Manipulation of charge carrier injection into organic field-effect transistors by self-assembled monolayers of alkanethiols

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-(2'-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, µFET = 4 × 10-4 cm2 V-1 s-1 and rr-P3HT, µFET = (1–2) × 10-2 cm2 V-1 s-1) are determined. The contact resistance obtained for the SAM-modified electrodes is at least one order of magnitude larger than for untreated contacts.

Harding University Course Catalog 1988-1989

Catalog of Harding University 1988-1989https://scholarworks.harding.edu/catalogs/1050/thumbnail.jp

Self-assembled monolayers in organic electronics

In de afgelopen jaren hebben organische (plastic) halfgeleiders steeds meer aandacht gekregen vanwege mogelijk gebruik in elektronische toepassingen zoals zonnecellen, LED’s en veldeffect transistors. Ze kunnen goedkoop op grote schaal gefabriceerd worden en hebben goede mechanische eigenschappen. Een veelbelovende techniek voor de fabricage van organische elektronica is ‘bottom-up self-assembled monolayer’ (SAMs), waarbij moleculen zichzelf formeren in complexe patronen en structuren zonder menselijke interventie. Ondanks het feit dat een SAM bestaat uit maar één laag moleculen, kunnen SAMs de macroscopische mechanische en elektrische eigenschappen van een oppervlak veranderen. SAMs zijn aangebracht op de elektrodes om de werkfunctie te manipuleren. De injectie van lading in de halfgeleider kan zo worden verbeterd of onderdrukt. Het regelen van de drempelspanning in OFETs (organic field-effect transistors) kan ook door middel van SAMs. SAMs die zijn gemaakt van geconjugeerde moleculen kunnen functioneren als halfgeleidende lagen in een OFET. Uit het onderzoek blijkt dat werkende OFETs met slechts een monolaag halfgeleider verrassend genoeg kunnen worden gemaakt door middel van spincoaten In recent years organic semiconductors have attracted considerable attention for application in electronic devices such as solar cells, light-emitting diodes and field-effect transistors. The advantages of the use of polymers are their unique electrical and mechanical properties and the opportunity to produce low-cost electronics on large area substrate such as glass and plastic. A promising technology for organic electronics is bottom-up self-assembled monolayer (SAM), where molecules self-organize into complex patterns and structures without human intervention. Despite being only a single molecular layer thick, the SAM can change the macroscopic mechanical and electrical properties of surfaces. SAMs are applied onto the metal electrodes to tune the work function. The charge injection into the semiconductor can be enhanced or suppressed. Controlling the threshold voltage in OFETs can be done by SAMs. SAMs made from conjugated molecules can act as semiconducting layers in an OFET, it is shown that monolayer OFETs can surprisingly be made by simple spin coating. The mobility of the semiconductor is comparable with that of a bulk layer of the same material. The SAMFETs can also made by solution process. The SAMFETs made with SiO2 as the gate dielectric cannot be used for flexible electronics as the substrate is not bendable. The fully functional circuits demonstrate long-range order over large areas of a bare polymer surface, which can be regarded as the start of flexible monolayer electronics is the main report of this thesis.

Photovoltaic action in a self-assembled monolayer of hemicyanine dyes on gold from dissociation of surface plasmons

Hemicyanine dye molecules, containing a thiol functionality, form a self-assembled monolayer (SAM) on thin films of gold. The combined SAM-gold layer system supports surface plasmons and can be converted into a diode using a liquid electrolyte top contact. Diodes fabricated on a quartz prism allow for incoupling of incident light to surface plasmons and show a spontaneous photocurrent under short-circuit conditions. Measurement of the short-circuit photocurrent as function of incident angle of the light shows that the photocurrent arises from dissociation of surface plasmons into pairs of charge carriers

Single-Layer Pentacene Field-Effect Transistors Using Electrodes Modified With Self-assembled Monolayers

Pentacene field-effect transistor performance can be improved by modifying metal electrodes with self-assembled monolayers. The dominant role in performance is played by pentacene morphology rather than the work function of the modified electrodes. With optimized processing conditions, hysteresis-free transfer curves with very small switch-on voltages are obtained for single-monolayer pentacene active channels.

Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold: Influence of Orbital Energy Level Alignment on Short-Circuit Current Generation

Photoinduced current generation at metal–organic monolayer interfaces is observed upon photoexcitation of a monolayer of hemicyanine molecules chemically adsorbed onto a gold electrode. A series of hemicyanines is investigated that bind to the gold via a thiol moiety, in an orientation such that the acceptor moiety of the hemicyanine is closer to the metal than its donor part. The quantum yield of short-circuit photocurrent generation in a diode using a liquid electrolyte as second contact, correlates with the strength of the donor moiety of the dyes. Modeling of the photocurrent generation using Marcus theory indicates that the net photocurrent results from asymmetry in the electron transfer rates of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with the electrodes. The quantum efficiency of short-circuit photocurrent generation decreases for the HOMO levels of the hemicyanine going deeper below the Fermi-level of the metal. The deeper HOMO level provides a larger driving force for back electron transfer from the metal to the photo-oxidized molecule and suppresses current generation in favor of quenching of the excited state

Synthesis and properties of small band gap thienoisoindigo based conjugated polymers

Using Stille and Suzuki polymerization reactions we incorporate thienoisoindigo (TII) as an acceptor co-monomer into a series of alternating p-conjugated copolymers with combinations of benzene, thiophene and carbazole as donor co-monomers. By changing the nature and length of the donor segments, the optical band gap of these soluble TII copolymers can be varied over a large range from 1.52 eV down to 0.87 eV. The semiconducting properties of the TII copolymers were established in bottom-gate bottom-contact field-effect transistors that provide hole mobilities for these materials in the range of 10-3 to 10-2 cm2 V-1 s-1

Photophysics of Self-Assembled Monolayers of a π-Conjugated Quinquethiophene Derivative

The photophysics of fully and partially covered self-assembled monolayers (SAMs) of a quinquethiophene (5T) derivative have been investigated. The monolayers behave as H-aggregates. The fluorescence of fully covered SAMs is weak and red-shifted, and the extinction is blue-shifted as compared to that of single molecules. The fluorescence of partially covered SAMs is dominated by that of single molecules on the surface. The extinction spectra are similar for fully and partially covered monolayers, which show that even the smallest islands are H-aggregates. The extinction spectra furthermore closely resemble those for 5T single crystals, which demonstrates that in oligothiophene crystals the intermolecular interactions within one layer molecules are stronger than the interlayer electronic coupling.