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In this work, crystallization kinetics and aggregate growth of poly(3-ethylhexylthiophene) (P3EHT) thin films are studied as a function of film thickness. X-ray diffraction and optical absorption show that individual aggregates and crystallites grow anisotropically and mostly along only two packing directions: the alkyl stacking and the polymer chain backbone direction. Further, it is also determined that crystallization kinetics is limited by the reorganization of polymer chains and depends strongly on the film thickness and average molecular weight. Time-dependent, field-effect hole mobilities in thin films reveal a percolation threshold for both low and high molecular weight P3EHT. Structural analysis reveals that charge percolation requires bridged aggregates separated by a distance of ≈2-3 nm, which is on the order of the polymer persistence length. These results thus highlight the importance of tie molecules and inter-aggregate distance in supporting charge percolation in semiconducting polymer thin films. The study as a whole also demonstrates that P3EHT is an ideal model system for polythiophenes and should prove to be useful for future investigations into crystallization kinetics. [ABSTRACT FROM AUTHOR]

Charge transport in semiconducting polymers is inextricably linked to their microstructure, making the characterization of polymer morphology at all length‐scales essential for understanding the factors that limit mobility in these materials. Indeed, charge transport depends both on the ability of polarons to delocalize at the approximately nanometer length‐scale and navigate a complex energetic and morphological mesoscale landscape. While characterization of the mesoscale morphology of polymers is well‐established, studies of the local chain packing and nanoscale disorder, which affect delocalization, can be significantly more difficult to carry out. Through infrared charge modulation spectroscopy and theoretical modeling, the effect of the local chain environment on polaron delocalization is directly measured and quantified. Using a series of polymers based on the model system, poly(3‐hexylthiophene), the link between disorder and polaron localization is systematically explored. Polaron delocalization is correlated with known trends in mobility, revealing that while charge delocalization is always beneficial, the formation of tie‐chains is necessary to reach the highest mobilities in semicrystalline polymers. The results provide direct evidence for the importance of both nanoscale (charge carrier delocalization) and mesoscale (tie‐chains) orders, demonstrating the need to distinguish the key length‐scale limiting charge transport in the design of new, high mobility polymers. Quantitative infrared studies of polaron absorption combined with a vigorous and complete theoretical model reveal rich information about the local, nanoscale packing of a series of polymers based on poly(3‐hexylthiophene). Correlating these insights to independent studies of carrier mobility, direct evidence for the effect of charge localization and crystallite connectivity on charge transport is shown. [ABSTRACT FROM AUTHOR]