Owing to their particular properties, displays based on organic light-emitting diodes (OLEDs) are an alternative to the established technology of liquid crystals (LCD). This novel technology offers advantages in particular in applications involving portable devices which are not plugged into the mains supply, such as mobile telephones, pagers and toys, for example.
The advantages of OLEDs include the extremely flat design, the property of generating their own light, which means that like liquid crystal displays (LCDs) they require no additional light source, the high luminous efficiency and the unrestricted viewing angle.
In addition to displays, however, OLEDs can also be used for lighting purposes, for example in large-area radiation emitters. Due to their extremely flat design, they can be used to build very thin lighting elements, which hitherto was not possible. The luminous efficiencies of OLEDs now exceed those of thermal radiation emitters, such as incandescent bulbs for example, and the emission spectrum can in principle be varied as desired through a suitable choice of emitter materials.
Neither OLED displays nor OLED lighting elements are limited to a flat, rigid design. Arrangements that are flexible or curved in any way are just as feasible owing to the flexibility of the organic functional layers.
An advantage of organic light-emitting diodes lies in their simple structure. This structure is conventionally as follows: a transparent electrode is applied to a transparent support, e.g. glass or plastic film. On top of this is at least one organic layer (emitter layer) or a stack of organic layers applied in succession. Finally a metal electrode is applied.
Organic solar cells (OSCs) have the same structure in principle (Halls et al., Nature 1995, 376, 498), except that in this case, conversely, light is converted into electrical energy.
The economic success of these novel electro-optical structures will depend not only on fulfilment of the technical requirements but substantially also on manufacturing costs. Simplified process steps leading to a reduction in manufacturing complexity and manufacturing costs are therefore of great importance.
TCO (transparent conducting oxide) layers such as indium-tin oxide (ITO) or antimony-tin oxide (ATO) or thin metal layers have hitherto conventionally been used as transparent electrodes in OLEDs or OSCs. The deposition of these inorganic layers takes place by sputtering, reactive surface atomisation (reactive sputtering) or thermal evaporation of the organic material in vacuo and is therefore complex and cost-intensive.
ITO layers are a substantial cost factor in the production of OLEDs or OSCs. ITO layers are used because they combine high electrical conductivity with high transparency. However, ITO has the following considerable disadvantages:
a) ITO can only be deposited in a complex, cost-intensive vacuum process (by reactive sputtering).
b) Temperatures of T>400° C. are necessary in the deposition process if high conductivities are to be obtained. In particular, the polymer substrates which are important for flexible displays do not withstand these temperatures.
c) ITO is brittle and forms cracks when shaped.
d) The metal indium is a raw material with only limited production, and supply shortages are predicted if consumption increases further.
e) The environmentally compatible disposal of electro-optical structures containing the heavy metal indium is a problem which has yet to be solved.
Despite these disadvantages, ITO layers are still used because of their favourable ratio of electrical conductivity to optical absorption and above all due to the lack of suitable alternatives. A high electrical conductivity is necessary to keep down the voltage drop across the transparent electrode in current-driven structures.
Alternatives to ITO as electrode materials have been discussed in the past, but hitherto no alternatives have been found which do not exhibit the disadvantages described above and which at the same time offer comparably good properties in electro-optical structures.
Thus a polymeric ITO substitute in which monomers are polymerised in situ on a substrate to form conductive layers has been described, such as e.g. in-situ polymerised poly(3,4-ethylenedioxy)thiophene, which is also abbreviated by specialists to in-situ-PEDT (WO-A 96/08047). However, as well as being likewise difficult to process on the substrate, these in-situ-PEDT layers have the disadvantage, particularly for applications in OLEDs, that firstly the material has a strong inherent colour and secondly that the electroluminescence efficiencies that can be achieved with it are low.
In addition, a complex of polyethylenedioxythiophene and polystyrene sulfonic acid, abbreviated by specialists to PEDT/PSA or PEDT:PSA, has been proposed as a polymeric ITO substitute (EP-A 686 662, Inganäs et al., Adv. Mater. 2002, 14, 662-665; Lee et al., Thin Solid Films 2000, 363, 225-228; W. H. Kim et al., Appl. Phys. Lett. 2002, Vol. 80, No. 20, 3844-3846). However, the conductivity of PEDT:PSA layers produced from formulations having a PEDT:PSA ratio such as 1:2.5 (percent by weight) is not particularly high, e.g. approx. 0.1 S/cm for an aqueous PEDT/PSA dispersion (available commercially as Baytron® P from H.C. Starck), and far removed from the desired value for ITO of 5000 to 10,000 S/cm. Although the conductivity can be raised to approx. 50 S/cm by adding additives such as e.g. dimethyl sulfoxide, N-methyl pyrrolidone, sorbitol, ethylene glycol or glycerol to such an aqueous PEDT/PSA dispersion, it is still well below the value for ITO. Furthermore, the fact that these formulations lead to relatively rough layer surfaces because of their coarse particle structure argues against the use of these layers as an ITO substitute in many electro-optical applications. In particular, for applications which are sensitive to short-circuits due to surface roughness, such as OLEDs and OSCs, these layers are thus not very suitable.
There was therefore still a requirement for a suitable substitute material for ITO which does not exhibit the disadvantages of ITO and at the same time offers equivalent properties in electrical or electro-optical structures.