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Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films having Narrow Bandgaps Hyeon Han, Seungwoo Song, June Ho Lee, Kun Joong Kim, Guan-Woo Kim, Taiho Park, and Hyun Myung Jang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03408 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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Chemistry of Materials

Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films having Narrow Bandgaps Hyeon Han,† Seungwoo Song,† June Ho Lee,† Kun Joong Kim,† Guan-Woo Kim,‡ Taiho Park,‡ and Hyun Myung Jang*,† †

Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea ‡

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea ABSTRACT: Ferroelectric photovoltaics (FPVs) are being extensively studied owing to their anomalously high photovoltages, coupled with reversibly switchable photocurrents. However, FPVs suffer from their extremely low photocurrents, which is primarily due to their wide bandgaps. Herein, we present a new class of FPV by demonstrating (i) a nearly optimum bandgap of ~1.55 eV and (ii) the ferroelectric polarization switching in the epitaxial hexagonal manganite thin films, h-RMnO3, where R = Lu and Y. According to the thickness-dependent photovoltaic measurements, the ITO/h-LuMnO3/Pt solar cell shows power conversion efficiency of ~0.11 % when the thickness of h-LuMnO3 layer is ~150 nm. We have shown that the PCE is 1 to 3 orders higher than those of classical FPVs such as undoped Pb(Zr,Ti)O3 and BiFeO3 under the standard AM 1.5G illumination. We have further elucidated that the depolarization-field effect arising from the switchable polarization dominates over the nonferroelectric internal field effect.

1. INTRODUCTION Currently, ferroelectric photovoltaic devices are being extensively investigated owing to their anomalously high photovoltages, coupled with reversibly switchable polarizations.1-4 In conventional semiconductor p-n junction solar cells, the photo-generated electron-hole pairs are separated by the builtin potential inside the p-n junction. In this case, the opencircuit voltage (Voc) is limited by the optical bandgap of the light absorbing semiconductor. In ferroelectric solar cells, on the contrary, the photo-generated electron-hole pairs are separated by the ferroelectric polarization originating from polar non-centrosymmetry. As a result, the output Voc can be a few orders of magnitude larger than the bandgap of ferroelectric materials.1,4 The most outstanding feature of ferroelectric photovoltaic (FPV) devices is that the photocurrent direction can be reversibly switched by changing the polarization direction with the aid of a bias electric field.2-4 Owing to the enhanced charge collection efficiency associated with the depolarization field, the FPV efficiency is known to be more pronounced in thin-film forms than in bulk forms.5-6 Several theoretical models were proposed to account for the FPV effect and associated high photovoltage outputs. These include the shift-current model for bulk photovoltaic effect,7-9 the domain-wall theory,1,10 the Schottky-junction effect,11 and the depolarization-field model.12 Until recently, the FPV effect has remained an academic interest rather than having practical applications owing to the very low output photocurrent densities in the order of nA cm-2 ~ μA cm-2. However, the research activity of FPV solar cells is re-spurred

by the two recent breakthroughs: (i) an unprecedented achievement in the power conversion efficiency (PCE) of 8.1 % by the bandgap tuning of Bi(Fe1/2Cr1/2)O3-based ferroelectric multilayers13 and (ii) the giant switchable photovoltaic effect observed in organometal trihalide perovskite (organicinorganic hybrid perovskite) devices.14-16 The observed low photocurrent density which is the main drawback of FPV solar cells is attributed primarily to the wide-bandgap (Eg) characteristics of typical ferroelectric materials applied to FPV devices: Eg of ~2.7 eV for BiFeO3 (BFO), ~3.6 eV for Pb(Zr,Ti)O3 (PZT), and ~3.5 eV for BaTiO3.8,17,18 Accordingly, extensive studies have been made to reduce Eg by suitable chemical modifications. However, this type of the bandgap tuning usually leads to deterioration of ferroelectric and dielectric properties.19 According to the Shockley and Queisser limit for the optimal Eg of p-n junction solar cells,20 materials with their Eg between 1.3 and 1.5 eV show the highest PCE, in general. Sbbased ferroelectric semiconductors such as SbSI and SbSBr possess a narrow Eg of ~2.0 eV which is still noticeably larger than the optimal Eg range (1.3~1.5 eV).21 Moreover, their ferroelectric polarizations begin to appear at a temperature below 20 oC, which inevitably leads to problems in their practical (room-temperature) applications.22 On the other hand, GeTe, SnTe, PbTe, and their mixtures are characterized by an extremely narrow Eg of ~0.2 eV.23,24 This is not desirable for use in p-n junction solar cells either as the bandgap reduction below 1.3 eV often leads to the decrease in the open-circuit voltage (Voc)20 with a concomitant decrease in the PCE.

