Highly Efficient Fluorescent Organic Light-Emitting Devices Using a

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Clusters, Radicals, and Ions; Environmental Chemistry

Highly Efficient Fluorescent Organic Light-Emitting Devices Using a Luminescent Radical as the Sensitizer Yingxin Chen, Lin Yang, Yan Huang, Ablikim Obolda, Alim Abdurahman, Zhiyun Lu, and Feng Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03410 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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The Journal of Physical Chemistry Letters

Highly Efficient Fluorescent Organic Light-Emitting Devices Using a Luminescent Radical as the Sensitizer aYingxin

Chen, bLin Yang, bYan Huang, aAblikim Obolda, aAlim Abdurahman, bZhiyun Lu*,

aFeng

Li*

aState

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

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

of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu, 610064, P. R.

China Corresponding Author *E-mail: ([email protected];[email protected] )

ACS Paragon Plus Environment

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ABSTRACT

In traditional fluorescent organic light-emitting diodes (OLEDs), the upper limit of internal quantum efficiency (IQE) is only 25% because 75 % triplet excitons created on the fluorescent dyes are non-luminous. Here, luminescent radicals are proposed as the sensitizer. Under ideal conditions, electrons and holes firstly recombine on the sensitizer molecule to create doublet excitons, then through energy transfer to generate singlet excitons on the fluorescent dye, finally via radiative decay to emit light. The upper limit of IQE can theoretically reach 100%. As an example, the maximum external quantum efficiency (EQE) of a fluorescent OLED sensitized by a luminescent radical, TTM-1Cz, has reached 8.1%, which is much higher than the upper limit of EQE of traditional fluorensenct OLEDs. Our results suggest a new route to realize highly efficient fluorescent OLEDs.

TOC GRAPHICS


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KEYWORDS doublet; luminescent radical; organic light-emitting device; sensitize; deep-red Organic light emitting diodes (OLEDs) have attracted much attention during the past three decades because of their special features, such as easy processing, lightweight, and flexibility.1,2Although facing the intense competition from other kinds of LEDs, they are the only ones that have been commercially used as the active-luminescent displays in the market. In OLEDs, the ratio of singlet to triplet excitons is expected to be 1:3 under electrical excitation according to the spin statistics.3,4 Generally, the transition of triplet excitons to the ground state is spin-forbidden, thus, triplet harvesting is always the focus of the research of OLEDs. Several strategies have been proposed to directly/indirectly utilizing triplet or avoid the creation of triplet.5-11 Besides the methods harvesting the triplet of emitters, the method harvesting both singlet and triplet of host then transferring the energy to the singlet of fluorescent dopant has also been proposed, which is termed as sensitizing. Forrest et al. used a phosphorescent molecule and Duan et al. used thermally activated delayed fluorescence (TADF) molecules as the sensitizers to excite the fluorescent dyes,12,13 almost 100% singlet exciton were created in the fluorescent dopants. One advantage of this kind of devices is that it increases the upper limit of internal quantum efficiency (IQE) of conventional fluorescent dopants-based devices to 100 % which are abundant and cheap.


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Figure 1. Schematic diagram for the sensitizing processes. DR stands for doublet radiation, FR for fluorescence radiation, and ET for energy transfer.

Here, another sensitizing route is proposed through utilizing a luminescent radical whose emission comes from the radiative decay of doublet excitons as the sensitizer. We have verified that the formation ratio of doublet exciton in radical-based OLEDs could reach 100 %.14 If there is no efficiency loss in the energy transfer process from the sensitizer to the fluorescent dopant, the upper limit of IQE of this kind of fluorescent OLEDs would be 100 %. Figure 1 shows the sensitizing processes. Under ideal conditions, electron-hole


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recombination creates doublet excitons on the sensitizer, then through energy transfer to create singlet excitons in the fluorescent dopant, finally via radiative decay to emit light. An open-shell molecule, (4-Ncarbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)-methyl (TTM-1Cz),15-16 was used as the sensitizer in the light-emtting layer, and 2-[4-(N-butyl-Nphenylamino)-2,6-dihydroxyphenyl] ‐ 4 ‐ [(4 ‐ (N-butyl-N-phenylamino)-2,6dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP)17 was chosen as the fluorescent emission dopant. The molecular structures of the two compounds are shown in Figure2 a. The maximum external efficiency (EQE) of a deep-red device based on this sensitizing route has achieved 8.1 %, which is much higher than the theoretic upper limit (5 %)18

of

traditional

fluorescent

devices.

