Goldschmidt's Rules and Strontium Replacement in Lead Halogen

Article pubs.acs.org/JPCC

Goldschmidt’s Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3 T. Jesper Jacobsson,*,† Meysam Pazoki,‡ Anders Hagfeldt,†,‡ and Tomas Edvinsson‡,§ Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland ‡ Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden § Department of Engineering Sciences, Solid State Physics, Uppsala University, Box 534, 75121 Uppsala, Sweden †

S Supporting Information *

ABSTRACT: During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a watersoluble lead compound. Replacing lead in the perovskite structure with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PbI2 and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal− halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the valence of Sr2+, results in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite. This is explained as a consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.

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INTRODUCTION Lead halogen perovskites have recently emerged as a promising class of solar cell materials. One of the first papers on the subject was published in 2009 by Miyasaka et al., 1,2 demonstrating a 3.8% efficient quantum dot sensitized solar cell using CH3NH3PbI3 as a light absorber and a liquid electrolyte based on iodide/tri-iodide as redox couple. A few years later, a number of advances were published,3−7 the efficiencies reached 10%, and the stabilities were greatly increased by switching from a liquid electrolyte to a solidstate hole conductor. Since then, the field has been one of the most hyped in materials science, the number of published articles has increased exponentially, and the record efficiency © 2015 American Chemical Society

now surpasses 20% according to NREL’s table of record efficiencies. The highest efficiencies for lead halogen perovskite solar cells are thus higher than what have been achieved for all other emerging solar cell technologies and start to catch up with commercial thin film technologies like CdTe and CIGS. The perovskite solar cells are further considered as a potentially cheap solar cell technology, explaining the immense interest currently seen for lead halogen perovskites. Received: July 5, 2015 Revised: October 23, 2015 Published: October 26, 2015 25673

DOI: 10.1021/acs.jpcc.5b06436 J. Phys. Chem. C 2015, 119, 25673−25683

Article

The Journal of Physical Chemistry C

table and thus has similar electronic properties. Solar cell efficiencies with tin perovskites have reached 6%.20 This is considerably lower than that for the lead-containing counterpart, but given the less attention given to the tin perovskites, it may be a promising way forward. Stability has, however, been an even bigger problem than that for the lead-containing perovskites due to the decrease in stability of the oxidation state of tin(II) compare to lead(II). Sn2+ has a tendency to oxidize to Sn4+, which turns out to be detrimental for the structural integrity of the perovskite and its advantageous photophysics.20,22 The elements so far investigated as replacements for lead are located in the immediate vicinity of lead in the periodic table, which is a natural first step, as they could be expected to have similar electronic structure. That may, however, be an unnecessarily restricted strategy. Re-evoking the classical notion of the Goldschmidt’s rules (GRs) of ion replacement in minerals, several interesting alternatives emerge. The original GRs are based on empirical observations of replacements in natural minerals while keeping the crystal structure more or less unchanged. These were later complemented by Ringwood23 by also including the contribution from differences in electronegativity. These notions are here, together with quantum mechanical extension, explored with respect to elemental exchange in lead halogen perovskites. Exactly how the different elements of the perovskite structure contribute to the photophysics required for an efficient solar cell is still not entirely clear. So far, the halogens, the organic ion, as well as the lead ion have been replaced individually while the perovskite still have maintained reasonably adequate photo physical properties. This invokes the question of the functionality of lead in the structure. In view of Goldschmidt’s rules, lead can be considered as a provider of the charge and size required to hold the perovskite crystal structure together. The electronics of the lead orbitals will, however, also be of importance. Lead is, according to DFT-calculations, contributing d-orbital electron density in the bottom of the conduction band,24 and a replacement of lead with an element with different energies and polarizability of the orbitals will therefore have an effect on, for example, the band gap. It is, however, unclear if this is the only effect that matters, and how close other elements will be at reproducing the electronic properties of lead in the perovskite structure. In this article, we utilize Goldschmidt’s rules together with quantum mechanical extensions of the possible bonding patterns for exploring possibilities to replace lead in the perovskite structure. As a first step, ions with the same charge and similar ionic radius as Pb2+ are investigated. Thereafter, the ability to replace lead in the structure without causing a detrimental effect of the photophysics are discussed, which narrows down the list. By this approach, a look at the periodic table provides a number of candidates where the most interesting would be barium and strontium, where barium has a slightly larger ionic radius (Ba2+ = 149 pm) and strontium has almost the same ionic radius as lead (Sr2+ = 132 pm, Pb2+ = 133 pm). Strontium would indeed be a highly interesting alternative to lead. It is the 15th most common element in the earth crust, it is mined in reasonable quantities, and it is fairly nontoxic, as illustrated by the fact that strontium acetate is the active ingredient in the toothpaste Sensodyne25 which frequently, although with a possible economically motivated enthusiasm, is recommended by dentists.

