responses. To unearth the microscopic origin of the enhanced photo-responses under a compressive strain, we have compared optical absorption properties and ferroelectric responses of the compressive-strained h-YbFO//MgO cell with those of the tensile-strained h-YbFO//Al2O3 cell. As shown in Figure 3a, the absorption coefficient of the h-YbFO//MgO cell is consistently larger than that of the h-YbFO//Al2O3 cell over a wide range of the photon energy



including the visible and near-infrared regions. The solar spectrum, as denoted by a blue line in Figure 3a, shows that the visible range (1.7 ~ 3.2 eV) occupies a major part of the solar irradiance. It is known that hexagonal ferrites (h-RFOs) possess a direct band-gap transition property with Eg values between ~1.1 and ~2 eV.24-26 These Eg values are distinctly narrower than those of other typical ferroelectric materials. Thus, h-RFOs are very promising materials for photovoltaic and optoelectronic applications. For both types of h-YbFO films, the onset of a rapid increase in the light absorption occurs at ~2 eV (Figure 3a). Therefore, the Taucmodel27 fitting was carried out with the extrapolated photon energy around 2 eV. The measured Eg (Figure 3b) decreases significantly from 2.07 eV for the h-YbFO//Al2O3 cell to 1.91 eV for the h-YbFO//MgO cell. The corresponding solar absorption rate28 (Figure S5) was estimated by extracting the solar irradiance and optical coefficients from Figure 3a. The solar absorption rate is 15.5 % in the tensile-strained h-YbFO//Al2O3 cell at the film thickness of 250 nm. However, this value remarkably increases to 22.0 % (i.e., 1.42 times) in the compressive-strained h-YbFO//MgO cell. Thus, the photon absorption from the solar illumination in the h-YbFO film is significantly enhanced by the compressive film strain, which consequently leads to the enhanced photocurrent density observed in the h-YbFO//MgO cell (Figure 2d). The polarization-electric field (P-E) hysteresis curves demonstrate that the h-YbFO film is ferroelectric at room temperature29 and Pr increases substantially from " %& '() for the h-YbFO//Al2O3 cell to " # %& '() for the h-YbFO//MgO cell (Figure 3c). To ensure the enhanced polarization in the h-YbFO//MgO cell, we have measured the P-E hysteresis loop by employing nine different electrodes for each cell type (Figure S6). The double-box poling test performed by a piezoresponse force micrscope (Figure S7) support the P-E hysteresis results. Moreover, the computed off-centering displacement (OCD) of Yb-ion along the polar c-axis increases with the in-plane compressive strain: from 35.6 pm under a tensile strain of +3 % to 38.8 pm under a compressive strain of -3 % (Figure S8). This ab initio result supports the


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experimental observation of Pr since the OCD along the c-axis in the YbO8 (RO8, in general) unit having trigonal D3d symmetry is responsible for the manifestation of the *+, hexagonal ferroelectricity.29,30 The present observation of Pr is also supported by the reported ab initio result on the strain-dependent Pr for hexagonal rare-earth ferrites.31 More explicitly, the computed Pr of h-DyFeO3 under a tensile strain of +3.92 % is 8.1 %& '() and this value increases to 11.2 %& '() under a compressive strain of the same magnitude.31 The spontaneous polarization, thus Pr, plays an important role in the photocurrent generation in FPVs since the rate of electron-hole recombination tends to reduce with increasing value of the depolarization field, thus, Pr. It can be shown that this difference in Pr also affects the magnitude of switchable photovoltaic responses (see Section 3 in Supporting Information). As shown in Table 1, Jsc of the ITO/h-YbFO//MgO heterojunction device is 2.1 times higher than that of the ITO/h-YbFO//Al2O3 device. On the other hand, we showed that the solar absorption rate of the compressive-strained ITO/h-YbFO//MgO cell is 1.42 times higher than that of the tensile-strained ITO/h-YbFO//Al2O3 cell (Figure S5). The discrepancy between these two values (2.1 vs. 1.42) can be attributed to the difference in Jsc caused by the depolarization field, thus, by Pr. To identify other possible causes that might influence the enhanced photovoltaic efficiency, we have examined the concentration of oxygen vacancies (Vo) by using x-ray photoelectron spectroscopy (XPS) method. This is because Vo defects are able to modulate the energy band and to influence the ferroelectric photovoltaic effect.32-35 According to the XPS O 1s and Fe 2p results (Figure S9), however, the Vo concentration in the tensile-strained h-YbFO//Al2O3 device is slightly higher than that in the compressive-strained h-YbFO//MgO device. If the Vo effect did dominantly contribute to the overall photovoltaic efficiency, Jsc and PCE of the h-YbFO//Al2O3 device would be even higher than those of the h-YbFO//MgO device. Thus, the XPS result indicates the Vo is not the cause of the enhanced PCE in the compressive-strained h-YbFO//MgO cell. In addition, according to the present XPS result, the Vo concentration in the tensile-strained film is higher than that in the



