JP2002299063A - Electroluminescent element with lead bromide system layered perovskite compound as luminescent layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electroluminescent element that has lead bromide system layered perovskite as a luminescent layer, is efficient, is drivable at a low voltage and presents luminescent colors ranging from violet to blue. SOLUTION: The element is of a three-layered lamination type, where an organic molecular layer with hole transport properties and an organic molecular layer with electron transport properties hold the lead bromide system layered perovskite luminescent layer in between.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of light emitting devices, and more particularly to a novel electroluminescent device using a lead bromide-based layered perovskite compound as a light emitting layer.

[0002]

2. Description of the Related Art An electroluminescent element is an element utilizing a phenomenon in which a substance emits light when an electric field is applied. As a material for an electroluminescent device, a material using an inorganic semiconductor such as gallium arsenide and a material using a light-emitting organic molecule have been conventionally known. Recently, a halide-based layered perovskite is expected to be applied to a light-emitting device because it has a small exciton emission with a small emission wavelength width in a visible region. The present inventors have previously reported a lead iodide-based layered perovskite as an electroluminescent device using a halide-based layered perovskite, but its emission color is limited to green (M.E.
ra, S. Morimoto, T. Tsutsui, and S. Saito, Appl. Ph
ys. Lett., 65, 676-678 (1994)).

The present inventors have proposed an electroluminescent device comprising a layered perovskite compound of a lead bromide type as an electroluminescent device using a layered perovskite of a halide type. The lead bromide-based layered perovskite compound is represented by the general formula A ₂ PbBr ₄ where A is an organic ammonium molecule, and forms a superlattice structure in which organic ammonium molecule A layers and lead bromide PbBr ₄ layers are alternately stacked. (David B. Mitzi, "Synthesis, Structure, and
Properties of Organic-Inorganic Perovskites and R
elated Materials, "Progress in Organic Chemistry,
Vol. 48, Edited by Kenneth D. Karlin, John Wiley &
Sons, Inc. (1999)), the present inventor has found that this compound exhibits strong exciton emission in the violet to blue region (M. Era, N. Kakiyama, T. Ano, and M. Nagano). , Tran
s. Mater. Res. Soc. Jpn., 24, 509-511 (1999)). FIG. 6 shows an absorption spectrum and an emission spectrum of a lead bromide layered perovskite having an alkylammonium molecule as an organic layer. The exciton emission peak wavelength ranges from 400 nm to 44
It can be seen that the emission color changes from 0 nm to a violet to blue emission color.

As described above, the lead bromide-based layered perovskite provides a new type of electroluminescent element having a wide range of light emission from purple to blue, but requires a relatively large driving voltage to obtain a predetermined current. And
The efficiency is not good in that the light emission intensity obtained by the predetermined current value is low.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an electroluminescent device which emits violet to blue light with high efficiency and can be driven at a low voltage by using a lead bromide-based layered perovskite as a light emitting layer. It is in.

[0006]

As a result of repeated studies, the present inventors have succeeded in producing a multilayer structure in which a light-emitting layer composed of a lead bromide-based layered perovskite compound is combined with an organic molecule having a carrier transporting property. And enabled efficient electroluminescence at low voltage.

Thus, according to the present invention, A is represented by the general formula A ₂ PbBr ₄ where A is an organic ammonium molecule, and a superlattice structure in which organic ammonium molecule A layers and lead bromide PbBr ₄ layers are alternately laminated is formed. The lead bromide-based layered perovskite compound is used as a light-emitting layer, and the two carrier transporting layers including an organic molecule layer having a hole-transporting property and an organic molecule layer having an electron-transporting property are used as the light-emitting layer. There is provided an electroluminescent element having a three-layer structure in which a light emitting layer of a compound is sandwiched.

[0008] In the electroluminescent device of the present invention, the organic ammonium molecule of A generally has the formula R- (CH ₂ ) _n -N
Represented by H _3, where, n is an integer from 0 or 1 to 6, R represents an alkyl group having 1 to 6 carbon atoms, a phenyl group or a 5-membered cycloalkyl group 7-membered ring from the ring, Or a cycloalkenyl group. In a preferred embodiment of the electroluminescent device of the present invention, a phthalocyanine, a diamine derivative, a fluorene derivative, or a polythiophene derivative is used as the hole-transporting organic molecule, and an oxadiazole derivative is used as the electron-transporting organic molecule. A triazole derivative, a perylene derivative, or a quinolinol metal complex is used.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In an electroluminescent device according to the present invention, two carrier transporting layers comprising an organic molecular layer having a hole transporting property and an organic molecular layer having an electron transporting property are composed of a lead bromide layered perovskite compound. Has a three-layered laminated structure sandwiching a light-emitting layer composed of FIG. 1 shows a cross section of a typical example of an electroluminescent device according to the present invention, in which the above-mentioned three-layered laminated structure is arranged between electrodes (anode and cathode). Generally, the thickness of the lead bromide-based layered perovskite light-emitting layer is about 1
The thickness is preferably from 0 to 50 nm, and the thickness of each of the hole transporting organic molecular layer and the electron transporting layer is preferably about 50 to 100 nm.

