Host and Dopant Materials for Idealized Deep‐Red Organic Electrophosphorescence Devices

Abstract

An idealized bipolar host material and dopant are synthesized for deep-red phosphorescent organic light-emitting diodes (PhOLEDs). The host material exhibits a low-lying lowest unoccupied molecular orbital and high thermal and morphological stability, while the dopant emits sharply at 616 nm with a full width at half-maximum of only 39 nm. The new host/guest combination for the deep-red device yields the highest device efficiencies to date. The performance of organic light-emitting diodes (OLEDs) can be greatly improved by using phosphorescent materials because both the singlet and triplet excitons can be harvested and the internal quantum efficiency can reach as high as 100%.1 To reduce the self-quenching and triplet–triplet (TT) annihilation and to achieve a high efficiency for phosphorescence organic light-emitting diodes (PhOLEDs), a proper combination of the host and dopant system is vital.2 For full-color applications, a search for efficient host and dopant materials are generally required for all three primary colors (red, green, and blue).3 The carbazole-based host material 4,4'-bis(9-carbazolyl)-2,2'-biphenyl (CBP)1, 2 has been widely used in red PhOLEDs. However, its relatively low glass transition temperature, Tg = 62 °C, results in poor morphological stability.4 In addition, the large energy gap impedes charge carrier injection from adjacent carrier-transporting layers. Therefore, a large driving voltage is usually required for the CBP-based devices.5 Several research groups have endeavored to develop efficient host materials for red PhOLEDs. Kwon et al. utilized a narrow bandgap beryllium complex as the host material and bis(2-phenylquinoline)(acetylacetonate)iridium (Ir(phq)2(acac)) as the red dopant for the devices, thus achieving an external quantum efficiency (EQE) of up to 21% with the Commission Internationale de l'Eclairage (CIE) coordinates of (0.62, 0.37).6 To facilitate charge injection, Shu et al. reported a fluorine-based bipolar host material with a maximum EQE of 19.9% and CIE coordinates of (0.64, 0.36).7 Reports for high-efficiency deep-red PhOLEDs with the CIE coordinate x ≥ 0.67 are still rare. The efficiency and brightness of deep-red PhOLEDs are hard to improve because of the energy gap law and the drop in luminous flux in the deep-red region.8 Yang et al. optimized PhOLEDs based on a bipolar triphenylamine/oxadiazole hybrid material as the host to realize a very high EQE of 21.6% and a maximum power efficiency (PE) of 16.1 lm W−1.9 We reported an efficient deep-red EL device that utilized bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide (BCPO) as the host and tris[1-phenylisoquinolinato-C2,N]iridium(III) (Ir(piq)3) as the dopant to achieve an EQE of 17.0%, and a PE of 20.4 lm W−1 along with CIE coordinates of (0.67, 0.33).10 Most recently, Kido et al. developed a series of host materials containing building blocks of carbazole and arylenes. The best performence of the resultant deep-red PhOLEDs exhibited an EQE of 18.4% and a PE of 20.3 lm W−1 with CIE coordinates of (0.67, 0.33).11 In order to reduce the power consumption, it is still a great challenge to design and synthesize ideal host and dopant materials for deep-red electroluminescent (EL) devices to improve the current and power efficiencies. Two key issues for the phosphorescent dopant of an idealized deep-red PhOLED with a CIE x ≥ 0.67 are the emission wavelength and the bandwidth of the emission. Based on the chromaticity diagram, a spectrally pure monochromatic light with a CIE x ≥ 0.67 should have a wavelength ≥ 612 nm. For an actual OLED, the EL spectrum is a band structure rather than monochromatic light. If the EL spectrum is symmetric and the emission maximum of the device is kept at 612 nm, its CIE x should be lower than 0.67 because human eyes are much more sensitive to wavelengths shorter than 612 nm and less sensitive to wavelengths longer than 612 nm in the EL spectrum. Thus, to achieve a CIE x of 0.67, the emission maximum of the device needs to shift to more