Origin and Control of Orientation of Phosphorescent and TADF Dyes for High‐Efficiency OLEDs

Abstract

It has been known for decades that the emitting dipole orientation (EDO) of emitting dyes influences the outcoupling efficiency of organic light-emitting diodes (OLEDs). However, the EDO of dopants, especially phosphorescent dopants, has been studied less than that of neat films and polymer emitting layers (EMLs) due to the lack of an apparent driving force for aligning the dopants in amorphous host films. Recently, however, even globular-shaped Ir complexes have been reported to have a preferred orientation in doped films and OLEDs. External quantum efficiencies (EQEs) higher than 30% have also been demonstrated using phosphorescent and thermally activated delayed fluorescent dyes (TADF) doped in EMLs. Here, recent results on the EDO of phosphorescent and TADF dyes doped in host films, and highly efficient OLEDs using these dyes are reviewed. The origin and control of the orientation of phosphors are discussed, followed by a discussion of future strategies to achieve EQEs of over 60% without a light extraction layer, from the material point of view.

Routes for Efficiency Enhancement in Fluorescent TADF Exciplex Host OLEDs Gained from an Electro‐Optical Device Model

“…This discovery opened the possibility to fabricate highly efficient fluorescence-based OLEDs with external quantum efficiencies (EQEs) similar to phosphorescence-based OLEDs, but with the advantages of a broader variety of emitter materials (no iridium and platinum complexes), higher color purity, and higher operational stability. [9] By adding TADF assistant dopants to fluorescent emitters in conventional hosts [5,10,11] or in a TADF exciplex host, [12] EQEs of up to 24% were reported. With TADF emitters in a conventional host, [6] an exciplex host, [7] or in a TADF exciplex host, [8] EQEs of up to 37% were obtained without light outcoupling structures, which are similar to the values obtained with the best phosphorescent OLEDs.…”

Molecular Orientation Effects in Organic Light‐Emitting Diodes

“…A number of reviews have comprehensively discussed the experimental techniques for determining the exciton dipole orientation in OLEDs. [25][26][27] The angle-dependent PL spectrometry, in particular, is the most widely used technique. Specifically, as briefly discussed above, one can deposit a layer of emitter on a dielectric substrate, and with the attached prism, one can detect the total angular distribution of far-field emission corresponding to all polarizations.…”

“…In this section, we briefly review the recent success in the OLED community. Note that since recent reviews, [25,26] have already covered phosphorescent molecules, we will focus on thermally activated delayed fluorescent (TADF) emitters. The molecular structures, the degree of horizontal orientation, the PLQY and EQE of the TADF emitters discussed herein, are reported in Figure 3 and Table 1.…”

Efficiency of Light Outcoupling Structures in Organic Light‐Emitting Diodes: 2D TiO2 Array as a Model System

“…[10,[20][21][22] Further highefficiency solutions are combinations of high refractive index substrates and light outcoupling structures. [30,31] Therefore, light outcoupling technologies will still be needed for high-efficiency applications. However, the high costs hamper economic applications.…”

“…[29] The remaining fraction of lost photons consist mostly of substrate and waveguide modes. [30,31] Therefore, light outcoupling technologies will still be needed for high-efficiency applications.…”