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We report a novel strategy to reduce one fine metal mask (FMM) step in a full-color organic light-emitting diode (OLED) display by introducing a common red layer (CRL) which replaces a hole transporting layer (HTL) with the same thickness of a red phosphorescent dye-doped layer. Because the dopant in the HTL acts as a hole trap, careful trap-level engineering is required for achieving efficient green and blue emission from the emitting layer while minimizing the red emission from the CRL. We investigated the characteristics of OLEDs depending on hole trap levels of the CRL with five different organic HTLs, and demonstrated efficient red, green and blue (RGB) emitting devices using the CRL. The electroluminescence spectrum of the devices with the CRL is nearly identical with those of the devices without the CRL. These results open up the possibility of simplified fabrication of practical full-color OLED displays with the reduced FMM steps, resulting in lower manufacturing cost.
© 2015 Optical Society of America
1. Introduction
Since the first organic light-emitting diodes (OLEDs) were demonstrated, they have attracted great attention for the next-generation full-color displays and solid-state lightings owing to their high flexibility, light weight, and low electric power consumption with bright and efficient emission [1–5]. By the efforts of decades on research and development of OLEDs, the state-of-the-art technology enabled the active-matrix OLEDs to be commercialized as a main display device in large-sized televisions as well as mobile devices [6]. However, complicated fabrication process via fine metal shadow mask (FMM) steps hinders OLED panels to be price-competitive compared with that of liquid crystal display (LCD) panels. In order to reduce the manufacturing cost of OLED panels, multilateral research and development, for instance, low-price materials, simplified fabrication processes, and effective device structures, have been achieved by both academic and industrial institutes [7–16].
Typically, a conventional OLED consists of electrodes and multilayer thin-films of functional organic materials, which are hole/electron injection layers (HIL/EIL), hole/electron transporting layers (HTL/ETL), electron and exciton/hole blocking layers (EBL/HBL), and red (R), green (G) and blue (B) sub-pixels in the emitting layer (EML) patterned by FMMs. Another method using white OLEDs with a color filter (CF) is also commonly used for the purpose of a simple process because all functional layers can be deposited through a single common mask [6]. However, the color gamut and power consumption can be a problem in this method due to the absorption of the CF. To increase the efficiency of white OLEDs, a tandem structure which is a stack of the two devices emitting primary colors each has been introduced, but the number of the layers in this structure is almost twice compared to the conventional OLED structure. It means a huge increase in the TAKT time, the organic material usage, and the fabrication cost of OLEDs. In the case of the FMM process, three FMMs are needed to form the R, G and B sub-pixels during thermal evaporation. Because the size of the sub-pixels and the interval between them are on the micrometer scale, aligning each sub-pixel separately with the FMM is a sophisticated and time-consuming process, which increases the TAKT time and fabrication cost. If one of the R, G and B sub-pixels can be made by using the common mask instead of an independent FMM, it will be much more efficient to fabricate OLED panels with reduced efforts. A few years ago, researchers in major OLED companies demonstrated the practicable OLED architectures using a common blue layer adjacent to the ETL, maintaining the overall performances of red and green pixels [12,13]. This common layer not only performed as a HBL, but also had an advantage in a micro-cavity effect. Chen et al. also reported OLEDs with a common blue layer on the HTL without any change in driving voltages and emission spectra of the other colors [14]. It was also adopted in polymer based LEDs, which fulfilled the roles of an EBL to curtail leakage current and a host layer with the energy transfer to red and green colors [17,18]. However, the efficiency of blue-emitting pixels in the papers reported so far was quite low because the common layer should be thin enough to suppress its own emission in the other color-emitting pixels. Besides, they also used blue fluorescent materials which can have 25% of the internal quantum efficiency (IQE) at the maximum, instead of phosphorescent materials of which the IQE can be 100% [19,20]. Moreover, those blue-emitting host materials generally possess a wide energy band gap, which causes inefficient carrier injection from adjacent layers. In particular, wide band gap host materials are required for deep-blue phosphorescence emission and their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are very high and low, respectively, resulting in increased driving voltages due to the poor charge carrier injection and transport properties [21].
