Polar analyte effects on charge transport and trapping In organic field effect transistor based chemical and vapor sensors

Organic thin film transistors (TFTs) based on the field effect transistor architecture provide a methodology for sensing by exhibiting a change in the transport properties such as shifts in mobility, threshold voltage and conductivity. Chemical recognition is achievable by various methods including the two processes, which we are studying, direct analyte interactions with the semiconductor and specific receptor molecules on the semiconducting surface. Previous work demonstrates the effects of carrier concentration, grain size (surface morphology), and channel length on the sensing response to analytes such as alcohols, which exhibit a moderate dipole moment. When the alcohol interacts with the organic channel the addition of a trap and a positive charge occurs at the grain boundaries. At low carrier concentrations the added charge has the effect of producing an increase in current for the sensing response. At higher carrier concentrations the occurrence of trapping overwhelms the effect of the positive charge and you see and reduction in current. Typically the mobility shifts, which occur during sensing are correlated with trapping for polar analytes. The magnitude of the mobility decreases are dependent on the dipole moment of the polar analyte. Another aspect of organic materials is the fine-tuning of the chemical sensitivity by modifying the surface with receptor sites to increase the partition coefficient. In our study we pull the polarization, molecular dipole moment, transport and trapping, and partition coefficient concepts together to produce a model, which describes how an OFET based sensor interacts with an analyte with and without receptor molecules and under aqueous conditions.