Solution-processed oxadiazole-based electron-transporting layer for white organic light-emitting diodes

Abstract

A novel alcohol-soluble electron-transporting small-molecule material comprising oxadiazole and arylphosphine oxide moieties, ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide) (OXDPPO), has been synthesized and characterized. Its single crystal structure, together with the photophysical, electrochemical and thermal properties, has been investigated. This material not only possesses a wide bandgap with a low HOMO level but also exhibits a strong π–π stacking with a distance of 3.35 Å. Moreover, this compound shows excellent thermal stability with a high glass transition temperature of 104 °C and a decomposition temperature of 384 °C. The unique solubility in 2-propanol makes it a good candidate for fabricating fully solution-processed multilayer organic light-emitting diodes (OLEDs). Efficient solution-processed white OLEDs have been fabricated with this compound as an electron-transporting layer (ETL). It was found that this ETL can greatly balance the electrons and holes in devices with the high work-function metal cathode (Al) and an increase in luminous efficiency of ∼70-fold can be achieved. The maximum luminous efficiency of devices with an ETL/Al configuration is even higher than that of devices using a Ca cathode.

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Introduction

Organic light-emitting diodes (OLEDs) have attracted much attention owing to their promising applications in full-color display panels, flexible displays, and solid-state lighting sources.^1–6 OLEDs can be fabricated either through a vacuum evaporation method or through a solution-processed one. In order to improve the device efficiency through balancing the charge injection and transport, OLEDs usually adopt a multilayer structure which involves using of a hole-transporting (or injecting) layer (HTL), an emitting layer (EML) and an electron-transporting (or injecting) layer (ETL) in device fabrication.^7,8 However, fabrication of multilayer devices through the vacuum evaporation process is complicated, and time and energy consuming compared with the solution process. Although highly efficient OLEDs have been produced via vacuum evaporation techniques, fully solution-processed devices are more desirable for large-size flat-panel displays and solid-state lighting due to their many unique advantages such as low-cost and facile manufacturing process, compatibility with flexible substrates, and easy processability over large areas by spin-coating, inkjet printing, or roll-to-roll coating.^5,9–12

In the past years, solution-processed hole-transporting materials, emissive materials and host materials have been developed.^13–18 Usually, charge/exciton management plays an important role in improving the performances of OLEDs.^19–21 For the purpose of facilitating electron injection, low work function metal cathodes such as Ba and Ca can establish ohmic contacts with the emissive layer to achieve high-efficiency devices. Nevertheless, these low work function metals are sensitive to both oxygen and moisture, resulting in reducing of device lifetime. Alternatively, the use of environment stable metals (such as Al or Ag) often shows poor device performances due to a large electron injection barrier existing between the cathode and the emitting layer.²² It is very necessary to introduce an ETL to reduce the electron injection barrier from the high work function metal cathode and to enhance the electron transport ability in devices. However, most of ETLs are deposited by vacuum thermal evaporation, which leads to a costly and complicated fabrication process.^23–29 Among the reported electron-transporting materials (ETMs), oxadiazole (OXD) is recognized as a good electron accepting component for building compounds with high electron mobility.^30–35 In addition, OXD derivatives exhibit wide band gaps because it restricts extensions of π-conjugation beyond the ring even if the molecule is co-planar.^36,37 Such a merit is significantly important in phosphorescent devices where the wide bandgap can avoid undesired triplet energy transfer from emitters to ETM and is also beneficial for exciton blocking.³⁸ Accordingly, OXDs are promising ETMs for OLEDs. Unfortunately, when used in devices, all the reported OXD-based ETMs were either deposited by vacuum evaporation (which is time and energy consuming) or blended into the EML (in which a continuous electron-transporting channel can hardly be formed).

Although there is of great interest for fabricating highly efficient multilayer OLEDs through sequential solution-processing, the existing problem is that the former layer would be dissolved during deposition of a latter layer. One general strategy towards overcoming this problem is to utilize the orthogonal solvents.³⁹ Therefore, developing alcohol/water (nonsolvent for most organic semiconductors) soluble ETMs will provide a good solution to this problem.

