Modeling observed capacitance–voltage hysteresis in metal–SiO2-thin film organic semiconductor devices

Recently, metal–oxide–organic semiconductor capacitors (MOOSCAPs) and organic field effect transistors (OFETs) using organic semiconductor (OS) materials such as arylamino–poly-phenylene-vinylene (arylamino–PPV), pentacene, etc., have been reported of being subject to capacitance–voltage (C–V) hysteresis. This hysteresis is exhibited with a simple parallel shift in the C–V. In this paper, the C–V hysteresis observed in these devices is modeled using a physics-based thermodynamic-variational approach and compared with the empirical values reported in literature. Our thermodynamic-variational model is based on minimizing the free energy of the device. The hysteresis effect is modeled as arising from residual polarization of the organic film at flat band which consequently produces a polarization charge layer at the SiO2/OS interface. In the arylamino–PPV OS film, polarization is associated with redistribution of the molecular π-orbitals. Consequently, such residual polarization is expected to be of opposite polarity when the sweep direction is alternated. The change in polarity thus is expected to produce a shift in the flat band voltage which can be related simply and quantitatively to the charge density in the layer.In order to compare these MOOSCAP results with metal–oxide–silicon (MOS) TCAD, we performed 3-D TCAD simulations of a MOS structure where the p-Si layer is used to approximate the properties of an OS such as arylamino–PPV. In this MOS structure the polarization charge layer is approximated as a sheet charge at the p-Si/SiO2 interface. This comparison signifies that hysteresis effects in a MOOSCAP structure can be estimated reasonably using TCAD simulations of the MOS structure.