M = Co, Ni, Fe, Mn; X = F, Cl, Br, I - ACS Publications - American

Article Cite This: J. Phys. Chem. C 2019, 123, 14303−14311

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A New Class of Bifunctional Perovskites BaMX4 (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I): An n‑Type Semiconductor with Combined Multiferroic and Photovoltaic Properties Jiawei He,† Hong-Jian Feng,*,† Qiang Zhang,† Zi-Xuan Chen,† Chong-Xin Qian,† Xiao-Wen Liang,‡ Yong-Hua Cao,† and Xiao Cheng Zeng*,§ School of Physics and ‡Library, Northwest University, Xi’an 710069, People’s Republic of China § Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States Downloaded via UNIV OF SOUTHERN INDIANA on July 21, 2019 at 18:16:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: We design a new class of bifunctional materials, namely, BaMX4 (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I), with corner-shared octahedral layered perovskite structures. Our joint experimental and theoretical study demonstrates that BaCoF4, a prototype in this new class, possesses a unique combination of multiferroic and photovoltaic properties. Moreover, BaCoF4 exhibits both an antiferromagnetic spin structure and ferroelectric polarization along the (001) direction. These combined electronic and optical features render BaCoF4 a promising bifunctional material for application in spintronic or photovoltaic devices. Density functional theory calculations suggest that the dominant point defects in BaCoF4 are mostly shallow-level donor defects, leading to fascinating n-type self-doping. As such, a BaCoF4 layer may be exploited for electron transport and light absorption altogether, which may enhance the photovoltaic performance in solar cells. To confirm this predicted feature, we incorporate BaCoF4 as an electron transport layer and fabricated a BaCoF4/ Cs0.05MA0.14FA0.81PbI2.55Br0.45-based solar cell device. Notably, the solar cell devices yield the champion power conversion efficiency of ca. 13.14%. We also investigate photovoltaic properties of other analogous materials and find that BaNiBr4 and BaMnCl4 also possess both multiferroic and photovoltaic bifunctionalities. Hence, this new class of BaMX4 offers diverse and tunable capability for various applications.

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INTRODUCTION Multiferroic materials, in which different ferroic orders such as ferroelectric, ferromagnetic, and ferroelastic orders coexist in the same phase,1,2 have captured significant research attention due to the novel existence of multiple ferroic degrees of freedom and the coupling between various ferroic orders.3 Multiferroic materials have a great potential in advanced device applications, for example, information storage, spintronics, and sensors.4,5 However, single-phase multiferroic materials are rare, largely because ferroelectric and magnetic orders are mutually exclusive of one another.6 To date, oxide perovskites have become a research hotspot as the multiferroic materials.7 Perovskite multiferroics such as BiFeO38−10 show ferroelectricity independent of the magnetic order since the two ferroic orders occur on different ions, thereby having little interaction among them. Other perovskite mutilferroics, for example, TbMnO311 and GdFeO3,12 display ferroelectricity that is related to the magnetic order where the spin−orbit coupling connects the spin configuration with the ferroelectric distortions. Transition-metal oxide perovskites can also offer a possibility to be new multiferroic materials. © 2019 American Chemical Society

In addition to the multiferroic applications, in recent years, hybrid organic−inorganic halide perovskites in the form of ABX3, where A represents a monovalent organic cation, for example, methylammonium (MA+) and formamidinium (FA+), B represents a divalent metal cation, for example, Pb2+, and X represents a halogen anion, for example, Cl−, Br−, and I−, have been widely investigated as ideal light harvesters.13−16 This is because hybrid organic−inorganic halide perovskites have exceptional optoelectronic properties such as large dielectric constant,17 high optical absorption coefficient,18 desirable and tunable band gap,18,19 low exciton binding energy,20 long carrier lifetime,21 large charge-carrier mobility,22 high tolerance for trap states,23 and bipolar behavior.24 However, large-scale applications of the perovskite photovoltaic techniques still face chemical and mechanical instability challenges including the instability against moisture/air and temperature, which mainly stem from the weak chemical Received: May 12, 2019 Revised: May 21, 2019 Published: May 22, 2019 14303

DOI: 10.1021/acs.jpcc.9b04502 J. Phys. Chem. C 2019, 123, 14303−14311

Article

The Journal of Physical Chemistry C

Figure 1. (a) Optimized crystal structure of BaCoF4. (b) Calculated magnetic structure of BaCoF4. (c) Calculated nearest-neighbor Heisenberg exchange coupling constants for BaCoF4 as a function of Ueff for the shown AFM magnetic structure.

