More work on inorganic transistors: Progress at Evonik

In the Science-to-Business Center Nanotronics of Creavis Technologies and Innovation, the strategic research unit of Evonik`s Chemicals Business Area, is developing materials for thin film transistors (TFTs). Apart from printed conductors Evonik develops dielectrics and inorganic, oxidic semiconductors.

 

In the field of printed dielectrics, Evonik has experience in evaluating derivates and special grades of PMMA for the use in TFTs. Due to the amorphous character of the polymer, film homogeneity of PMMA is much better than of many other polymeric systems. The huge variation of copolymers and different polymer architectures provide a lot of opportunities to tune this organic material to special solvent systems and to make this class of materials accessible to several printing technologies like inkjet or screen printing.

 

In the field of printed semiconductors, Evonik is developing new n-type inorganic oxides as high performing semiconductor materials that can be processed by several printing technologies. Compared to organic systems, these materials show a more stable and higher performance in the device, but usually processing temperature is above 150°C, and therefore low cost substrates like PET cannot be used.

 

Based on mobilities of non-solution processed films of ZnO between 55 to 155cm2/Vs (pp. 33-38, Physik Journal 5 (2006) Nr. 1) which up to now, could not be achieved with organic systems, Evonik obtained solution processable material systems with mobilities up to 0,1cm2/Vs at 150°C and more than 2cm2/Vs at 260°C (see figure 1) in 2008. These very promising values let them feel certain that they can reach mobilities higher than 1cm2/Vs at temperatures below 150°C in the near future (see figure 2).

 

Figure 3 shows the typical transfer characteristic of their n-type semiconductor materials in a bottom gate configuration. Due to good filming properties of the semiconducting materials they can also be used in top gate devices with Evonik's special grade PMMA. The combination of the two components (dielectric and semiconductor) provides the chance to offer not only one part of the device but a high performing material system for the application and printing process.

 

 

In the field of transparent conductors Evonik has developed a new mask-less process based on an inkjet-printing and screen-printing to structure transparent and conductive layers (TCL) applicable as electrodes for display, lighting, and sensor devices. This new approach is based on dispersions of nano-scaled indium tin oxide (ITO) material and features manufacturing advantages: reduction of process steps, increase of material yield, compatibility with variable layouts and designs as well as substrate size and scalability for large areas by using direct printing instead of vacuum sputtering and etching.

 

New sintering methods have been adjusted to the limited thermal stability of typical polymer-type substrates like PET, however they are equally suitable for glass-type substrates. The transparent conductor combines low sheet resistance below 100 Ω/sq. and a transmittivity above 90 % on glass. This approach can be part of a new enabling technology for all printed devices. The performance of a Novaled PIN OLEDTM stack that uses this ITO TCL used as an anode is shown as an example in figure 4.

Figure 4: Current efficiency of a Novaled PIN OLEDTM stack on an inkjet printed, transparent conductive ITO anode.

Evonik Industries AG is the creative industrial group from Germany, which operates in three business areas: Chemicals (formerly Degussa), Energy and Real Estate. They provide specialty chemicals, power generation from hard coal to renewable energies, and are one of the largest private residential real estate companies.

 

Creavis Technologies & Innovation addresses the establishment of profitable and sustainable new businesses for Evonik and their focus is to enhance the Evonik portfolio by building high-value businesses in new technologies and/or new markets.

 

Based on their developments, Creavis Technologies & Innovation will provide printing inks for semiconductors (p-type and n-type), conductors, transparent conductors, dielectrics, insulators and memory materials.

 

Furthermore, the company offers the adoption of these materials to suitable printing technologies such as gravure, flexo, ink jet, screen printing.

 

For more attend Printed Electronics Europe 2009 .

 

Top image: PolyIC