Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes

High-quality host materials are indispensable for the construction in the emitting layer of efficient organic light-emitting diodes (OLEDs), especially in a guest and host system. The good carrier transport and energy transfer between the host and emitters are out of necessity. In this work, a wide bandgap and bipolar organic compound, 2,2′-bis(4,5-diphenyl-(1,2,4)-triazol-3-yl)biphenyl (BTBP), conjugating two electron-transporting triazole moieties on a hole-transporting biphenyl core, was synthesized and characterized. The wide bandgap of 4.0 eV makes the promise in efficient energy transfer between the host and various color emitters to apply as the universal host, especially for blue emitters. The close electron and hole mobilities perform the same order of 10–5 cm2·V–1·s–1, identified as bipolar behavior and benefited for carrier balance at low bias. Although carrier transportation belongs to bipolar behavior at a low electrical field, the electron mobility is much faster than the hole one at a high electrical field and belongs to electron-transporting behavior. Employing the BTBP as the host matrix mixed with a phosphor dopant, iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2]picolinate, a high-efficiency sky-blue phosphorescent organic light-emitting diode (OLED) was achieved with a maximum current efficiency of 65.9 cd/A, maximum power efficiency of 62.8 lm/W, and maximum external quantum efficiency of 30.2%.


INTRODUCTION
Blue phosphorescence is one of the most important issues in the development of organic light-emitting diode (OLED) display and lighting. 1−5 Compared to green and red phosphorescent emitters, the blue ones have much higher triplet states, thus imposing much difficulty on molecule design, carrier-transporting characteristics, and reliability. 6−9 For the past few years, efficient blue phosphor emitters like iridium(III) bis[4,6-di-fluorophenyl-pyridinato-N,C 2 ]picolinate (FIrpic), fac-iridium(III) tris(mesityl-2-phenyl-1Himidazole) (fac-Ir(mpim) 3 ), fac-iridium(III) tris(N-phenyl,Nmethyl-pyridoimidazol-2-yl) (fac-Ir(pmp) 3 ), iridium(III) tris-[3 -(2, 6-di methy lpheny l )-7-methylimidazo[1,2f ]phenanthridine] (Ir(dmp) 3 ), iridium(III)[bis(4,6-difluorophenyl) pyridinato-N,C 2 ′-tetrakis(1-pyrazolyl)borate (FIr6), etc., have been disclosed to have an external quantum efficiency (EQE) over 20%. 10,11 Generally, to avoid backward energy transfer, host materials with triplet energy (E T ) higher than blue dopants were necessary for high efficiency. 12−16 For example, carbazole-based materials were typically used as the hosts for blue phosphor dopant (e.g., FIrpic with E T = 2.65 eV) due to the wider triplet energy (e.g., E T = 2.9 eV for N,Ndicarbazolyl-3,5-benzene (mCP); E T = 2.8 eV for 3,3′-di(9Hcarbazole-9-yl)-1,1′-biphenyl (mCBP)). 17−20 Well-known benchmark bipolar host materials mCP and mCBP have good hole-transporting characteristics and also demonstrate good efficiency performance with an EQE around 20% for the blue phosphorescent OLED. 10,21 However, typically, such a host material exhibited higher hole mobility than the electron one, which results in carrier imbalance and reduces the electroluminescence efficiency. Donor−acceptor-type bipolar organic materials were synthesized as the host of blue phosphorescent OLED for improving the efficiency and reducing the efficiency roll-off by connecting the holetransporting unit (e.g., carbazole moiety) and electron-transporting unit (e.g., cyano, triazole, phosphine oxide, benzimidazole, and oxadiazole moieties). 21−26 Many bipolar host materials such as mCP, mCBP, mNBICz, UGH2, CbzBiz, DPEPO, and so on were reported with the high-efficiency OLEDs. 