Triazine-based electron-transport material for stable phosphorescent organic light-emitting diodes

CHEN Ling-ling,WANG Lin-ye,XIAO Shu,ZOU Jian-hua,ZHU Xu-hui*,MA Dong-ge

(1.State Key Laboratory of Luminescent Materials and Devices,Institute of Polymer Optoelectronic Materials and Devices,South China University of Technology,Guangzhou 510640,China;2.Guangzhou New vision Opto-Electronic Technology Co.,Ltd.,Guangzhou 510730,China)

Abstract:An organic electron-transport compound for phosphorescent OLEDs is reported,which possesses the advantages of low molecular weight,enhanced glass transition temperature and electron mobility upon doping with 8-hydroxyquinolatolithium (Liq) as well as facile synthesis and purification.The analytically pure NaAN-m-TRZ (m/z = 611.73) is obtained through coupling the 4,6-diphenyl-1,3,5-triazin-2-yl unit via a 1,3-phenylene linker with 10-(naphth-2-yl)-anthracen-9-yl moiety.The residual bromo intermediate could be easily removed by column chromatography and/or recrystallization from CH2Cl2,hence eliminating a high-risk factor for OLED stability.Thermal analyses show that it exhibits a Tg of 157 ℃ and decomposition temperature of 353 ℃ at 1% weight loss.NaAN-m-TRZ has a HOMO level of -5.76 eV determined by the ultraviolet photoelectron spectroscopy measurement and an estimated LUMO level of -2.84 eV.Doping NaAN-m-TRZ with 50% (mass fraction) Liq yields impressive electron mobility of 6.23×10-5～7.19×10-4 cm2·V-1·s-1 @ E = (2～5)×105 V·cm-1 using space-charge-limited current model,which contributes to suppressing triplet-polaron annihilation in the phosphorescent OLEDs.Consequently,based on the single NaAN-m-TRZ∶Liq electron-transport layer,the top-emission green phosphorescent OLED involving Ir(ppy)2(m-mbppy) produces extraordinary durability with projected lifetime t97 of 2 567 h @ 1 000 cd·m-2 as well as a luminous efficiency of 72.2 cd·A-1 and power efficiency of 81 lm·W-1@ 1 000 cd·m-2.

Key words:triazines;anthracenes;low molecular weight;glass transition temperature;electron mobility

1 Introduction

Organic electron-transport materials (ETMs) are an integral part of organic light-emitting diodes[1],which critically influence the performance parameters such as the OLED efficiency,working voltage and lifetime.It is of considerable interest to develop high-mobility neat or doped ETMs with low molecular weight and increased thermal and morphological stability (e.g.Tgca.120 ℃).Such ETMs may be sublimable at a reduced temperature and likely survive the prolonged vacuum deposition processes and meet the display purposes.

In this context,we present an electron-transport molecular glass 2-(3-(10-(naphth-2-yl)anthracen-9-yl)- phenyl)-4,6-diphenyl-1,3,5-triazine(NaAN-m-TRZ,m/z=611.73),which nicely shows a low molecular weight and yet a highTgof 157 ℃.Among others,1,3,5-triazine (TRZ) is a desirable electron-deficient heterocyclic building block to construct electron-transport materials for optoelectronics because of its high electron affinity and electrochemical stability[2-3],and moreover effective doping with a lithium complex 8-hydroxyquinolatolithium (Liq)[4-5].On the other hand,anthracene (AN) is a fused aromatic ring of easy accessibility and modification at the 9,10-positions.Due to the intrinsically non-planar chemical structures,properly 9,10-disubstituted anthracenes are prone to form glasses[6-11]and play an essential role in the current OLED technology as the host materials for the blue fluorescent dopants[6-10].For instance,the widely studied bipolar host 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) exhibits aTgof 120 ℃ and HOMO/LUMO level of -5.5/-2.5 eV[8].In a p-i-n bottom-emission OLED involving MADN and the blue dopantp-bis(p-N,N-diphenylaminostyryl)benzene,we demonstrated a noticeable lifetimet95of ca.160 h @ 1 000 cd·m-2 [12].

