JP4214482B2 - Organic electroluminescence device - Google Patents
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本発明は、有機エレクトロルミネッセンス素子に関する。 The present invention relates to an organic electroluminescence element.
情報化社会の急速な進展に伴い、軽量で消費電力の小さなフラットパネルディスプレイ（ＦＤＰ）へのニーズが益々高まっている。ＦＤＰの候補として様々な表示方式が知られているが、その中でも、有機エレクトロルミネッセンス素子（以下、「有機ＥＬ素子」という場合がある）が注目されている。有機ＥＬ素子では、発光材料を選択することによって様々な発光色が得られる。また薄膜化が容易でプラスティック基板などのフレキシブル基板上にも作製できるため、フルカラーのフレキシブルディスプレイを実現する際に有利な特徴を持っている。 With the rapid development of the information society, there is an increasing need for a flat panel display (FDP) that is lightweight and consumes little power. Various display methods are known as candidates for the FDP, and among them, an organic electroluminescence element (hereinafter sometimes referred to as “organic EL element”) has attracted attention. In an organic EL element, various emission colors can be obtained by selecting a light emitting material. In addition, since it can be easily made into a thin film and can be manufactured on a flexible substrate such as a plastic substrate, it has an advantageous feature in realizing a full-color flexible display.
有機ＥＬ素子では、素子に注入された電子と正孔が発光層内部や発光層と他の有機層との界面で再結合して発光する。従って発光層は素子の発光効率を決定する重要な部分である。具体的には、正孔注入電極と電子注入電極から素子に注入された正孔と電子は、正孔輸送層と電子輸送層を通過して発光層へと運ばれそこで再結合する。この再結合によって発光層材料の一部が励起状態を形成する。この時、励起一重項状態と励起三重項状態とがそれぞれ２５％：７５％の割合で生成する。生成した励起状態から効率よく発光を得るために発光層の材料としては蛍光量子収率の高い蛍光色素や燐光色素が使用される。蛍光は励起一重項状態からの発光であるので再結合によって生成した励起状態の中で２５％だけが利用可能である。これに対して、励起三重項状態からの発光である燐光を用いれば、生成する励起状態の７５％を利用できるため非常に高い発光効率を実現することができる。 In an organic EL element, electrons and holes injected into the element recombine inside the light emitting layer or at the interface between the light emitting layer and another organic layer to emit light. Therefore, the light emitting layer is an important part that determines the luminous efficiency of the device. Specifically, holes and electrons injected into the element from the hole injection electrode and the electron injection electrode are transported to the light emitting layer through the hole transport layer and the electron transport layer and recombined there. A part of the light emitting layer material forms an excited state by this recombination. At this time, an excited singlet state and an excited triplet state are generated at a ratio of 25%: 75%, respectively. In order to efficiently obtain light emission from the generated excited state, a fluorescent dye or phosphorescent dye having a high fluorescence quantum yield is used as the material of the light emitting layer. Since fluorescence is emitted from an excited singlet state, only 25% of the excited state generated by recombination can be used. On the other hand, if phosphorescence, which is light emission from an excited triplet state, is used, 75% of the generated excited state can be used, so that very high emission efficiency can be realized.
室温において強い燐光を示す材料として、イリジウムや白金を中心金属にもつ有機金属錯体が知られている（非特許文献１参照）。これらの材料は有機ＥＬ素子の発光材料として用いられ高効率の発光を示す。しかしながら、励起三重項状態からの発光は本来禁制遷移のため、室温で強い燐光を示す材料は、イリジウム錯体（Ir(ppy)3）や白金ポルフィリン錯体など一部の特殊で高価な化合物に限られるといった問題があった。 As a material exhibiting strong phosphorescence at room temperature, an organometallic complex having iridium or platinum as a central metal is known (see Non-Patent Document 1). These materials are used as a light emitting material of the organic EL element and exhibit high efficiency light emission. However, since emission from the excited triplet state is essentially a forbidden transition, materials that exhibit strong phosphorescence at room temperature are limited to some special and expensive compounds such as iridium complexes (Ir (ppy) 3 ) and platinum porphyrin complexes. There was a problem.
また、従来から、燐光材料から蛍光材料へのエネルギー移動を用いて蛍光材料を発光させる有機ＥＬ素子も提案されてきた（非特許文献２、特許文献１）。
しかしながら、上記先行技術で検討されてきた燐光材料は、高価な金属錯体であった。また、上記先行技術では、燐光材料から蛍光材料へのエネルギー移動に適した有機化合物の検討は十分にされていなかった。 However, the phosphorescent material that has been studied in the above prior art has been an expensive metal complex. Further, in the above prior art, an organic compound suitable for energy transfer from a phosphorescent material to a fluorescent material has not been sufficiently studied.
このような状況に鑑み、本発明は、エネルギー移動を用いて高輝度の発光が得られ、低コストで製造が可能な有機エレクトロルミネッセンス素子を提供することを目的の１つとする。 In view of such a situation, an object of the present invention is to provide an organic electroluminescence element that can emit light with high luminance by using energy transfer and can be manufactured at low cost.
