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EQE Climbing Over 6% at High Brightness of 14350 cd/m2 in Deep-Blue OLEDs Based on Hybridized Local and Charge-Transfer Fluorescence Juewen Zhao, Bin Liu, Zhongyi Wang, Qing-Xiao Tong, Xiaoyang Du, Cai-Jun Zheng, Hui Lin, Si-Lu Tao, and Xiao-Hong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19646 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Materials & Interfaces

EQE Climbing Over 6% at High Brightness of 14350 cd/m2 in Deep-Blue OLEDs Based on Hybridized Local and Charge-Transfer Fluorescence Juewen Zhao‡,a, Bin Liu‡,b, Zhongyi Wangb, Qingxiao Tongb,*, Xiaoyang Dua, Caijun Zhenga, Hui Lina, Silu Taoa,* and Xiaohong Zhangc,* a

School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China b Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, P. R. China. c Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, P. R. China ‡

These authors contributed equally to this work. *Corresponding author

E-mail addresses: [email protected] (Q. X. Tong), [email protected] (S. L. Tao), [email protected] (X. H. Zhang)

Abstract Three deep blue emitters PPi-Pid, PPi-Xid and PPi-Mid based on a novel conjugated system Phenantroimidazole-π-indolizine have been designed and synthesized. Here indolizine with appropriate π-conjugation length was used as the acceptor profited from its high photoluminescence quantum yield and good electron-withdraw ability. Fluorescent organic light-emitting diodes (OLEDs) based on PPi-Pid, PPi-Xid and PPi-Mid achieved deep blue emissions with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.151, 0.076), (0.155, 0.052) and (0.153, 0.052), high brightness of 14350, 4377 and 4002 cd/m2 and high external quantum efficiencies (EQE) of 6.01, 3.90 and 4.28%, respectively. Moreover, it’s noticeable that all of the devices exhibited efficiencies increasing with brightnesses. Especially the PPi-Pid based device, exhibited high EQE over 6% at a high brightness of 14350 cd/m2. Such high brightness along with high EQE is very rare whether in deep blue fluorescent or thermally activated delayed fluorescent

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OLEDs. Keywords: Indolizine; conjugated system; deep blue; OLEDs; efficiency roll-off

1. Introduction To realize the practical application of organic light-emitting diodes (OLEDs) in flat panel displays and solid-state lightings, developing highly efficient blue emitters is always essential.1-5 Because in full-color displays, efficient deep-blue emitters are essential to produce picturesque displays and reduce power consumption.3, 6-8 Over the past several decades, many efforts have been devoted to develop high-performance blue emitters.9-13 Nevertheless, because the existing of opposing requirements for electrical and optical properties, it is hard to develop blue emitters with matching performance of red and green emitters.14-17 To obtain high efficiency, phosphorescent and thermally activated delayed fluorescent (TADF) emitters have attracted intense interest as they can approach 100% internal quantum efficiency (IQE) by utilizing both singlet and triplet excitons.18-19 However, phosphorescent and TADF blue OLEDs typically have poor color purity, short lifetime and sharp efficiency roll-off at high brightness, which is undesirable in the practical application.20-21 Considering fluorescent emitters possess a mild efficiency roll-off and much longer lifetime than phosphorescence, they are used in many commercial OLED products all the same. Hence, high performance fluorescent devices still play an essential role in OLEDs research. Generally, the color purity and efficiency of deep blue emission are closely bound


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up with material design. The energy gaps of deep blue emitters are intrinsically wider than the green and red emitters, which often lead to poor carrier injection and low luminous efficiency. Because the electron-donating (D) and electron-accepting (A) group often facilitates injection and transport of hole and electron respectively. D-A or D-π-A structure is an effective way to obtain molecules with good electrical properties.22-24 However, due to the intramolecular charge transfer (ICT) in such D-A or D-π-A molecules, the color purity would be undermined with spectral redshifts and broadenings. Besides, it is reported that photoluminescence quantum yield (Фf) increase with the molecular conjugation length in similar molecular systems.25-27 Normally, para-linkage mode would possess longer conjugation and larger overlap of frontier orbitals. However, longer linear conjugation may increase the probability of π-π stacking and undermine deep blue color purity.28-29 Thus, to suppress the unprofitable redshifts, many strategies like the meta-linkage mode, creation of a twisted conformation and doping into a host are commonly used to adjust conjugation lengths and ICT effect of molecules for deeper emissions. Herein, we proposed a novel D-π-A conjugated system using Phenantroimidazole (PI) and indolizine moieties. Here indolizine with appropriate π-conjugation length was used as the acceptor profited from its high fluorescence quantum yield and excellent electron transporting ability enabled by the nitrogen-containing heterocyclic conjugated structure.30-31 PI unit as the mild donor alternatively due to its potential bipolar transporting ability, although it has been commonly exploited as the acceptor



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for the blue emitters that reported extensively.32-37 In addition, we further tuned the π-conjugation degree by adopting meta-linkage model or introducing steric hindrance group methyl to suppress the π-π stacking and adjust the opposing requirements for optical and electrical properties. Hence, we designed and synthesized three blue D-π-A

molecules

using

PI

and

indolizine

moieties.

