Meta-substituted bipolar imidazole based emitter for efficient non-doped deep blue organic light emitting devices with a high electroluminescence

Journal of Photochemistry & Photobiology A: Chemistry 379 (2019) 72–78

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Research paper

Meta-substituted bipolar imidazole based emitter for efficient non-doped deep blue organic light emitting devices with a high electroluminescence

T

⁎

Amjad Islama,b, , Khurram Usmanb,c, Abdul Ghafar Wattoob,d, Tauseef Shahidb, Nadeem Abbasb,e, ⁎ Hafiz Muhammad Adeel Sharifc,f, Ahmad Hassan Siddiqueb, Manan Ahmedd,g, Ziyi Geb, , Xinhua Ouyanga a

College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, PR China Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China c International Academy of Optoelectronics at Zhaoqing, South China Normal University, PR China d Department of Physics, Khawaja Fareed University of Engineering and Information Technology, Rahim Yar Khan-64200, Pakistan e Materials School of Shenzhen University, Shenzhen Key Laboratory of Advanced Functional Materials, Shenzhen, 518060, PR China f Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, No. 18 Shuangqing Road, Haidian District, Beijing 100085, PR China g School of Chemistry, The University of New South Wales, Sydney 2052, Australia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Benzimidazole Phenanthroimidazole Bipolar Deep blue Electroluminescence OLED

Enormous endeavors have been made to explore pure and efficient deep-blue electroluminescent materials for organic light emitting devices (OLEDs). However, materials with deep-blue emission and high efficiencies for non-doped OLED devices are still limited. In view of this, a novel ambipolar meta-substituted emitter (TPA-BIPI) is synthesized by combining phenanthroimidazole, benzimidazole and triphenyl-amine units for OLED devices. Non-doped OLED device is constructed by employing TPA-BIPI as emitter which gives deep-blue emission at 442 nm with CIE of (0.149, 0.105). The EQE of the device is obtained to be as high as 4.53% with the current efficiency (CE) of 4.3 cd/A and a power efficiency (PE) of 3.7 lm/W. A low driven voltage of 3.4 V, a reduced efficiency roll-off and a high electroluminescence of 12,491 cd/m−2 is also achieved. This combination of donor and acceptor moieties indicates a strong potential to prepare efficient bipolar materials for OLEDs.

1. Introduction

avoid concentration-caused quenching and triplet–triplet annihilation (TTA) of the phosphors. [18–22] Consequently, apart from the phosphorescent dopant, host material also plays a crucial role in the device performance. However, host-guest system has a drawback of potential phase separation during fabrication process. Additionally, this system is complicated and increase the number of materials as well as cost of fabrication process. Due to these reasons, high performance fluorescent emitters for non-doped devices are urgently required. So far, many electroluminescent materials have been discovered through the attachment of electron-rich and electron-poor groups into a molecule to realize maximum balanced charge recombination for achieving excellent device performance [23–34]. However, efficient and cost-effective materials are still limited in numbers for non-doped OLED device fabrication. To achieve efficient deep-blue luminogens, it is suggested that extension of conjugation can be controlled through the modification of molecular linkage from para-to meta-modes. [35] Nevertheless,

Owing to the increase in the commercial demand of organic lightemitting diodes (OLEDs) in display and lighting technologies, the requirement of blue emitters has also been increased. High performance deep-blue fluorophores are still rare. [1–6] The existing blue emitters are less efficient as compared to red and green emitters, indicating that more energy is consumed by these blue emitters in practical applications, which motivates researchers to explore blue fluorophores with high photoluminescence quantum yield and high-power efficiency to minimize energy consumption. However, it is still a challenging task to obtain a highly efficient and pure blue emission. Phosphorescent OLEDs (PhOLEDs) have demonstrated a significant potential in lighting and display technologies, because PhOLEDs utilize both, singlet as well as triplet excitons to realize 100% internal quantum efficiency (IQE). [7–17] In PhOLEDs, a host-guest method is used in which phosphorescent dopant is doped in a fluorescent host to

⁎ Corresponding authors at: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China. College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, P. R. China E-mail addresses: [email protected] (A. Islam), [email protected] (Z. Ge).

https://doi.org/10.1016/j.jphotochem.2019.04.033 Received 10 December 2018; Received in revised form 20 April 2019; Accepted 21 April 2019 Available online 27 April 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.