compressive-strained film. This is supported by other investigations that the Vo concentration in most oxides increases when the lattice volume expands by virtue of the tensile strain, oxygen partial pressure control during growth, and others.36-38 Figure S10 demonstrates that our tensil-strained film (382.65 Å3) have larger unit cell volume than the compressive-strained film (366.45 Å3), which further verifies that the tensile strained h-YbFO//Al2O3 device have more Vo concentration than the compressive strained h-YbFO//MgO device. To further clarify the strain-dependence of the FPV efficiency, we have compared the PCE by fabricating h-YbFO films with different thicknesses (Figure S11). In the 150-nm-thick films, the h-YbFO//MgO heterojunction cell shows 3.0 times enhanced efficiency than the hYbFO//AlO cell. This enhancement ratio is bigger than 2.6 times observed in the 250-nmthick h-YbFO films. In contrast, in the strain-relaxed films at the thickness of 800 nm, this enhancement ratio is substantially reduced by a factor of 1.5. This result clearly indicates that the strain effect is reduced with increasing film thickness, demonstrating the important role of the film strain on the photovoltaic responses of FPVs. Considering all of these, one can conclude that the enhanced Jsc, thus, PCE observed in the compressive-strained ITO/hYbFO//MgO solar cell can be attributed to the enhanced solar absorption, combined with the enhanced depolarization field or Pr. In addition, the compressive-strain effect is increasingly important as the film thickness is decreasing.

2.4. Electronic Origin of Reduced Band Gap and Enhanced Optical Absorption. We have performed density-functional theory (DFT) calculations to atomistically understand the observed decrease in Eg and enhanced optical absorption under a compressive film strain. It is known that an optical absorption spectrum of hexagonal ferrite can be decomposed into three distinct peaks in which one peak at a higher energy (4.20 eV in the case of h-YbFO) is caused by the purely charge transfer (CT) transitions between the occupied O 2p orbitals and the unfilled Fe 3d orbitals while two peaks at a lower energy region (2.37 and 2.98 eV; Figure


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S12) are attributed to a combined effect of the d-d transitions and the dipole-allowed CT transitions.25,26 The bipyramidal FeO5 unit in hexagonal RFeO3 (such as h-YbFO) is represented by the trigonal-bipyramidal D3h symmetry. Accordingly, the Fe 3d states split into the three distinct irreducible representations (IRs), namely,

/ .

00 1 2 / 33

441 34

and

5// 301 40 . Among possible d-d transitions, the 6A1' → 4A1" transition corresponds to the absorption band in the vicinity of ~ 2 eV (blue absorption band in Figure S12) and the energy gap of this transition is mainly determined by the difference in the energy eigenvalue between the

/ .

678

and doubly degenerated 2 / 69: 1 69 8 ;: 8

crystal-field theory, the energy gap between

/ .

states (Figure 4). According to the

and 2 / states tends to reduce under an in-

plane (x-y) compressive strain since the eigenvalue of the 678 orbital decreases owing to less repulsive interaction between the Fe core and the two axial oxygen ligands along the z-axis while the eigenvalue of the degenerate 69: or 69 8 ;: 8 orbital increases due to enhanced repulsive interaction between the Fe core and the equatorial oxygen ligands on the x-y plane (Figure 4b). This prediction is clearly supported by the computed 3d-orbital-resolved densityof-states (DOS). As indicated in Figure 4c, the energy gap between

/ .

and 2 / orbitals

increases under a tensile strain (+3 %). In contrast, this gap tends to reduce substantially under a compressive strain (-3 %). Let us now focus on the dipole-allowed CT transition (Figure 4d) which is solely responsible for the high energy peaks in optical absorption spectra of hexagonal ferrites25,26 (4.20 eV in the case of h-YbFO; Figure S12). The two-site transition dipole matrix element for the CT transition between the σ-bonded Fe 3d and O 2p can be written as 〈=2 678 |20|? @7 〉1 where z denotes the direction of σ-bonding. According to the previous theoretical calculations,39 the magnitude of this matrix element increases with decreasing MeO separation in the vicinity of the equilibrium bond distance (BCD;E F " G ), where Me denotes a 3d transition-metal atom such as Mn and Fe and a.u. designates the atomic unit, i.e.,



