Physical modelling of bio sensors based on Organic Electrochemical Transistors

Organic Electrochemical Transistors are widely used as transducers for sensors in bioelectronics devices. Although these devices have been extensively studied in the last years, there is a lack of fundamental understanding of their working mechanism, especially concerning the de-doping mechanism.This thesis is dedicated to Organic Electrochemical Transistors modelling. First of all, a numerical steady state model was established. This model allows implementing the Poisson-Boltzmann, Nernst-Planck and Nernst equations to describe the de-doping process in the conductive PEDOT:PSS layer, and ions and holes distribution in the device. Two numerical models were proposed. In the first, Local Neutrality model, the assumption of electrolyte ions trapping in PEDOT:PSS layer was taken into consideration, thus the local neutrality was preserved. In the second model the ions were allowed to move freely under applied electric field inside conductive polymer layer, thus only global electroneutrality was kept. It was experimentally proven that the Global Neutrality numerical model is valid to explain the global physics of the device, the origin and the result of the de-doping process. The transition from totally numerical model to analytical model was performed by fitting the parametric analytical Boltzmann logistic function to numerically calculated conductivity profiles. As a result, an analytical equation for the Drain current dependence on applied voltage was derived. By fitting this equation to experimentally measured Drain current- applied voltage profiles, we could obtain the maximum conductivity of a fully doped PEDOT:PSS layer. The maximum conductivity is shown to be dependent not only on the material, but also on device channel size. Using the maximum conductivity value together with the Conventional Semiconductor model it is possible to extract the other parameters for the full description of the OECT: intrinsic charge carrier density, initial holes density, initial PSS- concentration and conductive polymer layer volumetric capacitance. Having a tool to make easy parameters extraction and characterization of any OECT, permits not only to increase the level of device description, but most importantly to highlight the correlation between external and internal device parameters.Finally it is shown how to make the whole description of the real OECT device, all the models were validated by fitting the modeled and experimentally measured data profiles.As a result, not only the purely theoretical model was presented in this thesis to describe the device physics, but also the prominent step was made on simple real device characterization.