上記目的を達成するため、本発明の有機エレクトロルミネッセンス素子は、キャリアの注入によって発光する発光層を備える有機エレクトロルミネッセンス素子であって、前記発光層が第１の有機化合物と第２の有機化合物とを含み、前記発光層を構成する有機化合物に占める前記第１の有機化合物の割合が５０モル％以上であり、前記第１の有機化合物が、４−フェニルベンゾフェノンまたは４，４’−ジブロモベンゾフェノンであり、前記第１の有機化合物の最低励起三重項状態のエネルギー準位よりも前記第２の有機化合物の最低励起一重項状態のエネルギー準位が低く、前記第２の有機化合物の励起一重項状態のエネルギー準位であって前記最低励起一重項状態のエネルギー準位よりも高い準位の中に、前記第１の有機化合物の最低励起三重項状態のエネルギー準位に対して±０．３ｅＶの範囲にある準位が存在し、前記発光層へのキャリアの注入によって前記第２の有機化合物が蛍光を発する。
また、本発明の他の有機エレクトロルミネッセンス素子は、キャリアの注入によって発光する発光層を備える有機エレクトロルミネッセンス素子であって、前記発光層が第１の有機化合物と第２の有機化合物とを含み、前記発光層を構成する有機化合物に占める前記第１の有機化合物の割合が５０モル％以上であり、前記第１の有機化合物が、４−フェニルベンゾフェノンまたは４，４’−ジブロモベンゾフェノンであり、前記第２の有機化合物がルブレン、クマリン５４０、クマリン６、ＤＣＭ、ローダミンＢ、またはナイルレッドであり、前記発光層へのキャリアの注入によって前記第２の有機化合物が蛍光を発する。
In order to achieve the above object, an organic electroluminescent device of the present invention is an organic electroluminescent device including a light emitting layer that emits light by carrier injection, and the light emitting layer includes a first organic compound and a second organic compound. And the proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more, and the first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone And the energy level of the lowest excited singlet state of the second organic compound is lower than the energy level of the lowest excited triplet state of the first organic compound, and the excited singlet state of the second organic compound. Of the first organic compound in a level higher than the energy level of the lowest excited singlet state. There are levels in the range of ± 0.3 eV relative to the energy level of the electromotive triplet state, wherein the second organic compound by injection of carriers into the light emitting layer emits fluorescence.
Further, another organic electroluminescence element of the present invention is an organic electroluminescence element comprising a light emitting layer that emits light by carrier injection, wherein the light emitting layer includes a first organic compound and a second organic compound, The proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more, and the first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone, The second organic compound is rubrene, coumarin 540, coumarin 6, DCM, rhodamine B, or Nile red, and the second organic compound emits fluorescence when carriers are injected into the light emitting layer.
本発明の有機エレクトロルミネッセンス素子では、前記第１の有機化合物が、ベンゾフェノン骨格またはケトクマリン骨格を含むことが好ましい。すなわち、別の観点では、本発明の有機エレクトロルミネッセンス素子は、キャリアの注入によって発光する発光層を備える有機エレクトロルミネッセンス素子であって、前記発光層が第１の有機化合物と第２の有機化合物とを含み、前記発光層を構成する有機化合物に占める前記第１の有機化合物の割合が５０モル％以上であり、前記第１の有機化合物がベンゾフェノン骨格またはケトクマリン骨格を含み、前記発光層へのキャリアの注入によって前記第２の有機化合物が蛍光を発する。 In the organic electroluminescence device of the present invention, it is preferable that the first organic compound includes a benzophenone skeleton or a ketocoumarin skeleton. That is, from another viewpoint, the organic electroluminescent element of the present invention is an organic electroluminescent element including a light emitting layer that emits light by carrier injection, and the light emitting layer includes a first organic compound and a second organic compound. And the ratio of the first organic compound to the organic compound constituting the light emitting layer is 50 mol% or more, the first organic compound contains a benzophenone skeleton or a ketocoumarin skeleton, and the carrier to the light emitting layer The second organic compound emits fluorescence by the injection of.
本発明によれば、第１の有機化合物（ホスト材料）から第２の有機化合物（ゲスト材料）へのエネルギー移動を用いることによって、高輝度の発光が得られる。本発明の有機ＥＬ素子では、第１の有機化合物の室温における燐光発光の強度が低くてもよいため、多様な有機材料の中から安価な有機化合物を用いることができ、低コストで製造が可能である。 According to the present invention, light emission with high brightness can be obtained by using energy transfer from the first organic compound (host material) to the second organic compound (guest material). In the organic EL device of the present invention, since the intensity of phosphorescence emission at room temperature of the first organic compound may be low, an inexpensive organic compound can be used from various organic materials and can be manufactured at low cost. It is.
