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Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes
Organic Electronics 15 (2014) 3773–3779
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Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes Jeong-A Seo a, Myoung Seon Gong a, Jun Yeob Lee b,⇑ a Dept. of Nanobiomedical Science and BK21 PLUS NBM Global Research Center for Regenerative Medicine, Dankook University, Chungnam 330-714, Republic of Korea b Dept. of Polymer Science and Engineering, Dankook University, 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 5 September 2014 Received in revised form 9 October 2014 Accepted 14 October 2014 Available online 27 October 2014 Keywords: Indoloacridine Dibenzofuran Host High triplet energy Thermal stability
a b s t r a c t High triplet energy materials derived from carbazole or a-carboline modified indoloacridine were synthesized and device characteristics of blue triplet emitter doped devices were investigated. The indoloacridine derived host materials showed a high triplet energy above 2.80 eV and a high glass transition temperature over 170 °C due to rigid nature of the molecular structure. The indoloacridine based host materials could approach high external quantum efficiency above 20% in blue phosphorescent organic light-emitting diodes. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The triplet energy of host materials is a key parameter to realize high external quantum efficiency in phosphorescent organic light-emitting diodes (PHOLEDs) because main mechanism of light emission of PHOLEDs is energy transfer from the triplet host materials to triplet emitters. The triplet energy of the triplet host materials should be higher than that of triplet emitters to avoid reverse energy transfer from the triplet emitters to the triplet host materials. Therefore, the triplet host materials of blue PHOLEDs must have a triplet energy higher than 2.80 eV as the triplet energy of blue triplet emitters is between 2.65 eV and 2.80 eV. The most widely used core structures of high triplet energy host materials for blue PHOLEDs were carbazole [1–8], dibenzofuran [9–13], and spirobifluorene [14–16] because of high triplet energy of the core structures.
⇑ Corresponding author. Tel.: +82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). http://dx.doi.org/10.1016/j.orgel.2014.10.020 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
Carbazole has been used as a hole transport type core of the triplet host materials due to high triplet energy and good hole transport properties. Many carbazole based triplet host materials were reported and the high triplet energy host materials derived from the carbazole core achieved high quantum efficiency in blue PHOLEDs [1–8]. Similarly, dibenzofuran compounds were also popular as the triplet host materials of blue PHOLEDs for high quantum efficiency [9–13]. Spirobifluorene compounds are another class of high triplet energy host materials for blue PHOLEDs because of high triplet energy and good thermal stability [14–16]. The spirobifluorene core has a molecular structure with two fluorene units directly connected via sp3 carbon. The two fluorene units are distorted each other and the conjugation of molecules is separated by the sp3 carbon linkage, increasing the triplet energy of the spirobifluorene. Additionally, the rigid molecular structure increased the thermal stability and glass transition temperature. Recently, similar concept was used for the molecular design of hole transport materials and indoloacridine core
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derived compounds were reported as the hole transport materials of PHOLEDs [17]. The indoloacridine type hole transport materials showed high glass transition temperature and high quantum efficiency in blue PHOLEDs [17– 19]. However, the indoloacridine moiety could not be adopted as the core structure of the triplet host materials because of excimer formation by the planar structure of the indoloacridine moiety although the synthesis of high triplet energy host materials was reported in the literature [18]. Therefore, a new design to solve the problem of the excimer formation while keeping the merits of high triplet energy and good thermal stability is required. In this work, indoloacridine derivatives, 8-(8-(9Hcarbazol-9-yl)dibenzo[b,d]furan-2-yl)-8-phenyl-8H-indolo [3,2,1-de]acridine(AcHCz) and 8-(8-(9H-pyrido[2,3-b]indol9-yl)dibenzo[b,d]furan-2-yl)-8-phenyl-8H-indolo[3,2,1-de] acridine(AcHCb), were synthesized as high glass transition temperature and high triplet energy host materials for blue PHOLEDs. It was shown that the two host materials AcHCz and AcHCb showed high triplet energy of 2.91 and 2.86 eV and high external quantum efficiency of 23.8% and 20.8% in blue PHOLEDs, respectively. 2. Experimental 2.1. General information Aluminium trichloride, periodic acid, n-butyllithium, copper iodide, trans-1,2-diamino cyclohexane (AldrichChem. Co.), benzoyl chloride, dibenzo[b,d]furan (Tokyo Chemical Industry Co.), iodine (Samchun pure chemical Co.), 9-(2-bromophenyl)-9H-carbazole, a-carboline (P&H Tech Co.) was used without further purification. Sulfuric acid, acetic acid, hydrochloric acid, potassium phosphate, 1,4-dioxane, dichloromethane, diethyl ether, n-hexane, ethyl acetate (Duksan Sci. Co.) was used as received. Tetrahydrofuran (Duksan Sci. Co.) was distilled over sodium and calcium hydride. 9H-carbazole (Sigma–Aldrich Co.) was purified by recrystallization using toluene. Detailed chemical analysis and photophysical measurement were carried out according to the method reported in our previous work [20]. 2.2. Synthesis 2.2.1. Dibenzo[b,d]furan-2-yl(phenyl)methanone(1) Anhydrous aluminum trichloride (7.59 g, 56.9 mmol) and benzoyl chloride (8 g, 56.9 mmol) in dichloromethane (40 mL) were added to dibenzo[b,d]furan (14.3 g, 85.4 mmol) in dichloromethane slowly and the solution was stirred at room temperature under a nitrogen atmosphere for 2 h. The solution was quenched with 1 M HCl and extracted with diethyl ether. The organic layer was dried over anhydrous MgSO4 and concentrated. White powder was obtained as a product and was purified by column chromatography using n-hexane as an eluent (7.00 g, yield 45%) [21]. 1 H NMR (400 MHz, DMSO): d 7.46 (t, 1H, J = 7.5 Hz), 7.59–7.63 (m, 3H), 7.73 (t, 1H, J = 7.5 Hz), 7.79–7.83 (m, 3H), 7.88 (d, 1H, J = 4.5 Hz), 7.94 (d, 1H, J = 4.0 Hz), 8.31 (d, 1H, J = 3.5 Hz), 8.62 (s, 1H).
