Effect of anode and cathode workfunction on the operating voltage and luminance of a single emissive layer organic light emitting diode

Organic light emitting diodes (OLED) are considered to be a promising candidate for light sources as well as for flat panel display because of their numerous advantageous. Two important parameters of an OLED among many other parameters to measure OLED performance are operating voltage and luminance. Those two parameters are believed to be strongly influenced by an anode and cathode workfunction of materials used as electrodes. In this paper, we study the effect of anode and cathode workfunction on the operating voltage and luminance of a single emissive layer organic light emitting diode. Devices with five different cathodes: Aluminum (Al), Calcium (Ca), Magnesium (Mg), Argentum (Ar), and Cuprum (Cu) and three different anodes: Indium Thin Oxide (ITO), Poly-(3,4-Ethylenedioxidythiophene)-Poly (Styrene Sulfonate) (PEDOT:PSS), Zinc Oxide (ZnO) are compared and analyzed. A 2 nm thick of Polyfluorene (PFO) is used as an emissive layer. SimOLED is used to simulate and analyze both electrical and optical characteristics. Current voltage luminance (IVL) characteristics are simulated under forward biased from 0 to 10 V. We found that the use of anode with higher workfunction can reduce operating voltage as well as increases device luminance. On the other hand, the use of lower workfunction of cathode can reduce operating voltage however it is not always increasing device luminance. The decrease of the operating voltage by increasing anode workfunction and decreasing cathode workfunction are ascribed to the barrier lowering of the holes and the electrons respectively.

I. Introduction

Since proposed in 1963 with a single layer organic structure [1] and revived in 1987 with multilayer organics structure [2], organic electroluminescent attract much attention due to their advantages for several applications such as for flat panel display as well as for lighting [3], [4]. Organic light emitting diode is considered to be a powerful candidate for numerous lighting applications. This is because it has width area which is much larger than solid state light emitting diode (LED) and their low driven voltage which is only a several volts. The simplicity of its fabrication processes is also promising for mass production. Two important basic parameters of an OLED performance are operating voltage and luminance. The operating voltage is defined as the voltage necessary to have a luminance of 1 Cd/m². There are several causes giving impact to the operating voltage and luminance. The application of multilayer structure such as hole transport layer, electron transport layer and hole injection layer can effectively reduce the turn operating voltage as well as to increase luminance [5]. Another important parameter which are considered can influence the operating voltage and luminance are anode and cathode workfunction. Large anode work function and low metal cathode workfunction are commonly employed to facilitate both hole and electron injections into emissive layer. Low energy barrier between the cathode and the lowest unoccupied molecular orbital (LUMO) is believed can improve electrons injection into the emissive layer. Similarly, the low energy barrier between the anode and the highest occupied molecular orbital (HOMO) will enhance the number of holes into the emissive layer, therefore higher luminance is expected. Impact of the cathode metal work function on the performance of OLED have been experimentally reported [6]. Inserting a very thin layer of Lithium Fluoride (LiF) between emissive layer and Al cathode in order to enhance electron injection has experimentally reported [7]. This method can reduce the operating voltage as well as increases the emission efficiency which is ascribed to the balance of the hole and electron recombination in the emissive layer. In this paper, further study the effect of cathode and anode workfunction to the operational voltage and luminance in a single emissive layer OLED are discussed.