Oxide Analogs of Halide Perovskites and the New ... - ACS Publications

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Cite This: J. Phys. Chem. Lett. 2019, 10, 1722−1728

Oxide Analogs of Halide Perovskites and the New Semiconductor Ba2AgIO6 George Volonakis,† Nobuya Sakai,‡ Henry J. Snaith,‡ and Feliciano Giustino*,† †

Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom

‡

J. Phys. Chem. Lett. Downloaded from pubs.acs.org by ALBRIGHT COLG on 03/29/19. For personal use only.

S Supporting Information *

ABSTRACT: The past few years witnessed the rise of halide perovskites as prominent materials for a wide range of optoelectronic applications. However, oxide perovskites have a much longer history and are pivotal in many technological applications. As of today, a rational connection between these important materials is missing. Here, we explore this missing link and develop a novel concept of perovskite analogs, which led us to identify a new semiconductor, Ba2AgIO6. It exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs2AgInCl6, but with a band gap in the visible range at 1.9 eV. We show that Ba2AgIO6 and Cs2AgInCl6 are analogs of the well-known transparent conductor BaSnO3. We synthesize Ba2AgIO6 following a low-temperature solution process, and we perform crystallographic and optical characterizations. Ba2AgIO6 is a cubic oxide double perovskite with a direct low gap, opening new opportunities in perovskite-based electronics optoelectronics and energy applications.

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perovskites are the most prominent materials to exhibit colossal magnetoresistance.20 Recently, there have been a few works that also explore oxide perovskites for photovoltaic applications. The oxide double perovskite Bi2FeCrO6 has been used in thin-film solar cells, achieving power-conversion efficiency of 8.1%.21 Furthermore, on the basis of calculations from first principles, alloys of oxide double perovskites like Ba2SbV0.25Ta0.75O6 have been proposed as potentially suitable materials for solar cells.22 Overall, the vast majority of existing perovskites are oxides (68%), while halides account for only 16% of known compounds.23 Somewhat surprisingly, today there is not much overlap between the fields of oxide and halide perovskites, and a clear connection between these two subfamilies is missing. Here, we extend our recent work on the design of lead-free halide double perovskites11,14,18 to the case of oxide perovskites. We identify Ba2AgIO6 as an oxide double perovskite having a band gap in the visible range and a band structure extremely similar to that of Cs2AgInCl6. Motivated by the recent discovery that Cs2AgInCl6 is an efficient lightemissive material,24 we explore the relation between these two compounds and the origin of the lower band gap of Ba2AgIO6. This quest leads us to establish that the link between oxide and halide perovskites is to be found in the electronic valency of the cations at the B/B′ site. Using this conceptual link we explain why Ba2AgIO6 and Cs2AgInCl6 have similar band structures. We also demonstrate that these compounds are both analogs of BaSnO3, a prominent transparent oxide

erovskites are one of the most common crystals and have been employed for a broad range of applications, such as transistors,1 solar cells,2 light-emitting devices,2 memories,3 catalysts,4 and superconductors.5 Within the perovskite family, halide perovskites have attracted tremendous scientific and technological interest over the past few years and revolutionized the field of emerging photovoltaics.6−8 Solar cells based on lead-halide perovskites have recently achieved recordbreaking power conversion efficiencies of more than 23%, surpassing state-of-the-art CIGS and thin-film silicon technologies.9 Moreover, the initial concerns over the stability of halide perovskite devices have been alleviated,10 with devices reaching stabilized efficiencies of more than 20%. Despite this enormous progress, it would be desirable to replace Pb with an environmentally friendly element. To this aim, halide double perovskites have recently been designed and synthesized as potential lead-free alternatives.11−14 Among the synthesized double perovskites, Cs2BiAgBr6 has the lowest band gap of 1.9 eV,12,13,15 but it is indirect, and Cs2AgInCl6 is the only direct gap semiconductor, but the band gap is relatively large, 3.3 eV.14,16,17 On the basis of a rational design strategy, recently we have shown that in order to match the remarkable optoelectronic properties of lead-based compounds, lead-free halide double perovskites must combine In as a monovalent cation and Sb or Bi as the trivalent cation.18 However, these compounds have not been synthesized yet. Historically, oxide ABO3 perovskites and A2BB′O6 double perovskites have been investigated for over a century, well before the emergence of halide perovskites. For example, cuprates have been a prototypical system for high-temperature superconductors for decades,19 while manganese-based oxide © XXXX American Chemical Society