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Contrary to the above-mentioned non-oxide ferroelectric semiconductors, a series of RMnO3-type hexagonal rare-earth manganites (R = Sc, Y, Dy~Lu) having polar P63cm space-group symmetry nearly meet the above criterion of the optimal Eg with a reasonably large spontaneous polarization (Ps ≥ ~5 µC cm-2) for suppressing a rapid recombination of the photo-generated electron-hole pairs. On the basis of this finding, we have chosen two typical hexagonal rare-earth managnite (h-RMnO3) thin films for demonstrating FPV effects: (i) h-LuMnO3 (h-LMO hereafter) as an example of lanthanide-based manganites and (ii) h-YMnO3 (h-YMO hereafter) as an example of non-lanthanide-based rare-earth manganites. Herein, we have first demonstrated the ferroelectric polarization switching in hexagonal manganites using the caxis-oriented hetero-epitaxial h-RMnO3 thin films grown on Pt(111)/Al2O3(0001) substrates. The observed remanent polarization is ~5 µC cm-2. We have subsequently shown that the PCE (~0.11 %) of 150-nm-thick h-RMnO3 films is 1 to 3 orders higher than those of classical ferroelectric photovoltaics such as undoped Pb(Zr,Ti)O3 and BiFeO3 under the standard AM 1.5G illumination.2,8,17,25 On the basis of the polingdirection-dependent current-voltage (J-V) responses, we have further shown that the contribution from the switchable depolarization field to the photovoltage is much greater than that from the unswitchable internal field which arises from the net built-in potential developed in the h-RMnO3-based heterojunction solar cell.

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by a solar simulator (69920, 1 kW Xe lamp with an optical filter, Oriel). The incident light intensity was calibrated with a Si solar cell (as a reference) equipped with an IR-cutoff filter (KG-5, Schott).

3. RESULTS AND DISCUSSION 3.1. Epitaxial Growth and Hexagonal Ferroelectricity Theta-2theta x-ray diffraction (θ-2θ XRD) patterns indicate that both hexagonal films (denoted by h-RMO hereafter) grown on a Pt(111)/sapphire(0001) substrate are highly c-axis oriented (Figure 1a). The heteroepitaxial growth of h- LuMnO3 (h-LMO) films was confirmed by examining the in-plane XRD phi-scans (Figure 1b). These patterns were obtained by keeping the Bragg angle at 1122 for h-LMO and (200) for Pt. The six peak that are 60o apart each other occur at the same azimuthal phi-angles for both h-LMO and Pt, demonstrating a six-fold hexagonal symmetry of the h-LMO layers. As shown in Figure 1b, the six-fold hexagonal symmetry was also confirmed in the h-YMnO3 (h-YMO) film.

