Emitter Orientation as a Key Parameter in Organic Light-Emitting Diodes

Abstract

The distinct preferential alignment, i.e., horizontal orientation with respect to the substrate plane, of the optical transition dipole moment vectors (TDMVs) of organic dye molecules is of paramount importance for extracting the internally generated power of organic light-emitting diodes (OLEDs) to the outside world. This feature is one of the most promising approaches for the enhancement of the electrical efficacy in stateof-the-art OLEDs, as their internal quantum efficiencies are already close to the ultimate limit. If one can achieve complete horizontal orientation of the TDMVs, it is possible to increase the efficiency by at least 50% because alignment strongly influences the power dissipation into the different optical modes present in such a thin-film device. Thus, this feature of organic light-emitting molecules can lead to advanced performance for future applications. Therefore, we present here a review of recent achievements, ongoing research, and future tasks in this particular area of organic electronics.

Bond Rotations and Heteroatom Effects in Donor–Acceptor–Donor Molecules: Implications for Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence

“…20,21 For all devices, the maximum EQEs achieved were in agreement with the measured PLQYs (approximated using EQE = PLQY × charge balance × outcoupling = 0.25 × PLQY), assuming there is no horizontal molecular alignment. 38 The best performance was observed for material III with a maximum EQE of 22.6% with CIE coordinates (0.15, 0.18) and full width at half maximum (fwhm) of 78 nm. The EL peak is at 461 nm ( Figure 8d) and does not shift with voltage where the maximum intensity is achieved ( Figure S6).…”

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

“…Therefore, we calculate the ELOS according to Equation (1) to judge the effectiveness of each light outcoupling approach. [40] With all that, we can fit the EQE versus thickness dependence of the planar OLEDs ( Figure 2, solid circles) and obtain the radiative quantum yield η rad and electrical efficiency γ to η rad = 0.66 and γ = 1.0, respectively. Next, we need to know the share of photons χ in the substrate-, WG-, and SPP modes of the planar reference OLED.…”

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.…”