Organic-inorganic heterostructure eiectroluminescent

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using a layered perovskite semiconductor (C6H5C2H4NH&Pb&,. M. Era, S. ... carrier-injection type light emitting diode (LED) of inorganic semiconductors.
Organic-inorganic heterostructure eiectroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH&Pb&, M. Era, S. Morimoto, T. Tsutsui, and S. Saito Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-shi, Fukuoka 816, Japan

(Received 14 March 1994; accepted for publication 9 June 1994) Using the combination of a layered perovskite compound (C,H,G~H,$H,),PbI, (PAPI), which forms a stable exciton with a large binding energy owing to its low-dimensional semiconductor nature and exhibits sharp and strong photoluminescence from the exciton band, and an electron-transporting oxadiazoie derivative, we fabricated an organic-inorganic heterostructure electroluminescent (EL) device. The EL spectrum of the device corresponded well to the photoluminescence spectrum of the PAP1 film; the emission was peaking at 520 run and half-width of the emission was about 10 nm at liquid-nitrogen temperature. Further, highly intense EL of more than 10 000 cd m z was performed at 2 A cm-a at liquid-nitrogen temperature in the device.

In organic electroluminescent (EL) devices, high EL efficiency has been attained by utilizing organic heterostructures.1-5 The adoption of organic heterostructure to the EL devices was very effective in the improvements of carrier injection from electrodes and carrier confinement in the emission region in organic EL devices. The mechanism of the organic EL device is basically identical to that of carrier-injection type light emitting diode (LED) of inorganic semiconductors. Accordingly, introduction of inorganic semiconductors into organic EL devices as emitter or carrier transport materials may be used. In other words, construction of organic-inorganic heterostructure EL devices is possible. If one carefully selects inorganic semiconductors which satisfy the condition of carrier injection and carrier confinement, efficient EL is expected to realize in the organicinorganic heterostructure devices. In fact, fairly intense blue EL was observed in an organic-inorganic heterostructure device with a diamine derivative (TAD) as a hole-transport layer and ZnSe as an emitter layer.6 Further, the hybridization of organic and inorganic semiconductors is expected not only to permit wide-range selection of emitter and carriertransport materials, but to provide a new approach to construct high performance EL devices taking advantages of organic and inorganic semiconductors. For example, high photoluminescent efficiency of organic materials, especially in blue region, can be combined with high carrier density and low resistivity of inorganic semiconductors. Layer-type perovskite compounds expressed by the chemical formula (RNI&PbX, (X;halogen) naturally form a quantum-well structure where the two-dimensional semiconductor layer of Pb& and the organic ammonium layer of RNHs are alternately piled up?111 The compounds form stable exciton with a large binding energy even at room temperature owing to their low-dimensional semiconductor nature, and many compounds of the layered perovskite family exhibit photoluminescence with narrow bandwidth from the exciton band. Further, the electronic state of this type compounds can be adjusted by the replacement of metal, RNHs, and X.7,41o The flexibility in material design makes it possible to tune the exciton emission to a desired color region, and to improve compatibility of contact between the com676

Appl. Phys. Lett. 65 (6), 8 August 1994

pounds and an electrode for carrier injection. In addition to these very specific electronic and optical properties, the compounds possess excellent film processability; one can easily obtain good optical quality thin films of the compounds by the spin-coating method. These advantages stimulated us in applying these compounds for EL devices. In this letter, we report EL properties of an organic-inorganic heterostructure device using bis (phenethylammonium) tetraiode plumbate [(C,H&H$IHa)aPbI,, referred as PAP1 hereafter], which has a strong photoluminescence and belongs to the layered perovskite compounds, as an emitter. Thin film of PAP1 was prepared by the spin-coating method from the acetonitrile solution. Figure 1 shows the absorption and fluorescence spectra of the PAP1 thin film. The exciton absorption and band gap are located at 510 and 450 nm, respectively. Further photoemission with narrow bandwidth due to the exciton band is observed at 520 nm. The spectral properties correspond well to those of the PAPI crystal,7,8 demonstrating the formation of layered perovskite structure in the spin-coated film. The organic-inorganic heterostructure device employed in this study consists of an indium-tin-oxid @TO) anode, a PAPI emitter, an electron transport layer of an oxadiazole derivative (OXD7)” and an MgAg cathode (Fig. 2). First, the PAPI film was spin coated on a glass substrate with IT0 electrode. Then, the OXD7 film and the MgAg electrode were succes-

Wavelength

(nm)

F’IG. 1. Absorption (solid line) and photoluminescence [dotted link) spectra of a PAP1 spin-coated liIm at room temperature.

