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Highly efficient electroluminescence from a heterostructure device combined with emissive layered-perovskite and an electron-transporting organic compound
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.
105
T. Hattori et a l . / Chemical Physics Letters 254 (1996) 103-108
~
1.2
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~ 0.8 . ~ .D
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............
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~=0.8
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.,.,.:;::::',:: c)
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I0
20
40
30
2theta (degrees)
................ "Z'.
0.6
.a=
0.8
0.4
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Fig. 2. X-ray diffraction profile of PhE-PbI4 spin-coated film. Inset shows the comparisons of X-ray diffraction profiles of spin-coated films of PhE-PbI4, CHE-PbI4 and PhBu-PbI4.
¢-,
350
e~
400
450 500 550 Wavelength (nm)
600
0 650
Fig. 1. Absorption and photoluminescence spectra of spin-coated films (solid lines) and crystals (dotted line) of layered perovskite at room temperature: (a) PhE-PbI4, (b) CHE-PbI4, and (c) PhBuPbl4.
The absorption and emission bands correspond well to the exciton absorption and emission bands of the crystals. The appearance of an intense exciton absorption band denotes that a layered perovskite structure was formed in the spin-coated films. In addition, exciton absorption and emission were also observed in a PhBu-PbI4 film which was spin-coated from a stoichiometric solution of lead iodine and phenylbutylammonium iodide (Fig. lc). Fig. 2 shows an X-ray diffraction profile of the PhE-PbI4 thin film. When the X-ray was incident almost parallel to the film plane, only diffraction peaks corresponding to the layer spacing of the perovskite were observed; the spacing of 1.6 nm, which is estimated from the diffraction peaks, agrees with the layer spacing in the single crystal of PhEPbI4 [5,14]. In addition to providing experimental evidence of the formation of the layered perovskite structure, the diffraction profile demonstrates that the layer structure is highly oriented parallel to the film plane in the perovskite film. In the other two per-
ovskite films, the diffraction peaks corresponding to the layer spacing were observed as shown in the inset of Fig. 2. From the diffraction peaks, the long spacings of CHE-PbI4 and PhBu-PbI4 were evaluated to be 1.7 and 2.0 nm, respectively. The large value of PhBu-PbI4 suggests that the incorporation of amines with longer molecular length causes the expansion of the interlayer distance. Fig. 3 shows the temperature dependences of the integrated intensity of PL in the perovskite thin films. The PL intensity is normalized in terms of the numbers of absorbed photons. Accordingly, the PL ,-, 101 PhBu-Pbl4 CHE-PbI4
o= 100
10-I
t--
~- 10-2 0
,
,
,
,
I
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f
i
,
,
B
.
.
.
.
i
20 30 lO0Ofr (K -l)
.
.
.
.
i
40
....
50
Fig. 3. Photoluminescence intensity of spin-coated films of layered perovskite as a function of reciprocal temperature: (O) PhBu-PbI4; (O) CHE-PbI4; ( • ) PhE-PbI4.
106
T. Hattori et al. / Chemical Physics Letters 254 (1996) 103-108
intensity used here is proportionally correlative with PL quantum efficiency. The PL peak intensities of the films of CHE-PbI4 and PhBu-PbI4 are three and five times larger than that of PhE-PbI4. In other words, the films of CHE-PbI4 and PhBu-PbI4 possess large PL quantum efficiency in comparison with PhE-PbI4. Employment of the perovskite films with such high PL efficiencies as an emissive layer in EL devices is expected to promote highly efficient EL. When the heterostructure EL devices with the perovskite thin films as an emissive layer were driven at 110 K, greenish emission due to exciton in the perovskite films was observed in all of the devices. The EL spectra correspond well to the exciton emission of the perovskite films in all EL devices, as demonstrated in Fig. 4. The EL devices with PhE-PbI4 or CHE-PbI4 as an emissive layer exhibited high luminance and high EL efficiency. Fig. 5 shows the relation between current density and EL intensity in the heterostructure EL devices at 110 K. The EL intensity increased . . . .
