Organic thin film devices based on conjugated small molecules and polymers are receiving huge attention due to their potential in optoelectronic, photovoltaic and sensor applications. The determination of charge transport properties, and of electronic excited states energy and distribution is still an open and controversial issue, despite the fact that it plays a crucial role in the applicability of the material under study. In the present work, all-organic and transparent devices have been fabricated and characterized. We fabricated flexible thin film field effects transistors (FET) by co-evaporating different proportions of -sexithiophene (6T) and -dihexylsexithiophene (DH6T) as active material on polyethylene terephtalate (Mylar, Du Pont), which acts both as a substrate and as the gate dielectric. The charge injecting properties of metal (Au) and organic (PEDOT:PSS) contacts have been tested. The morphology and structure of the organic layers have been investigated by X-ray diffraction and atomic force microscopy. The electronic transport properties of the FETs, the energy distribution of the excited electron energy levels, and the major transitions across the energy gap have been investigated as a function of the 6T/DH6T ratio by means of current-voltage measurements and photocurrent spectroscopy, revealing the possibility of tuning the threshold voltage of the device as a function of the ratio of the two molecules. We focused on the effects of interface states on the device transport properties (such as mobility and threshold voltage). These states are located both at the substrate/semiconductor and at the electrode/semiconductor interfaces, and we identify the hexyl chains of DH6T as the key factor controlling the electrical performance of the device. Their presence, in fact, affects the energy level alignment at the metal/organic interface and we propose a model to describe the higher charge carrier mobility observed in DH6T, based on the additionally available electron density on the hexyl chains, which induces a localized dipole-like charge distribution.

Tunable transport properties of organic field effect devices

FRABONI, BEATRICE;
2007

Abstract

Organic thin film devices based on conjugated small molecules and polymers are receiving huge attention due to their potential in optoelectronic, photovoltaic and sensor applications. The determination of charge transport properties, and of electronic excited states energy and distribution is still an open and controversial issue, despite the fact that it plays a crucial role in the applicability of the material under study. In the present work, all-organic and transparent devices have been fabricated and characterized. We fabricated flexible thin film field effects transistors (FET) by co-evaporating different proportions of -sexithiophene (6T) and -dihexylsexithiophene (DH6T) as active material on polyethylene terephtalate (Mylar, Du Pont), which acts both as a substrate and as the gate dielectric. The charge injecting properties of metal (Au) and organic (PEDOT:PSS) contacts have been tested. The morphology and structure of the organic layers have been investigated by X-ray diffraction and atomic force microscopy. The electronic transport properties of the FETs, the energy distribution of the excited electron energy levels, and the major transitions across the energy gap have been investigated as a function of the 6T/DH6T ratio by means of current-voltage measurements and photocurrent spectroscopy, revealing the possibility of tuning the threshold voltage of the device as a function of the ratio of the two molecules. We focused on the effects of interface states on the device transport properties (such as mobility and threshold voltage). These states are located both at the substrate/semiconductor and at the electrode/semiconductor interfaces, and we identify the hexyl chains of DH6T as the key factor controlling the electrical performance of the device. Their presence, in fact, affects the energy level alignment at the metal/organic interface and we propose a model to describe the higher charge carrier mobility observed in DH6T, based on the additionally available electron density on the hexyl chains, which induces a localized dipole-like charge distribution.
2007
9th European Conference on Molecular Electronics
156
160
R. DiPietro; B. Fraboni; P.Cosseddu; A.Bonfiglio; J.-O. Vogel; I. Salzmann; J. P. Rabe; N.Koch
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/52835
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