Highly efficient blue organic light-emitting diodes using quantum well-like multiple emissive layer structure

In this study, the properties of blue organic light-emitting diodes (OLEDs), employing quantum well-like structure (QWS) that includes four different blue emissive materials of 4,4′-bis(2,2′-diphenylyinyl)-1,1′-biphenyl (DPVBi), 9,10-di(naphth-2-yl)anthracene (ADN), 2-(N,N-diphenyl-amino)-6-[4-(N,N-diphenyl amine)styryl]naphthalene (DPASN), and bis(2-methyl-8-quinolinolate)-4-(phenyl phenolato) aluminum (BAlq), were investigated. Conventional QWS blue OLEDs composed of multiple emissive layers and charge blocking layer with lower highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy level, and devices with triple emissive layers for more significant hole-electron recombination and a wider region for exciton generation were designed. The properties of triple emissive layered blue OLEDs with the structure of indium tin oxide (ITO) /N,N′-diphenyl-N,N′-bis(1-naphthyl-phenyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) (700 Ǻ)/X (100 Ǻ)/BAlq (100 Ǻ)/X (100 Ǻ)/4,7-diphenyl-1,10-phenanthroline (Bphen) (300 Ǻ)/lithium quinolate (Liq) (20 Ǻ)/aluminum (Al) (1,200 Ǻ) (X = DPVBi, ADN, DPASN) were examined. HOMO-LUMO energy levels of DPVBi, ADN, DPASN, and BAlq are 2.8 to 5.9, 2.6 to 5.6, 2.3 to 5.2, and 2.9 to 5.9 eV, respectively. The OLEDs with DPASN/BAlq/DPASN QWS with maximum luminous efficiency of 5.32 cd/A was achieved at 3.5 V.


Background
Since the report by Tang and VanSlyke on organic lightemitting diodes (OLEDs), [1,2] OLEDs have become a popular research subject due to its several technical advantages such as reduced power consumption, compatibility with flexible substrates, high color rendering index, high contrast, and wide viewing angle. OLEDs have emerged as strong candidates for next-generation flat panel displays and solid-state lighting sources [3][4][5][6]. Many progresses have been made in improving the performance of OLEDs, including high power efficiency tandem organic light-emitting diodes based on bulk heterojunction organic bipolar charge generation layer [7]. However, improving the performance of blue OLEDs still remains as an open challenge [8][9][10]. Various methods have been developed to optimize blue OLED's performance. Such methods include replacing emitters from fluorescent to phosphorescent materials [11], including balancing the carrier ratio in the emissive layer (EML) [12], designing a better surface texture for improving external quantum efficiency [13], and reduced efficiency roll-off in OLEDs at ultrahigh current densities by suppression of triplet-polaron quenching [14].
Among various methods for enhanced efficiency, the QWS has proved to be an effective approach for high device performance [15,16], by confining charge carriers and exciton within the multi-emitting layer. Thus, the charge carrier recombination efficiency and exciton formation probability can be beneficially enhanced [17]. The organic molecules were insufficiently restricted by Van der Waals force among molecules in the organic quantum well. The main features of QWS were high electroluminescence (EL) efficiency [18], tunable EL zone [19], and great carrier balance [20][21][22][23].