In this paper, for highly efficient phosphorescent OLEDs (PHOLEDs) with a simplified process, we introduce a novel device architecture using a common red layer (CRL) on the HTL for the full-color OLEDs based on red, green and blue phosphorescent dyes. Because we utilized the phosphorescent dyes, the device shows high efficiency and brightness for all colors. Besides, red host materials generally have smaller band gap than blue host materials; thus, the carrier injection barrier in this CRL structure is much lower than prior common blue structures. As the CRL, we utilized five host materials possessing different HOMO energy levels, which are 2,7-bis(carbazol-9-yl)-9,9-spirobifluorene (Spiro-2CBP, −5.0 eV), N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD, −5.4 eV), 1,1-bis[(di-4-tolyamino)phenyl]cyclohexane (TAPC, −5.5 eV), 3,6-bis-biphenyl-4-yl-9-[1,1′,4′,1″]terphe-nyl-4-yl-9H-carbazole (BBTC, −5.68 eV), and 4,4′-N,N′-dicarbazole-biphenyl (CBP, −6.0 eV), doped with a typical red phosphorescent dopant, bis(1-phenyliso-quinoline)(acetylacetonate) iridium(III) [Ir(piq)2(acac)]. We also adopted these host materials in the HTL of each monochrome device for the simple device structure. We found that the device using BBTC as the CRL showed the highest performances among the five CRLs in terms of efficiencies and driving voltage. On the other hand, the device with TAPC showed the lowest efficiency in red-emitting device, and the device with a CBP CRL showed drastic increase of driving voltage compared to that without a CRL. The red-emitting CRL was used in blue and green devices, but no red emission was observed in green and blue devices for all of the host materials. We also investigated the current conduction property depending on hole trap energy levels of the CRLs using hole only devices.
2. Experimental section
All devices were fabricated on indium-tin-oxide-coated glass substrates. The substrates were sequentially cleaned with acetone and isopropyl alcohol, and then rinsed with deionized water. After drying in a vacuum oven at 120 °C, the patterned ITO substrates were treated in UV-ozone for 10 minutes. All the organic materials and cathode metals were deposited in succession by thermal evaporation without breaking vacuum. During the deposition of the doping layers, the deposition rates of both host and guest materials were controlled with a quartz crystal oscillator and source shutters.
The current–voltage–luminance (I–V–L) characteristics were measured at room temperature using a Keithley 236 source-measurement unit and a Keithley 2000 multimeter. The luminance and efficiencies were calculated from photocurrent data from a calibrated silicon photodiode (Hamamatsu S5227-1010BQ) and a photomultiplier tube, and electroluminescence (EL) spectra by using a Minolta CS-1000A spectroradiometer. The Commission Internationale de l’Eclairage (CIE) 1931 chromatic coordinates of the EL were also obtained by using the spectroradiometer. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of all materials used in this work were measured by AC-2 photoelectron spectrometer and calculated from their optical band gap characterized by a Beckman DU-70 UV-Vis spectrophotometer unless otherwise noted.
3. Results and discussion
Figure 1(a) shows the device structure of conventional OLEDs having R, G and B sub-pixels patterned by three FMM steps, and Fig. 1(b) depicts the CRL-employed OLEDs having G and B sub-pixels using two FMM steps by skipping the FMM for a red sub-pixel. For both devices, we used indium-tin-oxide (ITO) as an anode and molybdenum trioxide (MoO3) as a HIL. Five organic materials possessing different HOMO energy levels, i.e., Spiro-2CBP, α-NPD, TAPC, BBTC, and CBP, were used in the HTL and the host of CRL at the same time owing to their good hole mobility [22–25]. The common red layer is composed of each host material doped with Ir(piq)2(acac) (8 wt%) with the thickness of 15 nm. For the simple and efficient green- and blue-emitting layers, we used 1,3-bis(9-carbazolyl)benzene (mCP) as a common host which has high triplet energy level (ET) of 2.9 eV, and bis(2-phenyl-pyridinato-N,C2′)iridium(acetylacetonate) [Ir(ppy)2(acac)] and iridium(III)bis[(4,6-di-fluorophenyl)- pyridinato-N,C2′]picolinate (FIrpic) as the green and blue phosphorescent dopants, respectively [25].
