In the past decades an enormous research as well as commercial interest has focused on the new field of conjugated organic electronics materials. This interest, which has just recently received a new impulse thanks to the Nobel Prize in chemistry for Shirakawa, MacDiarmid and Heeger, is due to the fact that these materials combine a number of interesting properties which give rise to a broad variety of new applications. Probably the most important feature of conjugated organic electronics materials is their ability to transport charges, i.e. they can be conductors or semiconductors. On the other hand these materials are organic molecules or polymers and thus offer the possibility to be designed in such a way as to perfectly fit the desired requirements. In this context these materials are often described as synthetic metals. Depending on their molecular weight and solubility, conjugated organic electronics materials can be easily processed by common techniques like organic chemical vapour deposition (OCVD), spin-coating or simple ink-jet printing. Owing to their mechanical stability these materials can even be deposited on flexible substrates. These features provide the possibility of producing “plastic organic electronics” at very low costs. Potential applications for polymeric materials include fully printed polymer field effect transistors (PFETs), conjugated polymer photovoltaic cells, organic light emitting diodes (OLED) and lasers as well as polymer sensors, actors, batteries and components for fuel cells. Low molecular weight compounds can be used in vapour-deposited thin-film transistors, and as charge transporting or emitting molecular glasses in OLEDs, photovoltaic cells, photo copiers and laser printers. Moreover, small conjugated molecules or oligomers can be designed in order to mimic the functionality of single electronic units like transistors, diodes, resistors and switches on a molecular scale. It is thus possible to further reduce the dimensions of these electronic components to the nanometre level.
As mentioned above, one key application for conducting polymers, oligomers and conjugated small molecules is their use in OLEDs. Such light emitting devices with poly(para-phenylenevinylene) (PPV) as the emissive material have first been described in 1990 by Friend et al. and are of the general structure depicted in Scheme 1.1.
Scheme 1.1 General structure of one-layer OLED by Friend
A simple one-layer OLED consists of an emitter material which is sandwiched between a transparent bottom electrode (anode) and a metal electrode (cathode) on top. Usually an indium tin oxide (ITO) coated glass substrate serves as a transparent anode whereas the cathode normally consists of metals with low work functions (Al, Mg, Ca). The emitter material can either be a polymer like PPV or polyfluorene (PF) or a low molecular weight compound such as tris(8-hydroxyquinolinato)-aluminium (Alq3). If an external field is applied between the two electrodes, positive and negative charges are injected at the anode and cathode, respectively. These charges recombine within the emission layer by formation of localised excited states (excitons) which can then decay radiatively (see Scheme 1.2. a)).
OLED displays have certain advantages compared to liquid crystal displays (LCD) and other display technologies. OLED displays possess rather high power efficiencies because they are emissive displays and do not need an additional backplane illumination. Further advantages are fast response times (up to 104 times faster than LCD), high luminance at relatively low voltages, wide view angle and high contrast. Flexible and large area displays are also possible.
Despite these advantages the conventional one-layer OLED displays still have some drawbacks. One major problem is the unbalanced charge flow through the devices which leads to higher operating voltages and reduced lifetimes. The difference between the amount of positive (holes) and negative (electrons) charge carriers injected into the device is mainly caused by a mismatch between the HOMO and LUMO levels of the emitter material and the Fermi level of the adjacent electrode (Scheme 1.2, a)). Moreover, holes possess a higher mobility than electrons in many emitter materials which leads to nonradiative neutralisation of the positive charges at the cathode.
To circumvent this problem additional charge-injection layers between the emission layer and the corresponding electrodes may be introduced. Especially the insertion of a so-called electron-conducting/hole-blocking (ECHB) layer has proved to effectively inhibit the quenching of holes at the cathode (Scheme 1.2, b)). Materials with good electron transport properties are molecules or polymers with low-lying LUMO levels such as oxadiazoles (PBD), triazines, benzothiadiazoles, etc.. For the improved injection of holes materials with high-lying HOMO levels are needed. Typical hole transporting materials (HTM) are aromatic amines like TPD or electron rich polymers like poly(ethylenedioxythiophene) (PEDOT).