This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: Chem. Mater. 2019, 31, 947−954 pubs.acs.org/cm Band Gap Tuning of Solution-Processed Ferroelectric Perovskite BiFe1−xCoxO3 Thin Films Pamela Machado, Mateusz Scigaj, Jaume Gazquez, Estel Rueda, Antonio Sań chez-Díaz, Ignasi Fina, Martí Gibert-Roca, Teresa Puig, Xavier Obradors, Mariano Campoy-Quiles, and Mariona Coll* Institut de Ciència de Materials de Barcelona ICMAB-CSIC, Campus UAB Bellaterra E-08193, Barcelona , Spain * S Supporting Information ABSTRACT: Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO3 is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal− oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe1−xCoxO3 thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide. 1. INTRODUCTION exhibit very low photocurrents at air mass 1.5 global conditions All-oxide photovoltaics (PV) is an appealing and versatile (1<μA/cm2).21,22 The wide band gaps in ferroelectric approach to help overcome the most persistent challenges in perovskites, ABO3, are due to a large difference in electro- the current PV technologies: to increase the power conversion negativity between the transition metal cation (B) and the efficiencies (PCEs) and to lower cost processing with stable oxygen (O).23 Therefore, modifying the B−O bond properties and eco-friendly elements. Indeed, oxides are progressively can allow to effectively tune the band gap of the material.24 being integrated in solar cells as passivation components, Lowering the Eg and preserving the ferroelectric properties selective layers, and transparent conducting electrodes and to a are an appealing route to obtain photovoltaic devices with less extent as light harvesters.1−4 Recently, semiconductor higher PCE. BiFeO3 (BFO) is among the most attractive lead- ferroelectric oxides with a perovskite structure have received a free perovskite oxide materials to be used as the photoactive great deal of attention because they can be used as photoactive layer in ferroelectric photovoltaic devices. It is a ferroelectric layers,5−8 inspired by the rapid increase in PCE (up to 23.7%) material at room temperature with high saturation polarization of the analogous lead halide perovskites.9−13 An exceptional along the pseudocubic [111] direction (90 μC/cm2) and property of ferroelectric perovskite oxides is that its non- presents a relatively narrow band gap of 2.7 eV.25 Additionally, centrosymmetric structure provides a unique route to having a relatively high index of refraction, n,26,27 enhances spontaneously separate charge carriers, known as bulk light confinement and, as a result, increases photoabsorption.28 photovoltaic effect, achieving extremely large above band gap Within the various approaches to tune the Eg,29−32 open-circuit voltages on a single-phase material.14,15 The compositional tuning appears to be one of the most aforementioned aspect should allow to speculate that the PCE successful.33−35 Using this approach, a record PCE of 3.3% beyond the Shockley−Queisser limit might be obtained.16−18 has been reported in Bi2FeCrO6 (8.1% in multilayer Additionally, they possess a spontaneous electric polarization architectures)23,36 and 4.2% in BiMnO3-based composite which allows controlling the direction of the PV current.19 It is films,37 although they present weaker ferroelectricity than worth to note the high stability at ambient conditions of such BFO. Despite the fast progress in ferroelectric perovskite metal oxides, as well as their capability to be manufactured by oxides, there are still important fundamental issues that need to low-cost methodologies.20 Nonetheless, most of the conven- tional ferroelectric oxides with a perovskite structure typically Received: October 15, 2018 absorb in the ultraviolet energy range (band gap Eg ≈ 3−4 eV, Revised: January 15, 2019 far from the ideal value of ∼1.4 eV for the maximum PCE) and Published: January 16, 2019 © 2019 American Chemical Society 947 DOI: 10.1021/acs.chemmater.8b04380 Chem. Mater. 