Highly efficient blue organic light-emitting diodes using quantum well-like multiple emissive layer structure
© Yoon et al.; licensee Springer. 2014
Received: 7 February 2014
Accepted: 2 April 2014
Published: 24 April 2014
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.
Since the report by Tang and VanSlyke on organic light-emitting 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[3–6]. 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. However, improving the performance of blue OLEDs still remains as an open challenge[8–10]. Various methods have been developed to optimize blue OLED's performance. Such methods include replacing emitters from fluorescent to phosphorescent materials, including balancing the carrier ratio in the emissive layer (EML), designing a better surface texture for improving external quantum efficiency, and reduced efficiency roll-off in OLEDs at ultrahigh current densities by suppression of triplet-polaron quenching.
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. 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, tunable EL zone, and great carrier balance[20–23].
In this study, the performance of blue OLEDs with multiple emissive layers 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) was investigated. These emissive materials have different highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy levels. Emissive layers with different orders in the QWS-type OLEDs were investigated and optimized to achieve the best device performances. Luminous efficiency and I-V-L characteristics were observed considering the effects of QWS and the variation of recombination region in EML.
Indium tin oxide (ITO)-coated glass was cleaned in ultrasonic bath by regular sequences: in acetone, methanol, diluted water, and isopropyl alcohol. Hereafter, pre-cleaned ITO was treated by O2 plasma under condition of 2 × 10-2 Torr and 125 W for 2 min. Blue OLEDs were fabricated using the high vacuum (1.0 × 10-6 Torr) thermal evaporation and N,N′-diphenyl-N,N′-bis(1-naphthyl-phenyl)-(1,1′-biphenyl)-4,4′-diamine (NPB), BAlq, DPVBi, ADN, DPASN, 4,7-diphenyl-1,10-phenanthroline (Bphen), lithium quinolate (Liq), and aluminum (Al) were deposited at different evaporation rates of 1.0, 0.5, 0.5, 0.5, 0.5, 1.0, 0.1, 5.0 Ǻ/s.
Layer structures of OLED devices A, B, C, and D
ITO (1,800 Ǻ)/NPB (700 Ǻ)/DPVBi (300 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Ǻ)
ITO (1,800 Ǻ)/NPB (700 Ǻ)/ADN (300 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Å)
ITO (1,800 Ǻ)/NPB (700 Ǻ)/DPASN (300 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Ǻ)
ITO (1,800 Ǻ)/NPB (700 Ǻ)/BAlq (300 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Ǻ)
ITO (1,800 Ǻ)/NPB (700 Ǻ)/DPVBi (100 Ǻ)/BAlq (100 Ǻ)/DPVBi (100 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Ǻ)
ITO (1,800 Ǻ)/NPB (700 Ǻ)/ADN (100 Ǻ)/BAlq (100 Ǻ)/ADN (100 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Ǻ)
ITO (1,800 Ǻ)/NPB (700 Ǻ)/DPASN (100 Ǻ)/BAlq (100 Ǻ)/DPASN (100 Ǻ)/Bphen (300 Ǻ)/Liq (20 Ǻ)/Al (1,200 Ǻ)
With various DC voltage bias, the optical and electrical properties of blue OLEDs such as the current density, luminance, power efficiency, luminous efficiency, Commission Internationale deL'eclairage (CIExy) coordinates, and electroluminescence spectra were measured with Keithley 238 (Seoul, Korea), LMS PR-650 spectrophotometer and colorimeter (Photo Research Inc., CA, USA) and the IVL system (LMS Inc., Gyeonggi-do, Korea).
Results and discussion
Luminance of OLED devices measured at 5 to 7 V
Luminous efficiency of OLED devices measured at different current densities of 50 to 150 mA/cm 2
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 carrier transport mechanism from the quantum well-like and the blue emitting layer based on space charge limited current will be further examined.
This work was supported by the Nano-Convergence Foundation (Project Number: B-0030016) funded by the Ministry of Education, Science and Technology (MEST, Korea) and the Ministry of Knowledge Economy (MKE, Korea).
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