High-Efficiency Blue Phosphorescence Organic Light-Emitting Diode

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High-Efficiency Blue Phosphorescence Organic LightEmitting Diode with Ambipolar Carbazole-Triazole Host Tien-Lung Chiu, Hsin-Jen Chen, Yu-Hsuan Hsieh, Jau-Jiun Huang, and Man-Kit Leung J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04685 • Publication Date (Web): 03 Jul 2015 Downloaded from http://pubs.acs.org on July 9, 2015

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The Journal of Physical Chemistry

High-Efficiency Blue Phosphorescence Organic LightEmitting Diode with Ambipolar Carbazole-Triazole Host Tien-Lung Chiu*,†, Hsin-Jen Chen†, Yu-Hsuan Hsieh‡, Jau-Jiun Huang‡ and Man-kit Leung‡ †

Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Rd., Taoyuan, Taiwan 32003.

‡

Department of Chemistry, Institute of Polymer Science and Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Rd., Taipei, Taiwan 10617.

KEYWORDS: blue phosphorescent organic light-emitting diode, ambipolar host. ABSTRACT: An ambipolar material with high triplet energy of 3.1 eV, 9-(2-(4,5-diphenyl-4H1,2,4-triazol-3-yl)phenyl)-9H-carbazole (CbzTAZ), was synthesized by conjugating an electrontransporting 1,2,4-triazole and a hole-transporting carbazole and employed as the host doping with blue emitter iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) to be the emitting layer (EML) of an efficient blue phosphorescent organic light-emitting diode (PhOLED). By adjusting the ratio between CbzTAZ and FIrpic, an efficient OLED performance with a current efficiency (ηC) of 50 cd/A and an external quantum efficiency (ηEQE) of 23.8% can be obtained. The position of the recombination zone located inside the EML close to the electron-transporting layer (ETL) can be detected by partially doping FIrpic in one of the three EML regions. By fine-tuning the thickness of the ETL to balance electrons and holes in the EML,

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a highly efficient blue PhOLED with a maximum ηC of 52.1 cd/A, maximum power efficiency of 46.1 lm/W, and maximum ηEQE of 24.4% was obtained. In addition, the device demonstrated a favorable efficiency roll-off (ηC of 44.6 cd/A and ηEQE of 21%) at a high brightness of 1000 cd/m2.

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INTRODUCTION:

Currently, organic light-emitting diodes (OLEDs) grabs a lot of attentions to be one of the promised candidates as next generation lighting source because they efficiently produce substantially improved red, green and blue light.1−4 Several small-area white OLED lighting products are gradually being commercialized and mass-produced. However, low efficiency remains a problem particularly in blue OLEDs, whereas general white OLEDs with warm color temperatures are highly efficient.5,6 An urgent demand for efficient blue OLEDs has encouraged researchers to actively develop a host-dopant phosphorescent OLED (PhOLED) through the evolutions on host materials,7,8 blue emitter materials,9−11 and device engineerings.12,13 According to the spin statistics theorem,14 the electron-hole pair forms 25% singlet excitons (generating fluorescence) and 75% triplet excitons (generating phosphorescence).15 Therefore, PhOLEDs currently appear to be the most suitable structure for producing high-efficiency electroluminescent devices. We focused on characterizing a new host material and engineered layer structures for constructing an efficient blue PhOLED with the blue emitter iridium(III)bis[4,6-(difluorophenyl)-pyridinato-N,C2']picolinate (FIrpic). Currently, FIrpic is the most efficient blue emitter, and exhibits external quantum efficiency (ηEQE) of approximately 30%.16 Several studies


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have produced FIrpic-containing high-efficiency blue PhOLEDs with ηEQE values of more than 25% by using novel host materials such as CbBPCb,16 CzBPCb,16 CzOTCb,17 CbOTCb,17 mCPCN,18 26DCzPPy,19 SitCz,20 and mNBICz.21 These progressive host materials have been classified as bipolar materials that enable the concurrent transportation of electrons and holes. Therefore, we synthesized a bipolar material 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)9H-carbazole (CbzTAZ) by conjugating an electron-transporting 1,2,4-triazole (TAZ) with a hole-transporting carbazole.22 Similarly, Zhuang et al. synthesized four structurally isomeric carbazole/triazole molecules, and their blue devices exhibited a low current efficiency (ηC) of less than 21.1 cd/A.23 By employing CbzTAZ as the host material of a blue PhOLED, we obtained a high-efficiency performance (ηC of 52.1 cd/A, power efficiency (ηP) of 46.1 lm/W, and ηEQE of 24.4%) after employing device engineering schemes such as scanning the dopant concentration, probing the position of the main recombination zone (RZ), and fine-tuning the thickness of the electron transporting layer (ETL) to achieve a great carrier balance in the emitting layer (EML).