than 612 nm, depending on the bandwidth. A larger emission bandwidth requires a longer emission maximum in order to mantain the same CIE x value of 0.67.12 However, as the emission shifts to longer wavelength, the luminous efficiency decreases rapidly.13 As a result, in the search for a supreme deep-red dopant with a CIE x = 0.67, the dopant should have an emission maximum close to 612 nm and the emission bandwidth as narrower as possible in order to reach the highest luminous efficiency. Here, we design and synthesize a new deep-red emitter, Iridium(III) bis(4-methyl-2-(thiophen-2-yl)quinolinato-N,C3')(acetylacetonate) ((tmq)2 Ir(acac)), that shows a very sharp emission band with a relatively short-wavelength emission maximum compared with the well-known deep-red iridium complexes (Scheme 1). In addition, we also report a new type of bipolar host material, BIQS, composed of two 6H-indolo[2,3-b]quinoxaline moieties bridged by a tetraphenylsilane core for red PhOLEDs. The molecule features: i) two electron-withdrawing quinoxaline groups that decrease the lowest unoccupied molecular orbital (LUMO) level and reduce the energy barrier for electron injection in devices; ii) a proper singlet and triplet energy for efficient transfer of energy to deep-red emitters; and iii) a silane-linked, bulky, and rigid structure that leads to high amorphous thin-film retainability.4, 14 The combination of these two materials gives the highest efficiency for deep-red PhOLEDs reported so far. Synthesis and structures of BIQS and (tmq)2Ir(acac). Scheme 1 illustrates the synthetic routes for the preparation of BIQS and (tmq)2Ir(acac). The former was made in 73% yield using a Cu(I)-catalyzed coupling reaction15 of 6H-indolo[2,3-b]quinoxaline with bis(4-iodophenyl)diphenylsilane. The iridium complex, (tmq)2Ir(acac), was prepared by following procedures reported previously.1 The crystal structure of BIQS that was determined using X-ray diffraction is shown in Figure S1 (Supporting Information).16 In the crystals, the bulky mole­cule consists of a tetraphenylsilane group and two rigid 6H-indolo[2,3-b]quinoxaline units. The torsion angles between the N-phenyl ring and the indoloquinoxaline plane are 54.2° and 44.9°, respectively. The morphology of BIQS is exceptionally stable, as indicated by the high glass transition temperature (Tg) of 171 °C, probably due to the high molecular weight combined with the rigidity of the molecule.14 The morphological stability of a BIQS thin film coated on a silicon wafer (30 nm) was demonstrated by annealing the film at 100 °C for 15 h. The atomic force microscopy (AFM) image before annealing had a fairly even surface with a narrow root mean square (RMS) roughness of 0.24 nm, and the topography did not change after annealing, exhibiting a RMS roughness of only 0.22 nm (Supporting Information, Figure S2). The material is also very thermally stable with a decomposition temperature, Td, corresponding to 5% weight loss, of 502 °C. The room-temperature UV-vis absorption, fluorescence, and 77 K phosphorescence spectra of BIQS in solution are shown in Figure 1. The absorption spectrum in dichloromethane has two absorption bands at 398 nm and 354 nm. To assign these absorption bands, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations for the molecule were performed. The contour plots of several important occupied and unoccupied molecular orbitals of BIQS are depicted in Figure S3 (Supporting Information). The results of TDDFT calculations show that there are two transitions of BIQS with large oscillator strengths (f). The lower energy transition is a combination of highest occupied molecular orbital (HOMO)–1 to LUMO and HOMO to LUMO+1 transitions with the transition energy of 3.12 eV (397 nm, f = 0.03). This transition appears to match well with the low-energy absorption band at 398 nm. As shown in the electron contour plots, this absorption band consists of a indoloquinoxaline-centered π–π* transition and an intramolecular charge transfer from the N-phenyl groups to the indoloquinoxaline units. However, the later should be minor due to the large torsion angles