Journal of Photochemistry & Photobiology A: Chemistry 379 (2019) 72–78

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owing to their ambipolar characteristics, high photoluminescence quantum yield, high stability, twisted conformation, high triplet energy and pure blue emission [34–37,44,45]. To enlarge the imidazole molecules family and to develop efficient bipolar blue emitters for nondoped OLED devices, an ambipolar emitter (TPA-BIPI) was designed by combining phenanthroimidazole and benzimidazole with triphenylamine. As demonstrated in scheme 1, in the preparation of (TPA-BIPI) molecule, an intermediate product, 4′-(1-(3-bromophenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenylbiphenyl-4-amine (3) was synthesized by condensation reaction and then Suzuki coupling reaction with 4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenylboronic acid (4) to get the designed material TPA-BIPI in a high yield (> 70%). TPABIPI was further purified using sublimation process to realize highly pure material for device fabrication. The structure of the compound was confirmed through 1H, 13C NMR and mass spectroscopy (Figs. S1 and S2).

demonstration of this strategy to realizing efficient deep-blue emission is less in numbers. For example, Hung and co-workers showed that Nconnected benzimidazole molecules exhibited a significant advantage compared with the C-connected counterparts [36]. The sp3 hybridization of N1 and meta-linking mode enhance the triplet energy level. [37] Herein, we construct a non-doped OLED showing deep-blue electroluminescence by using a bipolar donor-acceptor (D-A) molecule, N,Ndiphenyl-4′-(1-(4′-(1-phenyl-1H-benzo[d]imidazol-2-yl)biphenyl-3-yl)1H-phenanthro[9,10-d]imidazol-2-yl)biphenyl-4-amine (TPA-BIPI), prepared from a combination of benzimidazole (BI), phenanthroimidazole (PI) and triphenyl-amine (TPA) units. Benzimidazole and phenanthroimidazole units are incorporated in the same molecule to construct non-coplanar and asymmetrical geometry of the molecule, which reduces intermolecular interaction, increases morphological stability and enhances the device efficiency. Molecular geometry plays an important role in determining the color of emission and excited state energy. [38–40] Also, the charge injection/ transportation properties of molecules with asymmetric structure are improved [41–43]. Consequently, the resulting asymmetrical TPA-BIPI molecule achieves better spatial separation of HOMO/LUMO level to realize wide bandgap and excellent charge injection as well as charge transportation. The TPA-BIPI fluorophore shows higher decomposition temperature (Td) and exhibits higher glass transition temperature (Tg) than its parent molecule (TPA-PPI) without benzimidazole group. Nondoped blue OLED based on TPA-BIPI has emitted a deep-blue light at 444 nm with CIE(x, y) of (0.14, 0.10). TPA-BIPI based OLED shows higher color purity as compared to TPA-PPI molecule (0.15, 0.11) without benzimidazole moiety. The device demonstrates an external quantum efficiency (EQE) of 4.53%, current efficiency (CE) of 4.3 cd/A at low operating (VON) voltage (3.4 V).

2.2. Electrochemical and thermal properties The cyclic voltammetry tool has been used to study the electrochemical efficiency of our material. With the help of onset potential (of initial oxidation connected to ferrocene) the highest occupied molecular orbital (HOMO) energy values were obtained (see the Fig. S3). HOMO energy level can be calculated with the equation given as, EHOMO=-([Eonset]ox + 4.8). HOMO energy level of TPA-BIPI molecule is -4.99 eV. The value of energy band gap (Eg) of this material is around 2.95 eV, acquired by using absorption spectrum. The difference between the HOMO and Eg gives out the value of the lowest unoccupied molecular orbital (LUMO) energy levels (= -2.04 eV) of our material. All above mentioned values are presented in the Table 1. Thermal stability of material showing electroluminescence is one of the most important characteristic needs to be taken into the account for an efficient OLED. The thermogravimetric analysis (TGA) and differential scanning colorimetry (DSC) has been utilized to examine the thermal stability of TPA-BIPI in nitrogen environment. The schematic is shown in Fig. 1 and Table 1 represents the thermal data. The TGA indicates a high thermal stability of our material with temperature range above 500 °C. For TPA-BIPI, a 5% (Td) loss of weight has been detected at 530.6 °C. The sublimation temperature of this material is at least 150 °C