10,27−31 Specifically, most of these bipolar hosts exhibited a faster intrinsic hole mobility compared to the intrinsic electron one. When doped with emitters as the emitting layer (EML), the effective hole mobility of this mixture was routinely greater than electron one. 29,30 It is rare to find a bipolar host with a favorable electron-transporting behavior, which is important to contrive the carrier balance in using the hole-transporting dopant or hole-beneficial OLED structure. The carrier balance inside the emitting layer benefits the efficiency enhancement. Hence, several approaches to achieve the carrier balance in the EML were reported by introducing a mixed host (MH) by coevaporating hole-and electron-transporting hosts or a double EML structure with two adjacent hole-and electron-transporting hosts. 32−35 In this study, a new compound, 2,2′-bis(4,5-diphenyl-(1,2,4)-triazol-3-yl)biphenyl (BTBP), was synthesized using an end group, triazoles, to connect with the biphenyl core because triazole derivatives are well-known electron transport materials, which have been widely used as electron-transporting layers for OLED applications, such as TAZ. However, the usage of triazole derivatives as a host was rare, especially for the blue host. For example, Adachi et al. reported 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole as a host for the Ir(ppy) 3 emitter. The green phosphorescent OLED showed an EQE of 15.4% and PE of 40 lm/W. 36 Su et al. reported a series of pyridine-TAZ hybrid molecules, with TAZ as the core and the pyridinyl groups as substituents on periphery, as the host of Ir(ppy) 3 . The green phosphorescent OLED showed a good performance with a CE of 73.6 cd/A, PE of 72.2 lm/W, and EQE of 21.8%. 37 In our previous works, the pristine TAZ was employed as a blue host doped with an FIrpic emitter to achieve an efficient phosphorescent OLED with a CE of 45.9 cd/A and EQE of 20.2%; 21 the C2 chirality of the biphenyl core was also demonstrated to allow the stacking of electrontransporting moieties (e.g., oxadiazole) and to make the through-space interactions feasible. 38 Hence, in this study, BTBP was synthesized, combining the viewpoint of our above two reports, and employed as the host for blue phosphorescent OLED, which showed efficient device performances. In particular, this material possesses a large bandgap, high glass transition temperature (T g ), and high E T of 3.1 eV. Easy synthesis, simple procedure, and cost less can be obtained. The single carrier device and time-of-flight measurement show that the material properties is bipolar with a favorable electrontransporting behavior. By suitable tuning of the device structure, a maximum current efficiency (CE amx ), maximum power efficiency (PE max ), and maximum EQE (EQE max ) of 65.9 cd/A, 62.8 lm/W, and 30.2%, respectively, can be obtained in our BTBP-based blue phosphorescent OLED.
2.1.1. Synthesis of Dimethyl Biphenyl-2,2′-dicarboxylate (2). A mixture of biphenyl-2,2′-dicarboxylic acid (1) (11.0 g, 45.4 mmol) in methanol (50 mL) was dripped in concentrated sulfuric acid (98%, 5 mL). The mixture was refluxed for 24 h, cooled down to the room temperature, and concentrated to give a white solid precipitate. The solid was collected and rinsed several times with methanol to afford pure solid 2 (9.9 g, 80% yield). Mp 329−330°C. 1 Figure S9). 2.1.4. Synthesis of (2Z,2′Z,N′2Z,N′2′Z)-N′2,N′2′-Bis(chloro-(phenyl)methylene)biphenyl-2,2′-bis(carbohydrazonoyl) dichloride (5). A mixture of 4 (2 g, 4.2 mmol) and PCl 5 (3.7 g, 17.9 mmol) in toluene (16 mL) was refluxed for 3 h under N 2 . The reaction was quenched by addition of water at room temperature. The product was extracted several times with toluene. The collected organic layer was dried over anhydrous MgSO 4 and concentrated under reduced pressure to give a white solid that was further recrystallized from CH 2 Cl 2 /EtOH to pure 5 (1.6 g, 71% yield). Mp 124−125°C; 1 H NMR (400 MHz, CDCl 3 , δ) ( Figure  2.2. OLED Fabrication. Figure 1 shows the layer structures and energy levels of the blue phosphorescent OLEDs. Our devices were fabricated on indium tin oxide (ITO) glass substrates treated