For

1-(4-(tert-butyl)phenyl)-2-(4'-(2-phenylimidazo[1,2-a]pyridin-3-yl)-[1,1'-biphenyl]-4yl)-1H-phenanthro[9,10-d] imidazole (PPi-Pid), para-linkage strategy is used to endowed this molecule with high Фf and carriers mobility but derivate from the deep-blue

emission

due

to

longer

conjugation

length.

While

for

1-(4-(tert-butyl)phenyl)-2-(3'-(2-phenylimidazo[1,2-a]pyridin-3-yl)-[1,1'-biphenyl]-4yl)-1H -phenanthro[9,10-d]imidazole (PPi-Mid), it has the most balanced carrier transporting ability and deeper lighting spectra under the assistance of meta-linkage strategy.

For

1-(4-(tert-butyl)phenyl)-2-(2-methyl-4'-(2-phenylimidazo[1,2-a]pyridin-3-yl)-[1,1'-bip henyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (PPi-Xid), the introduction of steric hindrance methyl group also made the spectra blue-shift, but weakened the carrier transporting ability due to the increase of twisted angle. With the three compounds as emitters, non-doped OLEDs achieved blue emissions with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.092), (0.152, 0.057) and (0.154, 0.058), maximum brightness of 17100, 9163 and 9165 cd/m2, high electroluminescent efficiencies of maximum current efficiencies (CE) of 3.95, 2.45 and 2.70 cd/A, power efficiencies (PE) of 2.59, 1.20 and 1.93 lm/W and external


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quantum efficiencies (EQE) of 4.95, 4.51 and 4.60% for PPi-Pid, PPi-Xid and PPi-Mid, respectively. Moreover, the doped devices based on PPi-Pid, PPi-Xid and PPi-Mid showed maximum brightness of 14350, 4377 and 4002 cd/m2, high maximum EQE of 6.01, 3.90 and 4.28% and deep blue emissions with CIE coordinates of (0.151, 0.076), (0.155, 0.052) and (0.153, 0.052) respectively. All of the devices show high brightness and unobservable efficiency roll-off, especially the PPi-Pid based device, high EQE over 6% was achieved when the luminance exceeds 10000 cd/m2. 2. Experimental 2.1 Methods All commercially available reagents and chemicals were used without further purification. 1H NMR and 13C NMR spectrometry was recorded with a Bruker av-300 spectrometer. Mass spectrometry (MS) was performed with Bruker micrOTOF-Q II spectrometer. Elemental analysis (EA) was performed on Elementar vario MICRO select Elemental Analyzer. Cyclic voltammetry (CV) was carried out using CHI-660E analyzer at room temperature. Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.10 M) was used as the supporting electrolyte and dichloromethane (DCM) as the solvent, respectively. Theoretical calculation of the compounds was carried out using the Gaussian09 program. Density functional theory (DFT) calculations were carried out with the ground state geometry optimized at the B3LYP/6-31G (d) level. Optical absorption spectra and fluorescence spectra were measured using Hitachi ultraviolet-visible (UV-Vis) spectrophotometer U-3010 and fluorescence spectrometer



F-4600, respectively. PLQY in solvents and in films were respectively measured in quartz cell and on quartz plate using Hamamatsu C11347-11 Quantaurus-QY. Transient PL decay spectra were measured using PTIQM-TM. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were measured by TA instrument TGA Q500 and DSC Q200 respectively. Indium tin oxide (ITO) coated glasses with a sheet resistance of 15 Ω/□ were used as substrates. The ITO glass substrates were washed by acetone, ethanol and deionized water in order, dried in an oven at 120 °C for about 2 hours and treated with ultraviolet-ozone for 25 minutes. Then the substrates were transferred to a vacuum deposition chamber. The OLED were fabricated by thermal evaporation under a base pressure of about 10-6 Torr. The deposition rate and film thickness were monitored by quartz crystal microbalance. Organic layers were deposited at a rate of 2-4 Å/s. MoO3, LiF and Al were deposited onto the organic stack at rates of 0.1, 0.1 and 10 Å/s, respectively. The voltage and current were measured by a computer-controlled Keithley 2400 source meter. The luminance characteristics were measured with a Spectrascan PR655 photometer under ambient atmosphere. CE and PE of the devices were calculated using the measured luminance, current, voltage and its lighting area. The EQE of the device was calculated from the current density, luminance, and EL spectrum, assuming a Lambertian distribution. 2.2 Synthesis Scheme 1 shows the synthetic routes of PPi-Pid, PPi-Xid and PPi-Mid. The compound 1, compound 2 and compound 3 were prepared as reported previously.38 A


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facile one-pot reaction was used to synthesize the target products with good yields.28, 39-40

The crude compounds were then purified using silica gel column chromatography

and characterized with 1H NMR spectroscopy, mass spectroscopy and elemental analysis.

