Denis Fichou

A field-effect transistor based on conjugated alpha-sexithienyl

Abstract Metal-insulator-semiconductor field-effect transistors (MISFETs) with vacuum evaporated α-conjugated sexithienyl (α-6T) were fabricated and characterized. As other organic-based FETs these devices most probably operate through the injection of majority carriers in an accumulation channel. The field-effect mobility is 10-3 cm2 V-1 s-1 after a 3 h heat treatment at 120°C in air, the highest reported value for a conjugated organic semiconductor.

Structural order in conjugated oligothiophenes and its implications on opto-electronic devices

Over the last ten years, conjugated oligothiophenes have emerged as one of the largest families of organic semiconductors with potential applications in electronics devices. Thin film transistors (TFTs), photovoltaic solar cells, light-emitting diodes (LEDs), light modulators, photochromic switches and laser microcavities are some examples of devices that have been fabricated with oligothiophenes as the active materials. The key advantage of well-defined oligomers over their parent polymers is the high degree of molecular and crystalline ordering they can achieve. Polycrystalline and highly oriented thin films can be easily prepared from solution or by vacuum deposition. In many cases it has even been possible to grow single crystals and elucidate their X-ray structure. The aim of this review is to describe the structure of oligothiophene crystals and thin films and to explore its implications on the performances of various electronic devices.

Gate voltage dependent mobility of oligothiophene field-effect transistors

Organic field-effect transistors, in which the active semiconductor is made of oligothiophenes of various lengths, have been fabricated and characterized. A method is developed to estimate the field-effect mobility μ corrected for the contact series resistance. The mobility is found to increase by a factor of nearly 100 from quaterthiophene (4T) to octithiophene (8T). More importantly, μ increases quasilinearly with gate voltage. The origin of this gate bias dependence is discussed. One explanation could be the presence of traps that limit charge transport. Alternatively, the gate-voltage dependence is tentatively attributed to a dependence of the mobility with the concentration of carriers in the accumulation layer.

Stoichiometric control of the successive generation of the radical cation and dication of extended α-conjugated oligothiophenes: a quantitative model for doped polythiophene

Abstract The stoichiometric oxidation of extended α-conjugated oligothiophenes (C4H2S)nH2 (nT, with n = 3 to 6) by FeCl3 in dilute CH2Cl2 solutions allows the control of the oxidation state of the charged species successively generated. The respective nT·+ radical cations (polarons) are quantitatively produced in the first oxidation step and characterized by Vis-NIR absorption and ESR spectroscopies. The 6T·+ radical cation salt precipitates as a stable, conducting (σ300K = 0.5 S cm−1) and paramagnetic (g = 2.003, ΔHPP = 4 G) black powder, with FeCl4− as the counter anion. In the second step of the FeCl3 oxidation process, 6T·+ is qunatitatively interconverted into the 6T++ dication (bipolaron), as revealed by the two isosbestic points at 1.51 eV (823 nm) and 1.09 eV (1135 nm) on the optical spectrum. A four-level molecular orbital (MO) diagram of the singly and doubly charged hexamer is proposed on the basis of the well-resolved optical spectra and transition assignment. We compare these quantitative results obtained on finite and isolated systems with the current polaron-bipolaron model of doped polythiophene.

The oligothiophene‐based field‐effect transistor: How it works and how to improve it

Metal‐insulator‐semiconductor field‐effect transistors (MISFETs) based on organic semiconductors, mainly conjugated organic polymers and oligomers, have been reported recently. Unlike conventional MISFETs, these devices work through the modulation of an accumulation layer at the semiconductor‐insulator interface. A model for organic MISFETs, derived by changing the classical equations according to this particular operating mode is proposed. The ohmic current, parallel to the channel current, and due to the nonrectifying character of source and drain contacts, has also been taken into account. According to this model, the characteristics of these organic devices can be improved by decreasing the doping level and the thickness of the semiconducting layer. Simple rules are deduced and applied to devices based on α‐conjugated sexithienyl.

