Modification of metal/semiconductor junctions by self-assembled monolayer organic films (original) (raw)
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Self-assembled monolayer effect on the characteristics of organic diodes
Synthetic Metals, 2009
In this paper, we study the effect of self-assembled monolayers (SAMs) on the electric behavior of organic diodes based on sexithiophene (6T) sandwiched between indium tin oxide (ITO) and aluminum. We have used molecules of SAMs based on a thiol with functional groups of oligothiophene (3T(CH 2 ) 6 SH). Wettability measurements have been performed to characterize ITO surface energy and its modification upon deposition of SAMs. The results of contact angle measurements and surface energies demonstrate the homogeneity and rigidity of grafting surface. The current vs. applied voltage characteristics of devices show that conduction in weak biasing follow Richardson-Schottky behavior. Beyond 1.5 V, J-V characteristics can be successfully modeled by space-charge limited current (SCLC) theory followed by a trap charge limited current (TCLC). The electrical as well as optical characteristics of 6T layer are clearly affected by the presence of the SAM. The differences between ITO/SAM and bare ITO samples are interpreted in terms of structural effect induced by the self-assembled monolayer of 3T(CH 2 ) 6 SH.
Organic Electronics, 2010
We report on the effect of the deposition of self-assembled monolayers (SAMs) on the source and drain electrodes on the contact resistance and mobility in organic thin film transistors (OTFTs). Ultraviolet photoelectron spectroscopy (UPS) shows a variation of the work function of the electrodes depending on the SAM. OTFTs were fabricated with solution processible Polyera ActivInkä N1400, TIPS-pentacene and TFB, giving access to both polarities of transistors and to both crystalline and amorphous materials. The transfer line method (TLM) was used to separately estimate the contact resistance and mobility. A clear correlation is found between the work function of the modified electrodes and the corresponding contact resistance. The effect on mobility is more puzzling, and tentatively attributed to morphological effects.
Organic Electronics, 2008
The aim of this work is to improve charge injection by interposing an appropriately oriented dipole layer between contact and semiconductor in organic thin-film transistors (OTFTs). OTFTs are fabricated with pentacene semiconductor and gold source and drain contacts. The contacts are modified with self-assembled monolayers (SAMs) made of alkane or fluorinated alkane thiols. Ultraviolet photoelectron spectroscopy (UPS) shows a respective decrease and increase of the work function of the electrodes. Consistent with these results, we observe an increase and a decrease, respectively, of the contact resistance of the OTFTs, and a further decrease when shortening the length of the fluorinated molecule.
Organic field-effect transistors as new paradigm for large-area molecular junctions
Organic Electronics, 2012
Self-Assembly Monolayers (SAMs) are considered a promising route for solving technological hindrances (such as bias-stress, contact resistance, charge trapping) affecting the electrical performances of the Organic Field-Effect Transistors (OFETs). Here we use an OFET based on pentacene thin film to investigate the charge transport across conjugated SAMs at the Au/pentacene interface. We synthesized a homolog series of p-conjugated molecules, termed Tn-C8-SH, consisting of a n-unit oligothienyl Tn (n = 1. . .4) bound to an octane-1-thiol (C8-SH) chain that self-assembles on the Au electrodes. The multi-parametric response of such devices yields an exponential behavior of the field-effect mobility (l), current density (J), and total resistivity (R), due to the SAM at the charge injection interface (i.e. Au-SAM-pentacene). The surface treatment of the OFETs induces a clear stabilization of different parameters, like sub-threshold slope and threshold voltage, thanks to standardized steps in the fabrication process.
4-[(3-Methylphenyl)(phenyl)amino]benzoic acid (MPPBA) self-assembled monolayer (SAM) molecules as hole injection is formed on p and n type Si and on indium-tin oxide (ITO) electrodes to investigate the effect on the electrical parameters of hole only organic device. The hole mobility improvement of organic device was attributed to an intermediate energy level formed between hole transport materials (HTL) (N,N-Bis(naphthalen-1-yl)-N,N-bis(phenyl)benzidine-NPB) and ITO when forming an ultrathin MPPBA layer, leading to increase of carrier mobility of the device. Space charge limited current (SCLC) technique is used to estimate the mobility of the NPB formed at the interface metal/organic Ohmic contact. The hole mobility of ITO/NPB/Al and ITO/MPPBA/NPB/Al devices were obtained as 1.80 × 10 −6 and 1.76 × 10 −3 cm 2 /Vs, at 1350 E (V/cm) 1/2 applied electric field, respectively. SAM modified devices has lower barrier height values. The electronic characteristic parameters of the ITO/(with or without MPPBA)/NPB/Al, Au/n-Si(or p-Si)/(with or without MPPBA)/Au contacts were calculated using current–voltage (I–V) measurements by Schottky type carrier injection.