materials with the layered perovskite structure may be good light-harvesting materials. Here, we investigate the multiferroic and photovoltaic properties of the prototype BaCoF4 compound by combining first-principles calculations and experimental measurements. The results show that BaCoF4 possesses desirable optoelectronic properties, including a desired band gap of 1.50 eV and broad light absorption in the visible range, rendering it a promising bifunctional material possessing both good multiferroic and photovoltaic features. We have also explored the defect properties of the bulk BaCoF4 and demonstrated that the intrinsic defects in BaCoF4 exhibit a significant n-type feature. In the experiment, considering the n-type self-doping of BaCoF4, we have fabricated bulk BaCoF4-based solar cells w i th the f o ll o wing arc h it ect ure: FTO/Ba Co F 4 / Cs0.05MA0.14FA0.81PbI2.55Br0.45/spiro-OMeTAD/Au. We have assessed the photovoltaic performance of the devices. Last, we report the investigation of phase stability and photovoltaic properties of BaMX4 (M = Mn, Fe, Co, Ni; X = F, Cl, Br, I) compounds.

bonding and inherent structural instability of the organic cation at the A site.25,26 Thus far, many research efforts have also been devoted to developing lead-free perovskites with high structural stability as light harvesters.27−29 Strategies include replacing Pb2+ with other divalent metal cations, for example, Ge2+ and Sn2+,30 or employing monovalent metal cations, for example, Cs+, instead of organic cations.31 Furthermore, new inorganic lead-free perovskite materials, such as CaZrS3,32 BaZrS3,32 Cs2TiBr6,33 Cs2AgBiBr6,28 and Cs2AgSbCl6,34 have been successfully synthesized. Most of these perovskites exhibit the corner-shared octahedral network structure. Some of these perovskites may be good candidates as light-absorbing materials. Since many multiferroic materials have perovskite structures with corner-shared octahedral network, designing a new class of bifunctional materials combining the advantages of both multiferroic and photovoltaic properties is highly plausible. Barium fluorides BaMF4 (M = Mn, Fe, Co, or Ni) are promising magnetic ferroelectrics that have attracted considerable attention since late 1960s.35−38 A previous experimental study demonstrated that the ferroelectric spontaneous polarizations (Ps) along the (001) direction are 8.0 and 6.7 μC/cm2 for BaCoF4 and BaNiF4, with the Curie temperatures of 1153 and 1593 K, respectively, although no obvious ferroelectric switching was found in BaFeF4 and BaMnF4.39 Moreover, the BaMF 4 (M = Mn, Fe, Co, Ni) compounds exhibit antiferromagnetic (AFM) structures with the Néel temperature (TN) in the range of 20−80 K.35 It is not precisely controllable to synthesize the quaternary silver-bismuth double perovskites Cs2AgBiX6 (X = Cl, Br, I),28,40−42 copper-antimony double perovskites Cs2CuSbX6,43 and chalcopyrite Cu[In,Ga]Se244 at relatively low temperatures through solution growth. Thus, ternary lead-free solar-absorbing materials are highly desired as solar devices with low toxicity. Indeed, since BaMF 4 compounds with the space group Cmc21 have the cornershared MF6 octahedral structures in the ac plane,45 this class of

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COMPUTATIONAL METHODS Our first-principles calculations were performed using Quantum ESPRESSO package (QE)46−48 and Vienna ab initio simulation package (VASP 5.4).49 The atomic structures, electronic structures, optical properties, and defect properties were computed using the VASP code. Generalized gradient approximation (GGA) in the form of Perdew−Burke− Ernzerhof (PBE) functional was used to treat the exchange− correlation interactions.50,51 The frozen-core projected augmented-wave (PAW) pseudopotential was used to describe the electron−core interactions.52 An energy cutoff of 400 eV was employed. The atomic structures of BaMF4 (M = Ni, Mn, Fe, Co; X = F, Cl, Br, I) were optimized using the experimental lattice parameters as an input, for which the atomic structures of BaMX4 compounds were fully relaxed until the maximum force on each atom was less than 0.02 eV/Å. A 6 × 2 × 4 k14304


