Research Article | Open Access
Improvement of Power Efficiency in Phosphorescent White Organic Light-Emitting Diodes Using p-Doped Hole Transport Layer
We investigated the optical and electrical properties of molybdenum trioxide- (MoO3-) doped (1,1-bis[(di-4-tolylamino)phenyl]cyclohexane) (TAPC) films with different doping concentrations. Based on our results, we have fabricated white organic light emitting diodes (WOLEDs) with multi-emitting layer structures that consist of 1,3-bis(9-carbazolyl)benzene as a host and three phosphorescent dopants: iridium(III) bis[4,6-difluorophenyl]-pyridinato-N, picolinate as a blue dopant, bis(2-phenylbenxothiozolato-N,)iridium(III) (acetylacetonate) as an orange dopant, and bis(1-phenylisoquinoline) (acetylacetonate) iridium(III) as a red dopant. We improved the power efficiency and decreased driving voltage of WOLEDs by employing a MoO3-doped TAPC layer as a hole transport layer. The MoO3-doped TAPC layer lowers the driving voltage by about 1.2 V and increases the power efficiency from to at , an approximately 27.4% increase. Furthermore, the WOLED has a high color-rendering index, which is about 86 with the Commission Internationale de l’Eclairage 1931 chromatic coordinates of (0.4303, 0.3893) and correlated color temperature of 3008 K.
White organic light-emitting diodes (WOLEDs) have attracted great attention due to their applications in full-color displays, backlights for liquid crystal displays (LCDs), and solid-state lightings [1–5]. Because WOLEDs have many merits, such as high efficiency, high color-rendering index (CRI) and flexibility of design, they are studied intensively as lighting sources. The improvement in power efficiency is especially important in lighting sources because energy saving is currently a crucial issue. To enhance the power efficiency of WOLEDs, one needs to increase the quantum efficiency and decrease the driving voltage of the device.
To achieve high quantum efficiency in phosphorescent OLEDs, charges and triplet excitons must be confined to the emission region of a device. Therefore, the hole transport material and electron transport material should have shallow lowest-unoccupied-molecular-orbital (LUMO) and deep highest-occupied-molecular-orbital (HOMO) energy levels, respectively. In addition, they also have a higher triplet energy level than that of the emitting material. For example, a device with 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) has much higher quantum efficiency than a device with other hole transport materials, such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD) and 4,4′-bis[N-(p-tolyl)-N-phenylamino]biphenyl (TPD), due to its higher triplet energy level compared with other hole transport materials .
To decrease the driving voltage of OLEDs, the potential energy barrier between the electrode and the organic material should be reduced and the carrier transport material should have high electrical conductivity. Electrical doping such as p-doping and n-doping can reduce the driving voltage of OLEDs because it decreases the contact resistance between the electrodes and organic materials as well as increases the electrical conductivity of organic materials . Recently, transition metal oxides such as molybdenum trioxides (MoO3) and tungsten trioxides (WO3) have been widely used as p-dopants to enhance hole injection and transport in organic devices [8–13].
In this paper, we investigate the electrical and optical properties of MoO3-doped TAPC films with different doping concentrations. Based on these results, we fabricated trichromatic WOLEDs with red, orange, and blue emission layers. By employing an MoO3-doped TAPC layer as a hole transport layer (HTL), we improved the power efficiency and decreased the driving voltage of WOLEDs. In addition, the WOLEDs show a high CRI of about 86 with the Commission Internationale de l’Eclairage (CIE) 1931 chromatic coordinates of (0.4303, 0.3893) and correlated color temperature (CCT) of 3008 K.
Films were grown on quartz glass and indium-tin oxide- (ITO-) precoated glass substrates for optical absorption spectra measurement and device fabrication, respectively. The substrates were cleaned with acetone and isopropyl alcohol and were rinsed with deionized water. All substrates were dried in an oven filled with nitrogen gas at 200°C for 30 minutes. After being dried, patterned ITO substrates were treated in ultraviolet ozone for 5 minutes. All layers were grown in succession by thermal evaporation without breaking vacuum. During the deposition of the doping layers, the deposition rates of both the host and guest materials were controlled with a quartz crystal oscillator and source shutters.
