Highly Efficient TADF Polymer Electroluminescence with Reduced

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Letter

Highly Efficient TADF Polymer Electroluminescence with Reduced Efficiency Roll-off via Interfacial Exciplex Host Strategy Xingdong Lin, Yunhui Zhu, Baohua Zhang, Xiaofei Zhao, Bing Yao, Yanxiang Cheng, Zhanguo Li, Yi Qu, and Zhiyuan Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Materials & Interfaces

Highly Efficient TADF Polymer Electroluminescence with Reduced Efficiency Roll-off via Interfacial Exciplex Host Strategy Xingdong Lin,† Yunhui Zhu,‡,ǁ Baohua Zhang,‡,* Xiaofei Zhao,‡ Bing Yao,‡,ǁ Yanxiang Cheng,‡,* Zhanguo Li,†,‡ Yi Qu,†,* and Zhiyuan Xie‡,*

†

School of Science, Changchun University of Science and Technology, Changchun

130022, P. R. China

‡

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ǁ

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

1 ACS Paragon Plus Environment

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ABSTRACT Solution-processed

organic

light-emitting

diodes

(s-OLED)

consisting

of

TAPC/TmPyPB interfacial exciplex host and polymer PAPTC TADF emitter is prepared, wonderfully simultaneous displaying ultralow voltages (2.50/2.91/3.51/4.91 V at luminance of 1/100/1000/1000 cd m-2), high efficiencies (14.9%, 50.1 lm W-1), and extremely low roll-off rates (J50 of 63.16 mA cm-2, L50 of ca. 15000 cd m-2). Such performance is distinctly higher than that of pure-PAPTC s-OLED. Compared to pure-PAPTC, the advanced emissive layer structure of TAPC:PAPTC/TmPyPB is unique in much higher PL quantum yield (79.5% vs. 36.3%) and nearly 4-fold enhancement in kRISC of the PAPTC emitter to 1.48 ×107 s-1.

KEYWORDS: TADF polymer, triplet concentration quenching, interfacial exciplex, efficiency roll-off, solution-processed OLEDs.


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Organic light-emitting diodes (OLEDs) based on the thermally activated delayed fluorescence (TADF) materials have drawn great research interests since the TADF emitters cannot only harvest the triplet for delayed fluorescence, but also are cheap with regard to the phosphorescent complexes, which are economically favorable in flat-panel display and lighting applications.1 As reported, distinct progress has been achieved for the thermally evaporated TADF-type OLEDs, e.g. achieving external quantum efficiency (EQE) of > 30% and power efficiency (PE) of >100 lm W-1, respectively.2 Nowadays, a lot of attention has been paid on developing TADF-type polymers towards solution-processed OLEDs (s-OLEDs) as they are more favorable for wet-process, flexible and large-area mass production of OLEDs.3-13 However, it is difficult to precisely control the distance and torsion angle between donor (D) and accepter (A) segments in TADF polymers, which determines the energy gap (∆EST) between the lowest singlet state (S1) and triplet state (T1) of the polymer materials, the reverse intersystem crossing (RISC) rate from the T1 to S1 and the photoluminescent quantum efficiencies (PLQEs) of TADF polymers. In spite of these challenges, distinct progress has been achieved in this topic.3, 5-7, 12 Besides, there are reports on solution-processed

TADF-type

OLEDs

with

TADF

dendrimer14-16

and

solution-processable TADF small molecules17-20. Up to now, TADF polymer based s-OLEDs still face severe challenges on realizing high EQEs, PEs, and especially low efficiency roll-off rate. For example, s-OLEDs based on pure TADF polymer emissive layer (EML) structure was first reported by Nikolaenko et al.3, in which the peak EQE was up to 10% but it suffered from quick roll-off rate (J50≈0.5 mA cm-2). Yang et al. reported conjugated polymer by attaching the efficient TADF units as side chains of the polycarbazole backbone.21 The device showed a moderate EQE of 4.3%. Alternatively, host-guest doped strategy 3 ACS Paragon Plus Environment


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was used to enhance electroluminescent (EL) performance of TADF polymer. For instance,

Yan

and

Bryce

et

al.

