Achieving 20% External Quantum Efficiency for Fully Solution

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Achieving 20% External Quantum Efficiency for Fully Solution Processed Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Dendrimers with flexible chains Dan Liu, Wenwen Tian, Yingli Feng, Xusheng Zhang, Xinxin Ban, Wei Jiang, and Yueming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22662 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Achieving 20% External Quantum Efficiency for Fully Solution Processed Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Dendrimers with flexible chains Dan Liu, † Wenwen Tian, † Yingli Feng, † Xusheng Zhang, † Xinxin Ban, † ‡ Wei Jiang*† and Yueming Sun† †

Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, Jiangsu Engineering

Laboratory of Smart Carbon-Rich Materials and Device, School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, China ‡

Jiangsu Key Laboratory of Function Control Technology for Advanced Materials, School of

Chemical Engineering, Huaihai Institute of Technology, Lianyungang, Jiangsu 222005, China * To

whom correspondence should be addressed. Emails: [email protected]

KEYWORDS: Organic light-emitting diodes (OLEDs), fully solution processed, non-doped, flexible chains, thermally activated delayed fluorescence (TADF)

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ABSTRACT

Actualizing high-efficiency thermally activated delayed fluorescent (TADF) organic lightemitting diodes (OLEDs) with fully wet processes is of great significance to the development of purely organic electroluminescence and the application of large-area OLED displays. Herein, new strategies are proposed to develop the TADF dendrimers with tunable colors by adjusting the way of linking branches to core and the numbers of peripheral branches. Due to an energy gradient and efficient exciton utilization in core-dendrons system, the solution processed OLEDs with the four dendrimers 5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz and 5CzBN-2Cz all give rise to low turn-on voltage and great device efficiency. Notably, 5CzBN-2Cz affords record-high fully solution-processed TADF OLEDs with external quantum efficiency of above 20%, which is significantly comparable to efficiency of TADF OLEDs based on vacuum deposition. The work offers a guideline for designing solution processable materials, paving the way toward practical applications of large area fully solution processed OLEDs.

INTRODUCTION

Organic light-emitting diodes (OLEDs) are being developed as a commercial technology for next-generation flat-panel displays and lighting because they offer the competitive strength of wide viewing angles, enhanced brightness, high response speed and light weight. Vacuum deposited OLEDs have been proved to be highly efficient by constructing the stacks of several organic functional layers.1 However, because of confined spaces and high operating costs of vacuum chambers, large-area fabrication and wide production are still limited. Therefore, the development of solution processed OLEDs has attracted extensive attention in the academia and industry due to high material utilization rate, simple manufacturing process and easy


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operation.2 Nonetheless, it is still a huge challenge to achieve a multilayer device layout because the solution deposition in subsequent layers inevitably re-dissolves or destroys the subjacent layers, thereby leading to much poorer device performance than vacuum-deposited counterparts.3 To break this bottleneck, there have been great efforts in developing fully solution processed OLEDs, such as thermos- or photo-crosslinking,4-6 insoluble interfacial layers, intermediate liquid buffer layers,7 sol-get reactions,8 orthogonal solvents etc.9-10 Among them, orthogonal solvent processing is a favorable way to obtain multilayer structure in solution-processed OLEDs. Then a series of materials have been successfully exploited as TADF emitters to make solution-processed devices feasible by orthogonal solvent processing, which include low molecular weight materials, dendrimers and polymers.11 However, admittedly, the device efficiencies are lower than those of vacuum-deposited devices due to solubility, low exciton utilization efficiency, poor morphology of films and so on. The efficient OLED materials used for solution processes still urgently need development. Several conditions could be satisfied: (1) good high-solubility in ordinary organic solvents, sufficient resistance to various alcohols applied to processing upper layers, (2) high quality and uniform films with free pinhole, restricted aggregation in the wet process and thermal stabilities during the device operation, and (3) efficient exciton utilization (nearly 100%) and great device performance, which points to the TADF materials. Dendrimers generally have high purity, absolute molecular weight, no structural defects, simple preparation technology and no batch variability, which can be superior over conventional polymers.12,13 So dendrimers are considered as one of the novel ideal emissive materials for wet processes. The first novel TADF dendrimers with carbazole dendrons and a triazine core were designed and synthesized by Yamamoto et al. The solubility of G2, G1 dendrimers was significantly greater than G1 dendrimers, and it provides researchers an effective


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way to design novel TADF dendrimers used for wet process.12 Furthermore, Yang et al. proposed a strategy for the preparation of solution-processed TADF emitters, which named multi-carbazole encapsulation. This method is beneficial to improve the ability of hole injection and transportation, and minimize the quenching effect of concentration.14 Besides, several selfencapsulated TADF molecules with alkyl chains were initially synthesized and fully characterized by our groups.15 The improved electroluminescent efficiencies revealed that the design method is promising for all non-doped solution-processed devices. Judging from these preceding studies, we speculate that the core-dendrons system has the potential to promote the development of solution-processable TADF dendrimers. To the best of our knowledge, alkyl chains are the most common solubilizing groups.16 This is because, firstly the total interaction energy between materials and solvent increased with the additional alkyl chains; secondly, the arrangement of molecules in a solid state may be disrupted and the interactions between π-conjugated systems are reduced due to the vibrational motions of alkyl chains.17 So it is a promising and facile strategy that linking the peripheral branches to the chromophore core by alkyl chains.18-20 However, our previous designed molecular resulted in red shift and only small increases in efficiency. In this study, new strategies for the design of coredendrons TADF dendrimers were presented to achieve bluish-green emission and high device efficiency simultaneously. Two approaches to modifying the TADF core are used: one approach is to link flexible branches to core by lithium-halogen exchange which can avoid the addition of oxygen and effect on the energy gap; the other approach is to increase branches which can encapsulate the emitter, decrease the aggregation-caused quenching (ACQ) effect and then improve the device efficiency. We designed a series of the dendritic TADF emitters employing 2,3,4,5,6-penta (9H-carbazol-9-yl) benzonitrile (5CzBN) as a fluorescent core and flexible


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aliphatic chains with carbazoles as dendrons, and modifying the 5CzBN with the different peripheral functional groups resulted in color tunable emission from bluish-green to yellowgreen. The device performances of fully solution-processed OLEDs were found to be highly enhanced and achieving maximum external quantum efficiency of 20.41%. And it is the recordbreaking efficiency based on all-solution-processed organic multilayer system among the TADFOLEDs so far.

RESULTS AND DISCUSSION

Materials and Synthesis. The four dendrimers were easily prepared by the nucleophilic substitution reaction between the pentafluoro benzonitrile and the carbazole derivatives. The commercial source of the chemical materials and device fabrication procedure are given in detail in the Supporting information. Scheme 1 shows the synthetic route and chemical structure of the dendrimers. Before device fabrication, the target product was separated and purified by silica gel column chromatography and recrystallization to give yellow powders. 1H NMR, 13C NMR, mass spectrometry, and elemental analysis were used to confirm the chemical structure of the new compounds.


