OLEDp - 2
نویسندگان
چکیده
A new carbazole-containing polyacetylene (PA) was used as a host material in the construction of red phosphorescent LEDs with a configuration of ITO/PEDOT:PSS/host:btp2Ir(acac)/BCP/Alq3/LiF/Al. Compared with PVK as a host in the same device, The PA as a host has enhanced the quantum efficiency up to 1.4 fold. The efficiency of the device can be further improved 4.5 fold by using a BAlq layer instead of BCP/Alq3 layers, and external quantum efficiency of 4.1% photons/electron has been achieved. INTRODUCTION Recently, the use of phosphorescent dyes has been popular in organic light-emitting devices (OLEDs) to overcome the fluorescent efficiency limit (25%) imposed by the formation of triplet excitons [1]. Phosphorescence in these dyes is strong due to the incorporation of a heavy metal atom with strong spin-orbit coupling which mixes singlet and triplet states. As the result, the internal quantum efficiency can theoretically reach 100% for such phosphorescent dye doped devices. Although an effective external quantum efficiency of 19% photons/electron and a luminance power efficiency of 70 L/W have been demonstrated in OLEDs based on small molecules [2,3], the results in polymers have been not as encouraging. Polymers have an important advantage over their small-molecule counterparts in that they can be solution processed [4]. This potentially allows a lower cost of production using room temperature spin-coating, screen printing or ink-jet printing techniques. However, there are only a few reports on utilizing polymer as host matrix in electrophosphorescent LEDs. So far most work has been done with poly(vinylcarbazole) (PVK) [5−8] and poly(fluorene) (PF) derivatives [9−11] as hosts. Recently, Gong et al [5] and Xie et al [6] reported external quantum efficiency of 10% from green-emitting Ir(PPy)3-doped PVK devices. This important result demonstrated that phosphorescent dye-doped polymer host could provide very high efficiency as in the case of small molecule as well. For red-emitting phosphorescent polymer LEDs, the host studied is mainly focused on PVK.[7-8] By doping btp2Ir(acac) into PVK, Chen et al [8] achieved an external quantum efficiency of 3.3%, which was clearly less than the results on green-emitting devices. The probable reason is the generally low absorption of btp2Ir(acac) molecule in the blue spectral region of PVK. Therefore, in order to realize full color display based on phosphorescent polymer LEDs, it is very important to develop new polymer host materials which can give efficient energy transfer from the host to the red dye. In this paper, a new carbazole-containing polyacetylene (PA; Fig. 1) is introduced as the host instead of PVK. Using the same device configuration, we show that the quantum efficiency can be increase by up to 1.4 times. Since the performance of an OLED is sensitive to both the arrangement and the choice of the constituent organic layers, further improvement is quite possible. In this work, we modify the device configuration by using a BAlq layer (Fig. 1) instead of BCP/Alq3 layers, which resulted in a great improvement in its EL performances. When the thickness of a BAlq reach its optimal value, a high external quantum efficiency of 4.1% ph/el can be obtained. OLEDp 2
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