reported

s-OLEDs

based

on

1,

3-bis(carbazol-9-yl)benzene (mCP) host doped with a Copo1 polymer. The resultant s-OLEDs achieved a record peak EQE of 20.1% but sharply declined to 5.3% at a luminance of 100 cd m-2.6 Adachi et al. reported s-OLEDs by employing 4,4′-cyclohexylidenebis[N,

N-bis(4-methylphenyl)benzenamine]

(TAPC):tris(4-carbazol-9-yl-phenyl)amine (TCTA) binary host doped with TADF polymers, i. e. pCzBP or pAcBP.22 They exhibited high EQEs (i.e. 8.1% ± 0.7%, 9.3% ± 0.9%) and low efficiency roll-off. However, these s-OLEDs suffered from high driving voltages and low PEs of 9.0 ± 0.7 lm W-1 and 20.3 ± 1.8 lm W-1. Recently, Yang et al. developed a novel conjugated TADF polymer PCzDP-10.12 The s-OLEDs with an EML consisting of mCP:DMAC-DP-Cz binary-host doped with PCzDP-10 achieved a high EQE of 16.1%, but a low PE of 14.3 lm W-1 and high efficiency roll-off. Therefore, host-guest doping tactic is still unsatisfied since the resultant TADF polymer s-OLEDs cannot simultaneously achieve high EQEs, low roll-off rate and high PEs. Most importantly, the underlying principles about host material choice23-32 and photophysical mechanisms of host-guest doped TADF polymer EML are rarely mentioned. Herein,

we

propose

to

use

an

interfacial

exciplex

host

of

TAPC/1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TmPyPB) with a home-made TADF polymer PAPTC7 doping into the hole-transporting TAPC to restrain triplet quenching of PAPTC emitter. The resultant s-OLEDs demonstrate superior EL performance, i.e. a high EQE of 14.9%, PE of 50.1 lm W-1, and maximum luminance of 26321 cd m-2, respectively. Most importantly, an extremely low efficiency roll-off is realized with a L50, J50 (luminance and current density at which the EQE of device declines to half of 4 ACS Paragon Plus Environment

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its maximum) of ca.15000 cd m-2 and 63.16 mA cm-2, even superior to the-state-of-the-art

thermally

evaporated

TADF-type

OLEDs.2 The

overall

performance is among the best results for the TADF polymer-based s-OLEDs.3, 5-6, 12, 21-22

We find that the TAPC:PAPTC/TmPyPB EML shows not only high PLQE of

79.5%, but also very fast reverse intersystem crossing rate (kRISC), i.e. 1.48 ×107 s-1, for triplet of PAPTC emitter, which favors to restrain the triplet concentration quenching and reduce the efficiency roll-off. Figure 1 shows PL and absorption spectra of the TAPC, PAPTC and TAPC:PAPTC (x wt.%) blended films, and also molecular formulas of them. PAPTC polymer is characteristic of short lifetime of TADF delayed component (< 1 µs) and low exchange energy (∆EST) of 0.13 eV.7 The s-OLEDs employing non-doped PAPTC as EML demonstrated a high EQE of 12.6 % and PE of 37.1 lm W-1. However, it is noticed that PLQE of pure PAPTC film is merely about 36.3%, which may limit its EL performance. Herein, high energy level TAPC is used as host to disperse PAPTC emitter, which may reduce interactions of the TADF segments in PAPTC polymer and thus suppress the unwanted intra-chain and/or inter-chain non-radiative decay. As shown in Figure 1(a), when the TAPC:PAPTC doping ratio is higher than 20 wt.%, the PL emission of TAPC is fully quenched and the TAPC:PAPTC doped films solely display the PL emission from PAPTC component, indicating complete energy transfer from TAPC to PAPTC. The PL emission peak of PAPTC is gradually blue-shifted from 505 nm to 497 nm with increasing the TAPC content. It may be attributed to that the aggregation effects of PAPTC polymer chains are effectively restrained with incorporating TAPC. The absorption spectra of TAPC, PAPTC and TAPC:PAPTC blend films are shown in Figure 1(b). As confirmed, the absorption profiles of the TAPC:PAPTC blend films are just superposition of TAPC 5 ACS Paragon Plus Environment


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and PAPTC absorption. Therefore, the function of TAPC in TAPC:PAPTC blended film is only to serve as host matrix to dilute PAPTC polymer in these films.

