Sign inversion of dominant photocarrier of poly(3-alkylthiophene) associated with solid to liquid phase transition

We have tried to confirm the sign inversion of electronic photocarrier in a fusible conducting polymer, regiorandom poly(3-octadecylthiophene), PAT18 associated with the solid-liquid phase transition by means of the time-of-flight (TOF) method. It was found that the hole mobility decreases with increasing temperature. At the temperature near the solid-liquid phase transition, the hole mobility decreases drastically, and eventually the melting temperature, it was impossible to evaluate the hole mobility from noisy transient photocurrent. On the other hand, transient photocurrent based on electron transport are observed above the melting point. The negative carrier mobility was evaluated in the range from 10^-6 to 10^-5 cm²/Vs, which is comparably the hole mobility at solid state. This fact suggests that the same mechanism, for example interchain hopping limits carrier transport, and the negative carrier is electron. This unique phenomenon is interpreted as modulation of electronic energy state caused by conformational change of the main chain. That is, the drastic conformational change of polymer backbone induced by the thermal activation of side chain vibration, which is responsible for the thermochromism in poly(3-alkylthiophene)s, may play a key role for this unique phenomenon.