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-4tolylamino)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,C ′ ] picolinate as a blue dopant, bis(2-phenylbenxothiozolato-N,C ′ )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 6.2 lm · W−1 to 7.9 lm · W−1 at 1, 000 cd · m−2, 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.


Introduction
White organic light-emitting diodes (WOLEDs) have attracted great attention due to their applications in fullcolor displays, backlights for liquid crystal displays (LCDs), and solid-state lightings [1][2][3][4][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 4,4bis[N-(p-tolyl)-N-phenylamino]biphenyl (TPD), due to its higher triplet energy level compared with other hole transport materials [6].
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 [7].Recently, transition metal oxides such as molybdenum trioxides (MoO 3 ) and tungsten trioxides (WO 3 ) have been widely used as p-dopants to enhance hole injection and transport in organic devices [8][9][10][11][12][13].
In this paper, we investigate the electrical and optical properties of MoO 3 -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 MoO 3 -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.

Experimental
2.1.Method.Films were grown on quartz glass and indiumtin 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.

Measurement.
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 [14].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.400 nm and 900 nm.However, the optical absorption spectrum of the MoO 3 -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 MoO 3 increases, as shown in the inset of Figure 1.This new peak is absent in the optical absorption spectra of undoped TAPC and MoO 3 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 MoO 3 -doped TAPC film indicates the formation of CT complexes between the TAPC and the MoO 3 , resulting in ptype doping of the TAPC.

Results and Discussion
To investigate the enhancement in hole injection and transport of the MoO 3 -doped TAPC layer, we fabricated hole-only devices with device structures composed of ITO/TAPC : MoO 3 (x vol.%, 100 nm)/Al (100 nm) and ITO/MoO 3 (5 nm)/TAPC : MoO 3 (x vol.%, 100 nm)/MoO 3 (5 nm)/Al (100 nm).The MoO 3 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 MoO 3 -doped TAPC layer by comparing above two devices [9,17].abilities with respect to the device with an undoped layer.In comparing devices with an undoped and a 5% MoO 3doped layer, the current density of the device with the MoO 3 layer (5 nm) in the electrode is higher than that of the device without the MoO 3 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 MoO 3 layer (5 nm).This result indicates that the hole injection barrier between ITO and the MoO 3 -doped TAPC layer is similar to that between ITO and the MoO 3 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 MoO 3 reduces the hole mobility of the doped film.The decrease in hole mobility has previously been observed in MoO 3 -doped N -diphenyl-N, N -bis(1-naphthyl)(1,1biphenyl)-4,4 -diamine (NPB) [18].Figure 2(b) shows the electrical conductivities of the MoO 3 -doped TAPC films as a function of MoO 3 doping concentration, which is derived from the ohmic regime of the J-V characteristics in Figure 2(a).The electrical conductivity of MoO 3 -doped TAPC film increases as the MoO 3 doping concentration increases.When the doping concentration is greater than 25%, the MoO 3 -doped TAPC films exhibit similar electrical conductivities.The electrical conductivity of the undoped, 5%, 10%, 25%, and 50% MoO 3 -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 MoO x -doped 4 ,4 -tri(Ncarbazolyl)triphenylamine (TCTA) or NPB [8].The above results indicate that MoO 3 -doped TAPC films can be used as HTLs in OLEDs to reduce the driving voltage and to improve the power efficiency.

Phosphorescent WOLEDs.
To investigate the effect of MoO 3 -doped TAPC layer in the OLEDs, we have fabricated WOLEDs with MoO 3 -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 MoO 3 -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 MoO 3 -doped TAPC layer and the emitting layer [19].We used 1,3bis(9-carbazolyl)benzene (mCP) as a host for the emitting layer and three phosphorescent dopants iridium(III) bis[4,6difluorophenyl]-pyridinato-N, C 2 ] picolinate (FIrpic) as a blue dopant, bis(2-phenylbenxothiozolato-N,C 2 )iridium (III) (acetylacetonate) (Ir(BT) 2 (acac)) as an orange dopant [20], and bis(1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq) 2 (acac)) as a red dopant [21].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 [22].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 MoO 3 (10 nm) as an HIL instead of a MoO 3 -doped TAPC layer (40 nm).Since the WOLEDs have three emitting layers with different colors, we For the device with 25% doping concentration, the driving voltage at 1,000 cd•m −2 , V 1,000 nit , is 5.6 V, and the current density at 10 V, J 10 V , is around 230 mA • cm −2 , whereas V 1,000 nit is 6.8 V and J 10 V is around 73 mA • cm −2 for the device with an undoped layer.This result indicates that the layer of MoO 3 -doped TAPC can inject and transport holes very efficiently due to the ohmic contact between ITO and the MoO 3 -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 L-J curve decreases when the current density of the device is greater than about 20 mA•cm −2 .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 [24]; 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 [25].
Figures 5(a 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 MoO 3 -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 [23].The electrical conductivity of TAPC increases with the MoO 3 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 electronhole 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 MoO 3 doping concentrations in Table 1.
International Journal of Photoenergy    Figures 6(a)-6(e) show the EL spectra, normalized by the main emission peak of FIrpic at 468 nm, for WOLEDs with different MoO 3 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 MoO 3 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 MoO 3 doping concentrations as shown in Figure 7.All devices show high CRI which are over 84 at J = 153 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/cm 2 when the MoO 3 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 [5].The higher current density than 153 mA/cm 2 may cause nearly red color instead of white color.We also compare the EL spectra with different MoO 3 doping concentrations at the same current density of J = 153 mA•cm −2 as shown in Figure 6(f).As the MoO 3 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 MoO 3 doping concentration increases.In other words, more holes reach the ETL side by inserting an MoO 3 -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.

Conclusion
We have demonstrated that an MoO 3 -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 MoO 3doped TAPC layer as an HTL.The WOLEDs with an MoO 3doped 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 MoO 3 -doped TAPC layer because the doped MoO 3doped 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.

Figure 3 :
Figure 3: (a) Chemical structure of the organic materials, (b) device structure, and (c) schematic energy level diagram of WOLEDs.

Figure 5 :
Figure 5: (a) Power efficiency versus luminance characteristics and (b) power efficiency and external quantum efficiency of WOLEDs at 1,000 cd•m −2 with different MoO 3 doping concentrations.

Figure 6 :
Figure 6: (a)-(e) Normalized EL spectra of WOLEDs with different current density at fixed doping concentrations and (f) with different MoO 3 doping concentrations at the same current density (J = 153 mA•cm −2 ).Each inset shows the corresponding CIE color coordinates.The arrows indicate the spectral and CIE color coordinates variations with increasing current density from 0.051 to 153 mA•cm −2 .

Figure 7 :
Figure 7: CRI and CCT of WOLEDs with different MoO 3 doping concentrations as a function of current density.

Table 1 :
Performance of the WOLEDs with different MoO 3 doping concentrations.
a At 10 V. b At luminance of 1,000 cd•m −2 .c At current density of 153 mA•cm −2 .