Advanced device architecture
The architecture of LEDs may vary from the basic scheme and may involve the use of multicomponent chromophore and of one or more transporting layers. The use of multilayer diode construction improves the performance of the devices by insuring good injection and transport of charges carried at both electrodes via hole- and electron injection layers (HIL and EIL) and hole-and electron-transporting layers (HTL and ETL). In multilayer devices poly(3,4-ethylenedioxidethiophene)-polystyrenesulfonic acid PEDOT:PSS is deposited near the anode and acts as a hole transporting layer.
PEDOT-PSS is an electrochemically stable conjugated polymer and has attracted a lot of attention because of its transparency and its high conductivity in doped state. This chemical is a semiconductor with a band gap of around 1.6 eV and is usually used as an antistatic coating.
Ultra-thin LiF or CsF layer are often deposited between the cathode and the organic polymer film. The LiF interfacial layer mainly improves the device performance by protecting the polymer during the cathode deposition and by reducing the number of quenching sites at the interface. Moreover the presence of LiF may also stabilize and prevent diffusion of metal atoms coming from the cathode and into the organic layer. The role of the thin CsF layer is different. The CsF dissociates upon deposition of the cathode forming free Cs atoms, which act as dopants of n-type in the polymer film. Such an n-doped surface region is likely to enhance electron injection and improve device performance.
Moreover, the OLED structure may also contain additional electron- and hole-blocking layers. The role of the blocking layers is to force the charge carriers to recombine within the bulk by adequately adjusting the HOMO and LUMO levels of these materials.