Charge Carrier Density in Organic Semiconductors Modulates the Effective Capacitance of Electrolyte Gated Organic Transistors: a Unified View of EGOFET and OECT
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A framework for electrolyte gate organic transistors (EGOTs) which unifies the view of interfacial capacitive coupling of EGOFETs with that of the “volumetric” capacitance in organic electrochemical transistors (OECTs) is proposed and validated against experimental data. The model is based on the distribution of voltage drop across the EGOT, and on an organic semiconductor channel with exponential disorder in the HOMO LUMO gap that acts as active layer. The application of a gate bias to the electrolyte where the channel is immersed yields a shift of the electrochemical potential of the channel which depends on the DOS of the organic semiconductor within the HOMO-LUMO gap. We solve the analytical equation for the shift of charge carrier density and extract the expression for the gate-voltage dependent effective capacitance where the weight of the in-series chemical (or quantum) capacitance with respect to the interfacial capacitance is modulated by the gate voltage. Our model predicts the shape of the whole transfer curve of an EGOT, which allows us to extract accurate values for the threshold voltage and the interfacial transconductance, without using ad hoc assumptions, reduction of experimental data and without invoking the concept of volumetric capacitance. The model is assessed on recent experimental data of immunosensors and sensors of relevant pollutant molecules, yielding yields the actual dependence of the dose curve of the analyte on the threshold voltage.