Unformatted text preview: JOURNAL OF APPLIED PHYSICS 104, 054501 2008 Marked improvement in electroluminescence characteristics of organic light-emitting diodes using an ultrathin hole-injection layer of molybdenum oxide
Toshinori Matsushima, Guang-He Jin, and Hideyuki Murataa
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Received 25 May 2008; accepted 26 June 2008; published online 2 September 2008 We show that the performance of organic light-emitting diodes OLEDs is markedly improved by optimizing the thickness of a hole-injection layer HIL of molybdenum oxide MoO3 inserted between indium tin oxide and N , N -diphenyl-N , N -bis 1-naphthyl -1 , 1 -biphenyl-4 , 4 -diamine -NPD . From results of the electroluminescence EL characteristics of OLEDs with various thicknesses of a MoO3 HIL, we found that the OLED with a 0.75-nm-thick MoO3 HIL had the lowest driving voltage and the highest power conversion efﬁciency among the OLEDs. Moreover, the operational lifetime of the OLED was improved by about a factor of 6 by using the 0.75-nm-thick MoO3 HIL. These enhanced EL characteristics are attributable to the formation of an Ohmic contact at the interfaces composed of ITO / MoO3 / -NPD. © 2008 American Institute of Physics. DOI: 10.1063/1.2974089
I. INTRODUCTION In recent years, organic light-emitting diodes OLEDs have been intensively studied due to their possible applications for low-cost, ﬂexible, large area, lightweight lightings, and displays.1,2 The practical applications of OLEDs require devices, which have a high power conversion efﬁciency together with long-term operational stability. To improve the power conversion efﬁciency and the stability, a reduction in driving voltage is crucial. To reduce the driving voltage, an organic or inorganic hole-injection layer HIL has been frequently used between an indium tin oxide ITO anode layer and an organic hole-transport layer HTL owing to a reduction in hole-injection barrier height.3–30 Molybdenum oxide MoO3 is a material frequently used as a HIL between an ITO layer and a HTL.5,7,8,12,13,18,19,21,22,24,27,28,30 Also, MoO3 is used as a p-type dopant in a HTL to reduce the driving voltage.31–34 The thickness of a MoO3 HIL inserted between an ITO layer and a HTL of N , N -diphenylN , N -bis 1-naphthyl -1 , 1 -biphenyl-4 , 4 -diamine -NPD was generally greater than 5 nm. Using current densityvoltage characteristics of hole-only -NPD devices with various thicknesses of a MoO3 HIL, we recently demonstrated that the use of a 0.75-nm-thick MoO3 HIL inserted between the ITO and the -NPD only leads to the formation of an Ohmic contact at the ITO / MoO3 / -NPD interfaces due to electron transfers from ITO to MoO3 and from -NPD to MoO3.22 This MoO3 thickness of 0.75 nm is much thinner than the previously reported values and it is crucial to the formation of the Ohmic contact. Since the reduction in holeinjection barrier height improves the stability of OLEDs,4,24,31,34 the formation of this Ohmic contact not only provides the lowest driving voltage but is also expected to improve the stability of OLEDs. In this study, we investia gated how the thickness of a MoO3 HIL inserted between the ITO and the -NPD affects the performance of OLEDs. We found that the optimized thickness of a MoO3 HIL in our OLEDs was 0.75 nm, which is in good agreement with the value obtained in the hole-only devices.22 The OLED with a 0.75-nm-thick MoO3 HIL exhibited the lowest driving voltage, the highest power conversion efﬁciency, and the longest operational lifetime when compared to the OLEDs with other MoO3 thicknesses. The origin of the enhanced device performance at the particular MoO3 thickness was discussed.