There are, however, problems that must be addressed before the lead halogen perovskites have a chance of becoming a competitive commercial technology. One is the growing concern of the use of lead,8−10 which is a well-known toxic heavy metal. Different perspectives can be taken concerning the use of lead. The techno-optimistic standpoint claims that the benefits likely outweigh the risks and that efficient recycling schemes could minimize leakage of lead to the environment. It further points out that lead is tolerated in existing technology, like car batteries, ammunition, and pianos, and that metals like cadmium and arsenic, which are arguably more toxic than lead, are accepted in commercial solar cell technologies like CdTe, CIGS, and GaAs.11 It has been pointed out that the lead in a single car battery would be enough to make perovskite solar cells covering an area of 710 m2,12 corresponding to a comparatively high benefit-to-lead ratio. A specific problem with lead halogen perovskites is their water solubility, and the more pessimistic viewpoint worries over the consequences of damaged solar cell modules and panels with potential exposure to water followed by dissolution and distribution of lead ions into buildings, soil, air, and water. Lead is, as a matter of fact, one of the more well-studied toxins found in the environment and is known to damage the nervous system and cause brain disorders. One among many examples of the detrimental effects of lead pollution is the previous widespread use of lead, in the form of tetraethyl-lead, in gasoline, which have been shown to cause a decrease in the IQ levels of Americans by several IQ units,13 which in turn have been correlated to increased levels of crime in the society.14 Lead is unquestionably a problem, and there is an ongoing process in the European Union to phase out lead from the technological circular flow of materials. A large fraction of the compounds on the European Chemicals Agency’s candidate list do indeed turn out to be lead compounds. Regardless of the preferred viewpoint, it is reasonable to anticipate problems to promote and to obtain permission for a new technology based on a water-soluble lead compound, intended to cover acres upon endless acres of land. It would consequently, from a toxicological, marketing, and legal perspective, be highly beneficial if the lead in lead halogen perovskite solar cells could be replaced by a less toxic element, of course, without seriously degrading the overall solar cells performance. Such a replacement may be possible, especially given that CH3NH3PbI3 have turned out to be representative of a larger class of similar compounds, with comparable properties suitable for PV applications, rather than a single golden compound. The iodine has been changed to, Br15 and Cl,16 and the methylammonium ions have been substituted with other organic ions, like formamidinium17 and ethylammonium.2 These exchanges affect, among other things, the band gap, the charge transport properties, and the defect chemistry. As long as the structure remains intact, the substitutions do, however, appear to cause gradual changes rather than abrupt and fundamental differences in the photo physical properties. So far, most of the best results have been achieved with CH3NH3PbI3, but by playing around with different compositions, the properties can be tailor-made for specific device architectures and tandem combinations. Some work has been directed toward replacing lead.18,19 Most of these reports have been directed toward exchanging lead with tin,20,21 which is in the same group of the periodic 25674


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the coupled cluster (CCSD) level using the Stuttgart−Dresden pseudopotentials in the Gaussian09 quantum mechanical package.27 Periodic crystal calculations were performed with the Quantum Espresso package28 on resources provided by SNIC through Uppsala Multidisciplinary Center for Advanced Computational Science (UPPMAX). The crystal DFT calculations were done using the General Gradient Approximation (GGA) exchange correlation functional and the Perdew−Burke−Ernzerhof (PBE) pseudopotential.29 The strontium 4s2/4p6/5s2, the lead 5d10/6s2/6p2, the nitrogen 2s2/2p3, the iodide 5s2/5p5, and the carbon 2s2/2p2 electrons were considered as valence electrons. The super cells for both the lead and the strontium perovskite, MAPbI3 and MASrI3, consisted of 48 atoms in a tetragonal Bravia lattice with starting cell parameters, a = 8.71 Å and c = 12.46 Å. The orientation of the different methylammonium ions (CH3NH3+) where chosen in a way where neighboring dipoles are perpendicular to each other, which is illustrated in the Supporting Information SI. This configuration of the methylammonium ions have been reported to, according to DFT calculations, constitute the most stable phase at room temperature for the lead perovskite CH3NH3PbI3.30,31 This is in line with experimental data indicating a tetragonal structure with semi random orientation of the methylammonium ions at room temperature. This structure could be described as being composed of four slightly distorted cubic unit cells with a cell parameter, a, around 6.31 Å composed of the Sr or Pb perovskite. For the lead perovskite, the tetragonal structure is at higher temperatures experimentally observed to transform into a cubic phase. All the unit cell vectors and atom coordinates were relaxed to have a total force lower than 0.08 Ry/au for the strontium perovskite and 0.04 Ry/au for the lead perovskite. The XcrysDen package32 was used for visualization of atoms and charge densities. Cut-off energies for plane wave function and charge density were set to 25 and 300 Ry, respectively. Brillouin zone sampling was carried out by a 3 × 3 × 3Monkhorst−Pack grid (MPG) for the relaxation procedure, a 4 × 4 × 4MPG for the self-consistent procedures and a 6 × 6 × 6MPG for the nonself-consistent procedures. Formation energies of CH3NH3PbI3 and CH3NH3SrI3 were quantified by calculating the energy difference of the perovskite phase and the precursors; PbI2 and MAI in the case of CH3NH3PbI3, and SrI2 and MAI in the case of CH3NH3SrI3. The PbI2, SrI2, and MAI energies were calculated using the same computational parameters, except that single elements of Pb or Sr were calculated in a cubic lattice with lattice constants large enough to exclude any possible interaction of atoms in the lattice. Theory and Goldschmidt’s Rules. Crystal Structure and Tolerance Factors. A perovskite is originally referring to a calcium titanium oxide mineral with the chemical formula CaTiO3, but now also to an entire class of compounds with the general formula ABC3 that crystallize with the same structure. A and B are two different cations, and C is an anion binding the two together, as illustrated in Figure 1a. The ideal structure is cubic, with the smaller B cation octahedraly coordinated by six anions, and the larger A anion coordinated to 12 cations in a cubocatahedral symmetry, which is illustrated in Figure 1a−c. The perovskites were first investigated by Goldschmidt in the 1920s33 in work related to tolerance factors. The tolerance factor, t, with respect to the ionic radius of the actual ions is