2. EXPERIMENTAL SECTION Fabrication of Thin Films and Solar Cells. An epitaxial Pt(111) film adopted as the bottom-electrode layer was grown on the Al2O3(0001) substrate using RF magnetron sputtering. Pulsed laser deposition (PLD) method was then used for the fabrication of hexagonal RMnO3 film layers on the Pt(111)/Al2O3(0001) substrate at a laser energy density of 1.5 J cm-2 with the repetition rate of 3Hz. The substrate were maintained at 850 oC. For the fabrication of solar cells having an ITO/h-RMO/Pt junction structure (Figure 3a), transparent ITO top electrodes were deposited by PLD through a shadow mask with circular apertures (100~200 µm in diameter). Characterizations of Thin Films and Solar Cells. We have performed XRD structural analysis to confirm a hexagonal phase as well as in-plane epitaxy in the PLD grown h-RMO layer. For ferroelectric characterization, P-E hysteresis loops with a virtual ground mode were obtained using a Precision LC system (Radiant Technologies, Inc.). For the experimental study of the optical bandgap, optical absorption spectra were recorded as a function of the photon energy using a doublebeam UV–Vis–NIR infrared spectrophotometer (JASCOV570). Ultraviolet photoelectron spectroscopy (UPS; AXIS Ultra DLD) measurements were used to estimate the work functions, the Fermi energies, and the valence-band edges of the four relevant materials adopted in the ITO/h-LMO/Pt and ITO/h-YMO/Pt hetero-junction cell. UPS measurements were carried out using He I (21.22eV) photon lines from a discharge lamp. X-ray photoelectron spectroscopy was used to measure the O1s signal of h-RMO thin films having some oxygenvacancy defects. The current density–voltage (J-V) characteristics were measured using a Keithley 2400 source meter under simulated AM 1.5G illumination (100 mW cm-2) provided

Figure 1. Structural and dielectric data for the 150-nm-thick h-RMnO3 (h-RMO) thin films grown on a Pt(111)/Al2O3(0001) substrate. a) Theta-2theta (θ−2θ) x-ray diffraction (XRD) patterns of the preferential [0001]-oriented h-LMO and h-YMO films prepared by pulsed laser deposition method. b) In-plane XRD phi-scan spectra of h-LMO, hYMO, and Pt layers. c) Unit-cell crystal structure of the hexagonal manganite (h-RMO) having the polar P63cm symmetry, where blue circles denote Mn ions, red circles for oxygen ions, and larger gold circles designate R (rare-earth) ions. d) Polarization-electric field (P−E) hysteresis loops obtained at 300K. Herein, the ac-frequency used to obtain the P-E loops is as high as 2 kHz to effectively remove possible contributions from mobile space charges. As depicted in Figure 1c, the hexagonal cell consists of the alternative stacking of two distinct layers: one layer of corner-linked MnO5 bipyramids (pale blue color) and the other layer of trivalent rare-earth cations (R3+). According to ab initio calculations, the optimized crystal structure of h-RMO with the polar P63cm symmetry is characterized by (i) the RO8 units having trigonal D3d site symmetry and (ii) the MnO5 bipyramids with the D3h site symmetry.26-28 Oxygen ions sur-


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rounding the central R-ion in the RO8 unit fall into two distinct groups: two apical (axial) oxygen ions (OA) along the hexagonal c-axis and six oxygen ions (OI) located at two different triangular in-planes. In the polar P63cm structure, an asymmetric vertical shift of the R-ion with respect to the two neighboring OA ions along the c-axis is primarily responsible for the off-centering hexagonal ferroelectricity in h-RMO.26-28 The polarization-electric field (P-E) hysteresis curve shows that the heteroepitaxial h-LMO film (150-nm-thick) grown on a Pt(111)/sapphire(0001) substrate is ferroelectric at room temperature (blue line; Figure 1d). The c-axis component of the remanent polarization (Pr), as obtained from the PE hysteresis loop, is ~5 with the coercive field (Ec) of ~600 at 300 K. As shown in Figure 1d, the P-E hysteresis loop of h-YMO is very similar to that of h-LMO with nearly the same Pr and Ec. This Pr value of the epitaxial hYMO film is slightly larger than the previously reported values (3~3.9 ).29,30 According to our recent ab initio study,26 the ferroelectric polarization of h-LMO originates from the coupling between the nonpolar zone-boundary K3 phonon and the polar zone-center Γ phonon. The observed same values of Pr (Figure 1d) suggests that the degree of the Γ coupling is rather insensitive to the nature of rareearth ion (R) in h-RMO. 3.2. Bandgap of h-RMnO3 Ultraviolet-visible-near-infrared absorption spectra of h-LMO and h-YMO films are shown in Figure 2a as a function of the

