JP2004146110A - Organic electroluminescent element containing dihydrophenazine derivative in positive electrode buffer layer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic EL (electroluminescent) element facilitating injection of positive holes from a positive electrode by using a positive electrode buffer material having high positive hole transporting capability and heat resistance, and having high luminous efficiency and a long service life. <P>SOLUTION: This organic electroluminescent element has at least a positive electrode buffer layer, a positive hole transportation layer and a luminescent layer, and is formed with a laminated structure. The element is characterized by including the dihydrophenazine derivative having a specific structure in the positive electrode buffer layer. <P>COPYRIGHT: (C)2004,JPO

Description

【００１６】
本発明の好ましい態様は、陽極バッファー層、正孔輸送層、発光層に加えて電子輸送層をも含む前記有機電界発光素子である。
本発明の別の好ましい態様は、一般式（２）［式（２）中、Ａｒ_３は置換若しくは無置換のアリーレンを示し、Ｒ_９〜Ｒ_３２はそれぞれ独立に、水素原子、置換若しくは無置換のアリール若しくはヘテロ環基、アルキル若しくはシクロアルキル、アルケニル、アルコキシ、アルケニルオキシ、アルキニルオキシ、アリールオキシまたはアミノ基を示す］で表わされる化合物、または一般式（３）［式（３）中、Ａｒ_４はアリーレンであり、Ａｒ_５は置換若しくは無置換のアリール若しくはヘテロ環基であり、Ｒ_３３及びＲ_３４はそれぞれ独立に水素原子、置換若しくは無置換のアリール若しくはヘテロ環基またはアルキル若しくはシクロアルキル基であり、ｎは２〜４の整数を示す］で表わされる化合物を正孔輸送層に正孔輸送材料として含むことを特徴とする前記有機電界発光素子である。
【００１７】
【化４】

【００１８】
本発明の別の好ましい態様は、一般式（１）で表わされる化合物が、同式においてＲ_１〜Ｒ_８が全て水素原子からなる化合物である前記有機電界発光素子である。
【００１９】
【発明の実施の形態】
以下、本発明を詳細に説明する。一般式（１）において、Ｒ_１〜Ｒ_８は既述の通りである。
Ａｒ_１およびＡｒ_２については、それらがアリールである場合の例としてフェニル、ビフェニル、ナフチル、アンスリル、ピレニル、フェナンスリル、フルオレニル、トリル、キシリル等が挙げられる。同じくヘテロ環基の例として、チエニル、ベンゾチエニル、３−フェニルベンゾチエニル、Ｎ−フェニルカルバゾール等が挙げられる。アルキル若しくはシクロアルキルである場合には炭素数１〜６であることが好ましく、その例としてメチル、エチル、ｎ−プロピル、イソプロピル、ｎ−ブチル、ｓｅｃ−ブチル、ｔｅｒｔ−ブチル、ｎ−ペンチル、ｎ−ヘキシル、シクロプロピル、シクロブチル、シクロペンチルおよびシクロへキシル等が挙げられる。Ａｒ_１およびＡｒ_２がアリール、ヘテロ環基の場合は置換されていてもよい。
【００２０】
それらの置換基としては、メチル、エチル、ノルマルプロピル、イソプロピル、シクロペンチル若しくはｔｅｒｔ−ブチル等のアルキル若しくはシクロアルキル、ビニル、アリル、ブテニル若しくはスチリル等のアルケニル、メトキシ、エトキシ、イソプロポキシ若しくはｔｅｒｔ−ブトキシ等のアルコキシ、ビニルオキシ若しくはアリルオキシ等のアルケニルオキシ、エチニルオキシ若しくはフェニルエチニルオキシ等のアルキニルオキシ、フェノキシ、ナフトキシ、ビフェニルオキシ若しくはピレニルオキシ等のアリールオキシ、ジメチルアミノ、ジエチルアミノ、フェニルナフチルアミノ若しくはジフェニルアミノ等の置換アミノ基、フェニル、ナフチル、アントラセニル、ビフェニル、トルイル、ピレニル、ペリレニル、アニシル、ターフェニル若しくはフェナンスレニル等のアリールまたはヒドロピレニル、ジオキサニル、チエニル、チアゾリル、チアジアゾリル、アクリジニル、キノリル、キノキサロイル、フェナンスロリル、ベンゾチエニル、ベンゾチアゾリル、インドリル若しくはジヒドロフェナジン５−イル等のヘテロ環基等が挙げられる。さらに、任意の場所で縮合環を形成していてもよい。その様な縮合環基の具体例としてビフェニルの２つのベンゼン環がオルト位で架橋しているフルオレニルを挙げることができる。これらのうちでも置換アミノ基若しくはジヒドロフェナジン−５−イルが好ましい。そして、ジヒドロフェナジン誘導体の具体例としては、下記の式（４）〜（１６）で表わされる化合物を挙げることができる。
【００２１】
【化５】

【００２２】
【化６】

（式中、ｔＢｕはｔ−ブチル基を示す。）
【００２３】
【化７】

【００２４】
【化８】

【００２５】
これらの化合物は、既知の方法を利用して合成することができ、例えば、Ｊ．　Ａｍ．　Ｃｈｅｍ．　Ｓｏｃ．，　１９５７，　７９，　６１７８に記載の方法または、日本化学会第８１春季年会講演予稿集（ＩＩ）、１２５２ページ、１Ｈ１−４３に記載の方法により得ることができる。
【００２６】
