<i>N</i>‐Heterocyclic Carbazole‐Based Hosts for Simplified Single‐Layer Phosphorescent OLEDs with High Efficiencies

Highly efficient single-layer organic light-emitting diodes (OLEDs) are demonstrated using new N-heterocyclic carbazole-based host materials. Phosphorescent OLEDs with a structure of ITO/MoO3/host/host:dopant/host/Cs2CO3/Al are fabricated in which the new materials act simultaneously as electron-transport, hole-transport, and host layer. Devices with maximum current and external quantum efficiencies of 92.2 cd A−1 and 26.8% are achieved, the highest reported to date for a single-layer OLED. Organic light-emitting diodes (OLEDs) have attracted considerable research attention due to their applications in flat-panel displays and solid-state lighting.1, 2 Phosphorescent OLEDs (or PhOLEDs) employing late transition metal complexes as emitters are particularly attractive due to their ability to harvest both singlet and triplet excitons, making it possible to achieve internal quantum efficiencies of 100%.1, 2 However, due to the long excited-state lifetimes of phosphorescent materials, these emitters must be doped into host matrices to prevent exciton quenching by triplet–triplet annihilation. This doped emissive layer is then typically sandwiched between additional hole- and electron-transport layers (the HTL and ETL), which may be manipulated in order to achieve balanced charge injection into the emission zone. To date, nearly all development strategies for achieving high efficiencies in PhOLEDs have focused on such a multilayer strategy. Unfortunately, the use of multiple organic layers in an OLED greatly increases the cost of the device, presenting a significant barrier to commercialization. The materials for each layer must be individually synthesized and carefully purified before being deposited sequentially on the substrate, resulting in an expensive and time-consuming fabrication process. Furthermore, care must be taken to match the appropriate energy levels of all adjacent layers, and to avoid exciplex formation and charge accumulation at every interface within the device. These present significant challenges to OLED mass production, making simplified device structures highly desirable. Despite this strong motivation, only a small number of reports describe the preparation of simplified single-layer OLEDs, in which a single layer of organic material is required for device functionality.3 Though recent work has shown that this design holds promise, the efficiency of all single-layer structures reported to date remains far behind those of more complex multilayer devices.4 This is due primarily to the difficulty in developing a host material capable of balanced carrier transport that also possesses highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels well-matched to the work functions of the anode and cathode, respectively. Bipolar host materials containing both electron- and hole-transporting functionalities show promise in this regard, as careful selection and modification of the transporting moieties can provide good carrier balance.5 Though such materials have been the subject of considerable recent research, examples of their use in single-layer OLEDs remain rare.3 Carbazole-based molecules have been used extensively as host materials in OLEDs due to their high triplet energy and hole-transporting functionality.6 In particular, 4,4′-N,N′-dicarbazolylbiphenyl (CBP) is perhaps the most widely used host material for phosphorescent emitters. It has also recently been demonstrated that CBP may be used directly as an HTL in both fluorescent and phosphorescent OLEDs.7 For example, a phosphorescent OLED with greater than 20% external quantum efficiency (EQE) at a high luminance of over 10 000 cd m−2 has been demonstrated in a bilayer device using CBP directly as hole-transport layer as well as host.7 Since no additional injection layers and exciton-blocking layers were needed, the resultant device structure was highly simplified. The simple structure also helped to eliminate redundant organic/organic interfaces near the exciton formation zones, at which charge-carriers could accumulate and ultimately quench excitons. Inspired by this, we sought to determine whether similarly high performance could be achievable in an even more simplified device structure, employing a single material as HTL, ETL, and host. However, electron transport by CBP is relatively inefficient, resulting in poor electron injection from commonly used Cs2CO3/Al or LiF/Al cathodes and making a discrete electron transport layer necessary. Thus, while CBP can be used to fabricate highly efficient double-layer devices, additional chemical modification to promote electron transport is required to achieve a new material capable of acting as HTL, ETL, and host. Following this device design strategy, we therefore sought to synthesize a material designed such that: i) the LUMO level is lowered relative to CBP, reducing the barrier to electron injection at the cathode, ii) the HOMO energy is not significantly changed, preserving efficient hole injection at the anode, and, iii) the triplet level remains