The possibility of replacing lead with strontium in the perovskite structure is here explored both theoretically and experimentally. Coupled cluster singlet and doublet (CCSD) calculations were performed on the metal ions and the binary precursor salts to analyze possible bonding patterns and coordination. We go through theoretical arguments of what could be expected of the strontium iodine-binding and compare it to what is known for the lead analogue. We then utilize density functional theory (DFT) to calculate the geometrical ground state structure of CH3NH3SrI3 and CH3NH3PbI3 to extract and compare the electronic structures and the optical properties. We further discuss practical problems encountered while exploring wet chemical synthesis procedures for making the methylammonium strontium perovskite To the best of our knowledge, no theoretical or experimental data concerning organic strontium halogen perovskites have been reported so far. The approach of utilizing Goldschmidt’s rules together with additional quantum mechanical considerations, here explored and exemplified, can provide a promising route toward replacing lead, as well as provide general insights into the photophysics of the metal halogen perovskites.

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METHODS Experiments. A range of experiments have been performed with the goal of synthesizing the methylammonium strontium perovskite, CH3NH3SrI3. Protocols for synthesizing the analogous lead perovskite were used as a template. The solution preparation and all syntheses were performed under ambient atmosphere. In one series of experiments, an equimolar precursor solution of CH3NH3I and SrI2 was prepared. One gram of CH3NH3I (6.29 mmol) and 2.15 g anhydrous SrI2 (6.29 mmol) were dissolved in 6.2 mL N−N-dimethylformamide (DMF) under magnetic stirring at 80 °C. This solution, as well as diluted solutions, was spin-coated and drop coated on glass substrates. The films were thereafter annealed at different temperatures in an oven or on a hot plate. Attempts were also performed to synthesize mixed strontium halide perovskites according to the method of Ball et al.26 CH3NH3I and SrCl2 with a molar ratio of 3:1 were dissolved in DMF with 40% weight ratio of total solvated salts. The solution was spin coated at 2500 rpm for 30 s on either fluorine doped thin oxide (FTO) or on glass coated with an Al2O3 scaffold. The deposited films were annealed at either 100 or 150 °C. Lead halogen perovskites were synthesized to get a reference for comparison. The synthesis was performed according to established procedures,3 based on spin coating of an equimolar DMF solution of PbI2 and CH3NH3I. In a typical synthesis, 1 g of CH3NH3I (6.29 mmol) and 2.89 g PbI2 (6.29 mmol) were dissolved in 6.2 mL DMF at 80 °C under magnetic stirring for 3 h. This forms an orange-yellow solution containing 40 wt % salts. Thin films of the lead perovskite were spin coated on soda lime glass at 2000 rpm for 60 s using the 40% precursor solution. The films were thereafter annealed at 150 °C for 10 min. XRD measurements were performed with a Siemens D5000 Diffractometer, using parallel beam geometry, an X-ray mirror, and a parallel plate collimator of 0.4°. Cukα with a wavelength of 1.54 Å was used as X-ray source and 2Θ scans were measured between 5° and 90° using a step size of 0.014°. Calculations. Computations concerning ionic binding symmetries were performed with atomic based basis sets at 25675


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Table 2. Pauling’S Rules for Cation and Anion Radius Ratio and Coordination

a

Figure 1. (a) Unit cell of a general cubic perovskite. (b) Lead halogen perovskite illustrating the octahedral coordination around the lead ions. (c) Lead halogen perovskite illustrating the cuboctahedral coordination around the organic ion. (d) CH3NH3PbI3 with the methylammonium ions explicitly given, even though not with optimized orientations. The structures are drawn using the software Vesta.40

rC/rA

coordination

coordination number

1.0

2-fold coordination triangular tetrahedral octahedral cubic dodacahedral

2 3 4 6 8 12

The ideal ratio for the coordination.

after simple arithmetics gives the size of the individual atoms. To determine the ionic radius is, however, harder as the bond distance cannot simply be divided into equal parts due to differences in the electronegativity between the ions. The division between atoms in bonds can instead be determined by quantum mechanical theory as the minimum in the electrostatic potential between the atoms. This in turn depends on the coordination number and the type of pairing ion. The difference in electronegativity of the element in itself is thus too crude a discriminator and has to be complemented with the bonding symmetries dictated by quantum mechanics and the way this polarizes the electron density around the atom. Identification of Candidates for Replacing Lead. For an initial identification of interesting candidates for lead substitution in the perovskite structure, experimentally estimated radii in the notion of Shannon41 can be used, which originate from crystals in halogen systems with coordination taken into account. In the following analysis, radii for metal cations determined in halogen systems with a 6fold coordination will be used if not otherwise stated. The perovskites of primary interest for solar cell applications are organic lead halogen perovskites, where the most intensely investigated compound so far is CH3NH3PbI3. The A cation is here an organic group, whereas the B cation is a Pb2+, and C is a halogen ion. An idealized structure of this compound is given in Figure 1d. The ionic radius of Pb2+ is 0.132 nm, for I− it is 0.206, and for CH3NH3+ it is 0.18 nm.42 This gives a tolerance factor, according to eq 1, of 0.81, which is consistent with a noncubic perovskite structure, and at room temperature, a tetragonal structure is observed.43 The tetragonal structure is also a consequence of the methylammonium cations not having a spherically symmetry. An idea of the expected structure can also be obtained by drawing upon Pauli’s rules, summarized in Table 2, where the additive radius of I− and CH3NH3+ in an effective packing around Pb2+ would give a tolerance factor of 132 pm/386 pm = 0.32, which although not strictly applicable also indicates a noncubic structure (a tetrahedral). The temperature is also important for the phase stability and a phase transformation of noncubic perovskites into a cubic structure is often seen at elevated temperatures. For