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...........................................................
10-7
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10-4
10-1
Fig. 5. EL intensity-current density characteristics in organic-inorganic heterostructure EL devices with layered perovskite as an emissive layer: ( © ) PhE-PbI4; ( D ) CHE-PbI4; (~,) PhBu-Pbl4. The current-density dependencies of external quantum efficiency of the EL devices also shown: ( 0 ) PhE-PbI4; ( m ) CHE-PbI4; ( • ) PhBu-PbI4.
0.4~ 0.2~
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,.z 0.2
o
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. . . .
r
. . . .
500 550 600 Wavelength (nm)
i
. . . .
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0 700
Fig. 4. Electroluminescence spectra (solid lines) of the organic-inorganic heterostructure devices with layered perovskite as an emissive layer at 110 K: (a) PhE-PbI4, (b) CHE-PbI4 and (c) PhBu-Pbl4. Dotted lines show photoluminescence spectra of the perovskite spin-coated films at 110 K.
superlinearly with the current density in all the devices. Maximum luminance of the EL devices reached 12000 cd m -2 at a current density of 400 mA cm -2 for PhE-PbI4 and 4800 cd m -2 at a current density of 50 mA cm-2 for CHE-PbI4, where the applied voltage was 24 V in both cases. The external quantum efficiency r/ext (photon/ electron) of the EL devices are also shown in Fig. 5. The efficiencies were estimated from the comparison of numbers of emitted photons in the devices with perovskite and that of a multilayered EL device with tris(8-hydroxyquinoline) aluminum as an emitter, whose efficiency was found to be 1.7%, at the same current density in the same experimental apparatus. As implied from the nonlinear relation between current density and EL intensity in Fig. 5, the efficiency was increased with current density. The maximum efficiencies of 2.8% for CHE-PbI4 and 0.9% for PhE-PbI4, respectively, were attained. The value of 2.8% in the device with CHE-PbI4 is comparable to the highest r/ext in organic EL devices which have
T. Hattori et al. / Chemical Physics Letters 254 (1996) 103-108
been reported. Further, the external efficiency was converted to intemal efficiency r/i, by multiplying by a factor of 2n 2, which is the correction factor of light-output efficiency from a device to outer space in the forward direction, where n is the refractive index of the organic layers of a device [15]. When the value of n is assumed to be 1.6, the internal efficiency r/in of the device is roughly estimated to be 14%. The value is about half of the theoretical maximum value (25%) of r/in due to the emission from a singlet exciton. The result suggests that efficient carrier recombination occurred in the emissive layer and that radiative decay of the created excitons was efficient, in other words, the CHE-PbI4 emissive layer possessed high PL efficiency. The highly efficient carrier recombination is supposed to be promoted by the carrier confinement in the emissive perovskite layer combined with the electron-transporting OXD7 layer, as we mentioned previously [11]. From the measurements of photoemission threshold, the ionization potentials IP of CHE-PbI4 and OXD7 were evaluated to be 5.5 and 6.5 eV, respectively. The large difference in IP demonstrates that a large barrier potential for holeinjection from the CHE-PbI4 layer to the OXD7 layer exists at the CHE-PbI4/OXD7 interface, The large hole injection barrier is effective for confining holes injected from the ITO anode within the CHEPbI4 emissive layer. On the other hand, electrons injected from the MgAg cathode are assumed to be transported to the C H E - P b I 4 / O X D 7 interface through the electron-transporting OXD7 layer. Then, electrons efficiently recombine with the confined holes in the CHE-PbI4 emissive layer. As a result, highly efficient EL was most likely to be attained in the heterostructure EL device with the CHE-PbI4 emissive layer. Meanwhile, it should be noted that luminance and EL efficiency in the device with PhBu-PbI4 were quite low despite the fact that the PL intensity of the PhBu-PbI4 film was the highest among the three perovskite films. The maximum values of luminance and quantum efficiency of the device were 180 cd m -2 and 0.2% at a current density of 25 mA cm -2 at the applied voltage of 30 V. This device was very resistive in comparison with the other devices; the current density of the device was two orders of magnitude less than those of the others in the whole
107
applied voltage region. The high resistivity of the device is most likely to originate from preventing carrier transport by a thick and resistive organic ammonium layer in the PhBu-PbI4 film. The high resistivity may depress the effective transport of the carrier injected from the electrodes, and the EL efficiency may be lowered.
