Riadh Hajlaoui

Temperature and gate voltage dependence of hole mobility in polycrystalline oligothiophene thin film transistors

We have performed current–voltage measurement on polycrystalline sexithiophene (6 T) thin film transistors at temperatures ranging from 10 to 300 K. A method is developed to extract the carrier mobility from an analysis of the transfer characteristics. In particular, data are corrected for contact resistance. The carrier mobility is found to increase quasilinearly with gate voltage at room temperature. The dependence becomes superlinear at low temperatures. The temperature dependence shows three domains. For 100 K<T<300 K, the mobility is thermally activated with an activation energy of around 0.1 eV. The activation energy reduces to 5 meV for 25 K<T<100 K. Finally, the mobility is practically temperature independent for temperatures lower than 25 K. The data are explained by a model where charge transport is limited by a high concentration of traps at grain boundaries. At high temperatures, charge transfer at boundaries occurs via thermionic emission, while tunnel effect takes place at low temperatures. ...

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.

All-organic field-effect transistors made of π-conjugated oligomers and polymeric insulators

All-organic field-effect transistors (FET) based on various oligothiophenes and insulators are reviewed. These devices generally work in the accumulation mode, but can also be operated in the depletion regime. The performances of organic FETs, as mirrored by their field-effect mobility μFET, are strongly dependent on the nature of both the semiconducting oligomer and the insulator. A general trend is the increase of μFET as the chain length increases. α-Substitution with alkyl chains also increases the mobility, whereas s-substitution leads to an almost complete disappearance of the field effect. The use of an organic insulator with a high dielectric constant, cyanoethylpullulane (CYEP), resulted in a considerable increase of μFET. All these behaviors are attributed to structural effects, and more specifically to changes in crystallinity. The study of the depletion regime gives indications on the concentration of dopants and free carriers. Free carriers are two to three orders of magnitude less numerous than dopants. Finally, measurements on freshly made and aged devices give information on the process of oxygen doping in thiophene oligomers.

Theory of the organic field-effect transistor

A method is described to derive the mobility of organic field-effect transistors (OFETs) by using the current-voltage characteristics at low drain voltage. After correction for the contact series resistance, it appears that the mobility of sexithiophene (6T) and octithiophene (8T) is gate bias dependent, which actually corresponds to charge concentration dependence. Temperature dependent measurements show that the mobility is thermally activated, which is interpreted in terms of polaron hopping transport. At temperature lower than around 100K, the activation energy is considerably reduced, and the mobility becomes almost temperature independent below 25K. These features mirror a change in the transport regime, from thermally activated hopping at high temperatures, to a more coherent mechanism in the intermediate regime, and eventually band-like transport at very low temperatures. The charge concentration dependence is attributed to the presence of traps, which can be identified to grain boundaries.

VERTICAL DEVICE ARCHITECTURE BY MOLDING OF ORGANIC-BASED THIN FILM TRANSISTOR

An organic thin film transistor (TFT) of circular geometry is constructed by the use of polymer-based substrate, insulator, and semiconductor. Owing to the plasticity of organic materials, this device is mechanically molded by pressing the central circular drain electrode. A shallow well is created, resulting in the localization of the transistor channel onto the wall of the well. The electrical characteristics of the transistor remain almost unchanged after this molding. The obtained vertical device architecture allows the use of a very large channel width over length ratio, while preserving a maximum drain electrode area to be devoted to the further deposition of liquid crystal or an electroluminescent film. Current output of the order of some milliamperes per square centimeter can be delivered, which makes these molded TFTs well adapted for the construction of display pixels.

Transient electroluminescence of monolayer and bilayer sexithiophene diodes

Abstract The transient electroluminescence of monolayer and bilayer sexithiophene-based diodes has been measured. The delay time of the luminescence onset of the monolayer diode corresponds to a hole mobility of 5 × 10−6 cm2 V−1 s−1 which is considerably lower than that obtained by field-effect measurements. This is interpreted in terms of strong transient trapping. The bilayer diode presents a twofold time-resolved response which is attributed to the different mobility of its constituent layers.

Structure effect on transport of charge carriers in conjugated oligomers

Abstract The carrier mobility μ 3ET of a series of unsubstituted, α alkyl-substituted and β alkyl-substituted thiophene oligomers nT has been measured on field-effect transistors based on these organic semiconductors. A large increase of μ FET with conjugation length n has been observed up to the hexamer 6T . For a given length n , alkyl substitution in α position leads to a steep rise of mobility, which reaches values close to 1 cm 2 V −1 s −1 . These important values, which require high efficiency in intermolecular charge transfer, are attributed to the long range structural ordering appearing in thin films of these molecules, as shown by X-ray characterizations. On the other hand, the decrease of μ FET observed in the case of β-substituted oligothiophenes is related to the large structural modifications originating from the pendant bulky alkyl groups. The occurrence of radical cation or dication charged states is also discussed for understanding the effect of conjugation length, n , on carrier mobility.

Structural, spectroscopic and device characteristics of octithiophene

Abstract As a step further in the design of efficient organic semiconductors, we report here on the structural, spectroscopic and device characterization of octithiophene (8T), the linear conjugated octamer of thiophene. The X-ray structure and optical spectra under polarized light of the 8T single crystal are described for the first time. Vapor deposited 8T behaves as a p-type semiconductor whose transport properties are investigated in thin film transistors (TFTs). In particular, we show that the use of high purity 8T enhances by two orders of magnitude its field-effect mobility in TFTs to reach μ FET = 1.8×10 −2 cm 2 /V.s, a value similar to that of non-substituted sexithiophene (6T).