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EXPERIMENTAL SECTION

An ambipolar CbzTAZ was synthesized by conjugating a hole-transporting carbazole to an electron-transporting 1,2,4-triazole moiety, and its synthetic route was shown in Fig. 1. The synthesized process was started from preparation of 3-(2-fluorophenyl)-4,5-diphenyl-4H-1,2,4triazole(2): A mixture of aniline (5.1 ml, 55.91 mmol) and aluminum trichloride (1.83 g, 13.72 mmol) was stirred at 140 oC under argon for 2.5 hours. Compound 1 (5 g, 20.81 mmol) in NMP (5.8 ml) was added to the mixture and then heated at 200 oC for 24 hours. The reaction mixture



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was poured into ice water and the precipitated crude product was collected and dried. The crude product was collected and purified by recrystallization from CH2Cl2 and EtOH. Yield: 5.92 g (90%), mp. 224–225 oC; 1H NMR (400 MHz, d6-DMSO) δ 7.56-7.46 (m, 2H), 7.40-7.35 (m, 8H), 7.28–7.17 (m, 4H); 13C NMR (100 MHz, d6-DMSO) δ 160.35, 157.88, 153.74, 150.14, 133.93, 132.49, 132.19, 129.45, 127.33, 126.51, 124.37, 115.57, 115.05; HRMS m/z [M]+ 315.1177 Anal. calcd for C20H14FN3: C, 76.18; H, 4.47; N, 13.33; found C, 76.11; H, 4.48; N, 13.51. Preparation of 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole (3): A mixture of Cs2CO3 (3.33 g, 10.21 mmol) and carbazole (1.55 g, 9.28 mmol) in DMSO (25 ml) was stirred at room temperature under argon for 30 min. 3-(2-Fluorophenyl)-4,5-diphenyl-4H-1,2,4-triazole(2) (3.08 g, 9.77 mmol) was added to the mixture and then heated at 160 oC for 72 h. The reaction mixture was poured into a mixture of ice-water and extracted with ether. Collected the organic layer and dried over MgSO4. Purified by recrystallization from CH2Cl2 and acetone. After the recrystallization, we sublimed CbzTAZ to investigate the material characteristics and fabricate the device. The purity of sublimated CbzTAZ was analyzed by HPLC to be 99.7%. Yield: 3.61 g (70%), mp. 218–219 oC; 1H NMR (400 MHz, CD2Cl2) δ 8.14–8.11 (m, 1H), 7.97–7.95 (m, 2H), 7.68–7.65 (m, 2H), 7.45–7.43 (m, 1H), 7.24–7.09 (m, 7H), 6.97 (d, J = 4.23, 2H), 6.89–6.87 (m, 2H), 6.81 (t, J=7.48, 1H), 6.64 (t, J = 7.52, 2H), 6.08 (d, J = 8.28, 2H);

13

C NMR (100 MHz,

CD2Cl2) δ 153.96, 153.49, 139.93, 136.49, 133.83, 132.98, 131.60, 129.41, 128.76, 128.72, 128.68, 128.17, 128.02, 127.02, 125.82, 125.68, 125.18, 123.73, 123.72, 120.17, 119.69, 110.18; HRMS m/z [M+H]+ 463.1905 Anal. calcd for C32H22N4: C, 83.09; H, 4.79; N, 12.11; found C, 82.91; H, 4.79; N, 12.13. By employing this CbzTAZ as the host in the EML of a blue PhOLED, the basic layer structure was configured using 75 nm of indium-tin oxide (ITO) as the anode; 50 nm of N,N'-diphenyl-


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N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) formed a hole transport layer (HTL); 10 nm of N,N-dicarbazolyl-3,5-benzene (mCP) also formed another HTL, which can ameliorate the energy transfer between NPB and FIrpic; 40 nm of CbzTAZ:x%FIrpic was the EML, where CbzTAZ was the host and FIrpic was the blue emitter; 45 nm of 3-(4-Biphenylyl)-4-phenyl-5tert-butylphenyl-1,2.4-triazole (TAZ) was the ETL and hole block layer; 1.2 nm of lithium fluoride (LiF) was the electron injection layer; and 100 nm of aluminum (Al) was the cathode. In particular, the purity of NPB, mCP, CbzTAZ, FIrpic and TAZ characterized using HPLC was 99.8%, 99.5%, 99.7%, 99.4%, and 99.2%, respectively. We fabricated a series of devices by varying the FIrpic concentration (x = 0, 6, 9, and 12) in the EML, partially doping FIrpic in one of the three EML regions to probe the main carrier RZ, and fine-tuning the TAZ ETL thickness (45, 47, and 50 nm) to balance carrier recombination in the EML. These organic and inorganic layers were deposited in high vacuum at approximately 5×10−6 and 2×10−5 Torr, respectively. The devices were fabricated using a multisource thermal evaporator and were then encapsulated inside a glove box. Their electroluminescent characteristics such as current density (J) versus voltage (V; J-V), brightness (B) versus V (B-V), ηC-J, ηP-J, ηEQE-J, and emission spectra were measured using a BJV system, comprising a multisource meter (Keithley 2400) and spectrometer (Minolta CS1000).