Each layer constituting the electroluminescent device can be manufactured by a conventionally known thin film forming method. The carrier transport layer and the electrode layer are generally manufactured by a vacuum evaporation method. Further, the lead bromide-based layered perovskite light-emitting layer is preferably produced by a spin coating method. That is,
Crystal samples of lead bromide-based layered perovskite, or solutions of organic ammonium hydrobromide and lead bromide (PbBr ₄ ) dissolved in a polar solvent such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO) By spin coating, a lead bromide-based layered perovskite layer having a superlattice structure in which organic ammonium molecular layers and lead bromide layers are alternately stacked is formed.

The organic ammonium molecule has a chemical structure in which ammonia is bonded to an organic molecule, and is composed of lead bromide (P
Various compounds that can form a layered perovskite by coordination with bBr ₄ ) can be used. Preferred organic ammonium molecules of the general formula R- (CH ₂₎ can be represented by _n -NH _3, wherein, n is an integer from 0 or 1 to 6, R represents an alkyl group having 1 to 6 carbon atoms , A phenyl group, or a 5- to 7-membered cycloalkyl or cycloalkenyl group. Thus, a preferred example of the lead bromide-based layered perovskite used in the light emitting layer of the electroluminescent device of the present invention is represented by the following formula (1).

[0012]

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The organic molecule used in the organic molecule layer having a hole transporting property among the carrier transporting layers of the electroluminescent device of the present invention is not particularly limited, and is known to exhibit a hole transporting property. Various organic molecules (organic compounds) are applicable. Preferred hole transporting organic molecules include
Examples include phthalocyanines, diamine derivatives, fluorene derivatives, and polythiophene derivatives. For example, a preferable hole transporting organic molecule includes copper phthalocyanine represented by the following formula (2).

[0014]

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The organic molecules used in the organic molecule layer having an electron transport property of the electroluminescent device of the present invention are not particularly limited, and various organic molecules (organic compounds) known to exhibit the electron transport property are provided. Is applicable. Preferred electron transporting organic molecules include oxadiazole derivatives, triazole derivatives, perylene derivatives, and quinolinol metal complexes. For example, preferred electron transporting organic molecules include oxadiazole derivatives represented by the following formula (3).

[0016]

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As described above, the present invention comprises a three-layer laminated device in which a lead bromide layered perovskite light-emitting layer is sandwiched between an organic molecular layer having a hole transporting property and an organic molecular layer having an electron transporting property. Can emit light efficiently at a low voltage, as will be shown in Examples described later. The reason for the lower drive voltage and lower luminous efficiency is presumed to be that the lead bromide-based layered perovskite compound had a large band gap and ionization potential, which made carrier injection difficult, but was eliminated by the three-layer structure. You. That is, 1) by laminating with the organic molecular layer,
The drive voltage was reduced because the injection of holes from the anode and the injection of electrons from the cathode were facilitated. 2) The hole transporting molecule not only transports holes, but also has a role of confining electrons in the light emitting layer due to its low ability to transport electrons. In addition, the electron transporting molecule plays a role of confining holes in the light emitting layer due to its low ability to transport holes as well as electrons. As a result, it is considered that electrons and holes are confined in the light emitting layer with high density and recombination is efficiently performed, so that light emission efficiency is increased.

[0018]

The following examples are provided to further clarify the features of the present invention, but the present invention is not limited to these examples. As an example, the copper phthalocyanine (CuPc) of the above formula (2) is used as the organic molecule having the hole transporting property, and the oxadiazole derivative (OXD7) of the above formula (3) is used as the organic molecule having the electron transporting property. A three-layer type electroluminescent device sandwiching a light-emitting layer composed of a lead bromide-based layered perovskite compound of formula (1) (CHEPbBr ₄ ) was fabricated, and its characteristics were evaluated. For comparison, a single-layer element composed only of the lead bromide-based layered perovskite and a two-layer element composed only of the oxadiazole derivative were also evaluated.

In the three-layer type device, a copper phthalocyanine (CuPc) layer is formed on a glass plate coated with indium tin oxide (ITO) as a transparent electrode as an anode by a vacuum evaporation method, and then a lead bromide-based layered perovskite (CHEPbBr) is formed. ₄ )
The layer was spin-coated from a dimethylformamide solution, and further prepared by depositing an oxadiazole (OXD7) layer and an aluminum-lithium alloy (AlLi) as a cathode. The two-layer device was manufactured by spin-coating a lead bromide-based layered perovskite on a substrate coated with ITO, and then depositing an OXD7 layer and an AlLi cathode. The single-layer type device is obtained by spin-coating a lead bromide-based layered perovskite on an ITO-coated substrate,
It was prepared by depositing an AlLi cathode. FIG. 2 shows the element structure of the three-layer element and the two-layer element. Each film thickness was measured by a stylus method using Veeco's Dektak.