In this paper, we report the synthesis, characterization, and investigation of the structural, photophysical, electrochemical and thermal properties of a new alcohol-soluble OXD-based ETM, namely ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide) (OXDPPO). Moreover, we have fabricated white OLEDs with OXDPPO as an ETL and Al as a cathode. The device performance with OXDPPO as an ETL using high work-function metal (Al) as the cathode was significantly improved in comparison with the device without the ETL, owing to enhancement in electron injection from the cathode with the help of OXDPPO. Furthermore, the maximum luminous efficiency of device with Al cathode using OXDPPO as an ETL was even comparable to that of the device with a Ca cathode. These results demonstrate that OXDPPO is a promising solution-processable ETM for high-performance OLEDs.

Experimental

Materials

Poly(N-vinyl carbazole) (PVK) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (CLEVIOS PVP 4083) used in our experiments were purchased from Sigma Aldrich and H. C. Starck Clevios GmbH, respectively, and were used without further purification. 1,3-Bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) was purchased from Nichem Fine Technology Co. Ltd. Iridium(III)bis(4,6-difluorophenylpyridinato-N,C2′)picolinate (FIrpic) and bis(2-phenylbenzothiazolato-N,C2′)(acetylacetonate)iridium(III) (Ir(bt)₂(acac)) were purchased from Xi'an Polymer Light Technology Corp. Other reagents were purchased from J&K Chemical and were used without further purification. Unless otherwise stated, all the solvents were purchased from Beijing Chemical Works and were used as received. Tetrahydrofuran (THF) was distilled from Na/diphenylketone.

Characterization

¹H NMR and ¹³C NMR spectra were carried out using a 400 and 101 MHz AC Bruker spectrometer, respectively. Ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) emission spectra were measured by a Hitachi U-3900H spectrophotometer and a Hitachi F-7000 fluorescence spectrophotometer, respectively. Element analysis (EA) was conducted on a FLASH EA 1112 element analyzer. The molecular weights of compounds 1 and 2 were determined on an APEXII FT-ICR analyzer (Bruker Daltonics Inc.) using electrospray ionization mass spectrometry (ESI-MS) and electron bombardment ionization mass spectroscopy (EI-MS), respectively. The mass spectra of 3 and OXDPPO were determined on a Bruker Daltonics BIFLEX III MALDI-TOF analyzer using the matrix-assisted laser desorption ionization time-of-flight (MALDI) mode. Cyclic voltammograms (CV) were recorded on a conventional three-electrode cell using a Pt button working electrode, a platinum wire counter electrode and a Ag/AgCl reference electrode on an electrochemical workstation (CHI860D, CH instruments) in acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as the supporting electrolyte at room temperature under an argon atmosphere. The redox potentials were obtained with ferrocene/ferrocenium (Fc/Fc⁺) as an internal standard in the redox system. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere on a TA Instruments SDT-Q600. The differential scanning calorimetry (DSC) analysis was obtained on a PerkinElmer Instruments DIAMOND DSC under a N₂ flow. A heat-cool-heat method was used with an initial heating rate of 20 °C min⁻¹, a cooling rate of 40 °C min⁻¹, and a final heating rate of 20 °C min⁻¹. Single crystal X-ray diffraction analysis was recorded on a Rigaku Saturn 724+ under a nitrogen atmosphere. The dimensions of a single crystal measured were 0.45 × 0.44 × 0.38 mm³. The atomic force microscopy (AFM) images were obtained from a Veeco DI Dimension V atomic force microscope operating in the tapping mode.

Synthesis

The synthesis of OXDPPO was outlined in Scheme 1. The intermediate 1 and 2 were obtained by using the general procedure as reported in the literature.⁴⁰ In the third step, the intermediate 3 was synthesized by a low-temperature reaction. Finally, the desired product was obtained with a high yield (92%). OXDPPO was readily soluble and stable in common organic solvents such as dichloromethane, methanol and 2-propanol. All the compounds were characterized by ¹H NMR, ¹³C NMR and MS. Moreover, the single-crystal determination on OXDPPO confirmed the proposed structure.

Scheme 1 Synthetic route for OXDPPO.