Figure2. a) The molecular structures of TTM-1Cz and SQ-BP. b) The absorption spectrum of SQ-BP and PL spectra of TTM-1Cz (excitation at 375 nm) and SQ-BP (excitation at 637 nm) measured in normal hexane


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The absorption and PL spectra of SQ-BP and PL spectrum of TTM-1Cz are shown in Figure 2b. In hexane solution, the emission peak of SQ-BP is 676 nm, indicating the deep red emission. There is a large overlap between the emission of TTM-1Cz and the absorption of SQBP, which means that an effective Förster-type energy transfer from TTM-1Cz to SQ-BP can be forecasted. We have performed the study about the energy transfer between them in films before preparing of OLEDs. Because both TTM-1Cz and SQ-BP have serious aggregation-caused quenching (ACQ) problem, the hydrogenated precursor molecule of TTM-1Cz, [4-(Ncarbazole)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methane (αHTTM-1Cz), was chosen as the host matrix. The similar molecular structures between TTM-1Cz and αHTTM-1Cz (only one hydrogen atom difference) help TTM-1Cz uniformly disperse into the host. αHTTM-1Cz has a band gap of 3.6 eV which is much larger than those of TTM-1Cz and SQ-BP. So αHTTM-1Cz just plays a role of diluting matrix. We put 2.5 wt.% TTM-1Cz and 97.5 wt.% αHTTM-1Cz in methylene dichloride, and then stirred them to mix them uniformly. After evaporating the solvent, the mixture powder was obtained, which is termed as TTM-1Cz-mix (2.5 wt.%). The component ratio of TTM-1Cz in the mixture powder before and after vacuum evaporation has been confirmed to remains stable.14 We used this mixture as one evaporating source to co-deposit with SQ-BP in the vacuum chamber to acquire the doped films in this work.


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Figure 3. (a) The PL spectra of the TTM-1Cz-mix: SQ-BP films, weight ratio of SQ-BP means WeightSQ-BP / (WeightSQ-BP + WeightTTM-1Cz), the excitation wavelength is 375 nm (b) The PL transient decay curves of the TTM-1Cz-mix: SQ-BP films monitored at 666nm. the excitation wavelength is 375 nm.

Figure 3a shows the PL spectra of the films of TTM-1Cz-mix and TTM-1Cz-mix: x wt.% SQ-BP (x:WeightSQ-BP / (WeightSQ-BP + WeightTTM-1Cz) = 0.5, 1.0, 2.0, 5.0 %). Because the emission of αHTTM-1Cz locates in UV region and there is no visible emission of αHTTM1Cz,14 the spectrum of TTM-1Cz-mix should be the emission of TTM-1Cz in film peaking at 666 nm. As SQ-BP is doped into the films and its concentration increases, the spectra become red-shifted and narrowed, which means the emission component of TTM-1Cz decreases while that of SQ-BP increases, thus there exists energy transfer from TTM-1Cz to SQ-BP. Figure 3b shows the transient PL decay curves of TTM-1Cz-mix: x wt.% SQ-BP films. With


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the concentration of SQ-BP increasing, the lifetime of the emission of TTM-1Cz gradually reduces, suggesting the efficient energy transfer. The film of TTM-1Cz-mix shows mono-exponential decay with a lifetime of 42.4 ns, while the films of TTM-1Cz-mix: SQ-BP displays multi-exponential decays which is fitted by

A1exp(-t/)+A2exp(-t/). The average lifetime is calculated by 1/τ=A1/((A1+A2) × τ1)+ A2/((A1+A2) ×τ2) ). Thus, energy transfer efficiencies (ΦET) can be obtained as:

1

(1)

τ0 = kr + knr 1

(2)

τavg = kr + knr + kET kET

ФET = kr + knr + kET =

𝜏0 ― 𝜏avg

(3)

𝜏0

Where τ0 is the lifetime of the film of TTM-1Cz-mix and τavg is the average lifetime of the film of TTM-1Cz-mix: SQ-BP, Kr is the radiative rate and Knr is the non-radiative rate, KET is the energy transfer rate. All the lifetimes, rates and energy transfer efficiencies are summarized in Table 1. As can be seen, when the concentration of SQ-BP is higher than 1.0 wt%, the energy transfer efficiencies are larger than 63.1%, indicating efficient energy transfer from host to guest occurs, which means the sensitizing process would be the dominant pathway of the emission of the guest.


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Figure 4 (a) EL spectra of the devices with different doping ratio. (b) External quantum efficiency versus current density with different doping ratio. (c) Brightness versus driving voltage with different doping ratio.