これらのジヒドロフェナジン誘導体は、陽極バッファー層を形成させるための材料に適している。本発明の有機ＥＬ素子は、低電圧駆動で高効率である。これは、本発明で使用される一般式（１）で表わされるジヒドロフェナジン誘導体のイオン化ポテンシャル（Ｉｐ）が小さいため、陽極バッファー層として用いると陽極からスムーズに正孔が注入され、発光層で効率よく電子と再結合することができるためである。また保存時及び駆動時の耐久性が高い。これは、ジヒドロフェナジン誘導体のＴｇが高く、有機ＥＬ素子作成時にアモルファス状態になり易いためである。これらのジヒドロフェナジン誘導体の中でも特に好ましくは、（５）若しくは（１５）式で表わされるようなビフェニル骨格を有するものがあげられる。ビフェニル骨格の導入により、素子の発光効率及び電流効率を向上させることが可能になるからである。
【００２７】
本発明の有機ＥＬ素子の構造としては、各種の態様があるが、基本的には陽極と正孔輸送層との間に、上記ジヒドロフェナジン誘導体を含有する有機層を挟持した構造であり、上記ジヒドロフェナジン誘導体層に加えて他の材料を用いた正孔輸送層、発光層、電子輸送層あるいは電子注入層等を組み合せることができる。
【００２８】
具体的な構成としては、（１）陽極／ジヒドロフェナジン誘導体層（陽極バッファー層）／正孔輸送層／発光層／陰極、（２）陽極／ジヒドロフェナジン誘導体層（陽極バッファー層）／正孔輸送層／発光層／電子輸送層／陰極、（３）陽極／ジヒドロフェナジン誘導体層（陽極バッファー層）／正孔輸送層／発光層／電子輸送層／電子注入層／陰極等の積層構造を挙げることができる。これらの場合、電子輸送層および電子注入層は必ずしも必要ではないが、この層を設けることにより、発光効率を向上させることができる。
【００２９】
本発明の有機ＥＬ素子は、上記のいずれの構造であっても、基板に支持されていることが好ましい。基板としては、機械的強度、熱安定性および透明性を有するものであればよく、ガラス、透明プラスチックフィルム等を用いることができる。本発明の有機ＥＬ素子の陽極物質としては、４ｅＶより大きな仕事関数を有する金属、合金、電気伝導性化合物およびこれらの混合物を用いることができる。具体例として、Ａｕ等の金属、ＣｕＩ、インジウムチンオキシド（以下、ＩＴＯと略記する）、ＳｎＯ_２、ＺｎＯ等の導電性透明材料が挙げられる。
【００３０】
陰極物質としては、４ｅＶより小さな仕事関数の金属、合金、電気伝導性化合物、およびこれらの混合物を使用できる。具体例としては、アルミニウム、カルシウム、マグネシウム、リチウム、マグネシウム合金、アルミニウム合金等があり、合金としては、アルミニウム／弗化リチウム、アルミニウム／リチウム、マグネシウム／銀、マグネシウム／インジウム等が挙げられる。有機ＥＬ素子の発光を効率よく取り出すために、電極の少なくとも一方は光透過率を１０％以上とすることが望ましい。電極としてのシート抵抗は数百Ω／ｍｍ以下とすることが好ましい。なお、膜厚は電極材料の性質にもよるが、通常１０ｎｍ〜１μｍ、好ましくは１０〜４００ｎｍの範囲に選定される。このような電極は、上述の電極物質を使用して蒸着やスパッタリング等の方法で、薄膜を形成させることにより作製することができる。
【００３１】
本発明の有機ＥＬ素子において使用される式（１）で表されるジヒドロフェナジン誘導体は十分に高いＴｇを有しているが、他の正孔輸送材料、発光材料、電子輸送材料、電子注入材料等についても、Ｔｇが８０℃以上のものが好ましく、９０℃以上のものがより好ましい。
【００３２】
本発明の有機ＥＬ素子に使用される正孔輸送材料については、有機ＥＬ素子の正孔輸送層に使用されている公知のものの中から任意のものを選択して用いることができる。例えば、一般式（２）若しくは（３）で表わされる化合物、カルバゾール誘導体（Ｎ−フェニルカルバゾール、ポリアルキレンカルバゾール等）、トリアリールアミン誘導体（ＴＰＤ）、芳香族第３級アミンを主鎖あるいは側鎖に持つポリマー、１，１−ビス（４−ジ−ｐ−トリルアミノフェニル）シクロヘキサン、スチルベン誘導体（日本化学会第７２春季年会講演予稿集（ＩＩ）、１３９２ページ、２ＰＢ０９８に記載のもの等）、ポリシラン等があげられる。これらのうち好ましくは前記一般式（２）若しくは（３）で表わされる化合物である。これらの式においてＡｒ_３は好ましくはビフェニレンまたはフルオレニレンであり、Ａｒ_４は好ましくはビフェニレンであり、Ａｒ_５は好ましくはフェニル、ビフェニル若しくはナフチルである。一般式（２）で表わされる化合物の具体例として以下に示す化合物を例示することができる。
【００３３】
【化９】

【００３４】
【化１０】

【００３５】
一般式（３）で表わされる化合物の具体例として以下の化合物を挙げることができる。
【００３６】
【化１１】

【００３７】
なお、本発明の有機ＥＬ素子における陽極バッファー層は、本発明のジヒドロフェナジン誘導体および／もしくは上記化合物の一種以上を含有する一つの層で構成されていてもよいし、また、異種の化合物を含有する複数の層を積層したものであってもよい。本発明のジヒドロフェナジン誘導体を高分子に分散させることによっても、これらの層を作成することができる。
【００３８】
本発明の有機ＥＬ素子に使用される他の電子輸送材料および電子注入材料については特に制限はなく、光導電材料において、電子伝達化合物として従来から慣用されているもの、有機ＥＬ素子の電子注入層および電子輸送層に使用されている公知のものの中から任意のものを選択して用いることができる。
【００３９】
このような電子伝達化合物の好ましい例として、ジフェニルキノン誘導体（電子写真学会誌、３０（３），　２６６（１９９１）等に記載のもの）、ペリレン誘導体（Ｊ．　Ａｐｐｌ．Ｐｈｙｓ．，　２７，　２６９（１９８８）等に記載のもの）や、オキサジアゾール誘導体（前記文献、Ｊｐｎ．　Ｊ．　Ａｐｐｌ．　Ｐｈｙｓ．，　２７，　Ｌ７１３（１９８８））、Ａｐｐｌ．　Ｐｈｙｓ．　Ｌｅｔｔ．，　５５，　１４８９（１９８９）等に記載のもの）、チオフェン誘導体（特開平４−２１２２８６号公報等に記載のもの）、トリアゾール誘導体（Ｊｐｎ．　Ｊ．　Ａｐｐｌ．　Ｐｈｙｓ．，　３２，　Ｌ９１７（１９９３）等に記載のもの）、チアジアゾール誘導体（第４３回高分子学会予稿集、（ＩＩＩ）Ｐ１ａ００７等に記載のもの）、オキシン誘導体の金属錯体（電子情報通信学会技術研究報告、９２（３１１），　４３（１９９２）等に記載のもの）、キノキサリン誘導体のポリマー（Ｊｐｎ．　Ｊ．　Ａｐｐｌ．　Ｐｈｙｓ．，　３３，　Ｌ２５０（１９９４）等に記載のもの）、フェナントロリン誘導体（第４３回高分子討論会予稿集、１４Ｊ０７等に記載のもの）等を挙げることができる。