Figure 1. (a) PL and (b) absorption spectra of TAPC, PAPTC, and TAPC:PAPTC blend films with different weight ratios.(c)The chemical structures of TAPC and PAPTC. The PL transient decay and PLQE characteristics of the TAPC:PAPTC blend films (10 wt.%-100 wt.% PAPTC) are investigated in details. The results are shown in Figure 2 and Table S1. As shown in Figure 2(a), the PL transient data of the TAPC:PAPTC blend films display bi-component exponential decay behaviors, which are well correlated with prompt and delayed fluorescence portions of TADF material emission, respectively. The detailed fitting results of these samples are summarized in Table S2. As PAPTC is gradually decreased from 100 wt.% to 10 wt.% in these TAPC:PAPTC films, the short lifetime τPF is almost unchanged but the delayed 6 ACS Paragon Plus Environment

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lifetime τDF is monotonically increased from 832.1 ns to 2075.7 ns. These phenomena are analogous to recent findings by Lee et al. in the 1,3,6,8-tetramethylcarbazole (MCz)-xanthone

(XT)

D-A

type

TADF

molecule:

2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) (5-100 wt.%) doping system.27 They confirms that non-radiative triplet exciton loss pathway does exist and compete with radiative RISC/ISC (reverse intersystem-crossing/intersystem-crossing) cycling in the non-doped MCz-XT film and the heavily-doped PPF:MCz-XT film samples. Given the similarity in concentration-dependent PL transient characteristics for our samples, it is postulated that the quenching mechanism in non-doped PAPTC and heavily-doped TAPC:PAPTC film samples is attributed to triplet accumulation quenching between intra-chain or inter-chain of PAPTC polymers (see Figure S1). The detailed parameters of various photophysical processes in the PAPTC and TAPC:PAPTC films are derived and summarized in Table S1. It is noticed that concentration-quenching rate (KCQ) of triplet excitons in pure PAPTC film is as high as 1.2×106 s-1. The KCQ of triplet excitons in TAPC:PAPTC films is gradually decreased with the increased content of TAPC. As shown in Figure 2(b) and Table S1, the PLQE of pure PAPTC film is as low as 36.3%. The PLQE of the TAPC:PAPTC films is gradually increased to 92.3% with decreasing the PAPTC content to 20 wt.%. It is also noted that compared to pure PAPTC film, contribution of prompt fluorescence (PF) to PLQE in TAPC:PAPTC films (100 wt.% → 20 wt.%) are increased very little with prompt fluorescent quantum yield (ФPF) from 8.7 % to 10.6 %, while the delayed fluorescent quantum yield (ФDF) is distinctly increased from 27.6 % to 81.7 %. The enhancement of PLQE of the TAPC:PAPTC (20 wt%) film is thus mainly attributed to the increased ФDF. The low PLQE of the pure PAPTC



polymer film originating from severe triplet quenching and the resultant low ФDF is well solved by using the TAPC:PAPTC host-guest doping strategy.

Figure 2. (a) PL transient decay and (b) PLQE results of the TAPC:PAPTC films with various PAPTC concentrations (10-100 wt.%). Inset in (b) shows the prompt fluorescence (PF) and delayed fluorescence (DF) components in PLQE for each samples.

The TAPC:PAPTC (20 wt.%) and pure PAPTC films are used as the EML in s-OLEDs to study their EL behaviors. The corresponding s-OLEDs have the structures of ITO/PEDOT:PSS/TAPC:PAPTC(20 wt.%)/TmPyPB/LiF/Al (Device A) and ITO/PEDOT:PSS/PAPTC/TmPyPB/LiF/Al (Device B) as shown in Figure 3(a, b). The corresponding energy-level diagram and the proposed EL mechanism are also shown in Figure 3(c, d). There is no interaction between the PAPTC emitter and the TmPyPB to form exciplex (Figure S2). It is reasonable since LUMO offset between PAPTC and TmPyPB is -0.23 eV in spite of a distinct HOMO offset of 1.30 eV between them. Thus, the injected holes from ITO/PEDOT:PSS and electrons from TmPyPB/LiF/Al directly recombine to form excitons within the PAPTC layer and then decay to emit light in the Device B (Figure 3d). However, the case in Device A is different. Interfacial exciplex between the hole-transporting TAPC and 8 ACS Paragon Plus Environment

