Spinodal Decomposition of Blends of Semiconducting and Ferroelectric Polymers

The operation of resistive switches based on phase-separated blends of organic ferroelectrics and semiconductors depends significantly on the microstructure of such systems. A wide range of analysis techniques are used to characterize spin-coated films of the ferroelectric random copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and the semiconducting polymer, regio-irregular poly(3-hexylthiophene) (rir-P3HT). The blend separates into amorphous rir-P3HT domains embedded in a crystalline P(VDF-TrFE) matrix. The rir-P3HT domains are continuous throughout the film, from the substrate/blend interface to the blend/air interface. We also investigate the rir-P3HT domain size and number as a function of composition and find - unexpectedly-a rather mono-disperse domain size distribution for a given rir-P3HT: P(VDF-TrFE) ratio. The domain size increases with rir-P3HT content, indicating that the solidification is not dominated by nucleation processes. Spinodal decomposition is therefore more likely to be responsible for the microstructure induced in the rir-P3HT: P(VDF-TrFE) blends. Since spinodal decomposition occurs spontaneously without the presence of a nucleation step, this can facilitate processing considerably, since the intricate control of nucleation processes (homogenous or heterogenous) is rendered unnecessary. Measurement of the lateral conductivity of the blends demonstrates that the rir-P3HT domains are electrically not connected, supporting the microstructural evidence. A perpendicular current through the film is measured using both Au and Ag electrodes as a function of blend composition. A model was used to interpret the electrical transport. The injection for Ag diodes poled into the ON-state preferentially occurs at the circumference of the rir-P3HT domains. An accumulation width over which the injection occurs is estimated to be of the order of a few hundred nm.