【００４０】
本発明の有機ＥＬ素子の発光層に用いる他の発光材料としては、高分子学会編高分子機能材料シリーズ“光機能材料”、共立出版（１９９１）、Ｐ２３６に記載されているような昼光蛍光材料、蛍光増白剤、レーザー色素、有機シンチレータ、各種の蛍光分析試薬等の公知の発光材料を用いることができる。
【００４１】
具体的には、アントラセン、フェナントレン、ピレン、クリセン、ペリレン、コロネン、ルブレン、キナクリドン等の多環縮合化合物、クオーターフェニル等のオリゴフェニレン系化合物、１，４−ビス（２−メチルスチリル）ベンゼン、１，４−ビス（４−メチルスチリル）ベンゼン、１，４−ビス（４−フェニル−５−オキサゾリル）ベンゼン、１，４−ビス（５−フェニル−２−オキサゾリル）ベンゼン、２，５−ビス（５−ターシャリー−ブチル−２−ベンズオキサゾリル）チオフェン、１，４−ジフェニル−１，３−ブタジエン、１，６−ジフェニル−１，３，５−ヘキサトリエン、１，１，４，４−テトラフェニル−１，３−ブタジエン等の液体シンチレーション用シンチレータ、
【００４２】
特開昭６３−２６４６９２号公報記載のオキシン誘導体の金属錯体、クマリン染料、ジシアノメチレンピラン染料、ジシアノメチレンチオピラン染料、ポリメチン染料、オキソベンズアントラセン染料、キサンテン染料、カルボスチリル染料、およびペリレン染料、独国特許２５３４７１３号公報に記載のオキサジン系化合物、第４０回応用物理学関係連合講演会講演予稿集、１１４６（１９９３）に記載のスチルベン誘導体、特開平７−２７８５３７号公報記載のスピロ化合物および特開平４−３６３８９１号公報記載のオキサジアゾール系化合物等が好ましい。また、第９回応用物理学会講習会予稿集、（２００１）Ｐ１７および“有機ＥＬ材料とディスプレイ”、シーエムシー（２００１）Ｐ１７０に記載されている公知のりん光材料、例えば、イリジウム錯体、白金錯体、ユウロピウム錯体等も発光材料として好ましい。
【００４３】
本発明の有機ＥＬ素子を構成する各層は、各層を構成すべき材料を蒸着法、スピンコート法およびキャスト法等の公知の方法で薄膜とすることにより、形成することができる。このようにして形成された各層の膜厚については特に限定されることはなく、素材の性質に応じて適宜選定することができるが、通常２ｎｍ〜５０００ｎｍの範囲に選定される。なお、ジヒドロフェナジン誘導体を単独で薄膜化する方法としては、均質な膜が得やすく、かつピンホールが生成しにくい等の点から蒸着法を採用するのが好ましい。蒸着法を用いて薄膜化する場合、その蒸着条件は、ジヒドロフェナジン誘導体の種類、分子累積膜の目的とする結晶構造および会合構造等により異なるが、一般的に、ボート加熱温度５０〜４００℃、真空度１０^−６〜１０^−３Ｐａ、蒸着速度０．０１〜５０ｎｍ／秒、基板温度−１５０〜＋３００℃、膜厚５ｎｍ〜５μｍの範囲で適宜選定することが好ましい。
【００４４】
次に、本発明のジヒドロフェナジン誘導体を用いて有機ＥＬ素子を作成する方法の一例として、前述の陽極／ジヒドロフェナジン誘導体／正孔輸送層／発光層／陰極からなる有機ＥＬ素子の作成法について説明する。適当な基板上に、陽極用物質からなる薄膜を、１μｍ以下、好ましくは１０〜２００ｎｍの範囲の膜厚になるように蒸着法により形成させて陽極を作製した後、この陽極上にジヒドロフェナジン誘導体の薄膜を形成させて陽極バッファー層とし、この陽極バッファー層の上に正孔輸送層および発光層をそれぞれ１μｍ以下の膜厚になるように形成させ、さらに陰極用物質からなる薄膜を蒸着法により、１μｍ以下の膜厚になるように形成させて陰極とすることにより、目的の有機ＥＬ素子が得られる。なお、上述の有機ＥＬ素子の作製においては、作製順序を逆にして、陰極、発光層、正孔輸送層、陽極バッファー層、陽極の順に作製することも可能である。
【００４５】
このようにして得られた有機ＥＬ素子に直流電圧を印加する場合には、陽極を正、陰極を負の極性として印加すればよく、電圧２〜４０Ｖ程度を印加すると、透明又は半透明の電極側（陽極又は陰極、及び両方）より発光が観測できる。また、この有機ＥＬ素子は、交流電圧を印加した場合にも発光する。なお、印加する交流の波形は任意でよい。
【００４６】
【実施例】
次に、本発明を実施例に基づいて更に詳しく説明する。
【００４７】
実施例１
２５ｍｍ×７５ｍｍ×１．１ｍｍのガラス基板上にＩＴＯを１５０ｎｍの厚さに蒸着したもの（東京三容真空（株）製）を透明支持基板とした。この透明支持基板を市販の蒸着装置（真空機工（株）製）の基板ホルダーに固定し、例示化合物（４）をいれたモリブデン製蒸着用ボート、ＮＰＤをいれたモリブデン製蒸着用ボート、トリス（８−ヒドロキシキノリン）アルミニウム（以下、ＡＬＱと略記する。）をいれたモリブデン製蒸着用ボート、弗化リチウムをいれたモリブデン製蒸着用ボート、およびアルミニウムをいれたタングステン製蒸着用ボートを装着した。真空槽を１×１０^−３Ｐａまで減圧し、例示化合物（４）が入った蒸着用ボートを加熱して、膜厚４０ｎｍになるように蒸着して陽極バッファー層を形成し、次いで、ＮＰＤ入りの蒸着用ボートを加熱して、膜厚１０ｎｍになるようにＮＰＤを蒸着して正孔輸送層を形成した。
【００４８】
次に、ＡＬＱ入りの蒸着用ボートを加熱して、膜厚５０ｎｍになるようにＡＬＱを蒸着して発光層を形成した。以上の蒸着速度は０．１〜０．２ｎｍ／秒であった。その後、弗化リチウム入りの蒸着用ボートを加熱して、膜厚０．５ｎｍになるように０．００３〜０．０１ｎｍ／秒の蒸着速度で蒸着し、次いで、アルミニウム入りの蒸着用ボートを加熱して、膜厚１００ｎｍになるように０．２〜０．５ｎｍ／秒の蒸着速度で蒸着することにより、有機ＥＬ素子を得た。ＩＴＯ電極を陽極、弗化リチウム／アルミニウム電極を陰極として、約４Ｖの直流電圧を印加すると、２．８ｍＡ／ｃｍ^２の電流が流れ、電流効率３．６ｃｄ／Ａ、発光効率２．３ｌｍ／Ｗで波長５２０ｎｍの緑色の発光を得た。また、５０ｍＡ／ｃｍ^２の定電流駆動を行ったが、１００時間経過後も発光が持続されていた。また、８０℃に加熱しても発光の低下は見られなかった。また、例示化合物（４）のＴｇは１１８℃であり、Ｉｐは４．９ｅＶであった。
