Mixing at the Charged Interface of a Polymer Semiconductor and a Polyelectrolyte Dielectric

Electrolytes dissolved in ion conducting polymers are finding applications as dielectric materials with very high capacitance for electronic devices, particularly organic thin film transistors. A key mechanistic question concerning their application is whether mixing of electrolyte with the organic semiconductor occurs under gate bias. Here, we quantitatively analyze the interfacial-mixing problem within the framework of polymer solution thermodynamics. The model system studied consists of lithium poly(styrene sulfonate) dissolved in poly(ethylene oxide) as the dielectric and poly(3-hexylthiophene) as the polymeric semiconductor. A distinct transition between doping mechanisms is observed as a function of gate voltage (VG). In situ optical spectroscopy, transistor measurements, and theoretical analysis strongly point to electrostatic double layer formation in one voltage regime (0 > V-G > -1.8 V) and electrochemical mixing across the interface in another regime (V-G < -1.8 V). The formalism developed also defines the maximum charge density (2 x 10(14)/cm(2)) achievable in the electrostatic double layer regime.