Photogenerated minority carrier trapping and inversion layer formation in polymer field-effect structures

We describe the results of a photocapacitance study of Metal-Insulator-Semiconductor (MIS) capacitors formed from semiconducting polymers. In this technique, capacitance-voltage (C-V) plots of the devices are obtained prior, during and after illumination with light of energy greater than the optical band gap of the semiconductor. When the capacitors are biased into depletion and simultaneously exposed to light, optically-generated minority electrons drift to and become trapped in states at the semiconductor-insulator interface, resulting in a significant shift of the CV plot to more positive voltages. For devices employing a polysilsesquioxane insulator clear and unambiguous evidence is obtained for the formation of an inversion layer at the interface. Upon terminating the illumination, the devices relax back to the initially-obtained dark C-V plots as the trapped electrons are thermally excited from their trapping states. By tracking the voltage required to maintain a constant capacitance the dynamics of charge detrapping can be followed leading to an estimate for the energy of the interface state.