by O 2 plasma prior to thin-film deposition. Here, the layer structure of OLED used was 50 nm 4,4′-cyclohexylidenebis[N,N-bis(4methylphenyl)benzenamine] (TAPC)/10 nm mCP/30 nm BTBP:x %FIrpic/y-nm diphenylbis(4-(pyridin-3-yl)phenyl)silane (DPPS)/1.2 nm LiF/100 nm Al, where TAPC and mCP were the first and second hole transport layers (HTLs), respectively, BTBP was used as the host and doped with a sky-blue dopant FIrpic as the EML, DPPS was the electron transport layer (ETL), and LiF and Al were used as the electron injection layer and cathode, respectively. In particular, the EML neighboring materials, mCP and DPPS, possess E T values of 2.9 and 2.7 eV, which are greater than that of FIrpic (2.65 eV) to block the triplet exciton diffusion. Moreover, a faster hole mobility material, 1,3,5-tris(carbazol-9-yl)benzene (TCP), possesses similar energy-level properties like mCP, employed to replace the mCP HTL for the device optimization process. All the thin-film deposition processes were conducted via thermal evaporation in a high vacuum chamber (∼8 × 10 −6 torr). Chemical structures and energy diagrams of all organic materials are also shown in Figure 1. After thin-film deposition, OLEDs were transferred into a glove box with O 2 and H 2 O less than 1 ppm for encapsulation. For evaluating the hole-and electron-transport characteristics, hole-and electron-only devices (HOD and EOD) were also fabricated. The HOD structure was glass substrate/Al (50 nm)/MoO 3 (10 nm)/mCP (10 nm)/BTBP (50 nm)/mCP (10 nm)/MoO 3 (10 nm)/Al (50 nm). The MoO 3 (10 nm)/mCP (20 nm) stack was used to block electrons and ensure the hole-only carrier transport. On the other hand, we used 4,7-diphenyl-1,10-phenanthroline (BPhen) exhibiting high electron mobility as the hole-blocking layer in our EOD device, which was glass substrate/Al (50 nm)/LiF (1.2 nm)/BPhen (10 nm)/BTBP (50 nm)/BPhen (10 nm)/LiF (1.2 nm)/Al (50 nm). In addition, charge-transporting properties of vacuum-deposited films of BTBP were also tested using the time-of-flight (ToF) technique. 39 In the ToF setup, time-resolved excitation of samples through the ITO electrode was performed using the Nd:YAG laser EKSPLA NL300 with the wavelength of 355 nm and pulse duration of 6 ns ( Figure S16a). Since films of BTBP are transparent at the emission wavelength of the laser used, a layer of pentacene as an absorber was deposited. Thus, ITO/pentacene (250 nm)/BTBP (2 μm)/Al was the structure of ToF samples. Charges were generated under the laser excitation in the layer of pentacene. Then, holes or electrons were transported through the layer of BTBP  depending on the polarity of external electric field applied to the sample using the electrometer Keithley 6517B ( Figure S16a). ToF photocurrent transients for holes or electrons at different electric fields were recorded using a Tektronix TDS 3032C oscilloscope. To obtain carrier transit times (t tr ), ToF photocurrent transients were plotted in log−log plots ( Figure S16b,c). Charge mobilities at different electric fields were calculated by the usual formula μ = d 2 /(U × t tr ), where d is the layer thickness, and U is the surface potential at the moment of illumination. Luminance−voltage−current density measurements were obtained via a Keithley 2400 multisource meter and Minolta CS-1000 spectrometer, whose setup was also used to record both the driving current and angular emission spectra to obtain the EQE value. 40 A Hitachi F-4500 fluorescence spectrophotometer with the excitation wavelength at 270 nm was used for obtaining the photoluminescence (PL) spectra. The values of highest occupied molecular orbital (HOMO) in Figure 1 were obtained from lowenergy photoelectron spectra measurement (Riken-Keiki AC-2 spectrometer) with organic thin films deposited on a glass substrate. The lowest unoccupied molecular orbital (LUMO) was deduced from HOMO levels and the absorption spectra were taken using a Hitachi U4100 spectrometer.

