基于含芴菲并咪唑衍生物的可湿法加工的蓝光有机电致发光器件

左红文, 欧阳新华 , 王 娟， 杨利营， 印寿根， 葛子义*

(1. 天津理工大学 材料科学与工程学院， 天津 300384；2. 中国科学院 宁波材料技术与工程研究所， 浙江 宁波 315201)

基于含芴菲并咪唑衍生物的可湿法加工的蓝光有机电致发光器件

左红文1，2, 欧阳新华2, 王 娟2， 杨利营1*， 印寿根1*， 葛子义2*

(1. 天津理工大学 材料科学与工程学院， 天津 300384；2. 中国科学院 宁波材料技术与工程研究所， 浙江 宁波 315201)

设计合成了一种新型A-D-A含芴菲并咪唑和炔基基团的蓝光分子(FI)，采用紫外吸收、光致发光光谱和循环伏安对材料进行了表征，发现该物质具有稳定的蓝光发射。采用旋涂的方法制备了非掺杂蓝光有机发光二极管，器件的最大电流效率为1.52 cd·A-1,最大功率效率为0.63 lm·W-1.

芴的衍生物; 湿法加工; 蓝光OLED

1 Introduction

Organic light-emitting diodes (OLEDs) possess high potential for flat panel displays and lighting applications[1]. Commonly, high-vacuum thermal evaporation is used for fabrication of small molecule-based OLEDs (SMOLEDs), and solution processing technology is used for those based on polymers (POLEDs)[2-3]. Thermal evaporation deposition allows for stacking an arbitrary number of functional layers, such as charge carrier transport or blocking layers. This in turn allows a much better charge carrier confinement within the active layer and hence could lead to a higher device efficiency and lifetime[4-6]. In contrast, solution-based deposition limits fabrication of composite device structures. Because the solvent used for one layer can redissolve or otherwise damage the previous layers[7-8]. However, compared with the vacuum deposition, solution-processing offers significant advantages in cost, throughput and ease of fabrication. It is compatible with roll-to-roll processing and inkjet printing[3,9]. Additionally, it is possible to realize co-doping of several dopants by mixing the dopants and host material in solution[2]. Recently, efficient OLEDs based on solution-processed small molecules have been reported. Parketal.[10]fabricated device with a structure of ITO/PEDOT∶PSS/9-(9-phenylcar-bazole-3-yl)-10-(naphtha-lene-1-yl)anthracene(PCAN)/ Bp-hen/LiF/Al, which showed a maximum current efficiency of 1.15 cd·A-1and a external quantum efficiency (EQE) of 1.24%. Heetal.[2]reported fluorescent SMOLEDs with spin-coating blends of N″-biphenyl-(1,1″-bi- phenyl)-4,4″-diamine(NPB) and tris-8-hydroxyquinoline-aluminum (Alq3) as the emitting layer exhibited maximum brightness and luminous efficiency exceeding 10 000 cd·m2and 3.8 cd·A-1, respectively. These values are comparable to those of thermally evaporated Alq3-based devices. Hence, solution processing technology for the fabrication of SMOLEDs is of great importance to the commercialization of OLEDs.

Fluorene and its derivatives also have been widely investigated as blue emitters due to their high fluorescent quantum yields and good thermal stabilities[11]. With the ease and variability of substitution at C-9 position, fluorene units facilitate the excellent solubility, and modulate the excimer formation without effecting its electronic properties[12]. In addition, an appropriate functionalization at C-2 and C-7 position is beneficial for conjugation extension which is necessary to realize the high photoluminescence quantum yield (PLQY)[13]. Kieffer’s groups reported, at 100 cd·m-2, an EQE could reach 7.0% with CIE coordinates of (0.151, 0.088) by using a fluorene-based oligomer[14]. Huangetal. reported that star-shaped fluorene hybrids SMOLEDs which were fabricated by spin-coating emitting layer exhibited maximum brightness and luminous efficiency which exceeded 7 000 cd·m-2and 0.9 cd·A-1, respectively[15]. Yuetal.[16]reported that an EQE is 5.8% with CIE coordinates of (0.156, 0.055) at 100 cd·m-2by exploring cyanofluorene acetylene emitter.