between the N-phenyl group and indoloquinoxaline unit. Another transition is a combination of HOMO–2 to LUMO and HOMO–3 to LUMO+1 with a transition energy of 3.81 eV (325 nm, f = 0.32) and likely correlates to the observed absorption band at 310 to 370 nm; this absorption band could also be assigned as an indoloquinoxaline-centered π–π* transition. The UV-vis absorption spectrum of BIQS is essentially the same in solvents with different polarities (Supporting Information, Figure S4) indicating that the absorptions are mainly the indoloquinoxaline-centered π–π* transitions. In contrast, the fluorescence spectra show a clear bathochromic shift and broaden with increasing solvent polarity. These results could be interpreted as the emission involving a rapid photoinduced electron transfer between the donor and acceptor orbitals.17 This phenomenon was also found in other efficient host materials reported previously.4, 14 The HOMO and LUMO levels of BIQS were estimated to be 5.93 and 2.88 eV based on the cyclic voltammetry (CV) data. The lower-lying LUMO level is attributed to the presence of electron-withdrawing quinoxaline groups and is 0.37 eV lower than that of CBP (2.51 eV). To study the carrier-transporting property of BIQS, a hole-only device consisting of the structure ITO/NPB (10 nm)/TCTA (5 nm)/BIQS (30 nm)/NPB (15 nm)/Al (100 nm), where ITO is indium tin oxide, NPB is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl, and TCTA is 4,4′,4″-tri(N-carbazolyl)-triphenylamine, and an electron-only device containing the layers ITO/BCP (15 nm)/BIQS (30 nm)/BCP (15 nm)/LiF (1 nm)/Al (100 nm), where BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, were fabricated. The current density versus voltage (I–V) curves (Supporting Information, Figure S5) of these two devices show similar I–V curves, indicating that BIQS is good at transporting both electrons and holes.10 The absorption (Abs.) and fluorescence (Fl.) spectra measured at room temperature in dichloromethane (10−5 M) and the phosphorescence (Ph.) spectrum of BIQS measured at 77 K in 2-methyltetrahydrofuran (10−5 M), the absorption, phosphorescence, and excitation (Exc., intensity at 620 nm) spectra of (tmq)2Ir(acac) measured at room temperature in dichloromethane (10−5 M), and the photoluminescence (PL) spectrum of (tmq)2Ir(acac) (7 wt%)-doped in a thin film of BIQS (30 nm) irradiated at 407 nm at room temperature. The UV-vis absorption spectrum of (tmq)2Ir(acac) exhibits four absorption bands (Figure 1). The relatively weak and broad bands in the 400–600 nm region, similar to those for 2-arylquinoline-based iridium complexes, likely arise from the 1MLCT (metal to ligand charge transfer) and 3MLCT.18 The excitation spectrum of this complex shows that the 400–600 nm absorption region contributes greatly to the observed emission. The phosphorescence spectrum of (tmq)2Ir(acac) in dichloromethane shows that the emission maximum appears at 611 nm with the full width at half-maximum (FWHM) of only 48 nm and a quantum yield (Φp) of 0.55. For comparison, the well-known red phosphor, (piq)2Ir(acac), exhibits an emission maximum of 620 nm with a broader FWHM of 58 nm and a lower quantum yield of 0.20.19 The FWHM is reduced to 39 nm when (tmq)2Ir(acac) (7 wt%) is doped in a BIQS thin film (Figure 1). The difference in the bandwidth in dichloromethane and in BIQS is likely caused by the difference in the excitation process of (tmq)2Ir(acac), although the exact reason is not clear. In dichloromethane, the emission occurs via direct excitation of the iridium complex, while in BIQS, (tmq)2Ir(acac) is excited mainly via energy transfer from BIQS. It is noteworthy that the fluorescence (λmax = 468 nm) and phosphorescence (λmax = 530 nm) emission bands of BIQS overlap very well with the MLCT absorption bands of (tmq)2Ir(acac), indicating a possible facile energy transfer from BIQS to (tmq)2Ir(acac). To understand the EL properties of BIQS and (tmq)2Ir(acac), we fabricated several PhOLED devices using these two materials as the host and dopant, respectively. The device configuration consisted of the following layers: ITO/NPB (20 