Potential mapping on organic devices: 3D simulations and experiments

Scanning Kelvin Probe Microscopy and Electric Force Microscopy offer a unique opportunity to measure local (surface) potentials with nanometer resolution on actual devices. Therefore, these techniques are becoming more and more popular for characterizing physical aspects of organic thin film devices [1] like charge transport in polymer transistors [2] and charge generation in organic solar cells [3]. Unfortunately these techniques are extremely sensitive to the entire experimental geometry which severely limits the obtainable resolution. Here we present a numerical model that enables a quantitative description of experimental curves on a number of relevant organic devices. In addition, a strong and unexpected charging of common dielectric materials is found. We have focused our experiments and simulations on a study of the surface potential of samples with a standard organic transistor layout without active layer. A potential difference was applied between two flat electrodes, i.e. source and drain in FET operation, separated by an insulating micron-sized channel. It appeared that the measured potential profile strongly depends on whether the tip is parallel or orthogonal to the channel. Moreover, only a fraction (~0.6-0.9) of the applied voltage was observed in measured voltage traces. In order to understand the electrostatic interaction between the tip and the surface, and to explain these results we simulated the entire system of tip consisting of apex, cone and cantilever and sample in three dimensions using a commercial finite element program. The 3D simulations work for arbitrary potentials and realistic tip and cantilever geometries. It is shown to nicely reproduce the experimental features and yields valuable information on experimentally obtainable resolution versus tip-sample distance and tip-channel orientation. The increasing resolution with decreasing tip-sample distance is found to be due to the relative increase of the contribution of the apex area to the total probe-sample capacitance. The simulations fit the experimental surface potential using reasonable numerical parameters. Using the numerical tool to interpret experimental potential profiles of channels on different substrate materials, we found, to our surprise, that supposedly charge-free, insulating substrates do not at all behave as ideal dielectrics, but show significant charging effects when bias-stressed under ambient conditions