Synthesis of 4-bromo-N′-(4-bromobenzoyl)benzohydrazide (1). Hydrazine monohydrate (2.89 g, 57.7 mmol) was added dropwise to a mixture of triethylamine (6.68 g, 66.0 mmol) and 4-bromobenzoyl chloride (14.49 g, 66.0 mmol) in 150 mL of chloroform in an ice-water bath. The resulting mixture was stirred for 4 h at room temperature. The solvent was removed and the solid was washed with petroleum ether and then a large amount of deionized water twice to afford 1 (9.89 g, 75%) as a white solid. ¹H NMR (400 MHz, DMSO): δ(ppm) 10.64 (s, 2H), 7.85 (d, J = 8.1 Hz, 4H), 7.74 (d, J = 7.6 Hz, 4H). ¹³C NMR (101 MHz, DMSO): δ (ppm) 165.43, 132.11, 132.03, 130.02, 126.21. MS (ESI) m/z: M⁺ calculated for C₁₄H₁₀Br₂N₂O₂ 398.05; found: 421.1 [(M + Na)⁺].

Synthesis of 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (2). A reaction mixture of 1 (8.50 g, 21.4 mmol) in 150 mL of SOCl₂ was heated to reflux under nitrogen atmosphere for 5–6 h. The SOCl₂ was evaporated and the residue was extracted with dichloromethane (3 × 60 mL) and water (50 mL). The organic layer was dried over anhydrous sodium sulfate and the solvent was removed. The crude product was purified by column chromatography on silica gel using petroleum ether : dichloromethane (5 : 1 in v/v) as eluent, yielding a white solid (5.97 g, 73%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.98 (d, J = 8.5 Hz, 4H), 7.67 (d, J = 8.5 Hz, 4H). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) 164.07, 132.51, 128.36, 126.64, 122.65. MS (EI) m/z: M⁺ calculated for C₁₄H₈Br₂N₂O 380.03; found: 380 (M⁺).

Synthesis of 2,5-bis(4-(diphenylphosphino)phenyl)-1,3,4-oxadiazole (3). A solution of 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole (5.00 g, 13.2 mmol) in 130 mL dry THF was cooled to −78 °C under argon, and n-BuLi (1.6 M n-hexane solution, 19.8 mL, 31.68 mmol) was added dropwise. The reaction solution was stirred for 1 h at −78 °C. Then, chlorodiphenylphosphine (5.7 mL, 31.68 mmol) was also added dropwise and the reaction was kept for additional 2 h at −78 °C. After that, the reaction solution was allowed to warm to room temperature overnight. After removing of THF, the crude product was extracted with CH₂Cl₂ (3 × 60 mL) and H₂O (50 mL). The organic layer was dried over anhydrous sodium sulfate and the solvent was removed. The crude oil was purified by column chromatography on silica gel using petrol ether : dichloromethane (3 : 1 in v/v) as eluent to afford 3 (4.52 g, 58%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 8.05 (d, J = 7.9 Hz, 4H), 7.41 (t, J = 7.3 Hz, 4H), 7.35 (d, J = 8.6 Hz, 20H). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) 164.39, 136.14, 136.03, 134.07, 133.87, 129.23, 128.79, 128.72, 126.72, 126.66. MS (MALDI-TOF) m/z: M⁺ calculated for C₃₈H₂₈N₂OP₂ 590.59; found: 591.2 [(M + H)⁺].

Synthesis of OXDPPO. The excess H₂O₂ (10 mL) was added to a solution of 3 (1.50 g, 2.54 mmol) in 130 mL CH₂Cl₂ and the mixture was stirred for 24 h at room temperature. The reacted solution was extracted with CH₂Cl₂ and H₂O. Then, the organic layer was dried over anhydrous sodium sulfate and the solvent was removed. The crude product was purified by column chromatography on silica gel using petrol ether : methanol (20 : 1 in v/v), yielding a white solid (1.45 g, 92%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 8.21 (d, J = 6.7 Hz, 4H), 7.84 (dd, J = 11.2, 8.3 Hz, 8H), 7.67 (dd, J = 12.0, 7.5 Hz, 8H), 7.55 (d, J = 7.1 Hz, 4H), 7.48 (dd, J = 9.9, 4.7 Hz, 8H). ¹³C NMR (101 MHz, CDCl₃): δ(ppm) 164.32, 137.36, 136.35, 132.94, 132.84, 132.38, 132.36, 132.19, 132.12, 132.02, 131.15, 128.81, 128.68, 126.94, 126.82, 126.67, 126.65. MS (MALDI-TOF) m/z: M⁺ calculated for C₃₈H₂₈N₂P₂O₃ 622.59; found: 623.2 [(M + H)⁺]. EA found: C, 71.83; H, 4.53; N, 4.42%. C₃₈H₂₈N₂O₃P₂ requires C, 73.31; H, 4.53; N, 4.50%.

