JP4729203B2 - Electroluminescent device using phosphorescence of lead halide layered perovskite compound - Google Patents
Electroluminescent device using phosphorescence of lead halide layered perovskite compound Download PDFInfo
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- H10K85/50—Organic perovskites; Hybrid organic-inorganic perovskites [HOIP], e.g. CH3NH3PbI3
Description
【０００１】
【発明の属する技術分野】
本発明は、発光素子の技術分野に属し、特に、燐光を利用する新規な電界発光素子に関する。
【０００２】
【従来の技術】
従来の発光素子としては、ガリウム砒素など無機半導体を用いた発光ダイオードや、発光性の有機分子を用いた有機電界発光素子がある。有機電界発光素子は、蛍光または燐光を発する有機発光層を陰極と陽極の間に配置し両電極間に電界を印加することにより発光する現象を利用する素子である。そのなかで燐光を用いたものとしては、白金錯体、イリジウム錯体、クマリン誘導体を発光材料として用いた有機電界発光素子が従来より知られている。
【０００３】
有機物における燐光は最低励起三重項状態から基底一重項状態への遷移に基づく発光であり、一方、蛍光は、有機物の場合、最低励起一重項状態から基底一重項状態への遷移による発光である。キャリヤ注入によって生成される励起状態の割合は励起三重項状態が3に対して励起一重項状態が１であることから、燐光を利用した電界発光素子は、蛍光を用いた一般的な電界発光素子に比べ原理的に高効率を実現できると期待される。しかし、従来より電界発光素子の燐光材料としては、白金錯体やイリジウム錯体などのような主に中心原子を重金属とした少数の錯体化合物に限られていた。
【０００４】
また、電界発光素子においては、電荷（電子および正孔）を輸送するための電子輸送層と正孔輸送層とを発光層の両側に積層するのが一般的な構成であるが、発光層自身に電荷を輸送する機能があればそれらの輸送層の一部を省略した簡易な構成とすることも可能であるが、そのような電界発光素子は殆ど見当らない。
【０００５】
【発明が解決しようとする課題】
本発明の目的は、多様な有機発色団に適用できるような新しい燐光発光系を探索し、それを利用して、上述したような錯体を用いる場合に比べて多くの材料を用いることができ且つ簡易な構成も可能な有機電界発光素子を提供することにある。
【０００６】
【課題を解決するための手段】
本発明者らは、先に、層状ペロブスカイト化合物の有機層にナフタレン発色団を導入すると、きわめて強い燐光が発せられるという新規な現象を見出している（M. Era, K. Maeda, T. Tsutsui, Chem. Phys. Lett., 296, 417-420 (1998)）。
【０００７】
本発明者は、この現象を詳細に解析し、更に発展させることにより上述のごとき目的を達成し得る新しいタイプの電界発光素子の構築に成功した。かくして、本発明に従えば、陰極と陽極の間に発光層が配置された電界発光素子であって、Ａを発色団を有する有機アンモニウム分子としＸをハロゲン原子として一般式Ａ2ＰｂＸ4で表わされ、発色団を有する有機アンモニウム分子Ａ層とハロゲン化鉛ＰｂＸ4層が交互に積層した超格子構造を形成しているハロゲン化鉛系層状ペロブスカイト化合物の薄膜から成りその層状ペロブスカイト化合物の有機アンモニウム分子の発色団が燐光を発する発光層を含み、該発光層に隣接して陰極側に電子輸送性有機分子層および陽極側に正孔輸送性有機分子層のうちの少なくとも一つが積層されていることを特徴とする電界発光素子が提供される。
【０００８】
【発明の実施の形態】
以下に、本発明の実施の形態を説明する。
本発明の電界発光素子は、ハロゲン化鉛系層状ペロブスカイト化合物の薄膜を発光層とするものである。ここで、ハロゲン化鉛系層状ペロブスカイト化合物とは、Ａを有機アンモニウム分子（発色団を有する有機アンモニウム分子）としＸをハロゲン原子とすると一般式Ａ2ＰｂＢｒ4で表わされ、有機アンモニウム分子Ａ層と臭化鉛ＰｂＢｒ4層とが交互に積層した超格子構造を形成することが知られている（David B. Mitzi, "Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials," Progress in Organic Chemistry, Vol. 48, Edited by Kenneth D. Karlin, John Wiley & Sons, Inc. (1999)）。図１は、本発明において用いられるこのようなハロゲン化鉛系層状ペロブスカイト化合物の超格子構造の模式図である。図において、ハロゲン化鉛の層は、八面体の各頂点の位置にハロゲン原子（黒丸）があり該八面体の中心に鉛（白丸）が位置している状態を横から見たものとして示されており、また、有機アンモニウム分子の層は、白丸がアンモニウム分子として且つ矩形が発色団として示されている。