【００４９】
実施例２
例示化合物（４）を例示化合物（５）に替えた以外は、実施例１と同じ方法で、有機ＥＬ素子を得た。ＩＴＯ電極を陽極、弗化リチウム／アルミニウム電極を陰極として、約４Ｖの直流電圧を印加すると、２．７ｍＡ／ｃｍ^２の電流が流れ、電流効率３．７ｃｄ／Ａ、発光効率２．８ｌｍ／Ｗで波長５２０ｎｍの緑色の発光を得た。また、５０ｍＡ／ｃｍ^２の定電流駆動を行ったが、１００時間経過後も発光が持続されていた。また、８０℃に加熱しても発光の低下は見られなかった。また、Ｉｐは４．８ｅＶであった。
【００５０】
実施例３
例示化合物（４）を例示化合物（１２）に替えた以外は、実施例１と同じ方法で、有機ＥＬ素子を得た。ＩＴＯ電極を陽極、弗化リチウム／アルミニウム電極を陰極として、約４Ｖの直流電圧を印加すると、３．２ｍＡ／ｃｍ^２の電流が流れ、電流効率３．１ｃｄ／Ａ、発光効率２．２ｌｍ／Ｗで波長５２０ｎｍの緑色の発光を得た。また、５０ｍＡ／ｃｍ^２の定電流駆動を行ったが、１００時間経過後も発光が持続されていた。また、８０℃に加熱しても発光の低下は見られなかった。また、Ｉｐは４．９ｅＶであった。
【００５１】
比較例１
実施例１で用いた例示化合物（４）を銅フタロシアニンに替えた以外は、実施例１と同様な方法で有機ＥＬ素子を作成した。ＩＴＯ電極を陽極、弗化リチウム／アルミニウム電極を陰極として、約４Ｖの直流電圧を印加すると、６．５ｍＡ／ｃｍ^２の電流が流れ、電流効率１．５ｃｄ／Ａ、発光効率１．２ｌｍ／Ｗとなり、実施例１〜３の素子と比較して電流効率および発光効率が低下した。
【００５２】
比較例２
実施例１で用いた例示化合物（４）を４，４’，４”−トリス｛Ｎ−（３−メチルフェニル）−Ｎ−フェニルアミノ｝トリフェニルアミンに替えた以外は、実施例１と同様な方法で有機ＥＬ素子を作成した。　ＩＴＯ電極を陽極、弗化リチウム／アルミニウム電極を陰極として、約４Ｖの直流電圧を印加すると、３．３ｍＡ／ｃｍ^２の電流が流れ、電流効率３．０ｃｄ／Ａ、発光効率２．３ｌｍ／Ｗとなり、実施例１〜３と比較して電流効率が低下した。
【００５３】
【発明の効果】
以上説明してきたように、本発明のジヒドロフェナジン誘導体はＴｇが９０℃以上と高いためアモルファス状態になり易く、また、Ｉｐが５ｅＶ以下と小さいことから、これは有機ＥＬ素子の陽極バッファー材料に適しており、有機ＥＬ素子の高効率および長寿命化を可能とすることができる。すなわち、本発明の有機ＥＬ素子を用いることにより、フルカラーディスプレー等の高効率なディスプレイ装置が作成できる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an organic electroluminescent device (hereinafter, abbreviated as an organic EL device).
[0002]
[Prior art]
In recent years, an organic EL element has attracted attention as a next-generation full-color flat panel display, and active research and development have been made. The organic EL element is an injection type light emitting element in which a light emitting layer is sandwiched between two electrodes, and emits light by injecting electrons and holes into the organic light emitting layer and recombining them. The materials used include a low molecular weight material and a high molecular weight material, and both indicate that a high-luminance organic EL device can be obtained.
[0003]
There are two types of light emitting layers of such organic EL devices. One uses an electron transporting material to which a fluorescent dye is added as a light emitting layer, and the other uses a fluorescent dye itself as a light emitting layer, which is disclosed by CW Tang et al. is there.
[0004]
The types using the fluorescent dye as the light emitting layer are roughly classified into three types. The first is a three-layer structure in which the light-emitting layer is sandwiched between an electron transport layer and a hole transport layer. The second is a two-layer structure in which a hole transport layer and a light-emitting layer are stacked. The eye is obtained by laminating an electron transport layer and a light emitting layer to form two layers. It is known that luminous efficiency is improved by adopting such a laminated structure.