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Figure 2. (a) Optical absorption spectrum of the BaCoF4 powder. The inset is the corresponding (Ahν)1/2 versus energy (hν) curve of BaCoF4. (b) Comparison between the theoretical and experimental optical absorption spectra of BaCoF4. (c) Electronic band structure of BaCoF4 calculated using the PBE functional. (d) The CBM-associated (left) and VBM-associated (right) charge densities. The yellow isosurface represents the charge density of 0.002e/Å3.

larger than the experimental value. The difference might have resulted from the difference between the theoretical and experimental crystal structures. The ferroelectric polarization arises from softening of a polar phonon mode, which does not lead to the charge transfer between the cations and anions. This geometric ferroelectricity stems from the size effect and structural distortions associated with the frozen, unstable polar phonon mode. Photovoltaic Properties of BaCoF 4 . The BaCoF4 powder was synthesized through the hydrothermal method. Experimental and calculated X-ray diffraction (XRD) patterns of BaCoF4 are displayed in Figure S1a. The experimentally measured positions of diffraction peaks are in good agreement with the theoretical results, showing that the synthesized BaCoF4 powder exhibits an ideal orthorhombic structure with the space group Cmc21. The high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) measurements of BaCoF4 are shown in Figure S1b. The measured interplanar spacing value of 3.665 Å is consistent with the spacing of the (040) plane in BaCoF4, further confirming the formation of BaCoF4 crystal. We then measured the absorbance of BaCoF4 powder to investigate the optical properties of BaCoF4. The normalized absorbance spectrum and corresponding Tauc plot of the BaCoF4 powder are presented in Figure 2a. Apparently, the BaCoF4 powder exhibits strong absorption in the 400−700 nm visible light region, and the optical band gap of ca. 1.49 eV can be obtained from the corresponding Tauc plot. A comparison of the measured optical absorption (red line) and theoretical optical absorption (black line) of BaCoF4 is shown in Figure 2b. The first experimental absorption peak (red arrow) lies at ca. 720 nm, while the theoretical one (black arrow) lies at ca. 1120 nm, leading to a 400 nm red shift of the calculated absorption spectrum. The BaCoF4 sample possesses two absorption peaks over the wavelength range of 520−1120 nm, consistent with the theoretical result.

point grid was adopted to optimize the atomic structures and calculate the electronic and optical properties. The lattice parameters of all these compounds are listed in Table S1. The defect properties were calculated using the 2a′ × 2b′ × 2c′ supercell of BaCoF4. The atomic positions of the defect structures were optimized until the total force on each atom was less than 0.02 eV/Å. The QE code was utilized to compute the multiferroic properties of BaCoF4. The local spin density approximation (LSDA) and LSDA + U scheme were adopted to describe the spin configuration.53,54 The electron−core interactions were described by the GBRV ultrasoft pseudopotential (USPP) in QE.55 The Hubbard parameter Ueff of the transition-metal d states was set to be 4 eV, following previous studies.56 The kinetic energy cutoff for wave functions was 40 Ry. A 6 × 2 × 4 Monkhorst−Park k-mesh was used to calculate the ferroelectric and magnetic properties. The spontaneous polarization was calculated by using the modern theory of polarization.57

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RESULTS AND DISCUSSION Multiferroic Feature of BaCoF4. The optimized crystal structure of BaCoF4 is shown in Figure 1a, where the Ba2+ cations are surrounded by distorted octahedra, and the CoF6 octahedra are separated by Ba2+ cations. As shown in Figure 1b, the DFT calculations suggest that antiferromagnetic (AFM) spin structures along the c axis in BaCoF4 and the magnetic moments of the nearest Co are antiparallel to each other, consistent with a previous study.53 Figure 1c shows the Heisenberg exchange coupling constant J for the nearest magnetic moments in the AFM structure of BaCoF4 versus the Hubbard parameter Ueff. J decreases rapidly as the Hubbard Ueff increases from 0 to 4 eV, implying that the spin interaction is reduced upon the increase in the effective Hubbard parameter. Furthermore, Ps along the (001) direction of BaCoF4 is calculated by using the modern theory of polarization. The calculated Ps of 10.6 μC/cm2 is slightly 14305


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Figure 3. (a) Calculated optical absorption spectra (based on the PBE functional) of BaCoF4. (b) Simulated theoretical maximal solar cell efficiency, that is, spectroscopic limited maximum efficiency (SLME) of BaCoF4 versus the film thickness.