FIG. 3 shows an emission spectrum of the three-layer device according to the present invention. Electroluminescence corresponding to exciton emission of a lead bromide-based layered perovskite is observed at around 410 nm. From this spectrum, it can be seen that light emission from excitons in the layered perovskite layer was obtained in the three-layer device as intended. Similar results were obtained in the two-layer type device, and light emission due to excitons of the lead bromide-based layered perovskite was observed.

In a single-layer device coated with only the lead bromide-based layered perovskite light-emitting layer, the anode and the cathode were electrically connected, and the device could not be driven. This indicates that lamination with an organic molecular layer having good film quality is important for preventing the conduction between the anode and the cathode and for driving the element stably.

Next, the current-voltage characteristics of the three-layer device and the two-layer device in which light emission caused by excitons was obtained were compared (FIG. 4). Comparing the voltage required to supply a current of 100 mA, the three-layer type can drive the element at a low voltage of 20 V, the two-layer type has a voltage of 72 V, and the three-layer type has a lower voltage of 1/3 or less. .

FIG. 5 shows a comparison between the three-layer type and the two-layer type electroluminescence intensity characteristics. Comparing on the high emission intensity side, it can be seen that the three-layer type element has higher emission intensity than the two-layer type element even at the same current value and is more efficient. When compared at a current value of 150 mA, the electroluminescence intensity of the three-layer type is 30 times that of the two-layer type. This indicates that the electroluminescence efficiency per unit current density of the three-layer type device is as high as 30 times.

[0024]

According to the present invention, the following effects can be obtained. (1) An electroluminescent device using a lead bromide-based layered perovskite can emit light efficiently at a low voltage. (2) The excited state of the lead bromide-based layered perovskite can be efficiently created by current injection, and not only the exciton emission but also the function of developing the layered perovskite via the excited state can be efficiently driven by current. it can. (3) Lamination with an organic molecular layer having good film quality can prevent conduction between the anode and the cathode, so that a stable element can be constructed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a three-layer stacked light emitting device of the present invention in which a lead bromide-based layered perovskite light emitting layer is sandwiched between an organic molecular layer having a hole transporting property and an organic molecular layer having an electron transporting property. .

FIG. 2 is a schematic view of a three-layer element and a two-layer element manufactured in an example.

FIG. 3 is an electroluminescence spectrum of a three-layer element manufactured in an example.

FIG. 4 is a diagram comparing current-voltage characteristics of a three-layer element and a two-layer element manufactured in an example.

FIG. 5 is a diagram comparing current / electroluminescence intensity characteristics of a three-layer element and a two-layer element produced in an example.

FIG. 6 shows an emission spectrum and an absorption spectrum of a lead bromide-based layered perovskite having an alkylammonium molecule as an organic layer. The solid line shows the emission spectrum, and the dotted line shows the absorption spectrum. The formula shown above is the chemical formula of the layered perovskite, where the value of n indicates the number of carbon atoms in the alkyl chain of the alkyl ammonium molecule.

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Claims (4)

[Claims] A lead bromide represented by the general formula A ₂ PbBr ₄ wherein A is an organic ammonium molecule, and having a superlattice structure in which an organic ammonium molecule A layer and a lead bromide PbBr ₄ layer are alternately laminated. A layered perovskite compound is used as a light emitting layer, and a carrier transport layer composed of an organic molecular layer having a hole transporting property and an organic molecular layer having an electron transporting property sandwiches the light emitting layer of the lead bromide based perovskite compound. An electroluminescent device having a three-layered laminated structure. 2. The method according to claim 1, wherein the organic ammonium molecule of A is R- (C
H _{2) n} -NH _{3 (n} is an integer from 0 or 1 to 6, R represents an alkyl group having 1 to 6 carbon atoms, a phenyl group or a 5-membered ring of the 7-membered cycloalkyl group or a cycloalkyl, 2. An electroluminescent device according to claim 1, wherein the electroluminescent device is represented by the following formula: 3. The electroluminescent device according to claim 1, wherein a phthalocyanine, a diamine derivative, a fluorene derivative or a polythiophene derivative is used as the hole transporting organic molecule. 4. The electroluminescent device according to claim 1, wherein an oxadiazole derivative, a triazole derivative, a perylene derivative, or a quinolinol metal complex is used as the organic molecule having an electron transporting property. .