Received: January 22, 2019 Accepted: March 13, 2019

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Figure 1. (a) Atomistic illustration of the crystal structure of Ba2BIO6 oxide double perovskites with B = Na, Ag. (b) DFT-PBE0 electronic band structure of Ba2NaIO6, Ba2AgIO6, and Cs2AgInCl6. Effective masses are shown in gray. O atoms in red, I in blue, Na/Ag in gray, and Ba in purple. The energy axis is referred to the valence band top.

perovskite25,26 that has been used in perovskite solar cells.27 Having established that Ba2AgIO6 is an oxide analog of Cs2AgInCl6 with a significantly lower band gap, we proceed to synthesize Ba2AgIO6 through a newly developed solution processing route and characterize its optical absorption and photoluminescence for the first time. Recently, we designed in silico lead-free double perovskites based on pnictogen atoms and noble metals.11 Among these compounds, three new perovskites were synthesized: Cs2BiAgCl6,11,13 Cs2BiAgBr6,12,13,15 and Cs2SbAgCl6.28,29 In our work we followed the synthetic route for the elpasolite Cs2BiNaCl6, which has been known since the early 1970s,30 and replaced Na with Ag. We similarly succeeded in synthesizing another new halide double perovskite Cs2AgInCl6,14 following the synthesis route of Cs2NaInCl6.31 In both cases the key to success was the close match between the effective ionic radii of NaI and AgI, which are 1.02 and 1.15 Å, respectively. To search for potential candidates in the family of oxides double perovskites we follow the same strategy. The starting point for our search is the Inorganic Crystal Structure Database (ICSD). We searched for A2BB′O6 oxide double perovskites with NaI as the B-site cation. There are only four compounds matching our criteria: Ba2NaOsO6, Ba2NaReO6, Ba2NaIO6, and Pb2NaIO6,32−35 where Os, Re, and I are in the unusually high +7 oxidation state. Osmium and rhenium are transition metals of limited interest for optoelectronic applications because of their low availability and high prices (the oxides sell for more than $240/g). Hence, we are left with iodine as the sole candidate for the B′-site cation, and we focus on the case of Ba2NaIO6 because we expect similar properties for Pb2NaIO6. We employ density-functional theory (DFT) with the PBE functional36 to optimize Ba2NaIO6 within the Fm3̅m space group and with Na and I arranged in a rock-salt configuration, as shown in Figure 1a. The relaxed lattice constant is 8.41 Å, which is in good agreement with the measured value of 8.34 Å.34 Next we proceed to replace Na with Ag and reoptimize the structure. The DFT-PBE lattice constant of Ba2AgIO6 is found to be 8.56 Å. We check the stability of Ba2AgIO6 by calculating the total energy differences for all decomposition routes into any compounds in the Materials Project database37 and find this double perovskite to be stable against decomposition. We investigate the electronic structure using

hybrid functionals, so as to overcome the well-known band gap underestimation within DFT-PBE. In our recent work, we found that the PBE0 hybrid functional is in good agreement with the measured band gaps of similar double perovksites;11,14,38,39 therefore, we also use PBE0 in the following. Figure 1b shows the DFT-PBE0 electronic band structure for the optimized Ba2NaIO6 and Ba2AgIO6. The replacement of NaI with AgI leads to a considerable narrowing of the band gap, by almost 3 eV, and at the same time the bands become more dispersive. Within DFT-PBE0, the electronic band gap of Ba2AgIO6 is 1.9 eV, which is well within the visible range. A measure of the band dispersions near the band extrema are the electron and hole effective masses, which are indicated in Figure 1b. The electron masses are relatively low, 0.5me and 0.3me for Ba2NaIO6 and Ba2AgIO6, respectively. In the case of holes, the replacement of Na by Ag reduces the effective mass from 1me to 0.4me. The flat band that is present for both compounds at the valence band top (vbt) is not taken into account for the calculation of the hole effective mass, although its presence is expected to hinder hole transport along the [100], [010], and [001] directions. The band gap of Ba2AgIO6 is found to be quasi-direct in our calculations, with the conduction band bottom (cbb) at Γ and the vbt at X lying only 5 meV higher than the vbt at Γ. Given the smallness of this difference, it is possible that more refined many-body calculations of electron−electron and electron− phonon effects will yield a direct band gap.40,41 We also find that the optical transition at Γ is forbidden, which is a feature in common with the halide double perovskites Cs2AgInCl6.17,42 The similarity is not limited to the nature of the optical transitions, but the entire band structures of Ba2AgIO6 and Cs2AgInCl6 are remarkably similar, as shown in Figure 1b. These similarities are no coincidence; let us see why. Ba2AgIO6 and Cs2AgInCl6 have AgI/IVII and AgI/InIII as the cations at the B/B′-site, respectively. All these cations share the same valency: d10s0, with occupied 4d states and unoccupied 5s states. Hence, in a simple ionic picture we expect 4d states to contribute to the valence band, while the 5s states should lie in the conduction band. In line with this expectation, the vbt of Ba2AgIO6 is composed of Ag 4d orbitals that are hybridized with O 2p orbitals, shown in Figure S1, in the same way as the valence band of Cs2AgInCl6 is derived from Ag 4d orbitals and Cl 3p orbitals.14 The same is true for the cbb, which contains 1723

