Radical-Based Organic Light-Emitting Diodes with Maximum External

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Clusters, Radicals, and Ions; Environmental Chemistry

Radical-based Organic Light-Emitting Diodes with maximum external quantum efficiency of 10.6% Zhiyuan Cui, Shaofeng Ye, Lu Wang, Haoqing Guo, Ablikim Obolda, Shengzhi Dong, Yingxin Chen, Xin Ai, Alim Abdurahman, Ming Zhang, Lei Wang, and Feng Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03019 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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The Journal of Physical Chemistry Letters

Radical-Based Organic Light-Emitting Diodes with Maximum External Quantum Efficiency of 10.6% a

Zhiyuan Cui, bShaofeng Ye, aLu Wang, aHaoqing Guo, aAblikim Obolda, aShengzhi Dong,

a

Yingxin Chen, aXin Ai, aAlim Abdurahman, aMing Zhang, bLei Wang*, aFeng Li*

a

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Qianjin Avenue 2699, Changchun 130012, P. R. China b

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan, 430074, P. R. China Corresponding Author *E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

A new luminescent radical, tris-2,4,6-trichlorophenylmethyl-1,5-diazarcarbazole (TTM-DACz), was synthesized and characterized. The photoluminescence quantum yield (PLQY) of TTMDACz in solid 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) matrix film (5 wt.%) is as high as 57.0%. Organic light-emitting diodes (OLEDs) employing TTM-DACz as the emitter were fabricated. By rational designing device structure and host-guest doping system, external quantum eﬃciency (EQE) of up to 10.6% of the optimized device with a red CIE chromaticity of (0.62,0.36) was obtained, which is among the highest values for the red OLEDs using nonphosphorescent materials as the emitters. This work will accelerate the development of luminescent radical materials for high-performance OLEDs.

TOC GRAPHICS


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After several decades of development, organic light emitting diodes (OLEDs) have shown great competitiveness not only in the field of flat-panel displays but also solid-state lighting sources owing to their excellent features such as flexibility, low power consumption, easy to process and so on.1-3 Recently, research on OLEDs has been focused on the improvement of device performance, especially the external quantum efficiency (EQE).4-6 According to the spin statistics of exciton formation under electrical exciton, the ratio of singlet excitons and triplet excitons is 1: 3.7 It is well known that only singlet can radiatively decay and the triplet exciton is non-emissive. Another important factor is the light outcoupling efficiency, which is generally adopted as ~20%. These factors determine the theoretically highest EQE of 5.0 %.8 To utilize of the non-emissive triplet excitons, various approaches have been proposed and realized such as phosphorescent OLEDs (PhOLEDs),9 hybridized local and charge-transfer (HLCT) processes,10,11 triplet-triplet annihilation (TTA)12,13 and thermally activated delayed fluorescence (TADF) materials.14,15 Thanks to these remarkable progresses, the value of internal quantum efficiency (IQE) has been improved largely which is up to 100% theoretically. However, the materials of PhOLEDs are mostly confined to expensive Ir and Pt complexes.16-18 And the materials of other OLEDs need relatively complex molecular design and synthesis. Different with the above OLEDs in which all of the luminescent materials are closed-shell molecules, a novel kind of OLEDs based on stable neutral radicals, a collection of open-shell molecules, was proposed and investigated in our previous work.19 The emission of luminescent radicals typically comes from the radiative decay of doublet excitons, which is spin-allowed. Thus, the upper limit IQE of radical-based OLEDs is theoretically 100 %, circumventing the harvest of triplet excitons.


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Stable organic radicals have attracted much interest in spintronics,20,21 conductive polymer,22 polarizing agents23,24 and organic magnetism25,26 considering their unique open-shell electron structure. However, the species of stable radicals possessing fluorescence are still limited in tris2,4,6-trichlorophenylmethyl (TTM), perchlorotriphenyl methyl (PTM) and biphenylmethyl (BTM) series (Chart 1). Juliáet al27-29 studied the effect of carbazole derivatives on the photochemistry and photophysical properties of TTM series radicals. Lambert and co-workers30,31 synthesized several PTM-cored luminescent radicals. Besides, Nishihara et al32,33 incorporated pyridine derivatives into the TTM core skeleton forming pyridine biphenylmethyl (PyBTM) series radicals. Recently, our group successfully introduced carbazole to BTM skeleton forming (Ncarbazolyl)bis(2,4,6-tirchlorophenyl) methyl radical (CzBTM) for the first time,34 and we investigated the performance of CzBTM doped OLED device at the same time. Chart 1. Classification of luminescent radical systems