0.529

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As can be deduced from Figure 4a, the increase in the two Fe-O axial-bond

distances (HBID;E ) caused by the change from tensile to compressive strain of 3 % is between 0.067 and 0.076

On the contrary, the corresponding decrease in the three Fe-O equatorial-

bond distances is as large as 0.119

Under a compressive strain, thus, the decrease in the

equatorial Fe-O σ-bonding distance has a dominant effect over the increase in the axial Fe-O distance. Since the intensity of an optical absorption spectrum (at a given wavelength) is proportional to the square of the dipole matrix element, i.e., |〈=2 678 |20|? @7 〉|) 1 one would expect an increase in the solar absorption in the vicinity of 4.20 eV under a compressive film strain. It is well known that the intensity of the d-d absorption band (e.g., the band at 2.37 eV due to the 6A1' → 4A1" transition) is enhanced by mixing these intra-atomic dd transitions with the inter-atomic CT transitions.26 Admitting the mixing of d-d and CT transitions, one would expect enhanced solar absorption intensity at a lower energy region as well, where the two peaks centered at 2.37 and 2.98 eV reflect distinct Fe d-d

transitions

(Figure S12). Thus, the enhanced solar absorption intensity under a compressive strain can be interpreted by adopting the mixing effect of the Fe d-d transitions with the dipole-allowed CT transitions between the σ-bonded Fe 3d and O 2p.

3. CONCLUSIONS In summary, we have presented h-YbFO thin-film solar cells having a narrow band gap and demonstrate their enhanced FPV effects by suitably exploiting the substrate-induced film strain. A compressively strained ITO/h-YbFO//MgO device shows ~3 times enhanced photovoltaic efficiency than that of a tensile strained ITO/h-YbFO//Al2O3 device or BiFeO3, a prototypic FPV. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive film strain. DFT studies further indicate that the


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compressive film strain promotes the strength of the Fe d-d transitions by combining these with the CT transitions between the oxygen 2p and unfilled Fe 3d orbitals.

4. EXPERIMENTAL / COMPUTATIONAL DETAILS 4.1. Thin-Films Fabrication. Pulsed laser deposition (PLD) method was used for the fabrication of highly [0001]-oriented hexagonal YbFeO3 (h-YbFO) film layers either on MgO (111) or Al2O3 (0001) substrates at a laser energy density of 1.5 J·cm-2 with the repetition rate of 5 Hz. The substrate was maintained at 830 oC. For the fabrication of solar cells having an ITO/h-YbFO/Pt heterojunction structure (Figure 2a), transparent ITO top electrodes were deposited by PLD through a shadow mask with circular apertures (100~200 µm in diameter). Epitaxial Pt (111) bottom-electrode layers (15 nm in thickness) were grown either on MgO (111) or Al2O3 (0001) substrates using RF magnetron sputtering at 550 oC. 4.2. Characterizations of Thin Films and Solar Cells. We have performed XRD structural analysis to confirm a hexagonal phase as well as in-plane orientation in the PLD grown h-YbFO layer using a high-resolution x-ray diffractometer (D8 Discover, Bruker) under Cu Kα radiation. Atomic-scale microstructures of the h-YbFO/substrate interfaces were examined by adopting high-resolution transmission electron microscopy method (JEMARM200F, JEOL with a Cs-corrector) under 200-kV acceleration voltage. For ferroelectric characterizations, P-E hysteresis loops and the PUND test were obtained using a Precision LC system (Radiant Technologies, Inc.). PFM (Veeco-DI Multimode V equipped with a Nanoscope controller V) images were obtained with a scan rate of 0.5 Hz and an AC voltage of 4 V at a frequency of 10.1 kHz. For the experimental study of the optical bandgap, optical absorption spectra were recorded as a function of the photon energy using a double-beam UV– Vis–NIR spectrophotometer (JASCOV-570). UV photoelectron spectroscopy (UPS; AXIS Ultra DLD) measurements were used to estimate the work functions, the Fermi energies, and the valence-band edges of the five relevant materials adopted in the distinct types of



heterojunction

cells,

namely,

ITO/h-YbFO/Pt

and

ITO/BiFeO3(BFO)/SRO.

