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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We studied the effect of direct charge trapping at different doping concentrations on the device performance in tris(8-hydroxyquinoline) aluminum (Alq3):10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T) as a host-dopant system of a fluorescent organic light-emitting diode. With increasing C545T doping concentration, trap sites could lead to the promotion of hole injection and the suppression of electron injection due to the electron-transport character of Alq3 host for each carriers, as confirmed by hole- and electron-only devices. Direct charge injection of hole carriers from the hole transport layer into C545T dopants and the charge trapping of electron carriers are the dominant processes to improve the charge balance and the corresponding efficiency. The shift of the electroluminescence (EL) spectra from 519 nm to 530 nm was confirmed the exciton formation route from Förster energy transfer of host-dopant system to direct charge trapping of dopant-only emitting systems. Variation in the doping concentration dictates the role of the dopant in the fluorescent host-dopant system. Even though concentration quenching in fluorescent dopants is unavoidable, relatively heavy doping is necessary to improve the charge balance and efficiency and to investigate the relationship between direct charge trapping and device performance. Heavy doping at a doping ratio of 6% also generates heavy exciton quenching and excimer exciton, because of the excitons being close enough and dipole-dipole interactions. The optimum device performance was achieved with a 4%-doped device, retaining the high efficiency of 12.5 cd/A from 100 cd/m(2) up to 15,000 cd/m(2).

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The effects of location and density in triplet exciton formation on the device performance were systematically investigated by the variation of quantum well (QW) structure in blue phosphorescent organic light-emitting diodes. The hole transporting material of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as an interlayer was sandwiched between front emitting layer (EML) and rear EML for the construction of QW structure. The variation of the number of the QW unit induces the different exciton formation in recombination zone (RZ) in which the change of exciton density in the QW structure gives rise to the different optical path length and micro-cavity effect of the EL spectra. As the number of the QW unit increases, the corresponding EL spectrum is blue-shifted. It is originated that the exciton density in the front EML increases and the corresponding optical path length is shorten from the EML to the anode. However, the RZ formation in the single EML without QW structure is widened and red-shifted with increasing the EML thickness because the width of the RZ increases toward to the cathode and the length of optical path is expanded from the anode. In addition, the device having 2 units of QW structure in the EML shows the highest device performance of current efficiency, due to the good charge balance and avoiding the serious triplet exciton quenching by the interlayer. The current efficiency of 11.7 cd/A (from 11.7 to 7.3), power efficiency of 5.5 lm/W (from 5.5 to 3.4), and the operation voltage of 9.7 V at 5000 cd/m2 were presented for 2 unit QW, while the current efficiency of 9.4 cd/A (from 9.4 to 4.7), power efficiency of 5.9 lm/W (from 5.9 to 1.6) and the operation voltage of 7.1 V at 5000 cd/m2 were suggested for 1 unit QW, both showing relatively large efficiency roll-off.