【０００９】
既述したように、本発明者らは、発色団であるナフタレンを有する有機アンモニウム分子と臭化鉛とから成る層状ペロブスカイト化合物がきわめて強い燐光を発することを見出していた。本発明は、この現象は、臭化鉛の無機半導体層と発色団が数ナノメータの非常に近い距離にあり、また、ナフタレン発色団の励起三重項状態が無機半導体層の励起子バンドよりエネルギーが低くナフタレン発色団の励起一重項状態のエネルギーが励起子バンドより大きいため、励起子バンドから発色団の励起三重項状態へのエネルギー移動が効率よく起こり燐光が増強されるためと考察した。事実、さらに検討を重ねた結果、臭素の他に塩素やヨウ素などのハロゲンから構成されるハロゲン化鉛と各種の発色団を有する有機アンモニウム分子とから成る他の層状ペロブスカイト化合物についても同様に強い燐光が認められ、吸収スペクトルから、それぞれのハロゲン化鉛系層状ペロブスカイトの励起子バンドおよびそれぞれの発色団の励起三重項状態と励起一重項状態のエネルギー準位を推算すると、いずれも、発色団の励起三重項状態が無機半導体層（ハロゲン化鉛系ペロブスカイト）の励起子バンドよりエネルギーが低く且つ発色団の励起一重項状態のエネルギーが励起子バンドより大きいという条件を満たしていることを確認した。図２は、このようなエネルギー準位を模式的に示すものであり、無機半導体（ハロゲン化鉛系層状ペロブスカイト）の励起子バンドは有機発色団の励起一重項状態（Ｓ1）と励起三重項状態（Ｔ1）との間のエネルギー準位を有するので、励起子バンドから発色団の励起三重項（Ｔ1）へのエネルギー移動が効率よく起こり、励起三重項状態（Ｔ1）から基底一重項状態（Ｓ0）への遷移による燐光が増強されることを示している。
【００１０】
この燐光の増強効果は、ハロゲン化鉛系層状ペロブスカイトの有機層（有機アンモニウム分子）に導入された発色団の励起三重項状態のエネルギー準位が無機半導体層（ハロゲン化鉛系ペロブスカイト）の励起子バンドのエネルギー準位より低く且つ発色団の励起一重項状態のエネルギー準位が励起子バンドのエネルギー準位より大きいという条件を満たしていれば、原理的にあらゆる有機発色団において観測される。ハロゲン化鉛系層状ペロブスカイトの励起子バンドは、ハロゲンを塩素、臭素、ヨウ素と代えることで、３５０ｎｍ、４００ｎｍ、５００ｎｍと変化させることができ、適切なハロゲン化鉛系層状ペロブスカイトを用いることで可視光のエネルギー領域に励起三重項状態を有するあらゆる発色団を燐光材料として利用することが可能になる。かくして、本発明に従えば、このような層状ペロブスカイト薄膜を発光層とし適切な電極およびキャリア輸送層を設けた素子を作製することで様々な発色団からの燐光を利用した電界発光を得ることができる。
【００１１】
さらに、本発明に従えば、有機系の発色団を用いているので発光層自身に電荷（キャリア）を輸送する機能があり、正孔輸送層および電子輸送層のいずれか一方を省略した簡単な構成の電界発光素子を得ることもできる。
図３は本発明の実施例として、キャリア輸送性有機分子層とハロゲン化鉛系層状ペロブスカイト発光層からなる電界発光素子の素子構造を模式的に示したものである。有機分子層および層状ペロブスカイト層は、厚さが数十ｎｍから１００ｎｍとなるようにするのが好ましい。既述したように、本発明に従えば、素子構造として、層状ペロブスカイトから成る発光層を陽極側に正孔輸送性有機分子層と、陰極側に電子輸送性有機分子層とではさんだ３層型素子（ｃ）のみならず、層状ペロブスカイト発光層と正孔輸送性有機分子層ないしは電子輸送性有機分子層のいずれか一方のみからなる２層型素子（ａまたはｂ）も可能である。
【００１２】
本発明の電界発光素子を構成する各層は、従来より知られた薄膜形成法により作製することができる。キャリア輸送層（正孔輸送性有機分子層、電子輸送性有機分子層）や電極層は、一般に、真空蒸着法により作製する。また、ハロゲン化鉛系層状ペロブスカイト発光層は、スピンコート法により作製することが望ましい。すなわち、発色団を有する有機アミンとハロゲン化鉛から作製したハロゲン化鉛系層状ペロブスカイトの結晶試料、あるいは発色団を有する有機アミンのハロゲン化水素酸塩とハロゲン化鉛とを、ジメチルホルムアミドやジメチルスルホキサイドのような極性溶媒に溶かした溶液からスピンコートすることにより発色団を有する有機アンモニウム分子層とハロゲン化鉛層が交互に積層した超格子構造から成るハロゲン化鉛系層状ペロブスカイト層が形成される。