Page 14 of 26

UPS

measurements were carried out using He I (21.22 eV) photon lines from a discharge lamp. Xray photoelectron spectroscopy was used to measure the O1s signal of h-YbFO thin films having some oxygen-vacancy defects. The current density–voltage (J-V) characteristics were measured using a source meter (Compactstat, IVIUM tech.) under simulated AM 1.5G illumination (100 mW cm-2) provided by a solar simulator (Sun 3000, Abet tech.). The incident light intensity was calibrated with a Si solar cell (as a reference) equipped with an IRcutoff filter (KG-5, Schott). 4.3. Computational Methods: We have performed ab initio DFT calculations on the basis of the generalized gradient approximations (GGA)40 implemented with the projector augmented-wave (PAW) method41 using the Vienna ab initio simulation package (VASP).42 In actual DFT calculations, we have adopted (i) a

Monkhorst-Pack k-point mesh43

centered at J-point, (ii) 500 eV plane-wave cutoff energy, and (iii) 0.01 eV/atom for the force convergence limit in structural relaxation. Structural optimizations were basically performed for the 30-atoms cell containing six formula units. i.e., a hexagonal unit cell. We have first tried to calculate the band gap using the GGA+U method. However, the large U value shows a probelm of a large deviation from the experimental lattice parameter presumably due to the felectron contribution. Therefore, the purpose of our DFT calculation using the GGA method is not to directly observe the band gap of h-YbFO but to estimate the degree of the crystal field splitting, which can indirectly predict the band gap and absorption properties.25


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■ ASSOCIATED CONTENT * Supporting Information Section 1: XRD (Theta-2theta, Phi scan) measurements, effective lattice mismatch calculation, P-E hysteresis loop and illuminated J-V curve of BiFeO3, PUND, PFM, Ab initio computed Yb-ion displacement, XPS, thickness-dependent J−V characteristics, decomposed optical absorption spectrum. Section 2: origin of asymmetric switchable photovoltaic responses. Section 3: analysis of switchable photovoltaic effect and built-in field effect.

■ AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by Pohang Steel Corporation (POSCO) through the Green Science Program (Project No. 2015Y060 and 2016Y038) and by the National Research Foundation (NRF) Grant funded by the Korean Government (MSIP) (Grant No. 2016R 1D1A1B 03933253).



? References (1) Rühle, S.; Anderson, A. Y.; Barad, H.-N.; Kupfer, B.; Bouhadana, Y.; Rosh-Hodesh, E.; Zaban, A. All-Oxides Photovoltaics. J. Phys. Chem. Lett., 2012, 3, 3755−3764. (2) 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, III, J. W.; Martin, L. W.; Ramesh, R. Above-bandgap voltages from ferroelectric photovoltaic devices. Nat. Nanotech., 2010, 5, 143−147. (3) Yuan, Y.; Xiao, Z.; Yang, B.; Huang, J. Arising applications of ferroelectric materials in photovoltaic devices. J. Mater. Chem. A., 2014, 2, 6027−6041. (4) Ji, W.; Yao, K.; Liang, Y. C. Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films. Adv. Mater., 2010, 22, 1763−1766. (5) Cao, D.; Wang, C.; Zheng, F.; Dong, W.; Fang, L.; Shen, M. High-Efficiency Ferroelectric-Film Solar Cells with an n-type Cu2O Cathode Buffer Layer. Nano Lett., 2012, 12, 2803−2809. (6) Kamalasanan, M. N.; Chandra, S.; Joshi, P. C.; Mansingh, A. Structural and optical properties of sol-gel processed BaTiO3 ferroelectric thin films. Appl. Phys. Lett., 1991, 59, 3547. (7) Spanier, J. E.; Fridkin, V. M.; Rappe, A. M.; Akbashev, A. R.; Polemi, A.; Qi, Y.; Gu, Z.; Young, S. M.; Hawley, C. J.; Imbrenda, D.; Xiao, G.; Bennett-Jackson, A. L.; Johnson, C. L. Power conversion efficiency exceeding the Shockley-Queisser limit in ferroelectric insulator. Nat. Photon., 2016, 10, 611−616. (8) Nechache, R.; Harnagea, C.; Li, S.; Cardenas, L.; Huang, W.; Chakrabartty, J.; Rosei, F. Bandgap tuning of multiferroic oxide solar cells. Nat. Photon., 2015, 9, 61−67. (9) Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant switchable photovoltaic effect in organometal trihalide perovskite devices. Nat. Mater., 2015, 14, 193−198. (10) Chen, B.; Shi, J.; Zheng, X.; Zhou, Y.; Zhu, K.; Priya, S. Ferroelectric solar cells based on inorganic-organic hybrid perovskites. J. Mater. Chem. A., 2015, 3, 7699−7705.