II. EXPERIMETAL Electronic mail: [email protected] We fabricated the OLEDs with various thicknesses of a MoO3 HIL Fig. 1 a according to the following steps. Glass substrates a size of 25 25 mm2 coated with an ITO layer 150 nm with a sheet resistance of 10 / sq SLR grade, Sanyo Vacuum Industries were used as a hole-injecting anode. The substrates were cleaned using the procedures, which included ultrasonication in acetone, in detergent, in pure water, and in isopropanol, and ﬁnally ultraviolet light irradiation in an ultraviolet-ozone treatment chamber. The cleaned substrates were set in a vacuum evaporator, which was evacuated to around 3 10−4 Pa. Under this pressure, a MoO3 HIL X nm , an -NPD HTL 60 nm ,4 an emitting electron-transport layer ETL of tris 8-hydroxyquinoline aluminum Alq3 65 nm ,35 an electron-injection layer of LiF 0.5 nm ,36 and an Al cathode layer 100 nm were successively vacuum deposited on the ITO anode layer. The thickness of the MoO3 HIL X nm was varied from 0 to 10 nm. The deposition rates were precisely controlled at 0.05 nm/s for MoO3, 0.1 nm/s for -NPD and Alq3, 0.01 nm/s for LiF, and 0.5 nm/s for Al using a quartz crystal microbalance. The active area of the OLEDs was deﬁned by the overlapped area of the ITO and the Al. Four OLEDs with an area of 2 2 mm2 were prepared on the glass substrate with a size of
© 2008 American Institute of Physics 0021-8979/2008/104 5 /054501/6/$23.00 104, 054501-1 054501-2 Matsushima, Jin, and Murata J. Appl. Phys. 104, 054501 2008 nance meter BM-9, TOPCON . To evaluate the operational stability, the times at which the luminance reduced to 90% of their initial luminance 90% lifetimes of the OLEDs were measured at a constant dc density of 50 mA / cm2. All measurements were conducted at room temperature. To investigate the hole-injection characteristics at the ITO / MoO3 / -NPD interfaces, we fabricated the hole-only -NPD devices with various thicknesses of a MoO3 HIL Fig. 1 b using the preparation conditions similar to those previously described. To prevent electron injection from the Al cathode, we used a MoO3 electron-blocking layer EBL with a high work function of −5.68 0.03 eV Ref. 22 between the -NPD and the Al. In fact, we observed no electroluminescence EL from the devices, meaning that only holes were injected in the devices. The current densityvoltage characteristics of the hole-only devices were measured using a semiconductor characterization system 4200, Keithley Instruments at room temperature. Thin layers of -NPD, Alq3, MoO3, and Al were prepared on a glass substrate coated with an ITO layer 150 nm using the same preparation conditions mentioned above. The thickness of all layers was 100 nm. Their ionization potential and work function energy levels were measured by using photoelectron spectroscopy AC-2, Riken Keiki . We estimated the electron afﬁnity energy levels of the layers of -NPD and Alq3 by subtracting their optical absorption onset energy levels from the measured ionization potential energy levels. The energy-level diagram of the OLED is depicted in Fig. 1 c .
III. RESULTS AND DISCUSSION
FIG. 1. Schematic structures of a OLEDs and b hole-only -NPD devices with various thicknesses X nm of MoO3 HIL and c energy-level diagram of OLEDs before making contact between each layer. The thickness X nm of MoO3 HIL inserted between ITO and -NPD was varied from 0 to 10 nm. A. Characteristics of OLEDs with various thicknesses of a MoO3 HIL 25 25 mm2. High-purity source materials of MoO3 6-N grade, Mitsuwa Chemicals , -NPD NN60615, Nippon Steel Chemical , Alq3 NA30516, Nippon Steel Chemical , LiF 206–30, Nacalai Tesque , and Al AL-011480, Nilaco were purchased and used as received. The completed OLEDs were transferred to a nitrogen-ﬁlled glovebox O2 and H2O concentration levels less than 2 ppm connected to the vacuum evaporator without exposing the OLEDs to ambient air. The OLEDs were encapsulated with a glass cap using an ultraviolet curing epoxy resin inside the glovebox. The current density-voltage-external quantum efﬁciency characteristics of the OLEDs were measured using a semiconductor characterization system 4200, Keithley Instruments and an integrating sphere installed with a calibrated silicon photodiode. The glass substrate was placed on a surface of the integrating sphere. Light emitted from the substrate surface was introduced through an aperture a diameter of 10 mm into the integrating sphere to eliminate the light emitted from the edge of the glass substrate. The luminance and the