To date, radical-based OLEDs are principally concentered on TTM series materials, and the research is still in infancy. According to our previous research, substituents with different electronwithdrawing properties such as carbazole and its derivatives have a certain influence on the molecule photophysical properties and stability, which determine the properties of OLED


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devices.35-37 More research efforts are needed to design and synthesize new compounds with superior properties to further enhance the performance of radical based OLED devices. In this work, a weakly electron donating group 1, 5-diazarcarbazole (DACz) was chosen to replace one of the chlorine atoms on the TTM forming TTM-DACz radical. Then, doped OLEDs using TTMDACz as the emitter were fabricated. Through constantly optimizing the device structure, an OLED shows a maximum EQE as high as 10.6% with a red CIE chromaticity around (0.62, 0.36), which surpasses most red OLEDs using non-phosphorescent materials as emitters. The photoluminescence quantum yields (PLQY) of TTM-DACz doped film achieves 57.0%.

Figure 1. Material characteristics. a) Molecular structure formula of TTM-DACz. b) Steric configuration of TTM-DACz calculated by DFT. c) EPR spectrum of TTM-DACz powder measured at room temperature. d) TGA analysis of TTM-DACz.


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The molecular structure formula of TTM-DACz is shown in Figure 1a, and Figure 1b displays the molecular steric configuration optimized by density functional theory (DFT) calculation. The result of electron paramagnetic resonance (EPR) depicted in Figure 1c confirms the unpaired electron in TTM-DACz. It is worth noted that the method of organic molecular-beam deposition (OMBD) in vacuum chamber was used to fabricate OLEDs, the thermostability of TTM-DACz must guarantee to avoid decomposing during the evaporation process. In view of this, thermogravimetric analysis (TGA) was carried out (see Figure 1d), which shows that the temperature corresponding to a mass loss of 5% reaches up to 225℃, meeting the requirements of OMBD.

Figure 2. Photophysics properties and orbital energy levels. a) Abs and PL spectra of TTM-DACz in dichloromethane solution. b) Frontier molecular orbitals of TTM-DACz (LUMO, SOMO, and HOMO). The absorption (Abs) spectrum along with photoluminescence (PL) spectrum of TTM-DACz in dichloromethane solution are shown in Figure 2a. Figure 2b presents the frontier molecular energy levels (i.e. LUMO, SOMO and HOMO) calculated by DFT using the B3LYP hybrid


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functional, 6-31G (d) basis set. Besides, time-dependent DFT (TDDFT) methods were used to predict the electron transition of TTM-DACz (see supporting information S3). The result shows that the Abs band centered at 565nm should be dominated by the transition from171β to 172β, the electron is excited from DACz unit to TTM part. The PL band centered at 605nm belongs to a transition from 172β to 171β, a kind of charge transfer (CT) emission. Further conﬁrmation of our assignment came from the PL spectra as well as the lifetime of excited states in different solvents (see Figure S1). With the increasing of solvent polarity, 25nm red shift of PL peak was observed and the PL lifetime rose at the same time.

Figure 3. Device performance of the optimized device. a) Diagram of the device structure. b) EL spectra of the device. c) Correlation curves of current density-voltage-luminance (J-V-L). d) EQE values as a function of voltage.


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What needs to be pointed out is that the pure thin film of TTM-DACz is not emissive on the account of aggregation-caused quenching. In view of this, host-guest doping system was chosen to realize the electroluminescence of TTM-DACz. The schematic energy band diagram of the fabricated OLED with a structure of ITO/MoO3 (3 nm)/TAPC (70 nm)/TTM-DACz: TPBi (x wt. %, 40 nm)/PO-T2T (70 nm)/LiF (0.8 nm)/Al (100 nm), where x is equal to 1, 2.5, 5, 7.5, 10, is shown in Figure 3a. TAPC, POT2T and TPBi play the roles of hole-transporting, electrontransporting and doping-host materials respectively. Molecular structures of the compounds used in the devices are shown in Chart 2. Combining the oxidation peak of cyclic voltammetry (CV) curve (Figure S2) with the optical gap from the edge of absorption spectrum, the energy levels of TTM-DACz were acquired to be -5.3 and -3.3 eV. Chart 2. Molecular structure of the compounds used in doped OLEDs

