Photocurrent studies of sexythiophene-based OFETs

Photocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport properties of organic thin-film transistors (OFETs). We have applied PC analyses to the study of the electronic density of states (DOS) distribution of the OFETs with crystalline-mixed films of two derivatives of a conjugated molecule [α-sexithiophene (6T), and its alkylated analogue α,ω-dihexylsexithiophene (DH6T)] as the active semiconductor. We have investigated the modifications in the DOS distribution associated to variations in the carrier density in the OFET channel and we have detected the formation of deep electrically active states in the below-band-gap region, associated to polaron states induced by a prolonged exposure of the device to atmosphere. A clear correlation between the PC results and the electrical characteristics of the corresponding FET devices has been observed.     

Low temperature transport study of C60 thin film FETs

Low temperature transport in C₆₀ thin film field-effect transistors has been studied using several samples with various gate voltages. Nearest neighbor hopping and variable range hopping transport have been observed in the low temperature transport measurements. Analyzing the temperature dependence of the transport types, it was found that the electrical properties of the film can be improved by thermally agitated evaporation of C₆₀.     

Fluctuating in the hopping rate of CuO thin films with respect to substrate temperature

Electrical transport properties in CuO thin films processed using d.c. magnetron sputtering technique is investigated to understand the correlation between the processing conditions and electrical properties. It is identified that the temperature dependent conductivity of the investigated films is controlled by the multi-phonon hopping conduction mechanism. A detailed analysis in terms of carrier hopping parameters is used to correlate electrical transport properties with the d.c. magnetron sputtering conditions.     

Photo-Fries-based photosensitive polymeric interlayers for patterned organic devices

This work reports on the investigation of the photosensitive polymer poly(diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PPNB), which undergoes the photo-Fries rearrangement upon illumination with UV-light, used as interfacial layers in organic electronic devices. Two cases were investigated: the use of a blend of PPNB with poly-vinylcarbazole (PVK) as an interlayer in para-sexiphenyl (PSP) based organic light emitting diodes (OLEDs) and the use of PPNB as gate dielectric layer in organic field effect transistors (OFETs). The photo-Fries rearrangement reaction causes a change of the polymer chemical structure resulting in a change of its physical and chemical properties. The electroluminescence spectra and emission of the PSP OLEDs are not affected when fabricated with a non-UV-illuminated PPNB:PVK blend. However, the electroluminescence is totally quenched in those OLEDs fabricated with UV-illuminated PPNB:PVK blend. Although the dielectric constant of PPNB increases upon UV-treatment, it is demonstrated that those OFETs built with UV-treated PPNB as gate dielectric have lower performance than those OFETs built with non-UV-treated PPNB. Furthermore, the effect of the UV-illumination of PPNB and PPNB:PVK blend on the growth of the small molecules C₆₀ and PSP has been studied by atomic force microscopy. Using photolithography, this kind of photochemistry can be performed to spatially control and tune the optical and electrical performance of organic electronic devices.     

Control of one-dimensional transport in multi-wall carbon nanotubes by wall destruction

We have investigated the damage in multi-wall carbon nanotubes (MWNTs) destroyed by electrical breakdown and focused ion beam bombardment (FIBB). The transport properties of a MWNT destroyed by electrical breakdown have been compared with those of a MWNT destroyed by FIBB. Also the Tomonaga–Luttinger transport (TLT) model has been applied to each type of destroyed MWNT. The MWNT destroyed by FIBB showed TLT behavior because of the weak destruction of the remaining walls. However, in the case of MWNTs destroyed by electrical breakdown, three-dimensional variable-range hopping (VRH) was observed in the low temperature transport. This suggests that the local damage has been caused by strong breakdown. There exists a clear difference between the effects of electrical breakdown and FIBB. Wall destruction by FIBB could be applied to control the one-dimensional transport of MWNTs.     

Magnetic and electrical properties of zinc selenide crystals containing disorder

The electrical and magnetic properties of ZnSe single crystals containing disorder have been studied between temperatures 290K and 900K. The study of the magnetic properties has been extended to low temperatures (100K). Paramagnetism has been found to appear at high temperatures (460–900K). From the fact that this paramagnetism is proportional to e^?δE/kT, it is suggested that localized states of single occupancy are created by thermal excitation. The study of the magnetic properties has been of help in ascertaining the nature of the transport (band conduction or hopping conduction) and in finding the hopping energy and excitation energy separately. It has also been shown from this that both band conduction and hopping conduction exist simultaneously in the sample. A study of the thermo electric power (t.e.p.) shows that below 450K current is carried by electrons in the conduction band and above by hopping of holes.     