RESULTS AND DISCUSSIONS
3.1. BTBP Properties. From differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements (Figure 2), the T g and decomposition temperature (T d ) of BTBP are 126 and 417°C, respectively. Such a high T g value is essential for achieving long operation lifetime of an OLED. Figure 3a presents the UV−Vis absorption, fluorescence (FL), and low-temperature phosphorescence spectra (LTPh at 77 K) of BTBP measured in THF and in solid-state film state. The absorption peaked at 248 nm is attributed to the π−π* transitions. Observation of almost identical absorption and emission patterns for BTBP in solution and in film state suggested that intermolecular π−π stacking effects are small in the matrix. The energy bandgap of 4.0 eV is estimated on the basis of the onset of the solid-state absorption spectrum. A high degree of spectral overlap between the fluorescence emission of BTBP and the MLCT absorption of FIrpic allows effective energy transfer from BTBP to FIrpic. The LTPh reveals E T values of 3.1 and 3.0 eV for BTBP solution and solid-state film, respectively, which are high enough to prevent the occurrence of energy back transfer, and therefore, BTBP can be considered as a useful host material for FIrpic (E T of 2.65 eV) in blue phosphorescent OLED application. The HOMO energy level of 5.8 eV of BTBP can be obtained from the photoelectrical spectrum (Figure 3b). After subtracting the energy bandgap of 4.0 eV, the LUMO energy level is very high, reaching 1.8 eV (Figure 1). Figure 3c shows the PL spectra of BTBP thin film on glass substrates doped by FIrpic with different concentrations (0, 3, 9, and 15%). The emission peak of the pure BTBP thin film was at 369 nm. When it was doped with FIrpic, although the dopant concentration was as low as 3%, obvious FIrpic emission (peaks at 471 and 495 nm in Figure 3c) was observed. The relative intensity at a long wavelength peak (495 nm) increased with dopant concentration, which may come from trivial absorption and/or intermolecular interaction of FIrpic molecules. When enlarging the BTBP PL emission at a short wavelength region (peak at 369 nm) (Figure 3d), one can see the decrease in BTBP emission with increasing dopant concentration. It showed efficient energy transfer from BTBP to FIrpic when the dopant concentration was higher than 15%.
X-ray crystallography is a useful method of understanding material properties. Figure 4 shows the X-ray crystallography in different view angles and material packing of BPTP. BPTP has the torsion angle Φ = 53.59° (Figure 4a), and it has a highly twisted geometric structure. Figure 4b shows the benzene ring with a triazole ring having the torsion angle Φ = 52.04°, and it may allow the benzene and triazole orbital to have some overlap and let this molecule have some intramolecular change transfer. The closest distance between two face-to-face triazole  rings is 3.111 Å, which allows the space π−π interactions. Figure 4c and Figure S17 show the molecule packing in crystal lattice; the distances between intermolecular triazole rings are 5.557 and 5.659 Å. The channel of intramolecular and intermolecular triazoles benefits the electron transportation to enhance the electron mobility, which is similar to our initial concept. Figure 5 shows the geometry and HOMO and LUMO diagrams obtained by DFT calculations at the B3LYP/6-31G(d,p) level using Gaussian 09, Revision A.02 (Gaussian Inc., Wallingford, CT, 2016). 41,42 The energy-minimized structure of BTBP is nonplanar, thus reducing the π−π interactions. The nonplanar structure of BTBP prevents its participation in the formation of excimers or exciplexes between the host and guest (or molecules from the neighboring layers). HOMO is distributed over the electronrich side of phenyl rings, whereas LUMO is mainly localized on the triazole rings and biphenyl core. The HOMO and LUMO of the molecule partly overlap, which may lead to intramolecular charge transfer effects. 43 Such spatial distribution leads to a low LUMO energy level of −0.99 eV, which is favorable for the efficient electron injection and efficient electron transport. The calculated HOMO−LUMO gap for the host material is of 4.83 eV and is larger than the experimental one, obtained from the UV−Vis absorption spectra. Figure 6a shows the J−E characteristics of HOD and EOD of BTBP. Under a low electric field (<5 × 10 5 V/cm), the current density increased linearly with increasing electric field, which came from free carrier conduction or leakage current. Also, the current density values were quite similar for HOD and EOD. When increasing the electric field higher than 5 × 10 5 V/cm, the current density of EOD increased dramatically and was orders of magnitude higher than that of HOD, which showed that BTBP was mainly an electron transport material. We can make a rough estimation of electron-transporting capability for real OLED application. For example, when applying 6 V on 30 nm EML, it corresponded to ∼2 × 10 6 V/ cm in electric field, and BTBP provided the (electron-only) current density of 10 mA/cm 2 . Carrier mobility in an organic thin film was typically modeled with the following Poole− Frenkel equation. where μ is the carrier (electron or hole) mobility, μ 0 is the field-free mobility, β is the Poole−Frenkel constant, and E is the electric field. With increasing electric field, the carrier mobility increased.
