Direct imaging of defect formation in strained organic flexible electronics by Scanning Kelvin Probe Microscopy

The development of new materials and devices for flexible electronics depends crucially on the understanding of how strain affects electronic material properties at the nano-scale. Scanning Kelvin-Probe Microscopy (SKPM) is a unique technique for nanoelectronic investigations as it combines non-invasive measurement of surface topography and surface electrical potential. Here we show that SKPM in non-contact mode is feasible on deformed flexible samples and allows to identify strain induced electronic defects. As an example we apply the technique to investigate the strain response of organic thin film transistors containing TIPS-pentacene patterned on polymer foils. Controlled surface strain is induced in the semiconducting layer by bending the transistor substrate. The amount of local strain is quantified by a mathematical model describing the bending mechanics. We find that the step-wise reduction of device performance at critical bending radii is caused by the formation of nano-cracks in the microcrystal morphology of the TIPS-pentacene film. The cracks are easily identified due to the abrupt variation in SKPM surface potential caused by a local increase in resistance. Importantly, the strong surface adhesion of microcrystals to the elastic dielectric allows to maintain a conductive path also after fracture thus providing the opportunity to attenuate strain effects

Single-Crystal Organic Charge-Transfer Interfaces probed using Schottky-Gated Heterostructures

Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the hetero-interfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN2 single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transport is due to electrons, whose mobility exhibits band-like behavior from room temperature to ~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The electron density decreases linearly with decreasing temperature, an observation that can be explained quantitatively based on the heterostructure band diagram. These results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces and show the potential of Schottky-gated organic heterostructures for the investigation of transport in molecular semiconductors.Comment: 37 pages, 9 Figures (including supplementary information

Interfacial charge transfer in nanoscale polymer transistors

Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel ($L < 100$ nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero-gate-voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface.Comment: 19 preprint pages, 6 figure

Space Charge at Nanoscale: Probing Injection and Dynamic Phenomena Under Dark/Light Configurations by Using KPFM and C-AFM

International audienc

Molecular Electronics

Molecular electronics describes the field in which molecules are utilized as the active (switching, sensing, etc.) or passive (current rectifiers, surface passivants) elements in electronic devices. This review focuses on experimental aspects of molecular electronics that researchers have elucidated over the past decade or so and that relate to the fabrication of molecular electronic devices in which the molecular components are readily distinguished within the electronic properties of the device. Materials, fabrication methods, and methods for characterizing electrode materials, molecular monolayers, and molecule/electrode interfaces are discussed. A particular focus is on devices in which the molecules or molecular monolayer are sandwiched between two immobile electrodes. Four specific examples of such devices, in which the electron transport characteristics reflect distinctly molecular properties, are discussed

Flexible oxide thin film transistors: device fabrication and kelvin probe force microscopy analysis

I transistor a film sottile basati su ossidi amorfi semiconduttori sono ottimi candidati nell'ambito dell'elettronica su larga scala. Al contrario delle tecnologie basate su a-Si:H a poly-Si, gli AOS presentano un'elevata mobilità elettrica (m > 10 cm^2/ Vs) nonostante la struttura amorfa. Inoltre, la possibilità di depositare AOS a basse temperature e su substrati polimerici, permette il loro impiego nel campo dell'elettronica flessibile. Al fine di migliorare questa tecnologia, numerosi TFT basati su AOS sono stati fabbricati durante 4 mesi di attività all'Università Nova di Lisbona. Tutti i transistor presentano un canale formato da a-GIZO, mentre il dielettrico è stato realizzato con due materiali differenti: Parylene (organico) e 7 strati alternati di SiO2 e SiO2 + Ta2O5. I dispositivi sono stati realizzati su substrati flessibili sviluppando una nuova tecnica per la laminazione e la delaminazione di fogli di PEN su supporto rigido. L'ottimizzazione del processo di fabbricazione ha permesso la realizzazione di dispositivi che presentano caratteristiche paragonabili a quelle previste per TFT costruiti su substrati rigidi (m = 35.7 cm^2/Vs; VON = -0.10 V; S = 0.084 V/dec). Al Dipartimento di Fisica dell'UNIBO, l'utilizzo del KPFM ha permesso lo studio a livello microscopico delle prestazioni presentate dai dispositivi analizzati. Grazie a questa tecnica di indagine, è stato possibile analizzare l'impatto delle resistenze di contatto sui dispositivi meno performanti e identificare l'esistenza di cariche intrappolate nei TFT basati su Parylene. Gli ottimi risultati ottenuti dall'analisi KPFM suggeriscono un futuro impiego di questa tecnica per lo studio del legame tra stress meccanico e degradazione elettrica dei dispositivi. Infatti, la comprensione dei fenomeni microscopici dovuti alla deformazione strutturale sarà un passaggio indispensabile per lo sviluppo dell'elettronica flessibile