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RESULTS AND DISCUSSION

Firstly, we utilized the CbzTAZ thin film and charge-only device to characterize the material properties in solid state. Secondly, to investigate the carrier dynamics and device electronics of blue PhOLEDs with the CbzTAZ host, ten devices with detailed layer structures (summarized in



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Table 1) were designed and fabricated to obtain the optimal ratio between CbzTAZ host and FIrpic dopant (D0–D3), to compare a reference blue OLED with a commercial TAZ host (D4), to probe the location of the recombination zone (D5–D7), and to optimize electron-hole balance in the EML (D8 and D9). To characterize the solid-state attributes of CbzTAZ, we deposited a thin film on quartz through thermal evaporation and measured its ultraviolet-visible (UV) absorption spectrum by using a spectrometer (Hitachi U4100); fluorescence (FL), low-temperature fluorescence (LTFL), and low-temperature phosphorescence (LTPh) spectra by using a fluorescence spectrophotometer (Hitachi F4500); and photoelectron spectrum by using a photoelectron spectrometer (Riken Keiki AC2), as shown in Figure 2. The energy bandgap and triplet energy (ET) were detected from the launching points of the UV absorption and LTPh spectra; they were at 3.6 eV (approximately at 350 nm) and 3.1 eV (approximately at 405 nm), respectively. A workfunction of 5.9 eV was obtained by detecting the photoemission threshold energy of the photoelectron spectrum, which is associated with the highest occupied molecular orbital (HOMO). The lowest unoccupied molecular orbital (LUMO) was calculated to be 2.3 eV. Compared with the original TAZ (LUMO, 2.7 eV; HOMO, 6.3 eV), the carbazole contributed its high triplet energy to level LUMO and HOMO up by approximately 0.4 eV and favorably modified the polarity from electron transporting to bipolarity. In addition, the bipolar property of CbzTAZ was observed according to the J-V performances of an electron-only device (EOD: 50 nm of Al; 1 nm of LiF; 100 nm of CbzTAZ; 1 nm of LiF; and 100 nm of Al) and a hole-only device (HOD: 100 nm of ITO; 1 nm of molybdenum trioxide (MoO3) as hole injection layer; 100 nm of CbzTAZ; 1 nm of MoO3; and 100 nm of Al), as shown in Fig. 3. Both devices exhibited a similar turn-on voltage of approximately 5 V, implying that a commensurate energy barrier for electron and hole


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transportation existed in each device. When the voltage was more than 5 V, their J-V behaviors were belong to trapped charge limited current and the EOD exhibited a greater current density (by approximately one order of magnitude) than that of the HOD. Comparing to the J-V curves of TAZ EOD and HOD, one can deduced that the carbazole moiety balanced electron and hole transportation by accelerating the holes and retarding the electrons because CbzTAZ EOD and HOD exhibited similar electron-transporting and hole-transporting capabilities. Therefore, we concluded that CbzTAZ is ambipolar and can simultaneously and equivalently transport electrons and holes. An energy diagram of the blue PhOLED in which CbzTAZ was used as the host in the EML is illustrated in Figure 4. To investigate the efficient energy transfer between host and dopant, various concentrations of FIrpic (0%, 6%, 9%, and 12%) were used, and the corresponding devices were named D0, D1, D2, and D3. Their electroluminescent behaviors, as represented by J-V and B-V curves, and efficiency-J (ηC-J, ηP-J, and ηEQE-J) curves, are shown in Figure 5(a) and 5(b), respectively. Figure 5(c) shows the normalized emission spectra of D0, D2 and D4 at 6 and 12 V. In addition, their electroluminescent characteristics, such as driving voltage at 20 mA/cm2, maximum brightness, maximum efficiency, and efficiencies at brightness of 100 and 1000 cd/m2 are summarized in Table 2. Without FIrpic emitter, D0 was unable to achieve a satisfactory optical performance. Its emission spectrum exhibited a peak at 386 nm when the driving voltage was 4 V (Figure 5(c)). The driving voltage increased with an increase in the FIrpic concentration from 6 to 12% because the increasing amount of FIrpic traps in EMLs hindered carrier transport. Regarding efficiency, D2, which had a 9% FIrpic, exhibited the great efficiency with a ηC of 50 cd/A, ηP of 43.7 lm/W, and ηEQE of 23.8%. Typically, an overdose of FIrpic, such as a 12% concentration (D3), quenches the exciton energy. The emission spectra of



