Identification of the Key Parameters for Horizontal Transition Dipole Orientation in Fluorescent and TADF Organic Light‐Emitting Diodes

Abstract

In organic light‐emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass‐transition temperature (Tg) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state‐of‐the‐art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum‐processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta‐analysis of 203 published emitter systems is conducted and combined with density‐functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host–guest systems with low‐MW emitters, orientation is mostly influenced by the host Tg, whereas the length and MW of the emitter become more relevant for systems involving higher‐MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.

Controlling the Emitter Orientation in Solution-processed Films through Introduction of Mesogenic Groups within a Multi-resonance Thermally Activated Delayed Fluorescence Emitter

“…of the TDM of absorption in the films can be calculated from the local maxima of the extinction coefficients in the horizontal and vertical directions (ko and ke), 3,4 𝑆 =…”

“…64 The resulting anisotropy factor a was derived by fitting the ARPL spectra of the samples at the peak wavelength to an optical model based on the transfer-matrix method, 65 which can provide a more accurate measurement of the orientation of the TDM of emission of the molecules. [3][4][5]66 Importantly, the anisotropic optical constants obtained from our ellipsometry measurements were used in this model to reflect the fact that the complex refractive index of the material shows non-negligible anisotropy. 65 The PL spectra of the annealed films show a slight blue-shift in the peak wavelength and an increased PL intensity (Figure S10).…”

“…2 For VD-OLEDs, small-molecule, low-weight materials are sublimed into the vapor phase via heating in a highvacuum environment (pressure < 10 -6 Torr) to afford multilayer structures of very high purity. 2 With appropriate heating and choice of materials, the vacuum-deposited films can exhibit high densities, high thermal stability, and the emitter molecules can show a high degree of horizontal molecular orientation, [2][3][4][5] which are critical for the device to be both stable and show a high maximum external quantum efficiency (EQEmax). These advantages make vacuum-deposition the preferred technique for OLEDs fabrication, and to date, commercial OLEDs are mainly produced by this method.…”

“…17,18 A general method to improve ηout is to horizontally orient the TDM of the emitters, as the emission of light proceeds predominantly in a direction perpendicular to the TDM. 19 It is possible to infer the average orientation of the TDMs within a film, 4 which is done typically by either angle-resolved photoluminescence (ARPL) or variable-angle spectroscopic ellipsometry (VASE) studies. [19][20][21] Yokoyama's fundamental work on small molecule orientation in vacuum-deposited films has demonstrated that to minimize the surface energy, the ''first'' layer of the molecules deposited on the interface adopts a horizontal orientation, but then the molecules can quickly undergo surface diffusion until the orientation is fixed by successively overlaying molecules.…”

“…24,25 There are now a number of examples of VD-OLEDs that contain highly horizontally oriented TADF emitters. 4 For SP-OLEDs, the TDM in polymer devices were found to show preferential horizontal orientation even before TDM orientation was investigated in VD-OLEDs. [26][27][28][29] However, when using small molecule-based emitters for SP-OLEDs, the absence of the strong molecular anisotropy imposed by polymer chains and the simultaneous condensation and solidification of the material during the spin-coating often lead to isotropic TDM orientation in the final film.…”