OLEDs fabrication and measurements

The OLEDs was fabricated with structure of indium tin oxide (ITO)/PEDOT:PSS (40 nm)/PVK : OXD-7 : FIrpic : Ir(bt)₂(acac) (80 nm)/OXDPPO (20 nm)/Al (80 nm). For comparison, we also fabricated devices with structures of ITO/PEDOT:PSS (40 nm)/PVK : OXD-7 : FIrpic : Ir(bt)₂(acac) (80 nm)/Al (80 nm) and ITO/PEDOT:PSS (40 nm)/PVK : OXD-7 : FIrpic : Ir(bt)₂(acac) (80 nm)/Ca (8 nm)/Al (80 nm). The pre-cleaning and hydrophilic treatment of ITO glass substrates (10 Ω per square) was carried out according to the reported procedures.⁴¹ For all the devices, PEDOT:PSS was firstly spin-coated onto the ITO glass substrates and annealed at 120 °C for 20 min in an oven to form a 40 nm thick film. Following this, a chlorobenzene solution containing the mixture of PVK : OXD-7 : FIrpic : Ir(bt)₂(acac) (10 : 4 : 1 : 0.02 in m m⁻¹) with a total concentration of 13.6 mg mL⁻¹ was spin-coated onto the PEDOT:PSS layer and then baked at 100 °C for 20 min in a nitrogen filled glove box (H₂O < 0.1 ppm, O₂ < 0.1 ppm) to form a 80 nm-thick EML. OXDPPO dissolved in 2-propanol with a concentration of 10 mg mL⁻¹ was spin-coated onto the EML and then baked at 80 °C for 20 min in a nitrogen filled glove box (H₂O < 0.1 ppm, O₂ < 0.1 ppm) to form a 20 nm thick ETL. Finally, Ca (8 nm) or Al (80 nm) were deposited onto the ETL or EML as the cathode by thermal evaporation under a vacuum of 3 × 10⁻⁶ torr. Film thickness was measured by an Ambios Technology XP-2 profilometer. The current density–luminance–voltage (J–L–V), luminous efficiency–current density (η–J) were measured using a Keithley 2612 B source-measurement unit and a silicon photodiode calibrated by a Spectrascan PR655 photometer. Electroluminescent (EL) spectra of devices with the OXDPPO/Al and Ca/Al configurations were recorded on a Maya 2000Pro spectrophotometer (Ocean Optics). EL spectrum of device using the bare Al cathode was measured by a Hitachi F-7000 fluorescence spectrophotometer. Commission Internationale de l'Eclairage (CIE) coordinates were calculated from the EL spectra.

Results and discussion

X-ray crystal structure

A single crystal of OXDPPO grown via slow evaporation of the CH₂Cl₂/acetone solution was determined by XRD with Mo K_α radiation.† Crystallographic data at 173.15 K for OXDPPO: triclinic, space group P1̄, Z = 2, unit cell dimensions with a = 8.6042(19) Å, b = 12.291(3) Å, c = 16.260(4) Å, α = 110.945(4)°, β = 91.262(2)°, γ = 94.507(3)°, volume for 1598.7(7) ų, F(000) = 658. A total of 20 966 reflections were collected in the range of 1.343° ≤ θ ≤ 27.502°, (hkl range indices: −11 ≤ h ≤ 11, −15 ≤ k ≤ 15, −21 ≤ l ≤ 21), 7308 independent reflections.

