A physical-based equivalent circuit model for an organic/electrolyte interface

The aim of this work is to develop an equivalent circuit model for the metal-organic semiconductor-electrolyte structures that are typically used as transducers in biosensor devices. In particular, a perylene derivative material is implemented in the active layer of a gold-semiconductor-electrolyte stack. Our approach is extending the standard range of the bias voltages applied for devices that operate in water. This particular characterisation protocol allows to distinguish and investigate the different mechanisms that occur at the different layers and interfaces: adsorption of ions in the semiconductor; accumulation and charge exchange of carriers at the semiconductor/electrolyte interface; percolation of the ionic species through the organic semiconductor; ion diffusion across the electrolyte; ion adsorption and charge exchange at the platinum interface. We highlight the presence of ion percolation through the organic semiconductor layer, which is described in the equivalent circuit model by means of a de Levie impedance. The presence of percolation has been demonstrated by environmental scanning electron microscopy and profilometry analysis. Although percolation is much more evident at high negative bias values, it is still present even at low bias conditions. The very good agreement between the model and the experimental data makes the model a valid tool for studying the transducing mechanisms between organic films and the physiological environment.