2019, 31, 947−954 Chemistry of Materials Article be investigated. In particular, efforts to develop new material humidity) to obtain thin films with a final thickness of ∼10 nm. compositions to lower the band gap and enhance the electrical Subsequently, a crystallization treatment at 900−1000 °C for 10−30 polarization via cost-effective and scalable processing methods min under O2 atmosphere was performed as described elsewhere (see are required. Chemical solution deposition (CSD) is a well- Figure S7).49 2.3. Thin-Film Characterization. 2.3.1. Structure Character- established low-cost route to prepare high-quality multicationic ization. The crystalline structure and phase purity of BFCO films oxides with a wide variety of structural phases including were studied by means of X-ray diffraction (XRD) using Siemens metastable materials.20,38,39 In particular, CSD has already Diffractometer D-5000 with Cu Kα λ = 1.5418 Å for θ−2θ scan been used to fabricate epitaxial ferroelectric BFO films.40,41 analysis in the range 20°−80°. Reciprocal space maps (RSMs) were Therefore, solution-processed BFO films are an attractive acquired using Bruker D8 Discover with 2D GADDS (General Area starting point to explore the full potential of band gap Detector Diffraction System) with Cu Kα λ = 1.5418 Å. tunability via cation substitution for increased solar PCEs at Scanning transmission electron microscopy (STEM) and electron reduced costs. energy loss spectroscopy analysis (EELS) were performed using a Nion UltraSTEM equipment from Oak Ridge National Laboratory, In this work, we investigate the chemical substitution of the USA, operated at 100 kV and equipped with a fifth-order Nion transition metal in CSD-BFO by cobalt ions to judiciously aberration corrector and an Enfina EEL spectrometer. engineer the optical band gap and examine its impact on the 2.3.2. Surface Morphology Characterization. Surface morphology ferroelectric properties and the photoresponse. The incorpo- and root-mean-square roughness (rms) were studied from topography ration of Co in BFO is predicted to display high ferroelectric images acquired from an atomic force microscopy (AFM) Agilent polarization (P) reaching 170 μC/cm2 for BiCoO3.42 Also, the 5100 instrument in dynamic mode. addition of cobalt in BFO can affect the transition metal− 2.3.3. Optical Characterization. Variable-angle spectroscopic oxygen bond characteristics and ultimately the film absorption, ellipsometry (VASE) measurements were carried out using a a fact that, to the best of our knowledge, has been barely SOPRALAB GES5E rotating polarized ellipsometer, which uses a Xe lamp as the light source and a charge-coupled device detector. investigated.43−46 Previous studies showed that the stabiliza- Ellipsometric spectra were collected at room temperature, and the tion of BiFe1−xCoxO3 (BFCO) phases is limited to a rather analyzer was fixed at 20° under different angles of incidence (70°, 72°, narrow growth window requiring high-pressure synthetic and 75°) in the energy range of 1.2−5 eV. Acquired data (ψ and Δ) conditions.47,48 Here, we use low-cost and scalable chemical were analyzed using WinElli II piece of software. The sample was solution to form stable and pure BFCO phase by epitaxial modeled with the structure void/rough layer/bulk film/substrate, and growth with thickness up to 100 nm. Varying stoichiometry the Tauc−Lorentz model was used to describe the optical properties enables tuning the band gap from 2.7 to 2.3 eV while of the film (see Figure S3). The rough layer assumes a medium preserving a remarkably large polarization Pr = 60 μC/cm2 at consisting of 50% air and 50% film, whose effective dielectric function room temperature. Photoresponse measurements at 520 nm is modeled using the Bruggeman approximation. The absorption coefficient, α, was calculated from the equation α = 4πκ/λ. The film show a factor four improvement in the short-circuit current thickness was set to 20 nm, obtained by X-ray reflectometry, and the when cobalt is incorporated with respect to the BFO system. roughness layer value was varied in each composition according to the This trend is also reflected in the external quantum efficiency AFM analysis (Figure S8). (EQE). With this comprehensive study, we demonstrate not 2.3.4. Surface Chemical Composition Characterization. X-ray only the complexity but also the potential of this system for photoelectron spectroscopy (XPS) with a Mg Kα source (1253.6 eV) future light-harvesting applications. and a pass energy of 160 eV for survey