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

Abstract

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.

A Brief History of OLEDs—Emitter Development and Industry Milestones

“…This emitter showed a PLQY of 70% and a highly horizontally oriented feature in host materials, which lead to an excellent device performance with an EQE of 22.1%, EL emission wavelength at 483 nm, and CIE coordinates of (0.17, 0.32). [ 118 ] Also this year, Tang and co‐workers reported a series of TADF emitters based on benzonitrile derivatives. [ 119 ] These emitters bear both 3,6‐di‐ tert ‐butylcarbazole and carbazole as donor moieties and differ in their arrangement of the donors.…”

What controls the orientation of TADF emitters?

“…Its constituents, DMAC as donor and TRZ as acceptor, are frequently used in other TADF emitters as well and can, thus, be considered as prototypical building blocks (Wong and Zysman-Colman, 2017). ICzTRZ is a newly synthesized TADF emitter (Zhang et al, 2020) 3 support this notion and, particularly, indicate that the TDM is parallel to the long molecular axis. These two TADF emitters were co-evaporated with up to nine different host materials (Figure 4).…”

Asymmetric Spirobiacridine‐based Delayed Fluorescence Emitters for High‐performance Organic Light‐Emitting Devices

“…The synthetic routes for the X‐SBA‐Ph derivatives are shown in Scheme S1 (Supporting Information). The precursors of A units, such as tBuTZ and tBuPhTZ, were prepared using the two‐step reaction of a Grignard reaction followed by a Suzuki‐Miyaura coupling reaction [29,30] . The D unit, SBA−Ph, was synthesized according to a literature procedure [31] .…”