2.2.2. (8-Iiododibenzo[b,d]furan-2-yl)(phenyl)methanone (2) A solution of dibenzo[b,d]furan-2-yl(phenyl)methanone (7 g, 25.7 mmol), iodine (3.26 g, 12.9 mmol), periodic acid (2.93 g, 12.9 mmol), distilled water (50 mL), sulfuric acid (0.5 mL) and acetic acid (250 mL) was stirred at 60 °C for 10 h under nitrogen, and was allowed to be cooled to room temperature. After overnight stirring, distilled water was added to the solution and a precipitate was filtered off and washed with water. The crude powder was dissolved in dichloromethane followed by addition of n-hexane. The precipitate was dried in vacuo after filtering. Product was obtained as a white powder after purification by column chromatography. (4.9 g, yield 48%). 1 H NMR (400 MHz, CDCl3): d 7.37 (d, 1H, J = 4.4 Hz), 7.52 (t, 2H, J = 7.6 Hz), 7.517.65 (m, 2H), 7.77 (d, 1H, J = 4.8 Hz), 7.83 (d, 2H, J = 4.0 Hz), 8.01 (d, 1H, J = 5.2 Hz), 8.27 (s, 1H), 8.31 (s, 1H). 2.2.3. 8-(8-Iododibenzo[b,d]furan-2-yl)-8-phenyl-8Hindolo[3,2,1-de]acridine(3) 9-(2-Bromophenyl)-9H-carbazole (3.23 g, 10.0 mmol) was dissolved in anhydrous tetrahydrofuran (35 mL) under nitrogen at 78 °C and n-butyllithium (2.5 M in hexane, 4.0 mL) was added dropwise slowly. The solution was stirred for 2 h at 78 °C, followed by addition of a solution of (8-iododibenzo[b,d]furan-2-yl)(phenyl)methanone (3.2 g, 8.0 mmol) in anhydrous tetrahydrofuran (50 mL) under nitrogen. The mixture was gradually warmed to room temperature and quenched with distilled water and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and concentrated. The crude powdery product was placed in another two-neck flask (250 mL) and dissolved in acetic acid (160 mL). A catalytic amount of sulfuric acid (10 mL) was then added and the solution was refluxed for 12 h. After cooling to room temperature, white powder was filtered off and was purified by column chromatography using dichloromethane/n-hexane as an eluent. Final product was obtained as a white powder (3.2 g, yield 64%). 1 H NMR (400 MHz, DMSO): d 7.03 (d, 2H, J = 3.5 Hz), 7.09 (t, 3H, J = 7.5 Hz), 7.20 (t, 1H, J = 7.5 Hz), 7.26 (t, 1H, J = 7.5 Hz), 7.31 (t, 1H, J = 7.5 Hz), 7.41 (t, 2H, J = 8.0 Hz), 7.50 (d, 1H, J = 4.0 Hz), 7.55 (d, 1H, J = 4.0 Hz), 7.59–7.64 (m, 2H), 7.79 (d, 1H, J = 5.0 Hz), 7.92 (s, 1H), 8.13 (d, 1H, J = 3.5 Hz), 8.27 (d, 1H, J = 4.0 Hz), 8.32 (t, 2H, J = 8.5 Hz), 8.45 (s, 1H). 2.2.4. 8-(8-(9H-carbazol-9-yl)dibenzo[b,d]furan-2-yl)-8phenyl-8H-indolo[3,2,1-de]acridine(AcHCz) 8-(8-Iododibenzo[b,d]furan-2-yl)-8-phenyl-8H-indolo [3,2,1-de]acridine (1.2 g, 1.92 mmol), 9H-carbazole (0.32 g, 1.92 mmol), copper iodide (0.11 g, 0.57 mmol), potassium phosphate (0.82 g, 3.84 mmol) and trans-1,2-diamino cyclohexane (0.06 g, 0.57 mmol) were dissolved in 1,4-dioxane (60 mL) under nitrogen. The solution was filtered and the filtered product was diluted with dichloromethane and washed with distilled water. The organic layer was dried over anhydrous MgSO4 and concentrated. The crude powdery product was purified by column chromatography using n-hexane as an eluent. Final product was obtained as a white powder (1.1 g, yield 87%).
J.-A Seo et al. / Organic Electronics 15 (2014) 3773–3779 1 H NMR (400 MHz, CDCl3): d 7.06–7.13 (m, 5H), 7.18– 7.39 (m, 13H), 7.47–7.54 (m, 4H), 7.70 (d, 1H, J = 8.4 Hz), 7.85 (d, 1H, J = 1.2 Hz), 7.92 (d, 1H, J = 3.6 Hz), 8.058.13 (m, 5H). 13C NMR (100 MHz, CDCl3): 57.3, 109.7, 111.3, 113.0, 113.6, 114.6, 118.5, 119.9, 120.3, 120.4, 121.3, 121.4, 122.2, 122.4, 122.6, 123.0, 123.3, 125.9, 126.0, 126.4, 126.9, 127.9, 128.0, 128.2, 130.3, 130.7, 132.0, 132.3, 132.6, 137.1, 137.7, 138.8, 141.7, 141.8, 146.2, 155.8. MS (FAB) m/z 663 [(M+H)+]. Anal. Calcd for C49H30N2O: C, 88.80; H, 4.56; N, 4.23. Found: C, 88.36; H, 4.61; N, 4.28.
