Optimization of purification procedures in organic semiconductor synthesis for improving the performances of organic field-effect transistors

The aim of the research activity focused on the investigation of the correlation between the degree of purity in terms of chemical dopants in organic small molecule semiconductors and their electrical and optoelectronic performances once introduced as active material in devices. The first step of the work was addressed to the study of the electrical performances variation of two commercial organic semiconductors after being processed by means of thermal sublimation process. In particular, the p-type 2,2′′′-Dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DH4T) semiconductor and the n-type 2,2′′′- Perfluoro-Dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DFH4T) semiconductor underwent several sublimation cycles, with consequent improvement of the electrical performances in terms of charge mobility and threshold voltage, highlighting the benefits brought by this treatment to the electric properties of the discussed semiconductors in OFET devices by the removal of residual impurities. The second step consisted in the provision of a metal-free synthesis of DH4T, which was successfully prepared without organometallic reagents or catalysts in collaboration with Dr. Manuela Melucci from ISOF-CNR Institute in Bologna. Indeed the experimental work demonstrated that those compounds are responsible for the electrical degradation by intentionally doping the semiconductor obtained by metal-free method by Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) and Tributyltin chloride (Bu3SnCl), as well as with an organic impurity, like 5-hexyl-2,2':5',2''-terthiophene (HexT3) at, in different concentrations (1, 5 and 10% w/w). After completing the entire evaluation process loop, from fabricating OFET devices by vacuum sublimation with implemented intentionally-doped batches to the final electrical characterization in inherent-atmosphere conditions, commercial DH4T, metal-free DH4T and the intentionally-doped DH4T were systematically compared. Indeed, the fabrication of OFET based on doped DH4T clearly pointed out that the vacuum sublimation is still an inherent and efficient purification method for crude semiconductors, but also a reliable way to fabricate high performing devices

Characterization and modelling of organic devices for simultaneous stimulation and recording of cellular electrical activity with Reference-Less Electrolyte-Gated Organic Field-Effect Transistors

The study of neuronal and neurodegenerative diseases requires the development of new tools and technologies to create functional neuroelectronics allowing both stimulation and recording of cellular electrical activity. In the last decade organic electronics is digging its way in the field of bioelectronics and researchers started to develop neural interfaces based on organic semiconductors. The interest in such technologies arise from the intrinsic properties of organic materials such as low cost, transparency, softness and flexibility, as well the biocompatibility and the suitability in realizing all organic printed systems. In particular, organic field-effect transistor (OFET) -based biosensors integrate the sensing and signal amplification in a single device, paving the way to new implantable neural interfaces for in vivo applications. To master the sensing and amplification properties of the OFET-based sensors, it is mandatory to gain an intimate knowledge of the single transistors (without any analytes or cells) that cannot be limited to basic characterizations or to general models. Moreover, organic transistors are characterized by different working principles and properties as respect to their inorganic counterpart. We performed pulsed and transient characterization on different OFETs (both p-type and n-type) showing that, even though the transistors can switch on and off very fast, the accumulation and/or the depletion of the conductive channel continues for times as long as ten seconds. Such phenomenon must be carefully considered in the realization of a biosensor and in its applications, since the DC operative point of the device can drift during the recording of the cellular signals, thus altering the collected data. We further investigate such phenomenon by performing characterizations at different temperatures and by applying the deep level transient spectroscopy technique. We showed that the slow channel accumulation (and depletion) is due to the semiconductor density-of-states that must be occupied in order to bring the Fermi energy level close to the conduction band. This is a phenomenon that can takes several seconds and we described it by introducing a time-depend mobility. We also proposed a technique to estimate the behavior, in time, of the position of the Fermi energy level as respect to the conduction band. To understand the electrochemical transduction processes between living cell and organic biosensor, we realized two-electrodes structure (STACKs) where a drop of saline solution is put directly in contact with the organic semiconductor. On these devices, we performed electrochemical impedance spectroscopy at different DC polarizations and we developed an equivalent circuit model for the metal-organic semiconductor-solution structures that are typically used as transducers in biosensor devices. Our approach was extending the standard range of the bias voltages applied for devices that operate in water. This particular characterization protocol allowed to distinguish and investigate the different mechanisms that occur at the different layers and interfaces: adsorption of ions in the semiconductor; accumulation and charge exchange of carriers at the semiconductor/electrolyte interface; percolation of the ionic species through the organic semiconductor; ion diffusion across the electrolyte; ion adsorption and charge exchange at the platinum interface. We highlighted the presence of ion percolation through the organic semiconductor layer, which is described in the equivalent circuit model by means of a de Levie impedance. The presence of percolation has been demonstrated by environmental scanning electron microscopy and profilometry analysis. Although percolation is much more evident at high negative bias values, it is still present even at low bias conditions. In addition, we analyze two case studies of devices featuring NaCl (concentration of 0.1M) and MilliQ water as solution, showing that both cases can be considered as a particular case of the general model presented in this manuscript. The very good agreement between the model and the experimental data makes the model a valid tool for studying the transducing mechanisms between organic films and the physiological environment. Hence this model could be a useful tool not only for the characterization and failure analysis of electronic devices, such as water-gated transistors, electrophysiological interfaces, fuel cells, and others electrochemical systems, but also this model might be used in other applications, in which a solution is in intimate contact with another material to determine and quantify, if undesired mechanisms such as percolation and/or redox corrosive processes occur. Lastly, the knowledge gain on OFETs and STACKs were put together to realize electrolyte-gated field effect transistors (EGOFETs). We then developed a model to describes EGOFETs as neural interfaces. We showed that our model can be successfully applied to understand the behaviour of a more general class of devices, including both organic and inorganic transistors. We introduced the reference-less (RL-) EGOFET and we showed that it might be successfully used as a low cost and flexible neural interface for extracellular recording in vivo without the need of a reference electrode, making the implant less invasive and easier to use. The working principle underlying RL-EGOFETs involves self-polarization and back-gate stimulation, which we show experimentally to be feasible by means of a custom low-voltage high-speed acquisition board that was designed to emulate a real-time neuron response. Our results open the door to using and optimizing EGOFETs and RL-EGOFETs for neural interfaces