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R1 H

+

F

F

F

F

R1

O-hexyl-Cz=

hexyl-Cz=

CN

CN

N

R2

R2

NaH, THF, 25oC

N

R2

N

R1

3

3

R1

N

R2

N

F

O

N

N

R1

R2

N

R1

5CzBN-O-Cz: 5CzBN-O-2Cz: 5CzBN-Cz: 5CzBN-2Cz:

R1= H, R1= O-hexyl-Cz, R1= H, R1= hexyl-Cz,

R2

R2= R2= R2= R2=

O-hexyl-Cz O-hexyl-Cz hexyl-Cz hexyl-Cz

Scheme 1. Synthetic route to 5CzBN series compounds. Computational investigations. In order to gain insights into the fundamental photophysical properties of the four 5CzBN series dendrimers, the energy levels of frontier orbital and electronic cloud distribution were calculated by time-dependent density functional theory (TDDFT) at B3LYP/6-31G (d) level. All calculated frontier orbital energy levels, energies of excited states, corresponding assignments of each frontier orbital transition and ΔEst are listed in Table S1 (Supporting Information). Figure 1 and Figure S1 show the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 5CzBN and these four molecules on their optimized geometries. The Figure clearly suggests that the LUMOs of these molecules are distributed primarily over the benzonitrile groups, whereas, the HOMOs are delocalized on the carbazolyl units of the emissive core. One difference is that the HOMOs of 5CzBN-O-Cz and 5CzBN-O-2Cz extended to the electron-donating methoxyl at 3-position carbazole. Compared with 5CzBN (-5.54 eV), the HOMO levels of 5CzBN-Cz (-5.27 eV) and 5CzBN-2Cz (-5.30 eV) are shallower because of the increasing electron donating ability of peripheral carbazole, and not only that, the HOMO levels of 5CzBN-O-Cz (-5.26 eV) and


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5CzBN-O-2Cz (-5.10 eV) are much shallower because of introducing of oxygen atom at emissive cores. It can be therefore concluded that more carbazole dendrons and oxygen atoms can enhance the electron donating ability, decrease the levels of HOMO, but hardly change the LUMO.

Figure 1. Optimized geometries, electron cloud distribution maps and calculated HOMO and LUMO energy levels for 5CzBN, 5CzBN-2Cz and 5CzBN-O-2Cz. For all four emitters, the major assignments for the S0→S1 and S0→T1 transition were not only HOMO→LUMO, but also HOMO-n→LUMO (Table S1). It is difficult to investigate the characteristics of electron excitation visually via certain orbitals. Further, in order to describe accurately the electron transition and investigate the characteristics of TADF, the highest occupied natural transition (HONTO) and lowest unoccupied natural transition orbital (LUNTO) has been particularly presented in Figure S2. At singlet and triplet excited states, the four dendrimers both exhibit conspicuous separation between the HONTO and LUNTO at their corresponding electron-donating and withdrawing subunits, analysis of the NTOs apparently demonstrates that the S0→S1 and S0→T1 transition are mainly comprised of charge transfer (CT) transition, they all have the characteristics of TADF. Meanwhile, the S1 state of these materials


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can be identified as 1CT states, and the T1 state are 3CT. But the LUNTO and HONTO of the S0→T1 are not totally separated, especially for 5CzBN oxygenic series. The HONTO extended to the electron-withdrawing benzonitrile. Subsequently, the calculated T1 energy of 5CzBN-O-2Cz and 5CzBN-2Cz is significantly lower than that of the other two, and the ΔEst of all dendrimers (5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz and 5CzBN-2Cz) was calculated as 0.12 eV, 0.09 eV, 0.18 eV and 0.17 eV, respectively. This implies that the introducing oxygen atoms at the fluorescent core and the number of carbazole dendrons significantly affected the energy splitting between the single-state and triple-state of 5CzBN series compounds. Absorption and Emission Behavior. The photophysical properties of TADF dendrimers were analyzed using ultraviolet-visible (UV/Vis) absorption and photoluminescence (PL) spectroscopies, and the normalized UV-Vis and PL spectra were depicted in Figure 2 and Figure S3. The absorption bands at 340-360 nm can be attributed to the π-π* band of Cz moieties. All compounds reveal intense S0→1CT absorption bands at 400-450 nm, which can be ascribed to intramolecular charge transfer from the carbazole-based donors to the benzonitrile acceptor (1CT). The energy bandgaps (Eg) of the four dendrimers were calculated as 2.66 eV, 2.56 eV, 2.76 eV, 2.73 eV from the edge of the absorption (Table 1). We further measured the PL and phosphorescence (Phos) spectra in toluene at room temperature and 77 K, respectively. Meanwhile the UV-Vis, PL and Phos spectra of neat films were also measured for comparison (Figure S4), and the films are all spin-coated according to the conditions of device preparation. And the peaks of absorption, fluorescence, phosphorescence emission and S1/T1 were summarized in Table S2. The PL spectra of 5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz and 5CzBN-2Cz in toluene show broad and featureless emission bands at 509 nm, 522 nm, 478 nm, 476 nm, respectively, indicating that their S1 states are identified as charge-transfer states. And it


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is also clearly confirmed by the spatial separation of their NTOs. Moreover, the maximum peaks of PL spectra of 5CzBN-Cz and 5CzBN-2Cz are blue shifted of about 31 nm and 46 nm compared with those of 5CzBN-O-Cz and 5CzBN-O-2Cz owing to the deficient oxygen atoms. Predictably, the neat films of the four compounds all exhibit red-shifted emission due to the intermolecular interaction, in addition, the emission color of four dendrimers can be observed and ranged from green-yellow to sky blue under 365 nm UV light. This finding reveals that avoiding the introduction of oxygen to core helps to significantly maintain and even blueshift the emissive spectrum of the emitters. The values of energy of the S1 states (1CT energies) of the four compounds measured from the onset of the steady emission spectra are 2.71 eV, 2.66 eV, 2.83 eV and 2.85 eV, respectively. Figure 2 also delineates the Phos spectra in a frozen toluene matrix at 77 K. The Phos spectra of 5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz and 5CzBN-2Cz are well resolved and show characteristic vibrational structures, indicating that their T1 states are local excited (3LE) states. The characteristic structured 3LE peak are at 490 nm, 504 nm, 475 nm and 476 nm, respectively. Thus, the experimental ΔEst of them are 0.08 eV, 0.07 eV, 0.17 eV and 0.18 eV, respectively, which is consistent with and even smaller than the calculated values. Then smaller ΔEst of 5CzBN-O-Cz and 5CzBN-O-2Cz may be resulted from the stronger electron donating ability by introducing oxygen atoms at the cores. Meanwhile the phos spectra of neat films show red-shifted vibrational emission (Figure S4, S5), which resulted from the molecular aggregation and intermolecular interaction. The vibrational structured 3LE states are beneficial to improve the RISC process. Basically, spin orbit coupling does not work between the 1CT and 3CT

states, and the orbitals involved in the two states are the same and so the matrix elements

disappeared, = 0.21-22 However, due to the 3LE state energetically close to the 1CT

state, the four compounds allow efficient crossing via a spin orbit charge transfer


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intersystem crossing process (SOCT), thus achieving efficient thermally activated reverse intersystem crossing RISC.