Here, a novel blue emitting material, 2,2′-(((9,9-dioctyl-9H-fluorene-2,7-diyl)bis (ethyne-2,1-diyl))bis (4,1-phenylene))bis(1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazole) was designed and synthsized, named as FI. The optical property of FI was characterized by UV-Vis absorption, photoluminescence (PL) and electroluminescence (EL) spectra. Through solution-process, a non-doped blue organic light-emitting diode based on FI as the emitter was fabricated. The results showed a luminance efficiency of 1.52 cd·A-1and a power efficiency of 0.63 lm·W-1.

2 Experiments

2.1 Materials and Measurements

2,7-dibromo-9,9-dioctyl-9H-fluorene,ethynyltrime-thylsilane,bis(triphenylphosphine)palladium(Ⅱ) chloride were purchased from Tokyo Chemical Industry Co., Ltd.. Cuprous iodide (CuI), dimethyl formamide (DMF), triethylamine (NEt3) were obtained from J&K Chemical company. All other reagents were from Sinopharm Chemical Reagent Co., Ltd.The solvents involving air-sensitive reagents were performed under an inert atmosphere of dry nitrogen. All other reagents were used as commercial sources, unless otherwise stated.1H NMR spectra were determined in CDCl3with a Bruker DRX 400 MHz spectrometer (TMS as internal standard). UV-Vis absorption spectra (UV) were recorded on a PerkinElmer Lambda 950 spectrophotometer. Fluorescence (PL) measurements were carried out with a FLSP920 spectrophotometer in a solution of 10-6mol·L-1. Differential scanning calorimetry (DSC) curves were obtained with Metler Toledo DSC822 instrument at 20 ℃·min-1under nitrogen flushing. Thermogravimetric analyses (TGA) were carried out using a PerkinElmer Pyris thermogravimeter under a dry nitrogen gas flow at a heating rate of 10 ℃·min-1. The electrochemical properties of derivatives was studied through cyclic voltammetry (CV) on a CHI 660D analyzer with a three electrode configuration, which a Pt disk is as the working electrode of 0.01 cm2, a Pt wire as the counter electrode, and an Ag/AgCl as the reference electrode, and in a dichloromethane solution containing 0.1 mol/L of tetrabutylammonium hexafluorophosphate as supporting electrolyte.

2.2 Synthesis of Compound

The synthetic route and molecular structure of FI is illustrated in Fig.1.

Fig.1 Synthetic route for FI

2.2.1 Preparation of ((9, 9-dioctyl-9H-fluorene-2,7-diyl)bis(ethyne-2,1diyl))bis(trime-thylsilane) (FAC)

A 500 mL flask was charged with 2,7-dibromo-9,9-dioctyl-9H-fluorene (5.48 g, 10.04 mmol), TMSA (3.21 mL, 22.5 mmol), CuI (0.136 g, 0.72 mmol) and Pd(PPh3)4(0.264 g, 0.376 mmol). The mixture was degassed and backfilled with argon before injecting dried NEt3(120 mL). Then it was sealed with a rubber septum and heated to 50 ℃ overnight. The solvent was evaporated under vacuum and the crude product was purified by column chromatography (V(heptane)/V(CH2Cl2)=80∶20) to yield 4.2 g (72%) of FAC.1H NMR (400 MHz, Acetone-d6)δ:7.58(d, 2H),7.43(t,4H), 1.92-1.90(m, 4H), 1.25-1.00(m, 20H), 0.82(t, 6H), 0.55-0.48(m, 4H), 0.28(m, 18H).