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electron-transporting TmPyPB couple is presented as illustrated in Figure S3.There are both a large HOMO offset of 1.33 eV and a LUMO offset of 0.74 eV at the TAPC/TmPyPB interfaces, which energetically guarantees the exciplex formation under PL and EL excitation. The TAPC:TmPyPB couple also displays distinctly slow PL transient decay behaviors as shown in Figure S5, which further verifies the exciplex nature of TAPC:TmPyPB couple. Thus, the actual host in Device A is the TAPC/TmPyPB interfacial couple and the PAPTC acts as the TADF dopants. Such a structure in Device A is advantageous as the excitons can be formed barrier-freely at the heterojunction of TAPC/TmPyPB interfaces and then be transferred to the PAPTC emitter. The proposed EL operation mechanism is illustrated in Figure 3(c), which is similar to our previous report to get high performance phosphorescent s-OLEDs.23

Figure 3. Device configurations, the corresponding energy level alignments7, 29 and EL operating mechanisms of Device A(a, c) and B(b, d). The EL characteristics of the two types of s-OLEDs based on the pure PAPTC and TAPC:PAPTC(20 wt.% PAPTC) EMLs are shown in Figure 4 and the key 9 ACS Paragon Plus Environment


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parameters are summarized in Table S3. EL spectra of them are the same and purely come from the PAPTC emission, confirming that excitons are totally confined within EML and are finally harvested by the PAPTC emitters. The light turn-on voltage at a luminance of 1 cd m-2 is about 2.50 V and close to the optical band gap (2.56 eV) of the PAPTC emitter, confirming a barrier-free exciton generation for both of two devices. However, Device A shows superior performance than Device B. Device B shows a maximum luminance 16477 cd m-2, an EQE of 11.5 % and a PE of 41.5 lm W-1, respectively. Device A exhibits much better EL performance with a maximum luminance of 26321 cd m-2, an EQE of 14.9 % and a PE of 50.1 lm W-1, corresponding to enhancement of 59.7 %, 29.6 % and 20.7 %, respectively. Most importantly, the efficiency roll-off characteristics is greatly improved for Device A as compared to Device B (Table S4). The EQE of Device A is declined to 0.99, 0.95 and 0.62 of its maximum value (14.9%) at luminances of 100, 1000, 10000 cd m-2, respectively. However, Device B shows distinctly faster efficiency roll-off with EQE declining to 0.99, 0.86 and 0.26 of its maximum value (11.5%) at luminances of 100, 1000 and 10000 cd m-2, respectively. Besides, the L50 and J50 for Device A are 15000 cd m-2 and 63.16 mA cm-2, much higher than 6500 cd m-2 and 25.07 mA cm-2 for Device B. Accordingly, nearly three-fold enhancement in efficiency roll-off is realized by such kind of interfacial exciplex host strategy.


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Figure 4. Comparison of s-OLEDs employing the TAPC:PAPTC(20 wt.% PAPTC) (Device A) and pure PAPTC (Device B) as the EML. a) J-V, b) CE-PE-L, c) EQE-L, d) EL spectra of them. The actual host in Device A is the TAPC/TmPyPB interfacial exciplex rather than pure TAPC. The PLQE of the TAPC:PAPTC(20 wt.%)/TmPyPB sample, i.e. the actual EML structure in Device A, is 79.5 %, about two times higher than 36.3% of pure PAPTC, but slightly lower than 92.3 % of TAPC:PAPTC(20 wt.%) film. It is still unclear why the PLQE of TAPC:PAPTC(20 wt.%)/TmPyPB sample is slightly inferior to that of TAPC:PAPTC(20 wt.%). One possible reason lies in different PL transient decay process in both samples, especially because of the long lifetime, singlet/triplet cycling TADF characteristics of TAPC/TmPyPB exciplex host. Anyway, such enhancement in PLQE of EML film structures well explains the device efficiency promotions of Device A with respect to that of Device B. Compared to Device B, the distinct improvement in restraining efficiency roll-off for Device A is contributed to at least two reasons. Firstly, the intrinsic triplet-concentration 11 ACS Paragon Plus Environment