Figure 2. Absorption, fluorescence and phosphorescence spectra of 5CzBN-O-Cz (a), 5CzBN-O2Cz (b), 5CzBN-Cz (c) and 5CzBN-2Cz (d) in toluene, measured in dilute solution with solutes concentration of 1×10-5 M, and set 20 ms delayed time for the Phos spectrum. (inset: the photo in 365nm UV lamp). Table 1. Basic photophysical and electrochemical parameters of 5CzBN derivatives. Dendrimers

Td, Tg

λabs

λem

Eg

S1/T1

ΔEst

HOMO

LUMO

[℃]

[nm]a)

[nm]b)

[eV]c)

[eV]d,e)

[eV]f)

[eV]g)

[eV]h)

5CzBN-O-Cz

415,120

346, 428

508

2.66

2.71/2.63

0.08

-5.27

2.61

5CzBN-O-2Cz

412,109

343, 425

522

2.56

2.66/2.59

0.07

-5.28

2.72

5CzBN-Cz

404,117

343, 416

479

2.76

2.83/2.66

0.17

-5.32

2.56

5CzBN-2Cz

396,105

342, 426

477

2.73

2.85/2.67

0.18

-5.34

2.61

a) Measured in toluene solution at 300 K. b) Measured in toluene solution at 300 K. c) Estimated from the absorption edges in toluene. d) Calculated from the onset of the fluorescence spectra in toluene at 300 K. e) Estimated from onset of the phosphorescence spectra in toluene at 77 K. f) The difference between S1 energy and T1 energy. g) Determined by the CV measurement. h) Calculated from the energy gap and HOMO.


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In order to study the influence of carbazole dendrons and oxygen atoms on photophysical properties more deeply, we measured the PL quantum yield (PLQY) in toluene before and after bubbling nitrogen gas and transient PL delay characteristics. The oxygen sensitive phenomenon and double-exponential decay indicate the four emitters all have the TADF property. The values of PLQY, the prompt and delayed lifetimes, some kinetic parameters such as radiative decay rate kr, nonradiative decay rate knr, fluorescence decay rate kF, internal conversion decay rate kIC, intersystem crossing decay rate kISC and reverse intersystem crossing decay rate kRISC were summarized in Table 2. Compared with the intense prompt component, the delayed emission can be ascribed to the RISC from sufficiently small ΔEst (Figure S6). The lifetimes of the delayed components of neat films were 2.4 µs (5CzBN-O-Cz), 4.2 µs (5CzBN-O-2Cz), 3.6 µs (5CzBNCz) and 4.5 µs (5CzBN-2Cz). The improved delayed fluorescence of 5CzBN-O-2Cz and 5CzBN-2Cz, indicate that the more peripheral carbazoles can significantly suppress exciton quenching to reduce the nonradioactive loss. It also can be verified by the calculated kr and knr, the value of kr is almost same, but the value of knr significantly got lower by almost an order of magnitude. It is explained that sufficient encapsulation of more peripheral groups can suppress the intermolecular interaction and boost the proportion of RISC, and further improve the PLQY. Besides, the kISC and kRISC of 5CzBN-O-Cz and 5CzBN-O-2Cz are also enhanced owing to the relatively small ΔEst, which can also be supported by the ΔEst in non-doped films (Table S2). And the fully encapsulation of the TADF core by carbazole dendrons is more efficient for improving RISC (5CzBN-O-2Cz and 5CzBN-2Cz) and reducing the internal conversion decay rate kIC,. It has been reported that the PLQY of the emissive core 5CzBN in non-doped film was only 0.21,15 herein through simple increasing and adjusting the chains, the PLQY of dendrimers improved above 0.50. And the gradual improvement of PLQY suggests that the intermolecular


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interaction and exciton quenching could be sufficiently suppressed with more alkyl chain linked carbazole dendrons. Table 2. Photophysical characteristic and kinetic parameters of 5CzBN derivatives.23-25 Dendrimers 5CzBN-O-Cz

ɸPL(%) Solva) Film 20/78 56

5CzBN-O-2Cz

25/85

77

5CzBN-Cz

18/79

59

5CzBN-2Cz

26/89

80

τp(ns) Rp 6.9, 21.9% 6.3, 10.6% 8.7, 17.2% 5.5, 10.3%

τd(µs) Rd 2.4, 78.1% 4.2, 89.4% 3.6, 82.8% 4.5, 89.7%

kr b) [105 s-1]

knr b) [104 s-1]

kF c) [106 s-1]

kIC c) [106 s-1]

kISC c) [107 s-1]

kRISC c) [106 s-1]

2.3

18

17.8

14.0

11.32

1.90

1.8

5.8

13.0

3.9

14.19

2.25

1.6

12

11.0

7.6

9.63

1.71

1.8

4.2

15.0

3.7

16.31

2.17

a) Measured in toluene at 300K and before (left) and after (right) N2 bubbling. b) Calculated radiative decay rate kr and nonradiative decay rate knr according to the equations: τ=1/ (kr+knr), and ϕPL=kr / (kr+ knr). where τ is the total lifetimes of the transient and the delayed components, ϕPL is the PLQY of films. c) Calculated fluorescence decay rate kF, internal conversion decay rate kIC, intersystem crossing decay rate kISC from S1 to S0 and the rate constant of reverse intersystem crossing process kRISC. The detailed calculation formulas were shown in the supporting information.

Electrochemical Behavior. The cyclic voltammetry (CV) measurement of the compounds was performed to assess the contribution of the peripheral encapsulation on the hole or electron injection and transporting properties of the dendrimers. As shown in Figure S7, the multiple irreversible oxidation curves are attributed to the corresponding carbazoles of emissive core and peripheral dendrons. According to the onset potential, the HOMO energy levels of 5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz and 5CzBN-2Cz are calculated to be -5.27 eV, -5.28 eV, -5.32 eV, and -5.34 eV, respectively. The results are further verified by DFT calculations, the donor ability of 5CzBN-O-Cz and 5CzBN-O-2Cz are stronger and the HOMO is shallower than those due to the introducing of oxygen atom at 3-position of the carbazole. Based on the energy bandgaps calculated by the absorption spectra, the LUMO energy levels of 5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz and 5CzBN-2Cz are calculated to be -2.61 eV, -2.72 eV, -2.56 eV and -2.61 eV, respectively (Table 1), which match well with the theoretical simulations. In a word, the


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shallower HOMO levels of these 5CzBN derivatives are close to the energy level of hole transport layer PEDOT: PSS (-5.2 eV), which would facilitate the hole injection into the emitter. Thermal Behavior. The thermal properties of the four compounds were investigated by thermogravimetric analysis (TGA) and different scanning calorimeter (DSC) under the nitrogen atmosphere. Their decomposition temperatures (Td) with 5% loss are 415℃, 412℃, 404℃ and 396℃, and they exhibit the glass transition temperature (Tg) of 120℃, 109℃, 117℃, 105℃, respectively (see Figure 3 and Table 1). It indicates that introducing more flexible chains reduces the glass transition temperature and decreases slightly the stability of films. But overall, they all could form uniform amorphous films by solution process for OLED fabrication. In development, we investigated the morphology of spin-coated non-doped films using atomic force microscopy (AFM), as shown in Figure 4 and Figure S8, the spin-coated thin films are greatly smooth with root-mean-square (RMS) values of 0.37 nm, 0.32 nm, 0.32 nm, 0.31 nm, respectively, which are better than 5CzBN (0.74 nm) obviously. The spin-coated film with 5CzBN is heterogeneous and crystal separating, on the contrary, no cracks or pinholes are observed in the films of dendrimers. It indicates that these dendrimers could all form morphologically stable and uniform amorphous films through spin coating, which is mainly due to the solubilizing function of alkyl chains groups.