Figure 2. a) Ultraviolet-visible-near infrared absorption spectra of h-LMO and h-YMO thin films. b) Tauc plots of h-RMO films for the indirect bandgap transition (left-hand side) and for the direct bandgap transition (right-hand side). c) Ab initio computed band structures of h-LMO (left-hand side) and hYMO (right-hand side) along high-symmetry k-points, Γ-KM-L-A-Γ. photon energy, which indicates the onset of absorption at the photon energy of ~1.5 eV. Then, the bandgaps of h-RMO are evaluated by adopting the Tauc plot of absorption spectra. The Tauc model is represented by / ∝ , where is the absorption coefficient, E is the photon energy (), is the optical bandgap energy, and is the photon-energydependent constant.31 The power-law exponent, n, depends on

the transition type, where n = 1/2 for a direct bandgap transition and n = 2 for an indirect bandgap transition. According to our Tauc-model fitting shown in Figure 2b, both direct and indirect transitions properly describe the absorption data in the vicinity of . In the case of an indirect bandgap transition, the value is determined by plotting / as a function of the photon energy and extrapolating / to 0 (the left-hand side of Figure 2b). For a direct bandgap transition, on the contrary, the value is determined by plotting as a function of the photon energy and extrapolating to 0 (the right-hand side of Figure 2b). The discrepancy in between these two distinct plots is small. For both h-LMO and h-YMO, this discrepancy is as small as 0.05 eV. Thus, the value is little dependent on the transition type. Since the transition nature of h-RMO is not clearly elucidated yet, it is reasonable to assign the following values of with the uncertainty limit deduced from these two distinct types of transitions, 0.025eV (=0.05/2): ! 1.555 0.025 % for h-LMO and ! 1.505 0.025 % for h-YMO (Figure 2b). Thus, both hexagonal manganites possess a nearly ideal bandgap opening needed for enhanced photocurrent generation. According to the previous studies, the peak at ~1.7 eV (Figure 2a) can be attributed to either (i) the transition from the occupied hybridized O 2p state with the &'( /&') () orbitals to the unoccupied Mn &* )+ ) state32 or (ii) the d-d transition between Mn 3&')() ,'( and 3&* )+ ) states.33 The computed electronic band structures of h-LMO and h-YMO are presented in Figure 2c. Herein, we have adopted the GGA+Ueff method34 with Ueff of 8.0 eV to evaluate the exchange-correlation functional. According to the computed band structure of h-LMO, the energy gap at the Brillouin zone center (Γ-point) is 1.48 eV. On the other hand, the value for the M-Γ indirect bandgap transition is 1.45 eV (the left-hand side of Figure 2c). (the left-hand side of Figure 2c). Similarly, the computed value of h-YMO is 1.38 eV for the direct zone-center transition and 1.32 eV for the K-Γ indirect bandgap transition (the right-hand side of Figure 2c). According to the Bloch’s selection rule for an interband transition, a vertical transition is preferred. Thus, the direct interband transition at the Brillouin zone center is expected to primarily account for the optical absorption intensity although the indirect M-Γ transition (in h-LMO) or K-Γ transition (in h-YMO) precedes this direct interband transition at a slightly lower energy. 3.3. Switchable Photovoltaic Responses To examine the ferroelectric photovoltaic (FPV) responses of h-RMO films, we have fabricated a solar cell having an ITO/h-RMO/Pt hetero-junction structure (Figure 3a), where ITO denotes a transparent indium tin oxide top-electrode layer. Two opposite electrical-poling directions were used to examine the switchable photovoltaic effect: ‘upward poling’ signifies the application of a positive voltage to the bottom electrode (Pt), whereas ‘downward poling’ denotes the application of a negative voltage to the bottom electrode. To ensure a complete polarization switching by the poling, we applied an electric field of ~ 1.4 MV cm-1 (Table 1) which is much stronger than the coercive field (Ec), ~0.6 MV cm-1 (Figure 1d). To find the film thickness corresponding to the optimum photovoltaic performances, we have examined the J-V responses of the h-LMO thin films having four different film thicknesses. For doing this, we have employed the same upward poling for all four h-LMO thin films.