【００１３】
本発明の電界発光素子に用いられる有機アンモニウム分子としては、発色団を有する有機分子にアンモニアが結合した化学構造からなり、ハロゲン化鉛に配位して層状ペロブスカイト構造を形成し得る各種の化合物が使用可能である。好ましい有機アンモニウム分子は、一般式Ａｒ・(ＣＨ2)ｎ・ＮＨ3で表わすことができ、ここで、ｎは０または１〜１２の整数を表わし、Ａｒはπ共役電子系を有する発色団を表わし、このような発色団の好ましい例としてはナフタレン、アントラセン、ピレン、フェナンスレン、トリフェニレン、スチルベンなどを挙げることができる。なお、このような有機アンモニウム分子は、例えば、発色団分子にグリニヤール反応あるいはウイリアムソン反応によりハロゲン化アルキル基を導入し、さらにアンモニアやフタルイミドカリウムなどとの反応によりアミノアルキル基とすることにより合成することができる。
【００１４】
ハロゲン化鉛としては、塩化鉛、臭化鉛、ヨウ化鉛を用いることができる。これらは、発色団からの燐光が増強されるように、層状ペロブスカイトの無機半導体層の励起子バンドが、有機アンモニウム分子の発色団の励起一重項状態よりエネルギー的に小さくなり、かつ励起三重項状態よりも大きくなるように有機アンモニウム分子の発色団に応じて選択される。このようなエネルギー準位は吸収スペクトルから確認できる。かくして本発明の電界発光素子の発光層に用いられるハロゲン化鉛系層状ペロブスカイトの好ましい１例として下記の式（１）で表わされるものがあげられる。
【００１５】
【化１】
【００１６】
本発明の電界発光素子のキャリア輸送層のうち正孔輸送層に用いられる有機分子は、特に限定されるものではなく、正孔輸送性を示すものとして知られた各種の有機分子が適用可能である。好ましい正孔輸送性有機分子としては、フタロシアニン類、ジアミン誘導体、フルオレン誘導体またはポリチオフェン誘導体などを挙げることができる。
【００１７】
本発明の電界発光素子のキャリヤ輸送層のうち、電子輸送層に用いられる有機分子は、特に限定されるものではなく、電子輸送性を示すものとして知られた各種の有機分子が適用可能である。好ましい電子輸送性有機分子としては、オキサジアゾール誘導体、トリアゾール誘導体、ペリレン誘導体またはキノリノール金属錯体などをあげることができる。
以上のように、発色団を有機層に導入したハロゲン化鉛系層状ペロブスカイトからなる発光層とキャリヤ輸送性有機分子層とを積層することにより、様々な発色団からの燐光を利用した電界発光素子が可能となる。
【００１８】
【実施例】
以下に本発明の特徴をさらに明らかにするために実施例を示すが、本発明はこの実施例に限定されるものではない。
実施例１
発光層として式（１）のナフタレンを発色団として有する有機アンモニウム分子からなる臭化鉛系層状ペロブスカイト薄膜を用い、電子輸送性の有機分子として以下の式（２）で示すオキサジアゾール誘導体を用いた２層型電界発光素子を作製し、その特性を評価した。
【００１９】
【化２】
【００２０】
素子は、透明電極のインジウム錫酸化物（ＩＴＯ）をコートしたガラス板上に、層状ペロブスカイト薄膜をジメチルホルムアミド溶液からスピンコート法を用いて製膜した後、オキサジアゾール誘導体薄膜および陰極としてマグネシウム銀（ＭｇＡｇ）合金を真空蒸着法することにより作製した。
図４に、上記のナフタレン発色団を有機層に導入した臭化鉛系層状ペロブスカイトの吸収スペクトル、および１０Ｋで測定した発光スペクトル、励起スペクトルを示す。まず吸収スペクトルをみると、臭化鉛系層状ペロブスカイト化合物特有の鋭い励起子吸収が４００ｎｍ付近に観測されており、またナフタレン発色団に起因した吸収も３００ｎｍに見られていることから、図１に示す層状ペロブスカイト構造が形成されていることが確認できる。
【００２１】
次に発光スペクトルを観察すると、４９２ｎｍにピークを有する発光が見られる。この発光スペクトルはナフタレン発色団からの燐光発光スペクトルとよく一致している。また発光の寿命を測定したところ４．３ｍｓｅｃと非常に長いことから、燐光発光であることが確認できた。この燐光はナフタレン分子からの燐光より非常に強く、通常ナフタレン分子からの燐光が観測されない室温においても観測された。このように層状ペロブスカイト構造に導入されたナフタレン発色団からの燐光は通常の状態に比べて増強されており、この層状ペロブスカイトが燐光発光材料として有望であることがわかる。
【００２２】
さらに、励起スペクトルを見てみる。この励起スペクトルから、ナフタレン発色団の吸収ピークがある２８０ｎｍ付近の光で励起しても燐光はほとんど観測されず、４００ｎｍ付近の臭化鉛系層状ペロブスカイトの励起子吸収バンドを励起した際に観測されることがわかる。このことから、ナフタレン発色団からの強い燐光は、臭化鉛系層状ペロブスカイトの励起子からナフタレン発色団の励起三重項状態への効率よいエネルギー移動によって生じていることがわかる。