以下に、本発明の実施の形態について説明する。本発明の有機ＥＬは、キャリアの注入によって発光する発光層を備える有機ＥＬ素子であって、発光層が第１の有機化合物と第２の有機化合物とを含み、発光層を構成する有機化合物に占める第１の有機化合物の割合が５０モル％以上であり、第１の有機化合物が、室温（２５℃）において安定な励起三重項状態を形成する有機化合物であり、発光層へのキャリアの注入によって第２の有機化合物が蛍光を発する。 Embodiments of the present invention will be described below. The organic EL of the present invention is an organic EL element including a light emitting layer that emits light by carrier injection, and the light emitting layer includes a first organic compound and a second organic compound, and the organic compound constituting the light emitting layer The proportion of the first organic compound is 50 mol% or more, and the first organic compound is an organic compound that forms a stable excited triplet state at room temperature (25 ° C.), and carriers are injected into the light emitting layer. As a result, the second organic compound emits fluorescence.
本発明の有機ＥＬ素子は、発光層を除いて、公知の有機ＥＬ素子と同様に形成できる。本発明の有機ＥＬ素子は、通常、正孔注入電極と電子注入電極とを含み、発光層は正孔注入電極と電子注入電極との間に配置される。２つの電極のうちの少なくとも１つの電極は、発光層から出射された光を透過させる導電膜で形成される。正孔注入電極には、たとえば、仕事関数の大きな金や白金、インジウム−スズ酸化物、アルミニウム−亜鉛酸化物などが用いられる。電子注入電極には、たとえば、マグネシウム−銀合金やリチウム、セシウム、ナトリウム、カルシウムおよびアルミニウムなどの仕事関数の小さな金属が用いられる。 The organic EL device of the present invention can be formed in the same manner as a known organic EL device except for the light emitting layer. The organic EL device of the present invention usually includes a hole injection electrode and an electron injection electrode, and the light emitting layer is disposed between the hole injection electrode and the electron injection electrode. At least one of the two electrodes is formed of a conductive film that transmits light emitted from the light emitting layer. For the hole injection electrode, for example, gold, platinum, indium-tin oxide, aluminum-zinc oxide or the like having a large work function is used. For example, a magnesium-silver alloy or a metal having a small work function such as lithium, cesium, sodium, calcium and aluminum is used for the electron injection electrode.
正孔注入電極と発光層との間には、正孔輸送層などの他の層が形成されていてもよい。また、電子注入電極と発光層との間には、電子輸送層などの他の層が形成されていてもよい。正孔輸送層および電子輸送層には、それぞれ、正孔輸送特性および電子輸送特性に優れた公知の有機材料を用いることができる。それぞれの層の厚さに特に限定はない。発光層の厚さは、たとえば５ｎｍ〜１０００ｎｍの範囲から選択してもよい。 Other layers such as a hole transport layer may be formed between the hole injection electrode and the light emitting layer. In addition, another layer such as an electron transport layer may be formed between the electron injection electrode and the light emitting layer. For the hole transport layer and the electron transport layer, known organic materials having excellent hole transport characteristics and electron transport characteristics can be used, respectively. There is no particular limitation on the thickness of each layer. The thickness of the light emitting layer may be selected from the range of 5 nm to 1000 nm, for example.
発光層を構成する有機化合物に占める第１の有機化合物（ホスト材料）の割合は、５０モル％以上であり、たとえば８０〜９９．９９モル％である。 The ratio of the 1st organic compound (host material) to the organic compound which comprises a light emitting layer is 50 mol% or more, for example, 80-99.99 mol%.
第１の有機化合物は、キャリアの再結合によって室温で安定な励起三重項状態を形成する有機化合物である。様々な材料を検討した結果、本発明者は、第１の有機化合物として、ベンゾフェノン骨格またはケトクマリン骨格を含む有機化合物を用いることによって、第１の有機化合物の三重項励起状態から第２の有機化合物の一重項励起状態へのエネルギー移動を効率よく生じさせることができることを見出した。すなわち、第１の有機化合物として、ベンゾフェノン、ベンゾフェノン誘導体、ケトクマリン、またはケトクマリン誘導体を用いることが好ましい。ベンゾフェノン骨格は以下の式（１）で表される。ケトクマリン骨格は以下の式（２）で表され、ケトクマリン骨格を有する化合物は、たとえば以下の式（３）で表される化合物やその誘導体であってもよい。なお、以下の式（１）〜（３）の環には、さらに他の環が縮合していてもよい。 The first organic compound is an organic compound that forms an excited triplet state that is stable at room temperature by carrier recombination. As a result of studying various materials, the inventor of the present invention uses the organic compound containing a benzophenone skeleton or a ketocoumarin skeleton as the first organic compound, so that the second organic compound is removed from the triplet excited state of the first organic compound. It has been found that energy transfer to a singlet excited state can be efficiently generated. That is, it is preferable to use benzophenone, a benzophenone derivative, a ketocoumarin, or a ketocoumarin derivative as the first organic compound. The benzophenone skeleton is represented by the following formula (1). The ketocoumarin skeleton is represented by the following formula (2), and the compound having the ketocoumarin skeleton may be, for example, a compound represented by the following formula (3) or a derivative thereof. In addition, the ring of the following formulas (1) to (3) may be further condensed with another ring.
