Organic Light Emitting Diodes: Effect of Annealing the Hole Injection Layer on the Electrical and Optical Properties

Abstract

Organic Light Emitting Diodes (OLED) are receiving increased attention due to tremendous application potential these devices hold in the areas of large area displays and lighting applications. However, the problems of efficiency, stability and shelf life are major challenges for making OLEDs an attractive alternative. The simple device structure involving anode, emissive layer and cathode is no longer the norm. Recently, various buffer layers like Hole Injection Layer (HIL), Hole transport Layer (HTL), Electron Injection Layer (EIL), Electron Transport Layer (ETL) etc. are being widely used as integral parts of the OLED architecture to enhance the performance parameters. The nomenclature of these layers is often confusing and sometimes used by different authors to mean different layers and a common and universal nomenclature for layers is still wanting. Applying a buffer layer, often called as the hole injecting layer (HIL) between anode and emissive layer is a general technique for increasing the efficiency and stability of organic light emitting diodes. Poly- (3,4-ethyhylene dioxythiophene): poly- (styrenesulphonate) (PEDOT:PSS) is a very common and popular such HIL used in OLEDs. In this chapter, a basic structure of OLEDs has been discussed in perspective with this HIL material and the effect of annealing this PEDOT: PSS layer on the characteristics of the device at different temperatures ranging from 100°C to 300°C in vacuum. Devices fabricated in clean room conditions are characterized for their electrical and optical properties. Equivalent circuits of the devices are deduced using impedance spectroscopy and discussed. Surface morphology of the HIL layers using atomic force microscopy (AFM) provides reasons for the variation of the device properties with the annealing of HIL.

Chemical Reactivities and Organic Light-emitting Diode Properties of some Polyaromatic Molecules

“…Among these nanostructures, scientists have concentrated on OLEDs, as they are superior in many respects to their successors, liquid-crystal display (LCD) and light-emitting diode (LED) [8][9][10][11]. In addition, in studies on OLED design, OLED design with high performance comes to the fore [12,13]. In order to produce high performance OLED, the materials in the OLED layer, which consists of many layers, such as the electron injection layer (EIL), the electron transfer layer (ETL), the hole blocking layer (HBL), the emission layer (EL), the electron blocking layer (EBL), the hole transfer layer (HTL), the hole injection layer (HIL) sandwiched between anode and cathode electrodes, should be carefully selected [14][15][16].…”