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.