2.2.2 Synthesis of 4,4′-((9,9-dioctyl-9H-fluorene-2,7-diyl)bis(ethyne-2,1-diyl))dibenzaldehyde (FDBM)

The obtained ((9, 9-dioctyl-9H-fluorene-2,7-diyl)bis(ethyne-2,1-diyl))bis(trimethylsilane) was subjected to desilylation by stirring with KOH (4.00 g, 35.6 mmol). This process is achieved in a mixed solvent of methanol (50.0 mL) and THF (100.0 mL) at room temperature for 12 h. After the removal of the solvent, it was extracted with dichloromethane and water for three times. Then, we dry it over MgSO4and reduce the volume. Finally, the product was used directly for the next step. Yield: 3.07 g (95%). The materials used for experiments include 4-bromobenzaldehyde (2.96 g, 16 mmol), 2,7-diethynyl-9,9-dioctyl-9H-fluorene (3.07 g, 7 mmol), Pd(PPh3)4(0.264 g, 0.376 mmol), CuI (0.136 g, 0.72 mmol), and TEA (120 mL) at 50 ℃ overnight. The residue was purified by flash column chromatography with 10%-20% CH2Cl2in hexanes to give product.Yield:2.42 g, 75%.1H NMR (400 MHz, Acetone-d6)δ:10.03(s, 2H), 7.89(d, 4H), 7.72-7.70(m, 6H), 7.54-7.52(m, 4H), 2.20-1.98(m, 4H), 1.25-1.06(m, 24H), 0.080(t, 6H).

2.2.3 Preparation of 2,2′-(((9,9-dioctyl- 9H-fluorene-2,7-diyl)bis(ethyne-2,1-diyl)) bis(4,1-phenylene))bis(1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazole) (FI)

A mixture of 4-(trifluoromethyl)aniline (5 mmol), phenanthrenequinon (1 mmol), ammonium acetate (5 mmol), and acetic acid (40 mL) were refluxed under nitrogen in an oil bath. After two hours, the mixture was cooled and filtered. The solid product was washed with acetic acid/water mixture (1∶1, 50 mL). Then, it was purified by chromatography using CH2Cl2as an eluent.1H NMR (400 MHz, Acetone-d6)δ: 8.87(d, 2H), 8.80(d, 2H), 8.72(d, 2H), 7.90(d, 4H), 7.76(t, 2H), 7.71-7.66(m, 8H), 7.57-7.50(m, 14H), 7.34-7.30(t, 2H), 7.16(d, 2H), 1.97(t, 4H), 1.55-1.05(m, 24H), 0.83-0.80(t, 6H).

2.3 Device Fabrication

The OLEDs were fabricated on glass substrates with indium tin oxide (ITO). The substrates were cleaned using deionized water, acetone and ethanol, after plasma treatment for 10 min. A poly(3,4-ethylenedioxythiophene)∶poly(4-styrenesulfonate)(PEDOT∶PSS) hole-injection layer was first spin-coated on the ITO substrate, and the films were infiltrated with FI (EML) by spin-coating a FI solution in toluene (10 mg·mL-1). After then hole blocking layer 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBi) were thermally evaporated. Finally, the cathode consisting of 1 nm LiF and 100 nm Al was deposited through a shadow mask with a device active area of 1 mm2.

3 Results and Discussion

3.1 Thermal Properties

The thermal properties of FI were measured by DSC and TGA under a nitrogen atmosphere, and the corresponding results were shown in Table 1 and Fig.2. While a endothermic melting transition temperature (Tm) was 244 ℃ for FI, it also showed

Table 1 Thermal properties, optical properties and energy levels of blue emitters

a:Measured in THF;

b:Measured as a solid film on quartz plates;

c:Estimated based on absorption onset and cyclic-voltammetry.EHOMO=-(Eox+4.4) eV,ELUMO=Eoptg+EHOMO.

