High external quantum efficiency in yellow and white phosphorescent organic light-emitting diodes using an indoloacridinefluorene type host material

Organic Electronics 15 (2014) 1843–1848

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High external quantum efficiency in yellow and white phosphorescent organic light-emitting diodes using an indoloacridinefluorene type host material Jeong-A Seo a, Myoung Seon Gong a, Jun Yeob Lee b,⇑ a b

Department of Nanobiomedical Science & WCU Research Center, Dankook University Graduate School, Chenan-si, Chungnam 330-714, Republic of Korea Department of Polymer Science and Engineering, Dankook University, 126, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 11 March 2014 Received in revised form 14 May 2014 Accepted 14 May 2014 Available online 28 May 2014 Keywords: High efficiency Yellow device Spiroacridinefluorene Host White device

a b s t r a c t High efficiency yellow phosphorescent organic light-emitting diodes were developed using spiro[fluorene-9,80 -indolo[3,2,1-de]acridine]-2,7-dicarbonitrile (ACDCN) as the host material for yellow emitting iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C20 )acetylacetonate (PO-01). The ACDCN host showed bipolar charge transport properties and efficient energy transfer to PO-01 dopant. Maximum external quantum efficiency of 25.7% and external quantum efficiency of 21.9% at 1000 cd/m2 were obtained using ACDCN as the host material. In addition, high external quantum efficiency of 20.9% was achieved in the two color white phosphorescent organic light-emitting diodes with the PO-01 and iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate doped ACDCN emitting layer. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Yellow phosphorescent organic light-emitting diodes (PHOLEDs) have been developed for application in two color white PHOLEDs with a blue emitter and a yellow emitter in the light-emitting layer because the combination of blue and yellow emission can make white emission color [1–12]. The external quantum efficiency of the two color white PHOLEDs depends on the external quantum efficiency of the blue and yellow PHOLEDs and high efficiency yellow PHOLEDs can improve the external quantum efficiency of the white PHOLEDs. There have been several approaches to develop high efficiency yellow PHOLEDs and most studies were directed to synthesize host and dopant materials for the yellow PHOLEDs. Various yellow phosphorescent dopant materials such as iridium(III) bis[2-(2-naphthyl)pyridine]acetylacetonate [2,3], iridium(III) bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-

⇑ Corresponding author. Tel.: +82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). http://dx.doi.org/10.1016/j.orgel.2014.05.017 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

phenyl-1H-benzoimidazole)acetylacetonate [4,5], and iridium(III) bis(4-phenylthieno[3,2-c]pyridine)acetylacetonate (PO-01) [6] have been synthesized and PO-01 was the most effective as the yellow phosphorescent emitter. Typical host material for the yellow PHOLEDs was 4,40 -N,N0 -dicarbazole-biphenyl (CBP), but the CBP host material could not give high external quantum efficiency [13]. Therefore, a mixed host of hole transport type and electron transport type hosts have been typically used as the host materials for the yellow PHOLEDs [7]. However, it is complicated to control the composition of the mixed host and to deposit at least three materials at the same time, so the development of bipolar host materials for the yellow PHOLEDs is required [14,15]. In this work, a bipolar host material derived from indoloacridine core, spiro[fluorene-9,80 -indolo[3,2,1-de]acridine]-2,7-dicarbonitrile (ACDCN), was synthesized as a host material for the yellow PHOLEDs and the device performances of the yellow PHOLEDs were investigated. It was demonstrated that the ACDCN host was effective as the host material due to bipolar charge transport properties and high external quantum efficiency of 25.7% was

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Fig. 1. Chemical structures of the materials used in this work.

