Signatures of Intracrystallite and Intercrystallite Limitations of Charge Transport in Polythiophenes

Abstract

Charge carrier mobilities in conjugated semicrystalline polymers depend on morphological parameters such as crystallinity, crystal orientation, and connectivity between ordered regions. Despite recent progress in the development of conducting polymers, the complex interplay between the aforementioned parameters and their impact on charge transport is not fully understood. By varying the casting solvents and thermal annealing, we have systematically modulated the crystallization of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[2,5-bis(3-hexadecylthiophen-2yl)thieno(3,2-b)thiophene] (PBTTT) thin films to examine the role of microstructure on charge mobilities. In particular, we achieve equal crystallinities through different processing routes to examine the role of structural parameters beyond the crystallinity on charge mobilities. As expected, a universal relationship does not exist between the crystallinity in either P3HT and PBTTT active layers and the charge mobility in devices. In P3HT films, higher boiling point solvents yield longer conjugation lengths, an indicator of stronger intracrystalline order, and therefore higher device mobilities. In contrast, the charge mobilities of PBTTT devices depend on the interconnectivity between crystallites and intercrystalline order in the active layer.

Thermal Fluctuations Lead to Cumulative Disorder and Enhance Charge Transport in Conjugated Polymers

“…Finally, we find that the effective activation energies, which govern the temperature dependence of charge mobility as , are the same for charge transport in both the π–π stacking and backbone directions. The value of E a is about 100 meV, consistent with reported values, ranging from 23 to 116 meV . The Arrhenius dependence of mobility also indicates that the mechanism of charge transport in P3HT may be thermally activated hopping and the Marcus theory–based model is valid.…”

“…Combining MD results with a site‐to‐site charge hopping model, we demonstrate the effect of thermal fluctuations on charge mobility for crystalline P3HT. Previous works have suggested that charge carriers may be strongly localized and the temperature dependence of charge mobility is Arrhenius for P3HT . We treat charge transport in crystalline P3HT as 2D random hopping processes in both the backbone and π–π stacking directions.…”

The Polymer Physics of Multiscale Charge Transport in Conjugated Systems

“…The formation of such physical connections depends on the polymer chain contour length, the polymer chain persistence length, and the characteristic spacing between neighboring ordered regions . For such purposes, polymers with high MWs and high crystallinity are needed. And, as described above, polymers with rigid coplanar backbones have higher persistence length, which should in turn also facilitate the formation of tie chains …”

Liquid Crystal Ordering on Conjugated Polymers Film Morphology for High Performance

“…, Copyright (2013). (e) Percolation pathway formed by a tie chain between adjacent lamellae . (f) Higher molecular weight polymers have a higher probability of forming tie chains .…”