Structural Transition and Interdigitation of Alkyl Side Chains in the Conjugated Polymer Poly(3-hexylthiophene) and Their Effects on the Device Performance of the Associated Organic Field-Effect Transistor

Abstract

Direct grazing-angle X-ray scattering evidence of the order–disorder transition and interdigitation of side chains in a conjugated polymer poly­(3-hexylthiophene) (P3HT) is presented. The free methyl ends of the side chains exhibit closest packing, as in n-alkane crystallization, and cause a structural mismatch due to the difference between their packing density and the areal density of the attached ends. This mismatch is resolved by increases in the tilt angle of the side chains and local interdigitation. In situ X-ray scattering and electrical measurements show that the structural transition and interdigitation of these side chains strongly affect its surface morphology as well as the charge transport properties of the resulting P3HT-based organic field-effect transistor. Since most conjugated polymers have side chains, the results of this study provide a deeper understanding of the effects of side chains on the structural and electrical properties of conjugated backbones. These results also provide a new perspective on the formation of a metastable polymorph consisting of interdigitated P3HT.

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“…Unlike PBOBDTBTD, the PBOBDTTh has a less rigid thiophene structure than the former's benzothiadiazole ring. For this reason, PBOBDTTh polymer is expected to be less rigid, as already verified by other authors who reported that rigidity of conjugated backbones decreases when the polymer has a higher degree of alkyl substitution in the main chain [25,26], additionally, it also has been observed that the introduction of more flexible bonds in the polymer backbone substantially increases chain flexibility [27]. Figure 4 displays the evolution as a function of annealing time at 140 8C of the peak detected at low q values for PBOBDTTh, associated with the plane perpendicular to the conjugated plane, i.e., associated with the side alkyl chain.…”

“…This effect is stronger in the case of alkyl branches as presented for PBOBDTBTD and PBOBDTTh. Figure 8 illustrates in a schematic representation with a benzodithiophene conjugated polymer for the presently studied polymer films how the average interplanar distances may be higher with the advance of annealing, explained by the evidence found by [26] on the structural mismatch between chain ends and the chain start of the adjacent backbone in the crystallite, which could be evidence of the absence of alkyl chain interdigitation in the ordered regions. Interestingly, the increase of interplanar distance during crystallization is less pronounced for PBOBDTTh than for the others.…”

“…Thus, a possible explanation is an inhibition of the interdigitation effect detected at longer annealing times due to chain confinement in thin solid films. Indeed, several authors have explained how the interdigitation effect occurs in alkyl chains of conjugated polymers [20,21,26,35,36], reducing the volume of the crystalline arranges and presenting differences in FWHM between regioregular and regiorandom polymers. One of the studies [26] even points out that the methyl chain end on P3HT exhibits the closest packing, as in n-alkane crystallization, and causes a structural mismatch due to the difference between their packing density and the areal density of the attached ends.…”

“…Indeed, several authors have explained how the interdigitation effect occurs in alkyl chains of conjugated polymers [20,21,26,35,36], reducing the volume of the crystalline arranges and presenting differences in FWHM between regioregular and regiorandom polymers. One of the studies [26] even points out that the methyl chain end on P3HT exhibits the closest packing, as in n-alkane crystallization, and causes a structural mismatch due to the difference between their packing density and the areal density of the attached ends. This structural mismatch may be an explanation for the likely interdigitation inhibition observed in the present work.…”

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“…However, high sidechain attachment density prevents sidechain interdigitation and long-range intermolecular organization. For example, the sidechain density in P3HT is too large to permit interdigitation when the backbone is well packed 119 and mostly results in two-dimensionally ordered, smectic-like structure without registry between layers. In contrast, the side-chain attachment densities of PBTTT and PQT are sufficiently low to permit the interdigitation of side chains, which preserves the packing structure by interlocking adjacent layers and thus promote three-dimensional order.…”