The mechanisms of charge balance and tuning of the color coordinates in blue phosphorescent organic light-emitting diodes were studied by controlling the emission zone and electroluminescence (EL) spectrum. On the basis of the findings, this paper discusses different hole-injection and mobility factors such as the thickness of the hole injection layer (HIL), the HIL materials, the surface treatment, and the insertion of a buffer layer, which mainly enabled tuning of the color coordinates. The approximately 500-nmwavelength shoulder peak in the EL spectrum was influenced by the width and location of the emission zone, following the optical micro-cavity effect. Different emission zones were created by different types of hole injection and mobility, which simultaneously enhanced the device efficiency and tuning of the color coordinates.Although organic light-emitting diodes (OLEDs) 1-12 have attracted considerable attention owing to their low cost, simple process, and high efficiency, the application of blue OLEDs 1-7 has been limited by their poor long-term stability and short lifetime. Much effort has been devoted to developing novel materials and device architectures for blue phosphorescent OLEDs (PHOLEDs), 1-7 which are mainly used to meet the demands for high-efficiency and long-term stability in flatpanel displays and not necessarily because of their color coordinates and gamut. There have been many studies on methods of controlling the color coordinates using a tandem structure 10 and the micro-cavity effect in top-emission OLEDs 13 utilizing semi-transparent electrodes. However, very few studies have used blue PHOLEDs for color coordinate control.Moreover, there has been a tendency toward a trade-off between the efficiency and the color coordinates. Hsu et al. found that top-emitting, deep-blue-color OLEDs for Commission Internationale de l'Eclairage (CIE) 8,10 color coordinates (0.135, 0.056) exhibited a current efficiency of 1.5 cd/A, while an OLED for (0.132, 0.139) exhibited an efficiency of 3.8 cd/A. 13 High-efficiency devices using blue OLEDs have poor color coordinates compared to the National Television System Committee (NTSC) color coordinates (0.14, 0.08). It is actually not easy to simultaneously improve the device efficiency and color coordinates. Chen et al. have suggested that a higher hole mobility through emission layer (EML) resulting in an electroluminescence (EL) intensity change, color tuning, and a wider recombination zone was responsible for a longer device lifetime in blue OLEDs. 8 Nevertheless, to the best of our knowledge, only a few previous studies 2,3 on blue PHOLEDs using iridium -(III) bis-[(4,6-difluorophenyl)-pyridinato-N,C 2 ] picolinate (FIrpic) as a dopant considered the color coordinate performance.Here, we carried out a detailed investigation of how the performance of the device and the color coordinates were affected by changing the different types of hole injection and by the mobility of holeinjection-layer (HIL). Adjusted emission zones improved the device efficiency a...

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The effects of the electron mobilities and energy levels of different electron transport layer (ETL) materials on the performances were systemically investigated in blue phosphorescent organic light-emitting diodes. The spatial control of recombination zone (RZ) which was accompanied with triplet exciton quenching affected the balance between holes and electrons in the emission layer, resulting in the variations of the device performances. An optical micro-cavity effect in the electroluminescence (EL) spectrum around 500 nm was noticed by employing tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) ETL. This was attributed to the broadening of the emission zone through the emission layer over the ETL, exhibiting the greenish color coordinates. The current efficiency of the device with 3-phenyl-4(10-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) ETL was much higher than that of the same structured device with any other ETL due to better charge balance as well as the suppression of triplet exciton quenching by the narrow RZ with low electron mobility and proper band alignment.

The charge balance mechanism in green fluorescent organic light-emitting diodes is investigated for different electron transport layers (ETLs) and electron mobilities. Carrier accumulation and an increase in the exciton recombination probability are shown to be critical for improving the current and power efficiencies by aligning the bands at the interface between the emitting layer (EML) and ETL. The peak in the electroluminescence (EL) spectra was found to shift slightly in response to changes in the width of the emission zone and reflected the electron mobility of the ETL. Higher electron mobility resulted in a wider recombination zone in the EML that was manifested by a blue-shift of the EL peak.Organic light-emitting diodes (OLEDs) 1-14 have the potential to be employed in applications such as flat panel displays, flexible electronics, and solid-state lighting. Considerable research has been devoted to uncovering exact OLED operation mechanisms and precisely determining their electrical and optical properties. The charge balance 1-14 is considered to be a key factor for controlling device performance, stability, and color coordinates; in a heterojunction device structure, holes in the hole transport layer (HTL) 1 are regarded as being transported too quickly, while electrons in the electron transport layer (ETL) 2 are transported too slowly, and as a result, holes simply pass through without generating excitons with the electrons in the emission layer (EML), which manifests as a low current efficiency.To overcome this discordance between the hole and electrons mobilities and to improve the carrier charge balance, many researchers have investigated novel structures such as a metal oxide buffer layer, 3,4 electron or hole blocking layers, and new materials for the ETL 5,6 that have a high electron mobility. In the early stages of OLED heterostructure development, tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) was used for both the EML and ETL in a single device. 2 Other materials that have been introduced for the ETL include 3-phenyl-4(10-naphthyl)-5-phenyl-1,2,4-triazole (TAZ), 5,7 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BPhen), 1,5,6 and 2-[3,5-bis(1-phenylbenzimidazol-2-yl)phenyl]-1-phenylbenzimidazole (TPBi) 5,7,8 in an effort to enhance the exiton recombination and current efficiencies by producing a higher electron mobility and adequate band alignment. The electron mobility also has the potential to control the exciton recombinationzone width in the EML. 9 Tang and Chen et al. 9 have shown that a higher hole mobility through the EML resulted in a wider recombination zone, which was beneficial for a longer device lifetime, and that the electroluminescence (EL) peak shifts when the width of the recombination zone changes. Both the higher hole mobility in the EML and the larger emission zone generate an additional contribution to the EL peak from adjacent layers such as the HTL and ETL. 9 In other work, Meerholz et al. 12 adjusted the width of the emission zone in the EML by copolymerization of the HT...

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