Figure 4. (a) Calculated chemical ranges for the formation of BaCoF4 (middle green region). Two representative points A (F-rich) and B (Corich) are used for calculating the formation energies of point defects. (b) Calculated transition energy levels of donor-like and acceptor-like point defects in BaCoF4. (c) Calculated formation energies of point defects versus the Fermi level at point A (F-rich). (d) Calculated formation energies of point defects versus the Fermi level at point B (Co-rich).

To show the photovoltaic performance of BaCoF4 more directly, the spectroscopic limited maximum efficiency (SLME)58,59 is computed based on the calculated optical absorption spectrum within the range of 0−3.5 eV (Figure 3a). As shown in Figure 3b, at 3 μm film thickness, the SLME value of BaCoF4 is ca. 5.5%, much lower than typical photovoltaic materials, for example, prototype Si and MAPbI3 (Figure 7b), due to the relatively small absorption coefficient resulting from the indirect band gap and the weaker dispersions of the CBM and VBM. The relatively large absorption coefficients in the infrared and near-infrared regions, compared with those of prototype Si and MAPbI3, render BaCoF4 a good lightabsorbing layer in multijunction tandem solar cells to efficiently absorb the infrared light. Although the photovoltaic performance of BaCoF4 is not as competitive as those of prototype Si and MAPbI3, the concurrent multiferroic properties, an unusual feature not seen in traditional photovoltaic materials, render this material a promising bifunctional semiconductor. Previous studies demonstrated that the crystal symmetry has a significant influence on the optical absorption of materials.58,60 Hence, for the synthesized

To further examine the photovoltaic properties of BaCoF4, we simulated the electronic characteristics of the bulk BaCoF4 via DFT calculations. The electronic band structure of BaCoF4 illustrated in Figure 2c shows that the bulk BaCoF4 possesses an indirect band gap of 1.50 eV, consistent with the measured optical band gap. The partial charge densities corresponding to the conduction band minimum (CBM) and valence band maximum (VBM) of the bulk BaCoF4 are depicted in Figure 2d. The CBM originates mainly from the F 2s states, while the VBM is mainly composed by the Co 3d states. The CBM and VBM charge distributions are mostly localized on different atoms, which would assist the separation of electron−hole pairs. Moreover, since the effective masses of electrons (me) and holes (mh), which determine carrier mobility, have an important influence on the photovoltaic characteristics of materials, the me and mh of BaCoF4 are calculated (see Table S2). Due to the relatively small me and relatively large mh corresponding to the (010) direction, BaCoF4 possesses a good electronic transport property along the (010) direction when the material is used as the light-absorbing layer in solar cells. 14306