To precisely obtain electric field dependences of hole and electron mobilities of BTBP, the ToF was used. By the ToF experiment, it is proved that the BTBP layer exhibits higher electron mobility in comparison to hole mobility at the same electric fields (Figure 6b). For example, hole and electron mobilities of 2.9 × 10 −5 and 5.8 × 10 −5 cm 2 ·V −1 ·s −1 , respectively, were observed at the electric field of 1.1 × 10 6 V·cm −1 . It should be noted that electric field dependences of hole and electron mobilities of BTBP contained certain errors since it was difficult to determine transit times due to the very strong charge dispersity in the BTBP layer evident from current transients (Figure S16b,c). For the same reason, charge mobilities were detected only at very high electric fields (>7 × 10 5 V·cm −1 ). Nevertheless, it was previously demonstrated that hosts with mobility reaching 10 −5 cm 2 ·V −1 ·s −1 showed good performances in blue OLEDs. 44,45 Electric field dependences of hole and electron mobilities of BTBP were well described by the Poole−Frenkel model (eq 1). Parameters μ 0 and β are presented in Figure 6b. A higher value of β for electrons demonstrates a faster increase in electron mobility with increasing electric fields in comparison to that of holes. These findings are in good agreement with the space-chargelimited current−voltage measurements described above.
3.2. Device Performance. 3.2.1. ETL Modification. In an OLED, the electron mobility of ETL was typically much lower than hole one of HTL. In addition, the wide-angle interference also depended on the length between the emissive dipole and reflective electrode (Al cathode in this case). These two factors made the ETL thickness very critical. Hence, we chose the dopant concentration (x%) at 15% because of the promised energy transfer (Figure 3d) and varied the ETL thickness, y   factor, from 30 to 40 nm with a 5 nm interval as devices A−C. Figure 7a shows the J−V characteristics of devices A−C. Obviously, the driving voltage increased with the thicker ETL thickness (Table 1), which describes that certain voltage drops on the ETL. Considering the efficiency performances ( Figure  7b), it can be easily identified that the optimized ETL thickness is 35 nm. Among devices A−C, device B shows favorable CE max , PE max , and EQE max of 57.2 cd/A, 51.5 lm/W, and 25.4%, respectively. An obvious efficiency roll-off phenomenon can be observed for these FIrpic-based OLEDs because the main EML suffers the severe effects of triplettriplet annihilation and triplet-polaron annihilation at high   current density and high brightness. This efficiency roll-off can be further improved by carefully managing triplet excitons in the EML, 46,47 but it is not the scope of this study. In Figure 7c, device B exhibits the greater emission intensity recoded in various view angles, especially in the angles from 0°to ±60°. Figure 7d,e shows the EL spectra of devices A−C at 4 and 10 V, respectively. One can see that the relative intensity of long wavelength increased with increasing ETL thickness because the increase in distance between the recombination zone and cathode resulted in a cavity resonance effect. With increasing driving voltage, the recombination zone shifted farther away from the cathode, leading to the increase in the long wavelength of the EL spectral profiles. These also describe the greenshift movement of color coordinates in the CIE diagram ( Figure 7f). For example, the color coordinate of the device B moves from (0.163, 0.417) to (0.179, 0.449) when the driving voltage increases from 4 to 10 V.

EML Modification.
