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Pure CsPbBr: Highly Luminescent Zero-Dimensional Perovskite Solids Makhsud I. Saidaminov, Jawaher Almutlaq, Smritakshi Phukan Sarmah, Ibrahim Dursun, Ayan A. Zhumekenov, Raihana Begum, Jun Pan, namchul cho, Omar F. Mohammed, and Osman M. Bakr ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00396 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016
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ACS Energy Letters
Pure Cs4PbBr6: Highly Luminescent ZeroDimensional Perovskite Solids Makhsud I. Saidaminov†, Jawaher Almutlaq†, Smritakshi Sarmah, Ibrahim Dursun, Ayan A. Zhumekenov, Raihana Begum, Jun Pan, Namchul Cho, Omar F. Mohammed* and Osman M. Bakr* Division of Physical Sciences and Engineering, KAUST Solar Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia AUTHOR INFORMATION Corresponding Authors *
[email protected] and
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ABSTRACT. So-called zero-dimensional perovskites, such as Cs4PbBr6, promise outstanding emissive properties. However, Cs4PbBr6 is mostly prepared by melting of precursors that usually leads to co-formation of undesired phases. Here, we report a simple low-temperature solutionprocessed synthesis of pure Cs4PbBr6 with remarkable emission properties. We found that pure Cs4PbBr6 in solid form exhibits a 45% photoluminescence quantum yield (PLQY), in contrast to its three-dimensional counterpart – CsPbBr3 – which exhibits more than two orders of magnitude lower PLQY. Such PLQY of Cs4PbBr6 is significantly higher than other solid forms of lower dimensional metal halide perovskite derivatives, including perovskite nanocrystals. We attribute this dramatic increase in PL to the high exciton binding energy, which we estimate to be ~353 meV, likely induced by the unique Bergerhoff–Schmitz-Dumont type crystal structure of Cs4PbBr6, in which metal-halide-comprised octahedra are spatially confined. Our findings bring this class of perovskite derivatives to the forefront of color-converting and light-emitting applications.
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TOC GRAPHICS
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The photoluminescence quantum yield (PLQY) of semiconductor materials correlates with their trap state densities and applicability for optoelectronic applications like solar cells, color converters, lasers, light-emitting diodes, and others.1–3 An ideal emitter exhibits near-unity PLQY, due to absence of trap states within its band gap.4 Recent reports on perovskite single crystals have estimated ultralow trap state densities – on the order of ~1010 cm-3.5,6 Yet those bulk perovskites are not optically emissive, most likely due to the shared octahedra of the crystal structure that prevent quantum confinement of the charge carriers. As a result, their excitons have extremely low binding energies (~15-50 meV which is in order of thermal energy at room temperature) and thus readily dissociate to free carriers.7–9 Quantum confinement, and the ensuing high PLQY, are realized physically in perovskite quantum dots due to their nanoscale physical sizes (stabilized by ligands),10,11 and in perovskite thin films due to the nanoscale grain sizes.12,13 Unfortunately, the PLQY of quantum dots markedly decreases when they are made into solid form,14 because of the loss of capping ligands causing further aggregation. On the other hand, in thin films perovskites, the abundance of grain boundaries are detrimental to the material’s stability.15 An alternative attractive approach to achieve quantum confinement is by reducing the dimensionality of perovskites.16–18 The general formula of perovskites can be written as AnBX2+n, where A is a monovalent cation like CH3NH3+, Cs+, etc., B is a divalent metal, usually Pb2+ or Sn2+, and X is a halogen anion. When n=1, the perovskite is referred to as a threedimensional (3D) perovskite. For instance, CsPbBr3 is a 3D perovskite, where the crystal structure is based on the corner-shared PbBr64- octahedra (Figure 1a). For n=2 to 4, the structure is often referred to as low-dimensional perovskites in which the octahedra form planes (if n=2, they are called 2D), chains (if n=3, 1D) and isolated dots (if n=4, 0D).19,20 Recent reports have
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demonstrated a remarkable increase in PLQY upon reducing the dimensionality of perovskites due to the increase in the exciton binding energy.17,21,22 For example, the PLQY of 2D lead bromide based perovskite solids was reported to be ~9%
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(compared to ~0.1% for 3D, vide
infra). Also, CsPb2Br5 with 2D-like structure showed significantly strong emission. Its PLQY was reported to be ~87% in solution and expected to be also high in solid form compared to the 3D CsPbBr3 (the actual PLQY for the CsPb2Br5 solid was not reported).23 These intriguing findings drew our attention and motivated us to explore the extreme case when those octahedra are isolated by Cs bridges to form 0D perovskites,24,25 for which we hypothesized that exceptionally high PLQY could be achieved. The Bergerhoff–Schmitz-Dumont type crystal structure (the proper name for the crystal phase)26 in Cs4PbBr6 (Figure 1b) adopts a configuration in which the Cs+ ions minimize the electronic overlap between adjacent octahedra, resulting in localized states that are confined to the individual octahedra.27 The optical properties of Cs4PbBr6 have been partially investigated in the literature, but issues like synthesis and coexistence of CsPbBr3 have hindered its proper characterization. In particular, the origin of luminescence in these mixtures has been a point of controversy in the literature. Some studies attributed its PL to the presence of CsPbBr3,27–29 while others – to the intrinsic property of Cs4PbBr6.30,31 In this study we present a simple solution-processed chemical method to produce pure Cs4PbBr6 solids, which enabled us to study and reveal the materials true remarkable optical properties.