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Chen, B.; Zheng, X.; Yang, M.; Zhou, Y.; Kundu, S.; Shi, J.; Zhu, K.; Priya, S. Interface band structure engineering by ferroelectric polarization in perovskite solar cells. Nano Energy, 2015, 13, 582−591. (12) Choi, W. S.; Chisholm, M. F.; Singh, D. J.; Choi, T.; Jellison Jr., G. E.; Lee, H. N. Wide bandgap tenability in complex transition metal oxides by site-specific substitution. Nat. Commun., 2012, 3, 689. (13) Bhatnagar, A.; Chaudhuri, A. R.; Kim, Y. H.; Hesse, D.; Alexe, M. Role of domain walls in the abnormal photovoltaic effect in BiFeO3. Nat. Commun., 2013, 4, 2835. (14) Alexe, M.; Hesse, D. Tip-enhanced photovoltaic effects in bismuth ferrite. Nat. Commun., 2011, 2, 256. (15) Harshan, V. N.; Kotru, S. Influence of work-function of top electrodes on the photovoltaic characteristics of Pb0.95La0.05Zr0.54Ti0.46O3 thin film capacitors. Appl. Phys. Lett., 2012, 100, 173901. (16) Qin, M.; Yao, K.; Liang, Y. C. Photovoltaic mechanisms in ferroelectric thin films with the effects of the electrodes and interfaces. Appl. Phys. Lett., 2009, 95, 022912. (17) Chatterjee, S.; Bera, A.; Pal, A. J. p-i-n Heterojunctions with BiFeO3 Perovskite Nanoparticles and p- and n- Type Oxides: Photovoltaic Properties. ACS Appl. Mater. Interfaces., 2014, 6, 20479−20486. (18) Han, H.; Song, S.; Lee, J. H.; Kim, K. J.; Kim, G.-W.; Park, T.; Jang, H. M. Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films Having Narrow Band Gaps. Chem. Mater., 2015, 27, 7425−7432. (19) Zhang, G.; Wu, H.; Li, G.; Huang, Q.; Yang, C.; Huang, F.; Liao, F.; Lin, J. New High Tc multiferroics KBiFe2O5 with narrow band gap and promising photovoltaic effect. Sci. Rep., 2013, 3, 1265. (20) 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 BiFeO3/La0.7Sr0.3MnO3/PMN-PT heterostructures. Nano Energy, 2015, 18, 315−324. (21) Chu, K.; Jang, B.-K.; Sung, J.H.; Shin, Y.A.; Lee, E.-S.; Song, K.; Lee, J.H.; Woo, C.-S.; Kim, S.J.; Choi, S.-Y.; Koo, T.Y.; Kim, Y.-H.; Oh, S.-H.; Jo, M.-H.; Yang, C.-H.