2. Results and discussion 2.1. Synthesis Scheme 1 presents the preparation route of TPA-BIPI molecule, which is consisted of triphenyl-amine, benzimidazole and phenanthroimidazole moieties. Phenanthroimidazole and its derivatives have been employed as electroluminescent materials in electronic devices

Scheme 1. Synthetic pathway of TPA-BIPI. 73

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Table 1 Optical, electrochemical and thermal data of TPA-BIPI. Compound

Abs.a (nm)

PLa (nm)

Tdb, Tgb, (oC)

HOMO (eVc, eVd)

LUMO (eVc, eVd)

Ege (eV)

Egf (eV)

PLQYg,h (%)

TPA-BIPI

377

457

530.6/162.94

−4.99/-4.88

−2.04/-1.57

2.95

3.29

94/87

a b c d e f g h

UV–vis absorption and PL spectra in THF solution. Loss of 5 wt %. HOMO was evaluated from the onset of oxidation potential; LUMO was obtained from HOMO and Eg. Achieved from DFT calculations using B3LYP/6-31 G(d,p). Obtained from UV absorption onset in film. Calculated using B3LYP/6-31 G(d,p). Absolute PLQY investigated using an integrated sphere in THF solution. PLQY in film.

both, HOMO & LUMO are totally separated from each other. This total separation of HOMO and LUMO energy levels could be justified by disruption of conjugation at the N1 position of phenanthroimidazole. This would be helpful to create a balanced charge transfer (CT) in the emissive layer as well as CT process, which was further investigated through solvatochromism effect. The separation of HOMO and LUMO levels reveal their strong bipolar features. This configuration is different from other synthesized imidazole containing materials. [37,44] HOMO and LUMO energy levels of -4.88 eV and -1.57 eV for TPA-BIPI were obtained. Calculated optical band gap (Eg) of 3.31 eV was achieved for TPA-BIPI. 2.4. Photophysical properties To evaluate the optical properties, UV–vis absorption and photoluminescence (PL) spectrum of TPA-BIPI molecule was measured and is displayed in Fig. 3. Photophysical data of the parameters is presented in Table 1. In tetrahydrofuran (THF) solution, UV–vis absorption spectrum of TPA-BIPI gave an absorption peak with maximum intensity at almost 377 nm (Fig. 3a). This peak at 377 nm is a characteristic peak observed for phenanthroimidazole moiety. [24,29–32] PL spectrum of TPA-BIPI material in THF solution and film is shown in Fig. 3b. TPA-BIPI emitted deep-blue light with maximum intensity peak at 457 nm. Similar UV–vis absorption and PL behavior has already been observed in the previously reported phenanthroimidazole-based molecules. [26,27,30,30,31,32] PL behavior of TPA-BIPI molecule was also investigated in solvents having different polarity (Fig. 3c). By increasing the polarity from low (hexane) to high (ethyl acetate), emission peak was changed to higher wavelength (from 390 in hexane to 457 nm in THF), illustrating the variations in dipole moment (solvatochromism). However, in ethanol, the change in peak was less compared to THF. From absorption onset, Eg of TPA-BIPI was achieved to be 3.02 eV.

