Forum Article pubs.acs.org/IC
Preface for the Halide Perovskites Forum Wayne L. Gladfelter*,† and Mercouri G. Kanatzidis*,‡ †
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
‡
K
One approach to improving the power conversion efficiency in halide perovskite solar cells is to use the internal electric field in ferroelectric domains to enhance charge separation. The challenge, of course, is to design and synthesize polar halide perovskites. Using computational methods, Rondinelli and coworkers described how cesium and rubidium cation ordering can work in concert with rotation of the SnI6 octahedra in the double perovskite (CsRb)SnI3 to form a ferroelectric structure.11 Palstra and co-workers reported the crystal structure of a polar phase that exists below 143 K.12 The compound (MA)3Bi2I9 consists of face-sharing octahedra of BiI6 groups that are related to the perovskite phase. At higher temperatures, two additional structurally characterized, nonpolar phases were found in which the distortions of the structure at low temperature disappear because of disordering of the bismuth lone pair and above 160 K because of disordering of the methylammonium ion. In a study of a compound having a formula related to (MA)3Bi2I9, Melot and co-workers reported that Cs3Bi2Br9 exhibits weak, room temperature photoluminescence with sharp peaks attributed to vibronic coupling to lattice phonons.13 With the aid of density functional theory, the atomic orbital character at the top of the valence band and bottom of the conduction band was found to consist primarily of bismuth orbitals. Thus, photoexcitation produces an electron−hole pair localized strongly on the bismuth centers. The contribution by Karunadasa and co-workers summarized a spectrum of synthetic approaches to new lead halide perovskite structures aimed at mitigating problems of moisture, heat, and light instability as well as lead toxicity.14 By replacing a fractional amount of the small cation with a bulky ammonium ion, they demonstrated control of the slab thickness in two-dimensional lead iodide structures with the added benefit of reducing the moisture sensitivity. Two-dimensional lead iodide slabs were also made by substituting axial iodides with thiocyanate ligands in (MA)2[PbI2(SCN)2]. These and related compounds exhibit interesting structural and property changes at high pressures. Within the tin iodide system, Kanatzidis and co-workers systematically varied the organic cation using ions that exceeded the limiting size, giving rise to the perovskite structure type shown in Figure 1.15 The new ASnI3 structures characterized in this paper separate into two structural categories. One group, exemplified by (Me4N)SnI3, is referred to as the “perovskitoids” and comprises one-dimensional chains of SnI6 face-sharing octahedra, which form nearly perfect hexagonal close-packed arrays along the [SnI3]−∞ chains. The second group, designated as hexagonal perovskite polytypes, blends the face-sharing pattern of the perovskitoids with the corner-sharing octahedra
nown since the late 1800s, complex metal halides including those with structures related to perovskite have been widely studied and found to exhibit a fascinating array of zero-, one-, two-, and three- dimensional networks depending on the formula.1,2 The semiconductor properties, including the intense colors, of many of these compounds attracted recent interest in their use as absorbers in photovoltaic and other optoelectronic devices. In 2009, Kojima et al. incorporated (MA)PbI3 (Figure 1), where MA = methylammonium, as the absorber in a liquid
Figure 1. Structure of (MA)PbI3. Key: Pb, green; I, purple; N, blue; C, black.
Grätzel solar cell.3 Although the overall power conversion efficiency of the cell was less than that of existing dye-sensitized solar cells, the use of a solid-state compound that could be readily deposited using liquid precursors attracted attention. Following a series of reports demonstrating solid-state devices with remarkable improvement in cell efficiencies in 2011 and 2012,4−8 the field exploded with activity. As described in the contribution by Park, over 1000 papers on perovskite solar cells were published in 2015 alone.9 This Inorganic Chemistry Forum brings together a combination of additional original research contributions along with a series of minireviews of selected topics connected to this exciting field. The paper by Seshadri and co-workers focused heavily on the electronic structures of halide perovskites and highlights the important role that the “inert pair of electrons”, the s2 electrons, plays in the electronic structure and stability of the numerous phases of divalent lead and tin, as well as trivalent bismuth.10 The importance of cation−cage interactions on the material properties and a discussion of the ionic mobility in these compounds are included, along with a discussion of the impact of replacing the methylammonium cation with functional organic cations, such as the monocation of tetrathiofulvalene. © 2017 American Chemical Society
Special Issue: Halide Perovskites Published: January 3, 2017 1
DOI: 10.1021/acs.inorgchem.6b02910 Inorg. Chem. 2017, 56, 1−2
Inorganic Chemistry
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observed in prototypical cubic perovskites. Variation of the organic cation leads to different ratios of the hexagonal and cubic components in the structure. Finally, several dimensionally reduced structures are described that are analogous to the Ruddleston−Popper phases, which exhibit perovskite slabs separated by the organic cations. The optical properties, including band-gap measurement, of these new compounds are presented. Intentional doping plays a critical role in controlling the properties of semiconductors. Many of the devices based on (MA)PbI3 incorporate a fraction of bromide or chloride into the structure. De Angelis and co-workers used theoretical methods to elucidate the changes caused by 1% and 8% chloride doping into (MA)PbI3 crystals.16 The presence of the chloride substituted on an iodide site in the structure was found to alter the local lattice dynamics and electronic structure, which may contribute to the observed enhancement of exciton dissociation and reduced carrier recombination in the doped crystals. Falaras and co-workers reported that adding a small amount of SnF2 to the usual mixture of CsI and SnI2 in the synthesis of CsSnI2.95F0.05 caused no change in the lattice constants measured by powder X-ray diffraction of the known phases of CsSnI3.17 While the substitution of a fluoride for an iodide in CsSnI3 was not expected because of the large size difference of the two halides, the precise fate of F− could not be determined. The authors did observe an increase in the stability of CsSnI3 in the doped versus undoped samples as well as changes in the Raman and photoluminescence spectra. The humidity, heat, oxygen, and even light exposure can result in degradation of the lead halide perovskites. In the first part of their paper, Yang and Kelly surveyed many of the observations of these environmental effects on the decomposition of lead halide perovskite films.18 In the second part, they focused on mechanisms in actual solar cells where factors other than the halide perovskite chemistry may be responsible for device failure. In the last part of their review, they discussed different approaches being taken to improve the cell stability. In addition to understanding the factors that affect the crystal chemistry of the halide perovskites, the successful construction of solar cells demands a thorough knowledge of how the film deposition process contributes to the film microstructure. In his contribution, Park highlighted the importance of controlling the grain size of (MA)PbI3 to increase the power conversion efficiency in devices.9 Controlling the concentration of (MA)I and the addition of a Lewis base such as dimethyl sulfoxide to the solution of precursors resulted in larger grains capped on the surface with a layer of (MA)I. We hope this Inorganic Chemistry Forum provides a snapshot of the rapidly moving cutting edge in this huge field of research, by bringing together an important set of papers and reports that focus on the inorganic chemistry of these materials, underpinning the understanding of their fascinating physical properties.