The performances of the radical-based OLEDs are summarized in table 1. The maximum EQE reaches up to 10.6% when the doping concentration of TTM-DACz is 5wt.%. Figure 3b shows the EL spectra of the optimized device at various voltages. The EL peaks are located at 608nm, and the intensity increases gradually accompanied by the voltage. Figure 3c displays the


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current density-voltage and luminance-voltage characteristics of the optimized OLED, and Figure 3d depicts the EQE values of the optimized devics as a function of voltage. The maximum EQE of 10.6% at 6.5 V is achieved, breaking the theoretical limit of 5.0%. This is mainly because the introduction of the weak electron donor substituent DACz improving the transmission efficiency of electrons so that the more balance of the population of hole and electron in the light emitting layer. Besides, the PLQY of TTM-DACz doped (5 wt.%) TPBi film reaches up to 57.0%. Table 1 Summary of the properties of the TTM-DACz based OLED Emitter

TTM-DACz: TPBi

wt. (%) a

Turn on voltage (V)b

Luminance (cd/m2) max

EQEmax (%) c

EL peak (nm)d

1

6

26.53

1.3

608

2.5

6.5

38.11

5.6

608

5

6.5

32.12

10.6

608

7.5

6

48.46

9.1

608

10 6 42.15 5.9 608 a Doping Concentration; b Visible to naked eye; c Max value of the external quantum efficiency; d Electroluminescence peak In summary, as part of a research effort directed to enhance the performance of OLEDs based on luminescent radical materials, we successfully introduced a weak electron donor substituent DACz to TTM synthesizing TTM-DACz radical. The EPR spectra, thermal stability, electrochemical, and photophysical properties of TTM-DACz were studied. OLEDs based on TTM-DACz were fabricated, and a maximum EQE of 10.6% was obtained, which surpassed most values of red OLEDs with non-phosphorescent materials as emitters currently. The results suggest that the introduction of weak electron-donating group DACz to TTM is advantageous for improving the device efficiency despite its detrimental to molecular and device stability to some extent. This work will contribute to the design of new TTM series materials with better electrontransport properties as well as thermal-stability so that the performance of OLEDs based on TTM


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series radicals will be further improved.

ATHOUR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (no. 51673080, 51573065) and National Key Basic Research and Development Program of China (Grant No. 2015CB655003). Supporting Information Available: < General information of experimental section; Details of the synthesis; OLED fabrication and testing; Photophysical and electrochemical properties; DFT predicted data>

REFERENCES (1) Tang, C. W.; Vanslyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1998, 51, 913-915. (2) Xu, R.; Li, Y.; Tang, J. Recent Advances in Flexible Organic Light-Emitting Diodes. J. Mater. Chem. C 2016, 4, 9116-9142. (3) Lee, S. M.; Kwon, J. H.; Kwon, S.; Choi, K. C. A Review of Flexible OLEDs toward Highly Durable Unusual Displays. IEEE Trans. Electron Devices 2017, 64, 1922-1931.


