The optical, structural, and electrical properties of thin layers made from poly(3‐hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater. 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X‐ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 150 °C. However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue‐shift of the solid‐state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low‐molecular weight P3HT layers.

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 the conformation and packing of the polymer chains in the thin layers for different molecular weights. 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 behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. 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 a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites.

Previous investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW: Mn = 7 200 g mol−1; high‐MW: Mn = 27 000 g mol−1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10−2 cm2 V−1 s−1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.

The morphology of thin films at the polymer-to-insulator interface is of great importance for OFET applications. In order to find a relation between the thickness dependence of structural order and the electrical parameters in low molecular weight (M w ∼ 2.5 kDa) poly(3-hexylthiophene) (P3HT), we have performed grazing-incidence X-ray diffraction and field effect mobility measurements. The samples were prepared from solutions with different concentrations by spin-coating mainly onto HMDS-pretreated Si/SiO 2 substrates, resulting in film thicknesses that vary between 10 and 200 nm. The X-ray diffraction curves display Bragg peaks of nanocrystallites diluted into an amorphous matrix where the orientational distribution of the crystallites changes significantly as a function of film thickness. The orientation of nanocrystals was found to be random for the thickest films. Reducing the film thickness, we found an increase in the alignment of the stacking direction of molecules along the surface normal. At same time the mean crystal size along the film normal decreases less than the decrease of film thickness. This is interpreted by a preferential pinning of nanocrystals at the film-to-insulator interface when the crystal size becomes in the order of the film thickness, i.e., below 25 nm. The model of pinning effect is supported by temperature-resolved X-ray measurements performed between room temperature and melting temperature. For films thicker than 25 nm the phase transition appears rather continuously with temperature, but it becomes sharp for thinner films. In contrast to X-ray measurements the field effect mobility is found to be constant within the whole investigated range. Our findings give evidence that the charge transport in low molecular weight P3HT is dominated by the ultrathin layer stabilized at the film-to-insulator interface. Despite the very uniform orientation of the crystallites within this layer, the field effect mobility remains low for all thicknesses. This is attributed to the presence of amorphous regions between highly crystalline domains, which ultimately limits the charge transport in the layer plane.

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Synthesis and electronic properties of three swivel-cruciform oligothiophene dimersbis(terthiophene) (BT3), bis(pentathiophene) (BT5) and bis(heptathiophene) (BT7)-with increased solubility in organic solvents are reported. We obtained a field-effect mobility of 3.7 6 10 25 cm 2 V 21 s 21 and a current on/off ratio of .10 3 for a solution-processed OFET device with dimer BT5 as p-type semiconductor.

A pentathiophene-based swivel cruciform, which allows rotation between the cruciform arms, was synthesized. Homogeneous microcrystalline films were processed from solution, and field-effect transistors utilizing this dimer gave hole mobilities up to 0.012 cm2/V.s.

The performance of highly soluble regioregular poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HTOT) as a semiconducting material in organic field-effect transistors (OFETs) is presented in comparison to that of the corresponding homopolymers. Transistors made from as-prepared layers of P3HTOT exhibit a mobility of ca. 7 × 10 -3 cm 2 V -1 s -1 , which is comparable to the performance of transistors made from as-prepared poly(3-hexylthiophene) (P3HT) and almost 6 times larger than the mobility of transistors prepared with poly(3-octylthiophene) (P3OT). On the other hand, the solubility parameter δ p of P3HTOT is close to that of the highly soluble P3OT. Moreover, compared to a physical blend of poly(3-hexylthiophene) and poly(3-octylthiophene), the mobility of P3HTOT devices is almost twice as large and the performance does not degrade upon annealing at elevated temperatures. Therefore, the copolymer approach outlined here may be one promising step toward an optimum balance between a sufficient processability of the polymers from common organic solvents, a high solid state order, and applicable OFET performances.