Silvan Pretl

Ionic liquid as an electrolyte for organic electrochemical transistor

The presented study is focused on the improvements of static and dynamic characteristics of organic electrochemical transistor (OECT). Especially the switching speed, the switching ratio and stability were examined. For the given study a special structure of the OECT has been developed for the evaluation of various materials and their influence on selected characteristics. Conductive polymer poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) was used as the main material for channel of OECT device. The influence of two ionic liquids (ILs) used as an electrolyte in OECT is compared. The results indicate a significant difference between ILs and their suitability to be used as electrolyte in the OECT.

Contact properties of PEDOT:PSS

With ongoing rapid development of printed and organic electronics, high attention is paid to studies of contact resistance, which is recognized as the crucial factor affecting the overall performance of devices based on organic semiconductors. This paper presents the combined analysis of contact resistance in metal-organic heterostructures based on simultaneous employment of both the conventional and the modified transfer length method (C-TLM, M-TLM). Three modifications of the PEDOT:PSS with composition ratios 5:10, 3:10 and 2:10 between PEDOT and PSS in the final polyelectrolytic dispersions were chosen as the model organic semiconductor materials. Their contact properties were investigated at junctions with Au plated copper electrodes. The test vehicles consisted of PEDOT:PSS deposited by dip coating on glass-epoxy substrates with collinear electrodes with distances scaled from 100 μm to 1000 μm. Total resistances derived from I-V measurement on each electrode pair taken in relation to their distances were subjected to TLM analysis with linear regression fitting giving the information of contact and sheet resistance. The most consistent results were obtained for the PEDOT:PSS (5:10) and confirmed substantial contribution of the contact resistance, which constitutes up to 83 % of the overall electrical resistance of tested structures in the case of the shortest electrode distance.

Electrical characterization of PEDOT:PSS

The purpose of this paper is to summarize the results of electrical measurements carried out on different types of metal-semiconductor heterojunctions based on PEDOT:PSS in order to examine the possibilities of either ohmic contacts or rectifying Schottky junctions formation. The character of created junctions was evaluated according to their I–V characteristics. Unusual electrical behaviour was observed in the case of heterojunctions formed on gold, platinum and carbon with a region of negative differential resistance. The phenomenon supposed to be partly responsible for such nonlinearity is existence of thin barrier layer on the surface of dried PEDOT:PSS, through which charge carriers could pass by tunneling. However, deeper investigation will be necessary to enlighten all the circumstances leading to origin of this behaviour.

Comparison of quasi-solid state and liquid electrolytes for organic electrochemical transistor

The study is focused on a comparison of quasi-solid and liquid electrolytes for organic electrochemical transistor (OECT). Especially influences of used electrolytes on the switching speed and the switching ratio were compared. For the given study a three electrode structure of the OECT has been used for the evaluation of various materials and their influence on selected characteristics. For the channel of the OECT device, conductive polymer poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT.PSS) was used. Two different ionic liquids and one quasi-solid state electrolyte were used and compared in OECT structure. The results show that both OECTs with ionic liquids are faster than the OECT with quasi-solid state electrolyte but have approximately 10 times smaller switching ratio.

Low-voltage electrochemical transistor based on ionic liquid-gated PEDOT:PSS with copper electrodes

One of viable routes to a broader employment of printed and flexible electronics resides in combination of novel functional printing techniques and existing industrial processes. We have investigated potential combination of industrial continuous foil metallization with screen printing to prepare low voltage electrochemical transistor. The transistor based on vacuum evaporated copper electrodes, screen printed channel from PEDOT:PSS and manually deposited ionic liquid exhibited low voltage operation with saturated output current at Vds = -1 V. Long term voltage stressing revealed an overall resistance decrease of the transistor channel under a constant bias with the open gate. On the contrary, repetitive gate switching increased the overall channel resistance.

Organic materials for humidity detection

This article deals with the research of different types of diketopyrrolopyrroles (DPPs) for humidity detection. Thin layers of various DPPs are exposed to water vapors and then responses of electrical properties of these layers are measured. Potentially suitable materials are deposited on substrates with interdigital electrodes and subsequently impedance humidity characteristics are measured. Comparison of these materials, their features and responses to an ambient humidity are discussed in more detail as well.

Planar heterostructures based on organic semiconductor

Research of multi-layer heterostructure systems utilizing electrically active organic compounds is highly topical issue essential for effective development of new generation of electronic devices. This paper presents the results of particular work in this field focused on rather experimental construction of metal-insulator-semiconductor heterojunctions based on Ta + Ta2O5 electrode system incorporating easily applicable solution processed organic semiconductor, allowing to adjust the interfacial energetic conditions in desired manner. As a result, a well defined rectifying MIS structure with the onset voltage of 1,5 V and the rectifying ratio of 6×104 has been obtained.

Characterization of the organic field-effect transistor based on solution processed P3HT

The purpose of this paper is to present results of measurements taken on organic field effect transistors based on P3HT solution processed organic semiconductor. All discussed samples have been prepared both under ambient atmospheric conditions and in the dry nitrogen environment on rigid Si substrates covered with thermally grown SiO2 The electrical behaviour of OFETs has been subsequently examined under different test conditions. In particular, the overall device performance in normal room atmosphere as well as in the vacuum has been of special interest.