[0005]
The electron transport layer in the organic EL device having each of the above structures contains an electron transfer compound and has a function of transferring electrons injected from the cathode to the light emitting layer. The hole transport layer is a layer containing a hole transport compound and has a function of transmitting holes injected from the anode to the light emitting layer.
[0006]
Further, by interposing a buffer layer between the electrode of the organic EL device having a three-layer structure and the charge transport layer, many carriers are transmitted to the light emitting layer through the charge transport layer with a lower electric field, so that the luminous efficiency is improved. Thus, an organic EL device having excellent light emission performance, such as improved light emission performance, can be obtained.
[0007]
However, these organic EL devices did not have sufficient performance for practical use. One of the major reasons is that the material used for the anode buffer layer has insufficient hole transportability. Therefore, the driving voltage is increased, and the luminous efficiency of the device is not practically sufficient. In addition, the durability of the material is insufficient. It is considered that when an inhomogeneous portion such as a crystal grain boundary exists in an organic layer of an organic EL device, an electric field is concentrated on the portion, leading to deterioration and destruction of the device. Therefore, the organic layer is often used in an amorphous state. The organic EL element is an electron injection element, and if the material used has a low glass transition point, heat generated during driving will result in deterioration of the organic EL element. Therefore, the glass transition point (hereinafter abbreviated as Tg). ) Is required.
[0008]
As the anode buffer layer used in such an organic EL device, although a wide variety of materials centering on amine compounds are known, there are few materials suitable for practical use. For example, none of known starburst amine derivatives and known triarylamine oligomers have practically essential characteristics such as high luminous efficiency and long life. Phthalocyanines are known as metal complex-based anode buffer layers, but are not suitable for red light-emitting devices because they absorb in the visible region. Further, a polythiophene derivative is known as a polymer-based anode buffer layer, but has not been put to practical use.
[0009]
Further, in Patent Document 1, as a compound based on an amine, a light-emitting element having one or more organic compound layers sandwiched between a pair of electrodes, at least one of the organic compound layers contains a dihydrophenazine derivative There is disclosed an organic EL element which is a layer to be formed. However, those layers are also specifically an electron transporting layer, a hole transporting layer, and a light emitting layer, and there is no description used for the anode buffer layer.
[0010]
As described above, the anode buffer material used in the conventional organic EL device does not correspond to the recent high performance of the full color flat panel display, and by using an excellent material, the efficiency and the efficiency of the organic EL device can be improved. There is a demand for a longer life.
[0011]
[Patent Document 1]
JP 2000-21574 A
[Problems to be solved by the invention]
The present invention has been made in view of such problems of the related art, and an object of the present invention is to inject holes from an anode using an anode buffer material having high hole transportability and heat resistance. To provide an organic EL device having high luminous efficiency and long life.
[0013]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to solve the above-mentioned problems of the conventional organic EL device. As a result, at least an anode buffer layer, a hole transport layer, and an organic electroluminescent device having a laminated structure having a light emitting layer have been developed. By using a specific dihydrophenazine derivative as the anode buffer layer, they found that an organic EL device having high efficiency and long life was obtained, and the present invention was completed.
[0014]
That is, the present invention has the following configuration.
The present invention relates to an organic electroluminescent device having at least an anode buffer layer, a hole transport layer, and a light emitting layer and having a laminated structure, wherein R 1 to R 8 are represented by the general formula (1): Is independently a hydrogen atom, a substituted or unsubstituted aryl or heterocyclic group, alkyl or cycloalkyl, alkenyl, alkoxy, alkenyloxy, alkynyloxy, aryloxy or amino group, and Ar ₁ and Ar ₂ are independently substituted Or an unsubstituted aryl or heterocyclic group or alkyl or cycloalkyl]] in the anode buffer layer.
[0015]
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[0016]
A preferred embodiment of the present invention is the above-mentioned organic electroluminescent device including an electron transport layer in addition to the anode buffer layer, the hole transport layer, and the light emitting layer.
Another preferred embodiment of the present invention provides a compound represented by the general formula (2) wherein Ar ₃ represents a substituted or unsubstituted arylene, and R _{9 to} R ₃₂ each independently represent a hydrogen atom, a substituted or unsubstituted arylene. An aryl or heterocyclic group, alkyl or cycloalkyl, alkenyl, alkoxy, alkenyloxy, alkynyloxy, aryloxy or amino group of the formula (3) [wherein Ar ₄ Is an arylene, Ar ₅ is a substituted or unsubstituted aryl or heterocyclic group, and R ₃₃ and R ₃₄ are each independently a hydrogen atom, a substituted or unsubstituted aryl or heterocyclic group, or an alkyl or cycloalkyl group. And n represents an integer of 2 to 4]. It is the organic electroluminescent device, which comprises Te.
[0017]
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[0018]
Another preferred embodiment of the present invention is the above organic electroluminescent device, wherein the compound represented by the general formula (1) is a compound in which R _{1 to} R ₈ are all hydrogen atoms.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail. In the general formula (1), R _{1 to} R ₈ are as described above.
Examples of Ar ₁ and Ar ₂ when they are aryl include phenyl, biphenyl, naphthyl, anthryl, pyrenyl, phenanthryl, fluorenyl, tolyl, xylyl and the like. Similarly, examples of the heterocyclic group include thienyl, benzothienyl, 3-phenylbenzothienyl, N-phenylcarbazole and the like. When it is alkyl or cycloalkyl, it preferably has 1 to 6 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, n -Hexyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and the like. When Ar ₁ and Ar ₂ are an aryl or heterocyclic group, they may be substituted.