achieved in the yellow PHOLEDs. In addition, white PHOLEDs fabricated using the ACDCN based yellow PHOLEDs showed high external quantum efficiency of 20.9%. 2. Experimental 2.1. General information The 1H nuclear magnetic resonance (NMR) were recorded on JEOL, JNM-ECS400 (400 MHz). 13C NMR spectra were recorded on, JEOL JNM-ECS400 spectrometer at a frequency of 100 MHz. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 380 Fourier transform spectrometer. Transmittance measurements were conducted using the KBr pellet method. The mass spectra were recorded using a JEOL, JMSAX505WA spectrometer in FAB mode. Elemental analysis of the materials was carried out using EA1110 (CE instrument). The DSC measurements were performed on a Mettler DSC822e differential scanning calorimeter under nitrogen at a heating rate of 10 °C/min. The photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (HITACHI, F-7000) and the ultraviolet–visible (UV–Vis) spectra were obtained by means of a UV–Vis spectrophotometer (Shimadzu, UV-2501PC). Sample was dissolved in tetrahydrofuran (THF) at a concentration of 1.0  105 M for UV–Vis and PL measurements. Triplet energy analysis was carried out using low temperature PL measurement in liquid nitrogen. The highest occupied molecular orbital of compounds was measured with a cyclic voltammetry. Cyclic voltammetry measurement of organic materials were carried out in acetonitrile solution with tetrabutylammonium perchlorate at 0.1 M concentration. Ag was used as the reference electrode and Pt was the counter electrode, Glassy carbon electrode was used as the working electrode. High performance liquid chromatography (HPLC) analysis of the synthesized materials was carried out using HPLC from Younglin Instrument. A mixed eluent of actonitrile:methanol (90:10) was used for analysis.

solution of 2,7-dibromo-9-fluorenone (1.2 g, 3.55 mmol) in THF (60 ml) was added slowly. The mixture was stirred for 1 h at 78 °C, and allowed to warm to room temperature. After overnight stirring, the reaction mixture was washed with 5 wt% Na2CO3 solution for 2 h and extracted ethyl acetate and diluted water. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromatography from ethyl acetate/n-hexane. The final white powdery product was obtained in 1.3 g. Yield 65%. 1H NMR (400 MHz, CDCl3): d 6.50 (d, 1H, J = 3.6 Hz), d 6.57 (d, 1H, J = 3.6 Hz), d 6.89 (t, 1H, J = 7.6 Hz), d 7.11 (t, 1H, J = 7.6 Hz), d 7.23 (s, 1H), d 7.37– 7.45 (m, 2H), d 7.51 (d, 2H, J = 4.8 Hz), d 7.63–7.71 (m, 3H), d 7.92 (d, 1H, J = 4.0 Hz), d 8.20 (d, 1H, J = 4.0 Hz), d 8.24–8.30 (m, 2H). MS (FAB) m/z 563 [(M + H)+]. Spiro[fluorene-9,80 -indolo[3,2,1-de]acridine]-2,7-dicarbonitrile (ACDCN) A mixture of ACDBr (1.3 g, 2.31 mmol), CuCN (0.52 g, 5.78 mmol) and N-methyl-2-pyrrolidone (38 ml) was reflux at 170 °C for 20 h under a N2 atmosphere. The reaction mixture was added to a solution of NaOH and the solution was stirred for 30 min. The reaction mixture was extracted with toluene and diluted water. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromatography from ethyl acetate/n-hexane. The final white powdery product was obtained in 0.7 g. Yield 67%. 1H NMR (400 MHz, CDCl3): d 6.40 (d, 1H, J = 3.6 Hz), d 6.47 (d, 1H, J = 3.6 Hz), d 6.89 (t, 1H, J = 7.6 Hz), d 7.11 (t, 1H, J = 7.6 Hz), d 7.40–7.47 (m, 4H), d 7.67 (t, 1H, J = 7.6 Hz), d 7.73 (d, 2H, J = 5.2 Hz), d 7.94– 7.98 (m, 3H), d 8.20 (d, 1H, J = 4.0 Hz), d 8.29 (d, 2H, J = 4.0 Hz). 13C NMR (100 MHz, CDCl3): d 57.3, 113.3, 114.1, 115.1, 118.5, 119.4, 121.2, 121.6, 121.8, 121.9, 122.6, 123.0, 123.5, 123.8, 126.1, 126.3, 127.3, 128.7, 129.2, 129.9, 132.6, 137.1, 138.7, 141.9, 156.3. MS (FAB) m/z 455 [(M + H)+]. Anal. Calcd for C33H17N3: C, 87.01; H, 3.76; N, 9.22. Found: C, 86.73; H, 3.79; N, 9.23. 2.3. Device fabrication