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Figure 3. a) A schematic representation of the ITO/h-RMO/Pt hetero-junction device. b) Thickness dependent J-V characteristics of the ITO/h-LMO/Pt devices. c) Jsc and Voc plotted as a function of the h-LMO layer thickness.

Figure 4. J-V characteristics of a) h-LMO and b) h-YMO devices under AM 1.5G illumination. Herein, black filled circles represent the J-V data under the dark-current condition. On the other hand, filled diamond squares denote the J-V data under the upward poling (abbreviated as ‘Up Pr’), and open triangles for the downward poling (or ‘Down Pr’). c) Zero-bias photocurrent density of hLMO and h-YMO devices plotted as a function of time. Figure 3b displays the effect of the film thickness on the J-V responses. Detailed experimental values of the thickness-dependent photovoltaic parameters are summarized in Table 1. In case of conventional p-n junction solar cells, it is well known that Voc is restricted by Eg and Jsc increases, in general, with increasing Voc according to the Shockley equation36: - ! -. /exp 3

4567 89 :

; 1
denotes thermal energy. However, as shown in Figure 3c, our h-LMO films do not follow the above equation, which is frequently observed in FPV thin films. In the present case, Jsc increases gradually with decreasing film thickness (t) for ? @ 150 nm. This tendency is also observed in La-doped PZT thin films and can be attributed to the enhanced internal field with decreasing thickness, which is a combined effect of the depolarization field and the Schottky-junction barrier.6 On the other hand, Voc is nearly independent of the film thickness for ? @ 150 nm, which presumably reflects the decrease in the depolarization-field strength with increasing film thickness, yielding ! AB ∙ ? D constant, where AB is the magnitude of the depolarization field. On the contrary, both Jsc and Voc decrease rather rapidly with decreasing thickness for ? E 150 nm. This type of observation was previously attributed to the decrease in the photovoltaic efficiency for the film thickness smaller than the width of an electron-depletion region which is formed by the Schottky contact at the ITOferroelectric layer interface.35 However, this does not apply to the present case as the ITO/h-LMO interface face is represented by an Ohmic-type contact with a small built-in potential

(See the next section for detailed discussion). On the other hand, the estimated minimum depth required for the maximum light absorption (~1.7 eV as shown in Figure 2a) is ~200 nm [! 1⁄α D 0.5 J 10K ]. Thus, for ? E 150 nm, the hLMO film is not able to absorb a sufficient amount of the visible light, which consequently leads to the pronounced decrease in Jsc and Voc with decreasing film thickness, as shown in Figure 3c at t = 70 nm. According to the thickness-dependent J-V responses (Figure 3c), the thickness required for the optimum photovoltaic performances is thus ~150 nm. Accordingly, our subsequent photovoltaic experiments were carried out using the hRMO films having the thickness of 150 nm. As shown in J-V curves, for both h-LMO and h-YMO films, the photocurrent direction depends on the poling direction (Figure 4a and b), showing a switchable ferroelectric photovoltaic response in hRMO. Figure 4c presents the time-dependent photocurrent under a zero bias-voltage. The ON and OFF states are repeatable, which clearly demonstrates the photo-induced current in the absence of any bias field. As presented in Table 2, the PCE of the h-LMO device under the upward poling is 0.11 % (under AM 1.5G illumination) which is 1 to 3 orders higher than those of classical ferroelectric photovoltaics such as pure (undoped) BFO and PZT.2,8,17,25 Similar to h-LMO, h-YMO is also characterized by the switchable ferroelectric photovoltaic effect (Figure 4b). Another prominent feature of the h-RMObased solar cells is that the PCE under the upward poling is remarkably higher than that under the downward poling (Table 2). These asymmetric photovoltaic responses (PCE, Jsc and Voc) between the upward poling and the downward poling can


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be attributed to either (i) difference in the Schottky-barrier height between the top and bottom interfaces (i.e., the net built-in potential) or (ii) asymmetric spatial distribution of defects, typically oxygen vacancies, in the ferroelectric layer.37

Table 1. Thickness-dependent photovoltaic parametersa of hLMO devices under AM 1.5G illumination. Thick ness

a

Poling Field -1

JSC

VOC

FF

PCE

(mA cm-2)

(V)

(%)

(%)

(nm)

(MV cm )

70

1.3

0.42

-0.48

26.6

0.054 ± 0.003

150

1.4

0.52

-0.71

29.8

0.110 ± 0.004

300

1.5

0.39

-0.68

27.4

0.073 ± 0.005

500

1.3

0.29

-0.74

29.5

0.063 ± 0.004

Average photovoltaic efficiencies for 3 to 4 different ITO electrodes.