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cation-on-cation antisites BaCo and CoBa, both of them might exhibit acceptor and donor characters because of the same valence of Ba2+ and Co2+. It can be seen that the (0/1−) and (1−/2−) transition levels for BaCo are at 1.47 and 1.59 eV, respectively, showing deep-level acceptor states. Moreover, BaCo has a (1+/0) transition level at 0.15 eV below the CBM and a (1+/2+) transition level at 0.25 eV below the CBM, indicating that BaCo is also a deep-level donor defect. As a consequence, the BaCo defect makes less contribution to the ptype or n-type conduction. The same conclusion could be drawn for CoBa. For the three interstitials, Bai is considered as a shallow-level donor with a (0/1+) transition level at 0.05 eV and a (1+/2+) transition level at 0.15 eV below the CBM, while Coi exhibits deep transition levels within the band gap [i.e., ε(0/1 + ) = 1.32 eV and ε(1 + /2 + ) = 1.22 eV]. Fi should be a deep-level acceptor since the (0/−1) transition level is at 1.28 eV above the CBM. To assess the influence of the chemical potentials of component elements on the formation energies of point defects, two typical chemical potential points, A (F-rich/Copoor) and B (Co-rich/F-poor), were considered (see Figure 4a). The formation energies (ΔH) of point defects versus the Fermi level (EF) at chemical potential points A and B are shown in Figure 4c,d, respectively. Twelve intrinsic point defects, including three vacancies (VBa, VCo, and VF), three interstitials (Bai, Coi, and Fi), and six antisites (BaF, BaCo, CoBa, CoF, FBa, and FCo), were considered to ensure a systematic study. Since BaF and Fi are donor and acceptor defects with the lowest ΔH, the Fermi level can be pinned by these two defects. It is clear that the formation of vacancy defects including VBa, VCo, and VF is difficult in both F-rich and Co-rich conditions, suggesting the strong interaction force of the constituent element in BaCoF4. Furthermore, under the F-rich/Co-poor condition, the ΔH values of Fi, CoF, Bai, BaCo, and BaF are all below 0 eV over the whole EF region, indicating that the formation of Fi, CoF, Bai, BaCo, and BaF is spontaneous in the growth of the BaCoF4 crystal. Besides, BaF may be the dominant defect, owing to the minimum ΔH value. Since the ΔH values of the other defects, for example, VBa, VCo, VF, and more are considerably larger than 0 eV, it is difficult to form these point defects in BaCoF4. It is obvious that BaCoF4 is intrinsically a good n-type semiconductor as the Fermi level of BaCoF4 is higher than the CBM. The fact that the main donor defects, for example, BaF and Bai, are shallow-level defects, while the main acceptor defect, that is, Fi, is a deep-level defect, also supporting this conclusion. Under the Co-rich/F-poor condition, the ΔH values of Fi, BaF, BaCo, Bai, and CoF are also below 0 eV over the whole EF region, suggesting spontaneous generation of these defects. In contrast to the F-rich condition, the ΔH of Fi is much smaller, indicating that Fi has the possibility to be the dominant defect. The high values of ΔH of the other point defects will prevent their generation. Since the Fermi level is close to the CBM, BaCoF4 is considered to be an n-type conductor, similar to the F-rich/Co-poor condition. From the above analysis, BaCoF4 exhibits obvious n-type conduction for both the Co-rich and F-rich conditions. In addition, since Fi is a deep-level acceptor defect that is harmful to n-type conduction, the growth of BaCoF4 should be controlled under the F-rich condition. Moreover, for both the Co-rich and F-rich conditions, CoF, BaCo, and Fi are deep-level defects. These deep-level defects may form nonradiative carrier recombination centers, which tends to have severe impact on

BaCoF4 with distorted CoF6 octahedra, seeking a new hightemperature phase with higher space symmetry may be a possible way to improve the optical absorption and photovoltaic performance. Defect Properties of BaCoF4. The point-defect properties of semiconductors are key factors that determine the electronic properties of semiconductors. Here, the defect properties of BaCoF4 are also studied using DFT calculations. To confirm the intrinsic point defects of BaCoF4, the formation energies of these point defects are determined by the chemical potentials of the corresponding elements. In thermodynamic equilibrium condition, the chemical potentials must be restricted to the range that not only facilitates the formation of BaCoF4 but also eliminates the possible generation of elemental phases and secondary phases, such as BaF2 (cubic phase, space group Fm3m), CoF2 (tetragonal, P42/mnm), and CoF3 (hexagonal, P321). To stabilize the BaCoF4 phase, the chemical potentials must satisfy μBa + μCo + 4μF = ΔH (BaCoF4 ) = − 16.73 eV

(1)

where μi represents the chemical potential of the corresponding element i, and ΔH (BaCoF4) is the formation enthalpy of BaCoF4. To avoid the formation of elemental phases, the following conditions should be satisfied −1.31 eV < μBa < 0

(2)

−6.89 eV < μCo < 0

(3)

−1.87 eV < μF < 0

(4)

Last, to exclude the formation of possible secondary phases, the following constraints must also be met μCo + 2μF < ΔH (CoF2) = − 4.41 eV

(5)

μCo + 3μF < ΔH (CoF3) = − 6.69 eV

(6)

μBa + 2μF < ΔH (BaF2) = − 12.28 eV

(7)

Under the above conditions, the chemical potentials of Co and F that promote the growth of single-phase BaCoF4 are confined in the central green region, as shown in Figure 4a, where μBa is related to μCo and μF through eq 1. The small chemical potential range, in accordance with the dissociation energy of 0.04 eV for BaCoF4, as calculated by E (BaF2) + E (CoF2) − E (BaCoF4), suggests that the growth condition must be strictly controlled to synthesize the pure crystal of BaCoF4. To further investigate the defect properties of BaCoF4, the transition energy levels of donor-like (blue) and acceptor-like (red) point defects were calculated, as shown in Figure 4b. Here, considering the computed formation energies of different point defects displayed in Figure 4c,d, seven possible defects, including CoBa, BaCo, CoF, BaF, Bai, Coi, and Fi, are chosen to examine the defect characters. It is apparent that BaF is a donor with a (0/1+) transition level at 0.02 eV above the CBM and (1+/2+) transition level at 0.09 eV below the CBM, while the (2+/3+) transition level at 0.2 eV under the CBM is a deep-level state in the band gap. Compared with BaF, CoF has a deeper (0/1+) transition level at 0.16 eV below the CBM, and the corresponding (1+/2+) and (2+/3+) transition levels are both deep transition levels within the band gap, which can be ascribed to the higher ionicity of the Ba2+ cation. For the 14307


