Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23417−23427
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High-Efficiency Red and Near-Infrared Organic Light-Emitting Diodes Enabled by Pure Organic Fluorescent Emitters and an Exciplex-Forming Cohost Yuan-Chih Lo,†,# Tzu-Hung Yeh,‡,# Chun-Kai Wang,† Bo-Ji Peng,† Jing-Lin Hsieh,‡ Chih-Chien Lee,‡ Shun-Wei Liu,*,§,∥ and Ken-Tsung Wong*,†,⊥ †
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10617, Taiwan § Organic Electronics Research Center and ∥Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan ⊥ Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan
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‡
S Supporting Information *
ABSTRACT: Three D-A-D-configured molecules DTPBT, DTPNT, and DTPNBT with high quantum yield of orange red (628 nm), red (659 nm), and deep-red/NIR (710 nm) fluorescence, respectively, were developed as emitting dopants in an exciplex-forming cohost (TCTA:3P-T2T) for highefficiency fluorescence-based organic light-emitting diodes (OLEDs). The obtained physical properties together with theoretical calculations analyzed from these new molecules establish a clear structure−property relationship, in which the feature of central acceptor 2,1,3-benzothiadiazole (BT), naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NT), and 2,1,3-naphthothiadiazole (NBT) plays the crucial role for governing the physical characteristics. The optimized device configured as ITO/HAT-CN/TAPC/TCTA/TCTA:3P-T2T:5% emitter/3P-T2T/LiF/Al gave a record-high efficiency of orange red (591 nm, 15%), red (647 nm, 10%), and deep-red/NIR (689 nm, 9%) electroluminescent devices. The effective harvest of triplet excitons with an exciplex-forming system in conjunction with efficient energy transfer between the exciplex and the dopant is beneficial for such high device efficiencies. More importantly, the stable exciplex-forming cohost and fast radiative decay rate of DTPNT render this particular device exhibiting high device stability as indicated by the low efficiency roll-off under high current densities (EQE (external quantum efficiency) values of 8.1% at 1000 cd m−2 and 6.8% at 10,000 cd m−2). These results reveal the potential of employing an exciplex-forming system as cohost for fluorescent dopants to furnish high-efficiency OLEDs with an emission wavelength extending to the red or even the NIR range. KEYWORDS: organic light-emitting diodes, organic fluorescent emitters, near-infared emission, exciplex
1. INTRODUCTION Organic light-emitting diode (OLED) devices have been successfully utilized in display and lighting technology. Beyond these daily used applications, there are more emerging applications, for example, OLEDs with an emission wavelength in the deep-red to near-infrared (NIR) range that can be employed in oximetry detection,1 photodynamic therapy,2,3 night vision technology,4,5 and information-secured display5,6 and optical communication.7,8 To fulfill these potential applications, the device efficiency needs to be significantly boosted, which will strongly depend on the innovative approaches for developing new deep-red to NIR-emissive materials. However, the fluorophore with an emission wavelength in this range typically suffers from low photoluminescence quantum yield (PLQY) mainly due to the energy gap law.9 Limited by efficient emitters, the current progress of © 2019 American Chemical Society
NIR OLEDs largely lags behind the ones with an emission wavelength in the visible region. Accordingly, there are great interests and challenges of designing new deep-red and NIR fluorophores in conjunction with device engineering to improve the efficiency of deep-red and NIR OLEDs. In OLEDs, the recombination of injected electrons and holes leads to 25% of singlet excitons and 75% of triplet excitons according to spin statistics. The triplet excitons of organic emitters usually are nonemissive due to the limitation of the selection rule. To achieve 100% of internal quantum efficiency (IQE), a promising approach is to introduce a heavy atom such as Ir10 or Pt11 into the molecular structure to create Received: April 17, 2019 Accepted: June 5, 2019 Published: June 5, 2019 23417