Scheme 1. Synthetic routes for PPi-Pid, PPi-Xid and PPi-Mid.

Synthesis of PPi-Pid (6), PPI-Xid (7) and PPi-Mid (8) Compound 6 was prepared by refluxing phenanthrene-9,10-dione (0.59 g, 2.79 mmol), compound 1 (1.05 g, 2.79 mmol), 4-(tert-butyl)aniline (0.48 mL, 3 mmol), and ammonium acetate (3.79 g, 78.92 mmol) in glacial acetic acid (20 mL) for 24 h under nitrogen atmosphere. After cooling to room temperature, the mixture was poured into a methanol solution under stirring. The separated solid was filtered off, washed with methanol, and dried by rotary evaporation. The solid was further purified by column chromatography (petroleum ether: CH2Cl2, 2:1) on silica gel to get compound 6 (1.47 g, 2.10 mmol, 76%). compound 7 (1.30 g, 1.84 mmol, 68%) was prepared from compound 4 (0.57 g, 2.70 mmol), compound 2 (1.05 g, 2.70 mmol), compound 5 (0.48 mL, 3 mmol) by a similar procedure. compound 8 (1.30 g, 1.87



mmol, 71%) was prepared from compound 4 (0.55 g, 2.70 mmol), compound 3 (1.02 g, 2.63 mmol), compound 5 (0.48 mL, 3 mmol) by a similar procedure. 1-(4-(tert-butyl)phenyl)-2-(4'-(2-phenylimidazo[1,2-a]pyridin-3-yl)-[1,1'-bipheny l]-4-yl)-1H-phenanthro[9,10-d]imidazole (PPi-Pid), 6, pale yellow solid, 76 %, 1H NMR δ(CD2Cl2, 400 MHz), 8.81 (2H, dd, J = 12.4, 8.1 Hz), 8.74 (1H, d, J = 8.3 Hz), 8.03 (1H, d, J = 6.9 Hz), 7.84 – 7.64 (13H, m), 7.53 (5H, dd, J = 11.7, 8.4 Hz), 7.34 – 7.21 (6H, m), 6.81 (1H, t, J = 6.8 Hz), 1.47 (9H, s).

13

C NMR (400 MHz, CDCl3) δ

153.36, 150.40, 144.90, 142.55, 140.50, 140.12, 137.47, 135.99, 134.05, 131.03, 130.06, 129.78, 129.25, 129.01, 128.52, 128.44, 128.33, 128.25, 128.17, 127.99, 127.58, 127.26, 127.21, 127.11, 126.69, 126.26, 125.60, 124.87, 124.84, 124.07, 123.28, 123.11, 122.71, 120.91, 120.62, 117.59, 112.41, 35.06, 31.43. Elemental analyses for CHN, Found: C, 86.70; H, 5.47; N, 7.98%; molecular formula C50H38N4 requires: C, 86.42; H, 5.51; N, 8.06%. HRMS, Found: [M+H]+ 695.3144; molecular formula C50H38N4 requires [M+H]+ 694.3169. 1-(4-(tert-butyl)phenyl)-2-(2-methyl-4'-(2-phenylimidazo[1,2-a]pyridin-3-yl)-[1, 1'-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (PPi-Xid), 7, pale yellow solid, 68 %, 1H NMR δ(CD2Cl2, 400 MHz), 8.81 (2H, dd, J = 12.4, 7.8 Hz), 8.75 (1H, d, J = 8.3 Hz), 8.06 (1H, d, J = 6.9 Hz), 7.82 – 7.64 (7H, m), 7.58 – 7.46 (9H, m), 7.36 – 7.22 (7H, m), 6.84 (1H, t, J = 6.7 Hz), 2.30 (3H, s), 1.47 (9H, s). 13C NMR (400 MHz, CDCl3) δ 153.36, 150.62, 144.89, 142.57, 141.79, 141.35, 137.43, 136.13, 135.23, 134.13, 131.47, 130.37, 130.31, 129.82, 129.59, 129.26, 128.61, 128.42, 128.35, 128.32, 128.28, 128.22, 127.61, 127.29, 127.09, 126.88, 126.69, 126.30, 125.60,