Polaronic transport in TiO2 thin films with increasing Nb content

The temperature dependence of the charge transport in TiO₂ films was investigated to establish the correlation between the Nb content and electrical properties. It was identified that temperature-dependent conductivity of the films is dominated by a phonon-assisted small polaron hopping model in the non-adiabatic regime. Applying the polaron hopping models of Mott, Schnakenberg and Emin to describe the observed behavior, temperature-dependent conductivity data of the films were analyzed. A detailed analysis in terms of small polaron hopping parameters in the investigated temperature regime was used to correlate electrical properties with the percentage of Nb.     

Multiphonon hopping of carriers in CuO thin films

We have performed a detailed study of the electrical conduction process in CuO thin films deposited by the sol-gel dip coating technique in a temperature range 280-420 K. The electrical conduction is analyzed within the framework of various hopping conduction models. Multiphonon hopping conduction mechanism is found to dominate the electrical transport in the entire temperature region. Our results are consistent with this model of hopping conduction mechanisms with weak carrier-lattice coupling.     

Extrinsic limiting factors of carrier transport in organic field-effect transistors

Extrinsic factors to disturb the carrier transport in pentacene field-effect transistors (FETs), as a representative of the high-mobility organic FETs (OFETs), have been comprehensively analyzed by using atomic-force-microscope potentiometry (AFMP), microscopic four-point-probe field-effect transistor (MFPP-FET) measurement, and other techniques. In the first part, by mainly using AFMP as a powerful tool to reveal the potential distribution in working OFETs, we show how and how much the formation of source/drain electrodes influences the apparent field-effect mobility both for top- and bottom-contact configurations. In the second part, we show the influence of irregular grain structures and regular grain boundaries. The films grown both at very low and high temperature ranges contain distinctive insulating parts, which make the apparent mobility very low. Within the moderate growth temperature range, the intrinsic field-effect mobility obtained by MFPP-FET measurement is proportional to the average grain size. This behavior is well explained by the polycrystalline model with the diffusion theory. According to the observations in this work, it is obvious that these extrinsic limiting factors must be carefully excluded to discuss the intrinsic mechanism of the carrier transport in OFETs.     

Recent advances on π-conjugated polymers as active elements in high performance organic field-effect transistors

As high-performance organic semiconductors, π-conjugated polymers have attracted much attention due to their charming advantages including low-cost, solution processability, mechanical flexibility, and tunable optoelectronic properties. During the past several decades, the great advances have been made in polymers-based OFETs with p-type, n-type or even ambipolar characterics. Through chemical modification and alignment optimization, lots of conjugated polymers exhibited superior mobilities, and some mobilities are even larger than 10 cm²·V⁻¹·s⁻¹ in OFETs, which makes them very promising for the applications in organic electronic devices. This review describes the recent progress of the high performance polymers used in OFETs from the aspects of molecular design and assembly strategy. Furthermore, the current challenges and outlook in the design and development of conjugated polymers are also mentioned.     

Electrical properties of organic field effect transistors with thin graphite/metal electrode directly grown by ICP-CVD at low temperatures

The high contact resistance of organic thin film transistors (OTFTs), due to the work function difference between metal electrode and organic channel, seriously decreases the electrical properties. Graphene electrode could reduce the contact resistance and improve the electrical performance of OTFTs. However, the high chemical vapor deposition (CVD) temperature (900–1000 °C) limits the available OTFT substrate in the case of direct graphene growth on S/D metal electrodes. Furthermore, the application of a transferred graphene electrode induces significant problems due to the transfer process. In this work, thin graphite sheet was directly grown on a metal electrode by the inductively coupled plasma-chemical vapor deposition (ICP-CVD) method at as low temperature as 400, 500 °C. We show that OFETs with thin graphite sheet/metal, grown at 400, 500 °C, exhibit much lower contact resistance than OFETs with metal-only electrode.     

Competition between band and hopping carrier transport in Ge : Mn thin films

Regularities of hole transport and its correlation with percolation magnetism caused by localized carriers simultaneously involved in the formation of the magnetic and electrical properties of Ge: Mn thin films are investigated. It is established that at temperatures of T > 22 K the activationless band carrier transport occurs in the Ge: Mn samples (2 at % Mn). At low temperatures, the hopping mechanism with a variable hopping length works.     

Spin, charge and lattice couplings in Cu-deficient oxysulphide BiOCu0.94S

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.94)S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electrical resistivity measurements. The Cu spins order in a ferromagnetic arrangement below T(C) = 250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures. In the magnetically ordered state the electrical transport can be described using three-dimensional variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin-charge couplings are clearly revealed when the resistivity departs from the hopping conduction and begins to increase with increasing temperatures above 250 K where the Cu spins become disordered.     

Spin-flip induced magnetoresistance in positionally disordered organic solids

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.     