power conversion efﬁciencies of the OLEDs were measured in the direction perpendicular to the substrate surface using a lumi- We observed a marked reduction in driving voltage of the OLEDs by using a 0.75-nm-thick MoO3 HIL. The current density-voltage characteristics of the OLEDs with various MoO3 thicknesses are shown in Fig. 2 a . The driving voltages of the OLEDs at current densities of 1, 10, and 100 mA / cm2 are plotted as a function of the MoO3 thickness in Fig. 2 b . The driving voltages at a certain current density decreased as the MoO3 thickness was increased from 0 to 0.75 nm. The OLED with the 0.75-nm-thick MoO3 HIL had the lowest driving voltage among the devices. On the contrary, the driving voltages increased as increasing the MoO3 thickness from 0.75 to 2 nm. The driving voltages were nearly unchanged in the MoO3 thickness ranging from 2 to 10 nm. The driving voltage of the OLED with the 0.75nm-thick MoO3 HIL was 7.0 V at a current density of 100 mA / cm2, which is higher than that of an OLED with a chemically doped HTL Ref. 6 but lower than those of conventional undoped OLEDs.3,4,10 From the results of x-ray photoelectron and ultraviolet/ visible/near-infrared absorption studies,22 we previously found that electron transfers occur from ITO to MoO3 and from -NPD to MoO3. These electron transfers are expected to induce the shift of the Fermi levels of MoO3 and ITO and the hole-transport level of -NPD. When the MoO3 thickness is 0.75 nm, these energy levels are matched and an 054501-3 Matsushima, Jin, and Murata J. Appl. Phys. 104, 054501 2008 FIG. 3. External quantum efﬁciency at current densities of 1, 10, and 100 mA / cm2 vs MoO3 thickness of OLEDs. FIG. 2. a Current density-voltage characteristics of OLEDs with various thicknesses of MoO3 HIL and b driving voltage at current densities of 1, 10, and 100 mA / cm2 vs MoO3 thickness of OLEDs. Ohmic contact is formed at the interfaces, resulting in the lowest driving voltage observed in the OLED with the 0.75nm-thick MoO3 HIL. On the other hand, a strong interfacial dipole layer IDL , i.e., negatively charged MoO3 and positively charged -NPD, is gradually formed at the MoO3 / -NPD interface as the MoO3 thickness is increased from 1 to 2 nm. This IDL may lower a hole-injection efﬁciency and increase the driving voltages of the OLEDs.37–45 In all OLEDs, we observed green EL originating from electrically excited Alq3 molecules. The shapes and the peak tops 528 2 nm of EL spectra of the OLEDs were consistent with those previously reported4,9,17 and they were almost independent of both operational current densities ranging from 1 to 100 mA / cm2 and MoO3 thickness ranging from 0 to 10 nm. However, the thickness of the MoO3 HIL inserted between the ITO and the -NPD markedly affected the external quantum efﬁciencies and the power conversion efﬁciencies of the OLEDs. The external quantum efﬁciencies and the power conversion efﬁciencies at current densities of 1, 10, and 100 mA / cm2 are plotted as a function of the MoO3 thicknesses in Figs. 3 and 4, respectively. The maximum external quantum efﬁciency of the OLED without MoO3 was 1.1% at a current density of 66 mA / cm2, which is in good agreement with those of previously reported OLEDs with an Alq3 emitter.46–48 However, the external quantum efﬁciencies decreased when the MoO3 HIL was used Fig. 3 . The decrease in the external quantum efﬁciency is ascribed to an excess number of injected holes in the Alq3 ETL Ref. 49 and/or exciton quenching by accumulated holes at the -NPD / Alq3 interface,50,51 resulting from enhanced hole injection from the ITO / MoO3 contacts. Furthermore, since the marked decrease in the driving voltage Fig. 2 overcomes the decrease in the external quantum efﬁciency Fig. 3 , the power conversion efﬁciencies increased by using the MoO3 HIL Fig. 4 . The operational lifetimes of the OLEDs were markedly dependent upon the MoO3 thickness. The luminance/initial luminance-time characteristics of the OLEDs were measured at a constant dc density of 50 mA / cm2 at room temperature Fig. 5 a . The initial luminance at 50 mA / cm2 was about 1500 cd / m2, which was slightly changed depending upon the MoO3 thickness. The 90% lifetimes of the OLEDs are plotted as a function of the MoO3 thickness in Fig. 5 b . The OLED with the 0.75-nm-thick MoO3 HIL had the longest 90% lifetime 190 h among the OLEDs investigated in this study. By ﬁtting the experimental luminance decay curve of this OLED with a stretched single exponential equation,52 the time