Enhanced properties of organic electroluminescent devices with cesium chloride ultra-thin layer

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• 作者：Zhaoyue L&uuml ; ^a ; ^b ; Zhenbo Deng^b ; Ying Hou^a ; Xiuli Zhang^a ; Haisheng Xu^a ; ^c ; ^{hsxu@ecust.edu.cn}
• 关键词：Cesium chloride ; Electron injection ; Trap site ; Hole blocking ; Carrier balance
• 刊名：Displays
• 出版年：2013
• 出版时间：January, 2013
• 年：2013
• 卷：34
• 期：1
• 页码：69-74
• 全文大小：1276 K

文摘

The effect of various positions and thicknesses of cesium chloride (CsCl) layer in organic light-emitting diodes is investigated. The basic device configuration is indium-tin-oxide (ITO)/N,N¡ä-bis-(1-naphthl)-diphenyl-1,1¡ä-biphenyl-4,4¡ä-diamine (NPB)/tris (8-hydroxyquinoline) aluminum (Alq₃)/aluminum (Al). The following results are demonstrated. When the CsCl layer is placed at the Alq₃/Al interface and closer to the Al cathode inside Alq₃ layer, the effective barrier to electron injection is reduced due to the reaction between Cs and Al. As a result, the injection of electrons and holes is more balanced and the efficiency is enhanced from 1.83 to 3.49 cd/A. And as an electron injection layer, CsCl can be comparable with lithium fluoride (LiF). If the distance of the CsCl layer is more than 20 nm from Alq₃/Al interface inside Alq₃ layer, the luminance and efficiency are enhanced but the injection is not increased, which is attributed to trap sites induced by the CsCl layer. The decrease of luminance and efficiency is observed when the CsCl layer is located at the NPB/Alq₃ interface. The reason is exciton quenching due to charge transfer from metal Cs to organic material Alq₃. The CsCl layer inside NPB layer plays a role of hole blocking. More balanced injection of carriers is expected. Subsequently, both luminance and efficiency are enhanced by 34 % and 60 % , respectively. When the cathode of lithium fluoride (LiF)/Al is used, the device efficiency is also improved from 4.0 to 5.0 cd/A by inserting the CsCl layer inside NPB layer. However, the improvement is not observed for the device with the CsCl layer inserted inside Alq₃ layer in LiF/Al-OLED.