Pentacene domains

Fig. 2.4. (A) Sketch of the cryostat insert for single-molecule spectroscopy by fluorescence excitation. The focus of lens L is placed in the sample S by the magnet/coil pair M, C. (B) Scan over the inhomogeneous line (a) with a 2 GHz region expanded (b) to show isolated single-molecule absorption profiles. (C) Three-dimensional pseudo-image of single molecules of pentacene in p-terphenyl. The measured fluorescence signal (z-axis) is shown over a range of 300 MHz in excitation frequency (horizontal axis, center = 592.544 nm) and 40 pm in spatial position (axis into the page). (D) Rotation of the data in (c) to show that in the spatial domain, the single molecule maps out the shape of the laser focal spot. Bar, 5 pm. For details, see [33]...

Quite often the mobilities determined for a given molecule, e.g. rabrene or pentacene, differ for different devices in different laboratories, reflecting the problems related to the extraction of the mobility data from the electrical characteristics. Typically, these characteristics are not only defined by the mobilities but also by the contact resistance and of course the presence of domain boundaries defects and impirrities in the organic semiconductor. The determination of the true intrinsic mobility of an organic semiconductor is still a challenge, which has only been overcome in a very few cases. 

In order to exclude the appearance of rotational domains in the initial stage of growth we have used a Cu(llO) substrate and have studied the growth of pentacene films on this substrate quite extensively. In fact, a highly ordered submonolayer and a saturated monolayer phase are found which exhibit a uniform alignment of the flat lying molecules with their long axes orientated parallel to the (110 )-azimuth direction of the substrate [46, 52]. 

The second planar aromatic hydrocarbon for which a more extensive investigation of adlayers fabricated by OMBD has been earned out is perylene. Especially the deposition on Cu(llO) substrates is of particular interest sinee this low symmetry surface may provide a useful template whieh rules out the formation of rotational domains in the initial stage of growth (as found for the case of pentacene on this substrate [52]). 

The interface between pentacene and a layer of PEDOT PSS being spin coated onto Au source and drain contacts was analyzed by AFM, grazing incident angle x-ray diffraction and UPS. It was found that the PEDOT PSS interlayer reduced the hole-injection barrier and simultaneously led to increased crystalline domains of the semiconductor. Both changes improved the OFET characteristics. 

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