Surface Electrostatic Properties of Organic Semiconductors

University of Minnesota Ph.D. dissertation. March 2016. Major: Material Science and Engineering. Advisor: C. Frisbie. 1 computer file (PDF); x, 183 pages.Understanding the surface/interface electrostatic properties of organic semiconductors has great implications for the fundamental transport properties of these materials and their performance in devices. Therefore, this thesis aims to correlate electrostatic properties with microstructure and mechanical strain in benchmark organic semiconductors. To this end, a number of scanning probe microscopy (SPM) techniques are employed to examine thin films and single crystals of prototypical organic semiconductors. In particular, strong variations of interfacial polarization at the organic/insulator interfaces are quantified by scanning Kelvin probe microscopy (SKPM). The roles of the dielectric type and deposition condition are identified. Moreover, striking lateral electrostatic heterogeneities are visualized in thermally deposited organic semiconductor bi-layers on various dielectrics, and are directly related to the complex microstructural motifs of the films. The mixed homoepitaxial growth modes, which give rise to the inhomogeneous microstructure, can be conveniently determined by combining two variants of lateral force microscopy (LFM), namely, friction force microscopy (FFM) and transverse shear force microscopy (TSM). Furthermore, a fundamental correlation is established between the surface electrostatic potential and mechanical strain. The effects of tensile and compressive strains in both elastic and plastic regimes are determined for the first time. Overall, organic semiconductors exhibit complex surface/interface electrostatic properties, which can be visualized by SPM and can be correlated with microstructure and mechanical properties

Alternative materials for flexible transparent conductive electrodes

This thesis investigates new alternative materials for flexible transparent electrodes: monolayer graphene and micron-scale metal mesh structures. Growth of graphene on copper foils by chemical vapour deposition (CVD) was investigated by commissioning and developing a CVD system in Tyndall. Initial growth runs resulted in poor graphene coverage. Several routes for growth improvement were examined: an acid pre-treatment, substrate geometry and growth pressure. Following this improvement, a continuous growth run was carried out displaying high monolayer graphene coverage. Graphene was transferred to Si/SiO2 (90 nm thermal oxide) and glass substrates using a wet chemical transfer process. This process involves the use of a polymer which acts as a support mechanism. However, polymer residue can have drastic effects on the electrical performance of CVD graphene films. Therefore an alternative method for polymer removal with the use of heated acetone (~ 60 oC) was investigated. Micron-scale platinum mesh structures were fabricated on rigid glass substrates using a range of metal deposition techniques; metal evaporation and lift-off; ALD and dry etching and sputter deposition and dry etching. Square, hexagonal, circular and a new asymmetric pentagonal tiling were utilised as metal meshes. Their performance were investigated along with the metal deposition technique. Evaporation and lift-off provided the most consistent technique in relation to transparency, haze and sheet resistance. Finally, asymmetric pentagonal platinum meshes were fabricated on flexible transparent substrates using metal evaporation and lift-off. All designs were bent around a radius of curvature (in air) of ~ 3.8 mm up to 1,000 bending cycles for both tension and compression and suggested good performance in comparison to literature. All three designs were used as transparent heaters via Joule heating. All heaters demonstrated good thermal characteristics such as low response times and high thermal resistances. Finally, a pentagonal metal mesh was used to de-ice a glass substrate