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Forum Article
REFERENCES
(1) Saparov, B.; Mitzi, D. B. Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116, 4558. (2) Stoumpos, C. C.; Kanatzidis, M. G. Halide Perovskites: Poor Man’s High-Performance Semiconductors. Adv. Mater. 2016, 28, 5778. (3) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050. (4) Im, J. H.; Lee, C. R.; Lee, J. W.; Park, S. W.; Park, N. G. 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale 2011, 3, 4088. (5) Chung, I.; Lee, B.; He, J. Q.; Chang, R. P. H.; Kanatzidis, M. G. Allsolid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485, 486. (6) Etgar, L.; Gao, P.; Xue, Z. S.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Gratzel, M. Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. J. Am. Chem. Soc. 2012, 134, 17396. (7) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, XX DOI: 10.1038/srep00591. (8) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643. (9) Park, N. G. Nonstoichiometric Adduct Approach for High Efficiency Perovskite Solar Cells. Inorg. Chem. 2016, 56, 3. (10) Fabini, D. H.; Labram, J. G.; Lehner, A. J.; Bechtel, J. S.; Evans, H. A.; Van der Ven, A.; Wudl, F.; Chabinyc, M. L.; Seshadri, R. Main-Group Halide Semiconductors Derived from Perovskite: Distinguishing Chemical, Structural, and Electronic Aspects. Inorg. Chem. 2016, 56, 11. (11) Gou, G.; Young, J.; Liu, X.; Rondinelli, J. M. Interplay of Cation Ordering and Ferroelectricity in Perovskite Tin Iodides: Designing a Polar Halide Perovskite for Photovoltaic Applications. Inorg. Chem. 2016, 56, 26. (12) Kamminga, M. E.; Stroppa, A.; Picozzi, S.; Chislov, M.; Zvereva, I. A.; Baas, J.; Meetsma, A.; Blake, G. R.; Palstra, T. T. M. Polar Nature of (CH3NH3)3Bi2I9 Perovskite-Like Hybrids. Inorg. Chem. 2016, 56, 33. (13) Bass, K. K.; Estergreen, L.; Savory, C. N.; Buckeridge, J.; Scanlon, D. O.; Djurovich, P. I.; Bradforth, S. E.; Thompson, M. E.; Melot, B. C. Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9. Inorg. Chem. 2016, 56, 42. (14) Slavney, A. H.; Smaha, R. W.; Smith, I. C.; Jaffe, A.; Umeyama, D.; Karunadasa, H. I. Chemical Approaches to Addressing the Instability and Toxicity of Lead−Halide Perovskite Absorbers. Inorg. Chem. 2016, 56, 46. (15) Stoumpos, C. C.; Mao, L.; Malliakas, C. D.; Kanatzidis, M. G. Structure-Bandgap Relationships in Hexagonal Polytypes and Low Dimensional Structures of Hybrid Tin Iodide Perovskites. Inorg. Chem. 2017, 56, 56. (16) Quarti, C.; Mosconi, E.; Paolo, U.; De Angelis, F. Chlorine Incorporation in the CH3NH3PbI3 Perovskite: Small Concentration, Big Effect. Inorg. Chem. 2016, 56, 74. (17) Kontos, A. G.; Kaltzoglou, A.; Siranidi, E.; Palles, D.; Angeli, G. K.; Arfanis, M.; Psycharis, V.; Raptis, Y. S.; Kamitsos, E. I.; Trikalitis, P. N.; Stoumpos, C. C.; Kanatzidis, M. G.; Falaras, P. Structural Stability, Vibrational Properties and Photoluminescence in Undoped and SnF2doped CsSnI3 Perovskites. Inorg. Chem. 2016, 56, 84. (18) Yang, J.; Kelly, T. L. Decomposition and Cell Failure Mechanisms in Lead Halide Perovskite Solar Cells. Inorg. Chem. 2016, 56, 92.
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DOI: 10.1021/acs.inorgchem.6b02910 Inorg. Chem. 2017, 56, 1−2