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124.87, 124.11, 123.37, 123.18, 123.16, 122.78, 120.98, 120.83, 117.61, 112.44, 35.10, 31.48, 20.63. Elemental analyses for CHN, Found: C, 86.41; H, 5.69; N, 7.90%; molecular formula C51H40N4 requires: C, 86.76; H, 5.78; N, 7.77%. HRMS, Found: [M+H]+ 709.3297; molecular formula C51H40N4 requires [M+H]+ 708.3326. 1-(4-(tert-butyl)phenyl)-2-(3'-(2-phenylimidazo[1,2-a]pyridin-3-yl)-[1,1'-bipheny l]-4-yl)-1H-phenanthro[9,10-d]imidazole (PPi-Mid), 8, pale yellow solid, 71 %, 1H NMR δ(400 MHz, CD2Cl2), 8.80 (2H, dd, J = 12.7, 4.7 Hz), 8.73 (1H, d, J = 8.3 Hz), 8.02 (1H, d, J = 6.9 Hz), 7.80 – 7.60 (12H, m), 7.57 – 7.43 (6H, m), 7.36 – 7.18 (6H, m), 6.80 (1H, t, J = 6.8 Hz), 1.45 (9H, s).

13

C NMR (400 MHz, CDCl3) δ 153.30,

150.26, 144.78, 142.44, 141.56, 140.13, 137.41, 135.91, 134.01, 130.40, 130.05, 129.99, 129.74, 129.66, 129.19, 129.10, 128.45, 128.40, 128.32, 128.21, 128.05, 127.56, 127.47, 127.21, 127.18, 127.07, 126.72, 126.22, 125.55, 124.81, 124.02, 123.27, 123.08, 123.06, 122.67, 120.86, 120.79, 117.52, 112.41, 35.00, 31.39. Elemental analyses for CHN, Found: C, 87.04; H, 5.48; N, 8.05%; molecular formula C50H38N4 requires: C, 86.42; H, 5.51; N, 8.06%. HRMS, Found: [M+H]+ 695.3153; molecular formula C50H38N4 requires [M+H]+ 694.3169. 3. Results &discussion 3.1 Photophysical properties The photophysical properties of the three emitters were investigated by UV-Vis and Photoluminescence (PL) spectrometry. Absorption and PL spectra of PPi-Pid, PPi-Xid and PPi-Mid are shown in Figure 1 (a) and Table 1. They exhibit similar absorption spectra with three major absorption bands. The absorption bands around



260 nm can be assigned to the π-π* transition of the imidazole ring.37,

41-42

The

short-wavelength absorption bands around 215 nm probably originate from the indolizine unit.30 The intramolecular charge transfer transition is also expected to appear around 347 nm.43 For the PL spectra, PPi-Pid, PPi-Xid and PPi-Mid respectively exhibited deep-blue emission peaks at 426, 416 and 416 nm in DCM while red-shifted to 473, 435 and 433 nm in the thin film due to the intramolecular π-π packing interaction. Compared to PPi-Pid, the emission spectra of PPi-Xid and PPi-Mid exhibited obvious blue-shift whether in solution and solid film as expected due to controlled molecular conjugation. PL quantum yields (PLQYs, Фf s) in DCM were estimated to be 0.93, 0.77 and 0.82 for PPi-Pid, PPi-Xid and PPi-Mid. While in the solid state, PLQY of PPi-Pid, PPi-Xid and PPi-Mid were estimated to be 0.50, 0.43 and 0.47, which may assigned to aggregation induced quenching.

(a)

200

(b) Abs in DCM

PL in DCM

PL in film

PPi-Mid

PPi-Xid

PPi-Pid

300

400

500

Wavelength (nm)

600

Normalized Intensity (a.u.)


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

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Fluo.

Phos. in 2Me-THF at 77 K

PPi-Mid

PPi-Xid

PPi-Pid

400

500

600

Wavelength (nm)

Figure 1. (a) Normalized absorption and PL spectra of PPi-Pid, PPi-Xid and PPi-Mid in DCM and PL spectra in solid-state neat film; (b) PL spectra of PPi-Pid, PPi-Xid and PPi-Mid in 2-MeTHF at 77K (phos. spectra (red-line) measured with 0.5 µs delay).

As shown in Figure 1 (b), the fluorescence and phosphorescence spectra of the three compounds were measured in 2-methyl tetrahydrofuran (2-MeTHF) at 77 K.


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The singlet energy levels (S1) of PPi-Pid, PPi-Xid and PPi-Mid were calculated to be 3.04, 3.13 and 3.07 eV according to the highest energy peaks of fluorescence spectra. From the energy peaks of phosphorescence spectra, the triplet energy levels (T1) were estimated to be 2.38, 2.46 and 2.47 eV.