【００２３】
図５は、このナフタレン発色団を導入した臭化鉛系層状ペロブスカイトを発光層とし、オキサジアゾール誘導体をキャリヤ輸送層とした２層型の電界発光素子の発光スペクトルである。測定温度は１００Ｋである。電界発光素子の発光スペクトルは、この温度でのナフタレン発色団を導入した臭化鉛系層状ペロブスカイトの燐光スペクトルとよく一致している。このことから、ナフタレン発色団を導入した臭化鉛系層状ペロブスカイトを発光層とし適当なキャリヤ輸送層と組み合わせることで、この層状ペロブスカイトの燐光を利用した電界発光素子を構築できることが明らかとなった。
【００２４】
本実施例では電子輸送性有機分子と組み合わせた２層型素子を用いたが、電界発光素子の構造としては、実施例に示された２層型に限らず、発光層である層状ペロブスカイト薄膜の電気的特性に応じて、正孔輸送性分子をキャリヤ輸送層に用いた２層型素子、正孔輸送性分子および電子輸送性分子をキャリヤ輸送層に用いた３層型素子を用いることができる。これらの素子のキャリヤ輸送層に用いられる有機分子としては、ホール輸送性分子としてフタロシアニン類、ジアミン誘導体、フルオレン誘導体、ポリチオフェン誘導体などが、電子輸送性分子としてオキサジアゾール誘導体、トリアゾール誘導体、ペリレン誘導体、キノリノール金属錯体などがあげられる。
【００２５】
実施例２
層状ペロブスカイトに導入される発色団としては、上記のナフタレン発色団に限らず、発色団の励起三重項状態が無機層の励起子バンドよりエネルギーが低く発色団の励起一重項状態のエネルギーが励起子バンドより大きい発色団であればいずれも燐光が増強される。さらに、層状ペロブスカイトの励起子吸収バンドは塩化鉛系、臭化鉛系、ヨウ化鉛系とハロゲンを代えることで、３５０ｎｍ、４００ｎｍ、５００ｎｍと変化させることができるため可視領域に燐光を有する多様な発色団を燐光材料として利用することが可能になる。その他の発色団においてもこの原理による燐光増強効果が観測されることを確認するため、スチルベン発色団を導入した臭化鉛系層状ペロブスカイトを作製し、その発光特性を評価した。
【００２６】
図６は、発色団としてスチルベンを用いた場合の臭化鉛系層状ペロブスカイト（化学式（３））の吸収スペクトルと発光スペクトルである。吸収スペクトルからわかるように、スチルベン発色団の励起一重項状態に対応した吸収は３００ｎｍ付近にあり、臭化鉛系層状ペロブスカイトの励起子バンドに対応した吸収はそれよりエネルギーの低い４００ｎｍ付近に存在する。そのため励起子バンドからのエネルギー移動はスチルベンの励起一重項状態へは起こらず、励起三重項状態へ優先的に起こり、事実、発光スペクトルではスチルベン発色団からの燐光が観測されている。
【００２７】
【化３】
【００２８】
【発明の効果】
本発明によれば、層状ペロブスカイトの無機半導体層の励起子バンドから有機層に導入された発色団の励起三重項状態へのエネルギー移動による燐光増強効果という新しい燐光発光原理を用いることにより、様々な発色団からの燐光を利用して多様な材料から簡易な構成の有機電界発光素子を得ることができる。
【図面の簡単な説明】
【図１】本発明の電界発光素子において用いられる層状ペロブスカイトの模式図である。
【図２】本発明の電界発光素子における燐光発光のメカニズムを模式的に示す図である。
【図３】ハロゲン化鉛系層状ペロブスカイトを発光層とした本発明の電界発光素子の模式図である。（ａ）正孔輸送性有機分子層と組み合せた２層型素子、（ｂ）電子輸送性有機分子層と組み合せた２層型素子、（ｃ）正孔輸送性有機分子層および電子輸送性有機分子層を組み合せた３層型素子。
【図４】本発明に従い、ナフタレン発色団を有機層に導入した臭化鉛系層状ペロブスカイトの吸収スペクトル（点線）、発光スペクトル（実線ＰＬ）、および励起スペクトル（実線ＰＬＥ）を示した図である。破線は、ナフタレン発色団を有する有機アンモニウム分子の吸収スペクトルである。
【図５】本発明に従い、ナフタレン発色団として有機層に導入した臭化鉛系層状ペロブスカイト薄膜を発光層とした２層型の電界発光素子の発光スペクトルを示した図である。実線が電界発光スペクトル、点線が燐光スペクトルを示す。
【図６】本発明に従い、スチルベン発色団を有機層に導入した臭化鉛系層状ペロブスカイト（化学式（３））の吸収スペクトルおよび発光スペクトルを示した図である。実線が電界発光スペクトル、点線が燐光スペクトルを示す。[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to the technical field of light-emitting elements, and particularly relates to a novel electroluminescent element utilizing phosphorescence.