Electrical transport properties of consolidated ZnSe quantum dots at and above room temperature

Here we report a comprehensive study on the prevailing conduction mechanism and dielectric relaxation behavior of consolidated Zinc Selenide quantum dots in the frequency range of 1 kHz ≤ f ≤ 1.5 MHz and in the temperature range of 298K ≤ T ≤ 573 K. The ac conductivity increases either with increase in temperature or with increase in frequency, which is explained by the Jonscher Power law. At higher temperatures, correlated barrier hopping is found to be the prevalent charge transport mechanism with a maximum barrier height of 0.88 eV. The dielectric constant of the sample is found to exhibit weak temperature dependence. DC conductivity study reveals the semiconducting nature of the sample and it is discussed in the light of polaron hopping conduction. From the impedance spectroscopic study, role of the grains and grain boundaries in the overall electrical transport properties have been elucidated by considering an electrical equivalent circuit (composed of resistances and constant phase elements). Electric modulus study reveals non-Debye responses of the sample in the experimental range.     

Spin-singlet small bipolarons in Nb-doped BaTiO3

The magnetic susceptibility and the electrical resistivity of n-type BaTi1-xNbxO3 have been measured over a wide temperature range. It is found that, for 0相似文献   

Polaron effect dependence of thermopower in organic semiconductors

A unified physical model for thermopower was presented in organic semiconductors, based on the Marcus theory and variable-range hopping theory. According to the proposed model, the characteristic of charge carrier thermoelectric transport in organic semiconductors has been investigated. In particular, polaron effects, energetic disorder, and carrier density dependence of the thermopower have been discussed in detailed. The calculation also shows a good agreement with the experimental data in organic semiconductors.     

Study of charge transport in P3HT:SiNW-based photovoltaic devices

Hybrid devices based on silicon nanowires (SiNWs) dispersed in a conjugated polymer poly(3-hexylthiophene) P3HT thin films have been realized. The carrier transport mechanism in inorganic/organic hybrid nancocomposites consisting of SiNW dispersed in P3HT layer was investigated by using I?CV characteristics and impedance spectroscopy measurements. The conduction mechanism in these hybrid nanocomposites has been identified to be thermionic emission at the interfaces. The electrical parameters of the structure have been investigated by modelization of the I?CV characteristics using an electrical equivalent circuit and have been extracted for the different SiNW volume ratios. The barrier height, the series resistance and the shunt resistance values of the diodes have been calculated as about 0.9 eV, several k?? and several M??, respectively. The diode behaves as non-ideal one because of the series resistance and the Donor/Acceptor interface layer. The impedance spectroscopy study, in the frequency range 100 Hz?C100 kHz, shows a typical behavior of disordered materials and indicative of a hopping transport in the investigated temperature range. The dc conductivity follows the Arrhenius law with an activation energy transition from 8.4 to 55.8?meV at about 294 K.     

Nonideal double-slope effect in organic field-effect transistors

With the development of device engineering and molecular design,organic field effect transistors(OFETs)with high mobility over 10 cm2 V-1-s-1 have been reported.However,the nonideal doubleslope effect has been frequently observed in some of these OFETs,which makes it difficult to extract the intrinsic mobility OFETs accurately,impeding the further application of them.In this review,the origin of the nonideal double-slope effect has been discussed thoroughly,with affecting factors such as contact resistance,charge trapping,disorder effects and coulombic interactions considered.According to these discussions and the understanding of the mechanism behind double-slope effect,several strategies have been proposed to realize ideal OFETs,such as doping,molecular engineering,charge trapping reduction,and contact engineering.After that,some novel devices based on the nonideal double-slope behaviors have been also introduced.     

AC impedance analysis of LaLiMo2O8 electroceramics

Complex impedance analysis of a new rare earth-based ceramic oxide, LaLiMo₂O₈, prepared by a standard solid-state reaction technique has been carried out. Material formation under the reported conditions has been confirmed by X- ray diffraction studies. A preliminary structural analysis indicates the crystal structure to be orthorhombic. Electrical properties of the material sample have been studied using AC impedance spectroscopy technique. Impedance spectrum results indicate that the electrical properties of the material are strongly dependent on temperature and it bears a good correlation with the sample microstructure (i.e. the presence of bulk, grain boundary, etc.) in different temperature ranges. Evidences of temperature-dependent electrical relaxation phenomena in the material have also been observed. The bulk resistance, evaluated from complex impedance spectrum has been observed to decrease with rise in temperature showing a typical negative temperature coefficient of resistance (NTCR)-type behavior like that of semiconductors. The DC conductivity shows typical Arrhenius behavior when observed as a function of temperature. The AC conductivity spectrum has provided typical signature of an ionically conducting system and is found to obey Jonscher's universal power law. Modulus analysis has indicated the possibility of hopping mechanism for electrical transport processes in the system with non-exponential-type conductivity relaxation.