Optical absorption spectra were measured using a UV/Vis spectrophotometer (BECKMAN DU-70). The current-voltage-luminance characteristics were measured using a Keithley-236 source measurement unit and a Keithley 2000 multimeter. The luminance and efficiencies were calculated from photocurrent measurement data obtained with a calibrated Si photodiode (Hamamatsu S5227-1010BQ), a photomultiplier tube, and electroluminescence (EL) spectra data obtained using a spectroradiometer (Minolta CS-1000A). The CIE 1931 chromatic coordinates and CCT are also obtained using a spectroradiometer. The CRIs were calculated using the procedure defined by CIE . The HOMO and LUMO energy levels of all materials used in this work were obtained from the references and the measured data by a photoelectron spectrometer (Hitachi AC-2) and UV/Vis spectrophotometer.
3. Results and Discussion
3.1. Characteristics of MoO3-Doped TAPC Films
Figure 1 shows the optical absorption spectra of MoO3, undoped TAPC, and MoO3-doped TAPC films with different MoO3 concentrations. All film thicknesses are 100 nm. The absorption spectrum of the undoped TAPC film shows an absorption peak at 310 nm and no absorption peaks between 400 nm and 900 nm. However, the optical absorption spectrum of the MoO3-doped TAPC film shows an additional absorption peak at 703 nm, and the intensity of this new peak increases as the doping concentration of the MoO3 increases, as shown in the inset of Figure 1. This new peak is absent in the optical absorption spectra of undoped TAPC and MoO3 films. This result is similar to other p-type doping results discussed in other literatures, and they explain that the new absorption peak is originated from the charge transfer (CT) complex between the hole transport material and dopant [12, 15, 16]. Therefore, the absorption peak at 703 nm in the MoO3-doped TAPC film indicates the formation of CT complexes between the TAPC and the MoO3, resulting in p-type doping of the TAPC.
To investigate the enhancement in hole injection and transport of the MoO3-doped TAPC layer, we fabricated hole-only devices with device structures composed of ITO/TAPC : MoO3 ( vol.%, 100 nm)/Al (100 nm) and ITO/MoO3 (5 nm)/TAPC : MoO3 ( vol. %, 100 nm)/MoO3 (5 nm)/Al (100 nm). The MoO3 layer (5 nm) can efficiently inject holes from electrode to organic layer so we can find that the cause of current increase is whether the enhancement of hole injection or hole transport of the MoO3-doped TAPC layer by comparing above two devices [9, 17]. Figure 2(a) shows the current density-voltage (-) characteristics of the hole-only devices with different MoO3 concentrations. The devices with a MoO3-doped layer exhibit dramatically enhanced hole injection and transport abilities with respect to the device with an undoped layer. In comparing devices with an undoped and a 5% MoO3-doped layer, the current density of the device with the MoO3 layer (5 nm) in the electrode is higher than that of the device without the MoO3 layer (5 nm); however, the current density of the devices with doping concentrations above 10% is almost the same regardless of the insertion of the MoO3 layer (5 nm). This result indicates that the hole injection barrier between ITO and the MoO3-doped TAPC layer is similar to that between ITO and the MoO3 layer when the doping concentration is greater than 10%. The current density of the device increases as the doping concentration increases. However, the current density of a 50% doped layer is slightly lower than that of a 25% doped layer. This result is different from those observed in the absorption spectra. This may be due to the fact that heavily doped MoO3 reduces the hole mobility of the doped film. The decrease in hole mobility has previously been observed in MoO3-doped -diphenyl--bis(1-naphthyl)(1,1′-biphenyl)-4,4′-diamine (NPB) . Figure 2(b) shows the electrical conductivities of the MoO3-doped TAPC films as a function of MoO3 doping concentration, which is derived from the ohmic regime of the - characteristics in Figure 2(a). The electrical conductivity of MoO3-doped TAPC film increases as the MoO3 doping concentration increases. When the doping concentration is greater than 25%, the MoO3-doped TAPC films exhibit similar electrical conductivities. The electrical conductivity of the undoped, 5%, 10%, 25%, and 50% MoO3-doped TAPC films is approximately 1.2 × 10−8, 4.1 × 10−7, 3.3 × 10−6, 6.9 × 10−6, and 6.2 × 10−6 S·cm−1, respectively. These values are similar to the reported values for MoOx-doped 4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) or NPB . The above results indicate that MoO3-doped TAPC films can be used as HTLs in OLEDs to reduce the driving voltage and to improve the power efficiency.