The ORTEP diagram of the single-crystal structure of OXDPPO is shown in Fig. 1a. The central 2,5-diphenyl-1,3,4-oxadiazole core is almost planar with very small dihedral angles between benzene and oxadiazole rings (0.18° and 3.9°). The P=O bond lengths [P(1)–O(1) = 1.4847(12) Å and P(2)–O(3) = 1.4840(12) Å] and average P–C bond lengths [1.804 Å and 1.805 Å for P(1) and P(2), respectively] are similar about each phosphorus atom and comparable to that reported for Ph₃P=O.⁴² At one end, the bridging 2,5-diphenyl-1,3,4-oxadiazole plane approximately bisects the Ph–P–Ph bond angle and the oxygen atom is approximately in-plane with the 2,5-diphenyl-1,3,4-oxadiazole perimeter (O–P–C–C torsion angle of 13.65°), similar to what has been observed previously for 9,10-bis(diphenylphosphine oxide) anthracene.⁴³ Fig. 1b shows the packing motif of OXDPPO molecules. A strong π–π stacking (d = 3.35 Å) among 2,5-diphenyl-1,3,4-oxadiazole moieties can be found in the crystal, which will be beneficial to the electron transport through hopping mechanism.

Fig. 1 (a) Thermal ellipsoid plot (ORTEP) diagram and atomic numbering for OXDPPO; (b) packing diagram of OXDPPO molecules.

Photophysical properties

Fig. 2 shows the UV-vis absorption, PL emission, and low-temperature phosphorescent spectra of OXDPPO in methanol solution, in thin film, and in CH₂Cl₂, respectively. The absorption spectrum shows two peaks in the dilute methanol solution located at 205 and 296 nm. The absorption peak at 296 nm could be attributed to the π–π* transitions of the 2,5-diphenyl-1,3,4-oxadiazole core. Two absorption peaks were also observed in the thin film, similar to those in the solution except for a slight red shift (6 nm) of the π–π* transition band. This phenomenon is due to the increased electron delocalization in the solid state. The energy gap of OXDPPO was 3.59 eV derived from the UV-vis absorption edge. Upon UV excitation, the PL spectrum has an emission peak at 361 nm in methanol solution and 370 nm in film. The triplet energy level E_T of OXDPPO was 2.63 eV deduced from the phosphorescent spectrum in CH₂Cl₂ at 77 K. This E_T value is larger than that of the FIrpic dye (∼2.6 eV) which is a commonly used blue phosphorescent emitter in white OLEDs.

Fig. 2 UV-vis absorption (room temperature), PL emission (room temperature), and phosphorescence (77 K) spectra of OXDPPO in methanol solution, in thin film, and in CH₂Cl₂, respectively.

Electrochemical properties and theoretical calculations

The electrochemical behavior of OXDPPO was investigated by cyclic voltammetry method in acetonitrile solution under argon atmosphere using Bu₄NPF₆ as the electrolyte at a potential scan rate of 10 mV s⁻¹ (see Fig. S1, ESI†). Its lowest unoccupied molecular orbital (LUMO) energy was calculated to be −2.83 eV from the onset reduction potential, assuming the absolute energy level of Fc/Fc⁺ redox couple to be 4.80 eV below vacuum energy level. Its highest occupied molecular orbital (HOMO) energy was estimated to be −6.42 eV according to its LUMO energy and its optical bandgap. The deep HOMO energy level of OXDPPO indicates that it could block holes and confine excitons within the emissive layer. To investigate the structure–property relationship of OXDPPO at the molecular level, the geometry optimization and electronic properties of this compound was carried out by Gaussian 09 D.01 program.⁴⁴ The geometry was optimized by density functional theory (DFT) at B3LYP/6-31G(d) level, and no imaginary frequency for the optimized structure was found. The electronic properties (HOMO and LUMO distributions) of OXDPPO (see Fig. S2, ESI†) were calculated at τHCTHhyb/6-311++G(d,p) level, and the calculated HOMO and LUMO levels were −6.59 eV and −2.63 eV, respectively. The spatial distribution of front molecular orbital was visualized using GaussView. The HOMO and LUMO levels are all distributed on the oxadiazole core, indicating that the 2,5-diphenyl-1,3,4-oxadiazole moiety mostly determines the HOMO/LUMO levels of the OXDPPO. What's more, there is no distribution on the diphenylphosphine oxide moieties, consistent with their weak conjugation degree. Usually, since electrons transport through LUMOs, OXDPPO can thus be expected to have an electron transport ability due to the dominant distribution of LUMO on the 2,5-diphenyl-1,3,4-oxadiazole core and the close π–π stacking among these moieties.