Fig. 1 Schematic diagrams of full-color OLEDs with (a) conventional structure and (b) CRL structure.
At the green and blue sub-pixels, 10 nm of TAPC layer was used next to the CRL as a triplet-exciton and electron blocking layer due to its high ET of 2.9 eV and low LUMO energy level of 2.0 eV [25]. As an ETL, 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) was selected owing to its high electron mobility of 1 × 10−3 cm2 V−1s−1 and higher triplet energy level (2.78 eV) compared to FIrpic (2.65 eV) so that it can efficiently transport electrons and prevent excitons from being transferred to ETL [25,26]. Lithium fluoride (LiF) and aluminum (Al) were used as the cathode. The chemical structures of hole transporting materials, schematic energy level diagram for all materials are shown in Fig. 2(a) and 2(b). To investigate the effect of the CRL in the OLEDs, R, G and B devices were fabricated with various structures as shown in Fig. 2(c). Device R1 represents a red sub-pixel for OLEDs with using CRL, and devices G1 and G2 stand for green sub-pixels with and without CRL, respectively. Similarly, devices B1 and B2 denote blue sub-pixels with and without CRL, while device B3 is a control device without TAPC layer (i.e., EBL) as compared to device B1 to show the function of EBL in this CRL structure. The CRL means the layerof HTL:Ir(piq)2(acac) (8 wt%, 15 nm) with one of the HTLs (i.e., Spiro-2CBP, α-NPD, TAPC, BBTC, and CBP). The thickness of HTL in the devices with CRL (i.e., G1 and B1) was reduced to 35 nm by adding the CRL (15 nm) to keep identical with the thickness of HTL without CRL (i.e., G2 and B2).
Fig. 2 (a) Chemical structures of the materials used in the HTL and CRL, (b) the energy level diagram and (c) the device structures of the red, green, and blue OLEDs.
Figure 3(a) and 3(b) show the current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the red devices with different HTLs and hosts. The current densities of R1 with Spiro-2CBP, α-NPD, TAPC and BBTC are higher than that of R1 with CBP at the same voltage. For instance, the current densities of R1 with Spiro-2CBP, α-NPD, TAPC, BBTC, and CBP at 10 V are 549, 407, 470, 492 and 189 mA cm−2, respectively. The turn-on voltage of R1 with Spiro-2CBP, α-NPD, TAPC, BBTC, and CBP is 3.3, 3.2, 3.2, 3.5 and 3.7 V, respectively, which roughly corresponds to the descending order of HOMO energy levels of HTLs. The driving voltage of device R1 with Spiro-2CBP at 1000 cd m−2 is 6.3 V which is the lowest value among those of other devices due to low HOMO energy level of Spiro-2CBP. However, the driving voltage of device R1 with BBTC at 1000 cd m−2 is 6.5 V which is lower than that of devices R1 with α-NPD (6.7 V), TAPC (6.8 V) and CBP (7.6 V) because BBTC is efficient host for red phosphorescence and has lower HOMO energy level than CBP [23]. The device with TAPC shows high luminance at low driving voltage but low luminance at high driving voltage because high hole mobility of TAPC causes accumulation of holes near the EML/ETL interface, resulting in triplet-polaron or triplet-triplet annihilation at high voltage region and resultant rapid decrease of luminance [27–29]. Figure 3(c), 3(d) and Fig. 3(e), 3(f) show the J–V and L–V characteristics of green and blue devices with (devices G1, B1) and without CRLs (devices G2, B2), respectively. For both green and blue devices, the current densities at a fixed voltage were decreased when the CRL was inserted, because the CRL doped with red dye has lower hole transporting property. However, the decrement of current density highly depends