Gilles Horowitz

Contact resistance and threshold voltage extraction in n-channel organic thin film transistors on plastic substrates

n-channel organic thin film transistors were fabricated on polyethylene naphthalate substrates. The first part of the paper is devoted to a critical analysis of eight methods to extract the threshold voltage from the transfer characteristic in the linear regime. Next, to improve electron injection and reduce contact resistance, self-assembled monolayers (SAMs) were deposited on the gold source and drain electrodes. The subsequent modification on the current-voltage characteristics of the transistors is analyzed by the transfer line method, using a threshold-voltage-corrected gate voltage. The improved performance of the device obtained with some of the SAM treatments is attributed to both a better morphology of the semiconductor film, resulting in an increased channel mobility, and to easier electron injection, which manifests itself through a lowering of the contact resistance. Interestingly, the modulation of the contact resistance exactly follows an opposite behavior to what reported in the case of p-c...

Organic Metal‐Semiconductor Field‐Effect Transistor (OMESFET) Fabricated on a Rubrene Single Crystal

2010 WILEY-VCH Verlag Gm While some applications are becoming a commercial reality, the basics of organic electronic devices still present many unclear aspects. Understanding their mode of operation is not straightforward, because the high density of defects in the active layer generally obscures the intrinsic mechanisms of the devices. However, as in the inorganic case, single crystals of conjugated molecules can be easily grown and efficient, flexible and large-scale devices can be fabricated and characterized. Many of the effects linked to the non-periodicity of the elemental structure and presence of defects can be worked around and issues regarding the intrinsic properties of a device elucidated. Therefore, in order to clarify the operating mechanism of the organic metal–semiconductor field effect transistor (OMESFET), we used rubrene, a well-know organic semiconductor that forms single crystals with an extremely low defect density. An OMESFET is a non-conventional organic field-effect device, already proposed for use as a low-operating-voltage variable resistor, which differs from the conventional organic field-effect transistor (OFET) by the absence of a gate dielectric. Instead, the gate is directly deposited on the semiconductor layer, on which it forms a blocking contact. This simple architecture requires fewer fabrication steps than a traditional insulated-gate OFET. The device performance depends on the nature of the non-injecting gate metal electrode. Even if its practical integration into complex electronic circuits has still to be demonstrated, its importance for fundamental investigations is obvious, as underlined in a recent paper published during the course of this work. The formation of a metal–organic interface and the injection of electrical charges from a ‘‘non-ohmic’’ contact into an organic active layer involve complex and still unresolved processes, and the analysis of these devices, combined with the characterization of single crystal asymmetric diodes, should help in clarifying some of the aspects underlying organic electronics. Besides its importance in many optoelectronic applications, such as solar cells, light emitting diodes, and logic circuits, the metal– organic interface is also the basic ingredient of OMESFETs; its analysis is therefore a required preliminary step. From the study of asymmetric metal–semiconductor–metal structures, we did not find any evidence for the formation of a depletion layer in the rubrene single crystal close to the non-ohmic contact. Instead, the dependence of the impedance on the voltage, combined with the analysis of the current-voltage (I–V) characteristics, reveals the effect of a built-in potential, arising from the difference between the work function of the two metal electrodes at both sides of the structure. As long as the diode is under reverse bias, that is, when a negative bias is applied to the high work function electrode, no current flow is registered, while when the system is polarized in forward bias, injection of holes occurs from the high work function electrode. The small-signal impedance response of the diode clearly reveals the contribution of a second RC element in series with the expected high resonance geometrical one. On this basis, efficient single crystal metal–semiconductor field effect transistors have been fabricated and characterized. Thanks to the high resistance of an Au/rubrene/Al structure under reverse bias, these voltage-controlled resistors show negligible leakage current and a high ON/OFF ratio, comparable to that found in other low-voltage field effect devices. The absence of a depletion layer in an organic single crystal diode suggests that the physical principles underlying the device operation must be revisited. A first simple attempt in this direction is given here, suggesting that the current modulation is an interface process in which charge injection is controlled by the gate potential. By laying a thin single crystal on a metal substrate and evaporating a top electrode on the opposite side of the crystal, we fabricated asymmetric metal–semiconductor–metal structures, through which the fundamentals of organic diodes can be investigated. Figure 1 shows the semi-logarithmic and linear plots of a typical I–V response of an ITO/rubrene/Al system. These single crystal diodes are hole-only devices with no detectable current in the reverse direction. A measurable current only flows when a positive bias is applied to the anode (high work function electrode, ITO or Au), in which case positive carriers are injected into the active layer thanks to a suitable energy level alignment between the metal Fermi level and the rubrene highest occupied molecular orbital (HOMO) level, while practically no electrons can pass over the high injection barrier between the aluminum and the lowest unoccupied molecular orbital (LUMO) level of the semiconductor. The curve in Figure 1 is qualitatively similar to that of an inorganic diode. However, the principles governing the operation of both devices are quite different. In the organic diode, the rectifying effect is not controlled by a single metal–semiconductor barrier; instead, it is a consequence of the work function difference between the metal electrodes at both sides of the rubrene single crystal and of the resulting built-in potential established in the organic semiconductor layer. Figure 2 shows a close view of the current onset of

Interface Modification for Tuning the Contact Resistance of Metal/ Organic Semiconductor Junctions

As the performance of organic field-effect transistors improves, the limitation due to charge carrier injection at source and drain electrodes becomes crucial. This review describes the various solutions that have been developed to work around this issue. The most widespread method consists of interposing between the electrodes and the organic semi- conductor film a self-assembled monolayer made of an appropriate reactive molecule. In that case, the reduction of the contact resistance may come either from an improved morphology of the semiconductor film, or to a better energy level alignment at the interface with the electrode. The respective inference of both aspects is discussed. Alternative ways to re- duce the contact resistance by an appropriate surface treatment of the electrodes prior to the deposition of the semiconduc- tor are also presented.

44.1: Introducing a Two Stage Fully Organic AMOLED Display; from Design to Fabrication

In this paper, we introduce the design and the fabrication of a fully organic AMOLED display array. for solving the need of large OTFTs area issue, the pixel structure is designed in a way such that a top-emitting OLED is stacked on top of the driving circuitry including two pentacene TFTs. The pixel component geometries are designed and selected based on circuit simulation using behavioral SPICE-like models for pentacene TFTs and OLEDs. Moreover a new method is developed to provide the electronic connection between the OLED and the driving circuit.