significantly large for use with phosphorescent dopants. Based on this concept, we describe the first examples of single-layer OLEDs with efficiencies competitive with traditional multilayer devices. In order to systematically lower the LUMO of CBP while leaving the HOMO level virtually unchanged, we designed two novel host materials 4,5′-N,N′-dicarbazolyl-(2-phenylpyridine) (CPPY) and 4,5′-N,N′-dicarbazolyl-(2-phenylpyrimidine) (CPHP), which have been fully characterized and examined by 1H and 13C NMR spectroscopy, mass spectrometry, density functional theory (DFT) calculations, X-ray crystallographic analysis, and ultraviolet photoelectron spectroscopy (UPS). We demonstrate that these simple structural changes are sufficient to drastically improve electron injection and transport, giving single-layer OLEDs with by far the highest efficiencies reported to date. These devices have the structure ITO/MoO3/host/host:dopant/host/Cs2CO3/Al, employing ITO/MoO3 and Cs2CO3/Al as composite electrodes and Ir(ppy)2(acac) as phosphorescent emitter. With this structure, a peak EQE of 26.8% and current efficiency of 92.2 cd A−1 have been achieved; these remain as high as 21.3% and 73.3 cd A−1 at the practical brightness of 100 cd m−2. CPPY and CPHP can be easily synthesized in two steps, first by palladium-catalyzed Suzuki coupling of 4-bromophenylboronic acid with the appropriate heteroaryl halide, followed by copper-catalyzed Ullman condensation in excellent yield (Figure 1). Both CPPY and CPHP show excellent thermal stability by thermogravimetric analysis, comparable to that of CBP (Table 1). As expected, the introduction of electronegative nitrogen atoms to the π-system of CBP lowers the LUMO energy, while leaving the HOMO level largely unchanged. Substitution of CBP with one or two nitrogen atoms was found to reduce the LUMO level by 0.19 and 0.33 eV respectively, with no significant change in the HOMO level in either case, as measured by UPS. Synthesis of CPPY and CPHP. Reagents and conditions: i): 4-bromophenylboronic acid (1 equiv.), K2CO3 (3 equiv.), Pd(PPh3)4 (5 mol%), 1:1 THF:H2O, 55 °C, 16 h; and, ii) carbazole (3 equiv.), Cu powder (4 equiv.), 18-crown-6 (0.2 equiv.), K2CO3 (8 equiv.), o- dichlorobenzene, 185 °C, 7 d. X-ray crystal structural analysis confirmed that CPPY and CPHP have structures essentially identical to that of CBP, with the two central aryl rings being virtually coplanar (Figure 2).8 In fact, the crystals of CPPY and CBP are isomorphous with similar unit cell parameters and an identical space group, P21/c. This is possible due to two-site disordering of the pyridyl nitrogen atom of CPPY over two inversion center-related sites. Although the crystal of CPHP contains CH2Cl2 solvent molecules and is thus not readily comparable with CPPY and CBP, it is reasonable to suggest that intermolecular interactions of CPHP in the amorphous solid should be similar due to the similar molecular size and shape of these three molecules. Crystal structures of CPPY (top, showing only one disordered site of the pyridyl N atom) and CPHP (bottom) with 50% thermal ellipsoids. Nitrogen atom: white. The absorption and emission spectra of CBP, CPPY, and CPHP are shown in Figure 3, and show a clear progression to lower energy as the number of aromatic nitrogen atoms is increased. The triplet energies of these materials were also determined from the first vibronic peak in their time-resolved phosphorescent spectra at 77 K, (Table 1) and suggest that these materials would be appropriate hosts for red, green, or even sky-blue emitters. Absorption (solid) and emission (dashed) spectra at 10−5M in CH2Cl2. λex = 340 nm. The electronic properties of CPPY and CPHP were also compared with those of CBP using DFT calculations9 at the B3LYP level of theory with 6-31g* as the basis set (Figure 4). Consistent with UPS data, almost no change is predicted in the HOMO level upon introduction of nitrogen atoms at the 2- and 6-positions of the CBP biphenyl ring system, as the electron density in the HOMO is primarily located on the carbazole functional groups. However, as the LUMO of CBP consists primarily of the π* orbitals of the biphenyl unit, the introduction of these electron-deficient nitrogen atoms is predicted to lower the LUMO level, as observed experimentally. This is further verified by cyclic voltammetry measurements in dimethylformamide (DMF) solution, which clearly indicate improved electron-accepting ability in the order CPHP > CPPY > CBP (see the Supporting Information). Examination of the frontier MO surfaces of these three molecules also reveals increasing bipolar character moving from CBP to CPPY to CPHP. As the central biaryl unit becomes more electron-deficient, the HOMO exhibits increased electron density on the carbazole group farther from the central heteroaromatic ring, with the LUMO showing increased contribution from the N-heterocycle. This imparts more charge-transfer character to CPPY and CPHP, accounting for the larger Stokes’ shift observed for these molecules. Frontier molecular orbital surfaces and calculated orbital energies for CBP (top), CPPY (middle), and CPHP (bottom). Isocontour