Organic thin films as active materials in field effect transistors and electrochemical sensing

Abstract

This PhD thesis is focused on Organic Electronics, an emerging field where different disciplines converge to gain insights into the properties of organic materials and their applications. Under the present work different organic materials have been realized and analysed for application both in Organic Field Effect Transistors and electrochemical sensing with Organic Electrochemical Transistors. An overview about Organic Electronic is reported with the most recent advancement of the last years: a state of the art of research about Organic Field Effect Transistors (OFETs) and Organic Electrochemical Transistors (OECTs) is given, with an overview on the emerging Organic Bioelectronics. The main motifs of the research performed are reported along the discussion. In the application of the supersonic molecular beam epitaxy method, thin films of Copper Phthalocyanine have been grown, reaching an unprecedented order in the crystalline structure, as the characterization by Raman spectroscopy and AFM have shown. A modified-pentacene molecule (2,3-CN2-TIPS-Pn films) has been used as active layer for the building of an OFET device, which showed an ambipolar behaviour with balanced electrons and holes mobility on the order of 2⋅10-3cm2/Vs. The charge transport properties of 2,3-CN2-TIPS-Pn films show the effectiveness of TIPS-Pn functionalization with cyano e− withdrawing groups to promote e- transport while maintaining equivalent h− transport. A second OFET device has been realized with tetracene organic thin films deposited on different dielectrics substrates: the devices have been characterized and the mobility measured. For the tetracene film deposited on the polystyrene substrate, we have found a mobility of 2⋅10-1 cm2/Vs, the highest retrieved up to now in literature for tetracene. The molecular structures of all the organic molecules used, have been deeply investigated by means AFM analysis and XRD-advanced algorithm tools. For the films made with the TIPS molecule, the GIXRD analysis revealed a favourable arrangement of the molecules in the TFT channel. The XRD analysis performed on the tetracene films revealed interesting correlation between the mobility of the film and the AFM and structural parameters: in particular the polystyrene film shows the best surface coverage and the highest alpha phase percentage of the molecular structure. New insights into the device physics of OECT have been discovered: in the sensing experiments with OECTs, the role of the gate electrode has been investigated. This clarified the two working principles an OECT can operate (faradaic or non-faradaic mode). We found that an OECT can switch between these two modes of operation simply changing the metal wire acting as gate electrode. In particular the faradaic operational mode lead to the possibility to exploit the transistor as a halide sensor, able to detect Na+ ions in solution with a sensibility up to 10μM. Then the role of electrolyte has been studied with micellar structures, which open unexplored horizons for the application of OECT with a new class of electrolytes. The ability of micelles to dope/dedope efficiently the PEDOT:PSS permitted to investigate the doping process of the polymer, that is one of the main issue today in organic electronics. The modulation signals have been correlated with the surface charge of the micelles, measured by the zeta-potential techniques and the injection of micelles into the polymer structure has been probed by an optical spectroscopy measurement, performed in-situ during the OECT current acquisition. As a consequence of the micelle experiment, bilayer structure, like liposomes, have been tested and detected for the first time with an OECT. Although this experiment is currently in progress, it seems particularly promising, mainly because the opportunity to exploit the ability of liposomes to trap and release drugs in a controlled way. A new nanoparticles-based sensor has been developed, able to detect the presence in solution of iron-oxide magnetic nanoparticles functionalised with different polymeric coatings: we provide the ability of OECTs to detect and monitor selectively, with an appropriate choice of the electrolyte, different nanosystems. We demonstrate an on-line sensing based on OECTs, with an easy sampling/sample preparation, for the detection of functionalized magnetic nanoparticles.

OECTs have promising applications in bioelectronics as well as in nanomedicine or neuroscience. They are becoming an ideal platform for both in-vitro and in-vivo biomedical applications, as well as for the development of protocells inside miniaturized electro-chemical laboratory.