spectra and 20 eV for high- resolution spectra was measured at the Instituto de Nanociencia de Aragon (INA), Zaragoza, Spain. To compensate charging effects, all 2. EXPERIMENTAL SECTION spectra were calibrated with respect to the C(1s) peak at 284.9 eV. 2.1. Preparation of BFCO Thin Films. Stoichiometric amounts 2.3.5. Electrical Characterization. In order to perform electrical of hydrated bismuth nitrate, Bi(NO3)3·5H2O, iron nitrate, Fe(NO3)3· measurements, 10 nm thick Pt contacts were ex situ deposited using a 9H2O, and cobalt nitrate, Co(NO3)2·6H2O were weighted and dc sputtering via a shadow mask forming an array of contacts of about dissolved in a solvent blend of 2-methoxyethanol and acetic acid to 50 × 50 μm2 each. Hence, the obtained metal films have the obtain 0.25 M precursor solutions.40 When high Co concentrations transparency of about 30% within the used range of laser wavelengths. were added, additional heating (50 °C) and stirring were applied to The ferroelectric characterization was performed at room temperature the precursor solution to obtain a homogeneous solution free of using an aixACCT TFAnalizer2000 system with a virtual ground precipitates. The viscosity of all 0.25 M BFCO precursor solution was method. The voltage is applied to the top Pt electrode, whereas the 0.003 mPa·s. BFCO thin films were prepared from spin-coating 15 μL bottom LSMO electrode is grounded. Ferroelectric loops were of BFCO precursor solutions on clean and preheated (70 °C) recorded using a triangular voltage waveform and the dynamic leakage monocrystalline (001)-strontium titanate (STO) substrates or current compensation method50 at the frequency of 2 kHz, after being La0.7Sr0.3MnO3 (LSMO)-buffered (001) STO substrates at 6000 precycled 10 000 times. The photoresponse was induced using a blue rpm for 1 min under N2 atmosphere with relative humidity <10%. sky research STEC multiwavelength system with a fixed parallel laser The spin-coated samples were exposed to a low-temperature thermal beam set at 44° angle to the sample surface. The available wavelengths treatment at 90 °C for 10 min and then at 270 °C for 4 min. were λ = 405, 450, 520, and 638 nm, whereas the power density was Subsequently, samples were introduced in a preheated tubular furnace varied and mentioned in the text. The photoresponse versus time was at 550−650 °C with a continuous O2 flow of 0.6 l min−1 for 30−60 measured using the TFAnalizer2000, whereas the I−V data were min and quenched to room temperature. This procedure leads to the recorded using the Keithley 2600B sourcemeter. EQE measurements film thickness of 20 nm. Thicker films (up to 100 nm) were prepared were performed with a supercontinuum white laser (Fiannium PM- by a multideposition process. SC) and a monochromator (Fiannium LLTF Contrast Vis-2), both 2.2. Preparation of LSMO Thin Films. LSMO metal−organic from Photon ETC. To electrically measure the cell, a Keithley 2450 solution of 0.1 M was prepared from stoichiometric amounts of Sourcemeter was used alongside a LabView 2015 programme to powder precursors of lanthanum acetate, (CH3COOH)3La, man- control all the pieces of the equipment and collect the data. An optical ganese acetate, (CH 3 COOH) 2 Mn, and strontium acetate, fiber was connected to the output of the monochromator, and the (CH3COOH)2Sr, dissolved in acetic acid and distilled water. A 15 other end was pointed toward the sample. After that, the sample was μL of 0.1 M metal−organic solution was spin-coated on (001)-STO illuminated with a range of frequencies, and the short-circuit current substrates at 6000 rpm for 1 min at ambient atmosphere (35−50% of was measured. 948 DOI: 10.1021/acs.chemmater.8b04380 Chem. Mater. 