[0020]
Examples of such substituents include alkyl or cycloalkyl such as methyl, ethyl, normal propyl, isopropyl, cyclopentyl and tert-butyl, alkenyl such as vinyl, allyl, butenyl and styryl, methoxy, ethoxy, isopropoxy and tert-butoxy. Alkenyloxy such as alkoxy, vinyloxy or allyloxy, alkynyloxy such as ethynyloxy or phenylethynyloxy, aryloxy such as phenoxy, naphthoxy, biphenyloxy or pyrenyloxy, substituted amino such as dimethylamino, diethylamino, phenylnaphthylamino or diphenylamino Groups, phenyl, naphthyl, anthracenyl, biphenyl, toluyl, pyrenyl, perylenyl, anisyl, Feniru or phenanthrenyl such aryl or Hidoropireniru, dioxanyl, thienyl, thiazolyl, thiadiazolyl, acridinyl, quinolyl, quinoxaloyl, Fenansuroriru, benzothienyl, benzothiazolyl, heterocyclic groups such as indolyl or dihydrophenazine 5-yl. Further, a condensed ring may be formed at an arbitrary position. A specific example of such a condensed ring group is fluorenyl in which two benzene rings of biphenyl are bridged at the ortho position. Among these, a substituted amino group or dihydrophenazin-5-yl is preferred. Further, specific examples of the dihydrophenazine derivative include compounds represented by the following formulas (4) to (16).
[0021]
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[0022]
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(In the formula, tBu represents a t-butyl group.)
[0023]
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[0024]
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[0025]
These compounds can be synthesized using known methods. Am. Chem. Soc. , 1957, 79, 6178 or the method described in The Chemical Society of Japan's 81st Annual Meeting Preprints (II), p. 1252, 1H1-43.
[0026]
These dihydrophenazine derivatives are suitable as materials for forming an anode buffer layer. The organic EL device of the present invention is driven at a low voltage and has high efficiency. This is because the dihydrophenazine derivative represented by the general formula (1) used in the present invention has a small ionization potential (Ip), so that when it is used as an anode buffer layer, holes are smoothly injected from the anode and the efficiency in the light emitting layer is improved. This is because they can recombine well with electrons. In addition, durability during storage and driving is high. This is because the dihydrophenazine derivative has a high Tg and tends to be in an amorphous state when an organic EL device is manufactured. Of these dihydrophenazine derivatives, those having a biphenyl skeleton represented by the formula (5) or (15) are particularly preferable. This is because the introduction of the biphenyl skeleton makes it possible to improve the luminous efficiency and the current efficiency of the device.
[0027]
The structure of the organic EL device of the present invention has various aspects, but is basically a structure in which an organic layer containing the dihydrophenazine derivative is sandwiched between an anode and a hole transport layer. In addition to the dihydrophenazine derivative layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, or the like using other materials can be combined.
[0028]
Specific configurations include (1) anode / dihydrophenazine derivative layer (anode buffer layer) / hole transport layer / emission layer / cathode, and (2) anode / dihydrophenazine derivative layer (anode buffer layer) / hole transport (3) anode / dihydrophenazine derivative layer (anode buffer layer) / hole transport layer / emission layer / electron transport layer / electron injection layer / cathode, etc. Can be. In these cases, the electron transport layer and the electron injection layer are not necessarily required, but by providing this layer, the luminous efficiency can be improved.
[0029]
It is preferable that the organic EL element of the present invention is supported by a substrate in any of the above structures. The substrate only needs to have mechanical strength, thermal stability and transparency, and glass, a transparent plastic film or the like can be used. As the anode material of the organic EL device of the present invention, metals, alloys, electrically conductive compounds, and mixtures thereof having a work function greater than 4 eV can be used. Specific examples include metals such as Au, and conductive transparent materials such as CuI, indium tin oxide (hereinafter abbreviated as ITO), SnO ₂ , and ZnO.
[0030]
As the cathode material, metals, alloys, electrically conductive compounds, and mixtures thereof having a work function of less than 4 eV can be used. Specific examples include aluminum, calcium, magnesium, lithium, magnesium alloy, aluminum alloy, and the like. Examples of the alloy include aluminum / lithium fluoride, aluminum / lithium, magnesium / silver, magnesium / indium, and the like. In order to efficiently extract light emitted from the organic EL element, it is desirable that at least one of the electrodes has a light transmittance of 10% or more. The sheet resistance as an electrode is preferably several hundred Ω / mm or less. The film thickness depends on the properties of the electrode material, but is usually selected in the range of 10 nm to 1 μm, preferably 10 to 400 nm. Such an electrode can be manufactured by forming a thin film using the above-mentioned electrode substance by a method such as vapor deposition or sputtering.
[0031]
Although the dihydrophenazine derivative represented by the formula (1) used in the organic EL device of the present invention has a sufficiently high Tg, other hole transport materials, luminescent materials, electron transport materials, and electron injection materials Also about Tg, what has a Tg of 80 ° C or more is preferred, and what has a Tg of 90 ° C or more is more preferred.
[0032]
As the hole transporting material used in the organic EL device of the present invention, any material can be selected from known materials used in the hole transporting layer of the organic EL device. For example, a compound represented by the general formula (2) or (3), a carbazole derivative (N-phenylcarbazole, polyalkylenecarbazole, etc.), a triarylamine derivative (TPD), or an aromatic tertiary amine is a main chain or a side chain. Polymer, 1,1-bis (4-di-p-tolylaminophenyl) cyclohexane, stilbene derivative (Chemistry of the 72nd Annual Meeting of the Chemical Society of Japan (II), p.1392, 2PB098, etc.) , Polysilane and the like. Of these, the compounds represented by the general formula (2) or (3) are preferred. In these formulas, Ar ₃ is preferably biphenylene or fluorenylene, Ar ₄ is preferably biphenylene, and Ar ₅ is preferably phenyl, biphenyl or naphthyl. Specific examples of the compound represented by the general formula (2) include the following compounds.
[0033]
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[0034]
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[0035]
Specific examples of the compound represented by the general formula (3) include the following compounds.
[0036]
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[0037]
The anode buffer layer in the organic EL device of the present invention may be composed of one layer containing the dihydrophenazine derivative of the present invention and / or one or more of the above compounds, or may contain a different kind of compound. A plurality of layers may be stacked. These layers can also be formed by dispersing the dihydrophenazine derivative of the present invention in a polymer.
[0038]
Other electron transporting materials and electron injecting materials used in the organic EL device of the present invention are not particularly limited, and in the photoconductive material, those conventionally used as electron transfer compounds, the electron injecting layer of the organic EL device Any of known materials used for the electron transport layer can be selected and used.
