Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX
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α‑Particle Detection and Charge Transport Characteristics in the A3M2I9 Defect Perovskites (A = Cs, Rb; M = Bi, Sb) Kyle M. McCall,†,‡ Zhifu Liu,‡ Giancarlo Trimarchi,† Constantinos C. Stoumpos,† Wenwen Lin,† Yihui He,† Ido Hadar,† Mercouri G. Kanatzidis,† and Bruce W. Wessels*,‡ Department of Chemistry and ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
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ABSTRACT: We have investigated the defect perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb) as materials for radiation detection. The phase purity of Bridgman-grown A3M2I9 single crystals was confirmed via high-resolution synchrotron X-ray diffraction, while density functional theory calculations (DFT) show surprisingly dispersive bands in the out-of-plane direction for these layered materials, with low effective masses for both holes and electrons. Accordingly, each of the four A3M2I9 defect perovskites showed response to 241Am αparticle irradiation for hole and electron electrode configurations, a remarkable ambipolar response that resembles the 3D halide perovskites. The electron response spectra were used to estimate the mobility−lifetime product (μτ)e for electrons in these materials, with Rb3Bi2I9 showing the lowest (μτ)e value of 1.7 × 10−6 cm2 V−1 and Cs3Bi2I9 the highest (μτ)e of 5.4 × 10−5 cm2 V−1. The rise time of the α-particle-generated pulse was used to estimate the electron mobility μe of the A3M2I9 defect perovskites, which ranged from 0.32 cm2 V−1s−1 for Rb3Sb2I9 to 4.3 cm2 V−1s−1 in Cs3Bi2I9. Similar analysis of the hole response spectra yielded (μτ)h values for each A3M2I9 compound, with Cs3Bi2I9 again showing the highest (μτ)h value of 1.8 × 10−5 cm2 V−1, while Rb3Bi2I9 showed the lowest (μτ)h with 2.0 × 10−6 cm2 V−1. Rise time analysis gave hole mobilities ranging from 1.7 cm2 V−1 s−1 for Cs3Bi2I9 to 0.14 cm2 V−1 s−1 for Cs3Sb2I9. Comparing the experimental electron and hole mobilities to the effective masses obtained from DFT calculations revealed sizable discrepancies, possibly indicating self-trapping of charge carriers due to electron−phonon interactions. The α-particle response of the A3M2I9 defect perovskites demonstrates their potential as semiconductor radiation detectors, with Cs3Bi2I9 and Cs3Sb2I9 showing the most promise. KEYWORDS: radiation detection, charge transport, semiconductor detector, halide perovskite fully deliver on the promise of this field, new wide-gap materials are needed. Semiconductor compounds must fulfill several strict prerequisites to be considered a serious candidate for roomtemperature radiation detection. The band gap should be between 1.5 and 2.5 eV, as it must be high enough to ensure low dark current at room temperature while allowing for a reasonable energy barrier for electron−hole pair production.9 A successful detector requires normally conflicting properties to coincide, as both a high mass density and high atomic number Z are required for sufficient stopping power to cause interactions with γ-rays.1 Charge carriers formed by this interaction must have optimal transport properties, given by the mobility−lifetime product, μτ, to be collected.10 To accomplish this, the defect concentrations must be extremely low, as traps and scattering centers dramatically inhibit carrier transport and thereby reduce detector resolution.11,12 Finally, the growth of large single crystals of these materials must be feasible for use as detectors.
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oom-temperature nuclear radiation detectors are in demand for a variety of applications, ranging from medical imaging and astronomy to nuclear proliferation testing and security screening.1 Both scintillators and semiconductor devices have been investigated for this purpose, but semiconductor materials offer superior resolution and response linearity to scintillators because they directly convert incident radiation into an electrical signal.2,3 Current semiconductor detector materials have disadvantages in either operating temperature or resolution that could potentially be resolved with a single room-temperature wide-gap material. The leading room-temperature semiconductor detector material, Cd 1−x Znx Te (CZT), is plagued by twinning and Te precipitates, which hinder the growth of large single crystals, limiting detector resolution and size due to the low yields of high-quality crystals.3 On the other hand, high-purity germanium detectors offer excellent resolution, but require impractical liquid nitrogen cooling.1 Alternative halide-based materials such as BiI3,4 HgI2,5 PbI2,6 and TlBr7 have shown limited promise for room-temperature operation, but have limitations such as carrier polarization effects in TlBr8 or the softness of HgI2 and PbI2, which inhibits device fabrication. To © XXXX American Chemical Society
Received: June 18, 2018
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DOI: 10.1021/acsphotonics.8b00813 ACS Photonics XXXX, XXX, XXX−XXX
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Article
possess MX6 octahedra, have demonstrated good response to γ-radiation, suggesting that the antimony analogues are also of interest.39 Our group has found that single crystals of each of the A3M2I9 compounds show photoresponse to visible laser excitation with the exception of Cs3Sb2I9.40 Thus, we studied the charge transport properties of these compounds in the context of radiation detection. In this work, we report the high-purity Bridgman growth of single crystals of the A3M2I9 compounds, including an improved synthesis for Cs3Sb2I9 through precursor purification that leads to laser photoresponse for the first time. These improvements result in the successful detection of 241Am alpha-particles by all four of the A3M2I9 compounds, with ambipolar 241Am α-particle response for both hole and electron collection detector configurations. We use these responses to investigate the charge transport characteristics of the A3M2I9 defect perovskites to estimate the mobility and mobility−lifetime products, which range from 0.14 to 4.27 cm2 V−1 s−1 and from 1.6 × 10−6 to 5.4 × 10−5 cm2 V−1, respectively. The results show that these defect perovskite materials are promising candidates for radiation detection, with the Cs-based compounds Cs3Bi2I9 and Cs3Sb2I9 showing the most potential.