Based on the ultraviolet photoelectron spectroscopy measurement,NaAN-m-TRZ showed a HOMO level of -5.76 eV.Introducing the 1,3,5-triazinyl moiety led to lowering the LUMO level to approximately -2.84 eV,relative to MADN and 9,10-di(2-naphthyl)anthracene (EHOMO=-5.8 eV,ELUMO=-2.6 eV)[6-7].The electron-only device (ITO/NaAN-m-TRZ:50% (mass fraction) Liq/Al) provided a high electron-mobility value of 6.23×10-5～7.19×10-4cm2·V-1·s-1@E= (2～5)×105V·cm-1.As a single Liq-doped electron-transport layer,NaAN-m-TRZ afforded efficient top-emission green phosphorescent OLED (PHOLED) with a luminous efficiency of 72.2 cd·A-1and a power efficiency of 81 lm·W-1and a low voltage of 2.8 V @ 1 000 cd·m-2.In particular,the projected lifetimet97reached 2 567 h @1 000cd·m-2,implying that NaAN-m-TRZ would be promising for OLED technology.

2 Experiments

2.1 Materials and instructions

All procedures involving air-sensitive reagents were conducted under dry nitrogen.2,4-diphenyl-6- (3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)- phenyl)-1,3,5-triazine and 10-bromo-9-(2-naphthyl)- anthracene were obtained according to the literature methods[4,13-14].All starting materials were purchased commercially and used directly without further purification unless otherwise stated.

1H NMR and13C NMR measurements were performed on Bruker 400 MHz and 126 MHz DRX spectrometers respectively with tetramethylsilane (TMS) as the internal reference,deuterated chloroform (CDCl3) as the solvent.The mass spectrum was measured by Waters ACQUITY TQD liquid chromatography-mass spectrometry using an APCI ion source.The elemental analysis was conducted on vario EL cube.Thermogravimetric analysis (TGA) was measured by Netzsch TG 209 at a heating rate of 20 ℃·min-1under a nitrogen flow.Differential scanning calorimetry (DSC) testing was carried on a Netzsch DSC 204 thermal analyzer under a nitrogen flow with a heating and cooling rate of 10 ℃·min-1and 20 ℃·min-1,respectively.UV-Vis absorption spectra were measured on a Shimadzu UV-2600 ultraviolet-Visible spectrophotometer.Photoluminescence spectra were measured on a HORIBA Fluorolog-3 fluorescence spectrophotometer.Ultraviolet photoelectron spectroscopy (UPS) characterization was performed on a thermal ESCALAB 250 X-ray photoelectron spectrometer.

2.2 Synthesis of 2-(3-(10-(2-naphthyl)anthracen-9-yl)phenyl)-4,6-diphenyl-1,3,5-triazine (NaAN- m-TRZ)[15]

Under nitrogen atmosphere,the catalyst Pd(PPh3)4(100 mg,0.086 mmol) was quickly added to a mixture of 10-bromo-9-(2-naphthyl)anthracene (2.5 g,6.52 mmol),2,4-diphenyl-6-(3-(4,4,5,5-tetramethyl-1,3,2- dioxaborolan-2-yl)phenyl)-1,3,5-triazine (2.84 g,6.52 mmol),Na2CO3aqueous solution (2 mol/L,7 mL,14 mmol),ethanol (7 mL) and toluene (60 mL).The reaction was heated and stirred at 90～100 ℃ for 14 h.After being cooled to room temperature,the crude product was concentrated and distilled water was added.The organic layer was extracted with CH2Cl2,separated,dried over anhydrous MgSO4,filtered and concentrated under reduced pressure.The crude product was subject to column chromatography over silica gel using petroleum ether/dichloromethane (5∶1 volumn ration) as elute to afford a light yellow solid.Yield:3.6 g (90%).Rf(10-bromo-9-(2-naphthyl)anthracene) = 0.9;Rf(NaAN-m-TRZ) = 0.5.1H NMR (400 MHz,CDCl3)δ：9.02～8.99 (m,1H),8.93～8.91 (m,1H),8.77～8.75 (m,4H),8.10 (dd,J= 8.4,3.6 Hz,1H),8.05～8.02 (m,2H),7.96～7.93 (m,1H),7.86～7.82 (m,1H),7.80～7.74 (m,5H),7.69～7.64 (m,1H),7.63～7.52 (m,8H),7.39～7.32 (m,4H).13C NMR (126 MHz,CDCl3)δ：171.76,171.65,139.67,137.27,136.72,136.65,136.53,136.13,135.49,133.44,132.80,132.55,131.65,130.29,130.09,130.02,129.55,129.02,128.93,128.62,128.30,128.13,128.03,127.93,127.11,126.98,126.48,126.27,125.34,125.24.MS (APCI):m/z612.34 (100%) [M+H]+Calcd.:612.24.Anal.calcd.:C 88.35,H 4.78,N,6.87;Found:C 88.04,H 4.68,N 6.66 for C45H29N3.HPLC purity:99.98%.