DOI: 10.1021/acsami.9b06612 ACS Appl. Mater. Interfaces 2019, 11, 23417−23427
Research Article
ACS Applied Materials & Interfaces
efficiency exciplex-hosted OLEDs have been realized. For example, in 2016, we reported a red OLED with an emission at 606 nm employing the blend of Tris-PCz:CN-T2T as a cohost doped with a pure organic red fluorescent emitter 2-tert-butyl4-(dicyanomethylene)-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)vinyl]-4H-pyran (DCJTB) to achieve 9.7% of EQEmax.24 In 2017, Yang et al. reported a high efficiency (11.9%) deep-red (656 nm) OLED utilizing m-MTDATA:o-CzOXD as an exciplex-forming cohost doped with a phosphorescent emitter (TP-BQ)2Ir(acac).28 In 2017, Kim et al. reported a series of red OLEDs employing the TCTA:B4PYMPM blend as the cohost material doped with DCJTB as a fluorophore and Ir(ppy)2tmd as a sensitizer to promote efficient energy transfer.13 The best performance among these OLEDs reached to 23.7% of EQEmax. These great results point out an attainable way of employing an exciplex-forming blend as a cohost material to accomplish efficient red- and NIR-OLED. The interfacial layer between TCTA and 2,4,6-tris(3-(1H-pyrazol1-yl)phenyl)-1,3,5-triazine (3P-T2T) was reported to give efficient exciplex emission, which can be further utilized to realize a bilayer-type exciplex-based OLED with 7.7% of EQEmax.29 A recent study revealed the good photostability of the TCTA:3P-T2T blend, which can serve as an excellent cohost system for the realization of high-efficiency (29.1% of EQEmax at 10 cd/m2) and high-stability (τ80 ≈ 1020 min with an initial brightness of 2000 cd/m2) devices.26 The result suggests the great potential of the TCTA:3P-T2T blend as an exciplex-forming cohost system for further applications. Herein, we reported the realization of high-efficiency (15, 10, and 9.0% of EQEmax) OLEDs with EL peaks centered at 591, 647, and 689 nm, respectively, using the TCTA:3P-T2T blend as an exciplex-forming cohost system for pure organic fluorescent emitters with donor-acceptor-donor (D-A-D) configuration. These devices demonstrated superior efficiency and limited efficiency roll-off at high brightness, giving the best example ever reported for an EL device hosted by an exciplex cohost system with an emission close to 700 nm. These results clearly manifest the bright potential of the exciplex-forming blend as a cohost system for achieving high-efficiency NIR OLEDs with pure organic fluorescent emitters.
effective spin-orbital coupling and sequentially facilitate the intersystem crossing (ISC) for the downconversion of the singlet state (S1) to the triplet state (T1) followed by the feasible and efficient phosphorescent relaxation process benefiting from the consequence of ground/excited state orbital mixing. Particularly, the success of Ir-based emitters was recognized as the cornerstone of modern OLED technology, rendering this technology widely commercialized. However, the rarity of precious metals may impede the cost-effective production of OLEDs in the near future. Therefore, it is important to find other means for harvesting triplet state excitons of organic emitters. A clever approach to reach 100% IQE is the utilization of organic fluorophores with thermally activated delayed fluorescence (TADF). The process can be realized by the manipulation of a subtle overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to minimize the energy gap (ΔEST) between S1 and T1 states, rendering the reverse intersystem crossing (RISC) possible through the assistance of thermal energy from the environment. The fundamental requirement for TADF can be feasibly satisfied by a sophisticated molecular design of pure organic materials, which can potentially reduce the production cost of OLEDs in the future, rendering the research on TADF-based OLEDs significantly attractive. Nowadays, high-efficiency red,12−17 green,15,18,19 and blue20,21 TADF-based OLEDs have been reported. It is reasonable to anticipate that the extension of an emissive wavelength of TADF-enabling