3.2 Thermal properties Thermal properties were investigated by using TGA and DSC under a nitrogen atmosphere at a heating rate of 20 °C min-1 (Figure S1 in Supporting Information). The decomposition temperatures (Td, 5% weight loss) of PPi-Pid, PPi-Xid and PPi-Mid were measured to be 421.8, 396.3 and 425.3 °C, respectively.37 The glass-transition temperatures (Tg) of PPi-Pid, PPi-Xid and PPi-Mid were not observed from 25 to 500 °C. The melting temperatures (Tm) of PPi-Pid, PPi-Xid and PPi-Mid were measured to be 318.5, 308.5 and 316.6 °C, respectively. The results show that all three emitters own excellent thermal stability, which is crucial to maintain the stability of the device in the case of high current density and brightness. Detailed data are summarized in Table 1. Table 1. Physical properties for PPi-Pid, PPi-Xid and PPi-Mid. Compound PPi-Pid PPi-Xid PPi-Mid

λabs (nm)a 212/259/ 332/364 215/258/ 331/364 218/257/ 338/365

λem (nm)b 426/474 416/436 417/433

Td/Tm (°C)c 421.8/ 318.5 396.3/ 308.5 425.3/ 316.6

HOMO (eV)d

LUMO (eV)e

Eg (eV)f

-5.50

-2.28

3.22

-5.57

-2.34

3.23

-5.58

-2.43

3.15

a

S1/T1 (eV)g

Фfh

3.04/ 2.38 3.13/ 2.46 3.07/ 2.47

0.93/ 0.50 0.77/ 0.43 0.82/ 0.47

Measured in DCM solution at room temperature (10-5 M). b measured in DCM solution/solid-state neat film. c Tg: glass transition temperature; Td: onset temperature



corresponding to 5% weight loss. d HOMO: calculated from the onset oxidation potentials of the compound. e LUMO: estimated from the equation: ELUMO = EHOMO + Eg. f Energy gap estimated from the optical absorption band edge of the DCM. g Singlet/triplet energy levels calculated according to the highest energy peaks of fluorescence spectra at 77 K. h Measured in DCM using 9,10-diphenylanthracene as a standard (Φf = 0.90 in cyclohexane)/the solid state quantum yield on the quartz plate using an integrating sphere apparatus.

3.3 Electrochemical properties CV was conducted to evaluate the HOMO levels of the three compounds. The HOMO levels were determined from the onset of the oxidation potential curves with respect to ferrocene (HOMO = - (4.8 + Eox) eV). As shown in Figure S1, PPi-Pid, PPi-Xid and PPi-Mid have almost the same HOMO level (-5.50, -5.57 and -5.58 eV, respectively), which is attributed to their same donor PI unit.35 The energy gaps (Eg) of PPi-Pid, PPi-Xid and PPi-Mid were estimated to be 3.22, 3.23 and 3.15 eV from their absorption onset. Then the LUMO levels were further calculated to be -2.28, -2.34 and -2.43 eV for PPi-Pid, PPi-Xid and PPi-Mid. 3.4 Theoretical calculations Density function theory (DFT) calculations of the three compounds were carried out at the B3LYP/6-31G (d) level. The Cartesian coordinates of the optimized structures were shown in the Supporting Information. As depicted in Figure 2, the twisted structures are beneficial to suppress the π-π stacking. The HOMOs and LUMOs of PPi-Pid, PPi-Xid and PPi-Mid mainly localized over PI group and the two benzene rings linkage. The overlaps between the HOMO and LUMO orbitals endow them with effective electronic communication between the donor and the benzene rings linkage and more efficient luminescence as emitters in OLEDs.


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Figure 2. DFT B3LYP/6-31G (d) calculated optimized structures and HOMO/LUMO distributions of PPi-Pid, PPi-Xid and PPi-Mid.

3.5 Electroluminescence Properties Non-doped devices were fabricated with the following structure: ITO/MoO3 (1 nm)/TAPC (70 nm)/TCTA (7 nm)/EML (20 nm)/ TPBi (40 nm)/LiF (0.8 nm)/Al (80 nm). Here, ITO (indium tin oxide) is used as anode, MoO3 is the hole injecting layer, TAPC (Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexan) serves as the hole-transporting layer,