Fig. 1. TGA (inset: DSC) plots of TPA-BIPI.

lower than this value. The material also demonstrated high glass transition temperature (Tg) ˜ 162.94 °C Fig. 1. The twisted shape of the material is thought to be the major reason for higher values of Tg, that increases the stability of the molecules at high temperatures. [45] 2.3. Theoretical calculations To obtain more information about the three-dimensional conformation and electronic structure of this molecule, theoretical calculations were applied using density functional theory (DFT) through gaussian software at B3LYP/6-31 G (d) level (Fig. 2). HOMO level of TPA-BIPI is populated on phenyl ring, TPA and phenanthroimidazole moieties (Fig. 2a). On the other hand, the LUMO level of TPA-BIPI is localized on phenyl ring, benzimidazole moiety and phenyl ring attached to the N-atom of imidazole ring at position 3 (Fig. 2b). No orbital overlapping between HOMO and LUMO is found in this molecule and

2.5. Electroluminescence performance High thermal stability, good deep blue emission and wide band gap (< 3.0 eV) of this material rendered it suitable to use as emitter in OLED device. To evaluate the electroluminescence performance of TPA-

Fig. 2. Optimized HOMO and LUMO energy level distributions of fluorescent material (TPA-BIPI) calculated by using DFT. 74

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Fig. 4. Electroluminescence characteristics of TPA-BIPI-based OLED: (a) EL spectra of the device; (b) Current density vs voltage vs luminescence (J-V -L) curves; (c) Current Efficiency vs luminescence vs EQE. Fig. 3. (a) UV–vis abs. in solution (b) PL spectra of TPA-BIPI in THF solution and film (c) PL spectra of TPA-BIPI in different solvents.

BIPI, a non-doped OLED device was constructed employing the structure of ITO/NPB (40 nm)/TCTA (5 nm)/TPA-BIPI (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al, where ITO (indium doped tin oxide) was worked as anode, NPB (N,N'-di-1-naphthyl-N,N'-diphenylbenzidine) was adopted as hole-transport material, TCTA (4,4′,4′'-tri(N-carbazolyl)-triphenylamine) was served as electron blocking material and 75

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TPBI (1,3,5-tris(phenyl-2-benzimidazolyl)benzene) was utilized as the electron-transport material and also hole-blocking material. LiF/Al was used as cathode. The device structure is shown in Fig. S4. Owing to the higher energy gap and higher triplet energy of TCTA, it was employed as exciton confinement layer and electron-blocking layer. Besides, to achieve higher performances as well as suppress efficiency roll-off, charge-balance in emissive layer is crucial. Considering the higher hole-transporting mobility as well as the relatively low electron-transporting ability of TPBi, the TCTA layer was utilized to balance the charges in the EML, which possess relatively low hole transporting mobility. As can be presented in Fig. 4a, EL spectrum of the device exhibited emission peak at 442 nm having CIE coordinates of (0.149, 0.105), which is very much in agreement to the NTSC standard for deep blue emission. The emission of TPA-BIPI device was found to be highly pure and stable. Fig. 4b presents the current-density versus voltage versus luminescence (J-V -L) characteristics. No hole-injection barrier was observed from NBP to the emissive layer. The TPA-BIPI device demonstrated a high electroluminescence of 12,491 cd m−2. The device observed a low VON of 3.4 V. Low driven voltage can be attributed to the higher mobility of the material. Apart from stability and color purity, the device realized high efficiency and reduced roll-off. The EQE of non-doped device was obtained to be as high as 4.53% (Fig. 4c). Even at the luminescence of 100 cd m−2, EQE value remained to be 4.34%. On the other hand, CE and PE of the device were achieved to be 4.3 cd/A and 3.7 lm/W. Roll-off in CE and EQE was also very less. At 100 cd m−2, CE remained to be 4.1 cd m−2. High EQE of device can be ascribed to the high PLQY (94%) of the material. EL performance data of device is displayed in Table 2. This value of EQE is among the excellent results of non-doped deep-blue OLEDs obtained till date (Table 3). [46–54]

Table 3 Comparison of this material with best reported materials. Compound

VON (V)

CE (cd/A)

PE (lm/W)

EQE (%)

Ref.