Fig.2 DSC trace of FI recorded at a heating rate of 20 ℃·min-1and TGA thermogram of FI recorded at a heating rate of 10 ℃·min-1

good thermal stability with the decomposition temperature (Td)(Td, corresponding to 5% weight loss) of 426 ℃. Here, we can attributed the high thermal stability of FI to its rigid and fused fluorene acetylene containing molecular structure[16]. In addition, the FI had also shown relatively high thermal decomposition temperature than typical fluorene derivatives based on blue-emitter DNF, in whichTdwas highly dependent on the acetylene conjugate aromatic linker[17].

3.2 Theoretical Calculations

To understand the electronic structures of this compound, the molecular geometry and frontier molecular orbitals were calculated at B3LYP/6-31G (d, p) level by using DFT in Gaussian 03 program. Fig.3 is the molecular orbital distribution and optimized geometries of FI. The highest occupied molecular orbital (HOMO) was mainly located at the fluorene and phenanthroimidazole moiety, while the lowest unoccupied molecular orbital (LUMO) was mainly delocalized over the fluorene unit. It is noted that LUMO and HOMO show a small part of overlapping, which would contribute for the balanced charge transfer as the emitter in OLED. The twisted geometry of FI could efficiently prevent recrystallization in the film due to the large torsional stresses, which can induce the formation of either an exciplex or excimer[18]. The energies of HOMO and LUMO predicated by DFT were -5.12 and -1.99 eV, respectively. Compared with electrochemical analysis of FI, there were some changes in energies of HOMO and LUMO with DFT calculations. Since DFT calculation was performed assuming that FI was in gaseous phase, the interaction between FI and the solvent molecules could be eliminated[19]. And the gradual decrease of values in LUMO was good agreement with the experimental results.

Fig.3 Molecular orbital distribution and optimized geometries of FI

3.3 Optical Properties

UV-Vis absorption and photoluminescence (PL) spectra of FI in tetrahydrofuran (THF) solution were explored as shown in Fig.4. FI showed a maximum UV-Vis absorption at 379 nm, which was attributed to the π-π*transition. The absorption of π-π*transition mainly depended on conjugation of aryl unit connected to fluorene core[18]. From Fig. 4, there was no considerable absorbance change of the FI both in the film state and in solution, which indicate the introduction of acetylene unit extends the conjugation, and keeps the co-planarity of conjugation[16]. Furthermore, FI exhibited a dual emission peak at 420 nm and 440 nm in dilute THF solution. Compared with the diluted solution, the emission in the film was slightly red shifted 15 nm, which can be attributed to the constitution of FI with a large steric hindrance. This could prevent close molecular packing in the solid film[18].

Fig.4 UV-Vis absorption and PL spectra of FI in THF solution and in the film state

3.4 Electrochemical Properties

Cyclic voltammetry analysis was carried out to measure the HOMO values of the FI, and the results were shown in Fig. 5. HOMO values were calculated from asEHOMO= -([Eonset]ox+4.4). HOMO value of FI was -5.55 eV. Such a low HOMO energy level greatly reduced the energy barrier for hole injection from ITO to the emitter. The energy band gaps of the compounds were estimated by analyzing absorption edge with a plot of UV-Vis curve. And the energy band gap of FI was found to be 2.95 eV. The LUMO level of FI calculated byEHOMO+Eoptgwas -2.59 eV.