TCTA

(4,4′,4″-tris(N-carbazolyl)-triphenylamine)

is

used

as

the

exciton-blocking layer, PPi-Pid, PPi-Xid and PPi-Mid respectively is the emitting layer, TPBi (1,3,5-tri(phenyl-2-benzimidazolyl)-benzene) works as the electrontransporting layer, LiF is the electron injecting layer and Al is used as cathode. Key data are summarized in Table 2. The PPi-Pid based device exhibited stable blue emission at 452 nm with CIE coordinates of (0.150, 0.092). The device exhibited a low turn-on voltage (Von,



voltage at 1 cd/m2) of 2.90 V. As shown in Figure 3, the maximum external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) were 4.95 %, 3.95 cd/A, 2.59 lm/W, respectively. The PPi-Xid based device also exhibited good performance, emitting deep-blue light with a emission peak at 440 nm and CIE coordinates of (0.152, 0.057) and exhibiting a Von, maximum EQE, CE, and PE of 3.30 V, 4.51 %, 2.45 cd/A, and 1.20 lm/W, respectively. The PPi-Mid based device showed a higher Von of 3.10 V, and deeper blue emission peak at 436 nm with CIE coordinates of (0.154, 0.058). The PPi-Xid and PPi-Mid based devices exhibited good CIE coordinates close to high definition television (HDTV) blue standard (0.15, 0.06).


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(b) 10

100

Max cd/m2

1000

Current Density (mA/m2)


(a) PPi-Mid

PPi-Xid

PPi-Pid

400

500

105

500

PPi-Pid PPi-Xid PPi-Mid

400

200

101

100

100

0

10-1 2

3

4

102

103

Luminance (cd/m2)

104

6

7

8

9

10

10-2

(d) 101

PPi-Pid PPi-Xid PPi-Mid External Quantum Efficiency (%)

External Quantum Efficiency (%)

10

0

Power Efficiency (lm/W)

10

1

100

101

5

Voltage (V)


10-1 100

103 102

600

(c) 101

104

300

Wavelength (nm)

Current Efficiency (cd/A)

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


Luminance (cd/m2)

Page 15 of 27

0

10

5

4

3

2 102

103

2

104

Luminance (cd/m )

100

101

102

103

104

Luminance (cd/m2)

Figure 3. (a) EL spectra at different luminance; (b) The current density-voltage-luminescence (J-V-L) curves; (c) CE and PE curves; (d) The EQE curves; of PPi-Pid, PPi-Xid and PPi-Mid non-doped devices. Table 2. Devices data for PPi-Pid, PPi-Xid and PPi-Mid.

EML

Von (V)a

Lmax (cd/m2)b

CE1000/5000/max (cd/A)c

EQE1000/5000/max (%)d

CIE(x,y)e

PPi-Pid PPi-Xid PPi-Mid CBP:40wt%PPi-Pid CBP:40wt%PPi-Xid CBP:40wt%PPi-Mid

2.90 3.30 3.10 3.15 3.27 3.38

17100 9163 9165 14350 4377 4002

3.22/3.42/3.96 1.94/2.36/2.46 2.20/2.25/2.70 3.38/3.65/4.13 1.53/-/1.82 1.60/-/1.93

3.88/4.19/4.95 3.83/4.51/4.21 4.08/4.36/4.60 4.92/5.33/6.01 3.63/-/3.90 3.76/-/4.28

0.150, 0.092 0.152, 0.057 0.154, 0.058 0.151, 0.076 0.155, 0.052 0.153, 0.052

a

Turn-on voltage; bMaximum luminance; cLuminous efficiency: at the brightness of 1000/5000 cd/m2 and maximum brightness; dExternal quantum efficiency: at the brightness of 1000/5000 cd/m2 and maximum brightness; eCIE coordinates at about 1000 cd/m2.

The doped OLED devices were constructed with the same device structures to further

improve

the

device

efficiency

and

color

purity.

Here,

CBP

(4,4'-Bis(carbazol-9-yl)biphenyl) is used as host. CBP: PPi-Pid or PPi-Xid or



PPi-Mid are served as EML. The doping concentrations were optimized to 40 wt%. Compared with the non-doped device of PPi-Pid, the doped device showed higher EQE of 6.01 % and deeper blue emission with CIE coordinates of (0.151, 0.076). As shown in Figure 4, the turn-on voltage of the device was 3.15 V. The performance of PPi-Pid based device is among the best of reported deep blue fluorescence OLEDs.37, 44-46

The PPi-Xid and PPi-Mid based device also exhibits good performance with

CIE coordinates of (0.155, 0.052) and (0.153, 0.052). The CIE coordinates of the doped devices is deeper compared with the non-doped ones. This change in emission energies can be attributed to the difference in polarity of the environment (i.e., of the host) in the solid films. Furthermore, it is noticeable that the doped devices show excellent efficiency stability compared with the non-doped ones.