emitters from the visible light region to deep-red, or even to NIR, can be realized, rendering high-efficiency NIR-OLEDs with pure organic emitters possible. In this regard, in 2017, Yuan et al. reported a device with an emission centered at 693 nm and a high external quantum efficiency (EQE) up to 10.19%, employing a new TADF emitter (APDC-DTP) as a dopant dispersed in a 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI) host.33 More recently, Adachi et al. reported a series of borondifluoride curcuminoid derivatives with an NIR emission and TADF character.22 The champion device employed one of these derivatives as a dopant dispersed in a 4,4′-bis(Ncarbazolyl)-1,1′-biphenyl (CBP) host showed 5.1% of EQEmax with electroluminescence (EL) peaking at 760 nm. In another approach, TADF can also be realized by the intermolecular charge-transfer process between a hole-transporting material (HTM) and an electron-transporting material (ETM). In a device, the injected holes and electrons can accumulate at the interface of HTM and ETM with large energy level offsets, leading to the formation of interfacial exciton called exciplex. At this charge-transfer state, the population of HOMO and LUMO is distributed on HTM and ETM, respectively, resulting in the reduced exchange energy of S1 and T1 states, rendering the RISC process and TADF feasible.23−26 There are versatile functions of the exciplex-forming blend, which can act not only as an emitting layer, for example, in 2016, Data et al. reported an exciplex blend composed of m-MTDATA as the donor and dibenzophenazine-based POZ-DBPHZ as the acceptor to make an NIR OLED with the EL peak centered at 741 nm and an EQE of 5%,27 but also as a host material for accommodating emissive dopants to tune the emission wavelength. The exciton energy of the exciplex-forming cohost system can be transferred through the F̈ rster resonance energy transfer (FRET) process to the dopant with matched spectral overlap or via the Dexter energy transfer (DET) process to the dopant with appropriate energy level alignments. High-
2. EXPERIMENTAL SECTION 2.1. Synthesis. The synthetic procedure and detail characterization of the final compounds are provided in the Supporting Information. 2.2. Material Characterization. Thermogravimetric analysis (TGA) was conducted under a nitrogen atmosphere at a heating rate of 10 °C/min on a platinum pan via a TA Instruments Q500 TGA (V20.13 Build 39). NMR spectra were measured in CDCl3 using a Varian (Utility 400) spectrometer for 1H NMR (400 MHz) and 13C NMR (100 MHz). Optical absorption measurements were conducted by a JASCO V-670 spectrophotometer. All organic thin films were deposited on quartz substrates by a thermal evaporator under a high vacuum of 5 × 10−6 torr. The deposition rate was fixed at 0.5 Å/s, while the thickness is 80 nm for each material. For transient photoluminescence (TRPL), the sample was put in the homemade holder by a spectrofluorometer with an excitation wavelength of 320 nm (FluoroMax Plus, HORIBA Jobin Yvon) in the nitrogen-filled chamber. The instrument response function (IRF) of TRPL is within 2 ns. Note that the TRPL was also measured in an ambient condition to analyze the fluorescent property of materials. For the PL spectrum and PL quantum yield (PLQY) measurement, the thin film was conducted using an ozone-free xenon arc lamp with the excitation source of 305 nm with a pulse frequency of 50 kHz and power density of ∼87 × 10−6 W/m2. For the absorption measurement, the organic 23418
DOI: 10.1021/acsami.9b06612 ACS Appl. Mater. Interfaces 2019, 11, 23417−23427
Research Article
ACS Applied Materials & Interfaces
Figure 1. Molecular structures of TCTA, 3P-T2T, HAT-CN, and TPAC and the newly synthesized DTPBT, DTPNT, and DTPNBT.
Figure 2. UV−vis absorption and photoluminescence spectra of DTPBT, DTPNT, and DTPNBT in (a) dichloromethane solution and (b) thin film. high vacuum level of 2 × 10−6 torr). The deposition rates of each organic material were about 0.5−1 Å/s. Finally, the encapsulation of the device was carried out with a bare glass in a glovebox (oxygen, moisture of
