Surface roughness of electrodes plays a key role in the dielectric breakdown of thin-fifilm organic devices. The rate of breakdown will increase when there are stochastic sharp spikes on the surface of electrodes. Additionally, surface having spiking morphology makes the determination of dielectric strength very challenging, specififically when the layer is relatively thin. We demonstrate here a new approach to investigate the dielectric strength of organic thin fifilms for organic light-emitting diodes (OLEDs). The thin fifilms were deposited on a substrate using physical vapor deposition (PVD) under high vacuum. The device architectures used were glass substrate/indium tin oxide (ITO)/organic material/aluminum (Al) and glass substrate/Al/organic material/Al. The dielectric strength of the OLED materials was evaluated from the measured breakdown voltage and layer thickness.
Organic light-emitting diodes (OLEDs) have drawn enormous attention in both academia and industry owing to their cumulative applications in high-quality flflat-panel displays and solid-state lighting. Recently, OLEDs have been commercialized as information displays for car audio systems, sub-displays of cellular phones, and large-screen TVs, promising large market opportunities. The superior properties of OLEDs start to dominate existing flflat-panel display technology, liquid-crystal display (LCD). OLEDs have striking topographies, such as high color purity, high luminance, a wide viewing angle, high contrast and response speed, low power consumption, a simple fabrication process, ultra-thin structure, light weight, flflexibility, and low cost. To make OLED displays and lighting more competitive and customer affordable, and the resultant products more energy saving and longer lasting, OLEDs with higher power effificiency are demanded.
Generally, OLED devices have a driving voltage of 3 to 5 V. Depending on the color of the light andthe thickness of organic layers, OL ED devices need higher voltage in order to achieve more brightnessHowever, device efficiency and lifetime start to decline at a higher driving voltage. This failureof OLED devices is mainly due to dielectric breakdown of the organic material. For a thin film of100 nm, a voltage of 10 V can produce an electric field of 1 million volts/cm (MV /cm). The producedelectric field is sufficient to cause dielectric breakdown of most of the OLED materials (201. In thisstudy, we attempted to discover the correlation between surface roughness of the electrode ancdielectric breakdown of OLED materials. We measured the dielectric strength of the OLED materialsby evaluating the breakdown voltage and layer thickness.
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Figure 2a,b shows the studied devices composed with glass/ITO/organic material/Al andglass /Al/organic material/Al for dielectric strength measurement, respectively. Figure 2c,d illustratesthe respective electric circuits of studied devices. Figure 3a,b shows the breakdown voltage of DeviceI and Device Il. Device I showed a breakdown voltage of 3.90 V, whereas Device ll showed 6.20 VThe calculated dielectric strengths were 0.32 and 0.52 MV/cm for Device I and Device ll, respectively.
The reason why devices fabricated with an ITO anode show low dielectric strength whencompared with devices with an Al anode may be attributed to two important factors in the designecdevice architecture. First, the Al anode has a low work function (4.3 eV) as compared to the ITOanode (5.2 eV). Second, the devices fabricated with an ITO anode more effectively transfer holesinto the organic layer because of 0.8 and 1.1 eV hole-injection barriers at the interface of ITO/CBPand ITO/BPhen, respectively, ie., 1.7 and 2.0 eV for the devices with Al as an anode, as shown inFigure 5a,b and Figure 6a,b . Furthermore, we observed the low charge injection region in Figures 3band 4b, below the voltage 2 V and 10 V, for Devices ll and IV, respectively. That current density (follows the externally applied voltage (V) linearly, as shown in Equation (2), may be the reason behind this.
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