4. Conclusion In this Letter, we demonstrated that highly efficient EL was attained in inorganic-organic heterostructure devices with the combination of an emissive layered perovskite and an organic carrier transport material. The highly efficient EL is most likely to have arisen from the carrier confinement within the emissive perovskite layer due to the organic carrier transport layer. The result demonstrates that the perovskite film is one of a class of promising emissive materials with highly efficient photoluminescence, and that suitable selection of emissive perovskites and an organic carrier transport material bring about highly efficient EL. Further, we believe that the combination of inorganic emissive materials and organic carrier transport materials is one of the promising approaches for constructing high performance light-emitting devices using various emission species of inorganic materials (exciton, biexciton, emissive center and so on).
Acknowledgement This work was partly supported by NEDO International Joint Research Grant, Japan and the Foundation Advanced Technology Institute.
References [1] T. Ishihara, J. Takahashi and T. Goto, Solid State Commun. 69 (1989) 933. [2] T. Ishihara, J. Takahashi and T. Goto, Phys. Rev. B 42 (1990) 11099. [3] X. Hong, T. Ishihara and A.V. Nurnikko, Phys. Rev. B 45 (1992) 6961. [4] T. lshihara, X. Hong, J. Ding and A.V. Nurmikko, Surface Sci. 267 (1992) 323.
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[5] T. Ishihara, J. Luminescence 60&61 (1994) 269. [6] C.-Q. Xu, S. Fukuta, H. Sakakura, T. Kondo, R. lto, Y. Takahashi and K. Kumata, Solid State Commun. 77 (1991) 923. [7] T. Kataoka, T. Kondo, R. lto, S. Sasaki, K. Uchida and N. Miura, Phys. Rev. B 47 (1993) 2010. [8] G.C. Papavassiliou, A.P. Patsis, D.J. Jagouvardos and I.B. Koutselas, Synth. Metals 55-57 (1993) 3889. [9] Y.I. Dolzhenko, T. Inabe and Y. Maruyama, Bull. Chem. Soc. Japan 59 (1986). [I0] X. Hong, T. lshihara and A.V. Nurmikko, Solid State Commun. 84 (1992) 657.
[11] M. Era, S. Morimoto, T. Tsutsui and S. Saito, Appl. Phys. Letters 65 (1994) 676. [12] M. Era, S. Morimoto, T. Tsutsui and S. Saito, Synth. Metals 71 (1995) 2013. [13] Y. Hamada, C. Adachi, T. Tsutsui and S. Saito, Japan. J. Appl. Phys. 31 (1992) 1812. [14] J. Calabrese, N.L. Jones, R.L. Harlow, N. Herron, D.L. Thoron and Y. Wang, J. Am. Chem. Soc. 113 (1991) 2328. [15] N.C. Greenham, R.H. Friend and D.D.C. Bradley, Advan. Mater. 6 (1994) 491.
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.
105
T. Hattori et a l . / Chemical Physics Letters 254 (1996) 103-108
~
1.2
a, ...........
~ 0.8 . ~ .D
106
............
Abs~iil,
PL
t04 ~
~ 0.4 <
80
70 "4 ,~
E -=
0.6 ~
1.2
b)
"~
• PL
60
~ 80 l ~
50
~
40
o t~ 2°t-
30 .~ 20
~=0.8
0.4 "~
10
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0.2~
0
1.2 z .z)
<
PhBu-PbI4
!ill'
~1~t I ; ~;._./'
o
1o
2theta(degrees)
0
.,.,.:;::::',:: c)
CHE-Pbl4 . t
I0
20
40
30
2theta (degrees)
................ "Z'.