2.3 Device fabrication and characterizations

To evaluate the potential of NaAN-m-TRZ as a doped electron transport layer,top-emission green phosphorescent OLED (PHOLED) was prepared.Patterned indium-tin oxide (ITO,15 Ω/square)-coated glass substrates were washed with deionized water,detergent,acetone,deionized water,and 2-isopropanol successively in an ultrasonic bath.After drying in a stream of nitrogen,ITO substrates were placed in a vacuum (4×10-5Pa) organic-metal composite evaporation coating instrument to evaporate the organic layer and metal electrode through a mask.For the vapor deposition of doped layers,the vapor deposition speed of each component is controlled by their independent quartz crystal oscillators.In the deposition of n-doped electron transport layers (ETM∶Liq),deposition rate of both ETM and Liq is 0.05 nm/s.After preparation,the devices were encapsulated immediately using epoxy resin and glass sheets under a nitrogen atmosphere.The effective emission area of the device is 9 mm2,which is determined by the overlapping area of the anode and cathode.The EL spectra and CIE coordinates of the encapsulated devices are measured by the Konica Minolta CS2000 spectroscopy system.The current density (J)-voltage (V)-luminance (L) curves of the device were measured by a computer-controlled source meter (Keithley 2400) and a multimeter (Keithley 2000) with calibrated silicon photodiode.The luminance decay characteristic curves of devices were measured under a constant current driving.Before testing,the device was aged at a current density of 20 mA·cm-2for 24 h.All measurements were performed at room temperature in an atmospheric environment.The hole-injection material OMET-P008 was available from Eternal Material Technology,EMT;the exciton blocking material EBL and host materials of the emitting layer HOST1 and HOST2 were available from e-Ray Optoelectronics Technology Co.,Ltd.,while the green phosphorescent dopant Ir(ppy)2(m-mbppy) from Lumtec.The light extraction layer CP501 was available from Toray Industries,Inc.

The electron mobility of 50%(mass fraction) Liq-doped NaAN-m-TRZ film was measured with electron-only device (ITO/ETM∶Liq(150 nm,1∶1 mass ratio)/Al)viathe space-charge-limited current (SCLC) method.

3 Results and discussion

3.1 Synthesis

NaAN-m-TRZ was facilely prepared in ca.90% yield by the Suzuki coupling of 2,4-diphenyl-6-(3- (4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,5-triazine[4]with 9-bromo-10-(naphthalen-2-yl)- anthracene (Fig.1).Due to the considerable difference of molecular polarity,the residual bromo intermediate could be easily removed from the target compound by the mixed eluent of petroleum ether/CH2Cl2(5∶1 volumn ratio) via column chromatography.Moreover,the target compound can be recrystallized from CH2Cl2,in which the bromo intermediate is highly soluble,thus ensuring further separation[4-5,16].The analytically pure NaAN-m-TRZ obtained was used directly for device characterizations without sublimation.

Fig.1 Synthetic route to NaAN-m-TRZ

Fig.1 shows the synthetic route to NaAN-m-TRZ:(i) 2-naphthaleneboronic acid,Pd(PPh3)4,K2CO3aqueous solution,ethanol,toluene,90 ℃;(ii)N- bromosuccinimide,DMF,reflux;(iii) 2,4-diphenyl- 6-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,3,5-triazine,Pd(PPh3)4,2 mol/L Na2CO3aqueous solution,ethanol,toluene,90 ℃.