given by eq 1, where rA, rB, and rC, are the ionic radius of the A, B, and C ions, respectively. rA + rC t= 2 (rB + rC) (1) A tolerance factor of 0.9−1 is compatible with the ideal cubic perovskite structure in Figure 1a. For a tolerance factor between 0.7 and 0.9, the A ion is too small, or the B ion is too large, for a cubic structure. This instead results in an orthorhombic, rhombohedral, or tetragonal structure. For a large A cation, t gets larger than one, which results in layered perovskite structures,34,35 and a wide range of stochiometries and superstructures are known, like for example Ruddlesden− Popper, Aurivillius, and Dion−Jacobson phases.36 The effect of tolerance factors are summarized in Table 1. The expected structure is also related to Pauling’s rules (PRs)37 given the expected coordination around a two component cation/anion system which is summarized in Table 2. The range of the tolerance factor in photovoltaic perovskite materials and its effects on the band gap has been recently discussed.38,39 The empirical rules for element substitution according to Goldschmidt33 are (i) if the ionic radius differs by less than 15%, a full substitution can occur, and if the size differs between 15 and 30%, limited substitution can occur; (ii) if the ionic charge differs up to one unit, the element can be exchanged if charge neutrality is maintained by charge balance of other contributing ions in the material. Ringwood came with a modification to these rules in 1955 which introduced the difference in electronegativity,23 and thus the bond character, as an important factor. That could explain why Na+ and Cu+ not substitute each other, despite having the same radius and charge, as well as other similar observations. In many of these notions, the radius is an important factor. An important concern, although maybe not immediately obvious, is how to accurately determine the radius of an ion. The covalent or metallic atomic radius of an element can be determined from the bond distance in dimers or crystals, which Table 1. Tolerance Factors for the Perovskite Structure tolerance factor

structure

comment

1.0

tetragonal/ortorombic/rhombohedral cubic various layered structures

A too small or B too large ideal perovskite structure Acation too large

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Figure 2. Orbital symmetries of LUMO, LUMO+1, and LUMO+2 depicted with an isodensity value of 0.06 for Pb2+ and Sr2+ and the corresponding bonding situation in PbI2 and SrI2 showing HOMO−1, HOMO, LUMO, and LUMO+1. The centered LUMO in SrI2 is shown with isodensity value of 0.05 to more clearly visualize the symmetric ring shape of the orbital.

a noble gas electron configuration. This indicates the bonding in the lead halogen octahedral to have a high degree of ionic character and a low amount of directional bonding. The electron configuration of Sr2+ is [Kr], also giving a spherical symmetric electron configuration and a preference for ionic bonds. Given this, and that Sr2+and Pb2+ have almost the same charge and radius, it is reasonable to assume that the structural motif of the halogen octahedral would remain if lead were replaced with strontium. The quantum mechanical bonding scheme must, however, also be taken into account. In Figure 2a−d, the bonding situation on a coupled cluster level (CCSD) are shown for the 2+ oxidized states of Pb and Sr, where the three lowest unoccupied orbitals (LUMO, LUMO+1, and LUMO+2) act as accepting orbitals in a stoichiometric unit cell with PbI3− and SrI3−. The orbital symmetries allow a 6coordinanted bonding in both cases, whereas the Sr2+ has a more distinct belt around it is middle from the s-orbital character of the LUMO and LUMO+1 orbitals. The bonding situation in the cation-halogen salt shows the HOMO−1 and HOMO binding to be delocalized onto the iodine, and the LUMO and LUMO+1 are distributed around the cation, as expected. The calculated bonding patterns of the cations and the precursor metal halogen salts show a natural extension to a 6-coordination, where the similar orbital extension supports that strontium perovskite may form a stable phase, where Sr plays the same role as Pb does in the lead perovskite. There is a possibility that the spherical configuration of inner electrons can be disturbed when the metal ions are placed in the periodic potential of the crystal lattice. Results of periodic DFT calculations of the charge distribution, ionic bare potential, and total potential of Sr and Pb perovskites, which are discussed in more detail in the next section, support the picture of a fairly spherical electron configuration around both Pb2+ and Sr2+ given by the calculations presented in Figure 2. There we actually see that insights from the molecular fragments prevail in the periodic environment. Concerning Synthesis of Strontium Perovskites. Apart from considerations of Goldschmidt’s rules and quantum mechanical bonding patterns, problems of a more practical nature can occur while synthesizing the perovskites, which have to do with the properties of the reactants. Examples of such problems are differences in solubility, kinetic pathways, geometry, and availability of replacement ions in the synthesis. Due to such things, it is reasonable to assume that the strontium halogen perovskite, CH3NH3SrI3, is harder to synthesize than the corresponding lead perovskite, at least if following the type of wet chemical procedures developed for the lead perovskites. The problem arises as a consequence of the crystal structure of the halogen salts, PbI2 and SrI2, which are the most accessible