Enhancement of the anisotropic photocurrent in ferroelectric oxides by strain gradients. Nat. Nanotechnol. 2015, 10, 972–979. (22) Xu, X.; Wang, W. Multiferroic hexagonal ferrites (h-RFeO3, R = Y, Dy-Lu): a brief experimental review. Mod. Phys. Lett. B, 2014, 28, 1430008. (23) Song, S.; Han, H.; Jang, H. M.; Kim, Y. T.; Lee, N.-S.; Park, C. G.; Kim, J. R.; Noh, T. W.; Scott, J. F. Implementing Room-Temperature Multiferroism by Exploiting HexagonalOrthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films. Adv. Mater., 2016, 28, 7430−7435. (24) Holinsworth, B.S.; Mazumdar, D.; Brooks, C.M.; Mundy, J.A.; Das, H.; Cherian, J.G.; McGill, S.A.; Fennie, C.J.; Schlom, D.G.; Musfeldt, J.L. Direct band gaps in multiferroic h-LuFeO3. Appl. Phys. Lett. 2015, 106, 082902. (25) Wang, W.; Wang, H.; Xu, X.; Zhu, L.; He, L.; Wills, E.; Cheng, X.; Keavney, D.J.; Shen, J.; Wu, X.; Xu, X. Crystal field splitting and optical bandgap of hexagonal LuFeO3 films. Appl. Phys. Lett. 2012, 101, 241907. (26) Pavlov, V.V.; Akbashev, A.R.; Kalashnikova, A.M.; Rusakov, V.A.; Kaul, A.R.; Bayer, M.; Pisarev, R. V. Optical properties and electronic structure of multiferroic hexagonal orthoferrites RFeO3 (R=Ho, Er, Lu). J. Appl. Phys. 2012, 111, 056105. (27) Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi B, 1966, 15, 627. (28) Drotning, W. D. Optical properties of solar-absorbing oxide particles suspended in a molten salt heat transfer fluid. Solar Energy, 1978, 20, 313−319. (29) Jeong, Y. K.; Lee, J.-H.; Ahn, S.-J.; Song, S.-W.; Jang, H. M.; Choi, H.; Scott, J. F. Structurally Tailored Hexagonal Ferroelectricity and Multiferroism in Epitaxial YbFeO3 ThinFilm Heterostructures. J. Am. Chem. Soc. 2012, 134, 1450−1453. (30) Jeong, Y. K.; Lee, J.-H.; Ahn, S.-J.; Jang, H. M. Epitaxially Constrained Hexagonal Ferroelectricity and Canted Triangular Spin Order in LuFeO3 Thin Films. Chem. Mater., 2012, 24, 2426−2428. (31) Zhao, H. J.; Xu, C.; Yang, Y.; Duan, W.; Chen, X. M.; Bellaiche, L. Predicted energetics and properties of rare-earth ferrites films grown on cubic (111)- and hexagonal (0001)oriented substrates. J. Phys: Condens. Matter., 2015, 27, 485901.


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Yi, H. T.; Choi, T.; Choi, S. G.; Oh, Y. S.; Cheong, S. W. Mechanism of the Switchable Photovoltaic Effect in Ferroelectric BiFeO3. Adv. Mater., 2011, 23, 3403−3407. (33) 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, 275201. (34) Hong, S.; Choi, T.; Jeon, J. H.; Kim, Y.; Lee, H.; Joo, H. Y.; Hwang, I.; Kim, J. S.; Kang, S. O.; Kalinin, S. V.; Park, B. H. Large Resistive Switching in Ferroelectric BiFeO3 NanoK Island Based Switchable Diodes. Adv. Mater., 2013, 25, 2339−2343. (35) Ge, C.; Jin, K.-J.; Zhang, Q.-H.; Du, J.-Y.; Gu, L.; Guo, H.-Z.; Yang, J.-T.; Gu, J.-X.; He, M.; Xing, J; Wang, C; Lu, H.-B.; Yang, G.-Z. Toward Switchable Photovoltaic Effect via Tailoring Mobile Oxygen Vacancies in Perovskite Oxide Films. ACS Appl. Mater. Interfaces 2016, 8, 34590−34597. (36) Petrie, J. R.; Mitra, C.; Jeen, H; Choi, W. S.; Meyer, T. L.; Reboredo, F. A.; Freeland, J. W.; Eres, G.; Lee, H. N. Strain Control of Oxygen Vacancies in Epitaxial Strontium Cobaltite Films. Adv. Funct. Mater. 2016, 26, 1564–1570. (37) Jang, S. Y.; Lee, D.; Lee, J.-H.; Noh, T.W.; Jo, Y.; Jung, M.-H.; Chung, J.-S. Oxygen vacancy induced re-entrant spin glass behavior in multiferroic ErMnO3 thin films. Appl. Phys. Lett. 2008, 93, 162507. (38) Adler, S. B. J. Am. Ceram. Soc. 2001, 84, 2117. (39) Moskvin, A. S.; Pisarev, R. V. Optical spectroscopy of charge transfer transitions in multiferroic manganites, ferrites, and related insulators. Low. Temp. Phys., 2010, 36, 489−510. (40) Perdew, J. P.; Burke, K.; Wang, Y. Generalized gradient approximation for the exchangecorrelation hole of a many-electron system. Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 16533. (41) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953−17979. (42) Kresse, G.; Furthműller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Mater. Phys., 1996, 54, 11169−11186.



(43) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B: Condens, Matter Mater. Phys., 1976, 13, 5188−5192.