All-organic thin-film transistors made of alpha-sexithienyl semiconducting and various polymeric insulating layers

Thin‐film transistors (TFTs) were fabricated with a variety of organic polymer insulators topped with a vacuum evaporated organic semiconductor, α‐sexithienyl (α6T). No field‐enhanced source‐drain current was obtained with polystyrene (PSt) and polymethylmethacrylate (PMMA). The field‐enhanced current is weak with polyvinyl chloride (PVC), but much stronger with polyvinyl alcohol (PVA) and cyanoethylpullulan (CYEPL), a cyanoethylated polysaccharide which possesses a high dielectric constant (er=18.5 at 10 kHz). In these last two instances, the field‐effect mobility surpasses the one measured on TFTs made on a SiO2 insulating layer. A strong correlation is found between the dielectric constant of the insulator and the field‐effect mobility.

Structural basis for high carrier mobility in conjugated oligomers

Abstract In order to understand the structural parameters which control the charge transport properties in conjugated molecular semiconductors, we have analysed the ohmic conductivity and field-effect mobility in a series of short conjugated thiophene oligomers, α - n T, ranging from the trimer, α-3T, to the octamer, α-8T. It is first shown that a critical conjugation length, corresponding to α-5T, is required for observing a field-enhanced current in thin film transistors based on these materials. Field-effect mobility, together with conductivity, then increases with conjugation length, up to a very high value of the order of 10 −1 cm 2 V −1 s −1 for α-6T. This high mobility value is confirmed by results from space-charge limited current analysis of Au/α-6T/Au sandwich structures, which furthermore indicate the presence of a shallow trapping of the charges, located at 0.2 eV above valence band, and associated to a hopping or to a multiple trapping mechanism for charge transport. A levelling of electrical properties appears for α-8T, which is interpreted by the increase of conjugation defects in thiophene oligomers as their chain length increases. Furthermore, the absence of any field-effect mobility in the distorted chains of para -sexiphenylene confirms that the extent of conjugation and absence of structural defects are the most significant parameters for achieving high carrier mobility in these molecular materials.

CONJUGATED POLYMERS AND OLIGOMERS AS ACTIVE MATERIAL FOR ELECTRONIC DEVICES

Abstract A series of α-conjugated thiophene oligomers α-nT (n = 4, 5, 6, 8) were synthesized and vacuum evaporated as thin films on various substrates in order to characterize their structural and electronic properties. Their absorption spectra show the presence of priviledged conformers in the solid state. As deposited, they behave as p-type semiconductors, as confirmed by the rectifying barrier obtained with Ag. Upon annealing, α-6T is shown to be the first example of a stable n-type organic semiconductor, as evidenced by the formation of a rectifying junction with Au. The comparison with polythiophene emphasized the interest of using oligomers for electronic devices.

Role of the semiconductor/insulator interface in the characteristics of π-conjugated-oligomer-based thin-film transistors

Abstract Thin-film transistors (TFT) were made with various π-conjugated oligomers, including an oligothiophene series (from tetrhienyl up to octithienyl) and two linear polyacenes, tetracene and pentacene. Furthermore, various inorganic and organic insulators have been used. The devices were characterized by measuring the drain current as a function of the source-drain bias for various source-gate voltages, which allow the determination of the field-effect mobility, μ FET . As expected, μ FET increases as the conjugation length of the oligomer increases. Importantly, the mobility also appears to be very dependent on the nature of the isolating layer, which shows that the charge transport of the device is predominantly governed by the properties of the semiconductor-insulator interface.

Tuning the Packing Density of 2D Supramolecular Self-Assemblies at the Solid-Liquid Interface Using Variable Temperature

The two-dimensional (2D) crystal engineering of molecular architectures on surfaces requires controlling various parameters related respectively to the substrate, the chemical structure of the molecules, and the environmental conditions. We investigate here the influence of temperature on the self-assembly of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C(12)) adsorbed on gold using scanning tunneling microscopy (STM) at the liquid/solid interface. We show that the packing density of 2D self-assembled HBC-C(12) can be precisely tuned by adjusting the substrate temperature. Increasing the temperature progressively over the 20-50 degrees C range induces three irreversible phase transitions and a 3-fold increase of the packing density from 0.111 to 0.356 molecule/nm(2). High-resolution STM images reveal that this 2D packing density increase arises from the stepwise desorption of the n-dodecyl chains from the gold surface. Such temperature-controlled irreversible phase transitions are thus a versatile tool that can then be used to adjust the packing density of highly ordered functional materials in view of applications in organic electronic devices.