Charge transport in nanoscale vertical organic semiconductor pillar devices

We report charge transport measurements in nanoscale vertical pillar structures incorporating ultrathin layers of the organic semiconductor poly(3-hexylthiophene)(P3HT). P3HT layers with thickness down to 5 nm are gently top-contacted using wedging transfer, yielding highly reproducible, robust nanoscale junctions carrying high current densities (up to $10^6$ A/m$^2$). Current-voltage data modeling demonstrates excellent hole injection. This work opens up the pathway towards nanoscale, ultrashort-channel organic transistors for high-frequency and high-current-density operation.Comment: 30 pages, 8 figures, 1 tabl

Interface properties of modified indium tin oxide based organic light emitting diodes withfunctional aromatic molecules

Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2011Includes bibliographical references (leaves: 89-96)xiii, 89 leavesThis thesis focused on modification and characterization of ITO substrates with carboxylic acid based self-assembled monolayers to improve OLED device performance. In this study, ITO was used as anode material in OLEDs. In order to modify ITO electrodes, MePIFA and DPIFA aromatic small molecules with double bound carboxylic acid have been used as self-assembly monolayer (SAM). Characterizations of modified ITO and unmodified ITO surfaces were performed via atomic force microscopy and scanning tunneling microscopy. In addition to surface characterization, I-V measurements of the modified and unmodified ITO were taken via spreading resistance microscopy and scanning tunneling microscopy. Moreover, in order to measure change in the surface potential after the modification of ITO surface with MePIFA and DPIFA SAM molecules, Kelvin Probe Force Microscopy was performed. Finally two different configurations of OLEDs devices were fabricated using thermal evaporator system in order to explore the effect of SAM modified ITO on electrical characterization of OLED devices. It was shown that OLED intensity, and turn on voltage were improved compared to OLED devices with unmodified ITO

Formation of double ring patterns on Co2MnSi Heusler alloy thin film by anodic oxidation under scanning probe microscope

Double ring formation on Co2MnSi (CMS) films is observed at electrical breakdown voltage during local anodic oxidation (LAO) using atomic force microscope (AFM). Corona effect and segregation of cobalt in the vicinity of the rings is studied using magnetic force microscopy and energy dispersive spectroscopy. Double ring forma-tion is attributed to the interaction of ablated material with the induced magnetic field during LAO. Steepness of forward bias transport characteristics from the unperturbed region of the CMS film suggest a non equilibrium spin contribution. Such mesoscopic textures in magnetic films by AFM tip can be potentially used for memory storage applications.Comment: 7 pages, 5 figure

Investigation of structural properties of organic thin films for solar cell and transistor applications

For the past several decades, organic materials including polymers, oligomers and small molecules have been of great interest for their various applications in the electronics and the semiconductor industry. The most appealing advantages of organic materials compared to their inorganic counterparts are their compatibility with flexible substrates and amenability to low-temperature and low-cost fabrication processes such as evaporation, spin-coating and printing. Moreover, the ability to be utilized in fabrication of lightweight and large-area devices is among other reasons for popularity of organic materials. A large number of studies have reported on various aspects of the development and optimization of organic electronics such as organic light emitting diodes (OLEDs), solar cells (OSCs) and thin film transistors (OTFTs). Although significant progress has been made during this period, some of the intrinsic electrical properties of organic materials such as low carrier mobility have continued to hinder the full development and maturation of the organic electronics industry. In order to manufacture organic electronic devices with high performance, more detailed studies of the structure and the morphology of the organic materials as well as the underlying physical charge transport mechanisms should be performed. Additionally, growth, deposition and assembly processes need to be established and optimized for the new organic semiconductor technology.;This work aims to advance the understanding of the effect of the structural properties of organic thin films on the charge carrier transport within the organic thin films as well as the charge carrier injection between the organic layers and the organic-inorganic materials such as metal or dielectric layers. Charge carrier transport mechanisms between different layers are crucial factors in determining the efficiency of organic electronic devices. These parameters rely largely on the molecular structure, morphology and ordering of the organic thin films. In order to investigate these intrinsic properties, several organic thin films were prepared using vacuum thermal evaporation method. Their morphology and structural properties were studied by the combination of various techniques including atomic force microscopy, X-ray reflectivity, spectroscopic ellipsometry and transmittance measurements. Based on the produced organic thin films, organic semiconductor devices such as OTFTs and OSCs were fabricated and their electrical and optical properties were characterized. Moreover, the effect of morphology and structure of the organic thin films on the organic device performance was studied. Ambipolar thin film transistors based on pentacene and PTCDI-C8 as the active layer and lithium fluoride (LiF) as the gate dielectric layer were fabricated and characterized. Conduction behaviors of these devices were modeled using Fowler-Nordheim (FN) tunneling theory. The results of this study suggest that the charge transport in OTFTs correlate not only with the organic semiconductor film structure, but also with the dielectric--semiconductor interfacial effects. Moreover, bilayer heterojunction OSCs based on CuPc/PTCDI-C8 as the donor/acceptor layers were fabricated and their electrical and optical properties were characterized. The effects of the active layers\u27 structures and morphologies as well as the buffer layers\u27 thickness variation on the device performance were studied. The results of this study emphasized the importance of the thin film structural properties on the device performance