What Controls the Orientation of TADF Emitters?

Naqvi, Bilal Abbas; Schmid, Markus; Crovini, Ettore; Sahay, Prakhar; Naujoks, Tassilo; Rodella, Francesco; Zhang, Zhen; Strohriegl, Peter; Bräse, Stefan; Zysman‐Colman, Eli; Brütting, Wolfgang

doi:10.3389/fchem.2020.00750

Cited by 58 publications

(99 citation statements)

References 36 publications

(49 reference statements)

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“…The anisotropy factor ( a ) for ICzTRZ was measured in several OLED‐relevant host materials ( Table 2 ). [ 33 ] The doped films (10 wt% ICzTRZ in the host) were obtained via vacuum‐deposition and the orientation of the emitter molecule was then measured using polarization‐ and angle‐dependent luminescence spectroscopy, after which the data were analyzed by optical simulation (see Supporting Information for details). ICzTRZ presents a nearly completely horizontal orientation in all the host materials tested (see Figure S14, Supporting Information) with the best results obtained from the DPEPO doped film, with an anisotropy factor of 0.06.…”

Section: Resultsmentioning

confidence: 99%

Efficient Sky‐Blue Organic Light‐Emitting Diodes Using a Highly Horizontally Oriented Thermally Activated Delayed Fluorescence Emitter

Zhang

Crovini

Santos

et al. 2020

Advanced Optical Materials

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employing a purely organic TADF emitter, 4CzIPN. [3] TADF molecules have an energy gap between the S 1 and T 1 states, ΔE ST , sufficiently small to allow the triplet excitons to convert into singlet excitons through reverse intersystem crossing (RISC), making possible their radiative decay from the singlet excited state, evidenced by a delayed fluorescence. To minimize ΔE ST and enhance k RISC , the overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the molecule must likewise be small, which is achievable in compounds containing electron-donating and electron-accepting moieties that are poorly electronically coupled giving the excited state an intra-molecular charge-transfer (CT) character. [4] The most common emitter designs rely on a large twist angle between the donor and acceptor coupled with the incorporation of aromatic spacer bridging moieties. [5] The external quantum efficiency (EQE) of the OLED is a function not only of the IQE but also of the light out-coupling efficiency. As the stack of an OLED device is composed of several different layers of materials included between two electrodes, total internal reflection at the interface between two media, and coupling to surface plasmon polaritons (SPP) at the interface with Organic thermally activated delayed fluorescent (TADF) materials can harvest 100% of the electrically generated excitons as a result of their small singlet-triplet energy difference. However, maximizing the external quantum efficiency (EQE) of a device also requires enhancing the light out-coupling efficiency. This work presents a new acceptor-donor-acceptor (ADA) emitter employing an indolocarbazole donor and diphenyltriazine acceptors that show nearly-completely horizontal orientation regardless of the host matrix, leading to a sky-blue organic light-emitting diode (λ EL = 483 nm, CIE coordinates of 0.17, 0.32) with EQE MAX of 22.1%, a maximum luminance of 7800 cd m −2 , and blue emission.

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Section: Resultsmentioning

confidence: 99%

Efficient Sky‐Blue Organic Light‐Emitting Diodes Using a Highly Horizontally Oriented Thermally Activated Delayed Fluorescence Emitter

Zhang

Crovini

Santos

et al. 2020

Advanced Optical Materials

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“…The robust molecular orientation of TPBi could be attributed to the shape of the molecules. 4,[23][24][25] To reduce the free energy on the surface, the molecules prefer configurations strongly interacting with the film surface during the deposition. Another point to reduce the surface free energy is to increase the entropy.…”

Section: Discussionmentioning

confidence: 99%

“…Investigation of the molecular orientation in organic films of electronic devices such as organic light-emitting diodes (OLEDs) has attracted much attention owing to their effects on the electrical and optical properties as well as the efficiency of the devices. [1][2][3][4] Some organic materials are spontaneously ordered during the formation process of thin films. Consequently, if the molecule possesses a permanent dipole moment (PDM), orientation polarization is induced in the resultant film.…”

Section: Introductionmentioning

confidence: 99%

Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules

Gunawardana

Osada

Koswattage

et al. 2021

Surface & Interface Analysis

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In this study, we investigated the spontaneous orientation polarization (SOP) in coevaporated films of polar and nonpolar molecules. SOP has been commonly observed in the evaporated films of organic light‐emitting diode materials, and it influences the device performances. However, the mechanism of the SOP formation is yet to be clarified. Herein, we characterized the molecular orientation of polar molecules of 1,3,5‐tris (1phenyl‐1H‐benzimidazol‐2‐yl) benzene (TPBi) in coevaporated films with nonpolar molecules of 1,4‐bis‐(triphenylsilyl) benzene (UGH‐2) or 4,4′‐bis (N‐carbazolyl)‐1,1′‐biphenyl (CBP). We measured the surface potential characteristics of the coevaporated films. We found that the molecular orientation of TPBi in both UGH‐2 and CBP hosts was enhanced, though the apparent enhancement factor was small in the UGH‐2 host. The enhancement of the molecular orientation is attributed to the reduction of the electrostatic interaction between polar molecules (TPBi), which deteriorates SOP as previously reported. In addition, our results suggest that the SOP of TPBi is robust even in the UGH‐2 host, in contrast to the random orientation of Ir complexes in the UGH‐2 host. Considering a polyhedral shape of Ir complexes, the robust SOP of TPBi in the host molecules with a weak van der Waals interaction, such as UGH‐2, is possibly due to its disk‐like molecular shape.

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“…[ 81 ] and, more recently, in the similarly light molecule DMAC‐TRZ studied by Naqvi et al. [ 106 ] It is worth noting that the studies by Mayr et al. and by Naqvi et al.…”

Section: Currently Pursued Strategies For Achieving Horizontal Orientationmentioning

confidence: 99%

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

et al. 2021

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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.

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What Controls the Orientation of TADF Emitters?

Cited by 58 publications

References 36 publications

Efficient Sky‐Blue Organic Light‐Emitting Diodes Using a Highly Horizontally Oriented Thermally Activated Delayed Fluorescence Emitter

Efficient Sky‐Blue Organic Light‐Emitting Diodes Using a Highly Horizontally Oriented Thermally Activated Delayed Fluorescence Emitter

Enhancement of the molecular orientation of TPBi in coevaporated films of UGH‐2 host molecules

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

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