[0002]
[Prior art]
Conventional light-emitting elements include light-emitting diodes using inorganic semiconductors such as gallium arsenide and organic electroluminescent elements using light-emitting organic molecules. An organic electroluminescent element is an element that utilizes a phenomenon in which an organic light emitting layer that emits fluorescence or phosphorescence is disposed between a cathode and an anode and light is emitted by applying an electric field between the two electrodes. Among those using phosphorescence, organic electroluminescence devices using platinum complexes, iridium complexes, and coumarin derivatives as light-emitting materials are conventionally known.
[0003]
Phosphorescence in an organic material is light emission based on a transition from the lowest excited triplet state to the ground singlet state, while fluorescence is light emission due to a transition from the lowest excited singlet state to the ground singlet state. The proportion of excited states generated by carrier injection is 3 in the excited triplet state and 1 in the excited singlet state. Therefore, the electroluminescent device using phosphorescence is a general electroluminescent device using fluorescence. It is expected that high efficiency can be realized in principle. However, conventionally, phosphorescent materials for electroluminescent elements have been limited to a small number of complex compounds mainly having heavy metals as the central atom, such as platinum complexes and iridium complexes.
[0004]
In addition, in an electroluminescent device, an electron transport layer for transporting charges (electrons and holes) and a hole transport layer are generally laminated on both sides of the light emitting layer. If it has a function of transporting charges, it is possible to adopt a simple structure in which a part of the transport layer is omitted, but such an electroluminescent element is hardly found.
[0005]
[Problems to be solved by the invention]
The object of the present invention is to search for a new phosphorescent light emitting system that can be applied to various organic chromophores, and to use it, more materials can be used than in the case of using the complex as described above and An object of the present invention is to provide an organic electroluminescent element that can be configured simply.
[0006]
[Means for Solving the Problems]
The present inventors have previously found a novel phenomenon that extremely strong phosphorescence is emitted when a naphthalene chromophore is introduced into an organic layer of a layered perovskite compound (M. Era, K. Maeda, T. Tsutsui, Chem. Phys. Lett., 296, 417-420 (1998)).
[0007]
The present inventor has succeeded in constructing a new type of electroluminescent device capable of achieving the above-mentioned object by analyzing this phenomenon in detail and further developing it. Thus, according to the present invention, an electroluminescent device in which a light emitting layer is disposed between a cathode and an anode, wherein A is an organic ammonium molecule having a chromophore and X is a halogen atom, and is represented by the general formula A 2 PbX 4 . It consists of a thin film of a lead halide layered perovskite compound that has a superlattice structure in which organic ammonium molecule A layers having chromophores and lead halide PbX 4 layers are alternately laminated. The molecular chromophore includes a light emitting layer that emits phosphorescence, and at least one of an electron transporting organic molecular layer on the cathode side and a hole transporting organic molecular layer on the anode side is laminated adjacent to the light emitting layer. An electroluminescent device is provided.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described.
The electroluminescent element of the present invention uses a lead halide layered perovskite compound thin film as a light emitting layer. Here, the lead halide layered perovskite compound is represented by the general formula A 2 PbBr 4 where A is an organic ammonium molecule (an organic ammonium molecule having a chromophore) and X is a halogen atom. It is known to form a superlattice structure in which PbBr 4 layers and PbBr 4 layers are alternately laminated (David B. Mitzi, “Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials,” Progress in Organic Chemistry, Vol. 48, Edited by Kenneth D. Karlin, John Wiley & Sons, Inc. (1999)). FIG. 1 is a schematic diagram of the superlattice structure of such a lead halide layered perovskite compound used in the present invention. In the figure, the lead halide layer is shown as a side view of a state in which a halogen atom (black circle) is located at each vertex of the octahedron and lead (white circle) is located at the center of the octahedron. The organic ammonium molecule layer is shown with white circles as ammonium molecules and rectangles as chromophores.