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Figure. 1. (a) Unit cell crystal structure of the hexagonal ferrite (h-RFO) having the polar P63cm symmetry, where navy blue circles denote Fe ions, orange circles for oxygen ions, and larger yellow circles designate rare-earth (R) ions. (b) Comparison of the thickness-dependent c-axis lattice parameter of the h-YbFO film grown on the Pt (111)/MgO (111) substrate with that of the h-YbFO film grown on Pt (111)/Al2O3 (0001) substrate. The lattice parameters were obtained from theta-2theta XRD peak positions presented in Figure S1a & S1b. (c) HR-TEM image of the h-YbFO/MgO interfacial region (upper panel) with the corresponding filtered HR image (lower panel). (d) HR-TEM image of the h-YbFO/Al2O3 interfacial region (upper panel) with the corresponding filtered HR image (lower panel). (e) SAED pattern of the h-YbFO film on MgO (111) with the zone axis of L

M N The aligned direction of (002) diffraction spots

in the SAED pattern is parallel to the out-of-plane direction in the corresponding HR-TEM image in (c), indicating that the h-YbFO film grown on an MgO (111) is well aligned along



the hexagonal c-axis. (f) SAED pattern of the h-YbFO film on Al2O3 (0001) with the zone axis of LM M N1 indicating that the h-YbFO films grown on an Al2O3 (0001) is also well aligned along the hexagonal c-axis.

Figure. 2. (a) A schematic representation of the ITO/h-YbFO/Pt heterojunction device. J-V characteristics of (b) ITO/h-YbFO/Pt/MgO and (c) ITO/h-YbFO/Pt/Al2O3 devices under AM 1.5G illumination. (d) J-V characteristics of the ITO/h-YbFO/Pt/MgO heterojunction device compared with those of the ITO/h-YbFO/Pt/Al2O3 and ITO/BFO/SRO/STO devices after the upward poling (i.e., up-polarization state). (e) Comparison of the average photocurrent density (JSC) and photovoltage (VOC) of the three relevant heterojunction devices used in the present study. Herein, error bars represent ±1 standard deviation. (f) Zero-bias photocurrent density of the h-YbFO/Pt/MgO and h-YbFO/Pt/Al2O3 devices plotted as a function of time.


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Table 1. Photovoltaic parameters of h-YbFO//MgO, h-YbFO//Al2O3 and BFO//STO thin-film heterojunction devices (~250 nm-thick) under AM 1.5G illumination. Here, BFO/SRO/STO is denoted by BFO//STO for simplicity.

Polarization

JSC

VOC

F.F.

PCE

direction

(mA cm−2)

(V)

(%)

(%)

Up

0.021

− 0.52

28.6

0.00312

Down

− 0.011

0.24

26.6

0.00070

Up

0.010

− 0.46

26.3

0.00121

Down

− 0.004

0.18

24.0

0.00017

Up

0.008

− 0.49

29.8

0.00117

Down

− 0.007

+ 0.36

31.3

0.00079

Device

h-YbFO//MgO

h-YbFO//Al2O3

BFO//STO



Figure. 3. (a) Comparison of the ultraviolet−visible−near infrared absorption spectrum of the h-YbFO film on MgO (filled black circles) with that of the h-YbFO film on Al2O3 (open red circles). (b) Tauc plots of h-YbFO/MgO and h-YbFO/Al2O3 structures for the direct band-gap transition. (c) Comparison of the polarization-electric field (P-E) hysteresis loop of the ITO/hYbFO/Pt/MgO capacitor (at 300 K) with that of the ITO/h-YbFO/Pt/Al2O3 capacitor.


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Figure. 4. (a) Comparison of the five ab initio computed bond lengths ( ) in the bipyramidal FeO5 unit under a tensile strain (+3%) with those in the FeO5 unit under a compressive strain (-3%). (b) Schematic orbital diagrams for the five Fe 3d states with the splitting into three distinct IRs, namely,

/ .

00 1 2 / 33

441 34

and 5// 301 40 on the basis of the crystal

field (CF) theory. (c) Calculated 3d-orbital-resolved DOS of tensile strained (top), unstrained (middle), and compressively strained (bottom) h-YbFO. The gap between O / and P. ′ states (marked with a red arrow) decreases as the strain changes from tensile to compressive. (d) Schematic presentation of the charge transfer (CT) transition.



Table of Contents


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Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal

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