Fibrillar Morphology of Derivatives of Poly(3-alkylthiophene)s by Solvent Vapor Annealing: Effects of Conformational Transition and Conjugate Length

Abstract

A fibrillar morphology was obtained, compared to the featherless pristine films, via solvent annealing the films of a series of derivatives of poly(3-alkylthiophene)s (P3ATs): poly(3-dodecylthiophene) (P3DDT), poly(3,3'''-didodecyl-quaterthiophene) (PQT12), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT12). Among the solvents used, including dichloromethane, chloroform, tetrahydrofuran, and carbon disulfide (CS2), CS2 was the best to induce fibril formation because its solubility parameter is closest to those of the P3AT derivatives. It was found that higher critical CS2 vapor pressures were needed to form crystal nuclei with increasing conjugation length and molecular weight of the P3AT derivatives; i.e., the critical vapor pressures for P3DDT 13.9k and PQT12 15.5k were 59.0% and 80.7%, respectively, and there were no nuclei of fibrils for pBTTT12 15.6k with the highest conjugation length, even at a CS2 vapor pressure of 98.3%. Meanwhile, at the highest vapor pressure, the fibril density decreased with increasing conjugation length and molecular weight of the P3AT derivatives. This is attributed to the rod-like conformation prevailing for polymers with larger conjugation length and higher molecular weight during solvent annealing, making the conformational transition toward coils more difficult and hindering diffusion of molecules. The results presented here are expected to be helpful for the design and processing of conjugated semiconductor polymers.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Thus, a smaller solubility parameter difference between solvent and solute determining a good solubility is important to satisfy the premise of ordered aggregation and enhance performances. Liu et al and Wang et al reported that with closer solubility parameters of solvent to polymer, the crystallinity and/or fibrillar morphology in films became improved [79,173]. Meanwhile, with the similar solubility parameters, a larger polarity of solvent led to a drop of carrier mobility.…”

“…It brings difficulty to the crystallization of chains during casting or post-annealing. Especially for conjugated polymers with larger conjugation lengths, Wang et al reported that (1) the critical nucleation vapor pressure was higher due to the higher critical vapor pressure for rod-to-coil transition during solvation and (2) the fibril density was reduced partly due to the slower motion of units during aggregation, which were the results of the larger conjugation lengths and enhanced rigidity for polymers ( Figure 7) [79]. Therefore, tuning the crystallization and morphology of conjugated polymers is challenging due to their conjugated molecular structures.…”

“…In the following, the effects of structural factors on the structures, morphology and charge transport in films will be discussed. The TEM images of films annealed at P CS2 = 98.3% with a increasing conjugation length from left to right and scale bars of 500 nm in (a), the variation of free exciton bandwidth W with P indicating the conformation change in (b) and dynamic light scattering (DLS) results of polymers in solution in (c) (Adapted with permission from [79]. Copyright 2013 American Chemical Society).…”

“…[107] Besides the electronic effects, the different conjugated structures of backbones could lead to big changes of rigidity among the family of conjugated polymers, which influence the structures and morphology in turn. The rigidity of chains could be increased by a more intense conjugation [68,79,109]. Zhao et al reported that the persistence length of pBTTT (P4) (9.0 nm) was much larger than rr-P3HT (4.8 nm) due to the introduction of fused thieno [3,2-b]thiophene moiety into backbones to strengthen the conjugation, implying a more rigid chains [68].…”

“…The conjugated polymer chains coupling with more intense conjugated units [79,99,110] or electron-rich units [99] display a longer conjugation length and more extended conformation which could be reflected by the red-shift max absorption in solutions [68,109]. These cause a regular stacking of planar chains to form ordered crystallites as efficient charge transport pathways, indicated in red-shifted absorption spectra and arc-like patterns in 2D-WAXS of films [99,[110][111][112].…”