Facile determination of bulk charge carrier concentration in organic semiconductors: Out-of-plane orientation hopping conduction characteristics in semicrystalline polythiophene

In this report, we demonstrate a straightforward two-terminal-device-based electrical measurement and analysis scheme that can simultaneously determine the bulk free charge carrier concentration and out-of-plane charge mobility in organic semiconductors by understanding the transition behavior of device current-voltage characteristics from ohmic to space charge limited conduction. As a model system, we characterize the properties of a semicrystalline poly(3-hexylthiophene) (P3HT) conjugated polymer film in which free carrier concentration is systematically controlled by adjusting oxygen doping level. The observed dependence of out-of-plane charge mobility on the carrier concentration is analyzed in the context of percolative variable range hopping conduction, and we identify the rate-limiting charge hopping process in P3HT and correlate it with the role of disordered polymer regions in mediating the charge transport between neighboring crystalline polymer lamellar domains.