Highly Efficient Blue Phosphorescent Organic Light-Emitting Diodes

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Highly Efficient Blue Phosphorescent Organic Light-Emitting Diodes Employing a Host Material with Small Bandgap Lei Zhang, Ye-Xin Zhang, Yun Hu, Xiao-Bo Shi, Zuo-Quan Jiang, Zhao-Kui Wang, and Liangsheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01304 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Applied Materials & Interfaces

Highly Efficient Blue Phosphorescent Organic Light-Emitting Diodes Employing a Host Material with Small Bandgap Lei Zhang, Ye-Xin Zhang, Yun Hu, Xiao-Bo Shi, Zuo-Quan Jiang, Zhao-Kui Wang*, and Liang-Sheng Liao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, China

E-mail: [email protected]; [email protected];

ABSTRACT Blue phosphorescent organic light-emitting diode (PhOLED) with a high maximum external quantum efficiency (EQE) of 26.6% was achieved using a new material, 2,8-bis(9,9dimethylacridin-10(9H)-yl)dibenzo[b,d]furan (DBF-DMS) with a small bandgap, as the host. The device with DBF-DMS showed improved performance compared with that with mCP, which is ascribed to the enhancement in carrier injection and transporting abilities and material stability of DBF-DMS. A lifetime of more than 100 h (time to 50% of the initial luminance, 1000 cd/m2 with an EQE of 19.6%) in the other DBF-DMS-based device is obtained by further utilizing better device structure. This is a report indicating that host material with a small bandgap like DBF-DMS can be successfully utilized toward blue PhOLEDs with high performance.

Keywords: blue phosphorescent organic light-emitting diode (PhOLED); efficiency; stability; small bandgap; lower driving voltage.

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INTRODUCTIONS Organic light-emitting diodes (OLEDs)1 are powerful candidates in the applications of displays and solid-state lightings.2,3 Currently, their practical application has leaned toward mobile phone, smart watch, MP3 player and plat panel TVs. However, there is still large room for the quality improvements in OLEDs such as their efficiency and stability. Especially, simultaneous improvement in both efficiency and stability of the blue devices still remains an issue which hinders the OLED’s commercialization progress. By developing various new fluorescent4 and phosphorescent3 materials, the efficiencies of the blue OLEDs have been improved.2-4 However, the internal quantum efficiency (IQE) was only 25% in fluorescent OLEDs due to the spin statistics rule.5 In 1998, Forrest et al. reported phosphorescent OLEDs (PhOLEDs).6 By harvesting all the excitons, 100% IQE is achievable.6 The large progress makes it possible for us to fabricate high-efficiency blue PhOLEDs. On the one hand, the performance of blue PhOLEDs can be improved by better device design. For example, Forrest and coworkers have demonstrated a device lifetime (T80) of 616±10 h for the two-unit stacked blue PhOLED by utilizing gradient doping in the emitting layer.7 On the other hand, the electroluminescent properties of the device are highly affected by the host material, which is the point we are focused on. To prevent the energy reverse from the excited guest, the host materials should possess a higher triplet energy (T1). And a narrow bandgap (Eg) and suitable frontier molecular orbitals (FMOs) of the host material can improve and balance the carrier injection/transporting, which should reduce the operational voltage so as to make improvements in the device performance.8-13 1,3-Di-9-carbazolylbenzene(mCP) has been successfully applied in blue PhOLEDs as the host material.9,14 Compared with the blue dopants, mCP has a higher T1 of 3.00 eV. So, the triplet excitons can be effectively confined on the guest. However, the large difference between the singlet and triplet energy (∆EST) of mCP is 0.49 eV, which make mCP a wide 2 ACS Paragon Plus Environment

