Surface and Interfacial Effects on Photo-Carrier Formation in Poly(3-hexylthiophene) Films

Poly(3-hexylithiophene) (P3HT) is one of the representative semiconducting polymers and used in a thin film geometry in organic optoelectronic devices. To improve the device performance, it is important to understand a thinning effect on the carrier formation in P3HT thin films. Here we examined the carrier formation process in thin P3HT films prepared in various sample geometries such as free-standing, Si supported and sandwiched films based on temperature-dependent femtosecond transient absorption spectroscopy. It was found that polarons (P), which were free carriers, were directly generated from polaron pairs (PP) among all samples. The rate constant for the transition process from PP to P remained almost constant in a lower temperature region, and then simply increased with temperature above a certain temperature, which was defined as T_PP→P. The T_PP→P value depended on the sample geometry. As a general trend, the T_PP→P values were in the ascending order of free-standing, supported and sandwiched films. While the T_PP→P values for the free-standing and supported films were lower than the corresponding bulk one, that for the sandwiched film was higher. These results clearly indicate that the effects of surface and substrate interface make T_PP→P lower and higher, respectively.