at which the luminance reached half of the initial luminance half lifetime was estimated to be approximately 3200 h at a current density of 50 mA / cm2 an initial luminance of 1440 cd / m2 . This value lies above half lifetimes of previously reported OLEDs with an Alq3 emitter operated under the similar conditions. The luminance of the OLED without MoO3 suddenly decreased at the initial stage 10 h of the device operation Fig. 5 a . However, the insertion of the MoO3 HIL between the ITO and the -NPD drastically suppressed this initial degradation. The OLED with the 0.75-nm-thick MoO3 HIL exhibited no initial degradation and had an about six times longer 90% lifetime 190 h than the OLED without MoO3 32 h . This result manifests that this initial degradation occurs near the ITO / -NPD interface. The origin of the initial FIG. 4. Power conversion efﬁciency at current densities of 1, 10, and 100 mA / cm2 vs MoO3 thickness of OLEDs. 054501-4 Matsushima, Jin, and Murata J. Appl. Phys. 104, 054501 2008 FIG. 5. a Luminance/initial luminance-time characteristics of OLEDs with various thickness of MoO3 HIL and b 90% lifetime and electric power vs MoO3 thickness of OLEDs. All OLEDs were operated at constant dc current density of 50 mA / cm2. degradation might be attributed to the migration of indium components into the -NPD HTL Refs. 53 and 54 and/or a chemical reaction between the -NPD and hydroxyl groups on the ITO surface.55 We would like to point out that after the initial degradation, all OLEDs exhibited the gradual decrease in luminance Fig. 5 a . The electric power E consumed by the operation of the OLEDs is expressed as an equation E = JV, where J is the current density and V is the driving voltage. The calculated electric powers of the OLEDs operated at 50 mA / cm2 are also plotted in Fig. 5 b . We found that the OLEDs with lower electric powers have longer operational lifetimes. This result indicates that the OLED degradation is related to Joule heat generated inside the OLEDs and it is consistent with the fact that operating OLEDs under a higher bias voltage at a higher temperature accelerates the degradation of OLEDs,56–59 indicating that the device degradation may rapidly proceed under high temperature conditions. Since electrochemically decomposed species act as nonradiative recombination centers and/or luminance quenchers in a carrier recombination zone, EL efﬁciencies of OLEDs gradually decrease as the amount of decomposed species increases with operational time.60–62 The incorporation of H2O molecules into the Alq3 emitting layer during its evaporation has been shown to induce an electrochemical decomposition of Alq3.63 From these considerations, we assume that the longterm degradation of the OLEDs originates from a thermally induced electrochemical decomposition of organic molecules. Decreasing the driving voltages electric powers may reduce a temperature increase inside the OLEDs and the probability of a thermally induced degradation process of the OLEDs. FIG. 6. a Current density-voltage characteristics of hole-only -NPD devices with various thickness of MoO3 HIL and b driving voltage at current densities of 10, 100, and 1000 mA / cm2 vs MoO3 thickness of hole-only -NPD devices. Solid line in a represents SCLC expressed as J = 9 / 8 r 0 eff V2 / L3 , where J is current density, r is relative permittivity 3.0 , 0 is vacuum permittivity 8.845 10−12 F / m , eff is carrier mobility 1.0 0.1 10−4 cm2 / V s , V is voltage, and L is cathode-anode spacing 100 nm . Ref. 22 B. Characteristics of hole-only devices with various thickness of a MoO3 HIL To obtain more detailed information on the change in the current density-voltage characteristics shown in Fig. 2 a , we analyzed hole-only -NPD devices, whose structure is shown in Fig. 1 b . The current density-voltage characteristics of the hole-only devices are shown in Fig. 6 a . The driving voltages of the hole-only devices at current densities of 10, 100, and 1000 mA / cm2 are plotted as a function of the MoO3 thickness in Fig. 6 b . The hole-only device with the 0.75-nm-thick MoO3 HIL exhibited the highest current density and a square law. These observations indicate that the current density-voltage characteristics of this device are controlled by a space-charge-limited current SCLC mechanism solid line in Fig. 6 a and that an Ohmic contact is formed at the ITO / MoO3 0.75 nm / -NPD interfaces. Details of these characteristics of the hole-only devices have been discussed in Ref. 22. Since the driving voltages