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bandgap.8 The coming unsuitable FMOs of mCP (HOMO= -5.90 eV, LUMO= -2.40 eV) introduce larger carrier injection barriers and unbalanced carrier recombination. The unsuitable FMOs by wide bandgap in commonly utilized host materials like mCP limited the device performance in blue PhOLEDs. In a word, it is necessary to make ∆EST smaller to develop new host materials with narrow bandgap. Recently, some materials with narrow bandgap by small ∆EST were reported.15-18 For example, Qiu et al. achieved an efficient green PhOLEDs by utilizing 2-biphenyl-4,6-bis(12phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine(PIC–TRZ) (∆EST=0.11 eV) as the host.10 Kido et al. reported an efficient green PhOLED with 2,4,6-tris(3-(carbazol-9-yl)phenyl)triazine(TCPZ) (∆EST = 0.14 eV) as the host.19 Wang et al. employed MTXSFCz(∆EST = 0.19 eV) in red PhOLEDs.20 However, there are few studies in blue PHOLEDs due to the tough mission of designing host molecules with a high triplet energy and a small ∆EST. Su et al. reported blue, green and red phosphorescent OLEDs by host (material 5) with heterocyclic cores (∆EST= 0.20 eV). The EQE of blue OLED can reach 14 % at a brightness of 1000 cd/m2.13 Huang et al. introduced SiPTCz (∆EST= 0.11 eV) in blue OLED and reduced the driving voltage.21 Herein, we successfully synthesize a host material with a narrow bandgap, 2,8-bis(9,9dimethylacridin-10(9H)-yl)dibenzo[b,d]furan (DBF-DMS), which has a ∆EST of 0.19 eV (T1=2.80 eV). With this material as a host, blue PhOLEDs with a maximum EQE of 26.6% has been achieved with lower driving voltage and improved stability. Further improved EQE (45.6%) has also been obtained in a two-unit tandem device. By utilizing a better device structure, a T50 lifetime (time to 50% of the initial luminance, 1000 cd/m2) more than 100 h was achievable in the other DBF-DMS based device.



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EXPERIMENTAL Synthesis of DBF-DMS: Under argon, 2.72 g of 9,9-dimethyl-9,10-dihydroacridine (13 mmol), 3.26 g of 2,8-dibromodibenzo[b,d]furan (10 mmol), 2.40 g of sodium t-butoxide (25 mmol), 0.18 g of Pd2(dba)3 (0.2 mmol), and 0.06 g of tri-t-butylphosphonium tetrafluoroborate (0.2 mmol) were dissolved in 100 mL of toluene .The reaction temperature was raised to 110 oC. After the completion of the raw material consumption, 50 mL water was added. Separated organic layer and extracted aqueous layer were achieved with twice 20 mL toluene treatment. Then, the separated organic layer was collected, dried, filtered and evaporated. Using petroleum ether/dichloromethane (4/1, v/v) as eluent, the resulting crude product was purified by chromatography on silica gel to get the target material (5.10 g, 87.48%).1H NMR (400 MHz, CDCl3): δ = 7.91 (d, J = 2.0 Hz, 2H), 7.89 (s, 1H), 7.87 (s, 1H), 7.49–7.45 (m,6H), 6.97–6.90 (m, 8H), 6.31 (d, J = 2.0 Hz, 2H), 6.29 (s, 2H), 1.70 (s, 12H).13C NMR (100 MHz, CDCl3): δ = 156.05, 141.21, 136.46, 130.95, 130.12, 126.35, 126.31, 125.23, 123.96, 120.67, 114.27, 114.08, 36.00, 31.22.HRMS (EI): m/z calcd for: 582.27, found: 582.19. Anal. Calcd for C43H26N2(%): C 86.57, H 5.88, N 4.81, O 2.75; found: C 86.78, H 5.62, N 4.84, O 2.70. Device and Characterization: Except for DBF-DMS, all the other materials were purchased from Lumtec Co., Ltd. OLEDs were fabricated under vacuum (~ 3 × 10-6 Torr) on ITO/glass substrates. Encapsulation was done with glass covers containing desiccant before test. The current density-voltage(J-V) curves of the devices were tested by Keithley 2400 SourceMeter. The electroluminescence (EL) spectra and intensities of the devices were recorded by a PR 655 photometer. A Lambda 750 spectrophotometer was used to achieve UV-Vis absorption spectra. A fluorescence spectrophotometer (Hitachi F-4600) was utilized to measure PL and phosphorescence spectra. There are two different modes (fluorescence and phosphorescence) of the machine. The delayed time to obtain the phosphorescence spectrum should be in the range of microsecond. So the phosphorescence spectrum will not be affected 4 ACS Paragon Plus Environment

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by fluorescence spectrum in our test. A spectrometer (Bruker400M) was operated to obtain 1

HNMR and

13

CNMR spectra. A TA SDT 2960 instrument was chosen to conduct

thermogravimetric analysis (TGA) under nitrogen. Cyclic voltammetry measurement was taken by a voltammetric analyzer (CHI600). Ultraviolet photoelectron spectroscopy (UPS) analysis was measured by KRATOS AXIS Ultra DLD. B3LYP/6–31G (D) atomic basis set was utilized to perform DFT calculations. And transient PL characteristics were detected with a transient spectrometer (FL-TCSPC).