on the material we used; for instance, the current density of the device G1-α-NPD and G1-TAPC are almost same with the device G2-α-NPD and G2-TAPC, which are 68 and 71 mA cm−2, 115 and 112 mA cm−2 at 10 V, respectively. In case of using BBTC, the current density of the device G1 (65 mA cm−2) is quite lower than that of the device G2 (111 mA cm−2) at 10 V. A drastic decrease of current density is observed in the device using Spiro-2CBP and CBP as the HTL and CRL. The device G2-Spiro-2CBP and G2-CBP exhibit 206 and 125 mA cm−2 at 10 V, respectively, whereas the device G1-Spiro-2CBP and G1-CBP shows 74 and 15 mA cm−2 at the same voltage, respectively. The blue-emitting devices with and without CRL also show the same trend in J-V characteristics among Spiro-2CBP, α-NPD, TAPC, BBTC and CBP. The current densities of B2 with Spiro-2CBP, α-NPD, TAPC, BBTCand CBP are 223, 62, 112, 103 and 100 mA cm−2 at 10 V, while those of B1 with Spiro-2CBP, α-NPD, TAPC, BBTC and CBP are 67, 67, 124, 63 and 13 mA cm−2 at 10 V, respectively. These results mean that the hole trap level of CRL is a dominant factor of transport property rather than injection barrier between MoO3 and HTLs. The hole trap depth (ΔEHOMO) can be calculated from the difference of the HOMO energy levels between the host and dopant materials. As shown in Fig. 4(a), the hole trap depth of Spiro-2CBP, α-NPD, TAPC, BBTC and CBP are −0.3, 0.1, 0.2, 0.38 and 0.7 eV, respectively. As the trap level becomes deeper, higher energy is required for the trapped carriers to escape the site [30,31].
Fig. 3 Current density–voltage and luminance–voltage characteristics for the (a), (b) red, (c), (d) green, and (e), (f) blue PHOLEDs with and without CRL.
Fig. 4 (a) Schematic energy levels of Ir(piq)2(acac) doped Spiro-2CBP, α-NPD, TAPC, BBTC and CBP with the hole trap depth (ΔEHOMO) calculated from the difference of the HOMO energy levels between the host and dopant materials. (b) Driving voltage changes with and without CRL as a function of ΔEHOMO.
Figure 4(b) shows the driving voltage changes of green (ΔVG) and blue devices (ΔVB) at 1000 cd m−2 by inserting the CRLs with different host materials making different hole trap levels. The increase of driving voltage is negligible in the devices with α-NPD (ΔVG: 0.1 V, ΔVB: 0 V), TAPC (ΔVG: 0 V, ΔVB: 0.1 V) and BBTC (ΔVG: 0.3, ΔVB: 0.3 V), but high in the device with Spiro-2CBP (ΔVG: 0.6, ΔVB: 0.9 V) and CBP (ΔVG: 1.6, ΔVB: 1.7 V) due to the deep trap energy levels. The reason for increased driving voltage of the device with Spiro-2CBP may be due to that the red dopant disturbs the transport of hole in Spiro-2CBP like the 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) in the α-NPD layer [32].
The effects of the depth of trap site on hole transport properties were confirmed with hole only devices (HODs), of which the structure is ITO/MoO3 (10 nm)/HTL (100 nm) or HTL:Ir(piq)2(acac) (8 wt %, 100 nm)/MoO3 (10 nm)/Al (100 nm) with the same HTLs (i.e., Spiro-2CBP, α-NPD, TAPC, BBTC and CBP). As expected, the voltage changes by doping and the voltages for a fixed current density are high in the sequence of trap depth as shown in the J–V characteristics of HODs in Fig. 5. For example, α-NPD and TAPC HODs show similar J-V characteristics, where J-V characteristics of Spiro-2CBP, BBTC and CBP HODs are shifted to higher voltage region by doping Ir(piq)2(acac). Interestingly, the current of the device with α-NPD is slightly increased by doping a red dopant. This may be because the red dopant serves as hole hopping sites and helps hole transport in α-NPD [33].