Experiment
Indium tin oxide (ITO)-coated glass was cleaned in ultrasonic bath by regular sequences: in acetone, methanol, diluted water, and isopropyl alcohol. Hereafter, pre-  Figure 1 shows the molecular structures of the different blue chromophores used in the OLED devices. We fabricated two types of blue OLEDs. The first type has a conventional device structure of ITO/NPB/DPVBi or ADN or DPASN/BAlq/Bphen/Liq/Al, where ITO, NPB, DPVBi (or ADN or DPASN), and Al are the anode, hole transporting layer, emissive layer, electron transporting layer, and cathode, respectively. The other type of blue OLEDs with a structure of ITO/NPB/DPVBi or ADN or DPASN/BAlq/DPVBi or ADN or DPASN/Bphen/Liq/Al was also fabricated for comparison studies. A list of OLEDs with different layer structures is summarized in Table 1.
With various DC voltage bias, the optical and electrical properties of blue OLEDs such as the current density, luminance, power efficiency, luminous efficiency, Commission   Results and discussion Figure 2a shows the current density-voltage characteristics measured for each conventional blue OLED devices. Device C has the highest current density, and its EML consists of DPASN which is a p-type emitting material with a higher hole mobility; device B, which also had a p-type material, shows the second highest current density device. However, device D with an n-type emitting material of BAlq shows the lowest current density. Consequently, we realized that a p-type semiconductor has more electron affinity than a n-type semiconductor [24]. Figure 2b shows the current density-voltage characteristics measured for each QWS triple emissive layer blue OLED device. Devices E, F, and G actually have similar current density characteristic, and this phenomenon is caused by different charge injection barriers between emitting materials. The energy band diagrams of devices A to G are shown in Figure 3. Although it is not so significant to compare other QWS blue OLED devices, the device G including DPASN shows the highest current density at 8 V because the hole and electron injection barriers of device G were higher than those of another  devices. As a result, the charge flow of charge injection barriers are interrupted, in turn decreasing its current density. Current densities of QWS blue OLED devices E, F, and G were lower than that of conventional OLED devices A, B, and C because electrons and holes are confined in the QWS which could possibly inhibit the current flow in EML. Figure 4a,b shows the luminance-voltage (L-V) characteristics of conventional blue OLEDs and QWS multiemissive layer blue OLEDs. Conventional blue OLEDs have higher luminance than QWS blue OLEDs. This is because QWS blue OLEDs consist of p-type emissive materials of DPVBi, ADN, and DPASN, and n-type emissive material of BAlq together. Consequently, n-type emissive materials influence on luminance much more than p-type emissive material although p-type emissive materials tend to have a higher luminance characteristic. Table 2 summarizes the luminance of each blue OLED device measured at 5, 6, and 7 V. Figure 5a,b shows the plot of luminous efficiency versus current density of conventional blue OLED device and QWS multi-emissive blue OLED devices. Table 3 summarizes the luminous efficiency of each device ranging from 50 to 150 mA/cm 2 . Luminous efficiency of QWS blue OLED devices is higher than that of conventional OLED devices. This phenomenon caused by emissive region of QWS OLED was evenly formed by DPVBi, ADN, DPASN, and BAlq. As a result, the power efficiency was enhanced because ADN and BAlq were emissive at different wavelengths. However, devices including DPASN show remarkable enhancement of efficiency. This can be explained by depth of QWS according to the difference of HOMO-LUMO energy level of emissive materials. HOMO and LUMO difference of DPVBi, BAlq, AND, and BAlq was 0, 0.1, 0.3, and 0.3 eV, respectively. This HOMO and LUMO level difference is not enough to confine charges and excitons in the emissive layer. Therefore, it was not enough to enhance luminous efficiency of OLED devices. However, when DPASN was used, luminous efficiency remarkably improved because HOMO and LUMO level difference of QWS OLED device was 0.7 and 0.6 eV between DPASN and BAlq. Therefore, QWS OLED devices need enough intermolecular HOMO and LUMO level difference of more than at least 0.3 eV.
The depth of QWS according to the difference of HOMO-LUMO energy level of emissive materials was concerned with the performance of the OLED. It is shown that the performance of OLEDs changes according to the depth of QWS ( Figure 6). Figure 6 shows the plot of external quantum efficiency (EQE) as a function of current density for conventional OLEDs and QWS OLEDs. EQEs of OLED devices measured at 100 mA/cm 2 were 2.71%, 2.21%, 1.99%, 1.75%, 2.53%, 1.81%, and 2.76%, respectively. QWS OLEDs having DPASN demonstrated a 38% enhancement in EQE. However, if QWS OLED devices include ADN and DPVBi, the EQE did not change or   Figure 7 shows the EL spectra of conventional OLED devices and QWS OLED devices at 5 V. We know that if QWS OLED devices include ADN and DPVBi, the full width at half maximum (FWHM) of EL spectra was increased. We can observe this result in Figure 7a,b. As the result demonstrates, the emission region formed at BAlq. However, when using DPASN at QWS OLED, the EL spectra remained almost the same.

Conclusions
Blue OLED with triple emissive layer structure achieved luminous efficiency of 5.23 cd/A at 3.5 V, which is 36% higher than that of the conventional blue OLEDs. Obviously, the quantum well-like structure is favorable for hole-electron recombination for efficient exciton generation in the multiple emissive layers of DPVBi, ADN, and DPASN with BAlq in the device. There was no significant improvement in the luminous efficiency (only about 3% and 4%) when DPVBi and ADN were used as the additional emitting layer to form a quantum well-like structure; a 36% improvement in luminous efficiency was realized in DPASN/BAlg/DPASN blue OLEDs. This result shows that blue OLEDs can only improve luminous efficiency under proper difference in HOMO and LUMO energy level between the central and surrounding emitting layers. The effect of layer thickness and combination of different emissive layers on charge   Figure 6 External quantum efficiency-current density characteristics measured for conventional OLEDs and QWS OLEDs.
carrier transport mechanism from the quantum well-like and the blue emitting layer based on space charge limited current will be further examined.