3.2 Thermal properties

NaAN-m-TRZ exhibited a decomposition temperature (Td) of 353 ℃,defined at an initial weight loss of 1% (Fig.2(a)).Differential scanning calorimetry (DSC) measurement revealed a glass transition temperature (Tg) of 157 ℃ (Fig.2(b)).With regard to the ternary triphenylphosphine oxide-2,6-pyridinylenetriazine molecular conjugate BPTRZ-Py-TPO (m/z=814.91,Tg= 123 ℃) we reported previously[5],it is noteworthy that NaAN-m-TRZ showed a remarkably increasedTgdespite its lower molecular weight.

Fig.2 Thermogravimetric analysis (a) and DSC diagrams (b) of NaAN-m-TRZ

3.3 HOMO/LUMO levels

The ultraviolet photoelectron spectroscopy measurement was performed to detect the HOMO level of NaAN-m-TRZ (Fig.3),yielding a value of -5.76 eV (EHOMO= -(φITO+ HOMOedge).The LUMO level was then roughly calculated to be ca.-2.84 eV,according toELUMO≈EHOMO+Eopt.Eoptrepresents the optical bandgap of ca.2.92 eV (Fig.4).Consequently,the presence of the 1,3,5-triazinyl moiety results in the reduction of the LUMO level and thus facilitates electron injection,relative to 9,10-di(2-naphthyl)- anthracene and 2-methyl-9,10-di(2-naphthyl)- anthracene (MADN)[6-9].

Fig.3 UPS spectra (a) at the low kinetic energy region and (b) at the valence band near the Fermi level for 10 nm NaAN-m-TRZ on ITO

Fig.4 Normalized UV-vis absorbance (Abs) and fluorescence (FL) spectra of NaAN-m-TRZ as film on quartz,spin-cast from CHCl3 solution (concentration:10 mg·mL-1,spin speed:2 000 r/min).Excitation:300 nm.

3.4 Electron-mobility measurement

The electron transport property of the Liq-doped NaAN-m-TRZ thin film was probed in the electron-only device (ITO/ETM∶Liq (1∶1 mass ratio,150 nm)/Al,Fig.5(a)).The ln(J/E2) versusE1/2followed a space-charge-limited current (SCLC) characteristic with field-dependent mobility (Fig.5(b)).We can deduce the electron mobility value of 6.23×10-5～7.19×10-4cm2·V-1·s-1atE=(2～5)×105V·cm-1by fitting the ln(J/E2)-E1/2curve,which is substantially improved against the triazine derivatives TRZ-m-Phen and BPTRZ-Py-TPO[4-5].The zero-field electron mobilityμ0and field-activation factorβof the NaAN-m-TRZ∶Liq device were 9.26×10-7cm2·V-1·s-1and 9.41×10-3(cm·V-1)0.5,respectively.

Fig.5 (a) J-V and (b) ln(J/E2)-E1/2characteristics of the electron-only device:ITO/NaAN-m-TRZ∶Liq(1∶1 mass ratio,150 nm)/Al.

3.5 OLED characterization

We characterized NaAN-m-TRZ as a single doped electron-transport layer in the top-emission green phosphorescent OLED:Ag/ITO/OMET-P008:p-dopant (4%)/HTL/EBL/HOST1∶HOST2∶Ir(ppy)2(m-mbppy) (1∶1∶0.3)/ETM∶Liq(1∶1)/Mg∶Ag(1∶9)/CP501.Ir(ppy)2(m-mbppy)=bis(2-phenylpyridine)(2-(4- methyl-3-phenylp-henyl)pyridine)iridium(Ⅲ) as the emitter.Commercially available OMET-P008,HTL,EBL,HOST1/ HOST2 and CP501 denoted respectively the hole-injection material,hole-transport layer,exciton-blocking layer,host materials and light- extraction layer[5].The current density (J)-voltage (V)-luminance (L),luminous efficiency (LE)-L,power efficiency (PE)-Lcharacteristics and electroluminescence (EL) spectrum were shown in Fig.6.At a luminance of ca.1 000 cd·m-2,LE = 72.2 cd·A-1,EQE (external quantum efficiency) = 17.9%,PE = 81.0 lm·W-1,J= 1.31 mA·cm-2,andV= 2.8 V.The EL spectrum originated from the iridium complex with an emission maximum at 528 nm and CIE coordinates (0.23,0.71),close to the NTSC standard (0.21,0.71).