ベンゾフェノン誘導体およびケトクマリン誘導体としては、たとえば、以下の式（４）〜（６）で表される化合物が挙げられる。 Examples of the benzophenone derivative and ketocoumarin derivative include compounds represented by the following formulas (4) to (6).
式（４）〜（６）において、Ｒ1、Ｒ2、Ｒ3、Ｒ5およびＲ6は、それぞれ独立に、ハロゲン原子、アルキル基、アリール基、芳香環、ヘテロ芳香環からなる群より選ばれる基であってもよい。また、Ｒ4は任意の有機基である。Ｒ1、Ｒ2、Ｒ3、Ｒ5およびＲ6は、ベンゼン環が維持される限り、ベンゼン環のいずれの位置に結合していてもよい。たとえば、Ｒ1およびＲ2は、それぞれ独立に、カルボニル基に対してオルト位、メタ位またはパラ位のいずれかの位置に結合する。 In formulas (4) to (6), R 1 , R 2 , R 3 , R 5 and R 6 are each independently selected from the group consisting of halogen atoms, alkyl groups, aryl groups, aromatic rings and heteroaromatic rings. It may be a group. R 4 is an arbitrary organic group. R 1 , R 2 , R 3 , R 5 and R 6 may be bonded to any position of the benzene ring as long as the benzene ring is maintained. For example, R 1 and R 2 are independently bonded to any position in the ortho, meta, or para position relative to the carbonyl group.
ベンゾフェノン骨格またはケトクマリン骨格を含む有機化合物は、第１の有機化合物に求められる特性を有すると共に、非常に安価であるため好ましい。本発明者は、ベンゾフェノン骨格を有する有機化合物が第１の有機化合物として特に好ましいことを見出した。ベンゾフェノン骨格を有する有機化合物は、安価であると共に高いエネルギー準位を有する励起三重項状態を有するため高効率にエネルギー移動を生じさせる。ただし、本発明の第１の有機化合物は、他の骨格を有する有機化合物であってもよい。 An organic compound containing a benzophenone skeleton or a ketocoumarin skeleton is preferable because it has characteristics required for the first organic compound and is very inexpensive. The inventor has found that an organic compound having a benzophenone skeleton is particularly preferable as the first organic compound. An organic compound having a benzophenone skeleton is inexpensive and has an excited triplet state having a high energy level, and thus causes energy transfer with high efficiency. However, the first organic compound of the present invention may be an organic compound having another skeleton.
ベンゾフェノン骨格を有する化合物の具体例としては、たとえば、４−フェニルベンゾフェノンや４，４’−ジブロモベンゾフェノンが挙げられる。 Specific examples of the compound having a benzophenone skeleton include 4-phenylbenzophenone and 4,4′-dibromobenzophenone.
第１の有機化合物の燐光寿命は、たとえば１マイクロ秒以上であり、好ましくは１ミリ秒以上であり、より好ましくは１００ミリ秒以上である。燐光寿命が長いほど、すなわち励起三重項状態の寿命が長いほど、第１の有機化合物の励起三重項状態から第２の有機化合物の励起一重項状態へのエネルギー移動の割合を高くでき、発光強度を高めることが可能となる。 The phosphorescence lifetime of the first organic compound is, for example, 1 microsecond or more, preferably 1 millisecond or more, and more preferably 100 milliseconds or more. The longer the phosphorescence lifetime, that is, the longer the lifetime of the excited triplet state, the higher the rate of energy transfer from the excited triplet state of the first organic compound to the excited singlet state of the second organic compound, and the emission intensity. Can be increased.
第１の有機化合物は、室温で光励起した時の燐光の強度が高い化合物である必要はなく、室温における燐光が実質的に生じない化合物であってもよい。たとえば、第１の有機化合物の燐光の量子収率が１％以下であってもよい。このような化合物は、安価で入手が可能である。 The first organic compound does not need to be a compound having high phosphorescence intensity when photoexcited at room temperature, and may be a compound that does not substantially generate phosphorescence at room temperature. For example, the phosphorescent quantum yield of the first organic compound may be 1% or less. Such compounds are available at low cost.
第２の有機化合物は、第１の有機化合物からのエネルギー移動によって励起される有機化合物であり、蛍光を発する有機化合物である。第２の有機化合物は、様々な蛍光材料から選択でき、高効率の発光特性を示す安価な蛍光材料を第２の有機化合物として用いることができる。 The second organic compound is an organic compound that is excited by energy transfer from the first organic compound, and is an organic compound that emits fluorescence. The second organic compound can be selected from various fluorescent materials, and an inexpensive fluorescent material exhibiting highly efficient light emission characteristics can be used as the second organic compound.