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D1 and D3 exhibited pure FIrpic emission with two peaks at 472 and 498 nm. These are similar to the emission spectrum of D2 shown in Figure 5(c). No CbzTAZ emission was detected, indicating that the exciton energy transfer from CbzTAZ to FIrpic was sufficiently effective, and the triplet energy back transfer from FIrpic to CbzTAZ did not occur because the ET of CbzTAZ (3.1 eV) was greater than that of FIrpic (2.7 eV). A clear intensity growth in the 498-nm peak, as represented by spectral redshift, was detected because the RZ inside the EML moved toward to the HTL side when a high voltage was applied. Brightnesses of higher than 10830 cd/m2 were obtained using D1–D3 at a driving voltage of 12 V. In addition, D4 was fabricated by replacing CbzTAZ with TAZ, and the electrical and optical performance of D4 was inferior to that of D0– D3, yielding a high driving voltage (11.5 V at 20 mA/cm2), low brightness (3041 cd/m2 at 12 V), and low efficiency (ηC of 41.8 cd/A, ηP of 32.5 lm/W and ηEQE of 19.1%). This was caused by an energy barrier in hole transport and electron-hole imbalance in the EML. The increase in the driving voltage was possibly caused by the energy barrier in hole transport. The HOMO barrier at mCP/TAZ was 0.4 eV, which is greater than that at the mCP/CbzTAZ interface. The electrontransporting and hole-blocking behaviors of TAZ situated the main exciton formation area close to the mCP/EML interface, resulting in a slight leakage light from HTL emission24,25 (400–450 nm) at a driving voltage of 12 V (Figure 5(c)). This indicated the electron-hole imbalance in the EML. These results rendered the brightness and efficiency of the TAZ device unsatisfactory. On the other hand, the small HOMO barrier between mCP and CbzTAZ did facilitate hole transport at the interface and penetrate into the CbzTAZ EML, which was verified by the absence of leakage light. Therefore, the entire carrier RZ of blue PhOLEDs (D1–D3) is located inside the EML. This evidence supports the bipolarity of CbzTAZ.


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Figure 6 illustrates the stability of the operational brightness of D2 and D4. The stability was obtained by fixing the current densities to emit an initial brightness of 1000 cd/m2. The driving current densities of D2 and D4 were fixed at 2.4 and 4.5 mA/cm2, respectively. The predominant factor influencing operational lifetime was the host material (CbzTAZ and TAZ). As expected, D4 with TAZ host exhibited a rapid declining curve for brightness, and its projected halfbrightness lifetime was 21 min. By contrast, the projected half-brightness lifetime for D2 with CbzTAZ host was 85 min. Therefore, we concluded that the ambipolar CbzTAZ host prolongs the device lifetime because of a stable electron-hole balance in the EML. The trend of an increase in the driving voltage is similar as that of the decay in brightness; D2 shows a gradual ascending trend for the increase in the driving voltage, whereas D2 exhibits a rapid ascent. According to the emission spectra, D4 exhibited electron-hole imbalance, as indicated by a leakage light of HTL emission, meaning that a portion of electrons was transported faster than holes and encounter holes, thus generating excitons in the HTL. The electron-hole imbalance also generated many charged molecules that impeded exciton formation. Conversely, the electron-hole balance might have reduced the formation of charged molecules and prevented heat generation and material degradation. This electron-hole imbalance may be a factor shortening the device lifetime of D4. To realize the position of the main RZ, we divided the CbzTAZ EML into three regions, R1– R3, and partially doped each region with 9%FIrpic to fabricate three PhOLEDs (D5–D7). Figure 7(a) and 7(b) shows their electroluminescent behaviors, as indicated by J-V and B-V curves, and efficiency-J (ηC-J, ηP-J, and ηEQE-J) curves, respectively. Figure 7(c) and 7(d) shows their normalized emission spectra at 5 and 12 V, respectively. Their electroluminescent characteristics, that is, driving voltage at 20 mA/cm2, maximum brightness, maximum efficiency, and