2019, 31, 947−954 Chemistry of Materials Article 3. RESULTS AND DISCUSSION 3.1. Structure of Solution-Processed BFCO Films. Solution-processed films obtained from nitrate precursors upon annealing result in pure phase and epitaxial (00l) BFCO for a range of cobalt substitutions starting from x = 0 to x = 0.3 (see the Supporting Information, Figure S1). High- resolution XRD RSMs for 20 nm pure BFO and BFCO with the maximum cobalt load (x = 0.3) (Figure 1) show two Figure 2. High-magnification Z-contrast images of (001)-oriented (a) BFO and (b) BFCO x = 0.3 on (100) STO substrates viewed along the [100]-crystallographic direction. Inset: background-subtracted EEL spectrum, extracted from a line scan spectrum image across the thickness of the film, showing the O K, Fe L, and Co L-edges. (α) dependence with photon energy (Figure 3b), it is found Figure 1. RSM around the (103) STO reflection for (a) BFO and (b) that the absorption edge monotonously shifts to lower energies BFCO x = 0.3 thin films. when increasing the Co content. Indeed, for x = 0.3, the absorption edge is at about 1.5 eV, well below the diffraction spots. The most intense one corresponds to (103) corresponding edge of pristine BFO films and in good STO and the weaker one corresponds to the (103) BFCO film, agreement with the theoretical data of the transition metal- confirming a heteroepitaxial growth. From the position of these substituted BFO.53 spots, the lattice constants of the films have been extracted Assuming a direct band gap for all the BFCO compositions, based on a pseudocubic symmetry. The in-plane lattice the optical band gap has been extracted from a linear constant of both films exactly matches that of the STO extrapolation of (αE)2 to zero and from this extrapolation, a substrate (a = 3.905 Å), whereas the out-of-plane lattice red shift is observed from 2.7 ± 0.1 eV for pure BFO to 2.3 ± constant increases from c = 3.976 Å for BFO to c = 4.047 Å for 0.1 eV for BFCO with x = 0.3 (Figure S4). This is the largest BFCO with x = 0.3. When increasing the film thickness, the band gap variation observed so far in BFO thin films by Co lattice parameter monotonously changes reaching a = 3.953 Å substitution.44,46,53 and c = 3.985 Å for 100 nm BFCO with x = 0.3. Lattice Figure 3c shows the extracted indices of refraction (n), constants for bulk BFCO system are reported for x = 0.1 and x indicating a ca 10% higher n for BFO and BFCO compared to = 0.2 (a ≈ 3.95 Å) and for x = 0.4 (a ≈ 3.75 Å).51 No the STO substrate. Pure BFO shows two spectral features at experimental data are available for bulk BFCO x = 0.3. The ∼2.8 and ∼4 eV that are linked to an absorption band peaking dramatic lattice constant change between x = 0.2 and x = 0.4 is at an intermediate energy (Kramers−Kronig consistency). attributed to a phase transition. Thus, according to these Previous SE investigations in unsubstituted BFO assigned values, it is likely that our films with x = 0.3 did not undergo a these features to charge-transfer transitions (M 3d−O 2p and phase transition. Further cobalt incorporation (x > 0.3 to x < M 3d−M 3d).31,54 The incorporation of cobalt slightly 0.9) enables epitaxial (00l) BFCO stabilization, although modifies the intensity and position of these two spectral segregated secondary phases are visible (see the Supporting features being more pronounced for BFCO with x = 0.3.50,51 Information, Figure S1). This is in good agreement with the Additional features are also seen in absorption and in n at reported difficulty to stabilize BFO with high cobalt content lower energies, in agreement with the aforementioned decrease without employing high-pressure synthesis.47,52 in the band gap. The modifications observed in the BFCO The crystalline quality of these two CSD-BFCO composi- spectra are believed to indicate that the metal-oxygen and tions was further investigated by STEM. This imaging mode metal−metal bonds have been altered upon cobalt substitution provides images where the intensity scales with the square of and consequently affect the band gap.29,43,53 the atomic number, referred to as Z-contrast images. Figure 2 3.3. Electrical Response of Solution-Processed BFCO shows a comparison of a Z-contrast image of 20 nm BFO and Vertical Devices. Figure 4 shows the ferroelectric hysteresis BFCO (x = 0.3) thin films. Both images show coherent loops (P−E) of ∼100 nm epitaxial BFCO thin films with x = 0, epitaxial films’ growth with a sharp interface with STO. BFCO x = 0.1, and x = 0.3 prepared on (001)LSMO//STO and films with x > 0.3 can present secondary phases at the film recorded in dark. Importantly, the P−E loops were also surface in agreement with XRD data (see Figure S2). The EEL recorded under illumination and overlap those acquired in spectrum acquired from the BFCO film shows the O K, Fe L, dark, ruling out the