The specific conductivity of thin films can be determined with various methods, e.g. by non-contact eddy current mapping or also after a direct contact using the four-point probe method (four-peak measurement). Well suited for comparisons like this is also, albeit with lower absolute measurement accuracy, a direct measurement with a simple ohmmeter (2 contact points). For meaningful and reproducible results is a sufficiently good contacting however mandatory. If layers < 100 nm are contacted directly with the measurement tips, the sensitive conductive layer can easily be damaged. At the same time it is very difficult to control if the contact was actually made since the contact area is extremely small. In order to reduce the transition resistance between the organic semiconductor layer and the sensor it is thus advantageous to use contact areas as wide as possible. By careful application of silver paste (silver nanoparticles in solvent, Aldrich) at a defined distance to the quartz plates previously coated with Electra 92, a comparatively large contact area between the silver layer and the conductive layer is obtained. A good electrical contact can then easily be established between measuring instrument (metal clamps) and silver layer and the resistance can be measured. The film thickness (d) of the conductive layer was previously determined profilometrically with a Dektak 150.
The specific resistance Rspec and Rsquare can be calculated as follows:
Rspec = R * A/l
If the calculated specific resistance Rspec is set in relation to the film thickness (d), Rsquare results:
Rsq = Rspec / d
The specific conductivity is calculated according to: L = 1/ Rspec = l/ R*A
R = measured resistance, Rspec = specific resistance, Rsq = R square,
A = the current-carrying area (cross section = height x width), l = length/ distance of electrodes (silver strip), L = specific conductivity, d = film thickness (height)
Depending on the respective layer thickness, measurements with Electra 92 (AR-PC 5090.02 and AR-PC 5091.02) resulted in well reproducible specific resistance values ranging from about 0.1 to 4 Ωm, which corresponds to a specific conductivity of 0.25 – 10 S/m.
Specific conductivity of Electra 92 layers on quartz in dependence on the film thickness