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efficiencies at the brightnesses of 100 and 1000 cd/m2, are summarized in Table 2. As 9% FIrpic was doped in R1–R3, the J-V lagged and efficiency increased. FIrpic dopants act as electron traps, thus hindering electron transport and increasing the driving voltage. The FIrpic-doped region close to the ETL impedes the electron transport, leading to an obvious increase in driving voltage. However, D7 (FIrpic doped in R3) exhibits a relatively distinguished efficiency (ηC of 47.5 cd/A, ηP of 42.1 lm/W, and ηEQE of 22.2%), indicating that the major RZ is located in R3. In addition, the FIrpic in R3 efficiently traps electrons that become photons. Regarding the tendency of the ηC-J and ηEQE-J curves of D5–D7 with increasing current density, D7 exhibits a rapid efficiency roll-off, D6 exhibits a relatively stable efficiency roll-off, and D5 exhibits an increase in efficiency initially and then turn a decrease at high current density. The tendency of these efficiency curves describes the movement of the RZ from R3 to R1 with increasing current density, which can be easily observed in the normalized emission spectra (Figure 7(c) and 7(d)). The main spectral profiles of D5–D7 exhibit FIrpic emission. The distinguishable differences in these spectral profiles are the intensities of the 498-nm peak and 380-nm leakage light peak that resulted from the internal microcavity effect and CbzTAZ emission, respectively. The dipole in R1, R3, and R3 emits a distinguished spectral profile based on the spacing between dipole and cathode, and this is internal microcavity effect. A larger spacing causes constructive interference with the wavelength occurs at a longer wavelength, resulting in a red shift in the spectral profile. As the driving voltage increases, the RZ moves toward the HTL, resulting in a red shift, compared with the FIrpic emission spectra in Figure 7(c) and 7(d). For D7 at a driving voltage of 5V, low CbZTAZ emission indicates that the major RZ in R3 (with FIrpic). As the driving voltage is increased to 12 V, a small portion of the RZ moves from R3 to R2 and R1 and is seen as CbzTAZ emission (Figure 7(d)). For D6 and D5 at a driving voltage of 5V, an obvious


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CbzTAZ emission was observed. This emission mainly originates from R3 and partially from R1 and R2. The portion of the RZ in R2 is greater than that in R1, and thus D5 shows greater CbzTAZ emission. As the driving voltage is increased, for the RZ dynamics in D6, a small portion of the RZ leaves R3 and enters R2, thus generating FIrpic emission, and another small portion of the RZ leaves R2 and enters R1, generating CbzTAZ emission. Therefore, the loss induced by the former transfer can be compensated slightly by the latter transfer. For D5, the RZ leaves R2 and enters R1, thus generating FIrpic emission, which reduces CbzTAZ emission. Therefore, D5 shows a greater decrease in CbzTAZ emission than that of D6. This can explain the stable efficiency roll-off in D6 and turn-over efficiency tendency in D5. To further improve the device efficiency by fine-tuning the major RZ located close to the EML/ETL interface for efficient carrier recombination, we increased the ETL thickness in D2 (47 and 50 nm, named D8 and D9, respectively). The J-V, B-V, and efficiency-J (ηC-J, ηP-J, and ηEQE-J) curves of D8 and D9 are presented in Figure 8 and their characteristics are summarized in Table 2. Comparing D2, D8, and D9 at a fixed current density of 20 mA/cm2 revealed that the driving voltage increased by 1.7 V when the ETL thickness was increased by 5 nm because the overall device thickness increased. A thicker ETL thickness delays electron transport, leading to the movement of the major RZ toward the EML/ETL interface. A HOMO barrier of 0.4 eV exists at this interface and blocks hole injection into the ETL. In addition, D8, which had a 47nm ETL, showed the most favorable efficiency (ηC of 52.1 cd/A, ηP of 46.1 lm/W, and ηEQE of 24.4%), thus implying that an optimal electron-hole balance occurs inside the EML close to the EML/ETL interface in which electrons and holes can meet to become excitons and then transform into photons, efficiently. Furthermore, D9, which had a 50-nm ETL, exhibited a



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slightly degraded efficiency because of a slight electron-hole imbalance compared with that in D8.



CONCLUSIONS

An ambipolar material CbzTAZ with a high triplet energy of 3.1 eV and a large energy bandgap of 3.6 eV was synthesized by conjugating an electron-transporting triazole with a holetransporting carbazole moiety, which was successfully employed as the host and doped with FIrpic to form the EML of an efficient blue PhOLED. Comparing to the carrier dynamics of TAZ, the CbzTAZ with a carbazole moiety does balance electron-hole transportation by accelerating the holes and retarding the electrons, leading the major RZ of device obviously moved from R1 (TAZ-OLED) to R3 (CbzTAZ-OLED). In addition, the CbzTAZ-OLED showed the efficient carrier recombination, exciton formation, and favorable operational lifetime because of carrier balance. To achieve the optimal carrier balance, fine-tuning the major RZ close to the EML/ETL interface by enlarging ETL thickness from 45 to 47 nm, the device yielded a maximum current efficiency of 52.1 cd/A, maximum power efficiency of 46.1 lm/W, and maximum external quantum efficiency of 24.4%.



AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel.:+886-3-4638800 ext. 7523 Notes


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The authors declare no competing financial interest

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ACKNOWLEDGEMENTS

This work was supported by the Ministry of Science and Technology (MOST), R.O.C., under Grant MOST 104-3113-E-155-001, 103-3113-E-155-001, 103-2221-E-155-028-MY3, 103-2622E-155-017-CC3, 102-2221-E-155-047 and 102-2622 -E-155-008-CC3. We also thank Prof. YuTai Tao, Institute of Chemistry, Academia Sinica, for AC-2 measurement.

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REFERENCES

1 Su, S.-J.; Cai, C.; Kido, J. Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes J. Mater. Chem. 2012, 22, 3447−3456. 2 Lv, Y.; Zhou, P.; Wei, N.; Peng, K.; Yu, J.; Wei, B.; Wang, Z.; Li, C. Improved holetransporting properties of Ir complex-doped organic layer for high-efficiency organic light-emitting diodes Org. electron. 2013, 14, 124−130. 3 Oh, C.-S.; Lee, C.-W.; Lee, J.-Y. Simple heteroatom engineering for tuning the triplet energy of organometallic host materials for red, green and blue phosphorescent organic light-emitting diodes Chem. Commun. 2013, 49, 3875−3877. 4 Sasabe, H.; Kido, J. Development of high performance OLEDs for general lighting J. Mater. Chem. C 2013, 1, 1699−1707.



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5 Kamtekar, K.-T.; Monkman, A.-P.; Bryce, M. R. Recent advances in white organic lightemitting materials and devices (WOLEDs) Adv. Mater. 2010, 22, 572−582. 6 Jou, J.-H.; Hsieh, C.-Y.; Tseng J.-R.; Peng, S.-H.; Jou, Y.-C.; Hong, J.-H.; Shen, S. M.; Tang, M.-C.; Chen, P.-C.; Lin, C.-H. Candle light-style organic light-emitting diodes Adv. Fun. Mater. 2013, 23, 2750−2757. 7 Baldo, M.-A.; O’Brien, D.-F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M.-E.; Forrest, S.-R. Highly efficient phosphorescent emission from organic electroluminescent devices Nature 1998, 395, 151−154. 8 Holmes, R.-J.; Forrest, S.-R.; Kwong, R.-C.; Brown, J.-J.; Garon S.; Thompson, M.-E. Blue organic electrophosphorescence using exothermic host–guest energy transfer Appl. Phys. Lett. 2003, 82, 2422. 9 Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent progresses on materials for electrophosphorescent organic light-emitting devices Adv. Mater. 2011, 23, 926−952. 10 Yook, K.-S.; Lee, J.-Y. Organic materials for deep blue phosphorescent organic lightemitting diodes Adv. Mater. 2012, 24, 3169−3190. 11 Chang, C.-H.; Ho, C.-L.; Chang, Y.-S.; Lien, I.-C.; Lin, C.-H.; Yang, Y.-W.; Liao, J.-L.; Chi, Y. Blue-emitting Ir(III) phosphors with 2-pyridyl triazolate chromophores and fabrication of sky blue- and white-emitting OLEDs J. Mater. Chem. C 2013, 1, 2639−2647. 12 Chiu, T.-L.; Lee, P.-Y. Carrier injection and transport in blue phosphorescent organic light-emitting device with oxadiazole host Intern. J. Mol. Sci. 2012, 13, 7575−7585.


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13 Jiang, M.-D.; Lee, P.-Y.; Chiu, T.-L.; Lin, H.-C.; Lee, J.-H. Optimizing hole-injection in organic electroluminescent devices by modifying CuPc/NPB interface Syn. Metal. 2011, 161, 1828−1831. 14 Baldo, M.-A.; O’Brien, D.-F.; Thompson, M.-E.; Forrest, S.-R. Excitonic singlet-triplet ratio in a semiconducting organic thin film Phys. Rev. B 1999, 60, 14422−14428. 15 Sun, Y.; Giebink, N.-C.; Kanno, H.; Ma, B.; Thompson, M.-E.; Forrest, S.-R. Management of singlet and triplet excitons for efficient white organic light-emitting devices Nature 2006, 440, 908−912. 16 Lee C.-W.; Lee, J.-Y. Above 30% external quantum efficiency in blue phosphorescent organic light-emitting diodes using pyrido[2,3-b]indole derivatives as host materials Adv. Mater. 2013, 25, 5450−5454. 17 Lee, C.-W.; Im, Y.; Seo, J.-A.; Lee, J.-Y. Carboline derivatives with an ortho-linked terphenyl core for high quantum efficiency in blue phosphorescent organic light-emitting diodes Chem. Commun. 2013, 49, 9860−9862. 18 Lin, M.-S.; Yang, S.-J.; Chang, H.-W.; Huang, Y.-H.; Tsai, Y.-T.; Wu, C.-C.; Chou, S.H.; Mondal, E.; Wong, K.-T. Incorporation of a CN group into mCP: a new bipolar host material for highly efficient blue and white electrophosphorescent devices J. Mater. Chem. 2012, 22, 16114−16120. 19 Ye, H.; Chen, D.; Liu, M.; Su, S.-J.; Wang, Y.-F.; Lo, C.-C.; Lien, A.; Kido, J. Pyridinecontaining electron-transport materials for highly efficient blue phosphorescent oleds with ultralow operating voltage and reduced efficiency roll-off Adv. Funct. Mater. 2014, 24, 3268–3275.



