Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Bipolar Poly(arylene phosphine oxide) Hosts with Widely Tunable Triplet Energy Levels for High-Efficiency Blue, Green, and Red Thermally Activated Delayed Fluorescence Polymer Light-Emitting Diodes Shiyang Shao,† Shumeng Wang,† Xiushang Xu,†,‡ Yun Yang,†,‡ Jianhong Lv,† Junqiao Ding,*,† Lixiang Wang,*,† Xiabin Jing,† and Fosong Wang† Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on April 25, 2019 at 22:31:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *
ABSTRACT: Polymer light-emitting diodes (PLEDs) based on thermally activated delayed fluorescence (TADF) emitters show great potential in developing high-efficiency solution-processed light-emitting devices without the use of noble metal complexes. However, a key challenge for the development of TADF-PLEDs so far is the lack of polymer hosts with suitable triplet energy levels (ETs) and good carrier transport capability. Here, we report the design, synthesis, and electroluminescent properties of a novel series of bipolar poly(arylene phosphine oxide) hosts based on electron-transporting arylphosphine oxide and hole-transporting carbazole units, which show widely tunable ETs in the range of 2.20−3.01 eV by finely tuning the conjugation extent of the polymer backbone. The tunable ETs make these polymers a universal host family for all of the blue, green, and red TADF emitters. TADF-PLEDs based on these polymer hosts show promising device efficiency with external quantum efficiencies up to 15.8, 17.1, and 10.1% for blue, green, and red emissions, respectively, which are among the highest efficiencies for TADF-PLEDs. These results open an avenue for the development of TADF-PLEDs with high efficiency and fullcolor emission in the future.
1. INTRODUCTION Thermally activated delayed fluorescence (TADF) emitters, which are capable of utilizing triplet excitons through the rapid reverse intersystem crossing process from the lowest triplet state (T1) to singlet state (S1), are recognized as the newgeneration materials for the application in organic lightemitting diodes (OLEDs).1−9 Up to now, great progress has been made for TADF-OLEDs based on vacuum-deposited small molecules where external quantum efficiencies (EQEs) over 30% have been reported.10−12 However, TADF-based polymer light-emitting diodes (PLEDs) that are suitable for simple, low-cost, and easily-scalable solution-processed technologies, such as ink-jet printing and roll-to-roll printing, are less developed. Although much effort have been made on TADF-PLEDs either by physically blending TADF emitters into polymer hosts13 or by covalently bonding TADF © XXXX American Chemical Society
molecules into polymer hosts with conjugated or nonconjugated backbone,14−29 their device performance still lags behind the small-molecule TADF-OLEDs.30,31 Recently, green TADF-PLEDs with maximum EQEs over 18% have been reported;28,32 nevertheless, the device efficiencies of blue and red ones are much inferior yet, with the best EQEs in the range of 4.8−12.1%.3,19,27 To develop efficient TADF-PLEDs, a key challenge is to develop polymer hosts with good carrier transport ability and suitable energy levels that match the TADF emitters. Because long-lifetime (microsecond scale) triplet states are involved in TADF emitters, the host materials play critical roles in Received: February 11, 2019 Revised: April 11, 2019
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DOI: 10.1021/acs.macromol.9b00077 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Molecular design and synthesis of the poly(arylene phosphine oxide) hosts.
polymer host systems with both high ET and good charge transport capability is highly desired for high-efficiency TADFPLEDs. In this work, we report a novel series of bipolar poly(arylene phosphine oxide) hosts with widely tunable triplet energy levels for high efficiency blue, green, and red TADF-PLEDs. The bipolarity of the polymer is realized by simultaneous incorporation of electron-transporting phosphine oxide units and hole-transporting carbazole units into the polymer skeleton. Meanwhile, the ETs of the polymer hosts can be tuned in a very wide range of 2.20−3.01 eV by finely modulating the conjugation extent of the polymer backbone, making them a universal polymer host family that are suitable for all of the blue, green, and red TADF emitters. TADFPLEDs based on these polymer hosts show promising device efficiency with EQEs up to 15.8, 17.1, and 10.1% for blue, green, and red emissions, respectively, which are among the highest efficiencies ever reported for TADF-PLEDs.31 The encouraging results make the poly(arylene phosphine oxide) hosts a promising platform for the development of highefficiency TADF polymers with full-color emission in the future.
reducing the intermolecular interactionsincluding triplet− triplet annihilation and triplet−polaron annihilation (TPA) which deteriorate the device efficiency especially at high current density.5 In general, there are two requirements for the polymer hosts. First, the triplet energy (ET) of the polymer host should be higher than that of the dopant to prevent back energy transfer from dopant to host which leads to energy dissipation of the excitons by nonradiative transition (Figure 1). Second, the polymer host should have balanced carrier transport capability to enhance the exciton recombination ratio in the emissive layer (EML). However, unlike small-molecule TADF-OLEDs where a large library of host materials has been established,5 there are only two main kinds of polymer hosts for TADF-PLEDs. One is the nonconjugated polymer hosts (such as polystyrenes17,24,28,33 and polynorbornenes26) which have high ETs that can be used for blue TADF emitters. However, the carrier transport ability of these polymers is poor; therefore, additional carrier transport units should be introduced to the polymer host24,26 (or external carrier transport materials should be mixed into the polymer host13) to achieve high device efficiency. The other kind is conjugated polymer hosts such as polycarbazoles (PCz),16,21 polyfluorenes,27 and their copolymer derivatives,15,32,34 which show moderate triplet energies (
