Article pubs.acs.org/crystal
Low-Temperature Solution-Grown CsPbBr3 Single Crystals and Their Characterization Yevgeny Rakita,† Nir Kedem,† Satyajit Gupta,† Aditya Sadhanala,‡ Vyacheslav Kalchenko,§ Marcus L. Böhm,‡ Michael Kulbak,† Richard H. Friend,‡ David Cahen,*,† and Gary Hodes*,† †
Department of Materials and Interfaces and §Departments of Veterinary Resources, Weizmann Institute of Science, Rehovot, 7610001, Israel ‡ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom S Supporting Information *
ABSTRACT: Cesium lead bromide (CsPbBr3) was recently introduced as a potentially high performance thin-film halide perovskite (HaP) material for optoelectronics, including photovoltaics, significantly more stable than MAPbBr3 (MA = CH3NH3+). Because of the importance of single crystals to study relevant material properties per se, crystals grown under conditions comparable to those used for preparing thin films, i.e., low-temperature solution-based growth, are needed. We show here two simple ways, antisolvent-vapor saturation or heating a solution containing retrograde soluble CsPbBr3, to grow single crystals of CsPbBr3 from a precursor solution, treated with acetonitrile (MeCN) or methanol (MeOH). The precursor solutions are stable for at least several months. Millimeter-sized crystals are grown without crystal-seeding and can provide a 100% yield of CsPbBr3 perovskite crystals, avoiding a CsBr-rich (or PbBr2-rich) composition, which is often present alongside the perovskite phase. Further growth is demonstrated to be possible with crystal seeding. The crystals are characterized in several ways, including first results of charge carrier lifetime (30 ns) and an upper-limit of the Urbach energy (19 meV). As the crystals are grown from a polar aprotic solvent (DMSO), which is similar to those used to grow hybrid organic−inorganic HaP crystals, this may allow growing mixed (organic and inorganic) monovalent cation HaP crystals.
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in a precursor-solution.13,14 However, organic elements in these compositions can act as “weak links” and decrease the stability (and thus reliability) of the entire structure, as was demonstrated in a comparison between the MAPbBr3 and CsPbBr3 HaPs.8 It was also shown that exchanging to the fully inorganic compound (i.e., CsPbBr3) does not necessarily exact a price in photovoltaic performance.9 To be able to use single crystals to study CsPbBr3 and compare it with its organic− inorganic hybrid analogs, the crystals should preferably be prepared under conditions that are comparable to those of the thin films used in device work. While the hybrid HaPs can indeed be grown as macroscopic crystals from solution, no such process was available for CsPbBr3 crystals. Here we report on this missing link. At standard temperature and pressure, the perovskite structure is the thermodynamically most stable structure of CsPbCl3 and CsPbBr3 (but not of CsPbI3).15 The only reported procedure of making millimeter-sized single crystals of these materials is by Bridgman growth, where precursors are melted (above 600 °C) in a quartz tube and passed through a multitemperature zone tube furnace).7,16 For hybrid organic−inorganic perovskites, the known compositions that can be created have a single stoichiometric composition (i.e., AMX3) regardless of precursor ratio and
INTRODUCTION In the past few years, halide perovskites (HaPs) with AMX3 stoichiometry (where A is methylammonium (MA), formamidinium (FA), or cesium (Cs); M = Pb or Sn; X = Cl, Br, I) have attracted attention because of their remarkable optoelectronic (especially photovoltaic) properties, compared to other competitive, low-cost, optoelectronic materials (e.g., various organic molecules and polymers, CdX, PbX (X = S, Se, Te), Cu(In,Ga)Se2).1,2 Some compositions of AMX3 yield small area photovoltaic light-to-electrical conversion efficiencies above 21%,2 and also relatively efficient and tunable light-emitting diodes,3,4 photodetectors/resistors,5,6 and high-energy radiation detectors.7 To help understand the remarkable optoelectronic properties of these materials, one would like to be able to study bulk material of controlled quality, prepared under conditions that are not too different from those used for making films employed in most HaP-based devices. Clearly, single crystals are a desirable form of materials for this purpose. CsPbBr3, has recently gained attention as a stable HaP,8,9 but hitherto no method was available to prepare macroscopic crystals by the low temperature solution methods that are used to make films for photovoltaic cells. For the hybrid organic− inorganic halide perovskites (where A is usually MA or FA), it was already demonstrated that single crystals with millimeter to centimeter dimensions can be grown using different solutionbased methods at low-temperature (around or below 100 °C): antisolvent vapor saturation,10,11 slow heating of a solution with a retrograde-soluble compound,12 or top-seeded slow-cooling © 2016 American Chemical Society
Received: May 20, 2016 Revised: August 15, 2016 Published: August 22, 2016 5717
DOI: 10.1021/acs.cgd.6b00764 Cryst. Growth Des. 2016, 16, 5717−5725
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Figure 1. (a) CsPbBr3 crystals grown by a VSA method: (i) growth from MeCN-saturated solution on a 50 °C hot plate; (ii) growth from MeOHsaturated solutions at RT. Both were grown for ca. 48 h; (iii) growth from an unsaturated DMSO precursor solution, using MeCN as an antisolventshowing (left) a yellow precipitate on top of which grew orange CsPbBr3. When illuminated with a UV light (right) the yellow precipitate shows a strong green fluorescence, indicating that it contains the CsBr-rich (Cs4PbBr6) compound. (b) CsPbBr3 crystals grown by an HRS method: growth at a heating rate of 10 °C/hour up to 80 °C (i) without and (ii) with seeding of carefully selected freshly grown crystals; (iii) before and after heating to 80 °C for the first time (i.e., without a preliminary heating/cooling cycle), showing the undesired Cs4PbBr6 fluorescent compound as a side product.