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Figure 3. TGA curve of 5CzBN-O-Cz(a), 5CzBN-O-2Cz(b), 5CzBN-Cz(c) and 5CzBN-2Cz(d) recorded at a heating rate of 10℃ min-1; Inset: DSC curve at a heating rate of 10℃ min-1. Alcohol Resistance Studies. Before device fabrication, the influence of different numbers of branches on the alcohol resistance was investigated by the UV-Vis absorption spectroscopy. Figure 5 shows the variations in absorption intensity of the four TADF dendrimers before and after spin-rinsing with isopropanol, which was used as solvents for processing the adjacent electron transport layer (ETL). Compared with 5CzBN,15 the absorptions of four TADF dendrimers after spin-rinsing were nearly invariable compared to those before spin-rinsing, so it clearly proves that the encapsulation with carbazole dendrons efficiently improves the resistance to isopropanol. Meanwhile, there were some differences between pre- and post-rinsed ratios of these dendrimers, as the numbers of carbazole branches increased, it is found that the resistance to isopropanol was increased distinctly upon going from 5CzBN-O-Cz, 5CzBN-Cz to 5CzBN-O-


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2Cz and 5CzBN-2Cz. In other words, the more carbazole branches can effectively encapsulate the central chromophore, and thus prevent them from being redissolved by isopropanol.

Figure 4. AFM images of 5CzBN(left), 5CzBN-2Cz(right) films and the device OLED device architecture(middle).

Figure 5. a) Absorption spectra of 5CzBN before and after rinsing with isopropanol. Reproduced with permission. [15] Copyright 2017, American Chemical Society. b) Absorption spectra of four dendrimers before and after rinsing with isopropanol. OLED Fabrication and Testing. In order to study the electroluminescence properties of four TADF dendrimers, at first, we constructed the non-doped OLED device A1-A4 using the solution processed non-doped emission layer and a vacuum-deposited electron transport layer (TPBi: 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene). The configuration of the device was ITO/ PEDOT: PSS (40 nm, spin-coated)/ EML (40 nm, spin-coated)/ TPBi (30 nm, vacuum-


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deposited)/Cs2CO3 (2 nm)/Al (100 nm) (EML = 5CzBN-O-Cz for device A1, 5CzBN-O-2Cz for device A2, 5CzBN-Cz for device A3 and 5CzBN-2Cz for device A4), in which PEDOT: PSS and TPBi served as hole and electron-transport materials, respectively; Cs2CO3 was utilized as an electron injection layer. The schematic energy-level diagrams and the fabrication process of the devices are shown in Figure 6. The four dendrimers are all spin-coated to serve as the emission layer (EML).

Figure 6. a) Energy-level arrangement and chemical structures of the organic materials used in the OLED devices. b) Device configurations and fabrication processes. Figure S9 shows the current density-voltage-luminance (J-V-L) characteristics, current efficiencies, EL spectra, external quantum efficiencies and power efficiencies of the four devices.


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The electroluminescent datas are summarized in Table 3. These devices show the bluish-green to green-yellow EL spectra (Figure S13a, Figure S14), which indicate emission from the dendrimers. As expected, because of the influence of oxygen atom in the emissive cores, the EL spectra of 5CzBN-O-Cz and 5CzBN-O-2Cz showed significant red-shift. And interestingly, as the number of carbazole branches, 5CzBN-O-2Cz and 5CzBN-2Cz also showed red-shifted EL spectra due to the effect of polarity of dendrons (shown in Figure S9b inset). Not only that, compared to the PL spectra, the EL spectra showed slight bathochromic shift, and the same phenomenon occurred on fully solution-processed OLEDs (Device B1-B4 and Device C1-C4) (Figure S14). Since the delayed fluorescence based on RISC exists at longer wavelengths than transient single state emission, the slight redshift in the electroluminescence spectra can be explained by the presence of larger amount of triplet excitons generated by the electrical excitation. The corresponding solution processed devices realized the maximum current efficiency (CEmax), power efficiency (PEmax) and maximum external quantum efficiency (EQEmax) of 32.18 cd A-1, 20.22 lm W-1, 11.05% (5CzBN-O-Cz), 45.00 cd A-1, 35.34 lm W-1, 14.74 % (5CzBN-O-2Cz), 37.76 cd A-1, 29.66 lm W-1, 13.15 % (5CzBN-Cz), and 45.90 cd A-1, 35.60 lm W-1, 16.08 % (5CzBN-2Cz), as well as the Commission International De L’Eclairge (CIE) coordinates of (0.35, 0.57), (0.38, 0.47), (0.21,0.47), (0.22,0.52), respectively (Fig. S13a). And it is noteworthy that the electroluminescent color ranged from bluish-green to green-yellow by the simple modification (e.g. introducing oxygen atoms and increasing the peripheral carbazole) in core-dendrons system. Besides, the modification in the TADF dendrimers affected the current density and efficiency. With the increase in carbazole dendrons, the current density decreased significantly due to more energy barriers, and the current efficiency increased with almost unchanged luminance. Further, to highlight the superiority of the 5CzBN core-dendrons


17


Page 18 of 34

system, we construct the device E1 with spin coated 5CzBN layer, during the course of the experiment, the 5CzBN material was spun coated poorly and the device efficiency also performed worse (11.5 cd A-1) (Figure S11 and Table 3). Therefore, the sufficient encapsulation of the 5CzBN can improve the solubility effectively and the fluorescence quantum yields as mentioned earlier, finally enhance the device efficiency.

Figure 7. a) Current density-voltage-luminance (J-V-L) characteristics of Device C1-C4; b) current efficiencies versus brightness characteristics (inset: the CIE of devices); c) the external quantum efficiencies versus luminance plots; d) the electroluminescence spectrum. Afterwards, inspired by the great film forming ability of the four dendrimers and great resistance to isopropanol, OLED devices based on fully solution process were fabricated with the structure of ITO/PEDOT-PSS (40 nm, spin-coated)/dendrimer (40 nm, spin-coated)/TPBi (30 nm,


18

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spin-coated)/Cs2CO3 (2 nm)/Al (100 nm) (device B1-B4). Electron-transporting material TPBi was spun from isopropanol. Using the spin-coated TPBi materials, the EQEmax reached 8.93% (5CzBN-O-Cz), 10.72% (5CzBN-O-2Cz), 9.42% (5CzBN-Cz), 10.73% (5CzBN-2Cz), respectively (Figure S10 and Table 3). Similar to Device A1-A4 with vacuum deposited ETL, the comparable turn-on voltages and luminescence efficiencies demonstrate that the TADF dendrimers have strong tolerance to isopropanol used for processing above adjacent ETL. But the device efficiencies are slightly lower than those of vacuum evaporated OLEDs. It could be due to its low solubility and weak electron transporting ability of TPBi. Based on that case, we chose

another

electron-transport

material

named

PO-T2T

(((1,3,5-triazine-2,4,6-triyl)

tris(benzene-3,1-diyl)) tris(diphenylphosphine oxide)), which can be spun from isopropyl alcohol well. Device C1-C4 showed higher CE, PE and EQE than device B1-B4, which were 40.50 cd A1,

31.50 lm W-1, 13.75% (5CzBN-O-Cz), 56.16 cd A-1, 45.24 lm W-1, 17.38 % (5CzBN-O-2Cz),