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quenching of PAPTC2 is largely restrained by using TAPC:PAPTC dispersed film structure to replace the pure PAPTC film counterpart, as confirmed by above-mentioned photophysical analysis shown in Figure 2 and Table S1. Secondly, the quenching of triplet excitons on PAPTC emitter may be further prohibited by using TAPC/TmPyPB interfacial exciplex host rather than single host, i.e. TAPC in this work. Previous studies on TADF-type OLEDs using DACT-II TADF emitter by Moon

et

al.

illustrated

TCTA:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine

that

(B3PYMPM)

binary

exciplex host couple can generate more singlet excitons (> 25 %) than the 4,4-N,N-dicarbazole-biphenyl (CBP) traditional single host due to efficient triplet-to-singlet RISC process of the exciplex couple.2 Subsequently, singlet/triplet excitons formed on the exciplex host are efficiently transferred to DACT-II emitter via Förster/Dexter transfer mechanisms. It was found that the RISC rate of DACT-II emitter in the TCTA:B3PYMPM:DACT-II film was increased by 1.5-fold compared to the CBP:DACT-II film, which is beneficial to reduce triplet accumulation on the TADF emitters and thus explained the distinct improvement in device efficiency roll-off characteristics. Similarly, we further compare the photophysical process in the TAPC:PAPTC(20 wt.%) and TAPC:PAPTC(20 wt.%)/TmPyPB structures in together. The RISC rate (kRISC) can be calculated as:2, 26

k RISC =

k p kd Φ d kISC Φ p

(1)

in which the kp, kd and kISC are the prompt, delayed and intersystem crossing decay rates of the PAPTC emitter. The Фd and Фp are PL quantum efficiency of delayed fluorescence component and prompt fluorescence component of the resultant film samples, respectively. The kRISC of PAPTC emitter is thus qualitatively dependent on 12 ACS Paragon Plus Environment

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the Фd/Фp of the corresponding films as demonstrated in formula (1), assuming that the kpkd/kISC of PAPTC emitter in the two samples are the same. A comparison of the TAPC:PAPTC(20 wt.%) and TAPC:PAPTC(20 wt.%)/TmPyPB films well reflects the TADF emission status of the PAPTC emitter in single host (TAPC) and interfacial exciplex host (TAPC/TmPyPB). The calculated ratio of the Фd/Фp is 7.7 for the TAPC:PAPTC(20wt.%) sample as shown in Table S1, while the Фd/Фp is 30.9 for the TAPC:PAPTC(20wt.%)/TmPyPB film deduced from its PL transient decay shown in Figure S6. Accordingly, the kRISC of PAPTC in TAPC:PAPTC(20 wt.%)/TmPyPB structure is enhanced by 4 times as compared to that in TAPC:PAPTC (20 wt.%) structure. The kRISC in the TAPC:PAPTC(20 wt.%) sample is 3.7×106 s-1, assuming independent kRISC on the PAPTC emitter concentration in TAPC:PAPTC films.27 The derived kRISC in the TAPC:PAPTC(20wt.%)/TmPyPB sample is as high as 1.48×107 s-1. Much faster kRISC is thus helpful to lower the concentration-induced accumulation effect of triplet excitons and decrease the possibility of triplet quenching of PAPTC. Accordingly, it is suggested that the combined high PLQE and kRISC for the TAPC:PAPTC(20wt.%)/TmPyPB structure contribute to realize high EQEs and low EQE roll-off performance in the resultant s-OLEDs. In summary, high-performance s-OLED based on TADF polymer, i.e. PAPTC, has been fabricated by employing an interfacial exciplex host of TAPC/TmPyPB to realize high light-emitting efficiency and low efficiency roll-off simultaneously. It demonstrates the satisfied overall EL performance, i.e. a high EQE of 14.9 %, PE of 50.1 lm W-1, and luminance of 26321 cd m-2. More importantly, the efficiency roll-off characteristics is excellent. The L50 and J50 are as high as 15000 cd m-2 and 63.16 mA cm-2, enhanced nearly 3-fold with respect to the s-OLEDs using pure PAPTC as the TADF emitter. We find that the TADF polymer emitter suffers from 13 ACS Paragon Plus Environment