CH3NH3PbI3, such a transformation has been reported around 55 °C.22,44 In the following analysis, the atoms directly neighboring Pb in the periodic system, i.e., tin, bismuth, and thallium are excluded, as they to some extent have been investigated before with the purpose of synthesizing lead free organic halogen perovskites. By comparing the ionic radius for metallic cations with an oxidation state of +II and filtering out the ones with an ionic radius within 15% of Pb2+ (132 pm), as given in halogenic 6-coordinated systems according to Shannon,41 a list of nine elements are found. These are Ba2+ (149 pm), Sr2+ (132 pm), Eu2+ (131 pm), Np2+ (124 pm), Dy2+ (121 pm), Tm2+ (117 pm), Yb2+ (116 pm), Hg2+ (116 pm), and Ca2+ (114 pm). They should, in principle, according to Goldschmidt’s rules, be able to substitute for lead in the perovskite structure. Three additional elements are in the proper size regime, Nd2+ (143 pm), Am2+ (140 pm), and Sm2+ (136 pm), but these are preferably found in 8- and 7-coordinated compounds. Lead is known to have two stable oxidation states, Pb2+ and Pb4+, but in the perovskite, CH3NH3PbI3, only the +II oxidation states is observed. The situation with mixed oxidation states in the perovskite is effectively hindered by a combination of the inert pair effect, and the difference in two between the two stable oxidation states. In the tin perovskite, CH3NH3SnI3, where the inert pair effect is weaker, mixed oxidation states are seen which have been found to be detrimental for the solar cell performance. To avoid compounds with mixed oxidation state, frustration in crystal structure, and vacancy formation, elements with two stable oxidation states with one charge unit difference could be filtered out. That reduces the nine candidates to three: Ba2+, Sr2+, and Ca2+. Case for Replacing Lead with Strontium. Of these three candidates, we have here decided to specifically focus on strontium. As the size of the Sr2+ ion is almost identical to the Pb2+ ion (132 vs 133 pm), this appears to be an interesting candidate for ion replacement without changing the room temperature crystal structure. Also the other elements would be interesting to investigate for this purpose, but that will be the focus of other studies. One of the basic structural motifs in the lead halogen perovskite is a halogen octahedral around a six coordinated Pb2+ ion. Another is a 12-fold cuboctahedral coordination around the organic cation. As mentioned above, if the tolerant factor deviates from 1, a slight buckling and distortion can occur, which can reduce the symmetry and to some degree also change the coordination around the A cation. The electronic configuration of the Pb2+ ion is [Xe]14 10 2 4f 5d 6s with a full 5d shell and a deep lying inert pair of 6s electrons, indicating a fairly spherical symmetric electron distribution. The same could be said for the I− ion, which have 25677


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Figure 3. (a) Crystal structure of SrI2. (b) Crystal structure of PbI2 illustrating the layered structure. (c) Crystal structure of SrI2 illustrating difference between the layered PbI2 structure.

the lead iodine, and the intercalation and formation of the perovskite structure must have a fairly low activation energy. This view of the mechanism of the synthesis is supported by the stepwise synthesis, where PbI2 is deposited separately and thereafter exposed to CH3NH3I,7,46 which works as well as the single step synthesis described here. The strontium iodine, SrI2, does instead crystallize in an orthorhombic structure where strontium is coordinated to seven iodine ions, as illustrated in Figure 3a. This is not a layered structure but rather an interconnected networkstructure, as illustrated in Figure 3c. Direct intercalation of methylammonium into the strontium iodine structure is consequently more difficult than what is the case for lead iodine. The 7-fold coordination to the strontium in the iodine further shows that a greater reorganization is required, which translates into a higher activation energy, while going from the strontium iodine to the perovskite compared to the case when the lead iodine is reacting. This implies that the wet chemical synthesis procedure used for CH3NH3PbI3 most likely must be rethought in order for CH3NH3SrI3 to be synthesized. Vaporbased synthesis may constitute a more favorable pathway in this case, which would be an interesting alternative to explore.

metal halogen precursors. These two salts have different solubility and crystal structures, which affect the kinetics and reaction paths while forming the perovskite by solution chemistry. Lead iodine, PbI2, crystallizes in a rhomohedral structure,45 composed of layers of face sharing, distorted, lead iodine octahedral, where the layers are held together by weaker van der Waals forces. The crystal structure of PbI2 is given in Figure 3b. In the typical synthesis procedure, an equimolar solution of PbI2 and CH3NH3I are dissolved in DMF, or some other solvent, spin-coated, and thereafter annealed, whereupon the perovskites forms. In the case of DMF and DMSO, the solubility of the methylammonium iodine is considerably higher than that for PbI2, and no reaction seems to occur at room temperature in the solvent. While the solution is spin-coated and the solvent is evaporated, PbI2 precipitate and crystallize first, which could be seen by the color of the film. Upon heat treatment, while the last of the solvent is evaporating, the methylammonium intercalates the layers in the PbI2 structure, where the extra iodine is enabling the structure to transform from face sharing lead iodine octahedral to corner sharing octahedral that surrounds the methylammonium ions. This proposed mechanism is illustrated schematically in Figure 4. In

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RESULTS AND DISCUSSION Structure Calculation. DFT calculations were performed on both the lead halogen perovskite and the corresponding strontium structure. The calculations indicate the tetragonal phase of strontium perovskite, CH3NH3SrI3, to be a stable phase with a formation energy of 0.402 Ry per unit cell (131.65 kJ/mol), compared to 0.414 Ry per unit cell (135.58 kJ/mol) for CH3NH3PbI3. It could further be concluded, by considering the same thermodynamic factors, that the formation energies are almost identical for the strontium and the lead perovskite, with a difference of merely 0.12 Ry per tetragonal unit cell (39.4 kJ/mol). The zero point energy (ZPE) can be estimated from half the energy of the difference between the lowest phonon energies and is on the order of 25 cm−1 and thus 0.3

Figure 4. Proposed mechanism for methylammonium lead iodine by intercalation of methylammonium iodine into solid PbI2..

this process, no covalent bonds needs to be broken, the solubility of the methylammonium must be higher than that for

