2, Lake Shore Cryotronics, Westerville, Ohio, United States
3, University of Washington, Seattle, Washington, United States
4, Temple University, Philadelphia, Pennsylvania, United States
Organic electronics utilize low-cost, solution-processable, and readily tunable semiconducting organic materials in a variety of applications. One common way to tune the important electronic properties of this class of materials is through molecular doping, that is, oxidizing or reducing the organic semiconductor to create an appreciable quantity of equilibrium charge carriers via polaron formation. To accomplish this doping, small organic or inorganic species must be incorporated into the polymer, causing changes in the molecular ordering. This molecular order is intimately linked with electrical properties in semiconducting polymers, making it essential to understand how incorporation of dopants change their physical structures. In this work, we discuss the properties of molecularly doped films of poly-3-hexylthiophene (P3HT) as the crystallinity of the polymer is systematically varied. The dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is introduced using sequential processing (SqP), a method where the polymer is first deposited and then the dopant is introduced in a second step that involves swelling, without dissolving the polymer layer.
Using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS), optical spectroscopy, electrical measurements, and theoretical modeling, we examine the molecular ordering of doped polymer films. The GIWAXS data shows that both the degree of crystallinity and orientation can be maintained after doping with F4TCNQ via SqP, so that more crystalline pure polymer films lead to more crystalline doped films. Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity while AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm2 V-1 s-1 in the most crystalline samples. Temperature-dependent conductivity measurements show that polaron mobility in SqP-doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity. Theoretical modeling of the polaron absorption suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. GIWAXS further showed that regardless of the P3HT crystallinity, doping led to an increase of the lamellar lattice spacing, a decrease in the π-stack spacing, and a loss of registry in the along-the-chain direction. With these observations, corroborated by the theoretical modeling, the GIWAXS data suggest that the F4TCNQ anions reside in the P3HT lamellae between the side chains and in the amorphous regions of the film, but do not π-stack within the polymer crystallites.