Transient photocurrent in poly(3-octadecylthiophene) near the solid-liquid phase transition

Poly(3-alkylthiophene)s (PATs) are conducting polymers possessing high processabilities such as solubility and fusibility. Most conducting polymers known to be p-type, or hole transporting semiconductors. PATs are also known as p-type semiconductors in the solid state. Previous studies have suggested that the dominant photocarrier inverts from positive to negative in the liquid state in PAT with relatively long alkyl side chain. In this study, we tried to confirm the sign inversion of the photocarrier in regiorandom poly(3-octadecylthiophene) (PAT18) by means of the time-of-flight 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 photocurrents 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/V s, 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