TPA-BIPI m-TPE-p-TPE TPA-BPI Cz-BTPE pTPE-2mTPE 4TPEDTPA TPETPAPI PyPC 1

3.4 4.1 2.8 4.9 3.7 4.1 3.14 3.2 3.1

4.3 2.8 2.63 3.74 4.03 8.0 3.56 2.38 3.21

3.7 2.0 2.53 2.55 2.79 5.8 2.8 1.89 3.15

4.53 1.9 3.08 1.9 2.17 3.7 3.80 3.72 2.46

This Work 40 41 42 43 44 45 46 47

2.6. Hole-only and electron-only devices Fig. 5. Current density versus voltage (J-V) curves of hole-only and electrononly devices of TPA-BIPI.

Charge mobility makes a significant impact on charge-balance present in the emission layer. Charge transport nature of TPA-BIPI was determined by measuring the current densities of the hole-only and electron-only devices with the configurations of ITO/HATCN(10 nm)/ TPA-BIPI(100 nm)/HAT(10 nm)/Al(150 nm) and ITO/Bphen(10 nm)/ TPA-BIPI(100 nm)/Bphen(10 nm)/LiF(0.5 nm)/Al(150 nm). The (J-V) current density versus voltage curves of single carrier (hole and electron) devices are presented in Fig. 5. HATCN (Hexaazatriphenylenehexacarbonitrile) was employed to block the electrons from the cathode to TPA-BIPI layer. In the same way, Bphen (4,7-Diphenyl-1,10-phenanthroline) was employed to block the holes from anode to the TPA-BIPI layer. J-V curves of the single carrier devices reveal the ambipolar transporting ability of TPA-BIPI. Generally, hole JSC is much higher (almost 1000 times) than electron JSC of organic semiconductors. [55] But the TPA-BIPI based device exhibits higher electron JSC compared to hole JSC (Fig. 5). It can be assigned to the change in the packing of molecules in the solid state. [56] Also, current density of single carrier device depends on interfacial properties of the material. This implies the bipolar nature of this material for the construction of high-performance OLEDs.

3. Conclusions In summary, we have prepared a meta-substituted bipolar fluorescence emitter for OLEDs. TPA-BIPI has exhibited a significant potential to explore efficient and stable deep-blue emitters. TPA-BIPI gave deepblue electroluminescence at 442 nm. The material demonstrated excellent performance for non-doped deep-blue OLEDs and these results are comparable with the excellent results achieved for non-doped devices so far. EQE of 4.53% was realized, CE of 4.3 cd/A and PE of 3.56 lm/W was achieved with CIE(x, y) (0.149, 0.105) at a low operating voltage of 3.4 V. Interestingly, a high electroluminescence of 12,491 cd/m−2 was obtained. Lastly, these results indicate a strong potential of imidazole-based derivatives as efficient fluorescence materials.

Acknowledgements This work was financially supported from the National Natural Science Foundation of China (21674123, 31700520), National Natural

Table 2 Electroluminescent performance data of the non-doped TPA-BIPI device. Device

TPA-BIPI

Von (V)

3.4

a

values at 100 cd m−2

maximum values ηC (cd A−1)b

ηP (lm W−1)

4.3

3.7

b

EQE (%)b

L (cd m−2)

4.53

12,491

b

V (V) 6

b

ηC (cd A−1)b

ηP (lm W−1)

4.1

2.2

b

EQE (%)b

CIE (x,y)c

λEL (nm)

4.34

(0.149, 0.105)

442

d

Abbreviations: a) Von = turn-on voltage at 1cd/m2; b) V = voltage; ηC = current efficiency; ηP = power efficiency; EQE = external quantum efficiency; L = luminescence; c) CIE = Commission International de I’Eclairage coordinates; d) λEL = maxima of electroluminescent spectra. 76

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Science Foundation of Fujian Province (2018J01592), New Century Excellent Talents in Fujian Province University (KLa17009A), International cooperation project of Fujian Agriculture and Forestry University (KXGH17003), and the Distinguished Young Scholars of Fujian Agriculture and Forestry University (No. xjq201729). Thanks to CAS-TWAS for providing funding.

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