Performance, Interfacial Properties and Applications of Printed Organic Diodes

Abstract

Organic semiconductor materials offer the intriguing possibility to fabricate diodes, transistors and circuits on flexible substrates using cost-effective mass printing processes. However, the fabrication of fully printed devices is challenging due to the properties of the organic materials and limitations set by the manufacturing processes. The published research in the field of organic electronics mainly concentrates on traditional fabrication methods such as photolithography and vacuum processing. With printing processes, it could be possible to manufacture relatively simple, high-volume products on a continuous production line.

Diodes are basic electronic components that are commonly used as switches and in rectifier circuits. Several groups have fabricated circuits based on organic diodes, but there are only a few reports on printed devices. For printed organic rectifying diodes, the published results usually exhibit relatively high leakage currents and/or poor stability in ambient atmosphere. The work done for this thesis concentrated on the fabrication, properties and applications of air stable, roll-to-roll processable organic diodes on a flexible polyester substrate.

The diode properties were examined using electrical characterization, atomic force microscopy, impedance spectroscopy and X-ray photoelectron spectroscopy. The data showed that a thin insulating interfacial layer of Cu2O and organic material at the rectifying contact reduces the diode reverse current up to 2 orders of magnitude without having a notable effect on the forward current. The high rectification ratio enabled the diodes to be used in a rectifier circuit, where rectification up to 10 MHz was observed. Furthermore, the diodes delivered output DC voltages of 10.4 V at 13.56 MHz from 10 V zero-to-peak AC input voltage in a printed charge pump circuit. In addition, an active-matrix driving circuit employing printed diodes with an electrophoretic front plane material and a contrast ratio of 4:1 was demonstrated.

The thesis discusses several aspects on how to improve the properties of the diodes. Future research should concentrate on examining the possibility to fabricate the interfacial layer, fabricating devices with printable semiconductors that exhibit higher charge carrier mobilities and examining the restrictions brought about by the printing process. The results of this thesis open up a path for further research in the area of printed organic rectifying diodes.