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triplet-concentration quenching seriously in its non-doped film state. This issue is well solved by dispersing it into suitable host such as TAPC, which effectively increases the delayed fluorescence contribution and thus leads to distinct total PLQE enhancement. In-situ formed interfacial exciplex host of TAPC/TmPyPB in TAPC:PAPTC doped s-OLEDs helps to further increase the reverse intersystem crossing rates of triplets in PAPTC emitters and thus favors to improve the efficiency roll-off characteristics. This work provides a novel approach to alleviate roll-off and potentially maximize the EL efficiencies of TADF polymer materials by solving triplet quenching and increasing RISC as much as possible.

■ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:xxxxxx. Experimental section; The calculating parameters and methods on photophysical analysis; The comparisons of EL performance and roll-off of device A and B; Absorption, steady-state and transient PL spectra of various films; J-V analysis on PAPTC transporting properties; Further confirmation on EL driving mechanisms of device A and B.

■AUTHOR INFORMATION Corresponding Authors *(B. Z.) E-mail: [email protected] *(Y. C.) E-mail: [email protected]


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*(Y. Q.) E-mail: [email protected] *(Z. X.) E-mail: [email protected] Notes The authors declare no competing financial interests.

■ACKNOWLEDGMENTS Zhang, Xie and Cheng thank the supports from the National Key Basic Research and Development Program of China (Nos. 2015CB655001, 2014CB643504) founded by MOST, the National Key R&D Program of China (No. 2016YFB0401001) and the National Natural Science Foundation of China (nos. 51325303, 51473162, 51773195, 21334006, 21404101). The financial support of the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200) is also acknowledged.

■REFERENCES (1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238. (2) Moon, C.-K.; Suzuki, K.; Shizu, K.; Adachi, C.; Kaji, H.; Kim, J.-J., Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching. Adv. Mater. 2017, 29, 1606448-1606452.



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(3) Nikolaenko, A. E.; Cass, M.; Bourcet, F.; Mohamad, D.; Roberts, M., Thermally Activated Delayed Fluorescence in Polymers: A New Route Toward Highly Efficient Solution Processable OLEDs. Adv. Mater. 2015, 27, 7236-7240. (4) Luo, J. J.; Gong, S. L.; Gu, Y.; Chen, T. H.; Li, Y. F.; Zhong, C.; Xie, G. H.; Yang, C. L., Multi-Carbazole Encapsulation as a Simple Strategy for the Construction of Solution-Processed, non-Doped Thermally Activated Delayed Fluorescence Emitters. J. Mater. Chem. C 2016, 4, 2442-2446. (5) Nobuyasu, R. S.; Ren, Z.; Griffiths, G. C.; Batsanov, A. S.; Data, P.; Yan, S.; Monkman, A. P.; Bryce, M. R.; Dias, F. B., Rational Design of TADF Polymers Using a Donor-Acceptor Monomer with Enhanced TADF Efficiency Induced by the Energy Alignment of Charge Transfer and Local Triplet Excited States. Adv. Opt.

Mater. 2016, 4, 597-607. (6) Ren, Z.; Nobuyasu, R. S.; Dias, F. B.; Monkman, A. P.; Yan, S.; Bryce, M. R., Pendant Homopolymer and Copolymers as Solution-Processable Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Macromolecules 2016, 49, 5452-5460. (7) Zhu, Y.; Zhang, Y.; Yao, B.; Wang, Y.; Zhang, Z.; Zhan, H.; Zhang, B.; Xie, Z.; Wang, Y.; Cheng, Y., Synthesis and Electroluminescence of a Conjugated Polymer with Thermally Activated Delayed Fluorescence. Macromolecules 2016, 49, 4373-4377. (8) Ban, X. X.; Zhu, A. Y.; Zhang, T. L.; Tong, Z. W.; Jiang, W.; Sun, Y. M., Highly Efficient All-Solution-Processed Fluorescent Organic Light-Emitting Diodes Based 16 ACS Paragon Plus Environment

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Highly Efficient TADF Polymer Electroluminescence with Reduced

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