10

Page 11 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Zhang, Q.; Tsang, D.; Kuwabara, H.; Hatae, Y.; Li, B.; Takahashi, T.; Lee, S. Y.; Yasuda, T.; Adachi, C. Nearly 100% Internal Quantum Efficiency in Undoped Electroluminescent Devices Employing Pure Organic Emitters. Adv. Mater. 2015, 27, 2096-2100. (5) Udagawa, K.; Sasabe, H.; Igarashi, F.; Kido, J. Simultaneous Realization of High EQE of 30%, Low Drive Voltage, and Low Efficiency Roll‐Off at High Brightness in Blue Phosphorescent OLEDs. Adv. Opt. Mater. 2016, 4, n/a-n/a. (6) Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R. Excitonic Singlet-Triplet Ratio in a Semiconducting Organic Thin Film. Phys. Rev. B 1999, 60, 14422-14428. (7) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151-154. (8) Gu, G.; Garbuzov, D. Z.; Burrows, P. E.; Venkatesh, S.; Forrest, S. R.; Thompson, M. E. High-External-Quantum-Efficiency Organic Light-Emitting Devices. Opt. Lett. 1997, 22, 396-398. (9) Yang, X.; Zhou, G.; Wong, W. Y. Functionalization of Phosphorescent Emitters and Their Host Materials by Main-Group Elements for Phosphorescent Organic Light-Emitting Devices. Chem. Soc. Rev. 2015, 44, 8484-8575. (10) Li, W.; Pan, Y.; Xiao, R.; Peng, Q.; Zhang, S.; Ma, D.; Li, F.; Shen, F.; Wang, Y.; Yang, B.; et al. Employing ∼100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and Charge-Transfer Excited State. Adv. Funct. Mater. 2014, 24, 1609-1614. (11) Li, W.; Pan, Y.; Yao, L.; Liu, H.; Zhang, S.; Wang, C.; Shen, F.; Lu, P.; Yang, B.; Ma, Y. A Hybridized Local and Charge-Transfer Excited State for Highly Efficient Fluorescent OLEDs: Molecular Design, Spectral Character, and Full Exciton Utilization. Adv. Opt. Mater. 2014, 2, 892-901 (12) Rothe, C.; Monkman, A. P. Triplet Exciton Migration in a Conjugated Polyfluorene. Phys. Rev. B 2003, 68, 338-344.


11


Page 12 of 14

(13) Chiang, C. J.; Kimyonok, A.; Etherington, M. K.; Griffiths, G. C.; Jankus, V.; Turksoy, F.; Monkman, A. P. Ultrahigh Efficiency Fluorescent Single and Bi-Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion. Adv. Funct. Mater. 2013, 23, 739-746. (14) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic LightEmitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238. (15) Chen, X. K.; Tsuchiya, Y.; Ishikawa, Y.; Zhong, C.; Adachi, C.; Brédas, J. L. A New Design Strategy for Efficient Thermally Activated Delayed Fluorescence Organic Emitters: From Twisted to Planar Structures. Adv. Mater. 2017, 29, 1702767. (16) Wong, W. Y.; Ho, C. L. Functional Metallophosphors for Effective Charge Carrier Injection/Transport: New Robust OLED Materials with Emerging Applications. J. Mater. Chem. 2009, 19, 4457-4482. (17) Zhou, G.; Wong, W. Y.; Suo, S. Recent Progress and Current Challenges in Phosphorescent White Organic Light-emitting Diodes (WOLEDs). J. Photochem. Photobio. C: Photochem. Rev. 2010, 11, 133-156. (18) Ho, C. L.; Wong, W. Y. Charge and Energy Transfers in Functional Metallophosphors and Metallopolyynes. Coord. Chem. Rev. 2013, 257, 1614-1649. (19) Peng, Q.; Obolda, A.; Zhang, M.; Li, F. Organic Light-Emitting Diodes Using a Neutral Pi Radical as Emitter: The Emission from a Doublet. Angew. Chem., Int. Ed. 2015, 54, 70917095. (20) Lee, E. C.; Choi, Y. C.; Kim, W. Y.; Singh, N. J.; Lee, S.; Shim, J. H.; Kim, K. S. A Radical Polymer as a Two-Dimensional Organic Half Metal. Chem.-Eur. J. 2010, 16, 12141-12146. (21) Zhang, J.; Yao, K. L. Radical Based Molecular Transport System as Good Molecular Spintronics Device Predicted by First-Principles Study. Comput. Mater. Sci. 2017, 133, 9398. (22) Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W. A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science 2018, 359, 1391-1395.