2.2. Synthesis The synthesis of intermediate compound was described in previous work. [16] 2.2.1. 2,7-Dibromospiro[fluorene-9,80 -indolo[3,2,1de]acridine] (ACDBr) 9-(2-Bromophenyl)-9H-carbazole (1.43 g, 4.44 mmol) in dry THF was treated with n-butyllithium (1.75 ml, 2.5 M) under argon atmosphere at 78 °C. After 30 min, a

Device structure of the yellow PHOLEDs was indium tin oxide (ITO, 150 nm)/poly(3,4-ethylenedioxythiophene): polystyrenesulfonate(PEDOT:PSS, 60 nm)/4,4-(cyclohexane-1,1-diyl)bis(N-phenyl-N-p-tolylaniline)(TAPC, 30 nm)/ ACDCN:PO-01 (25 nm)/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 35 nm)/LiF (1 nm)/Al (200 nm). Doping concentrations of PO-01 were 3%, 5%, and 10%. Hole only device with a device structure of ITO (150 nm)/ PEDOT:PSS (60 nm)/TAPC (30 nm)/ACDCN (30 nm)/Al and

J.-A. Seo et al. / Organic Electronics 15 (2014) 1843–1848

electron only device with a device structure of ITO(50 nm)/ Ca (5 nm)/ACDCN (30 nm)/TSPO1 (35 nm)/LiF(1 nm)/Al (200 nm) were fabricated to compared hole and electron densities in the emitting layer. PEDOT:PSS was deposited by spin coating and other materials were formed by vacuum thermal evaporation. White devices with a device structure of ITO (150 nm)/PEDOT:PSS (60 nm)/TAPC (30 nm)/ mCP:iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2] picolinate (FIrpic) (25  x nm)/ACDCN:PO-01 (x nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm) were also fabricated. The thicknesses of the yellow emitting layer were 3 nm and 5 nm. Doping concentrations of FIrpic and PO-01 were 15% and 3%, respectively. Chemical structures of the materials used in this work is shown in Fig. 1. All devices were sealed with a glass cover after Al deposition for device testing. Device performances were measured using Keithley 2400 source measurement unit and CS1000 spectroradiometer. 3. Results and discussion The ACDCN host was designed to show bipolar charge transport properties by combining the hole transport type indoloacridine core and electron transport type 2,7-dicyanofluorene unit. The indoloacridine core can play a role of hole transport unit as reported in other works [12] and the 2,7-dicyanofluorene unit can work as an electron transport moiety due to strong electron withdrawing character of two CN units. In addition, the indoloacridine core and 2,7-dicyanofluorene unit were connected through sp3 linkage, which can help to obtain high triplet energy. Although the indoloacridine core was applied as the core structure of high triplet energy hole transport materials, it was not used as the core structure of the host materials. Synthetic scheme of ACDCN is shown in Scheme 1. The ACDCN host was synthesized by ring closing reaction of 9(2-bromophenyl)-9H-carbazole with 2,7-dibromo-9-fluorenone followed by cyanation of ACDBr. Synthetic yield of the ACDCN was 67%. The ACDCN host was purified by vacuum train sublimation to obtain high purity level above 99%. Chemical structure of ACDCN was confirmed by 1H and 13C NMR, mass spectrometer, and elemental analysis.