0.6
.a=
0.8
0.4
0.4
0.2-~
Fig. 2. X-ray diffraction profile of PhE-PbI4 spin-coated film. Inset shows the comparisons of X-ray diffraction profiles of spin-coated films of PhE-PbI4, CHE-PbI4 and PhBu-PbI4.
¢-,
350
e~
400
450 500 550 Wavelength (nm)
600
0 650
Fig. 1. Absorption and photoluminescence spectra of spin-coated films (solid lines) and crystals (dotted line) of layered perovskite at room temperature: (a) PhE-PbI4, (b) CHE-PbI4, and (c) PhBuPbl4.
The absorption and emission bands correspond well to the exciton absorption and emission bands of the crystals. The appearance of an intense exciton absorption band denotes that a layered perovskite structure was formed in the spin-coated films. In addition, exciton absorption and emission were also observed in a PhBu-PbI4 film which was spin-coated from a stoichiometric solution of lead iodine and phenylbutylammonium iodide (Fig. lc). Fig. 2 shows an X-ray diffraction profile of the PhE-PbI4 thin film. When the X-ray was incident almost parallel to the film plane, only diffraction peaks corresponding to the layer spacing of the perovskite were observed; the spacing of 1.6 nm, which is estimated from the diffraction peaks, agrees with the layer spacing in the single crystal of PhEPbI4 [5,14]. In addition to providing experimental evidence of the formation of the layered perovskite structure, the diffraction profile demonstrates that the layer structure is highly oriented parallel to the film plane in the perovskite film. In the other two per-
ovskite films, the diffraction peaks corresponding to the layer spacing were observed as shown in the inset of Fig. 2. From the diffraction peaks, the long spacings of CHE-PbI4 and PhBu-PbI4 were evaluated to be 1.7 and 2.0 nm, respectively. The large value of PhBu-PbI4 suggests that the incorporation of amines with longer molecular length causes the expansion of the interlayer distance. Fig. 3 shows the temperature dependences of the integrated intensity of PL in the perovskite thin films. The PL intensity is normalized in terms of the numbers of absorbed photons. Accordingly, the PL ,-, 101 PhBu-Pbl4 CHE-PbI4
o= 100
10-I
t--
~- 10-2 0
,
,
,
,
I
10
f
i
,
,
B
.
.
.
.
i
20 30 lO0Ofr (K -l)
.
.
.
.
i
40
....
50
Fig. 3. Photoluminescence intensity of spin-coated films of layered perovskite as a function of reciprocal temperature: (O) PhBu-PbI4; (O) CHE-PbI4; ( • ) PhE-PbI4.
106
T. Hattori et al. / Chemical Physics Letters 254 (1996) 103-108
intensity used here is proportionally correlative with PL quantum efficiency. The PL peak intensities of the films of CHE-PbI4 and PhBu-PbI4 are three and five times larger than that of PhE-PbI4. In other words, the films of CHE-PbI4 and PhBu-PbI4 possess large PL quantum efficiency in comparison with PhE-PbI4. Employment of the perovskite films with such high PL efficiencies as an emissive layer in EL devices is expected to promote highly efficient EL. When the heterostructure EL devices with the perovskite thin films as an emissive layer were driven at 110 K, greenish emission due to exciton in the perovskite films was observed in all of the devices. The EL spectra correspond well to the exciton emission of the perovskite films in all EL devices, as demonstrated in Fig. 4. The EL devices with PhE-PbI4 or CHE-PbI4 as an emissive layer exhibited high luminance and high EL efficiency. Fig. 5 shows the relation between current density and EL intensity in the heterostructure EL devices at 110 K. The EL intensity increased . . . .
