Nenad Marjanovic

High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films

We report on organic n-channel field-effect transistors and circuits based on C60 films grown by hot wall epitaxy. Electron mobility is found to be dependent strongly on the substrate temperature during film growth and on the type of the gate dielectric employed. Top-contact transistors employing LiF∕Al electrodes and a polymer dielectric exhibit maximum electron mobility of 6cm2∕Vs. When the same films are employed in bottom-contact transistors, using SiO2 as gate dielectric, mobility is reduced to 0.2cm2∕Vs. By integrating several transistors we are able to fabricate high performance unipolar (n-channel) ring oscillators with stage delay of 2.3μs.

Nonvolatile organic field-effect transistor memory element with a polymeric gate electret

Organic field-effect transistors with a polymeric electret as gate insulator and fullerenes as a molecular semiconductor were fabricated. We observed an amplification of the drain–source current Ids on the order of 104 upon applying a gate voltage Vg. Reversing the gate voltage Vg features large metastable hysteresis in the transfer characteristics Ids(Vg) with a long retention time. The observation of a switchable channel current Ids is proposed to originate from charge storage in the organic electret. As such, this device is a demonstration of an organic nonvolatile memory element switchable with the gate voltage.

Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors

The fabrication and characterization of high-mobility, n-channel organic field-effect transistors (OFET) based on methanofullerene [6,6]-phenyl C61-butyric acid methyl ester using various organic insulators as gate dielectrics is presented. Gate dielectrics not only influence the morphology of the active semiconductor, but also the distribution of the localized states at the semiconductor-dielectric interface. Spin-coated organic dielectrics with very smooth surfaces provide a well-defined interface for the formation of high quality organic semiconductor films. The charge transport and mobility in these OFET devices strongly depend on the choice of the gate dielectric. The electron mobilities obtained are in the range of 0.05-0.2 cm2 V-1 s-1. Most of the OFETs fabricated using organic dielectrics exhibit an inherent hysteresis due to charge trapping at the semiconductor-dielectric interface. Devices with a polymeric electret as gate dielectric show a very large and metastable hysteresis in its transfer characteristics. The observed hysteresis is found to be temperature dependent and has been used to develop a bistable memory element.

Correlation between morphology and ambipolar transport in organic field-effect transistors

Attaining ambipolar charge transport in organic field-effect transistors (OFET) is highly desirable from both fundamental understanding and application points of view. We present the results of an approach to obtain ambipolar OFET with an active layer of organic semiconductor blends using semiconducting polymers in composite with fullerene derivatives. Clear features of forming the super position of both hole and electron-enhanced channels for an applied gate field are observed. The present studies suggest a strong correlation of thin-film nanomorphology and ambipolar transport in field-effect devices.

Electrical characteristics of metal-insulator-semiconductor diodes and transistors with space charge electret insulators: towards nonvolatile organic memories

Organic field-effect transistors (OFETs) consist of a gate dielectric and an organic semiconductor film. The performance of organic electronic devices substantially depends on the dielectric properties of the insulating gate layer. Only a few key parameters, primarily the dielectric constant and the resulting device capacitance, have been regarded to be of central importance. Many insulating layers are however not simple dielectrics, an example are space charge gate electrets with internally trapped charges. Space charge gate electrets affect the electrical characteristics of diodes and transistors in a much more sophisticated manner, they are for example the key element in flash memories. We present impedance measurements of an organic metal-insulator-semiconductor (MIS) diode and corresponding measurements of a related organic field effect transistor. Both devices have a comparable design, with polyvinylalcohol as gate electret and the methanofullerene PCBM as organic semiconductor. Pronounced electret effects of charge injection and trapping are observed by impedance measurements of the MIS structure and these effects are found equally expressed in the electrical characteristics of the OFET configuration, reflecting first steps towards organic flash memories

Photo-induced phenomena in organic field-effect phototransistors based on conjugated polymer/fullerene blends and organic dielectric

Photo-induced phenomena were investigated in photoresposive organic field-effect transistors (photOFETs) based on conjugated polymer/fullerene solid-state mixtures as active semiconductor layer and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as gate dielectrics. The devices were characterized both in under dark showing n-type transistor behaviour with linear and saturated mobility of 1.7 x 10-3 cm2/Vs and 2.7 x 10-2 cm2/Vs respectively, and under white light illumination condition, where large shifts in the threshold voltage in the transfer characteristics were obtained. A typical phototransistor behaviour in a wide range of illumination intensities are observed in these devices.

Space-charge electrets as gate dielectrics in organic electronic devices

The basic element in organic electronics is the organic field effect transistor, consisting of an organic semiconductor and a gate dielectric. Whereas organic semiconductors are widely investigated, much less attention is paid for the search on organic gate dielectrics. Space-charge and dipole electrets offer a huge potential as gate dielectrics in organic field effect transistors. An example is the development of a nonvolatile organic field effect transistor memory based on fullerenes as molecular semiconductor and polyvinylalcohol as space-charge electret. In polyvinylalcohol, most probably mobile ions are the charge carriers that are trapped with long retention times. The memory element is characterized by a large metastable hysteresis in the transfer characteristics, the drain source current versus gate voltage. Improvements and even further going applications are possible by the use of better space-charge electrets.

Interfacial Effects in Organic Field-Effect Transistors

We present results on various types of organic field-effect transistors (OFETs) fabricated using all solution-processed organic dielectric layers, organic semiconductors and hot wall epitaxially grown organic semiconductor films. A nanoscale control of morphology of the resulting semiconductor film can be obtained by in-situ pre-heating of the substrate and depositing the organic semiconductor at a lower rate. A correlation is found between the nanoscale morphology and charge transport properties of OFET. The general observation of hysteresis on OFET characteristics with organic dielectrics is also studied in detail by impendence spectroscopy.

Photoresponse Of Organic Field-Effect Transistors Based On Soluble Semiconductors And Dielectrics

Photoactive organic field-effect transistors, photOFETs, based on a conjugated polymer/fullerene blend, MDMO-PPV: PCBM (1:4), and polymeric dielectrics as polyvinylalcohol (PVA) or divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) with top source and drain electrodes were fabricated and characterized in dark and under AM1.5 illumination. With LiF/Al as top source and drain contacts the devices feature n-type transistor behavior in dark with electron mobility of 10 -2 cm 2 /Vs. Under illumination, a large free carrier concentration from photo-induced charge transfer at the polymer/fullerene bulk heterojunction (photodoping) is created. The device performance was studied with different illumination intensities and showed to be strongly influenced by the nature of the organic dielectric/organic semiconductor interface resulting in phototransistor behavior in BCB-based photOFETs and in phototransistor or photoresistor behavior for PVA-based photOFETs.

Photoinduced Effects in Conjugated Polymer/Fullerene-Based Organic Field-Effect Transistors (photOFETs)

Photoactive organic field-effect transistors, photOFETs based on conjugated polymer/fullerene blend and polymeric dielectrics are fabricated and characterised in dark and under AM1.5 illumination. High photoresponsivity of 5 A/W is obtained.