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Figure 1. Crystal structure of (a) CsPbBr3 and (b) Cs4PbBr6 featuring corner-shared and isolated PbBr64- octahedra, respectively. As aforementioned, one of the fundamental problems that needs to be addressed is the isolation of Cs4PbBr6 from any other additional phases. Figure 2a summarizes some of the explored parameters to arrive at pure Cs4PbBr6. We started with rapid precipitation from CsBr:PbBr2 (1:1 molar ratio) in dimethyl sulfoxide (DMSO) solution by adding antisolvent (dichloromethane). However, it resulted in a mixture of undesired phases – CsPbBr3 and CsPb2Br5 (Figure S1). Then, we turned to inverse solubility, in which precipitation occurs when the solution is heated; this phenomenon was recently utilized to synthesize a number of hybrid perovskite bulk crystals and crystalline films of high quality.32–37 We observed that CsBr:PbBr2 (1:1 molar ratio) in DMSO at 120 °C produces predominantly Cs4PbBr6 solid with some traces of CsPbBr3 (Figure 2b, top). Since Cs4PbBr6 is richer in cesium compared to CsPbBr3, we speculated that the increase of CsBr precursor concentration in solution would induce precipitation of only Cs4PbBr6 phase: but further increase in CsBr concentration to the molar ratio of 1.5:1 did not show any precipitation (Figure S2), which could be due to the limited solubility of CsBr in DMSO. Undissolved CsBr partially reacts with PbBr2, thus decreasing the final concentration of PbBr2 in solution (Figure S3). We further investigated the solubility of these perovskites, and found that
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DMSO dissolves CsPbBr3, but not Cs4PbBr6 (Figure S4). Thus, we turned back to the initial CsBr:PbBr2 (1:1) solution in DMSO. We collected the precipitate that was formed at 120 °C and washed it with DMSO. This step selectively removes CsPbBr3, leaving behind a pure Cs4PbBr6 phase. The powder X-ray diffraction (PXRD) of the purified sample confirms the presence of monophasic Cs4PbBr6, which is consistent with the calculated PXRD (Figure 2b, bottom). A high-resolution XRD scan at 2θ=13.5-18°C confirmed the absence of CsPbBr3 in our purified samples (Figure S5). The details of the optimized recipe can be found in Supporting Information. During the preparation of this manuscript, Rakita Y. et al. reported a highly luminescent side product formed during the synthesis of CsPbBr3, which according to XRD, consists of both Cs4PbBr6 and CsPbBr3.31 However, the problem of 0D purification from the coexisting 3D perovskite was not addressed, which, as we will show later, obscures the optical properties of the 0D Cs4PbBr6 and has a severe detrimental effect on the PLQY.
Figure 2. (a) Schematic representation of the optimization method for the synthesis and purification of Cs4PbBr6 (percentages represent the ratios of XRD peak intensities); (b) PXRD patterns of as-synthesized unwashed Cs4PbBr6 and pure Cs4PbBr6 along with the calculated spectra of the same. Insets: picture of the corresponding solids.
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Next, we studied the materials’ optical properties. The ground 3D CsPbBr3 is not luminescent under UV light, whereas the unwashed Cs4PbBr6 solid is green and the purified one is bright green under the same conditions (Figure 3a). Figure 3b shows the dependence of pure Cs4PbBr6’s PL intensity on the excitation wavelength. Cs4PbBr6 exhibits a single PL peak, centered at 520 nm (Figure 3c), in good agreement with recent report.31 Cs4PbBr6 has also a strong and stable emission at excitation wavelengths of λ>340 nm (Figure 3d). The absorption edge of pure Cs4PbBr6 at 540 nm (Figure 3e) is consistent with the literature values.30
Figure 3. (a) Ground CsPbBr3, unwashed Cs4PbBr6 and pure Cs4PbBr6 powders on the glass slides under ambient and UV light (365 nm); (b) PL intensity as a function of excitation wavelengths for the pure Cs4PbBr6; (c) the steady-state PL, (d) excitation spectrum at PL wavelength of 520 nm, and (e) absorption spectrum of pure Cs4PbBr6.
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To quantify the PL, we measured the PLQY of 0D Cs4PbBr6 and 3D Cs4PbBr6 following the method reported by de Mello et al.38 While we were not able to accurately measure the PLQY of 3D perovskite because of low luminescence (PLQY