Fig. 5 Current density–voltage characteristics of the hole only devices with pristine and Ir(piq)2(acac) doped Spiro-2CBP, α-NPD,TAPC, BBTC and CBP. (a)–(e) The driving voltage changes at the same current density by doping are high in Spiro-2CBP and CBP, while those in α-NPD and TAPC are small in accordance trap levels.
Figure 6 shows the external quantum efficiencies (EQEs) of red, green and blue devices with and without CRLs. The maximum EQEs of R1 with Spiro-2CBP, α-NPD, TAPC, BBTC, and CBP in Fig. 6(a) are 10.4, 9.0, 6.5, 10.5 and 9.2%, respectively, which is not as high as the conventional red phosphorescent OLEDs because device R1 has a thin HTL and no EBL. Nevertheless, device R1 with Spiro-2CBP and BBTC exhibit the highest EQE over 10% because Spiro-2CBP and BBTC are good red phosphorescent host materials as reported in the previous paper [23,34]. Figure 6(b) and 6(c) show the EQEs of green and blue devices with and without CRL with different HTLs and hosts, respectively. All devices exhibit high EQEs based on the phosphorescent emitters; the highest EQEs of both device B1 and B2 with BBTC are 18.6% and 18.7% at ~0.05 mA cm−2, corresponding to 29.3 cd A−1 and 29.9 cd A−1 in luminous current efficiency (LCE), respectively. Devices B1 and B2 with Spiro-2CBP, α-NPD, TAPC and CBP show slightly lower EQEs, but still higher than 10% in EQE. Green devices also show high EQEs around 12.3–13.7% at 1000 cd m−2 regardless of HTLs and CRL. This result shows that the insertion of CRL does not severely affect the efficiency of devices. Besides, the devices with CRL exhibit higher efficiencies compared with the devices without CRL in case of using BBTC. We attribute this improvement with CRL to the enhanced electron-hole balance. Because the hole mobility of BBTC is higher than that of CBP which possesses the hole mobility of ~10−3 cm2 V−1s−1 [23,24], we can know that the hole conductivity of them may be higher than electron conductivity of TmPyPB which has the electron mobility of ~10−3 cm2 V−1s−1 [26], meaning that hole density is higher than electron density in those devices. However, the thin CRL slightly reduces the hole mobility in the devices due to the trap sites caused by doping the red dye as mentioned, resulting in the improved electron-hole balance and higher efficiency [32]. The entire performances of all devices at a practical brightness (at 1000 cd m−2) are summarized in Table 1. Considering both efficiency and voltage, we can say that BBTC is the most appropriate material as a CRL.
Fig. 6 (a) External quantum efficiencies of (a) red, (b) green, and (c) blue PHOLEDs with and without CRL.
Table 1. Performances of devices with different CRLs.
Figure 7 shows the normalized EL spectra of red, green, and blue devices with BBTC measured at various voltages from 4 V to 8 V. As shown in Fig. 7, in each color of emission, there is no change in their EL peaks (at 632 nm for red, 520 nm for green and 470 nm for blue devices) according to the driving voltages. Also, the spectral shapes of device G1 and B1 are nearly same with those of device G2 and B2, respectively, meaning that the insertion of CRL does not affect the exciton recombination and emission zone of the devices. Furthermore, in devices G1 and B1, we can hardly observe the red emission originating from CRL. In other words, the color coordinates of devices G1, (0.31, 0.64), is slightly changed from that of G2, (0.32, 0.63), while those of B1 and B2 are almost same as (0.14, 0.29). The CIE coordinate of device R1 measures (0.69, 0.31). Because the color gamut is also one of the important factors for display devices, we compare the color gamut of the devices with and without CRL. In Fig. 7(d), the triangle with solid line displays the color gamut of the conventional OLED structure made by devices R1, G2, and B2, while that with dashed line means the color gamut of the OLED in the CRL structure made by devices R1, G1, and B1. The area of the device with CRL can represent 57% of the National Television Standards Committee (NTSC) 1953 colorgamut, which is only 1% lower than the area of the device without CRL. Therefore, we believe that the devices with CRL have enough possibility for the fabrication of full-color OLEDs. In case of device B3, it exhibits red emission from the CRL as well as blue emission as shown in Fig. 7(c). Also, device B3 has lower efficiency compared to other blue-emitting devices, because the excitons in blue-emitting layer transfer their energy to the adjacent red emitters or they are quenched without EBL. These results indicate that the EBL takes an important role in the OLEDs using CRL for both color purity and efficiency.