41.73 cd A-1, 34.48 lm W-1, 14.36 % (5CzBN-Cz), 63.02 cd A-1, 48.67 lm W-1, 20.41 % (5CzBN2Cz), respectively (Figure 7 and Table 3). The device performances are much better than the spin-coated TPBi materials for the following reasons: the PO-T2T possessed the superior solubility, then deeper LUMO levels reduced the electron injection barriers, meanwhile the deeper HOMO level also corresponds to the hole barrier layer, which prevents the holes from flowing into the electron transport layer. Moreover, we constructed the related devices D1-D4 using vacuum evaporated PO-T2T as the ETL to further illustrate that Device C series have more matching energy level structure by using PO-T2T as ETL. The device performances were shown in Figure S12 in details. The CEmax, PEmax, EQEmax reached 38.25 cd A-1, 35.86 lm W-1, 13.10% (5CzBN-O-Cz), 52.67 cd A-1, 41.37 lm W-1, 16.31% (5CzBN-O-2Cz), 45.27 cd A-1, 35.27 lm W-1, 15.45% (5CzBN-Cz), 51.81 cd A-1,


19


Page 20 of 34

32.55 lm W-1, 17.10% (5CzBN-2Cz), respectively. Obviously, the device efficiencies are higher than those of OLEDs using TPBi as the ETL. This is because that PO-T2T has the appropriate energy level. And it can be seen clearly that the efficiencies of Device C series are still highest, which is likely attributed to the different film morphology. The solution-processed devices may achieve better matching of electron and hole transfer rates due to the relatively inhomogeneous film. The detailed researches of the device performance of fully solution-processed OLEDs will be explored and studied in the follow-up experiment. For comparison, we also construct the Device E2 (TPBi as spin-coated EIL) and E3 (PO-T2T as spin-coated EIL) with 5CzBN under the same experimental conditions. Unfortunately, the device performances were barely satisfactory because of the poor solubility of emissive materials and the weak alcohol resistance of upper spin-coated EIL (as shown in Figure S11 and Table 3). Not just because of this, there are differences in energy transfer between 5CzBN and the coredendrons TADF dendrimers. As illustrated in Figure 8, it is worth mentioning that simple carbazole dendrimers have the feature of an electron-rich energy gradient. As we all know, the energy loss that inevitably exists in the devices always caused by serious quenching of excitons via the triplet-triplet annihilation, triplet-polaron annihilation (TPA) and so on, and which is triggered by the intermolecular collision between at least two molecules in general. Hence it can be effective to avoid direct contact between two emissive cores by core-dendrons molecular design. As shown in Figure 8a, the 5CzBN emitters collided intensely because of the direct contact and short molecular distance, the excitons were accumulated in the undiluted emitters and most of them cannot make efficient use with non-radiative transition. Finally, due to the particular morphological make-up, they have already been quenching during long decay lifetime, even worse in the solution-processed devices. However, in the core-dendrons system, the


20

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circumjacent carbazole dendrons possessed large energy gap and acted as the molecular barrier and every isolated TADF core from others, the holes and electrons transferred to 5CzBN cores from the dendrons. As can be seen from Figure 7, the current density of 5CzBN-O-2Cz and 5CzBN-2Cz is obviously lower than those of 5CzBN-O-Cz and 5CzBN-Cz. And more branches with large bandgap resulted in weak charge injection and transportation capability. Fortunately, owing to HOMO energy level of carbazole dendrons matched with those of PEDOT: PSS, the turn-on voltage almost unchanged with more branches. Eventually, the energy gradient and molecular barriers of peripheral carbazoles resulted in exciton decentralization and highly efficient exciton utilization, then the direct radiative transition occurred immediately with reduced emissive cores’ collision and exciton quenching, it improved the performance of nondoped OLEDs and reduced the ACQ and roll-off. Refer to Figure 7, the fully solution-processed Device C2 and C4 maintained the excellent efficiency of 43.6 cd A-1 and 49.5 cd A-1 at the luminance of 100cd m-2 and 38.3 cd A-1 and 31.5 cd A-1 at the luminance of 1000 cd m-2, respectively. The high efficiency and low roll-off of device C2 and C4 are resulted from the superiority of core-dendrons system with more branches’ encapsulation.


21


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Figure 8. Energy transfer analysis of solution-processed device with different emissive layers26 (5CzBN small molecules (a) and dendrimers (b)). Interestingly, Device C3 and Device C4 exhibit evidently red-shifted emission than the related devices using TPBi as the ETL. At first, it proved that it is not caused by exciplex by measured the EL spectra of films with blending the emitters and electron transporting materials (Figure S15). Then we preliminary speculate that it is caused by exciton recombination region and microcavity effect. To verify the opinion, OLED devices were fabricated with the structure of ITO/PEDOT-PSS (40 nm, spin-coated)/EML (40 nm, spin-coated)/PO-T2T (20-50nm, vacuumdeposited)/Cs2CO3 (2 nm)/Al (100 nm), ETL was vacuum deposited with different thickness. With the change in thickness of PO-T2T, the four dendrimers show slight electroluminescent spectral changes (Figure S16). As we all known, the charge transport ability and exciton recombination regions can be affected seriously by the thickness of ETL. In addition, microcavity structures existed in devices also affect the electroluminescence spectrum to a great extent,27 and eventually the combined effects resulted in the redshift of EL spectra of Device C3, C4.


22

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Table 3. Device performances of the solution-processed nondoped OLEDs Device

emitter

Vona

EL(nm)

CEmaxb

PEmaxc

EQEmaxd

CEe

Lmaxf

CIE(x,y)g

A1

5CzBN-O-Cz

3.5

524

32.18

20.22

11.05

24.4/29.1/7.9

13570

(0.35,0.57)

A2

5CzBN-O-2Cz

3.2

536

45.00

35.34

14.74

42.8/28.3/10.7

15300

(0.38,0.57)

A3

5CzBN-Cz

3.0

495

37.76

29.66

13.15

35.2/25.2/5.1

11020

(0.21,0.47)

A4

5CzBN-2Cz

3.1

506

45.90

35.60

16.08

44.9/35.6/9.5

13000

(0.22,0.52)

B1

5CzBN-O-Cz

3.2

536

27.94

20.42

8.93

15.8/16.8/7.8

16000

(0.37,0.58)

B2

5CzBN-O-2Cz

3.3

546

34.93

21.15

10.72

26.8/22.4/10.6

15000

(0.40,0.57)

B3

5CzBN-Cz

3.5

492

27.03

19.80

9.42

17.5/20.5/3.2

10000

(0.21,0.44)

B4

5CzBN-2Cz

3.4

495

30.63

22.64

10.73

26.4/17.7/4.7

11000

(0.21,0.48)

C1

5CzBN-O-Cz

2.8

523

40.16

31.50

13.75

38.7/32.6/11.7

20900

(0.37,0.58)

C2

5CzBN-O-2Cz

3.2

541

56.16

45.24

17.38

52.6/48.6/17.2

21630

(0.40,0.57)

C3

5CzBN-Cz

3.0

516

41.73

34.48

14.36

41.8/39.9/10.8

21500

(0.25,0.55)

C4

5CzBN-2Cz

3.6

515

63.02

48.67

20.41

56.7/43.5/14.8

25190

(0.25,0.52)

D1

5CzBN-O-Cz

3.3

534

38.25

35.86

13.10

40.2/17.7/8.8

14550

(0.36,0.57)

D2

5CzBN-O-2Cz

3.4

540

52.67

41.37

16.31

48.8/18.4/5.3

16780

(0.39,0.57)