3.2. Phosphorescent WOLEDs
To investigate the effect of MoO3-doped TAPC layer in the OLEDs, we have fabricated WOLEDs with MoO3-doped TAPC layer. Figures 3(a)–3(c) show the chemical structure of the organic materials, device structure, and schematic energy level diagram of WOLEDs, respectively. An MoO3-doped TAPC film was used as an HTL as well as a hole injection layer (HIL) in the device. We changed the doping concentration from 5 vol% to 50 vol%. To prevent exciton quenching in the EML by the dopants in the HTL, we inserted an undoped TAPC film as a buffer layer between the MoO3-doped TAPC layer and the emitting layer . We used 1,3-bis(9-carbazolyl)benzene (mCP) as a host for the emitting layer and three phosphorescent dopants iridium(III) bis[4,6-difluorophenyl]-pyridinato-N, C2’] picolinate (FIrpic) as a blue dopant, bis(2-phenylbenxothiozolato-N,C2’)iridium (III) (acetylacetonate) (Ir(BT)2(acac)) as an orange dopant , and bis(1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq)2(acac)) as a red dopant . We used tris[3-(3-pyridyl)mesityl]borane (3TPYMB) as an electron transport layer (ETL) because of its deep HOMO and high triplet energy level . This electron transport material can block holes and confine triplet excitons effectively [22, 23]. Lithium fluoride (LiF) and aluminum (Al) were used as an electron injection layer and cathode, respectively. For comparison, we also fabricated WOLEDs with an undoped TAPC layer (30 nm) as an HTL and MoO3 (10 nm) as an HIL instead of a MoO3-doped TAPC layer (40 nm). Since the WOLEDs have three emitting layers with different colors, we can easily find the variation of recombination region with MoO3 doping concentrations compared with monocolor OLEDs.
Figures 4(a) and 4(b) show the - and luminance-voltage (-) characteristics of the devices prepared with different MoO3 doping concentrations, respectively. It is clear that the current density and luminance of the device increase significantly with the insertion of the MoO3-doped TAPC layer compared with the device with an undoped layer. For the device with 25% doping concentration, the driving voltage at 1,000 cd·m−2, , is 5.6 V, and the current density at 10 V, , is around 230 , whereas is 6.8 V and is around 73 for the device with an undoped layer. This result indicates that the layer of MoO3-doped TAPC can inject and transport holes very efficiently due to the ohmic contact between ITO and the MoO3-doped TAPC layer and enhanced electrical conductivity. However, the increase in the luminance of all devices decreases in the high-current-density region as shown in the inset of Figure 4(b). The slope of - curve decreases when the current density of the device is greater than about 20 . As the current density of the device increases, many holes accumulate at the interface between the emitting layer and 3TPYMB because of the deep HOMO energy level of 3TPYMB and the relatively higher hole mobility of mCP with respect to the electron mobility ; the accumulated holes then disturb the radiative emission of triplet excitons. In other words, triplet-triplet (T-T) annihilation or triplet-polaron quenching may cause the decrease of the luminance in the high-current-density region .
