Ladder-like energy-relaying exciplex enables 100% internal quantum efficiency of white TADF-based diodes in a single emissive layer

ARTICLE https://doi.org/10.1038/s41467-021-23941-z OPEN Ladder-like energy-relaying exciplex enables 100% internal quantum efficiency of white TADF-based diodes in a single emissive layer 1234567890():,; Chunmiao Han 1, Ruiming Du 1, Hui Xu 1 ✉, Sanyang Han 2, Peng Ma1, Jinkun Bian1, Chunbo Duan Ying Wei 1, Mingzhi Sun 1, Xiaogang Liu 2 ✉ & Wei Huang 3,4 ✉ 1, Development of white organic light-emitting diodes based on purely thermally activated delayed fluorescence with a single-emissive-layer configuration has been a formidable challenge. Here, we report the rational design of a donor-acceptor energy-relaying exciplex and its utility in fabricating single-emissive-layer, thermally activated delayed fluorescencebased white organic light-emitting diodes that exhibit 100% internal quantum efficiency, 108.2 lm W−1 power efficiency, and 32.7% external quantum efficiency. This strategy enables thin-film fabrication of an 8 cm × 8 cm thermally activated delayed fluorescence white organic light-emitting diodes (10 inch2) prototype with 82.7 lm W−1 power efficiency and 25.0% external quantum efficiency. Introduction of a phosphine oxide-based acceptor with a steric group to the exciplex limits donor-acceptor triplet coupling, providing dual levels of high-lying and low-lying triplet energy. Transient spectroscopic characterizations confirm that a ladder-like energy relaying occurs from the high-lying triplet level of the exciplex to a blue emitter, then to the low-lying triplet level of the phosphine oxide acceptor, and ultimately to the yellow emitter. Our results demonstrate the broad applicability of energy relaying in multicomponent systems for exciton harvesting, providing opportunities for the development of third-generation white organic light-emitting diode light sources. 1 Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education) & School of Chemistry and Material Science, Heilongjiang University, Harbin, PR China. 2 Department of Chemistry, National University of Singapore, Singapore, Singapore. 3 Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Nanjing Tech University, Nanjing, China. 4 Frontiers Science Center for Flexible Electronics (FSCFE) & Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), Xi’an, China. ✉email: ; ; NATURE COMMUNICATIONS | (2021)12:3640 | https://doi.org/10.1038/s41467-021-23941-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-23941-z C onsiderable effort has recently been devoted to developing high-efficiency, white organic light-emitting diodes (WOLEDs) with compact design and large-area processing capability1–3. Thermally activated delayed fluorescence (TADF), based on purely organic emitters, enables theoretical 100% internal quantum efficiency for both singlet and triplet exciton harvesting4–6. A single-emissive layer (EML) design, comprising blue/yellow emitters or red/green/blue emitters, can simplify WOLED device structure effectively and can meet the demands of large-scale production, quality control, and low cost7–9. However, competition in exciton confinement between various color emitters makes it challenging to control emission color and device efficiency synchronously (Fig. 1)10,11. As an additional constraint, charge-transfer excited states of TADF dyes are highly sensitive to host-dopant interactions12–14 and interfacial effects15. Indeed, there are few reports of high-efficiency, purely TADF-based WOLEDs, among which multiple emissive layers are required to modify exciton allocation by spatially separating two or three host-dopant systems of different emission color16–18. High-efficiency WOLEDs with single-EML layout requires precise optimization of optical transition and energy transfer (ET) in a multi-emitter-doped host19. However, the narrow singlettriplet splitting energy (ΔEST) of TADF emitters limits energylevel regulation. The singlet-triplet energy gap between the blue and yellow TADF emitters, e.g., bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS, S1/T1 ≈ 2.8 eV)20 and 2,3,5,6-tetrakis(3,6-di-t-butylcarbazol-9-yl)−1,4-dicyanobenzene (4CzTPNBu, S1/T1 ≈ 2.2 eV)4, is approximately 0.6 eV, leading to nonradiative deactivation of excitons during energy transfer21–23. Exciplex S1Donor Yellow Emitter RISC S1 T2 0.3 eV T1Donor ISC 0.3 eV Energy S1Acceptor Blue Emitter Blue TADF Yellow TADF 0.3 eV T1 T1Acceptor S1 T1 D-A coupling Prohibited coupling Prohibited IC ET Inefficient ET 400 500 600 700 Wavelength (nm) Fig. 1 Proposed energy-relaying mechanism. Exciton allocation in singleemissive-layer, TADF WOLEDs comprising a D–A exciplex host, a blue emitter, and a yellow emitter. Inefficient energy transfer occurs from the exciplex host to the blue emitter and subsequently to the yellow emitter, due to a large energy gap between the blue and yellow emitters (~0.6 eV). The proposed exciplex, based on a high triplet donor and a low triplet acceptor, generates a large triplet gap between a high-lying T2 and a lowlying T1. This design enables efficient ladder-like (~0.3 eV ladder spacing) triplet energy transfer from T2 → blue emitter → T1 → yellow emitter. ET, IC, ISC, and RISC refer to energy transfer, internal conversion, intersystem crossing, and reverse ISC, respectively. 2 Presently, the performance of single-EML, TADF-based WOLEDs is not comparable to that of their phosphorescent counterparts24,25. We reason that a host matrix with dual levels of high-lying and low-lying triplet energy, which match the energy levels of blue and yellow TADF emitters, may improve performance23,26. In contrast, single-molecule-based hosts cannot offer two triplet excited states, due to fast internal conversion to the lowest triplet state (T1). Therefore, an exciplex host based on a bimolecular donor–acceptor (D–A) system is likely to provide two different triplet states upon appropriate D–A coupling27,28. In principle, a common exciplex comprises a donor and an acceptor with comparable excited energy levels and strong D–A coupling, lowering S1 and T1 levels of the exciplex below those of the donor and the acceptor. In contrast, an exciplex design featuring a large triplet energy gap between the donor and the acceptor limits donor–acceptor triplet coupling (Fig. 1)29. This leads to dual triplet levels of the exciplex with an energy difference of 0.6 eV. Therefore, a facile triplet energy transfer process may occur, namely the S1/T2 of the exciplex host → the S1/T1 of blue emitter → the T1 of the exciplex host → the T1 of yellow emitter. As an added benefit, the exciplex host with near-zero ΔEST can provide additional triplet-singlet conversion and enhance singlet-exciton utilization in the blue-TADF emitter. As a proof of concept, we designed and synthesized two exciplex hosts, mCP:pDPBITPO and mCP:DpPBITPO, with T2 and T1 energy levels of 3.0 and 2.5 eV, based o (...truncated)