D3

5CzBN-Cz

3.5

497

45.27

35.27

15.45

35.9/18.8/6.9

18810

(0.21,0.46)

D4

5CzBN-2Cz

3.5

503

51.81

32.55

17.10

44.4/32.0/9.9

20020

(0.22,0.50)

E1

5CzBN

3.0

514

11.5

9.0

3.64

7.2/4.7/-

5000

(0.27,0.55)

E2

5CzBN

6.7

485

0.22

0.08

0.08

0.17/-

150

(0.21,0.33)

E3

5CzBN

6.9

494

0.76

0.27

0.30

0.76/-

200

(0.23,0.41)

aV

= turn-on voltage at 1cd m−2, bCEmax = maximum current efficiency, cPEmax = maximum power efficiency, dEQEmax = maximum external quantum efficiency, eCurrent efficiency at the luminance of 100 cd m−2, 1000 cd m−2, and 10000 cd m−2, fLmax = maximum luminance, gCIE = the Commission Internationale de L’Eclairage coordinates. on

Deserve to be mentioned, the enhanced performance of Device C1-C4 almost comparable to vacuum-deposited counterparts. Figure 9 shows the maximum external quantum efficiencies versus the EL wavelength of all the reported solution-processed non-doped OLEDs including small molecules, dendrimers and polymers. The red color represents the fully solution processed TADF OLEDs. The blue color represents the solution processed TADF OLEDs with vacuum evaporated ETL. It is clearly shown that the device efficiencies with these dendrimers are among the best efficiencies which have been reported. Moreover, the current efficiencies, power


23


Page 24 of 34

efficiencies, EL wavelength and EQEs were summarized at Table 4 and Table S3. Importantly, as far as we know, this is the highest efficiencies (20.41%) of in the area of fully-solutionprocessed non-doped OLEDs (Table 4).12,14,15,28-51 And on the whole, the enhanced performance and ultrahigh EQEmax (20.41%) of Device C4 broke records of fully solution-processed nondoped TADF OLEDs, which were above 5-fold higher than those of 5CzBN-based device (Figure S11, Table 4). Moreover, it can be expected that improving the encapsulation of emitters with more branches and optimizing the way connected core and dendrons would further improve the device performance and make the emission blue shift in fully solution-processed fluorescent OLEDs. Detailed studies of the blue and deep blue TADF OLEDs with fully wet processes are currently underway.

Figure 9. The summarized maximum external quantum efficiencies (EQEmax) versus the electroluminance wavelength in solution-processed non-doped TADF OLEDs

12,14,15,28-51.

numbers correspond to references in which values were obtained.


24

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Table 4. Comparison of highly efficient TADF OLEDs based on fully solution process. emitter

EQEmax

PEmax

CEmax

Lmax

5CzBN-2Cz

20.41

48.7

63.0-

25190

Cz-CzCN

15.5

39.3

46.3

28000

MeG2TAZ

9.4

-

-

G2B

5.7

11.5

tBuG2B

17.0

LEP

10

ELpeak(nm)

CIE(x,y)

ref

(0.25,0.52)

this work

(0.25,0.52)

15

2235

-

28

14.0

-

(0.26,0.48)

29

40.7

46.6

4922

(0.27,0.52)

30

-

-

-

(0.32,0.58)

31

510

CONCLUSION

In summary, novel design strategies for core-dendrons TADF dendrimers were presented to realize highly efficient color tunable emission. By changing the linking method between the alkyl chains and the TADF core, the emission had with different colors without the loss of device efficiency. Moreover, with the increase in carbazole dendrons and more sufficiently encapsulated structure of 5CzBN-2Cz, the average interchromophore distance increased between the TADF cores and reduced the intermolecular interaction. Last but not least, the energy gradient and barriers of carbazole dendron isolated the excitons that were trapped at the cores and induced the radiation transition. Thus, ACQ was suppressed, and the exciton utilization was improved, thereby enhancing EQEs of the all-solution-processed devices. Given the comparable efficiency to vacuum-deposited small-molecular TADF fluorophore, we believe that the work provides a practical strategy to the development of TADF emitters based on solution process, and paves the way for the popularization of solution processed OLEDs and the application of large-area manufacturing.


25


Page 26 of 34

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website. Quantum chemical calculations. Devices measurements and characterization. Equations. Systhesis of materials. Calculated HOMO, LUMO, Bandgap, S1, T1, ΔEst, f values from DFT and TD-DFT at B3LYP/6-31g(d) level. Optimized geometries and calculated HOMO and LUMO density maps. The natural transition orbitals (NTOs) character of lowest excited triplet states for 5CzBN derivatives. The UV-Vis absorption, fluorescence, phosphorescence spectra, the transient PL delay and device performances of 5CzBN-O-Cz (a), 5CzBN-O-2Cz (b), 5CzBN-Cz (c) and 5CzBN-2Cz (d). AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We are grateful for the grants from the National Natural Science Foundation of China (21875036 and 51103023), the Fundamental Research Funds for the Central Universities (2242016K41082), We are also thankful for the National Basic Research Program (973 program, 2013CB932902), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Nanjing science and technology committee (2014-030002).


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REFERENCES (1) Wu, Z.; Yu, L.; Zhao, F.; Qiao, X.; Chen, J.; Ni, F.; Yang, C.; Ahamad, T.; Alshehri, S. M.; Ma, D. Precise Exciton Allocation for Highly Efficient White Organic Light-Emitting Diodes with Low Efficiency Roll-Off Based on Blue Thermally Activated Delayed Fluorescent Exciplex Emission. Adv. Opt. Mater. 2017, 5, 1700415. (2) Zhong, C.; Duan, C.; Huang, F.; Wu, H.; Cao, Y. Materials and Devices toward Fully Solution Processable Organic Light-Emitting Diodes. Chem. Mater. 2011, 23, 326-340. (3) Aizawa, N.; Pu, Y. J.; Watanabe, M.; Chiba, T.; Ideta, K.; Toyota, N.; Igarashi, M.; Suzuri, Y.; Sasabe, H.; Kido, J. Solution-Processed Multilayer Small-Molecule Light-Emitting Devices with High-Efficiency White-Light Emission. Nat. Commun. 2014, 5, 5756. (4) Duan, L.; Hou, L.; Lee, T.-W.; Qiao, J.; Zhang, D.; Dong, G.; Wang, L.; Qiu, Y. Solution Processable Small Molecules for Organic Light-Emitting Diodes. J. Mater. Chem. 2010, 20, 6392-6407. (5) Köhnen, A.; Riegel, N.; Kremer, J. H. W. M.; Lademann, H.; Müller, D. C.; Meerholz, K. The Simple Way to Solution-Processed Multilayer OLEDs - Layered Block-Copolymer Networks by Living Cationic Polymerization. Adv. Mater. 2009, 21, 879-884. (6) Huang, F.; Cheng, Y.-J.; Zhang, Y.; Liu, M. S.; Jen, A. K. Y. Crosslinkable HoleTransporting Materials for Solution Processed Polymer Light-Emitting Diodes. J. Mater. Chem. 2008, 18, 4495-4509. (7) Tseng, S.-R.; Lin, S.-C.; Meng, H.-F.; Liao, H.-H.; Yeh, C.-H.; Lai, H.-C.; Horng, S.-F.; Hsu, C.-S. General Method to Solution-Process Multilayer Polymer Light-Emitting Diodes. Appl. Phys. Lett. 2006, 88, 163501. (8) Kamino, B. A.; Bender, T. P. The Use of Siloxanes, Silsesquioxanes, and Silicones in