of the hole-only devices Fig. 6 b were changed in a manner similar to the driving voltages of the OLEDs Fig. 2 b , we conclude that the improved OLED performance discussed in Sec. III A was ascribed to the enhanced hole injection from the ITO / MoO3 anodes. The MoO3 layers with various thicknesses were prepared on a glass substrate coated with an ITO layer. The work functions of these samples were measured using AC-2 pho- 054501-5 Matsushima, Jin, and Murata J. Appl. Phys. 104, 054501 2008 FIG. 7. Work functions of glass/ITO 150 nm / MoO3 X nm samples as a function of MoO3 thickness X nm . The thickness X nm of MoO3 layer on ITO was varied from 0 to 10 nm. toelectron spectroscopy. The relationship between the work functions of the ITO / MoO3 anodes and the MoO3 thickness is shown in Fig. 7. The work functions of the ITO / MoO3 anodes markedly increased as the MoO3 thickness was increased from 0 to 2 nm due to an electron transfer from ITO to MoO3.22 When the MoO3 thickness was increased from 2 to 10 nm, the work functions of the ITO / MoO3 anodes gradually approached the work function level of an intrinsic MoO3 layer −5.68 0.03 eV Ref. 22 . At the MoO3 thickness of 0.75 nm, the ITO / MoO3 anode had a work function of −5.22 0.05 eV, indicating that a hole-injection barrier 0.18 eV was still present at the anode / -NPD interface. It should be noted that this hole-injection barrier height 0.18 eV was calculated from the difference in energy level between the work function of the ITO / MoO3 layer −5.22 0.05 eV and the ionization potential energy level of an intrinsic -NPD layer −5.40 0.01 eV Ref. 22 before these layers were in contact. As mentioned in the Sec. III A, we conﬁrmed that electron transfers occur both from ITO to MoO3 and from -NPD to MoO3 see Fig. 1 c .22 The electron transfer from -NPD to MoO3 induces an increase in free hole concentration in the -NPD HTL near the MoO3 / -NPD interface. This increase in free hole concentration leads to matching between the Fermi level of the ITO / MoO3 anode layer and the hole transport level of the -NPD layer,64,65 resulting in the formation of the Ohmic contact at the interfaces. We would like to emphasize again that the double electron transfers from ITO to MoO3 and from -NPD to MoO3 are indispensable to the formation of the Ohmic contact. The morphologies of the ITO / MoO3 surfaces were measured using an atomic force microscope SPA400, Seiko Instruments . The results are shown in Fig. 8. When the MoO3 thicknesses were 0 and 0.75 nm, the ITO / MoO3 surfaces had a relatively large grain structure with a large average surface roughness Figs. 8 a and 8 b . As the MoO3 thickness was increased from 1 to 5 nm, the grain structures gradually disappeared and the ITO / MoO3 surfaces became ﬂat Figs. 8 c and 8 d , suggesting that the ITO surface was probably covered with the MoO3 layer. In this case, depositing the -NPD on the ﬂat ITO / MoO3 surfaces would form a strong IDL which may impede the hole injection at the interface. To FIG. 8. Atomic force microscope images of glass/ITO 150 nm / MoO3 X nm samples: a X = 0 nm, b X = 0.75 nm, c X = 1 nm, and d X = 5 nm. clarify a detailed mechanism of the IDL formation is an interesting research topic and we are now investigating to elucidate the mechanism.
IV. CONCLUSIONS We investigated how the thickness of a MoO3 HIL inserted between an ITO anode and an -NPD HTL inﬂuences EL characteristics of OLEDs and hole-injection characteristics of hole-only -NPD devices. We found the optimized MoO3 thickness to be 0.75 nm, which provided the lowest driving voltage, the highest power conversion efﬁciency, and the longest lifetime for the OLED. Moreover, current density-voltage characteristics of the hole-only -NPD device with the 0.75-nm-thick MoO3 HIL were controlled by a SCLC mechanism, indicating that an Ohmic contact was formed at the ITO / MoO3 0.75 nm / -NPD interfaces. The formation of the Ohmic contact using the 0.75-nm-thick MoO3 HIL was ascribed to 1 an electron transfer from ITO to MoO3, 2 an electron transfer from -NPD to MoO3, and 3 incomplete coverage of an ITO surface by MoO3. Finally, we emphasize that the ﬁndings presented in this study are valuable in developing organic opto electronic devices as well as clarifying the underlying mechanisms of carrier transport in organic ﬁlms.
ACKNOWLEDGMENTS We thank the New Energy and Industrial Technology Development Organization NEDO of Japan for ﬁnancial support of this work.
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