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Figure 2. (a) Schematic illustration of the family of analogs including oxides (left), halides (right), single (top), and double (bottom) perovskites. We focus on compounds where the B-site cations have d10s0 valency. (b) DFT-PBE0 electronic band structures of single perovskite BaSnO3, its halide analog CsCdCl3, and (c) its double perovskite analogs Ba2InSbO6 and Ba2CdTeO6. We also show the band-folding effect for BaSnO3 with a supercell corresponding to the Fm3̅m lattice.

ABIIX3 halide perovskites, the +2 oxidation of the B-site cation restricts the possible halide double perovskite analogs to the first tier only. Thus, the double perovskite analog of CsCdCl3 is Cs2AgInCl6. This link, which is schematically illustrated in Figure 2a, explains why the electronic structures of Cs2AgInCl6 and Ba2AgIO6 are so similar, as we discussed in the first part of this work. To meaningfully compare the electronic structure of single and double perovskites, we consider a supercell of the single perovskite ABO3 cubic lattice, which corresponds to the fcc lattice of the double perovskite and contains two ABO3 units. In this representation, the band structure of double perovskites is a folded version of the single perovskite bands. This bandfolding effect is shown in Figure 2c for the case of BaSnO3. It is clear that the folded band structure of BaSnO3 is very similar to the band structure of all its double perovskite analogs Ba2InSbO6, Ba2CdTeO6, and Ba2AgIO6, as well as to that of the halide double perovskite Cs2AgInCl6, shown in Figures 1b and 2c. Upon folding, the indirect band gap of BaSnO3 becomes a direct gap, but the direct transition at Γ obviously remains forbidden. This mechanism explains why the optical transition at Γ for the direct gap double perovskites Cs2AgInCl6 and Ba2AgIO6 is inherently forbidden. More generally, for perovskites with B-site cations in their d10s0 electronic configuration, the analogs of an indirect-gap single perovskite will always be double perovskites with a forbidden direct gap. Given the close analogy between BaSnO3, Ba2AgIO6, and Cs2AgInCl6, it is natural to look for all the possible analogs of

5s orbitals for both compounds. Furthermore, as a consequence of the hybridization of Ag 4dx2−y2 orbitals with O 2px,y orbitals at the vbt of Ba2AgIO6, a nondispersive band emerges, as shown in Figure 1b. This hybridization leads to wave functions that are confined in two dimensions, as shown in Figure S2. We observed precisely the same feature for the case of Cs2AgInCl6.14 These strong similarities indicate that the valency of the atoms at the B/B′-sites controls the electronic structure of both compounds, even though one is a halide and one is an oxide. In the following we generalize this concept to identify halide/oxide analogs for single and double perovskites. To identify all the double perovskite analogs of an ABIVO3 perovskite, we follow the split-cation approach and consider structures like A2BIIIB′VO6, A2BIIB′VIO6, and A2BIB′VIIO6. Following the illustration in Figure 2a, we will refer to these analogs as first-tier, second-tier, and third-tier double perovskites, respectively. Within this framework, Ba2InSbO6, Ba2CdTeO6, and Ba2AgIO6 are the first-tier, second-tier, and third-tier double perovskite analogs of BaSnO3, respectively. By design, all these compounds are iso-electronic with BaSnO3, with their B-site cations in a d10s0 electronic configuration, like SnIV. On the basis of what we discussed above for Ba2AgIO6, we now employ the electronic configuration at the B-site to connect oxide with halide perovskites. For example, the analog of BaSnO3 is CsCdCl3, because Cd in its +2 oxidation state has the same d10s0 electronic configuration. Thus, their electronic band structures are expected to be similar, and this is confirmed by the calculations (see Figure 2b). For such 1724

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The Journal of Physical Chemistry Letters Table 1. Oxide and Halide Perovskite and Double Perovskite Analogsa

a These compounds share the same d10s0 electronic configuration. The dot means that the element is the same as in the row above. The full list of the references where the synthesis of these compounds was first reported is included in Table S1 of the Supporting Information.