Figure 5. Simulated data (a) Distortion of the PbI6 octahedron along the c-axis. (b) Distortion of the SrI6 octahedron along the c-axis. (c) Distortion of the PbI6 octahedron in the ab-plane. (d) Distortion of the SrI6 octahedron in the ab-plane. 25678


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Figure 6. (a) Calculated PDOS for the strontium halogen perovskites. (b) Calculated PDOS for lead halogen perovskites. (c) Comparison of the DOS for lead and strontium halogen perovskites. (d) Comparison of the PDOS for the organic part in the two perovskites. (e) Comparison of the PDOS for strontium and lead in the respective perovskites. (f) Comparison of the PDOS for the iodine in the two perovskites.

kJ/mol.47 The entropy contribution is here only a few percent of the total formation energy since no gas-to-solid transitions are present. The ZPE and the entropy contribution to the total formation energy are here neglected, and this approximation has previously also been shown to give meaningful trends in formation energies for perovskites while compared to experiments.48 The relaxed cell parameters were, within the resolution of the calculations, the same for both the strontium and the lead perovskite. Details of the variable cell relaxation of the strontium perovskite lattice are found in the SI. There are, however, some structural differences that may be of importance. Even seemingly small distortion can have a significant impact on the photo physics of the material. The distortion angles of the octahedron within the perovskite lattice, or the metal−halogen-metal bond angle, have, for example, been reported49,50 to influence the band gap in the tin perovskite, CH3NH3SnI3. After relaxation of the atomic coordinates for the Pb and Sr perovskite lattices, the distortion angle of SrISr parallel to the c-axis was found to be somewhat higher (172.28°) compared to the corresponding PbIPb angle (170.77°). There is also some distortion of the SrISr bonds in the ab-plane, which is lower (151.27°) than the corresponding PbIPb angle (153.89°). These bond angles are illustrated in Figure 5a−d. Density of States and Estimation of the Band Gap. From the periodic crystal calculations, the density of states (DOS) was extracted for both the lead and the strontium perovskite. This was in turn subdivided into partial density of states (PDOS) for the metal cation, the methylammonium cation, and the halogen, which is illustrated in Figure 6a,b. From these data, the band gap of CH3NH3SrI3 were estimated to 3.6 eV. There is some uncertainty to this number due to the well-known band gap problem associated with DFT calculations. Trends do, however, tend to be trustworthy, and the same calculations performed on CH3NH3PbI3 give a band gap of 1.6 eV, which is fairly close to the experimental value reported to be around 1.55 eV.5,43 This indicates that the errorcancelation due to underestimation of the band gap in pure

DFT and the neglect of the spin−orbit effect by the scalar relativistic pseudo potentials to be reasonably good in this case. The top of the valence band is mainly composed of energy levels originating from the iodine, or more specifically the I-5p electrons. This is the case for both the lead and strontium perovskite, as can be seen in Figure 6. The nature of the bottom of the conduction band does, however, differ between the two perovskites. In the lead perovskite, the bottom of the conduction band is primarily composed of energy levels originating from the lead ion, with a small amount of hybridization with the I-5p and I-5s orbitals. In the strontium perovskite, the energy states in the bottom of the conduction band are instead the result of a more extensive hybridization between the Sr-5s, the I-5p, and I-5s orbitals. Also the methylammonium contributes with states in the bottom of the conduction band. The shift of the conduction band edge in the strontium perovskite is thus large enough for the electronic properties of the organic ion to have a more significant influence on the band gap. The orbital overlap between the organic part and the strontium may, however, be weaker than the coupling between the halogen and the metal, and consequently have a smaller impact on the optical absorption. Another effect caused by exchanging Sr for Pb is that the shape of the PDOS for both the halogen and the organic ion is shifted and slightly distorted, as seen in Figure 6d,e. The lower electronegativity of Sr, compared to Pb, shifts the electronic cloud closer to the iodine atoms in the lattice, which in part can rationalize this behavior. This will perturb the local dipole moment as well as the bonding angles between the iodine octahedrons and consequently also their columbic interaction with the methylammonium dipoles. According to Figure 6c, a replacement of lead with strontium will result in an upward shift of the conduction band, resulting in a higher band gap. It has been reported that in perovskites with the general formula ABC3, the band gap decreases with increasing electronegativity of the B cation,51,52 and that the electronegativity of the B cation has a substantially larger impact on the band gap than the A cation. The antibonding contribution in the BC bond also impacts the band gap, and 25679


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Figure 7. Schematic illustration of charge density (a,b), bare potential (c,d), and total potential (e,f) inside the lattice for CH3NH3PbI3 to the left (a,c,e) and CH3NH3SrI3 to the right (b,d,f). Isolines are plotted to distinguish the regions of the same value in the plot.