value = 0.02. To evaluate the performance of these compounds in OLEDs, a series of devices were fabricated in which a thin layer of the doped host material was deposited between two undoped buffer layers of the same host, which act also as the ETL and HTL in this design (Scheme 1). These devices had a structure of ITO/MoO3 (1 nm)/host (35 nm)/host:Ir(ppy)2(acac) (8 wt%, 15 nm)/host (60 nm)/Cs2CO3 (1 nm)/Al, with Devices I, II, and III incorporating CBP, CPPY, and CPHP as host, respectively (see Table 2). All devices show green emission with a peak wavelength of 523 nm and Commision Internationale de l′Éclairage (CIE) coordinates of (0.32, 0.64) (see the electroluminescence spectrum in the Supporting Information), indicating that emission originates entirely from the Ir(ppy)2(acac) dopant in all cases. Simplified single-layer OLED structure. The performance of the three devices is compared in Figure 5. Remarkably, after optimization of each layer thickness it was possible to achieve a reasonably high efficiency single-layer OLED simply using CBP as host. Device I shows a peak EQE of 13.3% and current efficiency of 54.4 cd A−1 at 438 cd m−2, with a moderate turn-on voltage of 4.0 V. Device II, incorporating CPPY as host, outperforms the CBP-based device at low luminance, with a peak current efficiency of 74.9 cd A−1, EQE of 21.5%, and turn-on voltage of 3.8 V. However, due to significant efficiency roll-off, Device I shows better performance at higher luminance (greater than 200 cd m−2). The performance of Device III, however, shows excellent performance at all voltages examined, giving an exceptionally high peak EQE and current efficiency of 26.8% and 92.2 cd A−1, remaining as high as 21.3% and 73.3 cd A−1 at the practical brightness of 100 cd m−2. Furthermore, this device shows a much lower turn-on voltage of 3.0 V, confirming that the lower LUMO level of CPHP does indeed reduce the barrier to electron injection at the cathode. This is, to our knowledge, the most efficient simplified single-layer OLED reported to date by a factor of two or more3 and, most importantly, shows performance comparable to state-of-the-art devices based on conventional multilayer architectures.4 a) Current efficiency, b) power efficiency, and c) external quantum efficiency of Devices I, II, III, and IIIb. Since no organic/organic heterojunctions are present to facilitate exciton formation in these single-layer devices, there should be a distribution of exciton formation in the host. We thus sought to determine whether a broader emission zone doped with phosphorescent emitter could more effectively overlap with the exciton formation zone, thus further enhancing device efficiency. Device IIIb was fabricated with a structure of ITO/MoO3 (1nm)/CPHP (35 nm)/CPHP:Ir(ppy)2(acac) (55 nm)/CPHP (20 nm)/Cs2CO3 (1 nm)/Al, using CPHP as host, as in Device III, but incorporating a much wider 55 nm doped region. The performance of this device is also shown in Figure 5, and is compared with Device III. No significant improvement was achieved by broadening the emission zone, indicating that doping in a wider region does not necessarily enhance the efficiency of an already-optimized single-layer device. Based on the HOMO and LUMO levels of CPPY it was expected that the performance of Device II should have been between that of CBP and CPHP. To determine the origins of the significant efficiency roll-off in Device II we fabricated single-carrier hole-only devices to investigate the transport and injection of charge in the three different hosts. The performance of these three devices is compared in Figure 6. Surprisingly the electrical characteristics of the device with CPPY are markedly worse, which suggests either poor injection or transport of holes in this material. This most likely accounts for the significant efficiency roll-off in Device II due to poor electron–hole balance, particularly at high current and brightness. To determine if the poor electrical performance of CPPY was due to poor hole injection or transport we measured the energy-level alignment at the interface with the ITO/MoO3 anode using UPS. Figure 6 shows the UPS valance band spectrum of the frontier orbitals of the three different hosts deposited on MoO3. Although the HOMO level relative to vacuum is the same for the three materials (–6.05 eV), the energy-level alignment is significantly different for CPPY. The HOMO-derived peak in the valence band of CPPY is around 0.5 eV further from the Fermi level than for either CBP or CPHP, which indicates a significantly increased barrier to hole injection at the anode, consistent with the single-carrier and OLED performance data. Owing to the similar structures of the three hosts, the reason for this radically different energy-level alignment is likely quite subtle. Among this series of materials, CPPY alone possesses a transverse dipole moment perpendicular to the molecular long axis, yet exhibits molecular packing isostructural with CBP by X-ray crystallography. However, the presence of this dipole moment may result in a preferred molecular orientation at the organic/MoO3 interface, which may change the