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We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability. [ABSTRACT FROM AUTHOR]

Highlights: [•] Electrical and morphological characterization of flexible OTFT devices under applied bending-stress is reported. [•] Bending-stress modifies the lateral spacing between individual pentacene crystallites. [•] The deterioration of the metal electrodes is found to be responsible for irreversibilities in the device characteristics. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work is to contribute to establish whether morphological properties prevail over crystal lattice organization in determining the cause for the observed sensitivity of organic semiconductors to mechanical deformations in Organic Thin Film Transistors (OTFTs). To this aim, the morphology of the active layer (made by Pentacene) of OTFTs fabricated on flexible substrates was intentionally modified by properly changing the deposition rate. By comparing Atomic Force Microscopy (AFM), X-ray Diffraction (XRD) investigations and the electrical characterization of the devices, we can conclude that sensitivity is clearly dominated by morphological rather than lattice effects. In addition, these results suggest a simple method for obtaining, in devices based on evaporated small molecules, a fine tuning of the sensitivity to mechanical deformation that could be predictably adjusted in a range that goes from a minimum, practically negligible, value, desirable for applications in flexible electronic circuitry, to a maximum extent, useful for mechanical sensing applications. [Copyright &y& Elsevier]

Abstract: One of the open issues in organic electronics is the long-term stability of devices based on organic materials, as oxidation is believed to be a major reason for early device failure. The focus of our research is to investigate the effects of low energy ion implantation (N and Ne) in the reduction and control of the degradation of pentacene organic thin film transistors (OTFTs) due to the exposure to atmosphere (i.e. oxygen and water). Despite the strong molecular structure modifications induced by ion implantation, we have observed that a controlled damage depth distribution preserves the functionality of the device. The electrical properties of the pentacene layer and of the OTFT have been investigated by means of current–voltage and photocurrent spectroscopy analyses. We have characterized the structural modification induced by ion implantation and we have monitored the effectiveness of this process in stabilizing the device carrier mobility and threshold voltage over a long time (over 2000h). In particular, we have assessed by depth resolved X-ray photoemission spectroscopy analyses that, by selectively implanting with ions that can react with the hydrocarbon matrix (e.g. N+), it is possible to locally modify the charge distribution within the organic layer. [Copyright &y& Elsevier]

In this paper we report on the realization of flexible all-organic ambipolar field-effect transistors (FETs) realized on unconventional substrates, such as plastic films and textile yarns. A double layer pentacene-C60 heterojunction was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of soft lithography microcontact printing (μCP). Very interestingly growing C60 on a predeposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the film so it obtains n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C60. As a result, it was possible to obtain all-organic ambipolar FETs and to optimize their electrical properties by tuning the thicknesses of the two employed active layers. Moreover, it will be shown that modifying the triple interface between dielectric/semiconductor/electrodes is a crucial point for optimizing and balancing injection and transport of both kinds of charge carriers. In particular, we demonstrate that using a middle contact configuration in which source and drain electrodes are sandwiched between pentacene and C60 layers allows significantly improving the electrical performance in planar ambipolar devices. These findings are very important because they pave the way for the realization of low-cost, fully flexible and stretchable organic complementary circuits for smart wearable and textile electronics applications. [ABSTRACT FROM AUTHOR]

Abstract: We report on the realization of fully flexible and transparent n-type and ambipolar all-organic OFETs. A double layer, pentacene-C60 heterojunction, was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography MicroContact Printing (μCP). Interestingly, as demonstrated by atomic force microscopy and X-ray diffraction investigations, growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to obtain n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C60. As a result, it was possible to realize n-type and ambipolar all-organic OFETs by optimizing the thicknesses of the pentacene buffer layer. All devices, measured in air, worked in accumulation mode with mobilities up to 1×10−2 cm2/Vs and 3.5×10−4 cm2/Vs for p-type and n-type regimes, respectively. This is particularly interesting because it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. These results confirm the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials. [Copyright &y& Elsevier]

We present the electromechanical characterization of organic field-effect transistors (OFETs) fabricated with different organic semiconductors. Pentacene- and poly(3-hexylthiophene-2,5-diyl) (P3HT)-based OFETs have been investigated as strain sensors, and a clear correlation between the structural and morphological properties of the active layer with the device sensitivity has been observed. The highly disordered structure of polymeric films, as P3HT, confirmed by morphological and structural investigations, gives rise to a dramatic reduction of the device response to mechanical stimuli. Nevertheless, an unambiguous, fast, and reproducible response has been obtained also for this material, which, being solution processible, represents a valuable solution for the fabrication of low-cost pressure sensors for a variety of innovative applications. [ABSTRACT FROM PUBLISHER]