Table 2. Switchable photovoltaic parameters of h-LMO and hYMO devices (150-nm-thick) under AM 1.5G illumination Device

JSC

VOC

FF

PCE

(mA cm-2)

(V)

(%)

(%)

Up Pr

0.52

-0.71

29.8

0.110 ± 0.004

Down Pr

-0.34

0.41

30.3

0.042 ± 0.005

Up Pr

0.55

-0.66

29.7

0.108 ± 0.005

Down Pr

-0.31

0.32

28.8

0.029 ± 0.003

Polarization

h-LuMnO3

h-YMnO3

3.4. Characteristic Energy Levels and Dark J-V Curves To understand the observed asymmetric switchable photovoltaic responses in terms of the energy band diagram, we have first extracted all the characteristic energy levels relevant to the ITO/h-RMO/Pt solar cell: (i) the work function L (or the Fermi level, M for h-RMO), (ii) the valence-band edge (Ev), and (iii) the conduction-band minimum (Ec) from ultraviolet photoelectron spectra (UPS). The three distinct types of samples were examined for this purpose: (i) an ITO layer deposited on h-RMO/Pt/Al2O3, (ii) two distinct h-RMO films deposit-

ed on Pt/Al2O3, and (iii) a Pt layer on Al2O3. The work function (L) is evaluated by using the four Ecut-off values presented in UPS (Figure S1a in Supporting Information) and by subsequently applying these values to the following equation: L ! 21.22eV (He I) – Ecut-off. The results are: L ! 4.40 eV for ITO, L ! 5.30 eV for Pt, and L! M ! 4.62 eV for h-LMO and 4.63 eV for h-YMO (See Figure S1b for details). For the hLMO or h-YMO layer, O M , thus O value, is determined by a linear extrapolation of the low binding-energy region of UPS (Figure S1a).38 Finally, the electron affinity, Ec, can be evaluated by using the previously estimated bandgap (P O ) and O . Figure S1b graphically summarizes all the estimated characteristic energy levels. Dark J-V characteristics were examined to obtain information on the electrical nature of the two contacts relevant to the ITO/h-LMO/Pt hetero-junction cell. From the extracted values of L (Figure S1), one can predict a Schottkytype barrier at the Pt/h-LMO interface with an upward band bending but an Ohmic-type contact at the ITO/h-LMO interface with a downward band bending since LQ:R (4.40eV) < LSTUR (4.62eV) < LVW (5.30eV). Exactly the same conclusion can be drawn for the ITO/h-YMO/Pt cell as LSTUR (4.62eV) is nearly the same as LSXUR (4.63eV). A proposed band diagram for the ITO/h-RMO/Pt hetero-junction without electrical poling (a virgin sample) is presented in Figure 5a. The upward band bending at the dielectric h-RMO side of the Pt/h-RMO contact accompanies with the appearance of an electrondepletion region, thus, forming a resistive Schottky-type contact. On the contrary, the downward band bending at the hRMO side of the ITO/h-RMO contact induces the formation of an Ohmic-type contact featured by an electron-rich anti-barrier region (Figure 5a). A dark J-V curve analysis experimentally confirms these two distinct types of contacts in the virgin ITO/h-LMO/Pt cell (no poling). As presented in Figure 5b, both positive and negative branches of the J-V curve for the symmetric Pt/h-LMO/Pt junction show a nonlinear J-V response, indicating the presence of a Schottky-type barrier at the h-LMO side of two Pt/hLMO contacts. On the contrary, the positive branch of the asymmetric ITO/h-LMO/Pt junction shows a linear J-V response (Figure 5c) which indicates an Ohmic-type contact at the ITO/h-LMO. Thus, the dark J-V curve analysis clearly supports the schematic energy band diagram presented in Figure 5a.