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0 1994 American Institute of Physics

,_MgAg

(200 nm)

I -

OXD7 (50 w i PAPI (about 10 nm) -IT0 (100 m

C-4

(b) OXD7

PIG. 2. Device structure of an organic-inorganic haterostructure EL device using a PAP1 spin-coated tilm [a) and molecular structure of an oxadiazole derivative OXD7 (b).

sively vacuum deposited on the PAP1 film at 1O-5-1O-6 Torr. When the organic-inorganic heterostructure device was driven at liquid-nitrogen temperature, an intense green emission was observed. Figure 3 shows the EL spectrum of the heterostructure EL device at liquid-nitrogen temperature. The spectrum corresponded well to the photoluminescent spectrum of the PAPI spin-coated film. The emission is peaking at 520 nm, and its bandwidth is very narrow (half-width is about 10 nm). The EL intensity is increased with current density. The EL intensity was reached a luminance of more than 10 000 cd m2 at a current density of 2 A cm-’ when a voltage of 24 V was applied to the device at liquid-nitrogen temperature. The efficient EL in the heterostructure device is most likely to originate from the confinement of holes in the PAPI layer at the PAPUOXD7 interface. Energy diagram of the heterostructure device is depicted in Fig. 4. The ionization potential which indicates the top level of the valence band of the PAPI spin-coated film and valence level of the OXD7 vacuum-deposited film were determined by the photoemission measurement. The band gaps of PAP1 and OXD7 film were evaluated to be 2.8 and 3.7 eV by the absorption spectra, respectively, where the band gap of OXD7 was assumed to be equal to the HOMO-LUMO gap. As shown in the diagram, barrier potentials for hole injection from the PAPI

‘: 60 2 50 -

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.@ 40 is 30 .a & 20 10 0L 400

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500 Wavelength

600 (nm)

PIG. 3. Electroluminescence spectrum of an organic-inorganic heterostructure device using a PAPI spin-coated film at liquid-nitrogen temperature. The dotted line shows photoluminescence spectrum of a PAP1 spin-coated tilm. Appl. Phys. Lett., Vol. 65, No. 6, 8August

1994

PIG. 4. Energy diagram of ITO/PAPI/OXD7/MgAg

device.

layer to the OXD7 layer are large because of a large ionization potential of the OXD7 layer. Then, injected holes from the indium tin oxide (ITO) electrode are confined in the side of the PAP1 layer at the PAPI/OXD7 interface. On the other hand, electrons in the OXD7 layer are smoothly injected to PAP1 layer because there is no potential barrier for electroninjection from OXD7 to PAP. Further, injected electrons are presumed to be efficiently recombined to the confined holes in the PAPI layer. Consequently, efficient EL from the PAP1 layer would occur in the inorganic-organic heterostructure device. When the heterostructure device was driven at room temperature, EL efficiency was very small in comparison with that at liquid-nitrogen temperature. We assume that the lowering of EL efficiency was mainly caused by the thermal quenching of photoluminescence of the PAPI film. In a PAPI single crystal, the photoluminescence intensity was reported to be reduced to less than one tenth when temperature was increased from low temperature below 200 K to room temperature.7,8 The quenching is due to the thermal ionization of the excitons. Prevention of thermal quenching, in other words, further stabilization of excitons in this type compounds is required for the increase of EL efficiency at room temperature. In this study, we demonstrated that intense EL with narrow bandwidth was performed in the organic-inorganic heterostructure device using the combination of a photoluminescent layered perovskite compound and an organic electrontransport material. The layered perovskite compounds are, we believe, promising for the application to EL devices because of not only very specific optical and electronic properties but flexibility of material design and processability. Further, our success in the organic-inorganic heterostructure device shows that the adoption of organic-inorganic heterostructure possess high potential for new approaches to construct high performance EL devices. Very recently, we have found that the photoluminescence from the compound with Pphenethylamine as the organic ammonium layer was shifted to blue region by replacing iodine to bromine. Fabrication and characterization of the blue light emitting device using the compound are now in progress. Era

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et al.

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et al.