キャリアの再結合によって分子が励起状態となる場合、１：３の割合で励起一重項状態と励起三重項状態とが生じる。そのため、高い発光強度を得るためには、励起三重項状態を有効に利用することが重要である。しかし、通常の励起三重項状態からの発光（燐光）は禁制遷移であるため、室温で強い発光強度を示す材料は限られている。また、燐光発光を生じる物質であっても、励起状態の寿命が長いため、三重項−三重項消滅によって発光を伴わずに失活する割合が素子に流す電流密度を増加させると共に増加する。 When a molecule is excited by recombination of carriers, an excited singlet state and an excited triplet state are generated at a ratio of 1: 3. Therefore, it is important to effectively use the excited triplet state in order to obtain high emission intensity. However, since light emission (phosphorescence) from a normal excited triplet state is a forbidden transition, materials that exhibit strong emission intensity at room temperature are limited. Even in the case of a substance that emits phosphorescence, since the lifetime of the excited state is long, the ratio of deactivation without light emission due to triplet-triplet annihilation increases as the current density flowing through the element increases.
これに対して、本発明の有機ＥＬ素子の発光層では、第１の有機化合物（ホスト材料）の励起三重項状態から、第２の有機化合物（ゲスト材料）の励起一重項状態へエネルギー移動が生じ、第２の有機化合物の励起一重項状態からの発光（蛍光）が生じる。この蛍光発光は、許容遷移であるため、効率よく発光すると共に、励起一重項状態の寿命が短いという利点がある。また、第１の有機化合物は、必ずしも燐光の発光強度が高いものでなくともよいため、様々な材料が利用可能となる。 In contrast, in the light emitting layer of the organic EL device of the present invention, energy transfer from the excited triplet state of the first organic compound (host material) to the excited singlet state of the second organic compound (guest material). And emission (fluorescence) from the excited singlet state of the second organic compound occurs. Since this fluorescence emission is an allowable transition, it has the advantages that it emits light efficiently and has a short lifetime in the excited singlet state. Further, the first organic compound does not necessarily have high phosphorescence emission intensity, and thus various materials can be used.
本発明の有機ＥＬ素子では、第１の有機化合物の最低励起三重項状態のエネルギー準位よりも第２の有機化合物の最低励起一重項状態のエネルギー準位が低く、第２の有機化合物の励起一重項状態のエネルギー準位の中に、第１の有機化合物の最低励起三重項状態のエネルギー準位とほぼ等しい位置に吸収を生じるエネルギー準位が存在することが好ましい。このようなエネルギー準位の一例を図１に模式的に示す。 In the organic EL device of the present invention, the lowest excited singlet state energy level of the second organic compound is lower than the lowest excited triplet state energy level of the first organic compound, and the second organic compound is excited. It is preferable that an energy level that causes absorption exists in the energy level of the singlet state at a position substantially equal to the energy level of the lowest excited triplet state of the first organic compound. An example of such an energy level is schematically shown in FIG.
図１のエネルギー準位図に示すように、第１の有機化合物は、少なくとも最低励起一重項状態のエネルギー準位Ｅ１S1と最低励起三重項状態のエネルギー準位Ｅ１T1とを有する。また、第２の有機化合物は、少なくとも、最低励起一重項状態のエネルギー準位Ｅ２S1と、Ｅ２S1よりも高い準位の励起一重項状態のエネルギー準位Ｅ２Sn（ｎは１より大きい整数である）とを有する。図１に示すように、第２の有機化合物の励起一重項状態のエネルギー準位Ｅ２Snは、第１の有機化合物の最低励起三重項状態のエネルギー準位Ｅ１T1とほぼ等しいエネルギー準位である。具体的には、Ｅ２Snは、Ｅ１T1に対して±０．３ｅＶの範囲のエネルギー準位にあることが好ましい。このような有機化合物を選択することによって、第１の有機化合物のエネルギー準位Ｅ１T1から第２の有機化合物のエネルギー準位Ｅ２Snへのエネルギー移動がより生じやすくなる。本発明の有機ＥＬ素子では、エネルギー準位Ｅ１T1からのエネルギー移動が生じるエネルギー準位Ｅ２Snが、エネルギー準位Ｅ１T1よりもエネルギー的に高い位置にあってもよい。このようなエネルギー移動は、熱励起を伴うことによって生じる。 As shown in the energy level diagram of FIG. 1, the first organic compound has at least the lowest excited singlet state energy level E1 S1 and the lowest excited triplet state energy level E1 T1 . The second organic compound includes at least the energy level E2 S1 in the lowest excited singlet state and the energy level E2 Sn in the excited singlet state higher than E2 S1 (n is an integer greater than 1). There is). As shown in FIG. 1, energy level E2 Sn excited singlet state of the second organic compound is a substantially equal energy levels and the energy level E1 T1 of the lowest excited triplet state of the first organic compound . Specifically, E2 Sn is preferably in the energy level in the range of ± 0.3 eV relative to E1 T1. By selecting such an organic compound, energy transfer from the energy level E1 T1 of the first organic compound to the energy level E2 Sn of the second organic compound is more likely to occur. In the organic EL device of the invention, the energy level E2 Sn energy transfer from the energy level E1 T1 occurs may also be in the energetically higher than the energy level E1 T1. Such energy transfer is caused by thermal excitation.