2.2.5. 8-(8-(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]furan-2yl)-8-phenyl-8H-indolo[3,2,1-de]acridine(AcHCb) AcHCb was synthesized according to the same procedure as the synthesis of AcHCz except that a-carboline was used instead of 9H-carbazole. (AcHCb) yield 70%. 1H NMR (400 MHz, CDCl3): d 7.06–7.12 (m, 5H), 7.19–7.24 (m, 4H), 7.28–7.53 (m, 9H), 7.57–7.61 (m, 2H), 7.74 (d, 1H, J = 4.4 Hz), 7.92–7.94 (m, 2H), 8.07–8.14 (m, 4H) 8.38 (d, 1H, J = 3.6 Hz), 8.45 (d, 1H, J = 2.4 Hz). 13C NMR (100 MHz, CDCl3): 57.3, 110.4, 111.1, 112.8, 113.6, 114.6, 116.1, 116.3, 118.4, 120.6, 120.7, 120.8, 121.0, 121.2, 121.3, 122.1, 122.4, 122.9, 123.5, 125.8, 126.3, 126.4, 126.8, 126.9, 127.0, 127.1, 127.8, 128.0, 128.2, 128.4, 130.2, 130.5, 131.2, 132.0, 132.4, 137.1, 137.7, 138.8, 140.9, 141.6, 146.1, 146.7, 152.6, 155.7, 156.0. MS (FAB) m/z 664 [(M+H)+]. Anal. Calcd for C48H29N3O: C, 86.86; H, 4.40; N, 6.33. Found: C, 86.36; H, 4.41; N, 6.34. 2.3. Device fabrication and measurements The blue PHOLEDs were prepared by thermal deposition of 4,40 -cyclohexylidenebis[N,N-bis(4-methylphenyl) aniline] (TAPC, 20 nm), 1,3-bis(N-carbazolyl)benzene (mCP, 10 nm), AcHCz: iridium(III) bis[(4,6-difluorophenyl) pyridinato-N,C2]picolinate (FIrpic) or AcHCb:FIrpic (25 nm), diphenylphosphine oxide-4-(triphenylsilyl) phenyl (TSPO1, 35 nm), LiF (1 nm) and Al (200 nm) on a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 60 nm) coated indium tin oxide (ITO, 120 nm) substrate layer by layer. FIrpic was doped in the AcHCz and AcHCb hosts at a doping concentration of 10%. Single carrier devices with device structures of ITO (50 nm)/ PEDOT:PSS (10 nm)/TAPC (20 nm)/mCP (10 nm)/AcHCz or AcHCb (25 nm)/TAPC (5 nm)/Al (200 nm) (hole only device) and ITO (50 nm)/PEDOT:PSS (10 nm)/TSPO1 (10 nm)/AcHCz or AcHCb (25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm) (electron only device) were also prepared to compare hole and electron density in the emitting layer. Electrical measurements of the blue PHOLED and single carrier devices were performed using Keithley 2400 source measurement unit and luminance measurements were carried out using CS 2000 spectroradiometer in ambient condition after encapsulation of the devices. 3. Results and discussion The design of the two indoloacridine based host materials was intended to increase the triplet energy and to
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hinder the molecular packing of the indoloacridine core simultaneously. The indoloacridine core was linked to dibenzofuran based aromatic moieties via sp3 carbon for high triplet energy and carbazole or carboline modified dibenzofuran was introduced to prevent the molecular packing of the planar indoloacridine core. The carbazole or carboline modified dibenzofuran moiety can also increase the glass transition temperature of AcHCz and AcHCb host materials. Synthetic scheme of AcHCz and AcHCb host materials is shown in Scheme 1. The indoloacridine core was synthesized by ring closing reaction of 9-(2-bromophenyl)-9Hcarbazole with (8-iododibenzo[b,d]furan-2-yl)(phenyl) methanone using catalytic sulfuric acid. Iodinated indoloacridine core (3) was reacted with carbazole or carboline to produce AcHCz and AcHCb as final products. Column chromatography was used as a wet purification method and vacuum train sublimation was used as a final dry purification process to obtain high purity above 99%. Molecular orbital of AcHCz and AcHCb was calculated using Gaussian 09 program to investigate the electron distribution in the host materials. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distribution of the host materials calculated using B3LYP 6-31G⁄ basis sets is shown in Fig. 1. The HOMO of AcHCz was centered on the carbazole unit attached to the dibenzofuran moiety due to high electron density of the carbazole unit and the LUMO of AcHCz was localized on the dibenzofuran moiety because of relatively low electron density of dibenzofuran by high electronegativity of oxygen. The HOMO was shifted to the indoloacridine core in the AcHCb host owing to high electron density of indoloacridine compared to carboline. The LUMO was mostly dispersed over the dibenzofuran moiety because of electron-withdrawing character of oxygen of dibenzofuran. Photophysical properties of AcHCz and AcHCb host materials were characterized with ultraviolet–visible (UV–vis) and photoluminescence (PL) measurements. Fig. 2 shows UV–vis absorption, solution PL and low temperature PL emission spectra of AcHCz and AcHCb in anhydrous tetrahydrofuran solvent. The AcHCz and AcHCb showed similar UV–vis absorption spectra and weak UV– vis absorption peaks assigned to n-p⁄ transition of the indoloacridine, carbazole, carboline and dibenzofuran moieties were observed between 320 nm and 360 nm in addition to the strong UV–vis absorption peaks by p–p⁄ transition of the conjugated backbone structure. UV–vis absorption edges of AcHCz and AcHCb were 364 nm and 365 nm, which corresponded to an energy gap of 3.40 eV. PL emission peaks of the AcHCz and AcHCb were observed at 371 nm and 372 nm and the first phosphorescent peak positions of low temperature PL emission were 426 nm and 433 nm, respectively. Triplet energies calculated from the low-temperature PL spectra were 2.91 and 2.86 eV for AcHCz and AcHCb. The separation of conjugation by sp3 carbon linkage between indoloacridine and dibenzofuran allowed the high triplet energy of AcHCz and AcHCb. Electrochemical oxidation and reduction behavior of AcHCz and AcHCb was studied by cyclic voltammetry (CV) to measure ionization potential (IP) and electron
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Scheme 1. Synthetic scheme of AcHCz and AcHCb.
AcHCz
AcHCb
HOMO
LUMO
Fig. 1. HOMO and LUMO distribution of AcHCz and AcHCb.
affinity (EA). CV oxidation and reduction curves of AcHCz and AcHCb are shown in Fig. 3. Ferrocene was used as the standard material for CV measurements. Oxidation/ reduction potentials of AcHCz and AcHCb were 1.20/ 2.15 V and 1.24/2.21 V, respectively. As the oxidation happens in the carbazole unit of AcHCz and the indoloacridine unit of AcHCb, the oxidation potential was similar. The reduction potentials of AcHCz and AcHCb were also similar as the molecular simulation result of two host materials revealed that dibenzofuran is mostly reduced due to relatively low electron density compared to other moieties of the host materials. The IP/EAs of AcHCz and AcHCb calculated from oxidation and reduction potentials
using ferrocene as the standard material (IP: 4.80 eV) were 6.00/2.65 eV and 6.04/2.59 eV [22]. Thermal properties of AcHCz and AcHCb were analyzed using differential scanning calorimeter (DSC). DSC scan curves of AcHCz and AcHCb are presented in Fig. 4. Glass transition temperature could be measured from the inflection point of endothermic curves of DSC and the glass transition temperatures of AcHCz and AcHCb were 174 and 177 °C, respectively. The glass transition temperature of the indoloacridine derived host materials was much higher than that of other high triplet energy host materials derived from carbazole or spirobifluorene [1–8,14–16]. The rigidity of the indoloacridine core and bulkiness of
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Solution UV Solution PL LTPL
Normalized intensity (a.u.)
1.0 0.8 0.6 0.4
AcHCz AcHCb Heat flow (arb.unit)
(a)
endothermic
0.2 0.0 300
400
500
50
Wavelength (nm)
(b)
150
200
250
300
Temperature (°C) Fig. 4. DSC curves of AcHCz and AcHCb.
Solution UV Solution PL LTPL
1.0
Normalized intensity (a.u.)
100
0.8 0.6 0.4 0.2 0.0 300
400
500
Wavelength (nm)
Current (arb.unit)
Fig. 2. UV–vis, solution PL, and low-temperature PL spectra of AcHCz (a) and AcHCb (b).