The binary heavy-metal halides (BiI3, HgI2, TlBr, etc.) fulfill many of these prerequisites,13 but fall short in part due to their soft nature, which inhibits device fabrication. Related classes of materials that fulfill these requirements include the chalcohalides Hg3Q2X214 and Tl6SeI4,15 as well as the ternary metal halides. Heavy-metal ternary halides offer similar electronic structures to the binary halides while providing the opportunity to improve upon the mechanical properties. Several ternary halide compounds have shown promise, including TlSn2I5,16 Tl4HgI6,17 and TlPbI3,18 but none has yet reached the level of the more established HgI2 and TlBr. A promising subclass of ternary halides that meets these requirements is the halide perovskites AMX3.19,20 Halide perovskites have the formula AMX3 where A is a monovalent cation, M is a divalent metal, and X is a halide anion, and the perovskite structure is characterized by a framework of cornersharing MX6 octahedra. Halide perovskite semiconductors have attracted immense interest over the past few years,21−23 beginning in 2009 with the discovery that CH3NH3PbI3 could be used as a solution-processed solar cell absorption layer with extremely low cost.24 Perovskite solar cell efficiencies have since reached 22%, 25 and these materials have been investigated for a variety of optoelectronic applications such as light-emitting diodes26 and photodetectors.27 The success of these materials is due to their high absorption coefficients,28 long diffusion lengths,29 and high defect tolerance,30,31 which permits high performance under lenient synthetic conditions. In particular, CsPbBr3,32,33 FAPbI3,34 and CH3NH3PbBr3:Cl35 have shown promise as radiation detectors. These 3D perovskites have demonstrated potential, but until very recently had not achieved reproducible spectral resolution of γ-radiation that is competitive with the response of semiconductors such as TlBr or HgI2.36 Alternatives to these compounds are the defect perovskites A3M2X9 (A = Cs, Rb; M = Bi, Sb), which are derivative structures based on the MX6 octahedra characteristic of the AMX3 perovskite aristotype.37 These form different structures due to the trivalent Bi and Sb metals, which lead to a 2/3 occupancy of the M site of the ideal A3M3X9 perovskite formula. These M-site vacancies order to form 2D-layered structures along the (111) layer of the perovskite (Figure 1a) or a 0D structure with layers of isolated M2X9 bioctahedra in the case of Cs3Bi2I9 (Figure 1b).38
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EXPERIMENTAL METHODS A3M2I9 Solid-State Purification and Synthesis. BiI3 and SbI3. A 10 g amount of BiI3 (or SbI3) (5 N, Sigma-Aldrich or Alfa Aesar) was sealed in a long quartz tube (13 mm o.d.) and purified by sublimation under vacuum (10−3 mbar) at 420 °C for 48 h with one end outside the furnace. The material was transported from the hot zone to the cold zone, leaving highmelting impurities in the hot zone. This process was repeated three times, until no evidence of impurities remained at the hot zone after transport. CsI. A 10 g amount of CsI (5 N, Sigma-Aldrich) was sealed in a bent quartz tube for vapor transport in a two-zone furnace. The hot zone was heated to 950 °C, and the cold zone was left off, reaching 450 °C. The material transported over the course of 3 days, resulting in a clear ingot at the cold side and leaving impurities at the hot zone. RbI. The vapor transport method was attempted several times for 10 g of RbI (3 N, Sigma-Aldrich), but impurities in the material reacted with the tube and cracked it each time. Solution methods were used to obtain high-purity RbI. A 25 g sample of Rb2CO3 (5 N, Alfa Aesar) was slowly added to 50 mL of aqueous HI (57% by weight in H2O, Sigma-Aldrich 99.95%) under stirring, producing an essentially colorless solution in an endothermic reaction. This material had small amounts of black impurities due to I3− in the solution. The material was filtered and washed with toluene, then dried in a rotary evaporator at 60 °C. The powder was dried in a vacuum oven at 200 °C, and the black impurities boiled off, resulting in a white powder. This powder was purified in a bent fused silica tube by melting at 720 °C with one end out of the furnace, with impurities boiling off to the cold side. This was repeated, and the final product was clear with hints of pink in the ingot. A vertical zone refining was then conducted in a two-zone Bridgman furnace (800 °C hot zone, 400 °C cold zone, 20 mm/h translation) and drove the pink impurities to the top of the ingot. Pink regions were removed with a blade, and a second zone refining (800 °C, 400 °C, 5 mm/h) was conducted. The resulting material was clear except for a small portion of the top, which was removed.