nm)/TCTA (10 nm)/emitting layer (EML) (30 nm)/BCP (15 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm), where Alq3 is tris(8-hydroxyquinoline) aluminum. In the devices, TCTA was introduced into the hole-transport layer (HTL)/EML interface to serve as an exciton-blocking layer14 and as a buffer layer to reduce the gap of the HOMO levels between NPB and the host. Devices A, B, C, and D consist of different combinations of the host and dopant (7 wt%) layers including CBP/(piq)2Ir(acac), BIQS/(piq)2Ir(acac), CBP/(tmq)2Ir(acac), and BIQS/(tmq)2Ir(acac) in EML, respectively. The key data of these EL devices are plotted in Figure 2 and listed in Table 1. Both devices A and B have deep-red emission with the CIE coordinates of (0.68, 0.32). However, device B exhibits a much greater luminance efficiency, a lower turn-on voltage (Vd), and a higher current density than device A. For comparison, device B shows a maximum brightness (Lmax) as high as 55486 cd m−2 and a maximum EQE of 22.6%, exceeding the theoretical limit of 20%, while device A using CBP as a host exhibits a maximum EQE of only 10.7% similar to the previously reported values for CBP/(piq)2Ir(acac)-based devices.14, 19 The exceptionally high EQE could be attributed to the following two reasons. First, the emission bands of BIQS overlap very well with the MLCT absorption bands of (piq)2Ir(acac), providing an efficient route for energy transfer from the host to the dopant. Second, the donor–acceptor character of BIQS facilitates both the electron and hole transport in the host layer and provides a good balance of carriers transport in EL devices. The much higher current density of device B could be explained based on the relative energy level of the materials used in the devices (Supporting Information, Figure S6). The HOMO level of TCTA (5.9 eV) is almost equal to that of BIQS and CBP, so the hole can be transported from TCTA to the hosts smoothly. On the other hand, the energy barrier for electron injection from BCP into CBP is much larger than from BCP into BIQS due to the lower LUMO level of the latter. This result facilitates the electron injection and leads to a much higher current density for device B than for device A (Figure 2a). In the absence of a hole injection layer, device B has a low Vd of 2.8 V, which could be attributed to the good matching of the HOMO and LOMO levels of BIQS with the adjacent carrier-transporting layers. The combination of low turn-on voltage, high brightness, and high EQE provides an outstanding maximum current efficiency (CE) of 25.6 cd A−1 and maximum PE of 26.5 lm W−1. A similar trend was observed for devices C and D, which use (tmq)2Ir(acac) as the dopant and CBP and BIQS, respectively, as the host. Device D shows exceptional performance with a Vd of 3.0 V, Lmax of 56831 cd m−2, and maximum EQE of 23.9%. The EL λmax of device D is at 616 nm and the FWHM is only 39 nm (Figure 2b), similar to the value for the PL of (tmq)2Ir(acac) doped in BIQS. The device shows exceedingly high EL efficiencies with a maximum CE of 34.1 cd A−1 and PE of 30.0 lm W−1 and the CIE coordinates of (0.67, 0.33). Devices E and F are the optimized BIQS-hosted devices with 4 wt% of dopant (piq)2Ir(acac) and (tmq)2Ir(acac), respectively. The performance of device F is extraordinary, showing maximum EQE, CE, and PE of 25.9%, 37.3 cd A−1, and 32.9 lm W−1, respectively, with CIE values of (0.67, 0.33) at 8 V. At a brightness levels of 1000 and 5000 cd m−2, the EQE values for device F are still as high as 24.0 and 20.3%. a) Current density-voltage-luminance (I–V–L) characteristics of devices A–D. b) EL spectra of devices B and D. The inset is the external quantum efficiency versus the current density curves for devices A–F. It is interesting to compare the current and power efficiencies of devices D and E, both of which show very similar external quantum efficiencies of 23.8 ± 0.1% and the same CIE coordinates (0.67, 0.33). However, device D, which uses (tmq)2Ir(acac) as the dopant material, outperforms device E, which uses (piq)2Ir(acac) as the dopant, by ≈20% in the current and power efficiencies. This is