Figure 3. (a) All-electron energy levels of the 4d orbitals of Ag, Cd, In, Sn, Sb, Te, and I, compared to the energy level of the O 2p orbital. (b) Square modulus of the electron wave function at the valence band top for Ba2CdTeO6 and Ba2AgIO6 oxide double perovskites.

halide single perovskites CsCdCl3, CsCdBr3, CsHgCl3, CsCdBr3, and CsHgI3 were synthesized by Wells as far back as in 189243 and 1894.44 Around the same time he also reported the first synthesis of lead halide perovskites CsPbX3

the common parent compound BaSnO3. To this aim, we use the ICSD and collect all the single and double perovskites with a d10s0 electronic configuration that have been synthesized so far. The results of this search are reported in Table 1. The 1725

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Figure 4. (a) X-ray diffraction pattern of the as-synthesized Ba2AgIO6 (green points) and simulated pattern of the DFT-PBE optimized structure (red line). The inset is a photograph of the as-synthesized powder. The arrows indicate peaks that we tentatively assign to AgI impurities. (b) UV− vis absorption and photoluminescence spectra for Ba2AgIO6. The inset shows the corresponding Tauc plot for a direct onset. (c) Time-resolved photoluminescence decay of Ba2AgIO6 and corresponding biexponential fit.

(X = Cl, Br, I).45 The double perovskites Ba2CdTeO646 and Ba2InSbO647 have been synthesized and studied as possible transparent conducting oxides, and their band gaps were reported within the same range as the band gap of BaSnO3. While Ba2AgIO6 is not reported in the ICSD, we found mention of this compound in a 1964 paper by Sleight and Ward.47 However, detailed crystallographic and optical characterization were not reported, and to the best of our knowledge this compound has never been properly investigated. As shown in Figure 2b,c, DFT-PBE0 calculations for the BaSnO3 analogs, Ba2CdTeO3, Ba2InSb3, and BaSnO3, using the experimental lattice parameters, yield large gaps of 3.4, 3.2, and 3.1 eV, respectively, as in the experiments, and yield similar band structures for all compounds. However, when we compare to Ba2AgIO6 in Figure 1b, we see that in this case the gap, 1.9 eV, is considerably smaller than in all other cases. This anomalous and intriguing feature of Ba2AgIO6 can be explained as follows. In BaSnO3, the conduction band bottom is formed by antibonding s−s* orbitals of Sn-5s and O-2s, and the valence band top is composed of nonbonding O-2p orbitals.38 When we replace two SnIV atoms with combinations of either InIII/SbV, CdII/TeVI, or AgI/IVII in a double perovskites lattice, the atomic energy levels of the occupied 4d orbitals change as shown in Figure 3a. In all double perovskites except Ba2AgIO6, the O-2p energy is above the energy of the d states, therefore the character of the vbt is nonbonding O-2p orbitals, like BaSnO3. When we reach Ag,

the energy of the 4d orbitals happens to cross the O-2p level; hence, the vbt of Ba2AgIO6 is formed by hybridized Ag-4d and O-2p states. This transition is manifest in the electronic wave functions at the vbt, which are shown in Figure 3b. Here we can see that in Ba2CdTeO6 only nonbonding O-2p orbitals are at the vbt, while for Ba2AgIO6 the vbt also contains Ag-4d orbitals. This is a unique property of Ba2AgIO6 which has the effect of reducing the band gap and making the bands more dispersive and which makes this compound stand out among all the d10s0 oxide perovskites in Table 1. Having established that Ba2AgIO6 has a band structure that resembles Cs2AgInCl6 and a band gap well within the visible range, we proceed with the experimental synthesis and characterization. To this aim, we followed a novel two-step solution process to first form yellow AgIO4 crystals (see Figure S3) and subsequently form Ba2AgIO6 as detailed in the Supporting Information. The inset of Figure 4a shows the resulting brown powder. In order to characterize the structure, we measured its X-ray diffraction (XRD) pattern, as shown in Figure 4a. We could match the XRD data to a double perovskite lattice with a lattice constant of 8.45 Å, which is in good agreement with our DFT-PBE-predicted value of 8.56 Å. Furthermore, when we compare the XRD pattern simulated with the DFT-PBE structure, we obtain very good agreement with the experimental data (see the red line in Figure 4a). This confirms that we formed the double perovskite Ba2AgIO6. We note that this is a rare occurrence where an oxide perovskite is formed through a low-temperature solution process.46 In 1726

