Charge-carrier injection characteristics at organic/organic heterojunction interfaces in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) having various guest molecules doped in an organic host matrix layer are fabricated [the OLED structure is anode/hole-transporting layer (HTL)/guest–host emitting layer/hole-blocking layer/electron-transporting layer/cathode], and the dependence of current density–voltage (J–V) characteristics of the OLEDs on highest occupied molecular orbital (HOMO) levels of guest molecules are investigated. From the J–V characteristics of these OLEDs, we find two important results: (1) J–V characteristics of the OLEDs are controlled by the direct hole injection from the neighboring HTL to guest molecules, and (2) HOMO level alignment between the HTL and guest molecules provides efficient hole injection at this interface

Highly Efficient Light Harvesting of Eu(III) Complex in a Host-Guest Film by Triplet Sensitization

Lanthanide complexes are attractive light emitters owing to their ideal high color purity. Sensitization using ligands with high absorption efficiency is a powerful approach to enhancing photoluminescence intensity. However, the development of antenna ligands that can be used for sensitization is limited due to difficulties in controlling the coordination structures of lanthanides. Here, we demonstrate a system comprising triazine-based host molecules and Eu(hfa)3(TPPO)2 (hfa: hexafluoroacetylacetonato, TPPO: triphenylphosphine oxide), which shows drastically improved total photoluminescence intensity compared to conventional luminescent Eu(III) complexes. Time-resolved spectroscopic studies revealed that the energy transfer from the host molecules to Eu(III) occurs via the triplet states over several molecules with nearly unity efficiency. Our discovery paves the way for efficient light harvesting of Eu(III) complexes with simple fabrication using a solution process

Hot Exciplexes in U-Shaped TADF Molecules with Emission from Locally Excited States

Rapid reverse intersystem crossing and high color purity are vital characteristics of emitters with thermally activated delayed fluorescence in opto-electronic devices. We present a new approach, called “hot exciplexes” that enables access to both attributes at the same time. Hot exciplexes are produced by coupling facing donor and acceptor moieties to an anthracene bridge, yielding an exciplex with large T1 to T2 spacing. The hot exciplex model is investigated using optical spec-troscopy and quantum chemical simulations. Reverse intersystem crossing is found to occur preferentially from the T3 to the S1 state within only a few nanoseconds. Application and practi-cality of the model are shown by fabrication of organic light-emitting diodes with up to 32 % hot exciplex contribution and low efficiency roll-off

Multiple donor–acceptor design for highly luminescent and stable thermally activated delayed fluorescence emitters

Abstract A considerable variety of donor–acceptor (D–A) combinations offers the potential for realizing highly efficient thermally activated delayed fluorescence (TADF) materials. Multiple D–A type compounds are one of the promising families of TADF materials in terms of stability as well as efficiencies. However, those emitters are always composed of carbazole-based donors despite a wide choice of moieties used in linearly linked single D–A molecules. Herein, we developed a multiple D–A type TADF compound with two distinct donor units of 9,10-dihydro-9,9-dimethylacridine (DMAC) and carbazole as the hetero-donor design. The new emitter exhibits high photoluminescence quantum yield (PLQY) in various conditions including polar media blend and high concentrations. Organic light-emitting diodes (OLEDs) showed a reasonably high external quantum efficiency (EQE). In addition, we revealed that the multiple-D–A type molecules showed better photostability than the single D–A type molecules, while the operational stability in OLEDs involves dominant other factors

材料科学 : 遅延蛍光を利用した高効率有機発光ダイオード

九州大学最先端有機光エレクトロニクス研究センター(OPERA)では、電子を光へほぼ100％の効率で変換できる新しい有機発光材料(第三世代)の開発に成功しました。本研究成果は、蛍光材料(第一世代)、リン光材料(第二世代)それぞれの長所を併せ持った低コスト・高効率発光を可能とし、また無限の分子設計の自由度を最大限生かせる夢の発光材料の創出と位置付けることができます。この九州大学発の新しい発光材料を\u22Hyperfluorescence\u22(ハイパーフルオレッセンス)と命名します。これはレアメタルを使わない究極の発光効率を有する有機EL素子の実現につながります。(2012.12.12 九州大学プレスリリースより)The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal–organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 106 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs

Highly efficient organic light-emitting diodes from delayed fluorescence

The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal–organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 106 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.九州大学最先端有機光エレクトロニクス研究センター(OPERA)では、電子を光へほぼ100％の効率で変換できる新しい有機発光材料(第三世代)の開発に成功しました。本研究成果は、蛍光材料(第一世代)、リン光材料(第二世代)それぞれの長所を併せ持った低コスト・高効率発光を可能とし、また無限の分子設計の自由度を最大限生かせる夢の発光材料の創出と位置付けることができます。この九州大学発の新しい発光材料を"Hyperfluorescence"(ハイパーフルオレッセンス)と命名します。これはレアメタルを使わない究極の発光効率を有する有機EL素子の実現につながります。(2012.12.12 九州大学プレスリリースより

Triplet management for efficient perovskite light-emitting diodes

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