RESULTS AND DISCUSSIONS It is necessary to maintain a small ∆EST to develop a host material with narrow bandgap. In fact, the introduction of suitable electron-donating and electron accepting units can effectively decrease the overlap between the HOMO and LUMO so as to achieve a small ∆EST.18 In this work, the dimethyl-dihydroacridine units and the diphenylene-oxide units are utilized. The synthesis of DBF-DMS is straight forward (Scheme1).The target material was afforded via one-step Bulchwald-Hartwig C-N coupling reaction, starting from 2,8dibromodibenzo[b,d]furan and 9,9-dimethyl-9,10-dihydroacridine, which were commercial avaliable. DBF-DMS was characterized by

1

HNMR,

13

CNMR, EI-MS and element

analysis(Figure S1 and S2, Supporting Information). TGA measurement (Figure S3, Supporting Information) was also carried out. The decomposition temperature(Td) was observed at 394 oC.

Scheme 1. The synthetic route of DBF-DMS.



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We evaluated the FMOs of DBF-DMS by density functional theory (DFT) calculation. From Figure 1, the calculated HOMO and LUMO of DBF-DMS are mainly distributed on the substituted dimethyl-dihydroacridine units and the diphenylene-oxide units as expected, respectively. This implies a small ∆EST. By calculation, DBF-DMS has a S1 of 2.94 eV, a T1 of 2.90 eV, and a ∆EST of 0.04 eV, respectively. As shown in Figure S4, the HOMO energy of DBF-DMS is measured to be -5.80 eV by UPS. From Figure 2(a), the UV–Vis absorption spectra at room temperature were recorded both in toluene and in film. According to the absorption spectrum onset in neat film, DBFDMS has an energy gap of 3.25eV. So the LUMO of DBF-DMS is -2.55 eV. At 77 K, the phosphorescence spectrum of DBF-DMS was detected in 2-methyltetrahydrofuran solution. As a result, the T1 of DBF-DMS is 2.80 eV. The photoluminescence (PL) spectra of DBFDMS were recorded under three different conditions to confirm the singlet energy; To estimate a relatively fair ∆EST, PL spectrum of DBF-DMS in 2-methyl-THF(77 K) was chosen. A peak at 415 nm appears in the PL spectrum of 2-methyltetrahydrofuran at 77 K, so a S1 of 2.99 eV and a small experimental ∆EST of 0.19 eV can be confirmed of DBF-DMS. What is more, the experimental S1, T1 and ∆EST values accords with the calculated values. However, DBF-DMS possess no TADF character in such a small ∆EST (Figure S5, Supporting Information). With a high T1 and a narrow bandgap, DBF-DMS is assumed to be an idea host for common blue phosphorescent emitters. Here, tris[1-(2,6-diisopropylphenyl)-2-phenyl-1Himidazole]iridium(III) (fac-Ir(iprpmi)3) was used as a blue emitter. And mCP with a wide bandgap is also utilized as a conventional host material for comparison. Devices with following structures: ITO/HAT-CN (10 nm)/TAPC (45 nm)/Host: fac-Ir(iprpmi)312 vol.% (20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (100 nm) were firstly fabricated. The EQE, and power efficiency (PE) versus the luminance curves are shown in Figure 3a. The current density-voltage (J-V) and luminance-voltage (L-V) curves are described in Figure 6 ACS Paragon Plus Environment

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3b. Table 1 summarized the detailed electroluminescence data of the devices. The Commission Internationale de L'Eclairage (CIE) coordinates of DBF-DMS and mCP based devices were demonstrated to be the same (0.17, 0.40). At different current densities, device B1 and device B2 showed almost the same EL spectra. (Figure S6, Supporting Information). Compared with the mCP based device B1, the operational driving voltage is lower and the efficiency is much higher in DBF-DMS based device B2, as expected. DBF-DMS possess a narrow bandgap by smaller ∆EST, which introduced suitable FMO levels. This is really helpful to achieve an improved and balanced carrier injection.8-13 What is more, hole-only and electron-only devices were fabricated to measure the transporting abilities of DBF-DMS. Compared to mCP, DBF-DMS has much better abilities in carrier transporting as shown in Figure S7. We ascribed them to the main reasons for the low driving voltage in DBF-DMS based devices. In addition, the device B2 based on DBF-DMS host exhibits a maximum EQE of 26.6 %. As far as we know, this efficiency is higher than other efficiencies reported in blue PhOLEDs based on fac-Ir(iprpmi)3 emitter.22-23 At 1000 cd/m2, the device still affords an EQE of 25.7%, and a power efficiency (PE) of 47.6 lm/W. On another hand, the device with mCP only presents an EQE of 21.1%, and a PE of 31.3 lm/W. There is a 52.1% improvement in the power efficiency by replacing the mCP host with DBF-DMS. The good carrier injection/transporting abilities of DBF-DMS is favorable for the higher performance in DBFDMS based blue PhOLEDs. The success of the improved performance in DBF-DMS-based PhOLEDs indicates the advantages by employing host with a small bandgap for conventional phosphorescent emitters.20 The T50 lifetimes(time to 50% of the initial luminance of 1,000 cd/m2) in DBF-DMS and mCP based blue PhOLEDs are further measured as shown in Figure 4a. DBF-DMS based device B2 possess a T50 lifetime of 20.3 h, while mCP based device B1 exhibits a T50 lifetime of 12.3 h. The lifetimes are consisting with the corresponding voltage changes during the test (Figure S9, Supporting Information). Utilizing the method that has been reported7, 7 ACS Paragon Plus Environment