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20 Bin, J.-K.; Cho, N.-S.; Hong, J.-I. New host material for high-performance blue phosphorescent organic electroluminescent devices Adv. Mater. 2012, 24, 2911−2915. 21 Pan, B.; Wang, B.; Wang, Y.; Xu, P.; Wang, L.; Chen, J.; Ma, D. A simple carbazole-Nbenzimidazole bipolar host material for highly efficient blue and single layer white phosphorescent organic light-emitting diodes J. Mater. Chem. C 2014, 2, 2466−2469. 22 Leung, M.-K.; Hsieh, Y.-H.; Kuo, T.-Y.; Chou, P.-T.; Lee, J.-H.; Chiu, T.-L.; Chen, H.-J. Novel ambipolar orthogonal donor–acceptor host for blue organic light emitting diodes Org. Lett. 2013, 15, 4694−4697. 23 Zhuang, J.; Su, W.; Li, W.; Zhou, Y.; Shen, Q.; Zhou, M. Configuration effect of novel bipolar triazole/carbazole-based host materials on the performance of phosphorescent OLED devices Org. Electron., 2012, 13, 2210-2219. 24 Lee, J.; Lee, J.-I.; Song, K.-I.; Lee, S.-J.; Chu, H.-Y. Effects of interlayers on phosphorescent blue organic light-emitting diodes Appl. Phys. Lett., 2008, 92, 203305. 25 Chiu, T.-L.; Chuang, Y.-T. Spectral observations of hole injection with transition metal oxides for an efficient organic light-emitting diode J. Phys. D: Appl. Phys., 2015, 48, 075101.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Layer structures of blue PhOLEDs. HTL

Device #

NPB

EML CbzTAZ:%Firpic 40 nm

mCP

D0

0%

D1

6%

D2

9%

D3

12%

D4 50 nm

10 nm

D5 D6 D7

TAZ

45 nm

R1 9%

TAZ:9% R2 0%

R3 0%

0%

9%

0%

0%

0%

9%

D8

ETL

47 nm

9% 9%

D9

50 nm

Table 2 Summarized electroluminescent characteristics of blue PhOLEDs. Device #

b

Voltage

Luminance

Current efficiency

Power efficiency

EQE

(cd/A)

(lm/W)

(%)