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Molecular simulation of ACDCN was carried out to investigate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of ACDCN. Fig. 2 shows the HOMO and LUMO distribution of ACDCN. The HOMO of ACDCN was distributed over the indoloacridine core and the LUMO was localized over the 2,7-dicyanofluorene unit due to electron rich indoloacridine unit and electron poor 2,7-dicyanofluorene unit. Donor–acceptor character of ACDCN separated the HOMO and LUMO of ACDCN, which may reduce the bandgap of the ACDCN host material. The ionization potential and electron affinity of ACDCN were analyzed using CV oxidation and reduction scans. The oxidation and reduction curves were separately measured due to irreversibility of the oxidation and reduction of the ACDCN host. Fig. 3 shows the oxidation and reduction curves of ACDCN. Oxidation potential and reduction potential of ACDCN were 1.31 V and 1.37 V, which corresponded to an ionization potential of 6.11 eV and electron affinity of 3.43 eV. Therefore, the HOMO and LUMO of ACDCN can be estimated to be 6.11 eV and 3.43 eV from the ionization potential and electron affinity. Compared with other host materials, the LUMO was deepened due to strong electron deficiency of the fluorene unit by two CN units, which may facilitate electron injection from electron transport layer to the emitting layer. Photophysical properties of ACDCN were analyzed using UV–Vis absorption, solid PL and low temperature PL emission. Fig. 4 represents the UV–Vis, solid PL and low temperature PL spectra of ACDCN. UV–Vis absorption of ACDCN corresponding to n-p absorption was observed between 300 nm and 360 nm, while p–p absorption was observed below 300 nm. Solid PL emission of ACDCN appeared at 481 nm, which was well overlapped with UV–Vis absorption of PO-01 dopant with metal to ligand charge transfer absorption above 450 nm and ligand centered absorption below 450 nm. Therefore, it is expected that the energy transfer from ACDCN to PO-01 is efficient. Phosphorescent emission of ACDCN was observed above 470 nm and the triplet energy of ACDCN was 2.57 eV from the first phosphorescent emission peak at 482 nm. Singlet

Scheme 1. Synthetic scheme of ACDCN.

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HOMO

LUMO

Fig. 2. HOMO and LUMO distribution of ACDCN.

Current (arb. unit)

type indoloacridine core and electron transport type 2,7dicyanofluorene, the bipolar charge transport properties were observed. From the hole only and electron only device data, it can be expected that the ACDCN host can balance holes and electrons in the emitting layer. Yellow emitting PO-01 dopant was doped in the ACDCN host to fabricate yellow PHOLEDs. Doping concentration of PO-01 was changed from 3% to 10% to optimize the device performances of the yellow PHOLEDs. Current density– voltage–luminance curves of the ACDCN devices are shown in Fig. 6. The current density of the yellow PHOLEDs was increased according to the doping concentration of PO-01 due to better charge hopping at high doping concentration. However, the luminance was not increased at high doping concentration because of relatively low recombination efficiency in spite of high current density at 10% doping concentration. Turn-on voltage of the device was 3.0 V. External quantum efficiency–luminance–power efficiency curves of the yellow PHOLEDs are presented in Fig. 7. The external quantum efficiency was optimized at 3% doping concentration and a maximum external quantum efficiency of 25.7% was obtained. The maximum quantum efficiencies at 5% and 10% doping concentrations were 25.3% and 22.8%, respectively, due to concentration quenching effect at high doping concentration. The external quantum efficiency of the 3% PO-01 doped ACDCN device at 1000 cd/m2 was 21.9%. The high external quantum

Voltage (V) Fig. 3. Oxidation and reduction curves of ACDCN.

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energy of ACDCN was 2.86 eV from the PL emission of ACDCN dispersed in polystyrene (PS). Hole only and electron only devices of ACDCN were fabricated to compare hole and electron density. Current density–voltage curves of ACDCN are shown in Fig. 5. The hole current density of ACDCN was similar to electron current density, indicating that ACDCN possesses bipolar charge transport properties. As the ACDCN host has hole transport

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efficiency of the PO-01 doped ACDCN device can be explained by efficient energy transfer, charge balance, exciton and charge confinement in the emitting layer. The energy transfer from ACDCN to PO-01 was efficient considering the electroluminescence (EL) spectra in Fig. 8 because no ACDCN emission was observed in the EL spectra, implying efficient energy transfer from ACDCN to PO01. Extensive overlap of ACDCN PL emission with PO-01 absorption and high triplet energy of ACDCN induced the efficient energy transfer. Charge balance is another factor for the high external quantum efficiency of the ACDCN device. As shown in the single charge devices, holes and electrons were balanced in the ACDCN emitting layer, which enhanced the external quantum efficiency through high recombination efficiency. Exciton and charge confinement in the emitting layer also contributed to the high external quantum efficiency of the ACDCN devices. Triplet excitons of PO-01 is confined in the emitting layer because of high triplet energy of ACDCN (2.57 eV), mCP (2.90 eV) and TSPO1 (3.39 eV) which can suppress triplet exciton quenching of PO-01. In addition, there are large barriers of 0.97 eV for electron leakage and 0.68 eV for hole leakage, which can confine holes and electrons in the emitting layer and increase the external quantum efficiency of the ACDCN device. EL spectra of the ACDCN devices are shown in Fig. 8. All devices showed typical EL emission of PO-01 with a peak