Figure 5. A schematic band diagram and associated dark J-V curves. a) A proposed energy band diagram for the ITO/h-RMO/Pt cell without poling (a virgin sample). b) A dark J-V curve of the symmetric Pt/h-LMO/Pt junction cell. c) A dark J-V curve of the asymmetric ITO/h-LMO/Pt junction cell.



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Figure 6. Three distinct energy band diagrams across the ITO/h-RMO(150nm)/Pt hetero-junction for a) the virgin state (without poling), b) the up-polarization state (under upward poling), and c) the down-polarization state (under downward poling).

3.5. Modulated Energy Band Diagrams The schematic energy band diagram presented in Figure 5a can further be substantiated by exploiting the characteristic energy levels obtained previously ( L, M , O & P in Figure S1b). Figure 6a shows the energy band diagram of the virgin ITO/h-RMO/Pt hetero-junction cell (without poling), which is supplemented by numerical values for all the necessary energy levels. The Schottky barrier height Φ at the Pt/h-RMO junction is evaluated by ΦVW/[UR ! LVW L[UR ! 5.30 4.62 ! ^0.68 %. On the other hand, the Ohmic anti-barrier depth at the ITO/h-RMO contact is given by ΦQ:R/[UR ! LQ:R L[UR ! 4.40 4.62 ! 0.22 % (Figure 6a). Then, the overall energy difference between the two distinct junctions ( ∆ ) is given by ∆ ! aLVW P b aLQ:R P b ! LVW LQ:R ! 5.30 4.40 ! 0.90 %. This value is actually equal to the barrier-height difference (or called ‘the net builtin potential’) since ΦVW/[UR ΦQ:R/[UR ! ^0.68 0.22 ! ^0.90 % ! ∆. Due to this energy gradient caused by the non-zero net built-in potential, an internal biasfield develops along the cell. Accordingly, the photogenerated electrons tend to migrate to the ITO/h-RMO interface, whereas the photo-generated holes move towards the Pt/h-RMO interface. It is known that the ferroelectric polarization substantially alters the band structure of a FPV cell.3,37,39,40 Figure 6b schematically depicts the band diagram under the upward poling (obtained by applying a positive bias voltage to the bottom electrode). Under this condition, the polarization head is oriented towards the ITO/h-RMO interface. Under the upward poling, thus, the direction of the depolarization field (Edp) is parallel to the direction of the unswitchable net internal bias-field (Ebi-bottom+ Ebi-top), which accompanies with a substantial increase in the barrier height at the h-RMO/Pt interface as dΦVW/[UR ⁄de D fghi ^ gjkjlmmln o ≡ Enet. This results in the enhanced degree of band bending, thus, energy gradient, under the upward poling (Figure 6b). The enhanced degree of band bending, in turn, leads to an enhanced photocurrent density and eventually an increased PCE (Table 2). Under the downward poling, on the contrary, the polarization head is oriented towards the h-RMO/Pt interface (i.e., ‘DownPolarization’ in Figure 6c). This leads to a remarkable reduction in the barrier height, which consequently transforms a