energy level alignment. Recent studies have shown that the dipole moments of structurally similar materials can have a dramatic effect on their performance in electroluminescent devices,10 and we speculate that similar phenomena are at play here. a) Charge-transport characteristics of hole-only devices incorporating CBP, CPPY, and CPHP. Inset: device structure. b) UPS spectral data for the host materials. Inset: zoom region showing the energy level alignment of each host on ITO/MoO3. In summary, we have demonstrated new carbazole-based host materials 4,5′-N,N′-dicarbazolyl-(2-phenylpyridine) (CPPY) and 4,5′-N,N′-dicarbazolyl-(2-phenylpyrimidine) (CPHP), which can be used to give highly simplified single-layer OLEDs with unprecedented performance. Both materials are easily prepared in high yield by a two-step Suzuki coupling/Ullman condensation route. Devices based on these hosts using Ir(ppy)2(acac) as emitter exhibit maximum external quantum efficiencies of 21.5% and 26.8%, respectively, the highest reported to date for a simplified single-layer device. Experimental and theoretical studies confirm that the LUMO energies of these materials are notably lower than the commonly used host CBP, while the HOMO energies remain largely unchanged. This design facilitates improved electron injection and transport while preserving hole-transporting functionality, resulting in bipolar hosts with significantly improved device efficiencies. Based on these results we demonstrate that single-layer OLEDs with performance comparable to those of conventional multilayer devices can be achieved by careful control of charge transport within the host and the energy level alignment of the host material at metal/organic interfaces. General Experimental Information: All reactions were carried out under a nitrogen atmosphere. Reagents were purchased from Aldrich chemical company and used without further purification. Thin-layer and flash chromatography were performed on silica gel. 1H and 13C NMR spectra were recorded on Bruker Avance 400, 500 or 600 MHz spectrometers. Deuterated solvents were purchased from Cambridge Isotopes and used without further drying. Emission spectra were recoded using a Photon Technologies International QuantaMaster Model 2 spectrometer. UV-visible absorbance spectra were recorded using a Varian Cary 50 UV-visible absorbance spectrophotometer. Crystal structures were obtained at 180K using a Bruker AXS Apex II X-ray diffractometer (50 kV, 30 mA, Mo Kα radiation). The synthesis of 4,4′-dibromo-2-phenylpyridine has been reported previously.11 EL Device Fabrication: All materials were purified by train sublimation prior to deposition. Devices were fabricated in a Kurt J. Lesker LUMINOS cluster tool with a base pressure of 10−8 Torr without breaking vacuum. The ITO anode is commercially patterned and coated on glass substrates 50 × 50 mm2 with a sheet resistance less than 15 Ω square−1. Substrates were ultrasonically cleaned with a standard regiment of Alconox, acetone, and methanol followed by UV ozone treatment for 15 min. The active area for all devices was 2 mm2. The film thicknesses were monitored by a calibrated quartz crystal microbalance. Current–voltage (I–V) characteristics were measured using a HP4140B picoammeter in ambient air. Luminance measurements and EL spectra were taken using a Minolta LS-110 luminance meter and an Ocean Optics USB200 spectrometer with bare fiber, respectively. The external quantum efficiency of EL devices was calculated following the standard procedure.12 After deposition, single-carrier devices were transferred to a homebuilt variable-temperature cryostat for measurement at 298 K. UPS measurements were performed using a PHI 5500 MultiTechnique system, with attached organic deposition chamber with a basepressure of 10−10 Torr. Additional details regarding device fabrication, characterization and UPS measurements have been described elsewhere.13 Synthesis of 4,4′-Dibromo-2-phenylpyrimidine: To a 250 mL Schlenk flask with condenser and stir bar was added 4-bromophenylboronic acid (1.4 g, 7.0 mmol), 5-bromo-2-iodopyrimidine (2.0 g, 7.0 mmol), Pd(PPh3)4 K2CO3 g, mmol), and mL 1:1 The was to 55 with for 16 after which the was in and the layer with CH2Cl2. The organic layers were with and the purified by chromatography on silica as to give the as a solid g, 1H NMR = = for found Synthesis of To a 100 mL Schlenk flask with stir bar and condenser was added the mmol), carbazole g, mmol), K2CO3 g, mmol), Cu powder g, 18-crown-6 g, mmol), and 30 mL The was to at 185 for 7 at which the solvent was by vacuum The was then with and CH2Cl2. The organic layers were with and the purified on silica as to give the Synthesis of 1H NMR = = = = = = = = 13C for found Synthesis of 1H NMR = = = = = = = 13C NMR for found Supporting is from the or from the contains the crystallographic data for this These data can be obtained of charge from The Cambridge and to this The the and of for for with the synthesis of CPPY and CPHP. of to are as are but not or are as by the The is not for the or of by the than should be to the for the

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