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With the aim of understanding the relationships between organic small molecule field-effect transistors (FETs) and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly (3-hexylthiophene) (RR-P3HT) field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies (average size and average roughness) of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

In order to enhance the performance of regioregular poly(3-hexylthiophene) (RR-P3HT) field-effect transistors (FETs), RR-P3HT FETs are prepared by the spin-coating method followed by vacuum placement and annealing. This paper reports that the crystal structure, the molecule interconnection, the surface morphology, and the charge carrier mobility of RR-P3HT films are affected by vacuum relaxation and annealing. The results reveal that the field-effect mobility of RR-P3HT FETs can reach 4.17 × 10−2 m2/(V · s) by vacuum relaxation at room temperature due to an enhanced local self-organization. Furthermore, it reports that an appropriate annealing temperature can facilitate the crystal structure, the orientation and the interconnection of polymer molecules. These results show that the field-effect mobility of device annealed at 150 °C for 10 minutes in vacuum at atmosphere and followed by placement for 20 hours in vacuum at room temperature is enhanced dramatically to 9.00 × 10−2 cm2/(V · s).

This paper investigates the effects of concentration on the crystalline structure, the morphology, and the charge carrier mobility of regioregular poly(3-hexylthiophene) (RR-P3HT) field-effect transistors (FETs). The RR-P3HT FETs with RR-P3HT as an active layer with different concentrations of RR-P3HT solution from 0.5 wt% to 2 wt% are prepared. The results indicate that the performance of RR-P3HT FETs improves drastically with the increase of RR-P3HT weight percentages in chloroform solution due to the formation of more microcrystalline lamellae and bigger nanoscale islands. It finds that the field-effect mobility of RR-P3HT FET with 2 wt% can reach 5.78 ? 10?3 cm2/Vs which is higher by a factor of 13 than that with 0.5 wt%. Further, an appropriate thermal annealing is adopted to improve the performance of RR-P3HT FETs. The field-effect mobility of RR-P3HT FETs increases drastically to 0.09 cm2/Vs by thermal annealing at 150 ?C, and the value of on/off current ratio can reach 104.

This paper investigates the morphology and crystallization properties of the two crystalline phases of pentacene grown by thermal evaporation on p+-Si substrates at room temperature by the methods of atomic force microscopy and x-ray diffraction. This kind of substrate induces a thin film phase and a triclinic phase which are formed directly onto p+-Si substrates and constitute a layer consisting of faceted grains with a step height between terraces of 15.8 A (1 A=0.1 nm) and 14.9 A, respectively. Above the critical thickness of the thin film phase, lamellar structures are found with an increasing fraction with the increase of the film thickness. When the film thickness is fixed, the fraction of lamellar structures increases with the increase of annealing temperature. These lamellar structures are identified as the second phase with a interplanar distance of 14.9 A corresponding to the pentacene triclinic phase. Furthermore, the thin film phase consisting of several micrometre sized uniformly oriented grains at an annealing temperature of less than 80°C and a deposition rate of 0.6 A/s is observed.

The crystallization and microstructure change of self-organization and the related conduction mechanisms of polymer semiconductor active thin layer in polymer organic field-effect transistors (OFET) are investigated by synchrotron radiation grazing incident X-ray diffraction (GIXRD) for understanding the relationships between polymer self-organization and charge carry. The change of the crystalline microstructure of RR-P3HT clarifies the effect of SAMs for improving the interface between the insulator layer and the organic semiconductor layer. The self-organiztion of RR-P3HT modified by SAMs improves the crystalliztion to pack form the thiophene rings along the perpendicular direction of substrate and results in that the π-π interchains are stacked to parallel the substrate. The two-dimensional charge transport is improved. Furthermore, we find that two-dimensional, conjugated, and self-organized crystalline lamellae are easier to gain with slow grown film than with fast grown film.

The contact effect on the performances of organic thin film transistors (OTFTs) is studied here. We fabricate Bottom-gated top-contact Pentacene-OTFTs on heavily doped n type Silicon wafers with using Al modified by MoO3 as source and drain electrodes. Field effect mobility μef of the OTFT reaches 0.42cm2/V ·s,the threshold voltage and the on/off current ratio arrive at -5.0 V and 4.7×103 respectively. The electric potential distribution in the channel is qualitively investigated by means of middle probe method (MPM) and the output curve is simulated by the charge drift method. Considering the contact effect,the μef is greatly improved to 1.1 cm2/V ·s,which indicates the importance of the contact engineering in OTFTs.