Fig.5 Cyclic voltammetry curve of FI

3.5 Electroluminescent Properties

To investigate the emitting properties of FI, non-doped OLED devices A and B were fabricated by using PEDOT∶PSS as hole injecting layer (HTL), which could match the HOMO energy level of FI well. Device A with a structure of ITO/PEDOT∶PSS(30 nm)/FI(20 nm)/LiF(1 nm)/Al(100 nm) and the structure of Device B was ITO/PEDOT∶PSS(30 nm)/FI(20 nm)/TPBi(40 nm)/LiF(1 nm)/Al(100 nm). Indium tin oxide (ITO) and Al were utilized as the anode and cathode. TPBi with higher electron mobility (3×10-5cm-2·V-1·s-1) was used as hole blocking layer, and LiF was used as an electron injecting layer. The current density-voltage-luminance curves and current efficiency-current density-power efficiency curves of the FI-based devices were shown in Fig.6. The luminance efficiencies and power efficiency of device A were found to be 0.8 cd·A-1and 0.43 lm·W-1, correspondingly. Compared with device A, the luminance efficiencies and power efficiency of device B could reach 1.52 cd·A-1and 0.63 lm·W-1, respectively. EL spectra of FI under 20 mA·cm-2for device A and B were shown in Fig.7. The maximum emissionλmaxwere located at 480 nm and 470 nm, and the FWHM values are from 66 nm to 68 nm. Compared with the PL spectra, it is common that the 20 nm shift is observed in OLEDs[19]. When the current density was 20 mA·cm-2, the CIE coordinates were (0.25, 0.17) for FI, which was a sky-blue emission. The EL properties of the devices A and B were summarized in Table 2.

Fig.6 Characteristic curves of the devices with FI. (a) Current density-voltage-luminances. (b) Current efficiency-current density-power efficiency.

Fig.7 Normalized EL spectra for the blue OLEDs with FI

Table 2 Electroluminescent characteristics of FI

4 Conclusion

Fluorene derivatives FI with acetylene group moieties was successfully designed and synthesized for blue emitters in non-doped OLEDs. The organic light-emitting diodes were fabricated with the structure of ITO/PEDOT∶PSS(30 nm)/FI(20 nm)/TPBi (40 nm)/LiF(1 nm)/Al(100 nm), which exhibited a maximum current efficiency of 1.52 cd·A-1, power efficiency of 0.63 lm·W-1as well as a blue emission with CIE coordinates of (0.25,0.17). Our results provide a way to design and synthesize blue emitter based on fluorene derivatives for solution-processed OLEDs.

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1000-7032(2015)02-0834-07

Solution-processable Blue Organic Light-emitting Diodes Based on Fluorene of Phenanthroimidazole Derivatives

ZUO Hong-wen1,2, OUYANG Xin-hua2, WANG Juan2, YANG Li-ying1*, YIN Shou-gen1*, GE Zi-yi2*

(1.SchoolofMaterialsScienceandEngineering,TianjinUniversityofTechnology,Tianjin300384,China; 2.NingboInstituteofMaterialsTechnology&Engineering,ChineseAcademyofSciences,Ningbo315201,China)*CorrespondingAuthors,E-mail:liyingyang@tjut.edu.cn;sgyin@tjut.edu.cn;geziyi@nimte.ac.cn

A novel A-D-A blue emitter containing fluorene, phenanthroimidazole and acetylene group moieties was synthesized and characterized, named as FI. Its photophysical and photochemical properties were systematically investigated. The results showed FI exhibits excellent solubility, fluorescence, thermal stability and film formation, which is a good candidate for the fabrication of solution-processable organic light-emitting diodes. A non-doped blue organic light-emitting diode based on FI as the emitter showed the maximum luminance efficiency of 1.52 cd·A-1, and the maximum power efficiency could reach 0.63 lm·W-1.

fluorene derivatives; solution-processable; blue OLEDs

TN383+.1 Document code： A

左红文(1989-)，女，山西大同人，硕士研究生，2008年于山西大同大学获得学士学位，主要从事有机光电显示材料与器件的研究。E-mail: zuohongwenupup@163.com

杨利营(1973-)，男，河北石家庄人，研究员，2002年于天津大学获得博士学位，主要从事有机光电功能材料与器件的研究。E-mail: liyingyang@tjut.edu.cn

10.3788/fgxb20153607.0834

2015-04-09；

2015-05-03

国家自然科学基金(51273209，514111004)； 中国科学院对外合作项目(GJHZ1219)； 宁波市自然科学基金(201401A6105063); 中国科学院长春应用化学研究所高分子物理与化学国家重点实验室开放研究基金(201404)资助项目