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Figure 5. (a) Structure diagram of BaCoF4-based perovskite solar cells. (b) J−V curves in reverse and forward scan modes for the best performance device under one sun illumination (AM 1.5G 100 mW cm−2). (c) PCE histogram of 30 BaCoF4-based solar cells, where the curve represents the Gaussian function of the histogram. (d) PCE decay of the unencapsulated BaCoF4-based device stored in ambient air for 1000 h. (e) J−V curves of the unencapsulated device under different storage times in ambient air. (f) Cross-sectional SEM image of the BaCoF4-based device.

Photovoltaic Properties of Other Analogues of BaMX4. On account of the bifunctionality of BaCoF4, it is reasonable to explore the photovoltaic properties of the analogues of BaMX4. Since the CBM and VBM of BaCoF4 are mainly contributed by the anion F and metal cation Co, respectively, it is reasonable to design BaMX4 compounds with an appropriate band gap by replacing either M or X ion with homologous elements. Here, we computed the photovoltaic properties of BaMX4 (M = Ni, Mn, Fe, Co; X = F, Cl, Br, I), as shown in Figure 6a. It is known that the band gap of a good

the electrical and photovoltaic properties of BaCoF4. Hence, the growth condition of BaCoF4 should be strictly controlled to exclude these deep-level defects. With both the multiferroic and photovoltaic characters, BaCoF4 exhibits the n-type selfdoping, which may be exploited as an alternative material to enhance the electron transport and photovoltaic performance for solar cells. Incorporation of BaCoF4 as ETL into Perovskite Solar Cells. A recent research study61 has demonstrated charge transfer along the local dipole order of the ferroelectric materials, suggesting that the domain walls tend to provide channels for charge transfer. Furthermore, our previous work62 has incorporated PbTiO3 as an ETL into the perovskite solar cell successfully. Therefore, considering the novel n-type property of BaCoF4, we incorporated BaCoF4 as an electron transport layer (ETL) and further fabricated BaCoF4-based solar cells with the architecture of FTO/BaCoF 4 / Cs0.05MA0.14FA0.81PbI2.55Br0.45/spiro-OMeTAD/Au, as displayed in Figure 5a. The device performance of BaCoF4based solar cells was examined. The champion power conversion efficiency (PCE) of BaCoF4-based device reaches ca. 13.14% upon primary device optimization (see Figure 5b), with a short-circuit current density (Jsc) of 20.15 mA cm−2, open-circuit voltage (Voc) of 1.02 V, and fill factor (FF) of 63.94%. The good photovoltaic properties of BaCoF4-based devices provide compiling evidence that the n-type self-doping of BaCoF4 improves the photovoltaic performance of the solar cells via the enhancement of the electron transport. The PCE histogram of 30 devices is displayed in Figure 5c. The PCE of BaCoF4-based device is located from 9.7 to 13.1% with good reproducibility, and most of the PCE values are higher than 11.4%. Figure 5d shows the PCE decay of the unencapsulated BaCoF4-based device stored in ambient air for 1000 h. The device keeps ca. 92.9% of its initial efficiency with nearly invariable Jsc and FF and slightly decreased Voc (Figure 5e), related to the intrinsic instability of the organic−inorganic hybrid perovskite material and the interface in the device configuration. These results demonstrate good long-term stability of BaCoF4-based solar cells. Cross-sectional SEM image of BaCoF4-based solar cell is shown in Figure 5f. The thickness of the as-prepared BaCoF4 film is about 60 nm.