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present a retrograde-solubility in a variety of organic solvents,17 which allow an efficient and easy crystal growth. For the fully inorganic analogue (for our particular interest: CsPbBr3), on the other hand, the situation is more challenging. First, similar to compositions with other alkaline A atoms, such as RbBr:PbBr 2 , in the CsBr:PbBr 2 system 1:2 and 4:1 compositions, as well as the 1:1 perovskite one, are known.18 Based on the phase diagram of PbBr2:CsBr compositions,19 it is difficult to prepare a pure CsPbBr3 compound, since either the CsBr-rich Cs4PbBr6 or PbBr2-rich CsPb2Br5 is expected to be formed at slight deviation from a perfect 1:1 PbBr2:CsBr ratio. In addition, there is a large difference in the solubility of PbBr2 and CsBr in aprotic solvents, making a big difference in the reactivity of the dissolved species, which results in precipitation of CsBr- or PbBr2-rich compounds alongside the desired CsPbBr3 perovskite. Since, usually, CsBr is less soluble than PbBr2 in aprotic solvents like dimethylformamide, γ-butyrolactone, dimethyl sulfoxide (generally used for growing hybrid-halide perovskites crystals), CsBr-rich product, Cs4PbBr6, tends to form. Thus, growing the 1:1 (CsPbBr3) composition presents a serious challenge. Although Cs4PbBr6 was usually evident as the main or side product, we found conditions that allow growth of pure CsPbBr3 crystals, without the presence of additional phases. Two low-temperature (600 °C; Bridgman method). The crystals are grown in a matter of a few hours (using the HSR method) up to a few days (using the VSA method). For future studies and possible applications, the VSA method with MeCN as an antisolvent was found to be the most suitable one, as it provides the most controllable and reproducible way of growing crystals. Nevertheless, one may find the other antisolvents or, especially, the HSR method more appealingmostly due to its comparative simplicity and speed of preparation. Despite the similarity, the solution-grown crystals differ from the thermally grown ones in some respects, such as their conductivity. It is reasonable to assume that, due to the presence of a solvent and the lower temperature, the crystallization kinetics will be different and, thus, the defect density and resulting properties may be different. Therefore, apart from the simplicity, which will make the crystals more readily accessible for research purposes or for optoelectronic uses (e.g., X-ray or γ-ray detection as was previously demonstrated7), crystals from low-temperature solution-based growth will likely be closer in their properties to those of solution-deposited HaP films, which are used in solar cells and LEDs. Another issue that arises due to the use of a solution medium for crystal growth is occluded solvent molecules, which can lead to defects in the crystal. Although thermal analysis of the powder did not show clear signs of trapped solvent, similar analysis for single crystals showed features that may be 5723
DOI: 10.1021/acs.cgd.6b00764 Cryst. Growth Des. 2016, 16, 5717−5725
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GPa) and hardness (∼0.34 ± 0.02 GPa) of these solutiongrown crystals are quite similar to those of MAPbBr3 (∼19.6 ± 0.03 GPa and ∼0.36 ± 0.02 GPa, respectively)22an indication for the lack of importance of the A group (Cs+ vs MA+) in the dominating interatomic bonds. Since solutiongrown CsPbBr3 crystals show photoconductivity (Figure 5) and nonlinear absorption (Figure 3b), Cs can replace the organic cation to give a more reliable material for (low-cost) single crystal-related applications (e.g., photodetectors, as was demonstrated for MAPbI3, and MAPbCl3,5,6 or nonlinear absorbers for ult rafast photonics, as demonstrated for MAPbBr341). Regarding the strongly fluorescent precipitate, we verified it is Cs4PbBr6 or, at least, mostly Cs4PbBr6. It fluoresces at ∼0.1 eV higher energy than CsPbBr3 crystals. It was previously reported that Cs4PbBr6 can be luminescent at the green region (∼545 nm),42 but also that confined CsPbBr3 quantum dots in a CsBr matrix can show a similar effect.35 It was found that the emission of confined CsPbBr3 crystals in CsBr matrix can be blue-shifted by ∼0.1 eV from the bulk crystal, which is very similar to our observation.35 However, in order to get a 0.1 eV blue-shift due to quantum confinement, the CsPbBr3 particles need to be