27


Page 28 of 34

Organic Semiconducting Materials. Chem. Soc. Rev. 2013, 42, 5119-5130. (9) Huang, F.; Wu, H.; Cao, Y. Water/Alcohol Soluble Conjugated Polymers as Highly Efficient Electron Transporting/Injection Layer in Optoelectronic Devices. Chem. Soc. Rev. 2010, 39, 2500-2521. (10) Wang, S.; Zhang, B.; Wang, Y.; Ding, J.; Xie, Z.; Wang, L. Solution-Processed Multilayer Green Electrophosphorescent Devices with Self-Host Iridium Dendrimers as the Nondoped Emitting Layer: Achieving High Efficiency while Avoiding Redissolution-Induced Batch-toBatch Variation. Chem. Commun. 2017, 53, 5128-5131. (11) Zou, Y.; Gong, S.; Xie, G.; Yang, C. Design Strategy for Solution-Processable Thermally Activated Delayed Fluorescence Emitters and Their Applications in Organic Light-Emitting Diodes. Adv. Opt. Mater. 2018, 1800568. (12) Albrecht, K.; Matsuoka, K.; Fujita, K.; Yamamoto, K. Carbazole Dendrimers as SolutionProcessable Thermally Activated Delayed-Fluorescence Materials. Angew. Chem., Int. Ed. 2015, 54, 5677-5682. (13) Wang, Y.; Wang, S.; Ding, J.; Wang, L.; Jing, X.; Wang, F. Dendron Engineering in SelfHost Blue Iridium Dendrimers towards Low-Voltage-Driving and Power-Efficient Nondoped Electrophosphorescent Devices. Chem. Commun. 2016, 53, 180-183. (14) Luo, J.; Gong, S.; Gu, Y.; Chen, T.; Li, Y.; Zhong, C.; Xie, G.; Yang, C. 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. (15) Ban, X.; Zhu, A.; Zhang, T.; Tong, Z.; Jiang, W.; Sun, Y. Highly Efficient All-SolutionProcessed Fluorescent Organic Light-Emitting Diodes Based on a Novel Self-Host Thermally Activated Delayed Fluorescence Emitter. ACS Appl. Mater. Interfaces 2017, 9, 21900-21908.


28

Page 29 of 34 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


(16) Xu, H.; Xu, A.; Yue, A.; Yan, P.; Wang, B.; Jia, L.; Li, G.; Sun, W.; Zhang, J. A Novel Deep Blue-Emitting ZnII Complex based on Carbazole-Modified 2-(2-hydroxyphenyl) benzimidazole:

Synthesis,

Bright

Electroluminescence,

and

Substitution

Effect

on

Photoluminescent, Thermal, and Electrochemical Properties. J. Phys. Chem. C 2008, 112, 15517-15525. (17) Lei, T.; Dou, J. H.; Pei, J., Influence of Alkyl Chain Branching Positions on the Hole Mobilities of Polymer Thin-Film Transistors. Adv. Mater. 2012, 24, 6457-6461. (18) Cai, J.; Ye, T.; Fan, X.; Han, C.; Xu. H.; Wang, L.; Ma, D.; Lin, Y.; Yan, P. An Effective Strategy for Small Molecular Solution-Processable Iridium(III) Complexes with Ambipolar Characteristics: towards Efficient Electrophosphorescence and Reduced Efficiency Roll-Off. J. Mater. Chem. 2011, 21, 15405-15416. (19) Han, F.; Zhang, X.; Zhang, J.; Wei, Y.; Zhang, X.; Huang W.; Xu, H. 3D-Encapsulated Iridium-Complexed Nanophosphors for Highly Efficient Host-Free Organic Light-Emitting Diodes. Chem. Commun. 2016, 52, 5183-5186. (20) Han, C.; Xie, G.; Xu, H. Highly Efficient Solution-Processable Nanophosphor with Ambipolar Shell. Chem. Eur. J. 2018, 24, 2971-2979. (21) Dias, F.; Santos, J.; Graves, D.; Data, P.; Nobuyasu, R.; Fox, M.; Batsanov, A.; Palmeira, T.; Berberan-Santos, M.; Bryce, M.; Monkman, A. The Role of Local Triplet Excited States and DA Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices, Adv. Sci. 2016, 3, 1600080. (22) Rothe, C.; Monkman, P. Triplet Exciton Migration in a Conjugated Polyfluorene, Phys. Rev. B 2003, 68, 075208. (23) Liu, H.; Zeng, J.; Guo, J.; Nie, H.; Zhao, Z.; Tang, B. High-Performance Non-Doped


29


Page 30 of 34

OLEDs with Nearly 100% Exciton Use and Negligible Efficiency Roll-Off. Angew. Chem. Int. Ed. 2018, 57, 9290 –9294. (24) Zeng, X.; Luo, J.; Zhou, T.; Chen, T.; Wu, K.; Zou, Y.; Xie, G.; Gong, S.; Yang, C. Using Ring-Opening Metathesis Polymerization of Norbornene to Construct Thermally Activated Delayed Fluorescence Polymers: High-Efficiency Blue Polymer Light-emitting Diodes. Macromolecules 2018, 51, 5, 1598-1604. (25) Xie, G.; Luo, J.; Huang, M.; Chen, T.; Wu, K.; Gong, S.; Yang, C. Inheriting the Characteristics of TADF Small Molecule by Side‐Chain Engineering Strategy to Enable BluishGreen Polymers with High PLQYs up to 74% and External Quantum Efficiency over 16% in Light‐Emitting Diodes. Adv. Mater. 2017, 29, 1604223. (26) Zhang, D.; Cai, M.; Zhang, Y.; Zhang, D. and Duan, L.; Sterically Shielded Blue Thermally Activated Delayed Fluorescence Emitters with Improved Efficiency and Stability. Mater. Horiz. 2016, 3, 145-151. (27) Wittmann, H.; Grüner, J.; Friend, R.; Spencer, G.; Moratti, S.; Holmes, A. Microcavity Efffect in a Single-Layer Polymer Light-Emitting Diodes. Adv. Mater. 1995, 7, No.6. (28) Albrecht, K.; Matsuoka, K.; Yokoyama, D.; Sakai, Y.; Nakayama, A.; Fujita, K.; Yamamoto, K. Thermally Activated Delayed Fluorescence OLEDs with Fully Solution Processed Organic Layers Exhibiting Nearly 10% External Quantum Efficiency. Chem. Commun. 2017, 53, 2439-2442. (29) Matsuoka, K.; Albrecht, K.; Yamamoto, K.; Fujita, K. Mulifunctional Dendritic Emitter: Aggregation-Induced Emission Enhanced, Thermally Activated Delayed Fluorescent Material for Solution-Processed Multilayered Organic Light-Emitting Diodes. Sci. Rep. 2017, 7, 41780. (30) Matsuoka, K.; Albrecht, K.; Nakayama, A.; Yamamoto, K.; Fujita, K. Highly Efficient