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superlinearly with the current density in all the devices. Maximum luminance of the EL devices reached 12000 cd m -2 at a current density of 400 mA cm -2 for PhE-PbI4 and 4800 cd m -2 at a current density of 50 mA cm-2 for CHE-PbI4, where the applied voltage was 24 V in both cases. The external quantum efficiency r/ext (photon/ electron) of the EL devices are also shown in Fig. 5. The efficiencies were estimated from the comparison of numbers of emitted photons in the devices with perovskite and that of a multilayered EL device with tris(8-hydroxyquinoline) aluminum as an emitter, whose efficiency was found to be 1.7%, at the same current density in the same experimental apparatus. As implied from the nonlinear relation between current density and EL intensity in Fig. 5, the efficiency was increased with current density. The maximum efficiencies of 2.8% for CHE-PbI4 and 0.9% for PhE-PbI4, respectively, were attained. The value of 2.8% in the device with CHE-PbI4 is comparable to the highest r/ext in organic EL devices which have
T. Hattori et al. / Chemical Physics Letters 254 (1996) 103-108
been reported. Further, the external efficiency was converted to intemal efficiency r/i, by multiplying by a factor of 2n 2, which is the correction factor of light-output efficiency from a device to outer space in the forward direction, where n is the refractive index of the organic layers of a device [15]. When the value of n is assumed to be 1.6, the internal efficiency r/in of the device is roughly estimated to be 14%. The value is about half of the theoretical maximum value (25%) of r/in due to the emission from a singlet exciton. The result suggests that efficient carrier recombination occurred in the emissive layer and that radiative decay of the created excitons was efficient, in other words, the CHE-PbI4 emissive layer possessed high PL efficiency. The highly efficient carrier recombination is supposed to be promoted by the carrier confinement in the emissive perovskite layer combined with the electron-transporting OXD7 layer, as we mentioned previously [11]. From the measurements of photoemission threshold, the ionization potentials IP of CHE-PbI4 and OXD7 were evaluated to be 5.5 and 6.5 eV, respectively. The large difference in IP demonstrates that a large barrier potential for holeinjection from the CHE-PbI4 layer to the OXD7 layer exists at the CHE-PbI4/OXD7 interface, The large hole injection barrier is effective for confining holes injected from the ITO anode within the CHEPbI4 emissive layer. On the other hand, electrons injected from the MgAg cathode are assumed to be transported to the C H E - P b I 4 / O X D 7 interface through the electron-transporting OXD7 layer. Then, electrons efficiently recombine with the confined holes in the CHE-PbI4 emissive layer. As a result, highly efficient EL was most likely to be attained in the heterostructure EL device with the CHE-PbI4 emissive layer. Meanwhile, it should be noted that luminance and EL efficiency in the device with PhBu-PbI4 were quite low despite the fact that the PL intensity of the PhBu-PbI4 film was the highest among the three perovskite films. The maximum values of luminance and quantum efficiency of the device were 180 cd m -2 and 0.2% at a current density of 25 mA cm -2 at the applied voltage of 30 V. This device was very resistive in comparison with the other devices; the current density of the device was two orders of magnitude less than those of the others in the whole
107
applied voltage region. The high resistivity of the device is most likely to originate from preventing carrier transport by a thick and resistive organic ammonium layer in the PhBu-PbI4 film. The high resistivity may depress the effective transport of the carrier injected from the electrodes, and the EL efficiency may be lowered.
4. Conclusion In this Letter, we demonstrated that highly efficient EL was attained in inorganic-organic heterostructure devices with the combination of an emissive layered perovskite and an organic carrier transport material. The highly efficient EL is most likely to have arisen from the carrier confinement within the emissive perovskite layer due to the organic carrier transport layer. The result demonstrates that the perovskite film is one of a class of promising emissive materials with highly efficient photoluminescence, and that suitable selection of emissive perovskites and an organic carrier transport material bring about highly efficient EL. Further, we believe that the combination of inorganic emissive materials and organic carrier transport materials is one of the promising approaches for constructing high performance light-emitting devices using various emission species of inorganic materials (exciton, biexciton, emissive center and so on).
Acknowledgement This work was partly supported by NEDO International Joint Research Grant, Japan and the Foundation Advanced Technology Institute.
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