Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)


Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)

Zen, A.; Saphiannikova, M.; Neher, D.; Grenzer, J.; Grigorian, S.; Pietsch, U.; Asawapirom, U.; Janietz, S.; Scherf, U.; Lieberwirth, I.; Wegner, G.

Recently, two different groups have reported independently, that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented, one proposing carrier trapping at grain boundaries and the second putting emphasis on different conformation and packing of the polymer chains in the thin layers.
Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were used to investigate the structure and crystallization behaviors of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements revealed a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. In order to study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in disordered matrix. In contrast, films of the high molecular weight fraction exhibit partially-ordered domains throughout the whole sample area. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight is largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites.

Keywords: organic field-effect transistors; regioregular poly(3-hexylthiophene) (P3HT)

  • Macromolecules 39(2006)6, 2162-2171

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Publ.-Id: 7883