Origin of Free Charge Carrier Generation of the Second-order Process Photocurrent in Conjugated Polymer Studied by Two-Correlated-Pulse Technique

Kinetics of decay of polaron pairs as precursors of free charge carriers at photoexcitation of poly(3-octylthiophene) under applied electrical field have been observed directly within a picosecond time domain by using the two-correlated-pulse laser technique. Experiments have shown that polaron pairs were generated without any noticeable delay after the light absorption. At high enough excitation intensity, free charge carriers have been shown to take part in the second-order processes photocurrent produced by the recombination of two neighboring polaron pairs. It has been clarified that polaron pairs with two kinds of lifetime τfast, τdelay exists, and τdelay decreases by high electrical field strength, and that polaron pair is the main origin of the second-order photocurrent. The direct observation of photoluminescence within a picosecond time domain has been also discussed.