Direct route of ultrafast charge-pair generation upon below-band-gap excitation in P3HT: effect of external electric field

Abstract

The generation and decay mechanism of polaron pairs (PPs) are relevant for a better understanding of photophysical processes in organic semiconductors such as poly 3-hexylthiophene-2,5-diyl (P3HT). A widely suggested model assumes that ultrafast PP generation occurs mainly from hot-exciton dissociation within <100 fs. In our experiments presented here, we have performed a wavelength-dependent transient pump–probe study to investigate the generation mechanism of the PP states. Interestingly, we have found that a below-band-gap excitation can generate the PP states efficiently even with a very low absorption strength at the pump-laser wavelength. This process has been found in neat P3HT thin film as well as in a P3HT-only diode configuration. In addition, besides the fast PP dynamics, we have also observed longer-lived species, which can tentatively be assigned to delocalized PP states. To gain more information, we have studied the PP dynamics under the influence of a static external field in reverse and forward bias for different excitation fluences. From this, we conclude that the bimolecular annihilation process of delocalized PPs states competes with a recombination process. The external electric field speeds up the rate of the recombination process when the annihilation process is saturated at high fluences.