effects of photogenerated charges on and Co L-edges, confirming the presence of cobalt within the polarization.55 The shape of the ferroelectric loop at high BFCO film’s structure. applied electric fields is not well saturated due to the residual 3.2. Optical Properties of Solution-Processed BFCO contribution of the leakage current.56,57 The measured Films. The optical properties of the BFCO films were remnant polarization (Pr) and coercive field (Ec) for pure investigated by variable angle SE (Figure 3). Experimental BFO films are 26.1 μC cm−2 and 182 kV cm−1, respectively, data were fitted using the Tauc−Lorentz model (see being in agreement with previous reports of epitaxial CSD- Experimental Section and Figure S3). This study enabled to BFO films.40,58,59 Substitution of cobalt leads to enhanced Pr ≈ extract for the first time the complex refractive index (n and k) 60 μC cm−2 (50% increase) and larger Ec = 600 kV cm−1 (x = for the BFCO system. From the optical absorption coefficient 0.1) and 373 kV cm−1 (x = 0.3). It has been demonstrated that 949 DOI: 10.1021/acs.chemmater.8b04380 Chem. Mater. 2019, 31, 947−954 Chemistry of Materials Article Figure 3. (a) Optical microscopy images of the STO substrate, BFO, and BFCO samples. (b) Absorption coefficient, α, as a function of photon energy of BFCO thin films (x = 0 to x = 0.3) compared to STO. Bi2FeCrO6 and MAPbI3 (MA = methylammonium) band gaps are indicated with dashed lines for comparison. (c) Corresponding indices of refraction (n). lattice deformation in ferroelectric films such as BaTiO3 can impact the magnitude of the polarization.60,61 Similarly, here it is suggested that the incorporation of cobalt will promote lattice deformation enhancing the ferroelectric response. Photoresponse measurements have been carried out for BFO, as well as BFCO with x = 0.1 and x = 0.3. The samples have been measured with the contact configuration shown in Figure 5a. Here, we define the positive sign of the current (J), where positive carriers flow from the top Pt electrode to the bottom LSMO electrode (ground), as indicated by the arrows. I−V curves have been collected after applying prepoling voltages being different for each composition according to their ferroelectric coercive field found in the P(E) loops Figure 4. Ferroelectric hysteresis P−E recorded at 2 kHz at room temperature for BFO and BFCO thin films with different cobalt (Figure 4), namely, ±3 V for BFO, ±5 V for BFCO x = 0.1, contents. and ±7 V for BFCO x = 0.3 (Figure 5b). The observed linear J−V dependence is rather common in many ferroelectric Figure 5. (a) Scheme of the electrical configuration used to measure the photoresponse, (b) J−V curves for BFO and BFCO positively and negatively poled films illuminated under 405 nm, 1.5 W cm−2. (c) Time dependence of the short-circuit photocurrent for BFO and BFCO x = 0.1 and x = 0.3 films illuminated under 520 nm with a laser of 1.5 W cm−2. (d) EQE. 950 DOI: 10.1021/acs.chemmater.8b04380 Chem. Mater. 2019, 31, 947−954 Chemistry of Materials Article perovskite oxides and contrasts with the rectifying behavior of can show the typical rectifying behavior in structures such as high-performance solar cells.5,15,62 This characteristic can be Pt/BFO/SrRuO3/STO.19 The leakage current in BFO films is related to the dominant conduction mechanism that will be mainly attributed to deviations in oxygen stoichiometry and discussed below. Fe3+ to Fe2+ transition.70−72 XPS analysis has been performed Illuminating the BFO sample with a 405 nm wavelength in order to investigate the surface chemistry of BFO and laser and 1.5 W cm−2 intensity (below BFO band gap energy) BFCO films with x = 0.3 (see Figure S6). From the Fe 2p core- causes a drastic change in the I−V curve. The resulting short- level spectra, it is suggested that the predominant specie in all circuit current and open-circuit voltage are Jsc = 1.56 mA cm−2 the films are Fe3+. From the Co 2p core-level spectrum, the and Voc = −186 mV, respectively. The Voc is similar to those coexistence of Co3+ and Co2+ is suggested. Because no Co-rich reported in the high-quality PLD BFO/LSMO//STO secondary phases are identified in the structural analysis (i.e., system.63 Note that the negative Voc values irrespective of Co3O4, CoFe2O4), the