なお、第１の有機化合物の励起三重項状態のエネルギー準位から第２の有機化合物の励起一重項状態のエネルギー準位へのエネルギー移動は、エネルギー準位Ｅ１T1からエネルギー準位Ｅ２Snへのエネルギー移動に限定されず、他の準位間で生じてもよい。 Note that the energy transfer from the excited triplet state energy level of the first organic compound to the excited singlet state energy level of the second organic compound is from the energy level E1 T1 to the energy level E2 Sn . It is not limited to energy transfer, and may occur between other levels.
本発明の発光層では、いわゆるフェルスター型のエネルギー移動が生じると考えられる。フェルスター型のエネルギー移動は、双極子−双極子相互作用で生じるため、エネルギーの授受をする分子同士の距離が比較的離れていてもよい。そのため、本発明の発光層では、第２の有機化合物の量が少なくてもエネルギー移動が円滑に行われる。 In the light emitting layer of the present invention, it is considered that so-called Forster energy transfer occurs. Since the Forster-type energy transfer is caused by dipole-dipole interaction, the distance between the molecules that transfer energy may be relatively long. Therefore, in the light emitting layer of the present invention, energy transfer is performed smoothly even if the amount of the second organic compound is small.
本発明の有機ＥＬ素子では、第１の有機化合物の燐光スペクトルのピークと第２の有機化合物の吸収スペクトルのピークとが少なくとも一部で重なっていることが好ましい。これによって、第１の有機化合物の励起三重項状態から第２の有機化合物へエネルギー移動が生じる。 In the organic EL device of the present invention, it is preferable that the peak of the phosphorescence spectrum of the first organic compound and the peak of the absorption spectrum of the second organic compound overlap at least partially. This causes energy transfer from the excited triplet state of the first organic compound to the second organic compound.
本発明の有機ＥＬ素子では、発光層を構成する有機化合物に占める第２の有機化合物の割合が、０．０１モル％〜２０モル％の範囲であることが好ましい。第２の有機化合物の割合を比較的低い範囲とすることによって、濃度消光の発生を抑制できる。発光層における第１の有機化合物と第２の有機化合物とのモル比は、［第１の有機化合物のモル数］：［第２の有機化合物のモル数］＝８０：２０〜９９．９９：０．０１の範囲であることが好ましく、たとえば、９０：１０〜９９．９：０．１の範囲である。 In the organic EL device of the present invention, the ratio of the second organic compound to the organic compound constituting the light emitting layer is preferably in the range of 0.01 mol% to 20 mol%. By setting the ratio of the second organic compound in a relatively low range, the occurrence of concentration quenching can be suppressed. The molar ratio of the first organic compound to the second organic compound in the light emitting layer is [number of moles of the first organic compound]: [number of moles of the second organic compound] = 80: 20 to 99.99: A range of 0.01 is preferable, for example, a range of 90:10 to 99.9: 0.1.
本発明の有機ＥＬ素子では、第２の有機化合物は第１の有機化合物に応じて選択されるが、たとえば多環式芳香族化合物を用いることができる。多環式芳香族化合物としては、たとえば、ルブレン、クマリン５４０、クマリン６、ＤＣＭ、ローダミンＢ、ナイルレッドなどが挙げられる。 In the organic EL device of the present invention, the second organic compound is selected according to the first organic compound, and for example, a polycyclic aromatic compound can be used. Examples of the polycyclic aromatic compound include rubrene, coumarin 540, coumarin 6, DCM, rhodamine B, and Nile red.
第１の有機化合物と第２の有機化合物との好ましい組み合わせとしては、たとえば、実施例に示される、４−フェニルベンゾフェノンとルブレンとの組み合わせなどが挙げられる。 As a preferable combination of the first organic compound and the second organic compound, for example, a combination of 4-phenylbenzophenone and rubrene shown in the examples can be mentioned.
なお、本発明の有機ＥＬ素子の発光層は、典型的には第１の有機化合物と第２の有機化合物との混合物で構成されるが、本発明の効果が得られる限り、第１および第２の有機化合物に加えて他の物質を含んでもよい。 The light emitting layer of the organic EL device of the present invention is typically composed of a mixture of the first organic compound and the second organic compound. As long as the effects of the present invention can be obtained, the first and first Other substances may be included in addition to the two organic compounds.
本発明の有機ＥＬ素子は、一般的な有機ＥＬ素子と同様の方法で作製できる。具体的には、蒸着法、スパッタリング法、スピンコート法といった公知の方法を用いて、基板上に電極層および有機材料層を積層することによって作製できる。 The organic EL device of the present invention can be produced by the same method as a general organic EL device. Specifically, it can be produced by laminating an electrode layer and an organic material layer on a substrate using a known method such as an evaporation method, a sputtering method, or a spin coating method.
以下、本発明の有機ＥＬ素子の一例について説明する。この実施例では、図２に示す有機ＥＬ素子１０を作製した。 Hereinafter, an example of the organic EL element of the present invention will be described. In this example, the organic EL element 10 shown in FIG. 2 was produced.