[0009]
As described above, the present inventors have found that a layered perovskite compound composed of an organic ammonium molecule having naphthalene as a chromophore and lead bromide emits extremely strong phosphorescence. In the present invention, this phenomenon is caused by the fact that the inorganic semiconductor layer of lead bromide and the chromophore are at a very close distance of several nanometers, and the excited triplet state of the naphthalene chromophore is more energy than the exciton band of the inorganic semiconductor layer. It was considered that the energy of the excited singlet state of the naphthalene chromophore was lower than that of the exciton band, so that the energy transfer from the exciton band to the excited triplet state of the chromophore was efficient and phosphorescence was enhanced. In fact, as a result of further studies, phosphorescence of other layered perovskite compounds composed of lead halides composed of halogens such as chlorine and iodine in addition to bromine and organic ammonium molecules having various chromophores is similarly strong. From the absorption spectrum, the exciton bands of each lead halide-based layered perovskite and the energy levels of the excited triplet state and excited singlet state of each chromophore were estimated. It was confirmed that the triplet state satisfies the conditions that the energy is lower than the exciton band of the inorganic semiconductor layer (lead halide perovskite) and the energy of the excited singlet state of the chromophore is larger than the exciton band. FIG. 2 schematically shows such energy levels, and the exciton band of the inorganic semiconductor (lead halide layered perovskite) shows the excited singlet state (S 1 ) and excited triplet of the organic chromophore. Since it has an energy level between the state (T 1 ), energy transfer from the exciton band to the excited triplet (T 1 ) of the chromophore occurs efficiently, and the ground triplet from the excited triplet state (T 1 ) It shows that phosphorescence due to the transition to the term state (S 0 ) is enhanced.
[0010]
This phosphorescence enhancement effect is due to the exciton of the inorganic semiconductor layer (lead halide perovskite) in the excited triplet state energy level of the chromophore introduced into the organic layer (organic ammonium molecule) of the lead halide layered perovskite. In principle, it is observed in any organic chromophore as long as it satisfies the condition that it is lower than the energy level of the band and the energy level of the excited singlet state of the chromophore is larger than the energy level of the exciton band. The exciton band of the lead halide layered perovskite can be changed to 350 nm, 400 nm, and 500 nm by replacing the halogen with chlorine, bromine and iodine, and visible light can be obtained by using an appropriate lead halide layered perovskite. Any chromophore having an excited triplet state in the energy region can be used as a phosphorescent material. Thus, according to the present invention, electroluminescence utilizing phosphorescence from various chromophores can be obtained by producing an element having such a layered perovskite thin film as a light emitting layer and having an appropriate electrode and carrier transport layer. it can.
[0011]
Furthermore, according to the present invention, since an organic chromophore is used, there is a function of transporting charges (carriers) to the light emitting layer itself, and either a hole transport layer or an electron transport layer is omitted. An electroluminescent element having a configuration can also be obtained.
FIG. 3 schematically shows an element structure of an electroluminescent element comprising a carrier transporting organic molecular layer and a lead halide layered perovskite light emitting layer as an embodiment of the present invention. The organic molecular layer and the layered perovskite layer preferably have a thickness of several tens to 100 nm. As described above, according to the present invention, the device structure is a three-layer type in which a light-emitting layer made of layered perovskite is sandwiched between a hole transporting organic molecular layer on the anode side and an electron transporting organic molecular layer on the cathode side. Not only the element (c) but also a two-layer element (a or b) comprising only one of a layered perovskite light emitting layer and a hole transporting organic molecular layer or an electron transporting organic molecular layer is possible.
[0012]
Each layer constituting the electroluminescent element of the present invention can be produced by a conventionally known thin film forming method. The carrier transport layer (hole transporting organic molecular layer, electron transporting organic molecular layer) and the electrode layer are generally produced by vacuum deposition. The lead halide layered perovskite light emitting layer is preferably produced by a spin coating method. That is, a crystal sample of a lead halide layered perovskite prepared from an organic amine having a chromophore and a lead halide, or a hydrohalide salt of an organic amine having a chromophore and a lead halide, is converted into dimethylformamide or dimethylsulfone. By spin-coating from a solution in a polar solvent such as oxide, a lead halide layered perovskite layer consisting of a superlattice structure in which organic ammonium molecular layers with chromophores and lead halide layers are alternately stacked is formed. The
[0013]
The organic ammonium molecule used in the electroluminescent device of the present invention includes various compounds that have a chemical structure in which ammonia is bonded to an organic molecule having a chromophore and can form a layered perovskite structure by coordination with lead halide. It can be used. Preferred organic ammonium molecules can be represented by the general formula Ar · (CH 2 ) n · NH 3 , where n represents 0 or an integer from 1 to 12, and Ar represents a chromophore having a π-conjugated electron system. As preferred examples of such chromophores, naphthalene, anthracene, pyrene, phenanthrene, triphenylene, stilbene, and the like can be given. Such an organic ammonium molecule is synthesized, for example, by introducing a halogenated alkyl group into a chromophore molecule by a Grignard reaction or a Williamson reaction, and further converting it to an aminoalkyl group by reaction with ammonia, potassium phthalimide, or the like. be able to.