Thermal properties

The thermal properties of OXDPPO shown in Fig. 3 were investigated by TGA and DSC. TGA curve revealed a high decomposition temperature (T_d, 5% weight loss) at 384 °C, confirming the high thermal stability of OXDPPO. The DSC curve revealed a well-defined glass transition temperature (T_g) of 104 °C, which is much higher than other commonly used ETMs such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) (60 °C)⁷ and 4,7-diphenyl-1,10-phenanthroline (BPhen) (62 °C).⁴⁵ Both the high decomposition temperature and the high glass transition temperature indicate that OXDPPO can form thermally durable and morphologically stable amorphous thin film, which is important for fabricating high-quality OLEDs. In addition, the surface morphology of solution-deposited OXDPPO thin film was investigated by AFM (Fig. S3, ESI†). It demonstrates a smooth surface with a root-mean-square (RMS) roughness of 0.25 nm.

Fig. 3 TGA traces of OXDPPO recorded at a heating rate of 10 °C min⁻¹. The inset shows DSC curve of OXDPPO at a heating rate of 20 °C min⁻¹.

Space-charge-limited current (SCLC) carrier mobility

In order to investigate the electron-transporting property of OXDPPO, we have applied SCLC measurement to evaluate its electron mobility.^46,47 The electron-only devices have been fabricated with the structure of ITO/Al (45 nm)/OXDPPO (x)/Ca (8 nm)/Al (80 nm), where x is 120, 160, and 190 nm, respectively. The 8 nm Ca was used to decrease the electron injection barrier between the OXDPPO layer and the Al electrode, ensuring the ohmic contact required by SCLC method. The 45 nm Al on ITO was utilized to block hole injection from ITO electrode. Fig. 4 shows the electron mobility at different thicknesses of the OXDPPO thin film as a function of the square root of the electric field. The electron mobility increases with both elevating the electric field and thickening the OXDPPO film in the measured range. The electron mobility increasing with the film thickness can be explained by the fact that, the contribution of interfacial effect to the effective electron mobility of OXDPPO becomes weaker and bulk properties are predominant in a thicker film, resulting in higher electron mobility.^46,47 When the thicknesses of OXDPPO thin films are 120, 160 and 190 nm, the electron mobilities are found to be 2.7 × 10⁻⁵, 7.0 × 10⁻⁵ and 1.1 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, under an electric field of 3 × 10⁵ V cm⁻¹.

Fig. 4 Electron mobility at different thicknesses of OXDPPO thin films as a function of the square root of the electric field.

Characterization of white organic light-emitting diodes (WOLEDs)