Figure 1. Archetypical A3M2I9 defect perovskite structures: crystal structure of (a) Cs3Sb2I9 with 2D bilayers and (b) Cs3Bi2I9 with 0D Bi2I9 dimers, both viewed down the b-axis.
The rubidium analogues Rb3M2I9 share the 2D connectivity of the trigonal 2D Cs3Sb2I9 structure, but have distorted octahedra which lower the symmetry to monoclinic. The A3M2X9 are high-density wide-bandgap semiconductors that fulfill the strict requirements of radiation detection materials. Furthermore, the related Sb-doped BiI3 detectors, which also B
DOI: 10.1021/acsphotonics.8b00813 ACS Photonics XXXX, XXX, XXX−XXX
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the other surface with 1.5 μm silicon carbide sandpaper (MicroMesh), but no change in response was observed between the two orientations due to its 0D structure. The 2D compounds were not amenable to perpendicular-cut preparations, as Rb3M2I9 fractured along the layers so only thin surfaces (Oriented Perovskite Sheets. Science 1995, 267 (5203), 1473. (66) Cortecchia, D.; Yin, J.; Bruno, A.; Lo, S.-Z. A.; Gurzadyan, G. G.; Mhaisalkar, S.; Bredas, J.-L.; Soci, C. Polaron self-localization in white-light emitting hybrid perovskites. J. Mater. Chem. C 2017, 5 (11), 2771−2780. (67) Khalifah, P.; Osborn, R.; Huang, Q.; Zandbergen, H. W.; Jin, R.; Liu, Y.; Mandrus, D.; Cava, R. J. Orbital Ordering Transition in La4Ru2O10. Science 2002, 297 (5590), 2237. (68) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52 (15), 9019− 9038. (69) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347 (6225), 967−970. (70) Miyata, K.; Meggiolaro, D.; Trinh, M. T.; Joshi, P. P.; Mosconi, E.; Jones, S. C.; De Angelis, F.; Zhu, X. Y. Large polarons in lead halide perovskites. Sci. Adv. 2017, 3 (8), e1701217. (71) Saidaminov, M. I.; Haque, M. A.; Almutlaq, J.; Sarmah, S.; Miao, X.-H.; Begum, R.; Zhumekenov, A. A.; Dursun, I.; Cho, N.; Murali, B.; Mohammed, O. F.; Wu, T.; Bakr, O. M. Inorganic Lead Halide Perovskite Single Crystals: Phase-Selective Low-Temperature Growth, Carrier Transport Properties, and Self-Powered Photodetection. Adv. Opt. Mater. 2017, 5 (2), 1600704. (72) Shluger, A. L.; Stoneham, A. M. Small polarons in real crystals: concepts and problems. J. Phys.: Condens. Matter 1993, 5 (19), 3049. (73) Williams, R. T.; Song, K. S. The self-trapped exciton. J. Phys. Chem. Solids 1990, 51 (7), 679−716. (74) Ghosh, B.; Chakraborty, S.; Wei, H.; Guet, C.; Li, S.; Mhaisalkar, S.; Mathews, N. Poor Photovoltaic Performance of Cs3Bi2I9: An Insight through First-Principles Calculations. J. Phys. Chem. C 2017, 121 (32), 17062−17067. (75) Du, M.-H. Density Functional Calculations of Native Defects in CH3NH3PbI3: Effects of Spin−Orbit Coupling and Self-Interaction Error. J. Phys. Chem. Lett. 2015, 6 (8), 1461−1466. (76) Sebastian, M.; Peters, J. A.; Stoumpos, C. C.; Im, J.; Kostina, S. S.; Liu, Z.; Kanatzidis, M. G.; Freeman, A. J.; Wessels, B. W. Excitonic emissions and above-band-gap luminescence in the single-crystal perovskite semiconductors CsPbBr3 and CsPbCl3. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92 (23), 235210.
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DOI: 10.1021/acsphotonics.8b00813 ACS Photonics XXXX, XXX, XXX−XXX