AcHCz AcHCb -4
-3
-2
-1
0
1
2
3
Voltage (V) Fig. 3. Oxidation and reduction curves of AcHCz and AcHCb.
the dibenzofuran moiety substituted with carbazole or carboline increased the glass transition temperature of the host materials. Basic thermal and photophysical properties of AcHCz and AcHCb are summarized in Table 1. Current density of single carrier devices of the host materials were compared to separately monitor the hole and electron density in emitting layer. In general, the comparison of single carrier density in the device reflects both mobility and energy barrier for injection. In our single
carrier device case, the energy barrier factor was removed because there is no energy barrier for charge injection. The LUMO of the electron transport material (2.53 eV) is shallower than that of two host materials and the HOMO of the hole transport material (6.10 eV) is deeper than that of two host materials. Therefore, the current density of the single carrier devices can be correlated with charge transport properties of the host materials. Fig. 5 shows current density–voltage characteristics of hole and electron only devices of AcHCz and AcHCb. The hole current density of AcHCb hole only device was much higher than that of AcHCz device, while the electron current density of two electron only devices was similar although the electron current density of AcHCb was slightly higher than that of AcHCz device. The hole and electron current densities of the single carrier devices can be correlated with the molecular orbital distribution presented in Fig. 1. The HOMO of AcHCb is localized on the indoloacridine moiety and the hole transport is dominated by the orbital overlap of the indoloacridine core between molecules. The indoloacridine core has a planar geometrical structure, which facilitates hole hopping between AcHCb molecules via orbital overlap. In the case of AcHCz, the HOMO localization on the carbazole hinders the hole transport due to restricted orbital overlap between AcHCb molecules. The similar electron current density is originated by the similar LUMO distribution in the two host materials. Blue PHOLEDs were fabricated by doping a blue emitting FIrpic dopant in the AcHCz and AcHCb host materials at a doping concentration of 10%. Fig. 6 shows current density–voltage–luminance plots of the blue PHOLEDs with AcHCz and AcHCb host materials. The current density and luminance of the AcHCb device were higher than those of the AcHCz device. The high current density of the AcHCb device is mostly due to high hole current density of the AcHCb device as explained in the single carrier device data. The luminance was also increased in the AcHCb device because more excitons are generated in the emitting layer by the high electron density. As the electron current density was lower than that of hole current density in the single carrier devices, the exciton density is dominated by electron current density, resulting in high luminance in the AcHCb device with high electron current density. The
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Table 1 Photophysical, electrochemical and thermal properties of AcHCz and AcHCb.
c d e f g
Solution PL (nm)a
IP (eV)b
EA (eV)c
Band gap (eV)d
Triplet energy (eV)e
Tmf (°C)
Tgg (°C)
Tdh (°C)
AcHCz AcHCb
292, 328, 340 295, 335, 350
371 372
6.00 6.04
2.65 2.59
3.35 3.45
2.91 2.86
344 310
174 177
415 420
Data were measured in anhydrous tetrahydrofuran solution. IP (Ionization potential) was calculated from oxidation potentials by cyclic voltammetry. EA (Electron affinity) was calculated from reduction potential by cyclic voltammetry. Band gap was calculated from IP and EA. Triplet energy was calculated from the first emission peak of low-temperature PL spectra. Melting temperature. Glass transition temperature. Thermal decomposition temperature at 5% weight loss.
Current density (mA/cm2)
h
UV–vis (nm)a
30
4 AcHCz (hole)
3.5
AcHCb (hole) AcHCz (electron)
3
AcHCb (electron)
2.5 2 1.5 1 0.5 0
0
2
4
6
8
10
Quantum efficiency (%)
a b
Host
25 20 15 10
AcHCz
5
AcHCb
Voltage (V) 0
10
100
1000
10000
Luminance (cd/m2)
AcHCz
1000
AcHCb
30
100
25
10
20 15
1
10 0.1
5 0
2
4
6
8
10
12
0.01
Voltage (V) Fig. 6. Current density–voltage–luminance curves of the AcHCz and AcHCb.
two devices were turned-on at 3.0 V and the driving voltages at 1000 cd/m2 were 6.9 V and 6.6 V in the AcHCz and AcHCb devices, respectively. External quantum efficiency–luminance plots of the blue PHOLEDs are shown in Fig. 7. The AcHCz device showed higher quantum efficiency than the AcHCb device over all luminance ranges measured. Maximum quantum efficiencies of the AcHCz and AcHCb devices were 23.8% and 20.8%, and the quantum efficiencies at 1000 cd/m2 were 20.4% and 17.2%, respectively. Both AcHCz and AcHCb could demonstrate high external quantum efficiency above 20% in the blue PHOLEDs by the high triplet energy of the host materials. The higher quantum efficiency of the AcHCz device than the AcHCb device is attributed to better carrier balance in the emitting layer. As shown in the single carrier device data, the hole current density was higher than
AcHCz AcHCb
Intensity (arb. unit)
35
0
1
Fig. 7. External quantum efficiency–luminance curves of AcHCz and AcHCb devices.
10000
40
Luminance (cd/m2)
Current density (mA/cm2)
Fig. 5. Current density–voltage curves of hole-only and electron-only devices of AcHCz and AcHCb.
380
480
580
680
780
Wavelength (nm) Fig. 8. EL spectra of AcHCz and AcHCb.
electron current density, indicating that holes and electrons balance in the emitting layer would be improved in the device with low hole current density. Therefore, the AcHCz device can balance holes and electrons in the emitting layer and enhances the quantum efficiency of the blue PHOLEDs. The rather large efficiency roll-off of the blue PHOLEDs is due to poor electron transport properties of TSPO1 and instability of TSPO1 at high driving voltage. Fig. 8 shows electroluminescence (EL) spectra of the AcHCz and AcHCb blue PHOLEDs. Pure emission of FIrpic
J.-A Seo et al. / Organic Electronics 15 (2014) 3773–3779
without any emission from host or charge transport materials was observed. Color coordinates of the AcHCz and AcHCb devices were (0.15, 0.31) and (0.14, 0.31).