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exciton density profile in the emitting layers (EMLs) of device B1 and B2 was measured to confirm the recombination zone, which is highly related to the lifetime of the device. However, exciton density profile in the EMLs of device B1 and B2 show similar tendency. A weak difference between the recombination zones shouldn’t be the main reason for the different lifetime (Figure S10, Supporting Information). In fact, the nature of the prolonged lifetime in DBF-DMS based device is ascribed to the follows. As aforementioned, a small bandgap of DBF-DMS make it suitable FMO energy levels. The energy barriers for the hole/electron injection into the EML play very important roles in the device stability. In present case, the HOMO energy levels are -5.50 eV for TAPC, -5.90 eV for mCP, and -5.80 eV for DBF-DMS, respectively. It means that there is a much lower hole barrier of 0.30 eV between TAPC and DBF-DMS than that (0.40 eV) between TAPC and mCP. And the LUMO energy levels are -2.70 eV for TmPyPB, -2.40 eV for mCP, and -2.55 eV for DBF-DMS, respectively. So, the electron injection barrier is much lower in device B2. Lower carrier injection barriers in device B2 are favorable for slowing the device interface degradation effectively.24-28 In addition, the lifetime of OLEDs especially the blue PhOLEDs strongly depends on the materials stabilities. The polarized optical microscopic images of mCP and DBF-DMS after annealing (Figure S11, Supporting Information) reveal that DBF-DMS preserved better film thermal stability than that of mCP, which make DBF-DMS based device B2 a better stability.29-30 With DBF-DMS as the host, blue PhOLED achieved an improved efficiency and stability. However, the lifetime of device B2 is still too short. To further improve the device stability, new solutions are in need. Tandem structure is a very efficient method to enhance the lifetime of OLEDs besides the device efficiency.31-32 To further increase the device stability, tandem blue PhOLED was fabricated by using an intermediate connector of Bphen:Li (10 nm) 1 vol.% /HAT-CN (10 nm). The tandem device with two units demonstrates an ultrahigh EQE of 45.6% at 1000 cd/m2, which is 77.4% higher than the one-unit device (Figure 5a). Notably in Figure 5b, the 8 ACS Paragon Plus Environment

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T50 lifetime is increased to 45.0 h. In reality, the T50 lifetime of an OLED increases with decreasing currentdensity.24 Since a largely improved current efficiency to enable the same luminance at a decreased current density, the DBF-DMS based tandem blue PhOLED presents a prolonged operational lifetime. By utilizing tandem structure in device T1, the EQE and lifetime all increased to nearly two times as that of device B2. But considering the complicated fabrication method, the high driving voltage, and limited lifetime improvement of device T1, we have to conduct a new strategy. In fact, a short lifetime in the DBF-DMS based PhOLEDs may concerned with the material instabilities of TAPC and TmPyPB due to their lower glass-transition temperatures (78ºC for TAPC33, and 79ºC for TmPyPB34) or the narrow recombination zone (Figure S10, Supporting Information). Klubeket al. used a stable device structure to achieve a 50 h device lifetime in blue PhOLEDs.23 Inspired by that, we design a new device structure to chase the balance between the efficiency and the lifetime. We replaced TAPC with N,N’bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine(NPB)/4,4,4-tris(N-carbazolyl)triphenylamine(TCTA) , and TmPyPB with and tris-(8-hydroxyquinoline) aluminum (Alq3). A striking extension of the operational lifetime with 122.6 h can be achieved in device B4(DBF-DMS) as shown in Figure 4b. And the EQE of device B4 could maintain 19.6% at 1,000 cd/m2(Table 1), which is quite high compared with the reported PhOLEDs based on host with small ∆EST.20 While the mCP-based device B3 showed shorter lifetime of 22.7 h. It should be noted that the long lifetime in device B4 is achieved with a sacrifice of the device efficiency due to the low triplet energies of Alq3 and the lower carrier mobilities of NPB and Alq3 compared with TAPC and TmPyPB.35-37 Nevertheless, the improved lifetime of device B4 gives a hint that higher peformance in the DBF-DMS based blue PhOLEDs could be approached if we synthesize and select suitable transporting materials and utilize appropriate device structure in the future.