[0014]
As the lead halide, lead chloride, lead bromide, or lead iodide can be used. These are such that the exciton band of the inorganic semiconductor layer of the layered perovskite is energetically smaller than the excited singlet state of the chromophore of the organic ammonium molecule, and the excited triplet state so that phosphorescence from the chromophore is enhanced It is selected according to the chromophore of the organic ammonium molecule so as to be larger. Such energy levels can be confirmed from the absorption spectrum. Thus, one preferred example of the lead halide layered perovskite used in the light emitting layer of the electroluminescent device of the present invention is one represented by the following formula (1).
[0015]
[Chemical 1]
[0016]
The organic molecules used in the hole transport layer in the carrier transport layer of the electroluminescent device of the present invention are not particularly limited, and various organic molecules known to exhibit hole transport properties can be applied. is there. Preferable hole transporting organic molecules include phthalocyanines, diamine derivatives, fluorene derivatives or polythiophene derivatives.
[0017]
Among the carrier transport layers of the electroluminescent device of the present invention, the organic molecules used in the electron transport layer are not particularly limited, and various organic molecules known to exhibit electron transport properties are applicable. . Preferable electron transporting organic molecules include oxadiazole derivatives, triazole derivatives, perylene derivatives, quinolinol metal complexes, and the like.
As described above, an electroluminescent device utilizing phosphorescence from various chromophores by laminating a light emitting layer composed of a lead halide layered perovskite having a chromophore introduced into an organic layer and a carrier transporting organic molecular layer. Is possible.
[0018]
【Example】
Examples are given below to further clarify the features of the present invention, but the present invention is not limited to these examples.
Example 1
A lead bromide-based layered perovskite thin film composed of an organic ammonium molecule having naphthalene of formula (1) as a chromophore is used as a light emitting layer, and an oxadiazole derivative represented by the following formula (2) is used as an electron transporting organic molecule. The two-layer type electroluminescent device was manufactured and its characteristics were evaluated.
[0019]
[Chemical 2]
[0020]
The element was formed by forming a layered perovskite thin film from a dimethylformamide solution on a glass plate coated with indium tin oxide (ITO), which is a transparent electrode, using a spin coating method, and then using magnesium silver as an oxadiazole derivative thin film and a cathode. A (MgAg) alloy was produced by vacuum deposition.
FIG. 4 shows an absorption spectrum of a lead bromide-based layered perovskite in which the naphthalene chromophore is introduced into an organic layer, and an emission spectrum and an excitation spectrum measured at 10K. First, when looking at the absorption spectrum, sharp exciton absorption peculiar to the lead bromide-based layered perovskite compound is observed around 400 nm, and absorption due to the naphthalene chromophore is also observed at 300 nm. It can be confirmed that the layered perovskite structure shown is formed.
[0021]
Next, when the emission spectrum is observed, light emission having a peak at 492 nm is observed. This emission spectrum is in good agreement with the phosphorescence emission spectrum from the naphthalene chromophore. Further, when the lifetime of light emission was measured, it was as long as 4.3 msec, and thus it was confirmed that phosphorescence was emitted. This phosphorescence was much stronger than the phosphorescence from the naphthalene molecule, and was usually observed even at room temperature where no phosphorescence from the naphthalene molecule was observed. Thus, the phosphorescence from the naphthalene chromophore introduced into the layered perovskite structure is enhanced as compared with the normal state, and it can be seen that this layered perovskite is promising as a phosphorescent material.
[0022]
In addition, look at the excitation spectrum. From this excitation spectrum, almost no phosphorescence is observed even when excited with light near 280 nm with an absorption peak of naphthalene chromophore, and is observed when the exciton absorption band of a lead bromide-based layered perovskite near 400 nm is excited. I understand that This indicates that strong phosphorescence from the naphthalene chromophore is generated by efficient energy transfer from the exciton of the lead bromide-based layered perovskite to the excited triplet state of the naphthalene chromophore.
[0023]
FIG. 5 shows an emission spectrum of a two-layer electroluminescent device using the lead bromide-based layered perovskite introduced with the naphthalene chromophore as a light-emitting layer and an oxadiazole derivative as a carrier transport layer. The measurement temperature is 100K. The emission spectrum of the electroluminescent element is in good agreement with the phosphorescence spectrum of the lead bromide-based layered perovskite introduced with naphthalene chromophore at this temperature. From this, it became clear that an electroluminescent device utilizing phosphorescence of this layered perovskite can be constructed by combining a lead bromide-based layered perovskite introduced with a naphthalene chromophore as a light emitting layer and an appropriate carrier transport layer.
[0024]
In this example, a two-layer type element combined with an electron transporting organic molecule was used. However, the structure of the electroluminescent element is not limited to the two-layer type shown in the example, and a layered perovskite thin film as a light emitting layer is used. Depending on the electrical characteristics, a two-layer device using a hole transporting molecule as a carrier transport layer, or a three-layer device using a hole transport molecule and an electron transport molecule as a carrier transport layer can be used. . Organic molecules used in the carrier transport layer of these devices include phthalocyanines, diamine derivatives, fluorene derivatives, polythiophene derivatives, etc. as hole transporting molecules, oxadiazole derivatives, triazole derivatives, perylene derivatives as electron transporting molecules, And quinolinol metal complexes.