[2] [3] [4] [5]
4. Conclusion
[6] [7]
In conclusion, two indoloacridine-derived host materials, AcHCz and AcHCb, were successfully synthesized and the photophysical properties and device performances of two host materials were examined. Both AcHCz and AcHCb showed high triplet energy above 2.80 eV, high glass transition temperature above 170 °C and high maximum quantum efficiency above 20.0% in the blue PHOLEDs. It was revealed that the indoloacridine core would be suitable as the high triplet energy and thermally stable core for the development of host materials for blue PHOLEDs. Acknowledgment The present research was conducted by research fund of Dankook University in 2014. References [1] S.O. Jeon, S.E. Jang, H.S. Son, J.Y. Lee, Adv. Mater. 23 (2011) 1436.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes Jeong-A Seo a, Myoung Seon Gong a, Jun Yeob Lee b,⇑ a Dept. of Nanobiomedical Science and BK21 PLUS NBM Global Research Center for Regenerative Medicine, Dankook University, Chungnam 330-714, Republic of Korea b Dept. of Polymer Science and Engineering, Dankook University, 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 5 September 2014 Received in revised form 9 October 2014 Accepted 14 October 2014 Available online 27 October 2014 Keywords: Indoloacridine Dibenzofuran Host High triplet energy Thermal stability
a b s t r a c t High triplet energy materials derived from carbazole or a-carboline modified indoloacridine were synthesized and device characteristics of blue triplet emitter doped devices were investigated. The indoloacridine derived host materials showed a high triplet energy above 2.80 eV and a high glass transition temperature over 170 °C due to rigid nature of the molecular structure. The indoloacridine based host materials could approach high external quantum efficiency above 20% in blue phosphorescent organic light-emitting diodes. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The triplet energy of host materials is a key parameter to realize high external quantum efficiency in phosphorescent organic light-emitting diodes (PHOLEDs) because main mechanism of light emission of PHOLEDs is energy transfer from the triplet host materials to triplet emitters. The triplet energy of the triplet host materials should be higher than that of triplet emitters to avoid reverse energy transfer from the triplet emitters to the triplet host materials. Therefore, the triplet host materials of blue PHOLEDs must have a triplet energy higher than 2.80 eV as the triplet energy of blue triplet emitters is between 2.65 eV and 2.80 eV. The most widely used core structures of high triplet energy host materials for blue PHOLEDs were carbazole [1–8], dibenzofuran [9–13], and spirobifluorene [14–16] because of high triplet energy of the core structures.
⇑ Corresponding author. Tel.: +82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). http://dx.doi.org/10.1016/j.orgel.2014.10.020 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
Carbazole has been used as a hole transport type core of the triplet host materials due to high triplet energy and good hole transport properties. Many carbazole based triplet host materials were reported and the high triplet energy host materials derived from the carbazole core achieved high quantum efficiency in blue PHOLEDs [1–8]. Similarly, dibenzofuran compounds were also popular as the triplet host materials of blue PHOLEDs for high quantum efficiency [9–13]. Spirobifluorene compounds are another class of high triplet energy host materials for blue PHOLEDs because of high triplet energy and good thermal stability [14–16]. The spirobifluorene core has a molecular structure with two fluorene units directly connected via sp3 carbon. The two fluorene units are distorted each other and the conjugation of molecules is separated by the sp3 carbon linkage, increasing the triplet energy of the spirobifluorene. Additionally, the rigid molecular structure increased the thermal stability and glass transition temperature. Recently, similar concept was used for the molecular design of hole transport materials and indoloacridine core
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derived compounds were reported as the hole transport materials of PHOLEDs [17]. The indoloacridine type hole transport materials showed high glass transition temperature and high quantum efficiency in blue PHOLEDs [17– 19]. However, the indoloacridine moiety could not be adopted as the core structure of the triplet host materials because of excimer formation by the planar structure of the indoloacridine moiety although the synthesis of high triplet energy host materials was reported in the literature [18]. Therefore, a new design to solve the problem of the excimer formation while keeping the merits of high triplet energy and good thermal stability is required. In this work, indoloacridine derivatives, 8-(8-(9Hcarbazol-9-yl)dibenzo[b,d]furan-2-yl)-8-phenyl-8H-indolo [3,2,1-de]acridine(AcHCz) and 8-(8-(9H-pyrido[2,3-b]indol9-yl)dibenzo[b,d]furan-2-yl)-8-phenyl-8H-indolo[3,2,1-de] acridine(AcHCb), were synthesized as high glass transition temperature and high triplet energy host materials for blue PHOLEDs. It was shown that the two host materials AcHCz and AcHCb showed high triplet energy of 2.91 and 2.86 eV and high external quantum efficiency of 23.8% and 20.8% in blue PHOLEDs, respectively. 2. Experimental 2.1. General information Aluminium trichloride, periodic acid, n-butyllithium, copper iodide, trans-1,2-diamino cyclohexane (AldrichChem. Co.), benzoyl chloride, dibenzo[b,d]furan (Tokyo Chemical Industry Co.), iodine (Samchun pure chemical Co.), 9-(2-bromophenyl)-9H-carbazole, a-carboline (P&H Tech Co.) was used without further purification. Sulfuric acid, acetic acid, hydrochloric acid, potassium phosphate, 1,4-dioxane, dichloromethane, diethyl ether, n-hexane, ethyl acetate (Duksan Sci. Co.) was used as received. Tetrahydrofuran (Duksan Sci. Co.) was distilled over sodium and calcium hydride. 9H-carbazole (Sigma–Aldrich Co.) was purified by recrystallization using toluene. Detailed chemical analysis and photophysical measurement were carried out according to the method reported in our previous work [20]. 2.2. Synthesis 2.2.1. Dibenzo[b,d]furan-2-yl(phenyl)methanone(1) Anhydrous aluminum trichloride (7.59 g, 56.9 mmol) and benzoyl chloride (8 g, 56.9 mmol) in dichloromethane (40 mL) were added to dibenzo[b,d]furan (14.3 g, 85.4 mmol) in dichloromethane slowly and the solution was stirred at room temperature under a nitrogen atmosphere for 2 h. The solution was quenched with 1 M HCl and extracted with diethyl ether. The organic layer was dried over anhydrous MgSO4 and concentrated. White powder was obtained as a product and was purified by column chromatography using n-hexane as an eluent (7.00 g, yield 45%) [21]. 1 H NMR (400 MHz, DMSO): d 7.46 (t, 1H, J = 7.5 Hz), 7.59–7.63 (m, 3H), 7.73 (t, 1H, J = 7.5 Hz), 7.79–7.83 (m, 3H), 7.88 (d, 1H, J = 4.5 Hz), 7.94 (d, 1H, J = 4.0 Hz), 8.31 (d, 1H, J = 3.5 Hz), 8.62 (s, 1H).