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Page 17 of 27

(b) 100

Max cd/m2

1000

PPi-Mid

PPi-Xid

PPi-Pid

400

500

300

102 101

100

600

100 0

10-1 2

3

4

100

102

103

Luminance (cd/m2)

6

7

8

9

10 11

104

10-1

(d) 101



100


101

Power Efficiency (lm/W)


101

5

Voltage (V)

(c)

10-1 100

103

200

Wavelength (nm)

101

104


Luminance (cd/m2)

10

Current Density (mA/m2)


(a)

Current Efficiency (cd/A)

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


100

100

101

6

5

4

3 10

3

102

10

4

Luminance (cd/m2)

103

104

Luminance (cd/m2)

Figure 4. (a) EL spectra; (b) The J-V-L curves; (c) CE and PE curves; (d) The EQE curves; of PPi-Pid, PPi-Xid and PPi-Mid doped devices.

3.6 Single-carrier devices Hole- and electron-only devices were fabricated to investigate the bipolar properties of the three compounds. Hole and electron mobility of PPi-Pid, PPi-Xid and PPi-Mid were investigated by space charge limited current (SCLC) method. Hole-only device: ITO/TAPC (10 nm)/EML (80 nm)/TAPC (10 nm)/Al (80 nm); Electron-only device: ITO/TPBi (10 nm)/EML (80 nm)/TPBi (10 nm)/LiF (0.8 nm)/Al (80 nm). As depicted in Figure 5, all the compounds show bipolar transport properties. PPi-Pid provides a better hole- and electron-transporting behavior compared to PPi-Xid and PPi-Mid, which explains why the devices based on the PPi-Pid

achieved

relatively

lower

turn-on

voltage


and

higher

current


density/luminescence. Moreover, PPi-Mid possesses the most balanced carrier transport ability among the three compounds which well explains why the PPi-Mid based devices exhibit smallest efficiency roll-off among the three compounds (Figure 4 (d) and 5 (d)). It is noticeable that the introduction of methyl group to the benzene ring suppresses carrier transport properties, especially the hole transporting properties. 10-5 10-6

Mobility (cm2/Vs)

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

10-7 10-8 Hole only_PPi-Pid Electron only_PPi-Pid Hole only_PPi-Xid Electron only_PPi-Xid Hole only_PPi-Mid Electron only_PPi-Mid

10-9

10-10 10-11

10-12

800

1000 1200 1400 1600 1800

[Electric field (V/cm)]1/2

Figure 5. Carrier mobility characteristics of PPi-Pid, PPi-Xid and PPi-Mid.

In Figure 3 (d) and 4 (d), the EQEs of non-doped or doped OLEDs exhibit mild roll-off and maintain high EQEs at their maximum brightness. These performance at maximum brightness are among the best for either the non-doped or doped OLEDs (Table 3). The main reasons of EQE roll-off at high brightness are twofold: imbalance between the numbers of electrons and holes in EML and non-radiative exciton quenching processes.47 For the phosphorescent OLEDs, due to the longer lifetime of triplet excitons, the triplet-triplet annihilation would influence the efficiency roll-off.48 While, due to singlet excitons have less time to cause quenching processes, achieving mild or zero roll-off should be possible for many fluorescent OLEDs, provided that charge balance is maintained. As shown in Figure 3 (a) and 4 (a), all the devices exhibit stable EL spectra from 10 cd/m2 to their maximum


Page 18 of 27

Page 19 of 27 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


brightness, indicating that these devices possess good charge balance and can maintain the recombination zone in the EML. Thus, the high EQEs at relative high brightness may result from good device structure and balanced carrier transportation. Table 3. Key performance data for devices based on PPi-Pid, PPi-Mid and other high performance blue emitters from literature.

EML

Von (V)a

Lmax (cd/m2)b

CE (cd/A)c

EQE (%)c

CIE(x,y)d

Reference

PPi-Pid CBP:40wt%PPi-Pid mCBP:1wt%DABNA-1 2NBTPI Ph-BPA-BPI CBP:5wt%12 DPT-TPI PPi-Mid CBP:40wt%PPi-Mid CBP:5wt%3 TTP-TPI CBP:5wt%2

2.90 3.15 4.10e 2.90 2.52 2.80 2.90 3.10 3.38 2.80 3.10 2.80

17100 14350 200e 5000e 3100e 16220 3000e 9165 4002 18580 2000e 7250

3.95 4.13 4.00e 3.20e 2.15e 2.40e 2.70 1.93 1.30e -

4.95 6.01 5.00e 5.05e 3.30e 1.50e 1.25e 4.60 4.28 2.60e 0.65e 1.00e

0.150, 0.092 0.151, 0.076 0.150, 0.090 0.150,0.090 0.150, 0.080 0.150, 0.070 0.150,0.070 0.154, 0.058 0.153, 0.052 0.150, 0.060 0.160, 0.050 0.150, 0.040

This work This work Ref.20 Ref.42 Ref.49 Ref.44 Ref.50 This work This work Ref.44 Ref.50 Ref.44

a

Turn-on voltage; bMaximum luminance; cLuminous and external quantum efficiencies at maximum luminance; dCIE coordinates at 1000 cd/m2; eValues are estimates from the original figures.