12

Page 13 of 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Song, C.; Hu, K. N.; Joo, C. G.; Swager, T. M.; Griffin, R. G. TOTAPOL: A Biradical Polarizing Agent for Dynamic Nuclear Polarization Experiments in Aqueous Media. J. Am. Chem. Soc. 2006, 128, 11385. (24) Mao, J.; Akhmetzyanov, D.; Ouari, O.; Denysenkov, V.; Corzilius, B.; Plackmeyer, J.; Tordo, P.; Prisner, T. F.; Glaubitz, C. Host-Guest Complexes as Water-Soluble High-Performance DNP Polarizing Agents. J. Am. Chem. Soc. 2013, 135, 19275-81. (25) Ohira, S.; Nakayama, K.; Ise, T.; Ishida, T.; Nogami, T.; Watanabe, I.; Nagamine, K. Anomalous Magnetism in Organic Radical Ferromagnets 4-Arylmethyleneamino-2,2,6,6tetramethylpiperidin-1-yloxyl Just above T C Studied by the μSR Method. Polyhedron 2001, 20, 1223-1227. (26) Taku Matsumura.; Hideki Tanaka.; Katsumi Kaneko.; Masako Yudasaka.; Sumio Iijima.; Hirofumi Kanoh. Magnetism of Organic Radical Molecules Confined in Nanospace of SingleWall Carbon Nanohorn. J. Phys. Chem. C 2007, 111, 10213-10216. (27) Castellanos, S.; Gaidelis, V.; Jankauskas, V.; Grazulevicius, J. V.; Brillas, E.; LopezCalahorra, F.; Julia, L.; Velasco, D. Stable Radical Cores: A Key for Bipolar Charge Transport in Glass Forming Carbazole and Indole Derivatives. Chem. Commun. 2010, 46, 5130-5132. (28) Fajarí,L.; Papoular, R.; Reig, M.; Brillas, E.; Jorda, J. L.; Vallcorba, O.; Rius, J.; Velasco, D.; Juliá, L. Charge Transfer States in Stable Neutral and Oxidized Radical Adducts from Carbazole Derivatives. J. Org. Chem. 2014, 79, 1771-1777. (29) Velasco, D.; Castellanos, S.; López, M.; López-Calahorra, F.; Brillas, E.; Juliá, L. Red Organic

Light-Emitting

Radical

Adducts

of

Carbazole

and

Tris(2,4,6-

trichlorotriphenyl)methyl Radical that Exhibit High Thermal Stability and Electrochemical Amphotericity. J. Org. Chem. 2007, 72, 7523-7532. (30) Heckmann, A.; Dümmler, S.; Pauli, J.; Margraf, M.; Köhler, J.; Stich, D.; Lambert, C.; Fischer, I.; Resch-Genger, U. Highly Fluorescent Open-Shell NIR Dyes: The Time-Dependence of Back Electron Transfer in Triarylamine-Perchlorotriphenylmethyl Radicals. J. Phys. Chem. C 2009, 113, 20958-20966.


13


Page 14 of 14

(31) Heckmann, A.; Lambert, C.; Goebel, M.; Wortmann, R. Synthesis and Photophysics of a Neutral Organic Mixed-Valence Compound. Angew. Chem. Int. Ed. 2010, 43, 5851-5856. (32) Hattori, Y.; Kusamoto, T.; Nishihara, H. Luminescence, Stability, and Proton Response of an Open-Shell (3,5-Dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl Radical. Angew. Chem., Int. Ed. 2015, 126, 12039-12042. (33) Kusamoto, T.; Kimura, S.; Ogino, Y.; Ohde, C.; Nishihara, H. Modulated Luminescence of a Stable Open-Shell Triarylmethyl Radical: Effects of Chemical Modification on Its Electronic Structure and Physical Properties. Chem. Eur. J. 2016, 22, 17725-17733. (34) Ai, X.; Chen, Y.; Feng, Y.; Li, F. A Stable Room-Temperature Luminescent Biphenylmethyl Radical. Angew. Chem., Int. Ed. 2018, 57. (35) Obolda, A.; Ai, X.; Zhang, M.; Li, F. Up to 100% Formation Ratio of Doublet Exciton in Deep-Red Organic Light-Emitting Diodes Based on Neutral Pi-Radical. ACS Appl. Mater. Interfaces 2016, 8, 35472-35478. (36) Gao, Y.; Obolda, A.; Zhang, M.; Li, F. A Pure Red Organic Light-Emitting Diode Based on a Luminescent Derivative of Tris(2,4,6-trichlorotriphenyl)methyl Radical. Dyes Pigm. 2017, 139, 644-650. (37) Gao, Y.; Xu, W.; Ma, H.; Obolda, A.; Yan, W.; Dong, S.; Zhang, M.; Li, F. Novel Luminescent

Benzimidazole-Substituent

Tris(2,4,6-trichlorophenyl)methyl

Radicals:

Photophysics, Stability, and Highly Efficient Red-Orange Electroluminescence. Chem. Mater. 2017, 29 , 6733-6739.


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Radical-Based Organic Light-Emitting Diodes with Maximum External

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