Examining the Role of Structure On Charge Transport of Polymer Semiconductors

Charge carrier mobility in conjugated semi-crystalline polymers depends critically on the crystallinity, orientation of the crystals, and connectivity between ordered regions.  However, the complex interplay between these morphological parameters is not fully understood.  By varying the thermal annealing parameters and casting solvents, we have systematically studied charge transport within poly(3-hexylthiophene) and poly(2,5-bis(3-hexadecylthiophen-2-yl) thieno[3,2-b]thiophene) as a function of the crystallinity.  We found that charge mobility is not a trivial function of the solvent; moreover, there is not a universal relationship between crystallinity and mobility. We hypothesize that interconnectivity among crystalline regions and the number of tie-chains between crystallites dominates charge transport if the long range order is sufficiently high. We have utilized recently developed methodologies for estimating the thin-film modulus which rely on measuring internal stresses by analyzing buckling phenomena due to thermal expansion. Since the modulus is proportional to the interconnectivity of crystallites, these experiments elucidate the role of processing on the structure and consequently on charge transport of the polythiophene thin-films.