Figure 6. (a) Crystal structure of BaMX4. (b) Computed electronic band gaps of BaMX4. The range of optimal band gaps for photovoltaic materials is highlighted by the pale yellow horizontal bar.

light-absorbing material should be within the range of 0.9−1.6 eV to achieve high PCE values for solar cells. Figure 6b shows the calculated band gaps, based on the PBE functional, of all 16 barium halides. For BaMX4, most band gaps are out of the optimal range of band gaps. Only two barium halides, that is, BaNiBr4 and BaMnCl4, are predicted to possess suitable band gaps, in addition to the BaCoF4 compound. Note that the 14308


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Figure 7. (a) Calculated optical absorption spectra (based on the PBE functional) of BaCoF4, BaNiBr4, and BaMnCl4 compared with the optical absorption spectra of prototypes Si and MAPbI3. (b) SLME values of BaCoF4, BaNiBr4, and BaMnCl4 as functions of film thicknesses. The results of prototypes Si and MAPbI3 are shown for comparison.

bifunctional materials. Since the phase stability calculation suggests that BaNiBr4 has a robust tendency to decompose, it is important to explore new methods to synthesize the BaNiBr4 compound.

computed band gap of BaCoF4 based on the PBE functional is in good agreement with the experimental band gap. To further investigate the photovoltaic properties of BaNiBr4 and BaMnCl4, the optical absorption spectra and corresponding SLME have been computed. Figure 7a shows the calculated optical absorption spectra of BaCoF4, BaNiBr4, and BaMnCl4 compared with those of prototype Si and MAPbI3. Compared with BaCoF4, BaNiBr4 and BaMnCl4 exhibit larger visible light absorption coefficients, leading to higher SLME values (Figure 7b) than BaCoF4 (Figure 3b). Moreover, the SLME value of BaNiBr4 reaches 13% at 3 μm film thickness, which is higher than that of BaMnCl4. Nevertheless, the photovoltaic performance of both new compounds still needs to be further improved compared with those of Si and MAPbI3. To estimate the phase stability of both compounds, we calculated the decomposition enthalpy ΔH based on the equation ΔH =

E[BaM2 +X4VII]

−

E[BaX 2VII]

−

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04502.

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E[M2 +X 2VII]

Experimental and computational details, lattice parameters for BaMX4 (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I), effective masses of BaCoF4, XRD patterns and TEM image of BaCoF4, and decomposition enthalpy of BaMX4 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-J.F.). *E-mail: [email protected] (X.C.Z.).

that is, the energy difference between the BaMX4 compounds and the decomposed products (see Figure S2). A negative value of ΔH indicates the thermodynamic stable condition of BaMX4. From our theoretical calculations, BaMnCl4 exhibits high thermodynamic stability, whereas BaNiBr4 shows a robust tendency to decompose. Hence, special experimental conditions, for example, high temperature and high pressure, should be controlled to synthesize the BaNiBr4 compound.

ORCID

Hong-Jian Feng: 0000-0003-2637-4062 Xiao Cheng Zeng: 0000-0003-4672-8585 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.-J.F. was financially supported by the National Natural Science Foundation of China (NSFC) under grant nos. 51672214, 11304248, and 11247230, the Natural Science Basic Research Plan in Shaanxi Province of China (program no. 2014JM1014), the Scientific Research Program Funded by Shaanxi Provincial Education Department (program no. 2013JK0624), the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shaanxi Province of China, and the Youth Bai-Ren (100 Talents Plan) Project in Shaanxi Province of China. X.C.Z. is supported by the University of Nebraska Holland Computing Center.

CONCLUSIONS In conclusion, we report a class of bifunctional materials with combined advantages of multiferroic and photovoltaic properties. Our theoretical results indicate that BaCoF4 exhibits AFM spin structures along the c axis and a ferroelectric Ps value of 10.6 μC/cm2 along the (001) direction. Both DFT and experimental studies suggest that BaCoF4 is a promising candidate as a bifunctional material with good multiferroic and photovoltaic performances due to desirable optoelectronic properties such as suitable band gap, wide absorption range in the visible light region, and relatively small me along Γ−Y. The defect properties of BaCoF4 were further investigated by DFT calculations. We found that BaCoF4 exhibits fascinating n-type self-doping under both Co-rich and F-rich conditions, which can result in an enhanced electron transfer as well as enhanced performance of solar cells. Indeed, our experiments confirm the good photovoltaic performance of BaCoF4-based solar cells due to the n-type self-doping of BaCoF4. Finally, we showed that BaNiBr4 and BaMnCl4 are promising candidates for the

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REFERENCES

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M = Co, Ni, Fe, Mn; X = F, Cl, Br, I - ACS Publications - American

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