[0039]
Preferred examples of such an electron transfer compound include diphenylquinone derivatives (described in Journal of Electrophotography, 30 (3), 266 (1991)) and perylene derivatives (J. Appl. Phys., 27, 269 ( 1988)), oxadiazole derivatives (Jpn. J. Appl. Phys., 27, L713 (1988), supra), Appl. Phys. Lett. , 55, 1489 (1989), etc., thiophene derivatives (described in JP-A-4-212286, etc.), and triazole derivatives (Jpn. J. Appl. Phys., 32, L917 (1993)). ), Thiadiazole derivatives (43th Preprints of the Society of Polymer Science, (III) described in P1a007, etc.), metal complexes of oxine derivatives (IEICE Technical Report, 92 (311), 43 ( 1992)), polymers of quinoxaline derivatives (described in Jpn. J. Appl. Phys., 33, L250 (1994), etc.), and phenanthroline derivatives (the 43rd Polymer Symposium Proceedings, 14J07). Etc.).
[0040]
Other light-emitting materials used in the light-emitting layer of the organic EL device of the present invention include daylight fluorescent light as described in Polymer Functional Materials Series “Optical Functional Materials” edited by The Society of Polymer Science, Kyoritsu Shuppan (1991), P236. Known luminescent materials such as materials, fluorescent brighteners, laser dyes, organic scintillators, and various fluorescent analysis reagents can be used.
[0041]
Specifically, polycyclic condensed compounds such as anthracene, phenanthrene, pyrene, chrysene, perylene, coronene, rubrene, and quinacridone; oligophenylene-based compounds such as quarterphenyl; 1,4-bis (2-methylstyryl) benzene; , 4-Bis (4-methylstyryl) benzene, 1,4-bis (4-phenyl-5-oxazolyl) benzene, 1,4-bis (5-phenyl-2-oxazolyl) benzene, 2,5-bis ( 5-tert-butyl-2-benzoxazolyl) thiophene, 1,4-diphenyl-1,3-butadiene, 1,6-diphenyl-1,3,5-hexatriene, 1,1,4,4 A scintillator for liquid scintillation such as tetraphenyl-1,3-butadiene,
[0042]
JP-A-63-264692, metal complexes of oxine derivatives, coumarin dyes, dicyanomethylenepyran dyes, dicyanomethylenethiopyran dyes, polymethine dyes, oxobenzanthracene dyes, xanthene dyes, carbostyril dyes, and perylene dyes, Oxazine-based compounds described in Japanese Patent No. 2534713, stilbene derivatives described in the 40th Preliminary Lectures on Lectures on Applied Physics, 1146 (1993), spiro compounds described in JP-A-7-278537, and Oxadiazole compounds described in 4-363891 are preferred. In addition, known phosphorescent materials described in the ninth JSAP Seminar Proceedings, (2001) P17 and “Organic EL Materials and Displays”, CMC (2001) P170, for example, iridium complexes and platinum complexes , A europium complex and the like are also preferable as the light emitting material.
[0043]
Each layer constituting the organic EL element of the present invention can be formed by forming a material constituting each layer into a thin film by a known method such as an evaporation method, a spin coating method, and a casting method. The thickness of each layer formed in this way is not particularly limited and can be appropriately selected depending on the properties of the material, but is usually selected in the range of 2 nm to 5000 nm. In addition, as a method of thinning the dihydrophenazine derivative alone, it is preferable to employ a vapor deposition method from the viewpoint that a uniform film is easily obtained and a pinhole is hardly generated. When thinning using an evaporation method, the evaporation conditions vary depending on the type of the dihydrophenazine derivative, the target crystal structure and the associated structure of the molecular accumulation film, etc., but generally, the boat heating temperature is 50 to 400 ° C., It is preferable to appropriately select a degree of vacuum of 10 ⁻⁶ to 10 ⁻³ Pa, a deposition rate of 0.01 to 50 nm / sec, a substrate temperature of −150 to + 300 ° C., and a film thickness of 5 nm to 5 μm.
[0044]
Next, as an example of a method for producing an organic EL device using the dihydrophenazine derivative of the present invention, a method for producing an organic EL device comprising the above-described anode / dihydrophenazine derivative / hole transport layer / light-emitting layer / cathode will be described. I do. On a suitable substrate, a thin film made of a material for an anode is formed by a vapor deposition method so as to have a thickness of 1 μm or less, preferably in the range of 10 to 200 nm to form an anode, and then a dihydrophenazine derivative is formed on the anode. To form an anode buffer layer. On the anode buffer layer, a hole transport layer and a light emitting layer are formed to a thickness of 1 μm or less, respectively. A target organic EL device can be obtained by forming a cathode having a thickness of 1 μm or less to form a cathode. In the production of the above-mentioned organic EL device, it is also possible to reverse the production order and produce the cathode, the light emitting layer, the hole transport layer, the anode buffer layer, and the anode in this order.
[0045]
When a DC voltage is applied to the organic EL device obtained in this way, the anode may be applied with a positive polarity and the cathode may be applied with a negative polarity, and when a voltage of about 2 to 40 V is applied, a transparent or translucent electrode is applied. Light emission can be observed from the side (anode or cathode, and both). The organic EL element also emits light when an AC voltage is applied. The waveform of the applied AC may be arbitrary.
[0046]
【Example】
Next, the present invention will be described in more detail based on examples.
[0047]
Example 1
A transparent support substrate was prepared by depositing ITO to a thickness of 150 nm on a glass substrate of 25 mm × 75 mm × 1.1 mm (manufactured by Tokyo Sanyo Vacuum Co., Ltd.). This transparent support substrate was fixed to a substrate holder of a commercially available vapor deposition device (manufactured by Vacuum Kiko Co., Ltd.), and a molybdenum vapor deposition boat containing the exemplary compound (4), a molybdenum vapor deposition boat containing NPD, and Tris ( A molybdenum vapor deposition boat containing 8-hydroxyquinoline) aluminum (hereinafter abbreviated as ALQ), a molybdenum vapor deposition boat containing lithium fluoride, and a tungsten vapor deposition boat containing aluminum were mounted. The pressure in the vacuum chamber was reduced to 1 × 10 ⁻³ Pa, and the deposition boat containing Exemplified Compound (4) was heated to vapor-deposit it to a thickness of 40 nm to form an anode buffer layer. Was heated, and NPD was deposited to a thickness of 10 nm to form a hole transport layer.