2.2.2. (8-Iiododibenzo[b,d]furan-2-yl)(phenyl)methanone (2) A solution of dibenzo[b,d]furan-2-yl(phenyl)methanone (7 g, 25.7 mmol), iodine (3.26 g, 12.9 mmol), periodic acid (2.93 g, 12.9 mmol), distilled water (50 mL), sulfuric acid (0.5 mL) and acetic acid (250 mL) was stirred at 60 °C for 10 h under nitrogen, and was allowed to be cooled to room temperature. After overnight stirring, distilled water was added to the solution and a precipitate was filtered off and washed with water. The crude powder was dissolved in dichloromethane followed by addition of n-hexane. The precipitate was dried in vacuo after filtering. Product was obtained as a white powder after purification by column chromatography. (4.9 g, yield 48%). 1 H NMR (400 MHz, CDCl3): d 7.37 (d, 1H, J = 4.4 Hz), 7.52 (t, 2H, J = 7.6 Hz), 7.517.65 (m, 2H), 7.77 (d, 1H, J = 4.8 Hz), 7.83 (d, 2H, J = 4.0 Hz), 8.01 (d, 1H, J = 5.2 Hz), 8.27 (s, 1H), 8.31 (s, 1H). 2.2.3. 8-(8-Iododibenzo[b,d]furan-2-yl)-8-phenyl-8Hindolo[3,2,1-de]acridine(3) 9-(2-Bromophenyl)-9H-carbazole (3.23 g, 10.0 mmol) was dissolved in anhydrous tetrahydrofuran (35 mL) under nitrogen at 78 °C and n-butyllithium (2.5 M in hexane, 4.0 mL) was added dropwise slowly. The solution was stirred for 2 h at 78 °C, followed by addition of a solution of (8-iododibenzo[b,d]furan-2-yl)(phenyl)methanone (3.2 g, 8.0 mmol) in anhydrous tetrahydrofuran (50 mL) under nitrogen. The mixture was gradually warmed to room temperature and quenched with distilled water and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and concentrated. The crude powdery product was placed in another two-neck flask (250 mL) and dissolved in acetic acid (160 mL). A catalytic amount of sulfuric acid (10 mL) was then added and the solution was refluxed for 12 h. After cooling to room temperature, white powder was filtered off and was purified by column chromatography using dichloromethane/n-hexane as an eluent. Final product was obtained as a white powder (3.2 g, yield 64%). 1 H NMR (400 MHz, DMSO): d 7.03 (d, 2H, J = 3.5 Hz), 7.09 (t, 3H, J = 7.5 Hz), 7.20 (t, 1H, J = 7.5 Hz), 7.26 (t, 1H, J = 7.5 Hz), 7.31 (t, 1H, J = 7.5 Hz), 7.41 (t, 2H, J = 8.0 Hz), 7.50 (d, 1H, J = 4.0 Hz), 7.55 (d, 1H, J = 4.0 Hz), 7.59–7.64 (m, 2H), 7.79 (d, 1H, J = 5.0 Hz), 7.92 (s, 1H), 8.13 (d, 1H, J = 3.5 Hz), 8.27 (d, 1H, J = 4.0 Hz), 8.32 (t, 2H, J = 8.5 Hz), 8.45 (s, 1H). 2.2.4. 8-(8-(9H-carbazol-9-yl)dibenzo[b,d]furan-2-yl)-8phenyl-8H-indolo[3,2,1-de]acridine(AcHCz) 8-(8-Iododibenzo[b,d]furan-2-yl)-8-phenyl-8H-indolo [3,2,1-de]acridine (1.2 g, 1.92 mmol), 9H-carbazole (0.32 g, 1.92 mmol), copper iodide (0.11 g, 0.57 mmol), potassium phosphate (0.82 g, 3.84 mmol) and trans-1,2-diamino cyclohexane (0.06 g, 0.57 mmol) were dissolved in 1,4-dioxane (60 mL) under nitrogen. The solution was filtered and the filtered product was diluted with dichloromethane and washed with distilled water. The organic layer was dried over anhydrous MgSO4 and concentrated. The crude powdery product was purified by column chromatography using n-hexane as an eluent. Final product was obtained as a white powder (1.1 g, yield 87%).
J.-A Seo et al. / Organic Electronics 15 (2014) 3773–3779 1 H NMR (400 MHz, CDCl3): d 7.06–7.13 (m, 5H), 7.18– 7.39 (m, 13H), 7.47–7.54 (m, 4H), 7.70 (d, 1H, J = 8.4 Hz), 7.85 (d, 1H, J = 1.2 Hz), 7.92 (d, 1H, J = 3.6 Hz), 8.058.13 (m, 5H). 13C NMR (100 MHz, CDCl3): 57.3, 109.7, 111.3, 113.0, 113.6, 114.6, 118.5, 119.9, 120.3, 120.4, 121.3, 121.4, 122.2, 122.4, 122.6, 123.0, 123.3, 125.9, 126.0, 126.4, 126.9, 127.9, 128.0, 128.2, 130.3, 130.7, 132.0, 132.3, 132.6, 137.1, 137.7, 138.8, 141.7, 141.8, 146.2, 155.8. MS (FAB) m/z 663 [(M+H)+]. Anal. Calcd for C49H30N2O: C, 88.80; H, 4.56; N, 4.23. Found: C, 88.36; H, 4.61; N, 4.28.