Fig.6 (a) J-V-L,(b) LE-L,(c) PE-L curves,and (d) EL spectrum of the top-emission green PHOLED (Ag/ITO/OMET-P008:p-dopant(100 nm,4%)/HTL(15 nm)/EBL(5 nm)/HOST1∶HOST2∶Ir(ppy)2(m-mbppy) (30 nm,1∶1∶0.3)/NaAN-m-TRZ∶Liq(30 nm,1∶1 mass ratio)/Al/ CP501).

The anthracene derivatives inherently possess low triplet energy[10,17-19].We recorded the photoluminescence spectrum of the NaAN-m-TRZ thin film after a delay of 0.1 ms upon excitation at 77 K and observed a maximal emission peak of 467～470 nm (Fig.7),which obviously stemmed from the triplet-triplet annihilation[10,17-19].Therefore,theEtripletwas estimated between 1.32～1.8 eV.We note that under analogue conditions,the BPTRZ-Py-TPO∶Liq OLED showed a higher luminous efficiency of 77.4 cd·A-1(corresponding to an EQE of 18.7%) @ ca.1 000 cd·m-2,which features an increased triplet energy of 2.88 eV and yet reduced electron mobility[5].This points to the presumption that to some extent,triplet energy transfer from Ir(ppy)2(m-mbppy) to the doped electron layer NaAN-m-TRZ∶Liq may occur[20].Therefore,an exciton blocking layer should be placed between the phosphorescent emitter layer and the doped electron-transport layer NaAN-m-TRZ∶Liq and the results shall be reported in due course.

Finally,we stressed the top-emission green PHOLED under a constant current at an initial luminescence of ca.3 000 cd·m-2(Fig.8).Generally,hole transport is predominant in an OLED.Thanks to the enhanced electron mobility and hence suppressed triplet-polaron annihilation in the phosphorescent emitting layer[4,5,21],the lifetimet97was extended to about 400 h.

Fig.8 Luminance decay characteristic of the encapsulated top-emission green PHOLED (Ag/ ITO/OMET-P008:p-dopant(147 nm,4%)/HTL(15 nm)/EBL(5 nm)/HOST1∶HOST2∶Ir(ppy)2(m-mbppy) (30 nm,1∶1∶0.3)/NaAN-m-TRZ∶Liq(30 nm,1∶1 mass ratio)/Mg∶Ag(15 nm,1∶9)/CP501(70 nm)).The initial luminance was set as ca.3 000 cd·m-2.Prior to testing,The OLED aged at a current density of 20 mA·cm-2 for 24 h.

.

(1)

According to Eq (1),the extrapolatedt97@ 1 000 cd·m-2was remarkable as ca.2 567 h,assumingn=1.7[22].

4 Conclusion

In summary,we have described facilely available 1,3,5-triazine-based electron-transport molecular anthracene NaAN-m-TRZ (m/z=611.73).The residual bromo intermediate 9-bromo-10-(naphthalen-2-yl)-anthracene,a fatal risk factor for the OLED stability,could be easily removed by column chromatography and/or recrystallization,due to its low molecular polarity and high solubility in weakly polar solvents such as CH2Cl2.NaAN-m-TRZ exhibits aTg/Tdof 157/353 ℃ at 1% weight loss and HOMO/LUMO level of -5.76/-2.84 eV.The Liq-doped NaAN-m-TRZ possesses outstanding electron mobility of 6.23×10-5～7.19×10-4cm2·V-1·s-1@E= (2-5)×105V·cm-1.The top-emission green phosphorescent OLED comprising the single NaAN-m-TRZ∶Liq electron- transport layer provides an EQE of 17.9% (72.2 cd·A-1) and 81 lm·W-1@ ca.1 000 cd·m-2and CIE (0.23,0.71).The projected operational stabilityt97amounted to ca.2 567 h @ 1 000 cd·m-2.Our finding shall stimulate further interest in this sort of materials for optoelectronics.