band gap as seen in the lead perovskites.24 Replacing the organic ion may be more promising. Replacing the methylammonium with the slightly larger formamidinium ion in lead iodine perovskites has been reported to decrease the band gap somewhat,17 and theoretical arguments have been made that a smaller cation also could decrease the band gap.24 It has been suggested that the band gap of the strontium perovskite may be reduced if the organic cation would be exchanged by an inorganic counterpart, like cesium. Experimental efforts toward synthesizing CsSrI3 have turned out to be problematic due to solubility problems as previously reported in conjunction with synthesis of the cesium lead iodide perovskite22 and have here not been pursued. Changing the size of the cation results in geometrical changes, such as octahedral tilting and the tolerance factor, which can have a large effect on the band gap of the perovskite material.39 Theoretical investigation of the orthrombic cesium strontium perovskite could give insight into the band gap tuning of this class of perovskite materials, but is outside the scope of this article. Although a replacement of the organic cation may decrease the band gap, it also removes the dipole of the cation, which are most likely crucial for the performance of the perovskite solar cells. Charge Distribution. From the DFT calculations, the charge distribution was extracted, and the spatial distribution of the electronic charge over a unit cell for the two perovskites investigated is given in Figure 7a,b. Not surprisingly, the overall spatial charge distribution is similar for the two structures, with the exception of a higher charge density around lead compared to strontium due to the higher atomic number of lead. A closer inspection does, however, reveal the charge density around the SrI bonds to be slightly more localized than what is the case for the PbI bonds. This is further supported by the Löwdin charges of the atoms in the lattice, which were estimated from the PDOS calculations. The iodide charge was 53.4e in the lead perovskite and 53.7e in the strontium counterpart, where e is the electron charge. The Löwdin charge for the Pb and Sr ions in the perovskites were 81.7e and 36.4e, respectively. This shows that the SrI bond in the perovskite is somewhat more ionic compared to the corresponding PbI bond. This is rationalized by the higher difference in electronegativity, and that the iodine can donate electrons into empty p-orbitals of

an enhancement of the antibonding character of the BC bond widens the band gap.51 The origin of this effect is here seen to be a consequence of different orbital symmetry, and the resulting different possibility for hybridization. Strontium has a lower electronegativity than lead (0.95 versus 2.33), and there is a larger antibonding character of the strontium−iodine bond, which is of σ-character, than in the bond between lead and iodine, which has more π-character. The sum of these contributions is likely the main reason for the wider band gap of CH3NH3SrI3 compared to CH3NH3PbI3. The calculated band gap of 3.6 eV of the CH3NH3SrI3 is not an ideal band gap for a photoabsorber, which is a negative result from the perspective of utilizing this material as a replacement for CH3NH3PbI3 in solar cells. The calculated band gap can be seen as a lower limit, as a pure GGA functional (PBE) was used, but it is still clear that CH3NH3SrI3 is not a promising solar absorber material. The MASrI3 could still be of interest in PV-applications, but then rather as a UV-filter or as a transparent contact material. Experimental verification is, however, still needed to confirm this high band gap and the absolute band edges to assess the possible applications of this material. If the band gap should turn out to be somewhat lower, then CH3NH3SrI3 could, if the photophysics and transport properties are comparable to CH3NH3PbI3, still be an interesting material for a top cell in a tandem configuration or, depending of defect type, as a selective layer for electrons or holes. The lack of a diffuse frontier orbital in strontium that can hybridize with the iodide 5p orbitals can result in highly effective masses for the hole. The band structure for the strontium perovskite has been calculated, and is given in the SI, shows a low dispersion, thus confirming that the effective mass for the hole in the valence band is fairly high. This is an indication of poor hole conducting properties. The strong dipolar cation in the structure could, however, change by introducing states high in the conduction band as well as cause some ferroelectric effects. This leads to a lower electron effective mass in the conduction band, and indicates reasonable electron transport properties. The band gap and the effective mass may be engineered by replacing additional ions in the structure. A replacement of the iodine with another halogen would, however, likely increase the 25680


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Figure 8. Green line is our experimental data on the lead perovskite, CH3NH3PbI3. The purple line is XRD data simulated from DFT-structure for the lead perovskite. The red line is the coresponding simulated data for the strontium perovskite. The indexation is for the simulated lead perovskite and represent families of planes.

Attempts to make a mixed halide strontium perovskite using SrCl2 as a strontium source had the same results as the experiments mentioned above, resulting in a white product with the X-ray diffraction peaks of the starting materials. Different solvents including DMF, DMF/DMSO, DMSO, water, and isopropanol, and different heat treatments in the range of 100− 150 °C were used, but without changing the fact that the intended strontium perovskite not was formed. A likely reason for the problems in synthesizing CH3NH3SrI3, is the problems that the methylammonium ions have in intercalating the SrI2 structure, as outlined in the theory section. From the experimental effort up to this point, it can be concluded that CH3NH3SrI3 indeed is difficult to synthesize according to the wet chemical intercalation procedures used to make the now well-investigated CH3NH3PbI3. The synthesis approach must be rethought, and vapor-based methods seem to supply a more promising route, which we currently are pursuing. The lead perovskites were synthesized according to standard procedure. This gave a reference used for comparison with the products formed in the synthesis with strontium. The lead perovskite formed easily under these conditions. An XRD spectrum of the synthesized lead perovskite is given in Figure 8, which matches the expected diffraction pattern for CH3NH3PbI3. Due to obvious reasons, the XRD pattern of the strontium perovskite is not found in the databases. In order to obtain a reference pattern to compare with experimental data, a theoretical diffractogram was simulated based on the relaxed computed structure. This XRD pattern is given in Figure 8. The experimental XRD data for the lead perovskite, CH3NH3PbI3, well match the experimentally reported XRD data.43 The XRD data simulated from the atomic coordinates given from the DFT calculation in this paper deviate somewhat from the diffractogram in the sense that the peaks split into several closely grouped peaks. The reason for this is the way DFT calculations deal with temperature. At zero kelvin, which is used in the DFT simulations, the methylammonium ions have a preferential orientation where the overall energy is minimized. The methylammonium has a low rotational activation energy and at room temperature, it will appear as a time averaged ellipsoid. That will give the structure a higher symmetry while also experiencing it as the time average seen under experimental conditions. The calculated structure thus loses some of the symmetry found experimentally at room temperature, which results in a peak splitting in the simulated XRD-pattern. The simulated XRD diffractogram is, however, still a useful reference for comparison to experimental data in further synthesis work directed toward making the strontium perovskite. A more idealized simulated XRD spectrum based on

