发光学报, 2009, 30 (2): 162, 网络出版: 2009-12-30   

CBP有机薄膜对MEH-PPV聚合物发光二极管性能的影响

Performance Improvement of Polymer Light-emitting Diodes with An Organic Small Molecule Layer(CBP)
作者单位
1 北京航空航天大学 材料物理与化学中心, 北京100083
2 上海大学 新型显示技术实验室, 上海200072
摘要
以聚合物发光材料MEH-PPV为发光层的聚合物发光二极管的ITO阳极与发光层之间,加入一层溶解于氯仿中的有机小分子CBP,能显著改善发光器件的电流特性。在电压较低的时候能提高电流,在电压较高的时候能抑制电流,从而增加工作电压范围。此外,器件的电流效率也能得到显著的提高。实验结果表明,加入CBP层后,在低电压时,CBP层能够减缓空穴注入到发光层中,将其限制在CBP层,从而在器件中形成一个内电场,有助于电子的传输,降低开启电压,提高发光亮度。在电压较高时,CBP作为电子阻挡层,能阻挡电子漏泄到阳极,从而使在复合区的空穴与少数载流子电子的复合效率提高,改善器件的性能。
Abstract
Generally in semiconducting polymers, holes are majority carriers while electrons as minority,the carrier misbalance between electrons and holes further deteriorate in the fact that hole usually take possession of higher mobility and smaller injection barrier. To balance the electrons and holes in luminous layer, there are two commonly used stratagems. One is to improve the injection of electrons such as employ low work function metals such as barium and calcium as cathode though they are susceptible to degradation upon water vapor and oxygen or proposed to inser an insulating thin layer usually metal fluoride between polymer/electrode interfaces to build a bilayer cathode.Another is adding a layer into the polymer light-emitting diode such as hole buffer layer or electron-blocking layer.In this work, a layer of small organic molecule (CBP, dissolved in chloroform) was added into the polymer light-emitting diode between the anode (ITO) and luminous layer (MEH-PPV), using spin-coating method. The influence of different CBP concentrations was investigated. A typical single-organic-layer polymer light-emitting diode device which were constituted with anode (ITO), luminous layer (MEH-PPV) and cathode (LiF/Al) was employed as reference one. The layer of CBP acts as the hole buffer layer at low voltage and electron-blocking layer at high voltage. Comparing with the single-organic-layer (MEH-PPV) devices, the current efficiency of the composite structure devices is significantly enhanced. The experimental results showed that, below the voltage of 5 V, the current of the composite structure device is higher than that of the single-organic-layer device resulting in the reduction of the turn-on voltage of device. However, when the voltage is higher than 5 V, the current of the composite structure device is lower than that of the single-organic-layer device. That was because, in the low voltage, the CBP layer can slow down the injection speed of the holes into the luminous layer and restricted the holes in the CBP buffer layer. Thus, a built-in electric field was formed in the device, which could improve the transmission of electrons from the cathode to the anode, resulting in the increasing of current, reduce the turn-on voltage and improve the luminous efficiency. However, with increasing of voltage, the CBP layer acts as electron-blocking layer, which limits the electron leaking to the anode. Moreover, as the hole acts as majority carrier, the minority carrier, i.e. electrons, can still combine with holes efficiently which had little influence on the luminance intensity. Therefore, the electron-hole current density balance is improved, luminous efficiency is enhanced. When the CBP concentration is 20 mg/mL, the optimum of current efficiency and luminous intensity is achieved.

万晖, 李博, 孙三春, 魏斌, 张俊英. CBP有机薄膜对MEH-PPV聚合物发光二极管性能的影响[J]. 发光学报, 2009, 30(2): 162. WAN Hui, LI Bo, SUN San-chun, WEI Bin, ZHANG Jun-ying. Performance Improvement of Polymer Light-emitting Diodes with An Organic Small Molecule Layer(CBP)[J]. Chinese Journal of Luminescence, 2009, 30(2): 162.

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