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Chromenopyrazole-based Novel Bipolar Blue Host Materials for Highly Efficient Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Mallesham Godumala, Suna Choi, Seo Yeon Park, Min Ju Cho, Hyung Jong Kim, Dae Hyun Ahn, Ji Su Moon, Jang Hyuk Kwon, and Dong Hoon Choi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01207 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018
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Chemistry of Materials
Chromenopyrazole-based Novel Bipolar Blue Host Materials for Highly Efficient Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Mallesham Godumala,† Suna Choi,† Seo Yeon Park,† Min Ju Cho,† Hyung Jong Kim,† Dae Hyun Ahn,‡ Ji Su Moon,‡ Jang Hyuk Kwon*,‡ and Dong Hoon Choi*,‡ †
Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145 Anam-Ro, Sungbuk-gu, Seoul 02841 (South Korea)
‡
Dept. of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul, 02447, Korea.
ABSTRACT: New electron-acceptor cores are necessary for developing highly efficient bipolar hosts, particularly for blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, chromenopyrazole (CP) was used for the first time as an electron-acceptor core to design and synthesize two novel blue bipolar hosts, viz., 8-(9Hcarbazol-9-yl)-3-methyl-1-phenylchromeno[4,3-c]pyrazol-4(1H)-one (CzCP) and 8-(9H-[3,9'-bicarbazol]-9-yl)-3-methyl-1phenylchromeno[4,3-c]pyrazol-4(1H)-one (2CzCP). The influence of donor strength on the photophysical, electrochemical, and electroluminescent performances was systematically investigated. CzCP and 2CzCP both maintain high triplet energy (~3.0 eV), appropriate highest occupied and lowest unoccupied energy levels (HOMO/LUMO), and bipolar nature. Consequently, OLEDs containing CzCP as a host in the emissive layer exhibited state-of-the-art performance with external quantum efficiency of 27.9% and CIE color coordinates of (0.15, 0.21), thus achieving excellent performance among all reported blue host materials in TADF-OLEDs. This work highlights the importance of the CP unit in developing new host materials and paves the way for the realization of high-efficiency blue TADF OLEDs.
INTRODUCTION Thermally activated delayed fluorescence (TADF) phenomenon has been considered as a research hotspot in the field of organic light-emitting diodes (OLEDs) based-on the promising study by Adachi et al., who reported an external quantum efficiency (EQE) exceeding 20%.1 Unlike conventional fluorescent emitters, TADF materials have the capability to harvest electrically excited both singlet and triplet excitons through reverse intersystem crossing (RISC). This is attributed to the small energy splitting between the lowest singlet and triplet excited energy levels (∆EST) leading to an internal quantum efficiency of 100%.24 Therefore, small ΔEST is an essential property of TADF materials, which can be accomplished by controlling the spatial separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO). Increasing the steric hindrance between the donor (D) and acceptor (A) entities or breaking conjugation in the molecular geometry are the most efficient pathways to achieve effective spatial separation of the frontier orbitals.5-8 Since TADF materials (both hosts and emitters) are composed of only organic entities, the molecular design is unrestricted and more affordable. Analogous to its phosphorescent counterparts, TADF emitters should also be embedded in a suitable host(s)
matrix to alleviate the detrimental self-quenching property, the triplet–triplet annihilation, or the triplet–polaron annihilation triggered by their long-lived triplet excitons. Host materials are often the majority in emissive layers (60–99%) and are particularly responsible for the turn-on voltage and power efficiency, thus demonstrating the influence of hosts on the overall device performance.9-13 In principle, the ideal TADF host materials must fulfill the following criteria: the triplet energy (ET) should be higher than that of the dopant to impede the reverse energy transfer from the dopant to host; the HOMO and LUMO energy levels should be suitable for efficient charge injection and transportation; the small ΔEST should be maintained to enable the upconversion of accumulated triplet excitons to a singlet state for reducing the nonradiative triplet excitons decay etc.9-13 From the perspective of materials design of the host materials, electron-acceptor (A) entities have not been explored extensively, thus highlighting the importance of their development. To date, blue TADF-OLEDs have been extensively developed; however, efficient and stable blue TADF-OLEDs with CIE coordinates of x + y < 0.40 have less investigated.14-20 To obtain such pure-blue OLEDs, not only emitters but also host materials are responsible, as highly polar hosts will broaden and red-shift the electroluminescence spectrum of the emitter.21-22 Compared to emitters, suitable host materials for blue TADF-OLEDs are rare due to the prerequisites of
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high ET and large band gaps. Therefore, molecules like DPEPO, mCP, or mCBP (ΔEST between 0.7-1.0 eV), commonly used as hosts in blue-phosphorescent emissive layers are also being utilized in TADF OLED applications so far.23-25 Although these hosts achieved good results, some disadvantages like unipolar type behavior and uncomplimentary energy levels restricted their widespread applications.26-29 Predominantly, the only hole-transporting tendency of mCP and mCBP leads to chargerecombination near the interface between the emissive layer and the electron-transporting layer, which reduces the device efficiency. Therefore, to overcome the only hole transporting or electron transporting tendency of many familiar hosts, significant efforts have been devoted to develop new blue host materials with comparable hole and electron conductivities.9-13, 30 For instance, Adachi et al. used benzimidazobenzothiazole as a new electronacceptor core for developing two blue host materials 29CzBID-BT and 39Cz-BID-BT.9 By employing DPAC-TRZ as an emitter for both hosts, 29Cz-BID-BT showed the best EQE of 20.8 % with CIE color coordinates of (0.16, 0.34) and a relatively high turn-on voltage (Von) of 4.0 V. Later, our group achieved an EQE as high as 25.7 % by developing dibenzothiophene-based new host material, i.e. ZDN; however, these devices suffer from high Von values of 4.70 V.30 Xu and his co-workers reported phosphine-oxide based hosts via molecular engineering of DPEPO and achieved relatively lower Von of 2.8 V, but the EQE was only 23.0 %.14 Recently, although the same group demonstrated a lower Von of 2.5 V by developing a phosphanthreneoxide-related blue host material, i.e. DPDPO2A, with CIE color coordinates of (0.16, 0.23) by applying DMAC-DPS as the emitter; however, the maximum EQE was limited to 22.5 %.10 These results manifest that the simultaneous realization of high EQE (>25.0 %) and lower Von (
