May 17, 1996 - stable excitons with large binding energy due to their low dimensionality, and exhibit intense ... vices [10-12]. Previously, Era et al. reported ...
17 May 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 254 (1996) 103-108
Highly efficient electroluminescence from a heterostructure device combined with emissive layered-perovskite and an electron-transporting organic compound Toshiaki Hattori, Takahiro Taira, Masanao Era, Tetsuo Tsutsui, Shogo Saito Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University. Kasuga-shi, Fukuoka 816, Japan Received 5 February 1996
Abstract
Two PbI-based layered perovskite compounds, which possess cyclohexenylethylamine or phenylbutylamine as an organic ammonium layer, were newly found to exhibit efficient exciton emission due to their self-organized quantum well structure where a lead halide semiconducting layer and an organic ammonium dielectric layer are alternately piled up. We prepared heterostructure electroluminescent devices using the combination of the emissive layered perovskites and an electron-transporting oxadiazole. When the heterostructure devices were driven at 110 K, greenish emission, which corresponded well to the exciton emission, was observed. In the device using the perovskite with an organic layer of cyclohexenylethylamine, a high luminance exceeding 4000 cd m-2 and high external EL quantum efficiency of 2.8% were attained at a current density of 50 mA cm -2 at an applied voltage of 24 V. 1. Introduction
The organic-based layered halide perovskites (RNH3)2PbX 4 self-organize a quantum well structure where an inorganic semiconductor layer of lead tetrahalide PbX 4 is sandwiched by organic alkylammonium layers of RNH 3 [1-9]. The perovskites form stable excitons with large binding energy due to their low dimensionality, and exhibit intense exciton absorption and photoluminescence from the exciton band even at room temperature. Moreover, the spectral characteristics of the layered perovskites can easily be modified by replacement of RNH 3, metal and halide. This feature provides the tunability of emission color. In addition, the perovskites possess excellent film processability. By using the conventional spin-coating method, optically high-quality thin
films can be easily obtained. From the above-mentioned feature, the perovskites are expected to be a promising thin film material for light-emitting devices [10-12]. Previously, Era et al. reported electroluminescence due to the perovskite's exciton in an organicinorganic heterostructure electroluminescent (EL) device incorporating a thin film of a layered perovskite ( C 6 H s C 2 H a N H 3 ) 2 P b I 4 (referred as PhEPbI4) as an emissive layer [11]. The report suggested that a combination of the perovskite thin film and organic carrier-transport layer made it possible to confine injected carriers within the perovskite emissive layer, and, as a result, bright EL from the perovskite by the injection current was attained. We have succeeded in extracting two layered perovskites which exhibit efficient photolumines-
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S0009-261 4(96)003 10-7
T. Hattori et al./Chemical
104
((+WH,)
+‘bL
CHE-PbI4
(0 WW,)+‘bL. PhBu-Pbl4
A w,cJf~o~,:~cK”3~3 Scheme 1. Chemical structures of layered perovskite compounds and an oxadiazole derivative (OXD7) used in this study.
cence (PL): (C6H9C,H,NH3J2PbI, and (C6H,C, which are referred to as CHE-Pb14 H,NH,)PbI,, and PhBu-Pb14 hereafter, respectively. Their PL efficiency was several times that of PhE-Pb14 which was used as an emissive layer in the previous work. [l I] Further, highly efficient EL was performed in the heterostructure EL device consisting of CHE-Pb14 thin film as an emissive layer and electron transporting layer of an oxadiazole (OXD7). In this Letter, we demonstrate that a suitable combination of layered perovskite emissive and organic electron-transporting layers can bring about highly efficient electroluminescence in the organic-inorganic heterostructure electroluminescent device.
2. Experimental The chemical structures of the layered perovskites used in this study are shown in Scheme 1. Single crystals of PhE-Pb14 and CHE-Pb14 were grown from acetone-nitoromethane solutions of lead iodine and organic ammonium iodide (stoichiometric molar ratio of lead iodine : organic ammonium iodide = 1 : 2). Thin films of the perovskites were prepared on a substrate by the spin-coating technique after the perovskite crystals were dissolved in conventional
Physics Letters 254 (1996) 103-108
organic solvents at cont. = 40 mg/ml: acetonitrile for PhE-Pb14 and a mixed solvent of acetonitrile and dimethylformamide for CHE-Pb14. On the other hand, we prepared thin films of PhBu-Pb14 by spincoating from an acetonitrile solution of stoichiometric amounts of lead iodine and phenylbutylammonium iodide without obtaining crystals of PhBu-PbI4, because of the difficulty of its crystal growth. The thickness of the perovskite films was evaluated to be 20-30 nm by an interference microscope. The formation of the layered perovskite structure in the spin-coated films was confirmed by absorption spectra and X-ray diffraction measurements. The absorption spectra were measured using a spectrophotometer (Hitachi 3301, and X-ray diffraction measurements were carried out with a focusing X-ray diffractometer (Stoe Co. Powder Diffractometer System) at the Center of Advanced Instrumental Analysis, Kyushu University. Photoluminescence (PL) spectra of the perovskite thin films were measured in the temperature range from 300 to 30K. The excitation light from a deuterium lamp (10 W> was irradiated through a UV-pass filter (Toshiba UVD-33) to the samples which were mounted on the cold head in a cryostat (Iwatani CRT-105). Then, PL from perovskites films were detected with a 20 cm monochromator (Jovin Yvon H20-UV) and a photomultiplier (Hamamatsu Photonits C1556). The EL devices were composed of an indium tin oxide (ITO) anode, layered perovskite emissive layer, oxadiazole (OXD7) electron-transporting layer [ 131 and MgAg cathode. The chemical structure of 0XD7 is shown in Scheme 1. First, the perovskite thin films were spin-coated on IT0 substrates, and then OXD7 and MgAg were successively vacuum-deposited at 10M5 Torr. The EL intensity of the devices was measured with a photon-counter (Hamamatsu Photonics C767).
3. Results and discussion Figs. la and lb show absorption and PL spectra of crystals and thin films of the layered perovskites PhE-Pb14 and CHE-Pb14 at room temperature. In the films, sharp and intense absorption and PL with a small Stokes shift are observed at around 510 nm.
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T. Hattori et a l . / Chemical Physics Letters 254 (1996) 103-108
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