OLED optics

Applications of organic light-emitting diodes (OLEDs) have been rapidly devel oped since the first demonstration of green OLEDs by Tang and Vanslyke (1987). The original structure of an OLED consisted of a layer of organic electron transport layer (ETL)/emitting layer (EML) using 8-hydroxyquinoline aluminum (Alq3) and a hole transport layer (HTL) using an aromatic diamine. In order to inject charges and out-couple the emitted light, the organic layers were sandwiched by a transparent indium-tin-oxide (ITO) anode and a reflecting metal cathode. Such an OLED can operate at high brightnesses, which can meet the requirement for display and lighting applications. Unfortunately, the poor carrier injection efficiency from electrodes led to the high operating voltage even for standard luminance of 1,000 cd/m² for display applications, resulting in undesir able high power consumption and very short lifetime. In 1997, Hung et al. showed that a very thin 1-2 nm lithium fluoride (LiF) layer adjacent to the ETL and aluminum cathode which greatly improved the injection efficiency for OLEDs, resulting in a low operating voltage at high brightness (Hung et al. 1997). The improved injection efficiency is attributed to forming an ohmic contact at the metal/organic interface which facilitates carrier injection. A similar approach at the other side to reduce the injection barrier of holes between the anode and the HTL was demonstrated by simple ultraviolet ozone (UVO) treatment on ITO-coated glass substrates (Sugiyama et al. 2000; Lee et al. 2004). The UVO treatment increases the work function of ITO by removing carbon contaminants and creating a tin-deficient and oxygen-rich surface. Another well-adopted treatment to improve hole injection efficiency is to insert a high work-function conducting polymer poly(3,4-ethylenedioxythiophene): Poly(4-styrenesulphonate) (PEDOT:PSS) layer between the anode and the HTL (Jonas and Schrader 1991; Carter et al. 1997; Elschner et al. 2000). Nevertheless, even with the improved carrier injection with those interfacial modifications, the low device efficiency still remained a challenge for commercialization.