the lead. That give a higher covalent contribution and affects the band gap of the materials, as analyzed in the view of the symmetry of the orbitals above. The difference in ionic character is also analyzed via the bare potential, Vbare, and the total potential, Vtotal, calculated for the two perovskite structures according to their definitions given in eqs 2−4, where e is the elementary charge, ε is the dielectric constant of vacuum, r and r′ are space vectors, Vionic is the potential of the positively charged ionic centers, and n(r) is the self-consistent charge density at point r. For calculation of VBare, the summation runs over all ionic centers i and for the Hartree potential, VHartree, of electron number l, the summation runs over all electrons other than l. The total potential is the summation of the bare, the Hartree, and the exchangecorrelation potential, Vex‑cor, in which the latter is estimated from the GGA approximation in the present study. n

VBare(r ) = e ∑

i V ionic (r′)(r − r′)

i=l

l V Hartree (r ) = e ∑ i≠l

4πε |r − r′|2

∫Unitcell

(ni)(r′)(r − r′) 4πε |r − r′|2

(2)

dr ′

l Vtotal(r ) = VBare(r ) + V Hartree (r ) + Vex − cor(r )

(3) (4)

The calculated bare potential is given in Figure 7c,d and the corresponding total potential in Figure 7e,f. While comparing the potential distribution along the SrI and the PbI direction, it appears to change more abruptly in the strontium perovskite and to be more evenly distributed in the lead perovskite. This is in agreement with the charge density data in Figure 7a,b, and is consistent with a picture with a more covalent PbI interaction in the lead perovskite and a more ionic SrI interaction in the strontium perovskite. Experimental Results. Initial attempts toward synthesizing CH3NH3SrI3 using the same wet chemical protocol as for CH3NH3PbI3 were performed. One gram of CH3NH3I (6.29 mmol) and 2.15 g anhydrous SrI2 (6.29 mmol) were dissolved in 6.2 mL DMF under magnetic stirring. Both substances are easily soluble in DMF at these concentrations, and the SrI2 is more soluble than the PbI2. Films were both spin and drop coated and heat treated at various temperatures. The deposited films were white and nontransparent. They were also highly hygroscopic, and in ambient atmosphere, with time they transform into a brown, viscous slurry. XRD measurements on the formed product did not indicate the formation of the intended perovskite. Rather the unreacted starting materials, and possibly also some unstable amorphous phase, were obtained. 25681



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simple replacement of lead with strontium in an experimentally determined structure, while holding all other structural parameters fixed, is found in the SI, together with a more complete indexation.

AUTHOR INFORMATION

Corresponding Author

*Phone: +41 (0)76-4298224. E-mail: Jesper.jacobsson.work@ gmail.com (T.J.J.).

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Notes

CONCLUSIONS In this work, strategies for finding and evaluating possible elements that could replace lead in the lead halogen perovskites used for solar cells are explored. This is done by combining classical crystal chemistry and the notion of tolerance factors from Goldschmidt with quantum mechanical considerations and DFT calculations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radii of Pb2+ and Sr2+ are close to each other (Sr2+ = 132 pm, Pb2+ = 133 pm), wherefore replacing Pb2+ with Sr2+ may lead to a stable perovskite structure. CCSD calculations on the oxidized elements and the precursor salts show an orbital symmetry allowing 6-coordination to iodide in both the Pb2+ and Sr2+ case. DFT calculations indicate that the strontium perovskite, CH3NH3SrI3, is a stable phase with a formation energy and cell parameters closely matching the analogous lead perovskite. The band gap of the strontium perovskite is estimated to be 3.6 eV which is considerably higher than that for the lead perovskite, CH3NH3PbI3, (1.6 eV), which is caused by an upward shift in the conduction band. The band gap of the strontium perovskite is thus not suitable for direct solar cell applications. If it could be decreased by also interchanging the organic ions, then it cannot be excluded that strontium perovskites could be interesting as, for example, a top cell in tandem architectures. The charge density and the potential distribution was extracted for both perovskite structures, and the metal halogen interaction was found to be more ionic in the strontium perovskite, in line with the lower electronegativity of strontium. The lack of effective hybridization with the frontier orbital of strontium and the iodide 5p-orbitals results in a low band dispersion in the valence band, which is equivalent with a high effective hole and probably also poor hole conducting properties. The effective mass of the electrons in the conduction band was found to be reasonable low. Initial attempts were made toward synthesis of CH3NH3SrI3, which were based on wet chemical intercalation routes developed for CH3NH3PbI3 synthesis. From these experiments, it was concluded that other routes of synthesis needs to be explored, i.e., vapor phase or layer by layer synthesis. This is a consequence of the less open crystal structure of SrI2, which is the most readily accessible precursor salt, compared to the layered structure of PbI2. This introduces a larger energy barrier while going from the precursor salts to the perovskite as covalent bonds needs to be broken in the process.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Malin Johansson for sharing knowledge and protocols for perovskite synthesis and Marcus Lundberg for discussions about simulations. We acknowledge Uppsala Multidisciplinary Center for Advanced Computational Science (UPPMAX) for providing the computational resources under projects snic 2014-3-71, snic 2015-6-65, and snic 2015-6-83. We would also like to acknowledge Lennanders Stiftelse for financial support.

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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.5b06436. Relaxation parameters for the cubic strontium phase. Additional simulated XRD data. Band structure of CH3NH3SrI3, details of the formation energy calculations (PDF) 25682


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