No abstract

No abstract

Light-emitting electrochemical cells (LECs) show high technical potential for display and lighting utilizations owing to the superior properties of solution processability, low operation voltage, and employing inert cathodes. For maximizing the device efficiency, three approaches including development of efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction have been reported. However, most reported works focused on only one of the three optimization approaches. In this work, a combinational approach is demonstrated to optimize the device efficiency of LECs. A sophisticatedly designed yellow complex exhibiting a superior steric hindrance and a good carrier balance is proposed as the emissive material of light-emitting electrochemical cells and thus the external quantum efficiency (EQE) is up to 13.6%. With an improved carrier balance and reduced self-quenching by employing the host–guest strategy, the device EQE can be enhanced to 16.9%. Finally, a diffusive layer embedded between the glass substrate and the indium-tin-oxide layer is utilized to scatter the light trapped in the layered device structure, and consequently, a high EQE of 23.7% can be obtained. Such an EQE is impressive and consequently proves that the proposed combinational approach including adopting efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction is effective in realizing highly efficient LECs.

There is growing interest in the bis-tridentate Ir(III) emitters as they are expected to display both improved emission efficiency and improved photostability. Herein, we turned to the new emitters m2h-1−3 and m6h-1−3, bearing a pincer carbene ancillary and a chromophoric chelate derived from judiciously selected phenyl-pyrimidine-pyrazole entities (pzm2h F )H 2 and (pzm6h F )H 2 , which differ in terms of the location of phenyl and pyrazole substituents on the central pyrimidine. Density functional theory calculations revealed a notable change in the spin density distribution from the pyrimidine-pyrazolate entity in m2h to the pyrimidine-phenyl fragment in m6h. As a consequence, the m6h emitters exhibited both shortened emission lifetimes and improved stabilities during extensive photolysis in solution, while corresponding organic light-emitting diodes (OLEDs) doped with green-emitting m6h-1 and sky-blue-emitting m6h-2 and m6h-3 exhibited external quantum efficiencies of 17.6, 15.9, and 17.6%, respectively, superior to those of all of their m2h counterparts at a practical luminance of 10 3 cd/m 2 . This finding suggests a new methodology for fine-tuning the electronic transition that is important to high-performance and durable phosphorescent OLEDs. 49 increase of the emitting excited state energy, which is needed 50 to achieve blue emission, also reduces the energy separation 51 between this emitting excited state and the upper-lying metal-52 centered (MC) dd states. These quenching states are capable 53 of fostering a fast nonradiative decay, giving an unsatisfactory 54 efficiency. 19−21 Notably, one method for blocking this 55 nonradiative process is to employ metal phosphors with a 56 bis-tridentate architecture, 22−24 for which the imposed higher 57 rigidity and multiple metal−chelate coordination bonding are 58 expected to offer higher ligand-field strength and, hence, a 59 much destabilized MC dd excited state and more efficient blue 60 emission.

Donor-Donor’-Acceptor molecules where the Donor’ bridges the Donor and Acceptor have different possible interaction pathways for charge transfer. Here we study a series of Donor-Donor’-Acceptor molecules, having the same Acceptor...