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The Journal of Physical Chemistry Letters Figure 4b we show the optical absorption and photoluminescence (PL) measurements of the synthesized Ba2AgIO6 powder. Using a Tauc plot for a direct onset we estimate an optical band gap of 1.93 eV. This value matches nicely our predicted PBE0 gap of 1.9 eV. We also observe a broad photoluminescence signal, which is centered at 613 nm and matches the absorption onset. The PL broadening is likely related to the presence of defects and/or poor crystallinity, but the location of the PL peak appears to suggest emission via band-to-band recombination, because it matches well the band gap energy. Finally, in Figure 4c we show time-resolved PL data. A double exponential fit yields a fast time scale of 0.55 ns and a slower time scale of 3.24 ns. These values are slightly faster than those determined for halide double perovskites such as Cs2BiAgCl6, Cs2BiAgBr6, and Cs2AgInCl6.11,14,15,48 We note that such relatively narrow band gap and the low calculated effective masses make Ba2AgIO6 stand out among oxide perovskites, which typically exhibit wide band gaps and heavy effective masses.46 In summary, in this work we have established the links between oxide and halide single and double perovskites. The key to identify halide/oxide analogs is the valency of the B-site cations, which controls the electronic structure of these compounds. Using this new concept, we demonstrated that the previously reported halide double perovskite Cs2AgInCl6 is the analog of BaSnO3, and we identified for the first time the oxide double perovskite Ba2AgIO6 as a semiconductor with a band gap in the visible. We reported a novel synthesis route of Ba2AgIO6 via solution and the characterization of its crystallographic and optical properties for the first time. We found a gap around 1.9 eV and a broad photoluminescence signal at the same energy, in agreement with our calculations. While future work will be needed to improve sample quality, understand defect chemistry, and demonstrate device fabrication, we believe that the new semiconductor Ba2AgIO6 will make a promising candidate for optoelectronic applications, such as mixed oxide/halide perovskite solar-cells, novel oxide-based light-emitting devices, and photocatalysis. It will be interesting in future work to explore the tunability of the electronic and optical properties of this compound via cation and anion substitution.23

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Feliciano Giustino: 0000-0001-9293-1176 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Prof. A. Sleight for fruitful discussions. The research leading to these results has received funding from the Graphene Flagship (Horizon 2020 Grant No. 696656 - GrapheneCore2), the Leverhulme Trust (Grant RL2012-001), and the UK Engineering and Physical Sciences Research Council (Grant Nos. EP/M005143/1, EP/J009857/ 1, EP/M020517/1, and EP/L024667/1). The authors acknowledge the use of the University of Oxford Advanced Research Computing (ARC) facility (http://dx.doi.org/10. 5281/zenodo.22558), the ARCHER UK National Supercomputing Service under the “2D-PSC” Leadership project, and PRACE for awarding us access to MareNostrum at BSCCNS, Spain.

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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.jpclett.9b00193. Details on the computational methods, synthesis of Ba2AgIO6, and experimental methods; supporting figures showing the band structure of Ba2AgIO6 resolved by atomic orbitals, the square modulus of the valence band top wave function at the X high symmetry point of the Brillouin zone, and the XRD pattern for AgIO4 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+44) 01865 272380. ORCID

George Volonakis: 0000-0003-3047-2298 Nobuya Sakai: 0000-0003-2142-6564 Henry J. Snaith: 0000-0001-8511-790X 1727

DOI: 10.1021/acs.jpclett.9b00193 J. Phys. Chem. Lett. 2019, 10, 1722−1728

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DOI: 10.1021/acs.jpclett.9b00193 J. Phys. Chem. Lett. 2019, 10, 1722−1728