Molecular Orientation Effects in Organic Light‐Emitting Diodes

Abstract

It is well known that by horizontally aligning the transition dipole moments of exciton dipoles in the emitter films of organic light‐emitting diodes (OLEDs), a larger fraction of the radiative power can escape from the OLED stack, increasing the light outcoupling efficiency by up to 50 % compared to the isotropic counterparts. In this account, we review recent advances in understanding this phenomenon, with a special focus on the practical strategies to control the molecular orientation in vacuum‐deposited films of thermally activated delayed fluorescent (TADF) dyes. The role of molecular orientation in efficient OLED design is discussed, which has been experimentally proven to increase the external quantum efficiency exceeding 30 %. We outline the future challenges and perspectives in this field, including the potential to extend the concept to the solution‐processed films. Finally, the development of multiscale computer simulations is reviewed to assess their potential as a complementary approach to systematically screening OLED molecules in silico.

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

“…It is important to note that the advancement of computational modeling techniques has recently made it possible to model the orientation of vacuum-deposited organic molecules through DFT and molecular dynamics calculations. These have been recently reviewed elsewhere, [31,35,100] so here we have focused on results from experimental measurements. The vast majority of reports included in this review used ARLS.…”

“…[40,110] These reports have been reviewed in detail elsewhere. [29][30][31][32] Of the three strategies, the one that has been invoked most frequently is the improvement of the molecular shape. However, modifying the geometry of model emitters while preserving or improving their photophysical properties has presented challenges for the design of highly efficient, highly oriented emitters.…”

“…Understanding the impact of emitter orientation on outcoupling efficiency in detail, developing robust methods for measuring emitter orientation within the EML, and identifying the factors that influence orientation in the EMLs of state-of-the-art OLEDs have recently become topics of intense research. [28][29][30][31][32][33] While the polymers used in many solution-processed OLEDs have for a long time been known to show a high degree of orientation parallel to the plane of the film, the lowmolecular-weight emitters (referred to as "small molecules" by the community) were originally assumed to be randomly oriented. However, over the last decade, this has been debunked and by now there are many examples of small-molecule-based OLEDs that show enhanced light-outcoupling efficiency owing to preferential horizontal orientation of the emitter TDM.…”

Tetrabenzo[a,c]phenazine Backbone for Highly Efficient Orange–Red Thermally Activated Delayed Fluorescence with Completely Horizontal Molecular Orientation

“…[18,19] Furthermore,t he molecules with strong p-p,C -H•••p,a nd C-H•••N intermolecular interactions in the aggregated state can lead to aclose-packed and highly ordered alignment, rigid molecular structure,a nd restrain molecular motions and geometric vibrational deformation, thus suppressing the nonradiative pathways in the excited state. [20][21][22][23][24] In the past decade, tremendous efforts have been made in blue,g reen, and orange-red TADF-based OLEDs exceeding EQEs of 20 % by utilizing numerous acceptor units such as benzonitrile, triazine,p yridine,q uinoline,b enzophenone,a nd phenazine. [25][26][27][28][29] Especially,t he multi-ring fused-phenazine based TADF emitters are particularly interested in the design of orange-red TADF emitters due to their planarity,and highly conjugated backbone can facilitate pronounced intermolecular interactions when combined with strong triphenylamine (TPA) or PXZ donor units,i nducing the horizontal TDM orientation in as olid-state.…”

Nematic Triphenyltriazine Triesters and the Induction of the Columnar Mesophase by Fluorine Substitution

“…Donor-substituted triphenyl-s-triazines such as 3 are a promising major class of thermally activated delayed fluorescence (TADF) emitters, [9][10][11][12][13][14][15][16][17][18][19][20] and parent triphenyl-s-triazine has a high triplet energy of ca. 3.05 eV [21] that allows its consideration as anisotropic host [22][23][24][25][26][27][28] for blue-emitting TADF dopants whose triplet energy should be smaller than the host's to confine the excitons on the guest molecules. [29] We thus wondered whether simple triphenyltriazine triesters, accessible via trimerization of cheap methyl cyanobenzoates in triflic or chlorosulphonic acid, would exhibit columnar mesomorphism that could qualify them as large band gap anisotropic matrices for such emitters.…”