＜実施例１＞
まず、インジウム−スズ酸化物からなる透明電極（正孔注入電極１２）が形成されたガラス基板１１を用意した。そして、正孔注入電極１２上に、Ｎ，Ｎ’−ジ−１−ナフチル−Ｎ，Ｎ’−ジフェニル−１，１’−ビフェニル−４，４’−ジアミン（N,N'-di-1-naphthyl-N,N'-diphenyl-1,1'-biphenyl‐4,4'-diamine（NPD））からなる正孔輸送層１３（厚さ５０ｎｍ）を形成した。
<Example 1>
First, a glass substrate 11 on which a transparent electrode (hole injection electrode 12) made of indium-tin oxide was formed was prepared. Then, N, N′-di-1-naphthyl-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (N, N′-di-1) is formed on the hole injection electrode 12. A hole transport layer 13 (thickness 50 nm) made of -naphthyl-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD)) was formed.
次に、正孔輸送層１３上に発光層１４（厚さ５０ｎｍ）を形成した。発光層１４は、ホスト材料である９９．５モル％の４−フェニルベンゾフェノン（4-Phenyl Benzophenone（４ＰＢＰ））と、発光性のドーパントである０．５モル％のルブレン（５，６，１１，１２−テトラフェニルナフタセン）とによって形成した。４ＰＢＰとルブレンはアルドリッチ社製のものを用いた。４ＰＢＰとルブレンとを組み合わせた理由は、図３に示すように４ＰＢＰの燐光スペクトルとルブレンの吸収スペクトルとが良好な重なり合いを示すからである。 Next, the light emitting layer 14 (thickness 50 nm) was formed on the hole transport layer 13. The light-emitting layer 14 includes 99.5 mol% 4-phenylbenzophenone (4PBP) as a host material and 0.5 mol% rubrene (5, 6, 11, 11) as a light-emitting dopant. 12-tetraphenylnaphthacene). 4PBP and rubrene were manufactured by Aldrich. The reason for combining 4PBP and rubrene is that the phosphorescence spectrum of 4PBP and the absorption spectrum of rubrene show good overlap as shown in FIG.
次に、発光層１４のうえに、Ａｌｑ３（トリス（８−キノリノール）アルミニウム）からなる電子輸送層１５（厚さ１５ｎｍ）を形成した。次に、電子輸送層１５上に電子注入電極１６を形成した。電子注入電極１６は、ＬｉＦ層（厚さ０．５ｎｍ）とアルミニウム層（厚さ８０ｎｍ）とを順次積層することによって形成した。なお、すべての有機層および電極は、抵抗加熱方式の真空蒸着法で形成した。蒸着時の圧力は、１．３×１０-4Ｐａ（１×１０-6Ｔｏｒｒ）で行った。蒸着時の基板温度は室温とした。 Next, an electron transport layer 15 (thickness: 15 nm) made of Alq3 (tris (8-quinolinol) aluminum) was formed on the light emitting layer. Next, an electron injection electrode 16 was formed on the electron transport layer 15. The electron injection electrode 16 was formed by sequentially laminating a LiF layer (thickness 0.5 nm) and an aluminum layer (thickness 80 nm). All organic layers and electrodes were formed by resistance heating type vacuum deposition. The pressure during vapor deposition was 1.3 × 10 −4 Pa (1 × 10 −6 Torr). The substrate temperature during vapor deposition was room temperature.
このようにして作製された素子に電圧を印加して発光の観察を行った。具体的には、正孔注入電極にプラス、電子注入電極にマイナスの電圧を印可したところ、４．１Ｖの電圧を掛けたときに１０ｍＡ／ｃｍ2の電流密度が得られ１８８ｃｄ／ｍ2の黄色の発光が観測され、５．７Ｖの電圧では１００ｍＡ／ｃｍ2の電流密度が得られ１６０９ｃｄ／ｍ2という高輝度の黄色の発光が観測された。この素子の発光スペクトルを測定したところ、図４に示すように５６０ｎｍにピークを持つスペクトルを示した。このスペクトルは、図５に示すルブレンの蛍光スペクトルにほぼ完全に一致した。このことから、この発光はルブレンからの発光であることを確認した。 Light emission was observed by applying a voltage to the device thus fabricated. Specifically, when a positive voltage is applied to the hole injection electrode and a negative voltage is applied to the electron injection electrode, a current density of 10 mA / cm 2 is obtained when a voltage of 4.1 V is applied, and a yellow color of 188 cd / m 2 is obtained. A current density of 100 mA / cm 2 was obtained at a voltage of 5.7 V, and yellow light emission with a high luminance of 1609 cd / m 2 was observed. When the emission spectrum of this element was measured, a spectrum having a peak at 560 nm was shown as shown in FIG. This spectrum almost completely coincided with the fluorescence spectrum of rubrene shown in FIG. From this, it was confirmed that this luminescence was from rubrene.
