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美国科学家利用三层光发光层(3-EML)结构,让白光有机发光二极管(white organic light-emitting diode, WOLED)的发光效率大幅提升。相较于先前发表的各种WOLED,这种3-EML WOLED在照度(luminance)为1000 cd/m2时的最大发光效率为64±3 lm/W,演色系数(CRI)高达81,而典型WOLED的演色系数大多介于75到80之间。
普林斯顿大学的Yiru Sun指出,上述组件除了具有目前最高的发光效率之一,更重要的是也有很好的CRI,因此可以得到高品质的白光。CRI值反应了物体在光源照射下呈现的色 彩真实度。Sun表示,发展高CRI值的光源对室内照明应用非常重要,CRI值大于70就可视为近似阳光的高品质白光。
电致发光的WOLED通常只含有一层或双层发光层,电子与电洞组成的激子(exciton)便是在此形成。当光层中的激子累积时,往往会发生另科 学家头痛的「三重态-三重态消灭」(triplet-triplet annihilation)现象,即两个三重态激子产生交互作用,其中一个会释出能量,掉到基态,导致组件光效率降低。
Sun发现采用3-EML结构,三个发光层分别发出红、蓝及绿光,由于激子的产生分布会在整个发光层中,因此可以降低激子的局部态密度并抑制三重 态消灭效应。再加上选用与RGB三个主发光体(host)能阶匹配的磷光?质,因此获得到最佳的发光效率与演色性。3-EML结构的另一项优点是可以藉由 改变发光层厚度与?杂浓度,来调整色彩强度,产生较高品质的白光。
该研究小组接着将对组件的使用寿命进行测试,因为这是OLED面临的一大议题。孙表示,他们将研究更多能够应用在此三层发光层结构的?质与主发光 体材料,以求获得更高的发光效率及更长的使用寿命。详见Applied Physics Letters 91, 263503 (2007)。
Efficient white OLED emits clearly
High quality light has been produced by a white OLED using a novel three emission layer structure.
A white OLED (WOLED) boasting a higher quantum efficiency, power efficiency and colour rendering index. Princeton University, Yiru Sun
Left to right shows three 3x3 mm WOLEDs in an array running with low to high current density. Three dopants are incorporated for red, green and blue. The white colour CRI remains the same when current density changes.
Image Credit: Yiru Sun
A white OLED (WOLED) boasting a higher quantum efficiency, power efficiency and colour rendering index compared with previous devices has been unveiled. The US team used a three emission layer (EML) structure to obtain a peak power efficiency of 64±3 lm/W at a luminance of 1000 cd/m2 and CRI of 81. Typical CRI values for WOLEDs are between 75-80. (Applied Physics Letters 91 263503)
"The power efficiency we obtained is one of the highest values among WOLEDs reported so far," Yiru Sun, a researcher at Princeton University, told optics.org. "What's more, we have made no compromise in CRI for high power efficiency as we have also achieved a good quality of white colour balance."
The CRI value indicates the ability of a light source to reproduce the colours of an object it illuminates. "It is important to develop light sources with high CRI values particularly for indoor lighting applications," explained Sun. "A CRI value larger than 70 is considered to have a good quality of white light, similar to sunlight."
Until now, electrophosphorescent WOLEDs have been fabricated with only one or two emission layers, in which excitons are formed. The crucial hurdle facing scientists is triplet-triplet annihilation, which can occur at the emission layer from a pileup of excitons. During the process one triplet exciton is destroyed, reducing the efficiency of the device.
Sun found that introducing a 3-EML structure lowered the local density of excitons and reduced the exciton annihilation effect. What's more, she found that combining the 3-EML structure with carefully selected phosphorescent dopants allowed optimum efficiency and colour rendition to be achieved.
A white OLED (WOLED) boasting a higher quantum efficiency, power efficiency and colour rendering index. Princeton University, Yiru Sun
Yiru Sun
"The three sections of the EML are for red, blue and green (RGB) emission separately. Excitons are generated in multiple locations across the full width of the EML which reduces triplet-triplet annihilation," explained Sun. "We chose each of the three RGB dopants so as to have favourable energy alignment with the three hosts. By achieving optimum efficiency for all three colours we enhanced the efficiency of the whole device."
The 3-EML structure offers the additional advantage of allowing the colour intensity to be adjusted. "We can produce a balanced white colour by changing the thickness and doping concentration of the RGB emission region," added Sun.
Although lifetimes of the device were not measured, the team recognizes that this is one of the major issues facing OLEDs and plans to address this in the future. "We will investigate more advanced dopant/ host materials that can be incorporated into this structure to achieve higher efficiency and longer lifetime," concluded Sun.
This research was carried out under the supervision of lead researcher Stephen Forrest from the University of Michigan, US. |
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