2

(V) a

(cd/m ) 364

c

b

c

b

D0

7.9 , 7.8

D1

a

c

d

9.8 , 4.9 , 6.9

11650

47.3 , 46.1 , 40.9

D2

9.9 , 4.9 , 6.9

a

c

d

11550

50.0 , 48.9 , 43.5

D3 D4

a

c

d

a

c

d

10.0 , 5.0 , 7.0 11.5 , 6.4 , 9.1 a

c

d

D5 D6 D7

8.6 , 5.2 , 6.8 a c d 9.2 , 5.4 , 7.2 a c d 9.5 , 5.2 , 7.1

D8

10.3 , 4.9 , 7.0

D9

10.5 , 5.1 , 7.3

10830

1.0 , 0.5

0.9 , 0.2

c

b

0.7 , 0.4

b

c

d

41.2 , 29.6 , 18.6

b

c

d

43.7 , 31.5 , 19.9

b

c

d

b

c

d

32.5 , 17.6 , 7.7

b

c

d

14.6 , 14.4 , 10.7 b c d 24.8 , 15.9 , 10.6 b c d 42.1 , 28.4 , 16.3

b

c

d

46.1 , 32.2 , 20.0

46.3 , 44.7 , 40.1

3041

41.8 , 36.4 , 22.4

8315 10040 10780

24.1 , 23.4 , 23.0 b c d 28.0 , 27.4 , 24.4 b c d 47.5 , 45.3 , 35.9

a

c

d

9560

52.1 , 50.6 , 44.5

a

c

d

9077

51.9 , 50.5 , 43

b

c

a

d

c

d

22.6 , 22.0 , 19.8

b

c

d

23.8 , 23.3 , 20.7

b

c

d

40.7 , 28.2 , 18.0 b

c

b

c

d

b

c

d

b

c

d

21.8 , 21.2 , 18.9

d

b

c

19.1 , 16.0 , 9.6

d

b

c

d

12.6 , 11.4 , 12.3 b c d 14.2 , 14.0 , 12.8 b c d 22.2 , 21.9 , 18.0

b

c

d

24.4 , 23.7 , 20.9

b

c

d

44.1 , 30.9 , 18.3 2 b

c

b

c

b

c

d

b

c

d

b

c

d

24.3 , 23.7 , 20.6 2 d

2

20 mA/cm ; Maximum; Illuminate at 100 cd/m ; Illuminate at 1000 cd/m



F O

N N

F PhNH 2, AlCl3

Ph N

1

N

carbazole, Cs 2CO3 , N N

NMP

Ph

DMSO

N

N N 3(70%)

2(90%)

Normalized intensity (a.u.)

Figure 1 Synthetic route of CbzTAZ.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

1.0

UV FL LTFL LTPh

0.5

0.0

300

400 500 Wavelength (nm)

600

Photoelectron

3.0

3.5

4.0

4.5 5.0 5.5 Energy (e.V)

6.0

6.5

Figure 2 UV-vis absorption, FL, LTFL, LTPh, and photoelectron spectrum of solid-state CbzTAZ.


Page 19 of 22

2

2

Current densitty (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10

EOD } CbzTAZ HOD EOD } TAZ HOD

0

5

10 15 Voltage (V)

20

25

Figure 3 J-V curves of EODs and HODs with 100 nm CbzTAZ and TAZ.

Figure 4 Illustration of organic layer structure and energy diagram.


D0 D1 D2 D3 D4

50 40 30 20 10 2

4

6 8 Voltage (V)

Normailized intensity (a.u.)

0

10

4

10

3

10

2

2

2

(a)

60

Brightness (cd/m )

70 Current density (mA/cm )

1.0

10

12

10

1

10

0

(c)

50 (b)

D1 D2 D3 D4

C

40 P

30 20

EQE

10 D0

0 -3 10

10

-2

-1

0

10 10 10 2 Current density (mA/cm )

1

10

2

0.05 Normailized intensity (a.u.)

0.6 0.4

0.00 400

0.2 0.0

Page 20 of 22

6V 9%, D2 ] 12V 6V ] 9%, D4 12V

0%, D0

0.8

400

Wavelength (nm)


450

700

Figure 5 (a) J-V and L-V curves and (b) efficiency curves (ηC-J, ηP-J, and ηEQE-J) of D0－D4, (c) normalized emission spectra of D0, D2 and D4 at 6 and 12 V. Inset highlights emission spectra of D2 and D4 at 400−450 nm.

2

D2 D4

11 10

750

9

Voltage (V)

1000 Brightness (cd/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Efficiency (cd/A, lm/W, %EQE)


8 500

0

20

40 60 Time (mins)

80

7

Figure 6 Operational degradations in emission brightness and driving voltage of D2 and D4 at an initial brightness of 1000 cd/m2.


D5 D6 D7

60

3

10

2

10

40

10

20

10

0

Efficiency (cd/A, lm/W)

2

80

1 0

0

4

8 Voltage (V)

12

D5 D6 D7

0.6

Normalized Intensity (a.u.)

0.1

5V

0.0

0.4

400

440

0.2 0.0

400


700

P

 EQE

D5 D6 D7

40

25 20 15

30 20

10

10

5

0 -3 10

10-1

1.0 (c) 0.8

C

50 (b)

-2

-1

0

02 10

1

10 10 10 10 Current density (mA/cm2)

External quantum efficency (%)

4

10

(a)

Brightness (cd/m )

100

Normalized Intensity (a.u.)

1.0 (d) 0.8

0.1

12V D5 D6 D7

0.6

0.0

0.4

400

440

0.2 0.0

400


700

Figure 7 Electroluminance characteristics of D5－D7 such as (a) J-V and L-V curves and (b) efficiency curves (ηC-J, ηP-J, and ηEQE-J), normalized emission spectra at (c) 6 V and (d) 12 V.

2

D8 D9 Efficiency (cd/A, lm/W, %)

80 60 40 20 0

4

6

50 40

C P

10

4

10

3

10

2

10

1

10

0

10

-1

2

100

30 20

EQE

10 0

0.01 0.1 1 10 100 2 Current density (mA/cm )

8 10 Voltage (V)

12

Brightness (cd/m )

Insets of (c) and (d) highlight normalized emission spectra at 400−450 nm.

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current density (mA/cm2)

Page 21 of 22

Figure 8 J-V and L-V curves of D8 and D9. Inset shows efficiency curves such as ηC-J, ηP-J, and ηEQE-J of D8 and D9.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 22

Graphic TOC


High-Efficiency Blue Phosphorescence Organic Light-Emitting Diode

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