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Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with Fully SolutionProcessed Organic Multilayered Architecture: Impact of Terminal Substitution on CarbazoleBenzophenone Dendrimer and Interfacial Engineering. ACS Appl. Mater. Interfaces 2018, 10, 33343-33352. (31) 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. (32) Chen, X. L.; Jia, J. H.; Yu, R.; Liao, J. Z.; Yang, M. X.; Lu, C. Z. Combining ChargeTransfer Pathways to Achieve Unique Thermally Activated Delayed Fluorescence Emitters for High-Performance Solution-Processed, Non-doped Blue OLEDs. Angew. Chem., Int. Ed. 2017, 56, 15006-15009. (33) Sun, K.; Sun, Y.; Jiang, W.; Huang, S.; Tian, W.; Sun, Y. Highly Efficient and Color Tunable Thermally Activated Delayed Fluorescent Emitters and Their Applications for the Solution-Processed OLEDs. Dyes Pigm. 2017, 139, 326-333. (34) Hu, J.; Zhang, X.; Zhang, D.; Cao, X.; Jiang, T.; Zhang, X.; Tao, Y. Linkage Modes on Phthaloyl/triphenylamine Hybrid Compounds: Multi-Functional AIE Luminogens, Non-Doped Emitters and Organic Hosts for Highly Efficient Solution-Processed Delayed Fluorescence OLEDs. Dyes Pigm. 2017, 137, 480-489. (35) Bin, H.; Ji, Y.; Li, Z.; Zhou, N.; Jiang, W.; Feng, Y.; Lin, B.; Sun, Y. Simple Aggregation– Induced Delayed Fluorescence Materials based on Anthraquinone Derivatives for Highly Efficient Solution–Processed Red OLEDs. J. Lumin. 2017, 187, 414-420. (36) Zeng, X.; Luo, J.; Zhou, T.; Chen, T.; Zhou, X.; Wu, K.; Zou, Y.; Xie, G.; Gong, S.; Yang, C. Using Ring-Opening Metathesis Polymerization of Norbornene To Construct Thermally


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Activated Delayed Fluorescence Polymers: High-Efficiency Blue Polymer Light-Emitting Diodes. Macromolecules 2018, 51, 1598-1604. (37) Huang, B.; Ban, X.; Sun, K.; Ma, Z.; Mei, Y.; Jiang, W.; Lin, B.; Sun, Y. Thermally Activated Delayed Fluorescence Materials based on Benzophenone Derivative as Emitter for Efficient Solution-Processed Non-Doped Green OLED. Dyes Pigm. 2016, 133, 380-386. (38) Wang, J.; Peng, J.; Yao, W.; Jiang, C.; Liu, C.; Zhang, C.; He, M.; Liu, R.; Xia, X.; Yao, C. Carbazole-Dendrite-Encapsulated Electron Acceptor Core for Constructing Thermally Activated Delayed Fluorescence Emitters used in Nondoped Solution-Processed Organic Light-Emitting Diodes. Org. Electron. 2017, 48, 262-270. (39) Li, Y.; Chen, T.; Huang, M.; Gu, Y.; Gong, S.; Xie, G.; Yang, C. Tuning the Twist Angle of Thermally Activated Delayed Fluorescence Molecules via a Dendronization Strategy: HighEfficiency Solution-Processed Non-Doped OLEDs. J. Mater. Chem. C 2017, 5, 3480-3487. (40) Li, Y.; Xie, G.; Gong, S.; Wu, K.; Yang, C. Dendronized Delayed Fluorescence Emitters for Non-Doped, Solution-Processed Organic Light-Emitting Diodes with High Efficiency and Low Efficiency Roll-Off Simultaneously: Two Parallel Emissive Channels. Chem. Sci. 2016, 7, 54415447. (41) Liu, H.; Zeng, J.; Guo. J.; Nie. H.; Zhao. Z.; Tang. B. Z. Angewante A Versatile Molecular Design for High‐Performance Nondoped OLEDs with ~100% Exciton Utilization and Negligible Efficiency Roll‐Off. Angew. Chem., Int. Ed. 2018, 57, 9290-9294. (42) Huang, M.; Li, Y.; Wu, K.; Luo, J.; Xie, G.; Li, L.; Yang, C. Carbazole-Dendronized Thermally Activated Delayed Fluorescent Molecules with Small Singlet-Triplet Gaps for Solution-Processed Organic Light-Emitting Diodes. Dyes Pigm. 2018, 153, 92-98. (43) Ban, X.; Jiang, W.; Lu, T.; Jing, X.; Tang, Q.; Huang, S.; Sun, K.; Huang, B.; Lin, B.; Sun,


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Y. Self-Host Thermally Activated Delayed Fluorescent Dendrimers with Flexible Chains: an Effective Strategy for Non-Doped Electroluminescent Devices based on Solution Processing. J. Mater. Chem. C 2016, 4, 8810-8816. (44) Sun, K.; Sun, Y.; Huang, T.; Luo, J.; Jiang, W.; Sun, Y. Design Strategy of Yellow Thermally Activated Delayed Fluorescent Dendrimers and Their Highly Efficient Non-Doped Solution-Processed OLEDs with Low Driving Voltage. Org. Electron. 2017, 42, 123-130. (45) Sun, K.; Sun, Y.; Tian. W.; Liu. D.; Feng Y.; Sun. Y.; Jiang. W. Thermally Activated Delayed Fluorescence Dendrimers with Exciplex-forming Dendrons for Low-Voltage-Driving and Power-Efficient Solution-Processed OLEDs. J. Mater. Chem. C 2018, 6, 43-49. (46) Godumala, M.; Choi, S.; Kim, H. J.; Lee, C.; Park, S.; Moon, J. S.; Si Woo, K.; Kwon, J. H.; Cho, M. J.; Choi, D. H. Novel Dendritic Large Molecules as Solution-Processable Thermally Activated Delayed Fluorescent Emitters for Simple Structured Non-Doped Organic Light Emitting Diodes. J. Mater. Chem. C 2018, 6, 1160-1170. (47) 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. (48) Wang, Y.; Zhu, Y.; Lin, X.; Yang, Y.; Zhang, B.; Zhan, H.; Xie, Z.; Cheng, Y. Efficient Non-Doped Yellow OLEDs based on Thermally Activated Delayed Fluorescence Conjugated Polymers with an Acridine/Carbazole Donor Backbone and TRiphenyltriazine Acceptor Pendant. J. Mater. Chem. C 2018, 6, 568-574. (49) Yang, Y.; Wang, S.; Zhu, Y.; Wang, Y.; Zhan, H.; Cheng, Y. Thermally Activated Delayed Fluorescence Conjugated Polymers with Backbone-Donor/Pendant-Acceptor Architecture for Nondoped OLEDs with High External Quantum Efficiency and Low Roll-Off. Adv. Funct.


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Page 34 of 34

Mater. 2018, 28, 1706916. (50) Luo, J.; Xie, G.; Gong, S.; Chen, T.; Yang, C. Creating a Thermally Activated Delayed Fluorescence Channel in a Single Polymer System to Enhance Exciton Utilization Efficiency for Bluish-Green Electroluminescence. Chem. Commun. 2016, 52, 2292-2295. (51) Shao, S.; Hu, J.; Wang, X.; Wang, L.; Jing, X.; Wang, F. Blue Thermally Activated Delayed Fluorescence Polymers with Nonconjugated Backbone and Through-Space Charge Transfer Effect. J. Am. Chem. Soc. 2017, 139, 17739-17742.

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Achieving 20% External Quantum Efficiency for Fully Solution

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