To upgrade the device performance greater than device B (15% FIrpic), a series of devices D− F with increasing FIrpic concentrations (x%) in volume ratios of 18, 21, and 24%, respectively, were fabricated, their electroluminance (EL) performances were measured as shown in Figure 8, and their characteristics are summarized in Table 1. In Figure 8a, the J−V curves of devices D−F showed that the driving voltage decreased with increasing dopant concentrations. This is because the hole mobility was much lower than the electron mobility of BTBP, as shown in Figure 6, which resulted in the difficulty in hole transporting in the EML. Hence, the heavy doping ratio of FIrpic constructed a path beneficial for hole transportation and then reduced the driving voltage. Figure 8b shows the efficiencies versus current density curves of devices D−F. The highest CE, PE, and EQE of 61.1 cd/A, 55.0 lm/W, and 27.2%, respectively, were obtained at device E (21% FIrpic) under 3.5 V driving voltage, as shown in Table 1. In particular, this device also exhibits the enhanced EL intensity in various view angles from 0°to ±80° (  Figure 8c). In comparison with device B (15% FIrpic), the increasing FIrpic concentration resulted in the longer holetransporting path in the EML of devices D−F, leading to greater resonance in blue band (with a peak near 474 nm) of the EL spectrum no matter at 4 or 10 V (Figure 8d,e). Similar to the carrier dynamics and optical cavity effects in above devices A−C (Figure 7), with increasing driving voltage, the recombination zone shifted farther away from the cathode, leading to the increase in the long wavelength of the EL spectral profiles. These also describe the greenshift movement of color coordinates in the CIE diagram (Figure 8f). For example, the color coordinate of device E moves from (0.154, 0.400) to (0.169, 0.433) when the driving voltage increases from 4 to 10 V.
In addition, several approaches in the EML modification such as thickness adjustment (Figure 8), gradient dopant, and double EML structure ( Figure S18) were employed to further optimize the device efficiency. To accelerate the hole transportation, a faster hole mobility HT material, TCP, was employed to replace the mCP. Device G was fabricated and exhibited an efficiency improvement by introducing the 10 nm TCP HTL and also enlarging the EML thickness to 40 nm, which may result in the favorable carrier balance compared with that of other approaches. Device G showed a degraded J− L−V curve due to the thicker device structure. Although with the lagged J−L−V behavior (Figure 8a), the process of carrier injection, transportation, and recombination to become a photon was efficient, especially under a low driving J. At J ≈   (Figure 8b), device G exhibited a greater efficiency (CE max of 64.7 cd/A, PE max of 56.5 lm/W, and EQE max of 28.3%) than device E. In a case of lower J = 0.004 mA/cm 2 , device G with a luminance of 2.6 cd/m 2 showed maximum efficiency performances with a CE max of 65.9 cd/A, PE max of 62.8 lm/W, and EQE max of 30.2%. This high device efficiency could be also confirmed by the high photoluminescence quantum yield value of 92.9% for the BTBP:21%FIrpic thin film, which was the EML inside the device. The EL spectra at 4 and 10 V (Figure 8d,e) and the color coordinates (Figure 8f) were similar with those of devices D−F. In addition, to realize the carrier transportation and main carrier recombination zone inside the EML of device G ( Figure S19), a series of OLEDs with the undoped and partialdoped EMLs were fabricated and their results described that the whole EML was dominated the device efficiency and the main recombination zone was located toward HTL, which is because the BTBP host belongs to the electron-favorable bipolar behavior.

CONCLUSIONS
A wide bandgap organic compound, BTBP, was successfully synthesized by conjugating two electron-transporting triazole moieties on a hole-transporting biphenyl core. The material properties of BTBP were characterized to have a wide bandgap of 4 eV, high triplet energy over 3 eV, HOMO of 5.8 eV, LUMO of 1.8 eV, high glass transition temperature of 126°C, and ultrahigh decomposition temperature of 417°C. In addition, this BTBP showed the electron-favorable bipolar behavior with close hole and electron mobilities of 2.9 × 10 −5 and 5.8 × 10 −5 cm 2 ·V −1 ·s −1 , respectively, at a low electric field of 1.1 × 10 6 V·cm −1 . In particular, this BTBP exhibits bipolar behavior at a low electric field and electron-transporting behavior at a high electric field. We utilized this BTBP as a host and demonstrated a blue phosphorescent OLED to achieve the high-efficiency performances with CE max , PE max , and EQE max of 65.9 cd/A, 62. 8