resistive Schottky barrier at the h-RMO/Pt junction to an Ohmic contact. On the contrary, an Ohmic-type contact becomes a Schottky-type barrier at the h-RMO/ITO interface under the same downward poling (Figure 6c). This reverses the direction of the energy gradient, thus, the direction of photo-generated currents. Under the downward poling, however, the magnitude of the energy gradient becomes smaller owing to the significantly reduced net internal electric field (Enet). This is because the depolarization field (Edp) is now antiparallel to the unswitchable net built-in field (Ebi-bottom+ Ebi-top), producing a significantly reduced Enet. This leads to a reduced photocurrent density and consequently a reduced PCE value under the downward poling, which accounts for the observed difference in the PCE between upward and downward poling (Table 2). Similar to the polarization flipping by the electrical poling (Figure 6b and c), high-concentration oxygen-vacancy defects can also modulate the energy band and, thus, influence on the switchable ferroelectric photovoltaic effect by reducing the barrier height at the interface. This modulation would be possible by the migration of oxygen vacancies to the polarization-head direction during the electrical poling.3,37,41 According to our estimate based on the x-ray photoelectron spectrum, however, the concentration of oxygen vacancies in the present h-LMO film is less than 3 at.% (See Figure S2 in Supporting Information). Essentially the same conclusion was obtained for the h-YMO film. Thus, the modulation of the energy band and the switchable photovoltaic effect are arising from (i) the net built-in potential caused by the barrier-height difference and (ii) the depolarization field by the switchable polarization (Figure 6b and c), rather than from the migration of oxygen vacancies. 3.6. Switchable Depolarization Field Versus Built-in Internal Field The contribution of the switchable ferroelectric polarization can be estimated by separating the total photovoltaic response into two independent components: (i) the switchable opencircuit voltage component (Vdp) mainly caused by the depolarization field (or switchable polarization) and (ii) the unswitchable voltage component (Vbi) arising from the nonferroelectric internal bias field. Both Vdp and Vbi can be estimated by using the following equations.8


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AB ! |q | and rs ! |q ^ |

(2)

where q and are the open-circuit voltage obtained after positive poling and negative poling, respectively. Thus, Vdp and Vbi values of the h-LMO film can be separately estimated using the above equations and the photovoltaic data shown in Figure 4a (or Table 2). They are: AB ! |0.71 0.41| !

0.56 and rs ! |0.71 ^ 0.41| ! 0.15 . Thus, for the h-LMO film, the contribution from the switchable polarization to the photo-voltage is about 3.7 times bigger than that from the unswitchable internal field. Similarly, Vdp and Vbi values of the h-YMO film are (from Table 2): AB ! |0.66 0.32| ! 0.49 and rs ! |0.66 ^ 0.32| ! 0.17 . In this case, the contribution from the switchable polarization is about 2.9 times bigger than the nonferroelectric contribution. Thus, for both hexagonal films, the contribution from the switchable ferroelectric polarization (or depolarization field) dominates over the contribution from the unswitchable (nonferroelectric) internal bias field which stems from the net built-in potential (i.e., barrier-height difference).

■ ACKNOWLEDGMENT This work was supported by Pohang Steel Corporation (POSCO) through Green Science Program (Project No. 2015Y060) and by the National Research Foundation (NRF) Grant funded by the Korea Government (MSIP) (Grant No. 2013R 1A2A2A 01068274).

4. CONCLUSIONS We have presented a new class of FPV by demonstrating (i) a nearly ideal bandgap of ~1.55 eV and (ii) the ferroelectric polarization switching in the heteroepitaxially grown rareearth manganite thin films, h-LuMnO3 and h-YMnO3. According to the thickness-dependent J-V responses, the ITO/hLuMnO3/Pt hetero-junction solar cell shows its optimum PCE of ~0.11 % when the thickness of ferroelectric h-LuMnO3 layer is ~150 nm. We have shown that the PCE of the hRMnO3 is 1 to 3 orders higher than those of classical ferroelectric photovoltaics such as undoped Pb(Zr,Ti)O3 and BiFeO3 under the condition of standard AM 1.5G illumination. We have further shown that the depolarization-field effect caused by the switchable ferroelectric polarization dominates over the unswitchable internal bias-field effect which is arising from the net built-in potential developed in the ITO/h-RMnO3/Pt junction structure. We judge that the present discovery of hRMO-based photovoltaics may open a new avenue to searching for oxide ferroelectrics towards high-efficiency solar cell applications.

■ ASSOCIATED CONTENT Supporting Information. Brief statement in nonsentence format listing the contents of the material supplied as Supporting Information.; UPS(Ultraviolet photoelectron spectroscopy) spectra and an energy level diagram, XPS(X-ray photoelectron spectroscopy) data. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author * E-mail : [email protected]

Notes The authors declare no competing financial interest.



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