2.2.5. 8-(8-(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]furan-2yl)-8-phenyl-8H-indolo[3,2,1-de]acridine(AcHCb) AcHCb was synthesized according to the same procedure as the synthesis of AcHCz except that a-carboline was used instead of 9H-carbazole. (AcHCb) yield 70%. 1H NMR (400 MHz, CDCl3): d 7.06–7.12 (m, 5H), 7.19–7.24 (m, 4H), 7.28–7.53 (m, 9H), 7.57–7.61 (m, 2H), 7.74 (d, 1H, J = 4.4 Hz), 7.92–7.94 (m, 2H), 8.07–8.14 (m, 4H) 8.38 (d, 1H, J = 3.6 Hz), 8.45 (d, 1H, J = 2.4 Hz). 13C NMR (100 MHz, CDCl3): 57.3, 110.4, 111.1, 112.8, 113.6, 114.6, 116.1, 116.3, 118.4, 120.6, 120.7, 120.8, 121.0, 121.2, 121.3, 122.1, 122.4, 122.9, 123.5, 125.8, 126.3, 126.4, 126.8, 126.9, 127.0, 127.1, 127.8, 128.0, 128.2, 128.4, 130.2, 130.5, 131.2, 132.0, 132.4, 137.1, 137.7, 138.8, 140.9, 141.6, 146.1, 146.7, 152.6, 155.7, 156.0. MS (FAB) m/z 664 [(M+H)+]. Anal. Calcd for C48H29N3O: C, 86.86; H, 4.40; N, 6.33. Found: C, 86.36; H, 4.41; N, 6.34. 2.3. Device fabrication and measurements The blue PHOLEDs were prepared by thermal deposition of 4,40 -cyclohexylidenebis[N,N-bis(4-methylphenyl) aniline] (TAPC, 20 nm), 1,3-bis(N-carbazolyl)benzene (mCP, 10 nm), AcHCz: iridium(III) bis[(4,6-difluorophenyl) pyridinato-N,C2]picolinate (FIrpic) or AcHCb:FIrpic (25 nm), diphenylphosphine oxide-4-(triphenylsilyl) phenyl (TSPO1, 35 nm), LiF (1 nm) and Al (200 nm) on a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 60 nm) coated indium tin oxide (ITO, 120 nm) substrate layer by layer. FIrpic was doped in the AcHCz and AcHCb hosts at a doping concentration of 10%. Single carrier devices with device structures of ITO (50 nm)/ PEDOT:PSS (10 nm)/TAPC (20 nm)/mCP (10 nm)/AcHCz or AcHCb (25 nm)/TAPC (5 nm)/Al (200 nm) (hole only device) and ITO (50 nm)/PEDOT:PSS (10 nm)/TSPO1 (10 nm)/AcHCz or AcHCb (25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm) (electron only device) were also prepared to compare hole and electron density in the emitting layer. Electrical measurements of the blue PHOLED and single carrier devices were performed using Keithley 2400 source measurement unit and luminance measurements were carried out using CS 2000 spectroradiometer in ambient condition after encapsulation of the devices. 3. Results and discussion The design of the two indoloacridine based host materials was intended to increase the triplet energy and to
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hinder the molecular packing of the indoloacridine core simultaneously. The indoloacridine core was linked to dibenzofuran based aromatic moieties via sp3 carbon for high triplet energy and carbazole or carboline modified dibenzofuran was introduced to prevent the molecular packing of the planar indoloacridine core. The carbazole or carboline modified dibenzofuran moiety can also increase the glass transition temperature of AcHCz and AcHCb host materials. Synthetic scheme of AcHCz and AcHCb host materials is shown in Scheme 1. The indoloacridine core was synthesized by ring closing reaction of 9-(2-bromophenyl)-9Hcarbazole with (8-iododibenzo[b,d]furan-2-yl)(phenyl) methanone using catalytic sulfuric acid. Iodinated indoloacridine core (3) was reacted with carbazole or carboline to produce AcHCz and AcHCb as final products. Column chromatography was used as a wet purification method and vacuum train sublimation was used as a final dry purification process to obtain high purity above 99%. Molecular orbital of AcHCz and AcHCb was calculated using Gaussian 09 program to investigate the electron distribution in the host materials. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distribution of the host materials calculated using B3LYP 6-31G⁄ basis sets is shown in Fig. 1. The HOMO of AcHCz was centered on the carbazole unit attached to the dibenzofuran moiety due to high electron density of the carbazole unit and the LUMO of AcHCz was localized on the dibenzofuran moiety because of relatively low electron density of dibenzofuran by high electronegativity of oxygen. The HOMO was shifted to the indoloacridine core in the AcHCb host owing to high electron density of indoloacridine compared to carboline. The LUMO was mostly dispersed over the dibenzofuran moiety because of electron-withdrawing character of oxygen of dibenzofuran. Photophysical properties of AcHCz and AcHCb host materials were characterized with ultraviolet–visible (UV–vis) and photoluminescence (PL) measurements. Fig. 2 shows UV–vis absorption, solution PL and low temperature PL emission spectra of AcHCz and AcHCb in anhydrous tetrahydrofuran solvent. The AcHCz and AcHCb showed similar UV–vis absorption spectra and weak UV– vis absorption peaks assigned to n-p⁄ transition of the indoloacridine, carbazole, carboline and dibenzofuran moieties were observed between 320 nm and 360 nm in addition to the strong UV–vis absorption peaks by p–p⁄ transition of the conjugated backbone structure. UV–vis absorption edges of AcHCz and AcHCb were 364 nm and 365 nm, which corresponded to an energy gap of 3.40 eV. PL emission peaks of the AcHCz and AcHCb were observed at 371 nm and 372 nm and the first phosphorescent peak positions of low temperature PL emission were 426 nm and 433 nm, respectively. Triplet energies calculated from the low-temperature PL spectra were 2.91 and 2.86 eV for AcHCz and AcHCb. The separation of conjugation by sp3 carbon linkage between indoloacridine and dibenzofuran allowed the high triplet energy of AcHCz and AcHCb. Electrochemical oxidation and reduction behavior of AcHCz and AcHCb was studied by cyclic voltammetry (CV) to measure ionization potential (IP) and electron
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Scheme 1. Synthetic scheme of AcHCz and AcHCb.
AcHCz
AcHCb
HOMO
LUMO
Fig. 1. HOMO and LUMO distribution of AcHCz and AcHCb.
affinity (EA). CV oxidation and reduction curves of AcHCz and AcHCb are shown in Fig. 3. Ferrocene was used as the standard material for CV measurements. Oxidation/ reduction potentials of AcHCz and AcHCb were 1.20/ 2.15 V and 1.24/2.21 V, respectively. As the oxidation happens in the carbazole unit of AcHCz and the indoloacridine unit of AcHCb, the oxidation potential was similar. The reduction potentials of AcHCz and AcHCb were also similar as the molecular simulation result of two host materials revealed that dibenzofuran is mostly reduced due to relatively low electron density compared to other moieties of the host materials. The IP/EAs of AcHCz and AcHCb calculated from oxidation and reduction potentials
using ferrocene as the standard material (IP: 4.80 eV) were 6.00/2.65 eV and 6.04/2.59 eV [22]. Thermal properties of AcHCz and AcHCb were analyzed using differential scanning calorimeter (DSC). DSC scan curves of AcHCz and AcHCb are presented in Fig. 4. Glass transition temperature could be measured from the inflection point of endothermic curves of DSC and the glass transition temperatures of AcHCz and AcHCb were 174 and 177 °C, respectively. The glass transition temperature of the indoloacridine derived host materials was much higher than that of other high triplet energy host materials derived from carbazole or spirobifluorene [1–8,14–16]. The rigidity of the indoloacridine core and bulkiness of
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Solution UV Solution PL LTPL
Normalized intensity (a.u.)
1.0 0.8 0.6 0.4
AcHCz AcHCb Heat flow (arb.unit)
(a)
endothermic
0.2 0.0 300
400
500
50
Wavelength (nm)
(b)
150
200
250
300
Temperature (°C) Fig. 4. DSC curves of AcHCz and AcHCb.
Solution UV Solution PL LTPL
1.0
Normalized intensity (a.u.)
100
0.8 0.6 0.4 0.2 0.0 300
400
500
Wavelength (nm)
Current (arb.unit)
Fig. 2. UV–vis, solution PL, and low-temperature PL spectra of AcHCz (a) and AcHCb (b).