Fig. 7 EL spectra at different driving voltages (4–8 V) of the devices using BBTC in the structure of (a) R1, (b) G1 and G2, (c) B1, B2, and B3 (B3 at 7 V), and (d) their CIE color coordinates at 8 V and the comparison of color gamut with and without CRL.
We also investigated the effect of the doping concentration in the CRL on the performance of the G/B pixels as well as the R pixel. Because the CRL fulfills the roles of the HTL in the G/B pixels, the doping ratio changes the trap density of the HTL. Thus, we fabricated RGB devices with the CRL possessing a low doping ratio of Ir(piq)2(acac) (3 wt%) in BBTC. As shown in Fig. 8, the current density is slightly increased in all the devices with the 3 wt% CRL compared to the doping ratio of 8 wt%. It is attributed to the reduced trap density in the HTL due to the lowered doping ratio in the CRL. Meanwhile, the EQEs of the devices with the 3 wt% CRL were kept similar with those of the devices with the 8 wt% CRL. In detail, the EQEs of the red and blue devices were decreased by less than 5% in the entire current density range, and even the green device exhibited slightly increased efficiency due to improved hole injection. The EL specta of the devices with the lower doping ratio in the CRL were not changed significantly compared with the original devices. It is mainly attributed to the thin CRL layer which has a high hole mobility as well as a good performance as a red host material. Although the driving voltages of the RGB devices for 1000 cd m−2 are similar as shown in Table 1, the R pixel shows much higher current density compared to G/B pixels regardless of the doping ratios due to the thin total thickness of the R pixel compared to the G/B pixels. But this large difference in driving condition can be improved by more precise optimization of the thickness, doping ratio, and materials. Varying the pixel sizes is also one good solution for the balanced driving condition of full-color AMOLEDs.
Fig. 8 Comparison of the doping ratio of the CRL in terms of (a) the current density–voltage and (b) the external quantum efficiency–current density characteristics.
4. Conclusion
In conclusion, we demonstrated full-color (RGB) phosphorescent OLEDs by using a common red dye-doped layer in the HTL, which can simplify the conventional OLED fabrication process by reducing one FMM step. We utilized and compared five different host materials possessing high hole mobility and different HOMO energy levels for the CRLs, which were Spiro-2CBP, α-NPD, TAPC, BBTC and CBP. The hole trap depth made by the energy level difference between the HOMO energy levels of each host material and a red dye was an important factor for the device performances. The increase of driving voltage by inserting the CRL was negligible in the devices with α-NPD, TAPC and BBTC, but high in the device with Spiro-2CBP and CBP due to the deep trap energy level for hole carriers. For all devices, the efficiencies and the EL spectra were not significantly changed by inserting the CRL and even slightly increased in case of using BBTC compared with the devices without CRL due to improved electron–hole balance. We found that BBTC was the most appropriate material for the CRL structure with good efficiency and low voltage increase, while the device with TAPC showed low efficiency in red pixel and the device with CBP required high driving voltage. The difference in the NTSC 1953 color gamut between the device with and without CRL was only 1%. From above results, we believe that the host material for the CRL should have high hole mobility, bipolar transport property, low HOMO energy level difference between the host and the red dopat for high performance device with the CRL. The CRL architecture presented here bring us one step closer to fabricating highly efficient full-color OLEDs in a more efficient way.
Acknowledgments
This work was partly supported by the ICT R&D program of MSIP/IITP[10041416, The core technology development of light and space adaptable energy-saving I/O platform for future advertising service]. This work was also supported by the Industrial strategic technology development program funded by the Ministry of Trade, Industry and Energy of Korea (10041556), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2011-0022716) funded by the Ministry of Education.
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