Sharma Rahul K, Katiyar Monica, Rao I V Kameshwar, Unni K N Narayanan
Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India.
Samtel Centre for Display Technologies, Kanpur, Kanpur 208016, India.
Phys Chem Chem Phys. 2016 Jan 28;18(4):2747-55. doi: 10.1039/c5cp06637a.
If an organic light emitting diode is to be used as part of a matrix addressed array, it should exhibit low reverse leakage current. In this paper we present a method to improve the on/off ratio of such a diode by simultaneous application of heat and electric field post device fabrication. A green OLED with excellent current efficiency was seen to be suffering from a poor on/off ratio of 10(2). After examining several combinations of annealing along with the application of a reverse bias voltage, the on/off ratio of the same device could be increased by three orders of magnitude, specifically when the device was annealed at 80 °C under reverse bias (-15 V) followed by slow cooling also under the same bias. Simultaneously, the forward characteristics of the device were relatively unaffected. The reverse leakage in the OLED is mainly due to the injection of minority carriers in the hole transport layer (HTL) and the electron transport layer (ETL), in this case, of holes in tris-(8-hydroxyquinoline)aluminum(Alq3) and electrons in 4,4',4''-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA). Hence, to investigate these layers adjacent to the electrodes, we fabricated their single layer devices. The possibility of bulk traps present adjacent to electrodes providing states for injection was ruled out after estimating the trap density both before and after the reverse biased annealing. The temperature independent current in reverse bias ruled out the possibility of thermionic injection. The origin of the reverse bias current is attributed to the availability of interfacial hole levels in Alq3 at the cathode work function level in the as-fabricated device; the suppression of the same being attributed to the fact that these levels in Alq3 are partly removed after annealing under an electric field.
如果将有机发光二极管用作矩阵寻址阵列的一部分，它应表现出低反向漏电流。在本文中，我们提出了一种在器件制造后同时施加热和电场来提高此类二极管开/关比的方法。一个具有出色电流效率的绿色有机发光二极管的开/关比很差，仅为10² 。在研究了几种退火与施加反向偏置电压的组合后，同一器件的开/关比可以提高三个数量级，特别是当器件在80°C的反向偏置（-15 V）下退火，然后在相同偏置下缓慢冷却时。同时，器件的正向特性相对不受影响。有机发光二极管中的反向泄漏主要是由于空穴传输层（HTL）和电子传输层（ETL）中少数载流子的注入，在这种情况下，是三（8-羟基喹啉）铝（Alq3）中的空穴和4,4',4''-三（N-3-甲基苯基-N-苯基氨基）三苯胺（m-MTDATA）中的电子。因此，为了研究与电极相邻的这些层，我们制造了它们的单层器件。在估计反向偏置退火前后的陷阱密度后，排除了电极附近存在体陷阱为注入提供状态的可能性。反向偏置下与温度无关的电流排除了热电子注入的可能性。反向偏置电流的起源归因于在制造的器件中，Alq3中的界面空穴能级在阴极功函数水平处的可用性；而这种情况的抑制归因于在电场下退火后，Alq3中的这些能级部分被去除这一事实。