[0025]
Example 2
The chromophore introduced into the layered perovskite is not limited to the naphthalene chromophore described above, and the excited triplet state of the chromophore is lower than the exciton band of the inorganic layer, and the excited singlet state energy of the chromophore is the exciton. Any chromophore larger than the band enhances phosphorescence. Furthermore, the exciton absorption band of the layered perovskite can be changed to 350 nm, 400 nm, and 500 nm by replacing lead chloride, lead bromide, and lead iodide with halogen, so that various phosphorescence in the visible region can be obtained. The chromophore can be used as a phosphorescent material. In order to confirm that the phosphorescence enhancement effect by this principle is observed in other chromophores, a lead bromide-based layered perovskite into which a stilbene chromophore was introduced was prepared, and the luminescence property was evaluated.
[0026]
FIG. 6 shows an absorption spectrum and an emission spectrum of a lead bromide-based layered perovskite (chemical formula (3)) when stilbene is used as the chromophore. As can be seen from the absorption spectrum, the absorption corresponding to the excited singlet state of the stilbene chromophore is in the vicinity of 300 nm, and the absorption corresponding to the exciton band of the lead bromide-based layered perovskite is present in the vicinity of 400 nm having a lower energy. . Therefore, energy transfer from the exciton band does not occur in the excited singlet state of stilbene but preferentially occurs in the excited triplet state, and in fact, phosphorescence from the stilbene chromophore is observed in the emission spectrum.
[0027]
[Chemical 3]
[0028]
【The invention's effect】
According to the present invention, various phosphorescence emission principles such as a phosphorescence enhancement effect by energy transfer from an exciton band of an inorganic semiconductor layer of a layered perovskite to an excited triplet state of a chromophore introduced into an organic layer can be obtained. An organic electroluminescent device having a simple structure can be obtained from various materials by utilizing phosphorescence from the chromophore.
[Brief description of the drawings]
FIG. 1 is a schematic view of a layered perovskite used in an electroluminescent device of the present invention.
FIG. 2 is a diagram schematically showing the mechanism of phosphorescence emission in the electroluminescence device of the present invention.
FIG. 3 is a schematic view of an electroluminescent device of the present invention using a lead halide-based layered perovskite as a light emitting layer. (A) Two-layer device combined with a hole-transporting organic molecular layer, (b) Two-layer device combined with an electron-transporting organic molecular layer, (c) Hole-transporting organic molecular layer and electron-transporting organic A three-layered element that combines molecular layers.
FIG. 4 shows an absorption spectrum (dotted line), emission spectrum (solid line PL), and excitation spectrum (solid line PLE) of a lead bromide-based layered perovskite in which a naphthalene chromophore is introduced into an organic layer according to the present invention. . The broken line is the absorption spectrum of an organic ammonium molecule having a naphthalene chromophore.
FIG. 5 is a diagram showing an emission spectrum of a two-layer type electroluminescent device using a lead bromide-based layered perovskite thin film introduced as an naphthalene chromophore in the organic layer according to the present invention as a light emitting layer. A solid line shows an electroluminescence spectrum, and a dotted line shows a phosphorescence spectrum.
FIG. 6 is a diagram showing an absorption spectrum and an emission spectrum of a lead bromide-based layered perovskite (chemical formula (3)) in which a stilbene chromophore is introduced into an organic layer according to the present invention. A solid line shows an electroluminescence spectrum, and a dotted line shows a phosphorescence spectrum.
Claims (1)
前記ハロゲン化鉛系層状ペロブスカイト化合物が、下記の式（１）または（３）で表され、
前記発光層に隣接して陰極側に積層される電子輸送性有機分子層および陽極側に積層される正孔輸送性有機分子層のいずれか一方を省略し得る、
ことを特徴とする電界発光素子。
The lead halide layered perovskite compound is represented by the following formula (1) or (3):
Either one of the electron transporting organic molecular layer laminated on the cathode side adjacent to the light emitting layer and the hole transporting organic molecular layer laminated on the anode side may be omitted.
An electroluminescent element characterized by the above.
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| KR102452189B1 (en) * | 2016-11-30 | 2022-10-11 | 가부시키가이샤 한도오따이 에네루기 켄큐쇼 | Light-emitting element, light-emitting device, electronic device, display device, and lighting device |
| TWI746745B (en) * | 2016-12-22 | 2021-11-21 | 日商住友化學股份有限公司 | Composition, film, laminated structure, light emitting device, and display |
| CN110872233B (en) * | 2018-09-03 | 2023-04-07 | 北京理工大学 | Organic ammonium salt, perovskite material, preparation method and application thereof |
| WO2019236011A1 (en) * | 2018-06-08 | 2019-12-12 | National University Of Singapore | Perovskites and uses thereof |
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