presence of Co2+ is very likely to be the prepoling applied voltage result from the asymmetric compensated by the formation of oxygen vacancies, which device structure. Illuminating the cobalt-substituted BFO films could contribute to the Ohmic conduction identified above. with the same 405 nm wavelength laser results in average Jsc Clearly, boosting the efficiency in this system is still a values of 1.07 mA cm−2 (x = 0.1) and 0.4 mA cm−2 (x = 0.3) challenge and requires further investigation. Future steps to and Voc values of −61 mV (x = 0.1) and −13 mV (x = 0.3) unlock the full potential of the BFCO-based PV device might evidencing that both J sc and V oc decrease with the be achieved through optimizing microstructures (porosity and incorporation of cobalt. The decrease in Jsc is assigned to an surface precipitates) of the active material,59,73,74 performing increased leakage current observed when increasing the cobalt A-site cosubstitution and oxygen annealing to minimize doping (see Figure S5) as well as a small reduction in fluctuations of the oxygen stoichiometry,75−79 and performing absorption coefficient at this excitation wavelength (see Figure interface engineering through the incorporation of selective S4). On the other hand, the decrease in Voc may be due to a layers80−82 and transparent oxide electrodes83 to facilitate combination of factors including increased charge recombina- charge collection and extraction. tion due to defects, decrease in photocurrent, and lowering the band gap. Importantly, the magnitude of Jsc and Voc can be 4. CONCLUSIONS modulated by applying a voltage of a particular polarity. This In this work, we have demonstrated the viability of solution effect is stronger in BFCO compared to that in pristine BFO, processing to stabilize BFCO films by epitaxial growth. The consistent with the higher value of (switchable) remnant incorporation of cobalt up to x = 0.3 allows a band gap polarization seen in Figure 4. The different magnitude of the modulation from 2.7 to 2.3 eV while showing large photocurrent observed for the same sample under different ferroelectricity. The photocurrent response has been evaluated signs of the applied voltage indicates the presence of two using a relatively simple device geometry pursuing an oxide- remnant states.5,64 based heterostructure, Pt/BFCO/LSMO//STO. Upon cobalt Then, the photoresponse of the samples BFO, BFCO x = incorporation, the photocurrent is enhanced according to the 0.1, and BFCO x = 0.3 can be compared when illuminated band gap narrowing, although EQE is, however, still low with a laser of 520 nm (below the tuned band gap energy upon compared to state-of-the-art ferroelectric perovskite oxides. cobalt substitution) and without being prepolarized (Figure Further elemental chemical substitution investigations together 5c). The Jsc of BFCO is larger by a factor of four compared to with interfacial engineering should be performed to increase that of unsubstituted BFO, which is in agreement with the the efficiency. Overall, the BFCO system is a potential band gap decrease observed by the incorporation of Co. When photoactive material, and we believe it will provide a robust comparing the BFCO samples with different cobalt additions, platform to study the underlying physical properties of these the sample with Co x = 0.3 shows slightly lower photocurrent systems such as the correlation between cation substitution than the sample with Co x = 0.1. This could be attributed to and optical and electronic properties. The field of ferroelectric the higher leakage currents in x = 0.3 as mentioned above perovskite oxides is young and dynamic and is expected to (Figure S5). Finally, the EQE measurements further confirm contribute toward a stable, robust, and efficient photovoltaic that BFCO films exhibit an enhanced photocurrent, hence a (all-oxide) device . The band gap tuning possibilities could also higher photoresponse, compared to unsubstituted BFO offer new opportunities when combined with other conven- between 440 and 590 nm (Figure 5d). Incident photon to tional photovoltaic devices. charge carrier efficiency (IPCE) for BFCO films at 520 nm and 100 mW cm−2 is still low (0.02%); however, it is one order of magnitude higher than pristine BFO (<0.003%). It is well-known that BFO shows larger leakage currents ■ ASSOCIATED CONTENT * Supporting Information S compared to other perovskite oxides.65,66 We have evaluated The Supporting Information is available free of charge on the the dominant conduction mechanism by analyzing the J−E ACS Publications website at DOI: 10.1021/acs.chemma- characteristics at room temperature (see Figure S5b). From the ter.8b04380. linear dependence in the log J−log E plot, the data can be fitted with a slope close to 1 suggesting that in our working Additional XRD θ−2θ scans; AFM topographic images conditions, that is under ∼30 kV/cm, the dominant and n and k refractive index for BFCO with x = 0 to x = mechanism most likely is the Ohmic conduction (bulk- 0.9 thin films; table of calculated c parameters; STEM limited).67,68 In fact, former studies on epitaxial BFO films image of BFCO x > 0.3; leakage curves for BFO, BFCO have reported that the most common conduction mechanisms x = 0.1, and x = 0.3; XPS spectra of BFO and BFCO x = are bulk-limited (space-charge-limited current and Poole− 0.3; XRD and AFM of optimized BFO/LSMO//STO Frenkel emission), although interface effects cannot be films; and current density and IPCE versus laser photon neglected.65,69 Note that improved devices based on BFO energy of BFO films (PDF) 951 DOI: 10.1021/acs.chemmater.8b04380 Chem. Mater. 2019, 31, 947−954 Chemistry of Materials Article ■ AUTHOR INFORMATION Corresponding Author (11) National Renewable Energy Laboratory (NREL). https://www. nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181221.pdf, accessed 14/01/2019. *E-mail: mcoll@icmab.es. (12) Rappe, A. M.; Grinberg, I.; Spanier, J. E. Getting a Charge out ORCID of Hybrid Perovskites. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 7191− Mariona Coll: 0000-0001-5157-7764 7193. Author Contributions (13) Coll, M.; Gomez, A.; Mas-Marza, E.; Almora, O.; Garcia- The manuscript was written through contributions of all Belmonte, G.; Campoy-Quiles, M.; Bisquert, J. Polarization Switching authors. All authors have given approval to the final version of and Light-Enhanced Piezoelectricity in Lead Halide Perovskites. J. the manuscript. Phys. Chem. Lett. 2015, 6, 1408−1413. (14) Yang, M.; Bhatnagar, A.; Alexe, M. Electronic Origin and Notes Tailoring of Photovoltaic Effect in BiFeO3 Single Crystals. Adv. The authors declare no competing financial interest. ■ Electron. Mater. 2015, 1, 1500139. (15) Bhatnagar, A.; Roy Chaudhuri, A.; Heon Kim, Y.; Hesse, D.; ACKNOWLEDGMENTS Alexe, M. Role of Domain Walls in the Abnormal Photovoltaic Effect We would like to acknowledge Prof. Josep Fontcuberta for in BiFeO3. Nat. Commun. 2013, 4, 2835. fruitful discussions and comments on this study and for (16) Kim, D. J.; Alexe, M. Bulk Photovoltaic Effect in Monodomain providing access to his experimental facilities. This research BiFeO3 Thin Films. Appl. Phys. Lett. 2017, 110, 183902. was supported by the Spanish MINECO (“Severo Ochoa” (17) Spanier, J. E.; Fridkin, V. M.; Rappe, A. M.; Akbashev, A. R.; Programme for Centres of Excellence in R&D SEV-2015-0496 Polemi, A.; Qi, Y.; Gu, Z.; Young, S. M.; Hawley, C. J.; Imbrenda, D.; and FIP-NANOSOLE-2016, MAT2017-83169-R, MAT2014- et al. Power conversion efficiency exceeding the Shockley-Queisser 51778-C2-1-R, MAT2015-70850-P, MAT2017-85232-R limit in a ferroelectric insulator. Nat. Photonics 2016, 10, 611. (18) Alexe, M.; Hesse, D. Tip-Enhanced Photovoltaic Effects in (AEI/FEDER, EU)), the Generalitat de Catalunya (2017- Bismuth Ferrite. Nat. Commun. 2011, 2, 256. SGR-1519, 2017 SGR-1377), the European Research Council- (19) Lee, D.; Baek, S. H.; Kim, T. H.; Yoon, J.-G.; Folkman, C. M.; ERC (FOREMAT under grant agreement no. 648901), and Eom, C. B.; Noh, T. W. Polarity Control of Carrier Injection at “L’Oreal UNESCO For Women in Science”. M.C., J.G., and Ferroelectric/Metal Interfaces for Electrically Switchable Diode and I.F. acknowledge RyC contracts (2013-12448, 2012-11709, Photovoltaic Effects. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, and RYC-2017-22531). I.F. is grateful to MINECO for 84, 125305. MAT2015-73839-JIN project. The HRTEM microscopy (20) Bretos, I.; Jiménez, R.; Ricote, J.; Calzada, M. L. 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