Electronic and structural properties of functional interfaces in organic thin film transistors

Main aim of this PhD thesis was the investigation of electronic effects taking place at the functional interfaces (i.e. organic/dielectric and organic/metal) of n-type organic thin-film transistors (OTFT)s. At present, OTFTs are an emerging technology of particular interest for the development of low-cost, flexible and large-area electronics. 
In general, it is widely recognized that the mechanisms occurring at the interfaces separating the main device components (i.e. electrodes, organic channel, gate dielectric) play a critical role in the overall performances. Moreover, while p-channel organic transistors have been largely investigated over the last two decades, a deeper understanding of interface interactions in n-channel OTFTs, which are essential to achieve the full potential of organic electronics, is still lacking. 
In this PhD work, the attention was focused on n-type OTFT based on N,N′-bis(n- ctyl)-x:y,dicyanoperylene-3,4:9,10-bis(dicarboximide) PDI-8CN2, as semiconductor, SiO2, as dielectric gate and Gold as metal for the electrodes.
Nowadays, PDI-8CN2 is considered one of the most appealing n-type organic semiconductors thanks to its ability to combine high charge mobility (up to 0.1 cm2 V-1 s-1), good electrical stability in air and solution processability. Despite these appealing features, at the state-of-the-art, there is a very poor knowledge about the phenomena arising at PDI-8CN2/SiO2 and PDI-8CN2/gold interfaces.
To carry out this work, several complementary high-resolution techniques have been used. Concerning the PDI-8CN2/SiO2 system, thin-film transistors have been fabricated at the Department of Physics of the University “Federico” II and at the Institute of Research CNR- Spin in Naples. Electron trapping mechanisms and charge density redistribution at the PDI-8CN2/SiO2 interface have been investigated by means of both electrical transport measurements (Bias Stress (BS) effect analysis) and optical spectroscopy techniques (Polarization Resolved Second Harmonic Generation (PR-SHG), Photoluminescence (PL) and Excitation-Resolved Photoluminescence (PLE) spectroscopy). Thin film morphology and crystal structure, instead, have been studied by Atomic Force Microscopy (AFM) and X-Ray Diffraction (XRD) analyses. 
The electronic phenomena occurring at the PDI-8CN2/Au interface have been deeply analyzed using photoemission spectroscopy (PES) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments. These experiments have been carried out at the Institute of Physical and Theoretical Chemistry of the University of Tubingen and at the third-generation synchrotron radiation source Bessy II in Berlin. 
In Chapter 1, OTFT applications and working basic principles are introduced. Moreover, a short description of recent achievements in the field of n-type organic materials is given. 
Chapter 2 is focused on the main physical and chemical processes occurring in general at organic/metal and organic/dielectric interfaces. Physisorption and chemisorption mechanisms are introduced and the effects of interface interactions on charge density distribution and orientation of the organic molecules at metal surface are presented. Moreover, the influence of the surface and bulk properties of the dielectric on the OTFT response is also considered.
Chapter 3 deals with the results obtained investigating charge transport and trapping mechanisms in PDI-8CN2 thin film transistors, deposited on bare and HMDS-functionalized SiO2 substrates. 
Despite the film morphology is poorly affected by the hydrophobic degree of the SiO2 interface as demonstrated by AFM images, the PDI-8CN2 OTFT electrical performances are strongly improved by the dielectric HMDS-treatment, which, in particular, provides a reduced sensitivity on hysteresis and bias stress (BS) phenomena. Moreover, in long-term (more than 1 day) BS measurements carried out in vacuum conditions, we also found that BS-induced IDS(t) decay tends to saturate. It is noteworthy to highlight that, in the field of organic transistors, this phenomenon was never reported in the past. To explain the BS experimental results, a model based on the occurrence of reduction-oxidation reactions involving PDI-8CN2, water molecules and surface silanol hydrolysis was proposed. According to this model, BS effect arises from the neutralization of negatively charged species in PDI-8CN2 molecules by means of H+ ions, diffusing from the SiO2 layer to the active channel (Proton migration) after the application of a positive gate voltage. 
Chapter 4 is aimed at presenting the results achieved by the investigation of PDI-8CN2/SiO2 interface by means of optical spectroscopy measurements. Dependence of SHG susceptibilities on both the film thickness and the dielectric surface treatments pointed out a non-uniform distribution of charge carriers in PDI-8CN2, whose spatial profile was affected by hydrophobic passivation of SiO2 surface by HMDS. An interpretation model based on the presence of a net charge localized in SiO2 accompanied by a charge redistribution in the organic semiconductor, according to a Debye-Huckel screening mechanism, was developed. This model was reinforced by PL and PLE characterizations, which support the presence of mobile charge carriers in PDI-8CN2 even in absence of any applied VGS. Such an interpretation seems to confirm the hypothesis of the VGS-driven proton migration phenomenon as physical origin of BS effect in OTFT based on PDI-8CN2/SiO2 interfaces.
In Chapter 5, the analysis of the PDI-8CN2/Au interface is reported. The structural and electronic features of PDI-8CN2 thin films deposited on Au(111) single crystals were investigated from the monolayer to the multilayer regime. From combined AFM and XPS analyses, 2D growth mode with molecular stepped terraces was deduced. This morphology was obtained without heating the substrate and adopting a low deposition rate. The formation of a chemical bond (chemisorption) between PDI-8CN2 and gold was pointed out from thickness dependent XPS measurements. From a detailed XPS peak fit analysis, CN groups were identified as the main species involved in the chemisorption process. This finding was further supported by excitation photon energy dependent XPS measurements. A thickness dependent molecular reorientation was deduced from NEXAFS measurements. These experiments demonstrated that PDI8-CN2 molecules on top of the gold single crystal are arranged with the molecular core adopting prevalently a flat lying position. However, with increasing film thickness, molecules are subjected to a reorientation and the upright standing position (c-axis orientation) is mainly assumed.