AcHCz AcHCb -4
-3
-2
-1
0
1
2
3
Voltage (V) Fig. 3. Oxidation and reduction curves of AcHCz and AcHCb.
the dibenzofuran moiety substituted with carbazole or carboline increased the glass transition temperature of the host materials. Basic thermal and photophysical properties of AcHCz and AcHCb are summarized in Table 1. Current density of single carrier devices of the host materials were compared to separately monitor the hole and electron density in emitting layer. In general, the comparison of single carrier density in the device reflects both mobility and energy barrier for injection. In our single
carrier device case, the energy barrier factor was removed because there is no energy barrier for charge injection. The LUMO of the electron transport material (2.53 eV) is shallower than that of two host materials and the HOMO of the hole transport material (6.10 eV) is deeper than that of two host materials. Therefore, the current density of the single carrier devices can be correlated with charge transport properties of the host materials. Fig. 5 shows current density–voltage characteristics of hole and electron only devices of AcHCz and AcHCb. The hole current density of AcHCb hole only device was much higher than that of AcHCz device, while the electron current density of two electron only devices was similar although the electron current density of AcHCb was slightly higher than that of AcHCz device. The hole and electron current densities of the single carrier devices can be correlated with the molecular orbital distribution presented in Fig. 1. The HOMO of AcHCb is localized on the indoloacridine moiety and the hole transport is dominated by the orbital overlap of the indoloacridine core between molecules. The indoloacridine core has a planar geometrical structure, which facilitates hole hopping between AcHCb molecules via orbital overlap. In the case of AcHCz, the HOMO localization on the carbazole hinders the hole transport due to restricted orbital overlap between AcHCb molecules. The similar electron current density is originated by the similar LUMO distribution in the two host materials. Blue PHOLEDs were fabricated by doping a blue emitting FIrpic dopant in the AcHCz and AcHCb host materials at a doping concentration of 10%. Fig. 6 shows current density–voltage–luminance plots of the blue PHOLEDs with AcHCz and AcHCb host materials. The current density and luminance of the AcHCb device were higher than those of the AcHCz device. The high current density of the AcHCb device is mostly due to high hole current density of the AcHCb device as explained in the single carrier device data. The luminance was also increased in the AcHCb device because more excitons are generated in the emitting layer by the high electron density. As the electron current density was lower than that of hole current density in the single carrier devices, the exciton density is dominated by electron current density, resulting in high luminance in the AcHCb device with high electron current density. The
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Table 1 Photophysical, electrochemical and thermal properties of AcHCz and AcHCb.
c d e f g
Solution PL (nm)a
IP (eV)b
EA (eV)c
Band gap (eV)d
Triplet energy (eV)e
Tmf (°C)
Tgg (°C)
Tdh (°C)
AcHCz AcHCb
292, 328, 340 295, 335, 350
371 372
6.00 6.04
2.65 2.59
3.35 3.45
2.91 2.86
344 310
174 177
415 420
Data were measured in anhydrous tetrahydrofuran solution. IP (Ionization potential) was calculated from oxidation potentials by cyclic voltammetry. EA (Electron affinity) was calculated from reduction potential by cyclic voltammetry. Band gap was calculated from IP and EA. Triplet energy was calculated from the first emission peak of low-temperature PL spectra. Melting temperature. Glass transition temperature. Thermal decomposition temperature at 5% weight loss.
Current density (mA/cm2)
h
UV–vis (nm)a
30
4 AcHCz (hole)
3.5
AcHCb (hole) AcHCz (electron)
3
AcHCb (electron)
2.5 2 1.5 1 0.5 0
0
2
4
6
8
10
Quantum efficiency (%)
a b
Host
25 20 15 10
AcHCz
5
AcHCb
Voltage (V) 0
10
100
1000
10000
Luminance (cd/m2)
AcHCz
1000
AcHCb
30
100
25
10
20 15
1
10 0.1
5 0
2
4
6
8
10
12
0.01
Voltage (V) Fig. 6. Current density–voltage–luminance curves of the AcHCz and AcHCb.
two devices were turned-on at 3.0 V and the driving voltages at 1000 cd/m2 were 6.9 V and 6.6 V in the AcHCz and AcHCb devices, respectively. External quantum efficiency–luminance plots of the blue PHOLEDs are shown in Fig. 7. The AcHCz device showed higher quantum efficiency than the AcHCb device over all luminance ranges measured. Maximum quantum efficiencies of the AcHCz and AcHCb devices were 23.8% and 20.8%, and the quantum efficiencies at 1000 cd/m2 were 20.4% and 17.2%, respectively. Both AcHCz and AcHCb could demonstrate high external quantum efficiency above 20% in the blue PHOLEDs by the high triplet energy of the host materials. The higher quantum efficiency of the AcHCz device than the AcHCb device is attributed to better carrier balance in the emitting layer. As shown in the single carrier device data, the hole current density was higher than
AcHCz AcHCb
Intensity (arb. unit)
35
0
1
Fig. 7. External quantum efficiency–luminance curves of AcHCz and AcHCb devices.
10000
40
Luminance (cd/m2)
Current density (mA/cm2)
Fig. 5. Current density–voltage curves of hole-only and electron-only devices of AcHCz and AcHCb.
380
480
580
680
780
Wavelength (nm) Fig. 8. EL spectra of AcHCz and AcHCb.
electron current density, indicating that holes and electrons balance in the emitting layer would be improved in the device with low hole current density. Therefore, the AcHCz device can balance holes and electrons in the emitting layer and enhances the quantum efficiency of the blue PHOLEDs. The rather large efficiency roll-off of the blue PHOLEDs is due to poor electron transport properties of TSPO1 and instability of TSPO1 at high driving voltage. Fig. 8 shows electroluminescence (EL) spectra of the AcHCz and AcHCb blue PHOLEDs. Pure emission of FIrpic
J.-A Seo et al. / Organic Electronics 15 (2014) 3773–3779
without any emission from host or charge transport materials was observed. Color coordinates of the AcHCz and AcHCb devices were (0.15, 0.31) and (0.14, 0.31).
[2] [3] [4] [5]
4. Conclusion
[6] [7]
In conclusion, two indoloacridine-derived host materials, AcHCz and AcHCb, were successfully synthesized and the photophysical properties and device performances of two host materials were examined. Both AcHCz and AcHCb showed high triplet energy above 2.80 eV, high glass transition temperature above 170 °C and high maximum quantum efficiency above 20.0% in the blue PHOLEDs. It was revealed that the indoloacridine core would be suitable as the high triplet energy and thermally stable core for the development of host materials for blue PHOLEDs. Acknowledgment The present research was conducted by research fund of Dankook University in 2014. References [1] S.O. Jeon, S.E. Jang, H.S. Son, J.Y. Lee, Adv. Mater. 23 (2011) 1436.
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