due to the fact that (tmq)2Ir(acac) has a narrower emission band with shorter emission maximum than (piq)2Ir(acac). In conclusion, we have successfully prepared a novel host material, BIQS, and a new iridium complex, (tmq)2Ir(acac), for deep-red PhOLEDs. BIQS shows a relative low-lying LUMO that facilitates electron injection, leading to a significantly lower operating voltage and higher current density. In addition, the material exhibits suitable singlet and triplet energies to provide efficient energy transfer to deep-red emitters. The deep-red emitter (tmq)2Ir(acac) shows a very sharp emission band with a proper emission maximum resulting in very high luminous efficiency. The application of BIQS as the host for deep-red iridium complexes (piq)2Ir(acac) and (tmq)2Ir(acac) gave idealized deep-red PhOLEDs. In particular, device F with the best performance device of the BIQS/(tmq)2Ir(acac) system shows a brightness of 58 688 cd m−2 and a maximum external quantum efficiency, current efficiency, and power efficiency of 25.9%, 37.3 cd A−1, and 32.9 lm W−1, respectively. These efficiency data appear to be the highest for deep-red PhOLEDs reported to date. We are currently investigating the lifetimes of the devices using the related materials as the host and/or dopant. Preliminary results show that the operational lifetime (t1/2) of device E is estimated to be more than 2000 h at an initial luminance of 500 cd m−2. OLED Fabrication and Measurements:10 The EL devices were fabricated by vacuum deposition of the materials at 10−6 Torr onto a UV-ozone cleaned ITO glass with a sheet resistance of 15 Ω square−1. The deposition rate for the organic compounds was 1–2 Å s−1. The cathode, consisting of Al/LiF, was deposited by evaporation of LiF with a deposition rate of 0.1 Å s−1 and then by evaporation of Al metal with a rate of 4 Å s−1. The effective area of the emitting diode was 9.00 mm2. Current, voltage, and light intensity measurements were made simultaneously using a Keithley 2400 source meter and a Newport 1835-C optical meter equipped with a Newport 818-ST silicon photodiode. Electroluminescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer. Procedure for the Synthesis of Bis(4-(6H-indolo[2,3-b]quinoxalin-6-yl)phenyl)diphenylsilane, (BIQS): Bis(4-iodophenyl)diphenylsilane (2.94 g, 5.00 mmol) and 6H-indolo[2,3-b]quinoxaline (2.41 g, 11.0 mmol) were dissolved in 1,4-dioxane (30 mL). Potassium phosphate (4.90 g, 23.1 mmol), copper(I) iodide (0.11 g, 0.58 mmol), and (±)-trans-1,2-diaminocyclohexane (0.25 g, 2.20 mmol) were then added to the solution. The mixture was stirred at 110 °C for 2 d. After cooling to ambient temperature, the solution was passed through to a flash column packed with silica and celite. The solvent was evaporated on a rotary evaporator and the residue was washed with ether and then hexane. The crude product was sublimated to give light yellow solid, BIQS (2.81 g, 73%). Procedure for the Synthesis of Iridium(III) bis(4-methyl-2-(thiophen-2-yl)quinolinato-N,C3')(acetylacetonate), ((tmq)2Ir(acac)): Iridium trichloride hydrate (2.26 g, 6.40 mmol) and 4-methyl-2-(thiophen-2-yl)quinoline (3.18 g, 14.1 mmol) were dissolved in 2-ethoxyethanol (16.0 mL) and water (5.5 mL). The mixture was heated under a nitrogen atmosphere at 110 °C for 15 h and then cooled to room temperature. The deep-red precipitate was collected and washed with methanol, ethyl ether, hexane, and dried in vacuum to give the corresponding cyclometalated Ir(III)-μ-chloro-bridged dimer (3.90 g, 90%). The iridium dimer (0.41 g, mmol), g, mmol), and g, mmol) were with 2-ethoxyethanol mL) and the mixture was heated in a nitrogen atmosphere at 100 °C for h. After cooling to room temperature, the red precipitate was collected and washed with methanol, ethyl ether, and hexane, and using a silica column with a mixture as the to give a red by the red solid, (tmq)2Ir(acac) g, is from the from the The the of of the of and the of the of for of this research and the for of the of for providing of to are as are not are made as by the The is not for the of by the than should be to the corresponding for the