The PPi-Pid based device exhibits high EQE of 6.01 % which exceeds the limit of typical fluorescent OLED (assuming out-coupling efficiency of 20 %). The transient PL decay of PPi-Pid, PPi-Xid and PPi-Mid was measured in neat film (Figure S2, Supporting Information), only prompt lifetimes are observed, thus the possibility of TADF can be excluded for three emitters. Then, the PL and absorption spectra were measured in different polar solvents to further investigate the charge transfer (CT) property. As shown in the inset of Figure 6 (Figure S3), an obvious solvatochromic effect with a redshift of 34 nm (21, 20 nm) was observed, when in different solvents from low to high polarity. The dipole moment of the excited state



can be estimated from the slope of a plot of stokes shift (υa-υf) versus solvent polarity function f, according to the Lippert-Mataga equation: hcሺ߭a ߭f ሻ = hcሺ߭ao ߭fo ሻ +

2ሺߤ݁ − ߤ‫ ݋‬ሻ2 fሺε, nሻ ܽ3

where f is the orientational polarizability of solvents, µe is the excited-state dipole moment, µo is the ground-state dipole moment; ε and n are the solvent dielectric and the solvent refractive index, respectively; a is the solvent cavity (Onsager) radius, derived from the Avogadro number (N), molecular weight (M), and density (d=1.0 g/cm3); f(ε, n) and a can be calculated respectively as follows: fሺε, nሻ =

ɛ−1 n² − 1 − 2ɛ + 1 2n² + 1

,

ܽ=ሺ

3M 1/3 ሻ 4Nπd

As shown in Figure 6 (Figure S3), two recognizable excited states were observed in low polarity and high polarity solvents, whose dipole moments µe were calculated to be 6.3 and 13.0 D, respectively. The small µe of 6.3 D can be attributed to the usual excited state, which is a LE like state, while the larger µe of 13.0 D should be treated as a hybridized local and charge transfer (HLCT) state.51-54 In this case, due to the intercrossing and coupling between LE and CT states, the HLCT state with an obvious CT feature in the excited state may be formed, which provided a weak binding energy and resulted in a larger singlet/triplet ratio than 1/3 of spin statistic.51-52, 55-56


Page 20 of 27

Page 21 of 27

6500 -1

6000 5500


7000

va-vf (cm )

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


1.0 0.8 0.6 0.4 0.2

34 nm

Hexane Triethylamine Butyl ether Diethyl ether Ethyl acetate DCM DMF Acetone Acetonitrile

0.0 350 400 450 500 550 600 650 Wavelength (nm)

5000 PPi-Pid

4500

-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30

f

Figure 6. The solvatochromic spectra (inset) and fitted linear correlation of the Stokes shift as a function of solvent polarity for PPi-Pid in different polar solvents.

4. Conclusions In conclusion, three novel deep blue materials PPi-Pid, PPi-Xid and PPi-Mid with HLCT state in D-π-A framework were designed and synthesized. With excellent photophysical properties and good thermal stabilities, PPi-Pid, PPi-Xid and PPi-Mid are suitable for application in OLED. The color purity were successfully tuned without scarifying device efficiencies and electrical properties. The non-doped/doped devices based on PPi-Pid, PPi-Xid and PPi-Mid achieved blue emission with CIE coordinate of (0.150, 0.092)/(0.151, 0.076), (0.152, 0.057)/( 0.155, 0.052) and (0.154, 0.058)/( 0.153, 0.052) and high EQE of 4.95%/6.01%, 4.51%/3.90% and 4.60%/4.28% respectively. All of the devices exhibited high brightness along with high EQE, especially the PPi-Pid based device, exhibited high EQE over 6% at high brightness of 14350 cd/m2. The outstanding feature presented here can provide us with a good reference for future high performance deep-blue emitter design. Supporting Information TGA, DSC and CV curves; Transient PL decay spectra; Solvatochromic spectra; Cartesian coordinates of the optimized structures; 1H NMR,


13

C NMR and HRMS


Page 22 of 27

spectra. Acknowledgements The work was supported by the National Natural Science Foundation of China (NSFC Grant Nos. 61775029, 51673113 and 51533005), "Science Fund for Distinguished Young Scholars of Sichuan Province (No.2015JQ0006)" "the Fundamental Research Funds for the Central Universities (ZYGX2016Z010 and ZYGX2015J048)." Reference (1) Zhang, B.; Tan, G.; Lam, C. S.; Yao, B.; Ho, C. L.; Liu, L.; Xie, Z.; Wong, W. Y.; Ding, J.; Wang, L. High-Efficiency

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