[0048]
Next, the deposition boat containing ALQ was heated to deposit ALQ to a thickness of 50 nm to form a light-emitting layer. The above deposition rates were 0.1 to 0.2 nm / sec. Thereafter, the deposition boat containing lithium fluoride is heated to perform deposition at a deposition rate of 0.003 to 0.01 nm / sec so that the film thickness becomes 0.5 nm, and then the deposition boat containing aluminum is heated. Then, an organic EL element was obtained by vapor deposition at a vapor deposition rate of 0.2 to 0.5 nm / sec so as to have a film thickness of 100 nm. When a DC voltage of about 4 V is applied using the ITO electrode as an anode and the lithium fluoride / aluminum electrode as a cathode, a current of 2.8 mA / cm ² flows, a current efficiency of 3.6 cd / A and a luminous efficiency of 2.3 lm / W. As a result, green light emission with a wavelength of 520 nm was obtained. In addition, the device was driven at a constant current of 50 mA / cm ² , but the light emission continued even after 100 hours. In addition, no decrease in light emission was observed even when heated to 80 ° C. The Tg of Exemplified Compound (4) was 118 ° C and Ip was 4.9 eV.
[0049]
Example 2
An organic EL device was obtained in the same manner as in Example 1, except that the exemplified compound (4) was replaced with the exemplified compound (5). When a DC voltage of about 4 V is applied using the ITO electrode as an anode and the lithium fluoride / aluminum electrode as a cathode, a current of 2.7 mA / cm ² flows, and a current efficiency of 3.7 cd / A and a luminous efficiency of 2.8 lm / W As a result, green light emission with a wavelength of 520 nm was obtained. In addition, the device was driven at a constant current of 50 mA / cm ² , but the light emission continued even after 100 hours. In addition, no decrease in light emission was observed even when heated to 80 ° C. Ip was 4.8 eV.
[0050]
Example 3
An organic EL device was obtained in the same manner as in Example 1, except that the exemplified compound (4) was replaced with the exemplified compound (12). When a DC voltage of about 4 V is applied using the ITO electrode as an anode and the lithium fluoride / aluminum electrode as a cathode, a current of 3.2 mA / cm ² flows, a current efficiency of 3.1 cd / A, and a luminous efficiency of 2.2 lm / W. As a result, green light emission with a wavelength of 520 nm was obtained. In addition, the device was driven at a constant current of 50 mA / cm ² , but the light emission continued even after 100 hours. In addition, no decrease in light emission was observed even when heated to 80 ° C. Ip was 4.9 eV.
[0051]
Comparative Example 1
An organic EL device was prepared in the same manner as in Example 1 except that the exemplified compound (4) used in Example 1 was changed to copper phthalocyanine. When a DC voltage of about 4 V is applied using an ITO electrode as an anode and a lithium fluoride / aluminum electrode as a cathode, a current of 6.5 mA / cm ² flows, a current efficiency of 1.5 cd / A and a luminous efficiency of 1.2 lm / W. The current efficiency and the luminous efficiency were lower than those of the devices of Examples 1 to 3.
[0052]
Comparative Example 2
Same as Example 1 except that Exemplified Compound (4) used in Example 1 was changed to 4,4 ′, 4 ″ -tris {N- (3-methylphenyl) -N-phenylamino} triphenylamine When a DC voltage of about 4 V was applied using the ITO electrode as the anode and the lithium fluoride / aluminum electrode as the cathode, a current of 3.3 mA / cm ² flowed, and the current efficiency was 3.0 cd. / A, the luminous efficiency was 2.3 lm / W, and the current efficiency was lower than in Examples 1 to 3.
[0053]
【The invention's effect】
As described above, the dihydrophenazine derivative of the present invention has a high Tg of 90 ° C. or higher and is likely to be in an amorphous state, and has a small Ip of 5 eV or less. Therefore, it is suitable for an anode buffer material of an organic EL device. Accordingly, it is possible to increase the efficiency and the life of the organic EL element. That is, by using the organic EL element of the present invention, a highly efficient display device such as a full-color display can be produced.

Claims (4)

An organic electroluminescent device having at least an anode buffer layer, a hole transport layer, and a light emitting layer and having a laminated structure, wherein R 1 to R 8 each independently represent hydrogen represented by the general formula (1): An atom, substituted or unsubstituted aryl or heterocyclic group, alkyl or cycloalkyl, alkenyl, alkoxy, alkenyloxy, alkynyloxy, aryloxy or amino group, and Ar ₁ and Ar ₂ are independently substituted or unsubstituted Represents an aryl or heterocyclic group or alkyl or cycloalkyl] in the anode buffer layer.

The organic electroluminescent device according to claim 1, further comprising an electron transporting layer in addition to the anode buffer layer, the hole transporting layer, and the light emitting layer. Formula (2) [In the formula (2), Ar ₃ represents a substituted or unsubstituted arylene, and R _{9 to} R ₃₂ each independently represent a hydrogen atom, a substituted or unsubstituted aryl or heterocyclic group, alkyl or A cycloalkyl, alkenyl, alkoxy, alkenyloxy, alkynyloxy, aryloxy or amino group] or a compound represented by the general formula (3) [wherein Ar ₄ is arylene and Ar ₅ is substituted Or an unsubstituted aryl or heterocyclic group, R ₃₃ and R ₃₄ are each independently a hydrogen atom, a substituted or unsubstituted aryl or heterocyclic group or alkyl or cycloalkyl, and n is an integer of 2 to 4. The compound represented by the following formula] is contained in the hole transport layer as a hole transport material. The organic electroluminescence device according to.

The organic electroluminescent device according to claim 1, wherein in the general formula (1), R _{1 to} R ₈ are all hydrogen atoms.