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The successful applications of DBF-DMS in blue PhOLEDs inspire us to utilize it in white PhOLEDs. Furthermore, white devices with fac-Ir(iprpmi)3 as blue emitter and PO-01 as yellow emitter, in which the host are DBF-DMS(device W2) or mCP(device W1), are fabricated. The device structures and EL spectra of device W1 and W2 are shown in the supporting information. Device W2 shows a maximum EQE of 22.4%, which is drastically higher than device W1 (Figure 6). However, as listed in Table S1, the CRI for device W1(48.6) and device W2(47.7) are poor mainly due to the lack of strong red and deep blue emitters. Further work will be done in the future to add red and deep blue emitters to achieve a better CRI. In this work, the good device efficiency still proves the superiority and the versatility of DBF-DMS as efficient host in white PhOLEDs in the future.

CONCLUSIONS In summary, a new host material DBF-DMS with a narrow bandgap (T1 =2.80 eV) is synthesized. With sky-blue emitter fac- Ir(iprpmi)3 as the dopant, the DBF-DMS based PhOLED demonstrates a maximum EQE of 26.6%. The PHOLEDs with DBF-DMS as the host have improved performance than those with mCP, which is ascribed to the enhancements in carrier injection/transporting abilities and material stability of DBF-DMS. A T50 lifetime of 122.6 h is obtained by utilizing suitable transporting materials and device structure. And the successful trial fabrication of the white PhOLEDs suggests the potential application of the narrow bandgap host (DBF-DMS) in high-performance PhOLED displays and lighting.

Supporting Information More properties like 1H and

13

C NMR spectra of the material DBF-DMS were measured

which could be achievable in the supporting information. More detailed device performance based on DBF-DMS and mCP could also be found on the ACS Publications website.


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ACKNOWLEDGEMENTS The authors thank financial support from the Natural Science Foundation of Jiangsu Province (No. BK20130288), and the Natural Science Foundation of China (Nos. 61575136,61307036 and 21202114). We also thank the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Collaborative Innovation Center of Suzhou Nano Science and Technology for fund support.

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Figures and Tables

Figure 1. (a) Molecular structure of DBF-DMS; (b) HOMO and (c) LUMO calculated by Gaussian 03 at the B3LYP/6-31G(d) level.



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Figure 2. (a) UV–Vis absorption spectra in toluene and neat film at room temperature;(b) fluorescence spectra in toluene, neat film at room temperature and 2-methyltetrahydrofuran at 77 K; (c)phosphorescence spectrum in 2-methyltetrahydrofuran at 77 K of DBF-DMS.


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Figure 3. (a) EQE and power efficiency versus luminance, (b) current density and luminance versus voltage of device B1 and B2.



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Figure 4. Lifetimes of (a) device B1, device B2 (b) device B3 and device B4 at an initial brightness of 1000 cd/m2.


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Figure 5. The (a) EQE-luminance characteristics; (b)lifetime at an initial brightness of 1000 cd/m2 of tandem device T1.



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Figure 6. (a) EQE and power efficiency versus luminance, (b) current density and luminance versus voltage of device W1 and W2.


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Table 1. Device characteristics in blue PHOLEDs.

Device

V (V)a)

PE(lm/W)b)

CE (cd/A)b)

B1

5.03

44.1, 31.3

54.5, 49.8

23.1, 21.1

12.3

B2

4.01

61.7, 47.6

62.5, 60.3

26.6, 25.7

20.3

B3

12.7

4.9, 4.6

18.7, 18.5

7.7, 7.6

22.7

B4

7.82 9.81

22.3, 19.1 39.2, 33.9

48.2, 47.6 105.8, 105.8

19.9, 19.6 45.6, 45.6

122.6

T1

EQE (%)b)

a)

T50 (h)

45.0

Driving voltage at 1000cd/m2; b)Efficiencies in the order of maximum, and at 1000 cd/m2.



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Graphical abstract


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Highly Efficient Blue Phosphorescent Organic Light-Emitting Diodes

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