OLEDs were fabricated with the architecher of ITO/MoO3 (3 nm) / Di-[4-(N,N-ditolylamino) -phenyl] cyclohexan (TAPC) (50 nm)/ SQ-BP: TTM-1Cz-mix (1.0, 2.0 and 5.0 wt. %,


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40 nm)/ 2,4,6-tris[m-(diphenylphosphinoyl)phenyl]-1,3,5-triazine (PO-T2T, 30 nm) / LiF (0.8 nm) / Al (100 nm), where TAPC, TTM-1Cz-mix, and PO-T2T are employed as holetransporting, host, and electron-transporting materials, respectively. Figure 4a displays the electruluminescent (EL) spectra of the devices with various SQ-BP concentrations at 7V. As the concentration of SQ-BP increases from 0 to 5.0 wt.%, the peak of the spectra shifts from 668 to 684 nm and the spectra also becomes narrowed, which suggests the contribution of TTM-1Cz emisson decreases while that of SQ-BP increases, thus the energy transfer from TTM-1Cz to SQ-BP happens. The positions and shapes of the EL spectra of these devices remain stable as the voltage increases from 7V to 10V, as shown in Figure S1a, b and c, respectively. Figrue 4b shows the external quantum efficiency (EQE) curves as a function of current density of the devices. The maximum EQEs of the devices with SQ-BP concentration of 1.0, 2.0 and 5.0 wt.% are 7.3%, 8.1% and 6.3%, respectively, which are all much higher than the upper limit of 5% of the fluorensenct OLEDs,17 The device with SQ-BP concentration of 2.0 wt.% has the highest maximum EQE. It is the tradeoff result between the energy transfer efficiency and ACQ. Figure 4c depicts the brightness curves of the devices as a function of voltage. The turn-on voltages for the devices are around 6 V, and the device with SQ-BP concentration of 2.0 wt.% has the largest value of 16.5 cd/m2. The current density-voltage characteristics of the devices are shown in Figure S2. Table 1: The life-times of the PL decay curves of TTM-1Cz Films

A1

τ1

A2

τ2

τavg

kET

kr+knr

ΦET

TTM-1Cz-mix：


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wt%SQ-BP

(ns)

(ns)

(ns)

(s-1)

(s-1)

(%)

0.0

1

42.4

42.4

0.5

0.48

10.6

0.52

36.5

23.9

1.7×107

1.0

0.54

7.0

0.46

25.4

15.5

4.1×107

2.0

0.70

5.1

0.30

21.2

9.9

7.6×107

76.0

5.0

0.84

2.7

0.16

22.7

5.8

1.5×108

88.2

40.5 2.4×107

63.1

The average lifetimes are calculated according to equation 1/τavg=A1/((A1+A2) × τ1)+ A2/((A1+A2) ×τ2

In summary, a luminescent radical with the doublet excited state, TTM-1Cz, was used as the sensitizer. Effective energy transfer from the sensitizer to the fluorescent emitter, SQ-BP, was verified. The maximum EQE of a fluorescent OLED sensitized by TTM-1Cz has reached 8.1%, which is much higher than the upper limit of EQE of traditional fluorensenct OLEDs. Our results suggest OLEDs using traditional fluorescent dyes as emitters are expected to have 100 % upper limit of IQE through the luminescent radical sensitizing route, in which heavy metal complexes are not needed.

ACKNOWLEDGMENT We are grateful for the National Natural Science Foundation of China (Grant No. 51673080, 21672156) and National Key Basic Research and Development Program of China (973 program，Grant No. 2015CB655003) Founded by MOST.


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Published online: ((will be filled in by the editorial staff)) Supporting Information Available OLED fabrication and testing date. REFERENCES (1) C. Tang, S. VanSlyke, Organic Electroluminescent Diodes, Appl. Phys. Lett. 1987, 51, 913915. (2) J. Burroughes, D. Bradley, A. Brown, R. Marks, K. Mackay, R. Friend, P. Burns, A. Holmes, Light-emitting Diodes Based on Conjugated Polymers, Nature. 1990, 347, 539-541. (3) M. Pope, C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford University Press, Oxford 1999. (4) M. Segal, M. Baldo, R. Holmes, S. Forrest, Z. Soos, Excitonic Singlet-triplet Ratios in Molecular and Polymeric Organic Materials, Phys. Rev. B. 2003, 68, 075211. (5) Y. Ma, H. Zhang, J. Shen, C. Che, Electroluminescence from Triplet Metal—ligand Chargetransfer Excited State of Transition Metal Complexes, Synth. Met. 1998, 94, 245 –248. (6) M. Baldo, D. OÏBrien, Y. You, A. Shoustikov, S. Sibley,M. Thompson, S. Forrest, Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices, Nature 1998, 395, 151 – 154.


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Highly Efficient Fluorescent Organic Light-Emitting Devices Using a

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