＜比較例＞
０．５モル％のルブレンを添加しなかった以外は上記実施例１と同様に素子を作成した。そして、正孔注入電極にプラス、電子注入電極にマイナスの電圧を印可したところ、３．６Ｖの電圧を掛けたときに１０ｍＡ／ｃｍ2の電流密度が得られ１５ｃｄ／ｍ2の青緑色の発光が観測され、５.０Ｖの電圧で１００ｍＡ／ｃｍ2の電流密度が得られ２４９ｃｄ／ｍ2の青緑色の発光が観測された。これは実施例１に比べて約１０分の１の発光輝度であった。この素子の発光スペクトルを測定したところ、図６に示すように５００ｎｍにピークを持つスペクトルを示した。このスペクトルは、４ＰＢＰの燐光スペクトルにほぼ完全に一致した。このことから、この発光は４ＰＢＰからの燐光であることを確認した。
<Comparative example>
A device was prepared in the same manner as in Example 1 except that 0.5 mol% of rubrene was not added. When a positive voltage was applied to the hole injection electrode and a negative voltage was applied to the electron injection electrode, a current density of 10 mA / cm 2 was obtained when a voltage of 3.6 V was applied, and a blue-green light emission of 15 cd / m 2 was obtained. Was observed, and a current density of 100 mA / cm 2 was obtained at a voltage of 5.0 V, and blue-green light emission of 249 cd / m 2 was observed. This was a light emission luminance of about 1/10 compared to Example 1. When the emission spectrum of this element was measured, a spectrum having a peak at 500 nm was shown as shown in FIG. This spectrum almost completely matched the phosphorescence spectrum of 4PBP. From this, it was confirmed that this luminescence was phosphorescence from 4PBP.
以上、本発明の実施形態について例を挙げて説明したが、本発明は上記実施形態に限定されず、本発明の技術的思想に基づいて他の実施形態に適用できる。 The embodiments of the present invention have been described above by way of examples, but the present invention is not limited to the above-described embodiments, and can be applied to other embodiments based on the technical idea of the present invention.
本発明は、有機ＥＬ素子およびそれを用いた各種の電子機器に適用できる。 The present invention can be applied to an organic EL element and various electronic devices using the organic EL element.
１０ 有機エレクトロルミネッセンス素子
１１ ガラス基板
１２ 正孔注入電極
１３ 正孔輸送層
１４ 発光層
１５ 電子輸送層
１６ 電子注入電極
DESCRIPTION OF SYMBOLS 10 Organic electroluminescent element 11 Glass substrate 12 Hole injection electrode 13 Hole transport layer 14 Light emitting layer 15 Electron transport layer 16 Electron injection electrode
Claims (7)
前記発光層が第１の有機化合物と第２の有機化合物とを含み、
前記発光層を構成する有機化合物に占める前記第１の有機化合物の割合が５０モル％以上であり、
前記第１の有機化合物が、４−フェニルベンゾフェノンまたは４，４’−ジブロモベンゾフェノンであり、
前記第１の有機化合物の最低励起三重項状態のエネルギー準位よりも前記第２の有機化合物の最低励起一重項状態のエネルギー準位が低く、
前記第２の有機化合物の励起一重項状態のエネルギー準位であって前記最低励起一重項状態のエネルギー準位よりも高い準位の中に、前記第１の有機化合物の最低励起三重項状態のエネルギー準位に対して±０．３ｅＶの範囲にある準位が存在し、
前記発光層へのキャリアの注入によって前記第２の有機化合物が蛍光を発する有機エレクトロルミネッセンス素子。 An organic electroluminescence device comprising a light emitting layer that emits light by carrier injection,
The light-emitting layer includes a first organic compound and a second organic compound;
The proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more,
The first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone ;
The energy level of the lowest excited singlet state of the second organic compound is lower than the energy level of the lowest excited triplet state of the first organic compound,
In the energy level of the excited singlet state of the second organic compound that is higher than the energy level of the lowest excited singlet state, the lowest excited triplet state of the first organic compound There is a level in the range of ± 0.3 eV with respect to the energy level,
An organic electroluminescence device in which the second organic compound emits fluorescence when carriers are injected into the light emitting layer.
前記発光層が第１の有機化合物と第２の有機化合物とを含み、
前記発光層を構成する有機化合物に占める前記第１の有機化合物の割合が５０モル％以上であり、
前記第１の有機化合物が、４−フェニルベンゾフェノンまたは４，４’−ジブロモベンゾフェノンであり、
前記第２の有機化合物がルブレン、クマリン５４０、クマリン６、ＤＣＭ、ローダミンＢ、またはナイルレッドであり、
前記発光層へのキャリアの注入によって前記第２の有機化合物が蛍光を発する有機エレクトロルミネッセンス素子。 An organic electroluminescence device comprising a light emitting layer that emits light by carrier injection,
The light-emitting layer includes a first organic compound and a second organic compound;
The proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more,
The first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone;
The second organic compound is rubrene, coumarin 540, coumarin 6, DCM, rhodamine B, or Nile Red;
An organic electroluminescence device in which the second organic compound emits fluorescence when carriers are injected into the light emitting layer.
前記第２の有機化合物がルブレンである請求項５に記載の有機エレクトロルミネッセンス素子。 The first organic compound is 4-phenylbenzophenone;
The organic electroluminescence device according to claim 5, wherein the second organic compound is rubrene.
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