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Wavelength (nm) Fig. 8. Electroluminescence spectra of ACDCN:PO-01 devices according to doping concentration of PO-01.

maximum between 559 nm and 562 nm without any emission from ACDCN. This result confirms the efficient energy transfer from ACDCN to PO-01. Color coordinates of the ACDCN:PO-01 devices at 1000 cd/m2 were (0.48, 0.52), (0.49, 0.51) and (0.50, 0.50) at 3%, 5% and 10%, respectively. Slight red-shift of the EL spectra was observed at high doping concentration due to intermolecular interaction. The high external quantum efficiency of the yellow ACDCN device prompted us to develop two color white PHOLEDs by combining the yellow PHOLEDs with sky blue PHOLEDs. The ACDCN:PO-01 emitting layer was stacked on mCP:FIrpic emitting layer to fabricate the white PHOLEDs. The thickness of the yellow emitting layer was controlled to manage the EL spectrum of the white PHOLEDs. Current density–voltage–luminance curves of the white PHOLEDs are presented in Fig. 9. The thickness of the yellow emitting layer had little effect on the current density and luminance of the device due to small variation of thickness, and similar current density and luminance were observed irrespective of the thickness of the yellow emitting layer. External quantum efficiency–luminance curves of the white PHOLEDs are shown in Fig. 10. Maximum external quantum efficiency of the white PHOLEDs with 5 nm thick yellow emitting layer was 20.9%, while that of the white

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Fig. 9. Current density–voltage–luminance curves white PHOLEDs with ACDCN:PO-01 emitting layer according to the thickness of ACDCN:PO-01 emitting layer.

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emission intensity of the blue and yellow emission. The blue intensity was relatively strong in the white PHOLEDs with 3 nm thick yellow emitting layer due to high probability of exciton formation in the blue emitting layer. The color coordinates of the white PHOLEDs with 3 nm and 5 nm yellow emitting layers were (0.39, 0.46) and (0.41, 0.47), respectively.

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Luminance (cd/m2) Fig. 10. External quantum efficiency–luminance curves white PHOLEDs with ACDCN:PO-01 emitting layer according to the thickness of ACDCN:PO-01 emitting layer.

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In conclusion, a bipolar host material with indoloacridine and 2,7-dicyanofluorene, ACDCN, was designed and synthesized as the host material for yellow PHOLEDs. The ACDCN host showed a triplet energy of 2.57 eV for efficient energy transfer to yellow emitting PO-01 triplet emitter and narrow HOMO/LUMO gap for efficient charge injection. The ACDCN host material allowed high external quantum efficiency of 25.7% in the yellow PHOLEDs and 20.9% in the white PHOLEDs. Therefore, the novel design combining the indoloacridine core and 2,7-dicyanofluorene was effective to improve the external quantum efficiency of yellow and white PHOLEDs. References

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Wavelength (nm) Fig. 11. Electroluminescence spectra of white PHOLEDs with ACDCN:PO01 emitting layer according to the thickness of ACDCN:PO-01 emitting layer.

PHOLEDs with 3 nm thick yellow emitting layer was 18.6%. The yellow emission was relatively strong in the device with thick yellow emitting layer as can be seen in the EL spectra of Fig. 11, which affected the external quantum efficiency of the white PHOLEDs because external quantum efficiency of the yellow emitting layer is higher than that of the blue emitting layer (19.7%). Therefore, high external quantum efficiency was obtained in the device with thick yellow emitting layer. Fig. 11 shows EL spectra of the white PHOLEDs with different yellow emitting layer thicknesses. The EL spectra were normalized to compare the change of the relative

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