Advanced Materials, 2009
Principal goals in organic thin-film transistor (OTFT) gate dielectric research include achieving: (i) low gate leakage currents and good chemical/thermal stability, (ii) minimized interface trap state densities to maximize charge transport efficiency, (iii) compatibility with both p-and n-channel organic semiconductors, (iv) enhanced capacitance to lower OTFT operating voltages, and (v) efficient fabrication via solution-phase processing methods. In this Review, we focus on a prominent class of alternative gate dielectric materials: self-assembled monolayers (SAMs) and multilayers (SAMTs) of organic molecules having good insulating properties and large capacitance values, requisite properties for addressing these challenges. We first describe the formation and properties of SAMs on various surfaces (metals and oxides), followed by a discussion of fundamental factors governing charge transport through SAMs. The last section focuses on the roles that SAMs and SAMTs play in OTFTs, such as surface treatments, gate dielectrics, and finally as the semiconductor layer in ultra-thin OTFTs. He is interested in structure and function at the nanoscale, and theory of chemical processes, and tries to unite structure and function in molecular nanostructures, based on theoretical notions, exemplary calculations, and (importantly) collaborations. Interest areas are molecular electronics, self-assembly, nonlinear response, and exact and approximate theories of quantum dynamics and using nanoscience to attack the energy problems facing this world.
Improved organic thin-film transistor performance using novel self-assembled monolayers
Applied Physics Letters, 2006
Pentacene-based organic thin-film transistors have been fabricated using a phosphonate-linked anthracene self-assembled monolayer as a buffer between the silicon dioxide gate dielectric and the active pentacene channel region. Vast improvements in the subthreshold slope and threshold voltage are observed compared to control devices fabricated without the buffer. Both observations are consistent with a greatly reduced density of charge trapping states at the semiconductor-dielectric interface effected by introduction of the self-assembled monolayer.
Electrical, magnetic and optical properties of organic self-assembled-monolayer diodes
the workshop highlights fundamental physical phenomena in hybrid systems consisting of organic and inorganic materials. Combined with modern nanolithography and self -organization processes, hybrid systems are expected to exhibit novel physical properties. They may even lead to the development of, e.g., new photonic and spintronic nanodevices. For this, a detailed understanding of (i) chemical, electronic and magnetic properties of interfaces, (ii) the efficient energy and charge transfer as well as (iii) the dynamics at interfaces plays a key role. The workshop brings together experts and young researchers from, both, theory and experiment to discuss current achievements and stimulate further developments in the emerging field of hybrid nanosystems.
Journal of Materials Chemistry, 2007
Charge carrier injection into two semiconducting polymers is investigated in field-effect transistors using gold source and drain electrodes that are modified by self-assembled monolayers of alkanethiols and perfluorinated alkanethiols. The presence of an interfacial dipole associated with the molecular monolayer at the metal/semiconductor interface changes the work function of the electrodes, and, hence, the injection of the charge carriers. The FET characteristics are analysed with the transfer line method and the hole injection into poly(2-methoxy-5-(29-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and regio-regular poly(3-hexyl)thiophene (rr-P3HT) is investigated. The device parameters are corrected for the contact resistances of the electrodes and the mobilities of the polymers (MEH-PPV, m FET = 4 6 10 24 cm 2 V 21 s 21 and rr-P3HT, m FET = (1-2) 6 10 22 cm 2 V 21 s 21 ) are determined. The contact resistance obtained for the SAM-modified electrodes is at least one order of magnitude larger than for untreated contacts.
Self-assembled monolayers on organic semiconductors
Self-assembled monolayers (SAMs) are widely used in a variety of emerging applications for surface modification of metals and oxides. Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors. Remarkably, SAM growth results in a pronounced increase of the surface conductivity of organic materials, which can be very large for SAMs with a strong electron-withdrawing ability. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10 −5 S per square, two orders of magnitude greater than the maximum conductivity typically achieved in organic field-effect transistors. The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors with molecular self-assembly. In particular, SAM-induced conductivity shows sensitivity to different molecular species present in the environment, which makes this system very attractive for chemical sensing applications.