at most 10 nm in diameter.43 Based on the Scherrer equation, which allows to estimate the particle size based on XRD peak broadening, and a closer look on the (121) peak fwhm of the CsPbBr3 (Δ(2θ) ∼ 0.2°) in the Cs4PbBr6 pattern (Figure 7d at 2θ = 21.4°), the CsPbBr3 particles are much larger than 10 nm. Therefore, it is very unlikely that the strong luminescence originates from confined CsPbBr3 crystals in a Cs 4 PbBr6 matrix. To exclude the possibility that the luminescence is correlated to a solvent-effect with the Cs4PbBr6 or CsPbBr3, we stored the samples in ambient atmosphere at room temperature or moderately heated to remove residues of the solvent. No clear PL-intensity loss was observed.
modifying the growth conditions) or extrinsically (by adding known impurities to the solution medium). The fact that CsPbBr3 is clearly more stable than MAPbBr3, but with similar mechanical and optoelectronic properties, suggests that the methods described in this paper may allow production of more reliable optoelectronic components, with respect to analogous hybrid HaPs. Because high dipole moment aprotic solvents (e.g., DMSO, DMF, GBL) are commonly used in synthesis of HaPs (for both thin films and single crystals), the presented growth method is a potential path to grow perovskite structures with both organic and inorganic A group, similar to recently reported Cs0.2FA0.8PbI2.84Br0.16 thin films.44 A combination of organic (e.g., MA and FA) and inorganic (e.g., Cs, Rb) monovalent cations may lead us to better understanding of the organic cation’s role in halide perovskites. The present work is an important step toward this goal.
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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.cgd.6b00764. Images of reaction systems during or after different reaction steps; schematic view of the VSA setup and the HRS sequence; rocking-curve of a single crystal; EDS spectrum; thermal analysis (TGA, DTA, and DSC) of nonpulverized crystals; PDS results from a single crystal and a thin film of CsPbBr3; impedance spectroscopy; detailed description of characterization methods and equipment; list of other tested antisolvents (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
SUMMARY Simple and reproducible procedures to grow high-quality CsPbBr3 single crystals from solution without the usually observed related byproducts, Cs4PbBr6 or CsPb2Br5, were described. Two separate methods were found to be suitable: (1) slow vapor saturation of an antisolvent (VSA) and (2) slow heating of a solution with a retrograde-soluble compound (HRS). For both methods a precursor (PbBr2 and CsBr) DMSO solution was first titrated with one of the following antisolvents: MeCN or MeOH. The presaturation step was important to prevent precipitation of the undesired Cs4PbBr6 alongside to the desired CsPbBr3. For the VSA method, MeCN was found to be more suitable than MeOH, while for the HRS method, MeOH is preferable. The low-temperature solution-grown method presented here provides a readily available alternative to obtain high-quality, millimeter-sized, CsPbBr3 single crystals to the thermally grown method (>600 °C). The absence of symmetry-breaking (based on SHG spectroscopy) and the 19 meV upper limit of the Urbach energy suggest high quality of the solution-grown crystals. The dielectric constant (at sub-MHz dielectric response) was found to be ∼40. The order of magnitude lower resistivity, compared to thermally grown CsPbBr3, and the low excited carrier lifetime, compared to other hybrid HaPs, suggest the presence of appreciable dopant (charged defect) density in the CsPbBr3 crystals formed by this growth method. If true, this implies the possibility to control the crystal doping: intrinsically (by
Funding
G.H. and D.C. acknowledge the Israel Ministry of Science and the Israel National Nano-Initiative for partial support and G.H. acknowledges the Leona M. and Harry B. Helmsley Charitable Trust. A.S., M.L.B., and R.H.F. would like acknowledge EPSRC for their support. A.S. would also like to acknowledge support from an India-UK APEX project. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Isai Feldman, Dr. David Ehre, Dr. Thomas M. Brenner, and Arava Zohar from the Weizmann Institute of Science for fruitful discussions and technical support. D.C. holds the Sylvia and Rowland Schaefer Chair in Energy Research.
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