Figures 5(a) and 5(b) show the power efficiency (PE) versus luminance characteristics and the PE and external quantum efficiency (EQE) of the devices at 1,000 cd·m−2 with different MoO3 doping concentrations, respectively. The PE of the device with an MoO3-doped TAPC layer is much higher than that of the device with an undoped layer. The PE of the device with undoped, 5%, 10%, 25%, and 50% MoO3-doped layer is 9.8, 11.2, 11.4, 11.0, and 10.7 lm·W−1 at 100 cd·m−2, respectively. These values are comparable to the reported values for the WOLEDs with similar emitting colors which are blue, orange, and red . The PE of the device with a 5% doping concentration is around 7.9 lm·W−1 at 1,000 cd·m−2, which is 27.4% higher than that of the undoped device, which is around 6.2 lm·W−1. The PE of the device with a MoO3-doped TAPC layer increases up to a doping concentration of 5% and then decreases at 1,000 cd·m−2 as shown in Figure 5(b). This pattern is related to the EQE variation tendency with the doping concentration. The hole mobility of TAPC is much higher than the electron mobility of 3TPYMB . The electrical conductivity of TAPC increases with the MoO3 doping concentration. Therefore, the hole density is much higher than the electron density in the emitting layer as the doping concentration increases, a pattern that leads to an electron-hole imbalance, and, thus, the EQE decreases as the doping concentration increases. This electron-hole imbalance can be improved by employing an electron transport material with a high electron mobility or n-doped organic layer. We summarize the performance of phosphorescent WOLEDs with different MoO3 doping concentrations in Table 1.
aAt 10 V.|
bAt luminance of 1,000 cd·m−2.
cAt current density of 153 mA·cm−2.
Figures 6(a)–6(e) show the EL spectra, normalized by the main emission peak of FIrpic at 468 nm, for WOLEDs with different MoO3 doping concentrations at various current densities. The orange (~560 nm) and the red (~620 nm) emission peaks of the EL spectrum increase irrespective of the MoO3 doping concentrations as the current density increases, so the CIE 1931 chromatic coordinates move to red region. The hole can be easily trapped in the red and orange emitting layer due to the deep HOMO level of Ir(piq)2(acac) and Ir(BT)2(acac) compared with that of mCP. As the current density increases, the amounts of trapped hole increase in the red and orange emitting layer, resulting in the color shift as shown in each inset of Figures 6(a)–6(e). The CRI increases as the current density increases regardless of MoO3 doping concentrations as shown in Figure 7. All devices show high CRI which are over 84 at mA·cm−2 due to obvious three emissive zones and a broad resulting spectrum. On the other hand, the CCT decreases as the current density increases because of red and orange emission increase. The ranges of CCT and CIE 1931 chromatic coordinate of the devices are from 2400 K to 3400 K and from (0.4596, 0.3826) to (0.4134, 0.3729), respectively, at 153 mA/cm2 when the MoO3 doping concentrations change from 0% to 50%. These values are comparable to the values of incandescent bulb which are 2854 K and (0.448, 0.408), respectively . The higher current density than 153 mA/cm2 may cause nearly red color instead of white color. We also compare the EL spectra with different MoO3 doping concentrations at the same current density of mA·cm−2 as shown in Figure 6(f). As the MoO3 doping concentration increases, the intensity of the red emission peak is reduced and shifted from 623 nm (50%) to 619 nm (undoped). This result indicates that the recombination region is gradually shifted to the cathode side as the MoO3 doping concentration increases. In other words, more holes reach the ETL side by inserting an MoO3-doped TAPC layer. In addition, the increase in absorption near 703 nm as the doping concentration increases as shown in Figure 1 slightly affects the decrease in the red emission of the WOLEDs.
We have demonstrated that an MoO3-doped TAPC layer can increase the electrical conductivity of TAPC films and improve hole injection from the ITO to the TAPC layer. Therefore, the power efficiency and driving voltage of phosphorescent WOLEDs are improved by using an MoO3-doped TAPC layer as an HTL. The WOLEDs with an MoO3-doped TAPC layer show 1.2 V lower driving voltage and a power efficiency of 7.9 lm·W−1 at 1,000 cd·m−2, which is about 27.4% higher than that of a device with an undoped layer. The device structure is also simplified by introducing an MoO3-doped TAPC layer because the doped MoO3-doped TAPC layer can act as a HTL as well as a HIL. In addition, the WOLEDs show a high CRI, 86, because of clear three emissive zones and a broad resulting spectrum.
This work was financially supported by the grant from the Industrial Source Technology Development Program (KI002110) and the Industrial Strategic Technology Development Program (KI002104, Development of Fundamental Technologies for Flexible Combined-Function Organic Electronic Device) of the Ministry of Knowledge Economy (MKE) of Korea. This work was also supported in part by the Ministry of Education, Science, and Technology (MEST) through the BK21 Program.
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