WOLED with OXDPPO as the ETL to improve the device's efficiency has been fabricated, where the cathode is Al. To compare the device performances, another two WOLEDs without ETLs using either Al or Ca as the cathodes were also fabricated as the control devices. The device configuration and the chemical structures of the relevant materials are shown in Fig. 5. The luminance–voltage–current density (L–V–J) characteristics of the three WOLEDs are shown in Fig. 6a and b. The luminous efficiency–current density (η–J) characteristics of the three WOLEDs are demonstrated in Fig. 6c. Obviously, the device with the bare Al cathode (/Al) exhibited a very poor performance due to the poor electron injection from high work-function Al (∼4.3 eV) to the emitting layer (PVK : OXD-7 : FIrpic : Ir(bt)₂acac). So only a very low luminance (L_max = 151 cd m⁻²), a high turn-on voltage (V_turn-on = 11.21 V) and a low luminous efficiency (η_max = 0.28 cd A⁻¹) can be obtained (see Fig. 6b). By using the low work-function metal Ca (2.8 eV) as the cathode (/Ca/Al), the device showed much improved performance with η_max of 18.47 cd A⁻¹. When OXDPPO was introduced into the device with Al cathode (/OXDPPO/Al), significantly improved performance can be achieved with a maximum luminous efficiency of 19.65 cd A⁻¹, which is nearly 70-fold larger than that of the bare Al cathode device and is even higher than that of the bare Ca cathode device. This indicates the important role of OXDPPO in balancing electrons and holes in OLEDs. The lower current density of the OXDPPO based device (Fig. 6a) can be attributed to the thicker organic layer thickness than the Ca/Al device. Besides the luminous efficiency, device with the OXDPPO/Al structure showed the lowest turn-on voltage of 5.10 V. Such a turn-on voltage is much lower than that of the device with the bare Al cathode and even a little lower than the device with the Ca/Al cathode (see Table 1), indicating that the electron injection were significantly facilitated by using OXDPPO as the ETL. When the luminance reached up to 1000 cd m⁻², the device with the OXDPPO/Al configuration still showed a high luminous efficiency of 18.96 cd A⁻¹. As a result, the device with an OXDPPO/Al structure can be a good replacement to the Ca cathode, which avoids the using of unstable low-work function metal as electrodes in OLEDs. The EL spectrum of device with the OXDPPO/Al configuration exhibited emission bands in the sky-blue and the orange-red regions (see Fig. S4 (a), ESI†). Thus, white emission with the corresponding CIE coordinates of (0.30, 0.43) can be obtained. We also obtained the EL spectra of control devices using Al or Ca as the cathode, and the corresponding CIE coordinates were (0.20, 0.37) and (0.29, 0.41), respectively (see Fig. S4 (b) and (c), ESI†). The CIE coordinates of device with the OXDPPO/Al configuration were better than the control devices. This can be ascribed to the carrier injection and transport being more balanced when OXDPPO was introduced into the device as ETL.

Fig. 5 Device configuration and chemical structures of relevant materials.

Fig. 6 (a) J–L–V characteristics of devices with the OXDPPO/Al and Ca/Al configurations; (b) J–L–V characteristics of device using the bare Al cathode; (c) η–J curves of devices with ETL using Al as the cathode and without ETL using either Ca or Al as the cathode. The filled symbols stand for current density and empty ones for brightness in (a) and (b).

Table 1 Performances of WOLEDs with and without the ETL

ETL and cathode structure	Von^a [V]	Lmax^b [cd m^−2]	ηmax^c [cd A^−1]	η1000^d [cd A^−1]
a Turn-on voltage which is defined as the voltage at a brightness of 1 cd m^−2.b Maximum luminance.c Maximum luminous efficiency.d Luminous efficiency at a brightness of 1000 cd m^−2.
/Ca/Al	5.70	29 907	18.47	16.48
/OXDPPO/Al	5.10	20 954	19.65	18.96
/Al	11.21	151	0.28	—

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Conclusions

In conclusion, a novel alcohol-soluble electron-transporting material OXDPPO based on oxadiazole and arylphosphine oxide moieties was synthesized and confirmed by various spectroscopic studies. OXDPPO exhibited a high decomposition temperature and a high glass transition temperature (104 °C) revealed by the TGA and DSC studies. It shows an excellent film-forming ability with RMS roughness of 0.25 nm. All-solution-processed WOLEDs with OXDPPO as ETLs using air stable Al cathode had an outstanding performance with a maximum luminous efficiency of 19.65 cd A⁻¹ which is nearly 70-fold higher than the device using bare Al cathode and is comparable to the device with Ca cathode. Furthermore, the driving voltage of the device using Al cathode can be lowered after introduction of OXDPPO as the ETL. In other words, OXDPPO can effectively improve the electron injection and balancing electrons and holes in devices, thus enhancing the OLEDs' performance. These results suggest that OXDPPO would be a good candidate for the application in multilayer EL devices with high efficiency through a solution process.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51273020, 51373022) and Research Fund for the Doctoral Program of Higher Education of China (20130006110007). We acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and Gaussian 09 (D.01).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Cyclic voltammogram, HOMO and LUMO orbital distributions, AFM, and EL spectrum of device with ETL using Al as cathode and control devices using Al or Ca as the cathode. CCDC 1044786. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra04599a

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