Transport Properties Of Field-effect Transistors Research Articles

Visualizing transport in thiazole flanked isoindigo-based donor–acceptor polymer field-effect transistors

Replacing S with Se in fluorinated isoindigo (IID) copolymers enhances charge transport properties in field-effect transistors (FET). Nonlinear optical imaging from IID FETs helps visualize transport.

The effects of P3HT crystallinity in bilayer structure organic solar cells

Because of favorable charge transport properties with a lower possibility of recombination, bilayer structure solar cells have received significant ongoing attention. The simplest approach to obtain better transport properties of a charge carrier is to increase the crystallinity of the active layer. In this work, we investigated the effect of the crystallinity of poly(3-hexylthiophene) (P3HT) on the PCE of bilayer solar cells. The self-assembled nano-fibril morphology induced by the addition of the non-soluble acetonitrile (AN) solvent induces a significant improvement of charge transport properties in field-effect transistors (FET). However, the self-assembled nano-fibril morphology of P3HT did not have a beneficial effect on the bilayer solar cells. Since the P3HT nano-fibrils were generally placed in parallel with the substrate, the FET device which uses lateral transport could produce an enhancement of the device properties. However, because of the difficulty of charge transport in the vertical direction between horizontally aligned fibrils, the properties of the solar cell device did not improve with increased P3HT crystallinity. Therefore, we concluded that simply increasing the crystallinity may not be sufficient to deliver a PCE enhancement in bilayer structures because of the anisotropic transport properties of the semiconducting polymer which depend on the orientation of the backbone.

Electronic transport properties of inner and outer shells in near ohmic-contacted double-walled carbon nanotube transistors

We investigate electronic transport properties of field-effect transistors based on double-walled carbon nanotubes, of which inner shells are metallic and outer shells are semiconducting. When both shells are turned on, electron-phonon scattering is found to be the dominant phenomenon. On the other hand, when outer semiconducting shells are turned off, a zero-bias anomaly emerges in the dependence of differential conductance on the bias voltage, which is characterized according to the Tomonaga-Luttinger liquid model describing tunneling into one-dimensional materials. We attribute these behaviors to different contact conditions for outer and inner shells of the double-walled carbon nanotubes. A simple model combining Luttinger liquid model for inner metallic shells and electron-phonon scattering in outer semiconducting shells is given here to explain our transport data at different temperatures.

Low-frequency noise in individual carbon nanotube field-effect transistors with top, side and back gate configurations: effect of gamma irradiation

We report on the influence of low gamma irradiation (104 Gy) on the noise properties of individual carbon nanotube (CNT) field-effect transistors (FETs) with different gate configurations and two different dielectric layers, SiO2 and Al2O3. Before treatment, strong generation–recombination (GR) noise components are observed. These data are used to identify several charge traps related to dielectric layers of the FETs by determining their activation energy. Investigation of samples with a single SiO2 dielectric layer as well as with two dielectric layers allows us to separate traps for each of the two dielectric layers. We reveal that each charge trap level observed in the side gate operation splits into two levels in top gate operation due to a different potential profile along the CNT channel. After gamma irradiation, only reduced flicker noise is registered in the noise spectra, which indicates a decrease of the number of charge traps. The mobility, which is estimated to be larger than 2 × 104 cm2 V−1 s−1 at room temperature, decreases only slightly after radiation treatment, demonstrating high radiation hardness of the CNTs. Finally, we study the influence of Schottky barriers at the metal–nanotube interface on the transport properties of FETs, analyzing the behavior of the flicker noise component.

The effect of processing additive on aggregated fullerene derivatives in bulk-heterojunction polymer solar cells

The effects of processing additive on fullerene aggregation in polymer BHJ solar cells were investigated using new fullerene derivatives bearing a thiophene moiety and alkyl groups. Although new fullerene derivatives showed quite similar electronic transport properties in field-effect transistors, the photovoltaic performances were significantly limited by their aggregative nature. Processing with 1% CN additive, however, changed the aggregated morphology of BHJ films to a smoother and homogeneous morphology, improving photovoltaic performance. The result indicates that processing additive not only influences on polymer side, but also significantly affects fullerene acceptor component.

Effects of substituted side-chain position on donor-acceptor conjugated copolymers

The optical properties and electrical properties of a series of low-band-gap conjugated copolymers, in which alkyl side chains were substituted at various positions, were investigated using donor–acceptor conjugated copolymers consisting of a cyclopentadithiophene derivative and dithienyl-benzothiadiazole. With substituted side chains, the intrinsic properties of the copolymers were significantly altered by perturbations of the intramolecular charge transfer. The absorption of poly[2, 6-(4,4-bis(2-octyl)-4H-cyclopenta-[2,1-b:3,4-b′]dithiophene)-alt-4, 7-bis(4-octyl-thiophene-2-yl)benzo-2,1,3-thiadiazole] [PCPDT-ttOTBTOT (P2)], which assumed a tail–tail configuration, tended to blue shift relative to the absorption of poly[2,6-(4,4-bis(2-octyl)-4H-cyclopenta-[2,1-b:3,4-b′]dithiophene)-alt-4,7-bis (thiophene-2-yl)benzo-2,1,3-thiadiazole] [PCPDT-TBTT (P1)]. The absorption of poly[2,6-(4,4-bis(2-octyl)-4H-cyclopenta-[2,1-b:3, 4-b′]dithiophene)-alt-4,7-bis(3-octyl-thiophene-2-yl)benzo-2,1,3-thiadiazole] [PCPDT-hhOTBTOT (P3)], which assumed a head–head configuration, was blue shifted relative to that of P2. The electrical transport properties of field-effect transistors were sensitive to the side chain position. The field-effect mobility in P2 (μ2 = 1.8 × 10−3 cm2/V s) was slightly lower than that in P1 (μ1 = 4.9 × 10−3 cm2/V s). However, the mobility of P3 was very low (μ3 = 3.8 × 10−6 cm2/V s). Photoexcitation spectroscopy showed that the charge generation efficiency (shown in transient absorption spectra) and polaron pair mobility in P1 and P2 were higher than in P3, yielding P1 and P2 device performances that were better than the performance of devices based on P3. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Effects of Substituted Side Chains on the Optical and Electrical Properties of D-A Conjugated Copolymers

ABSTRACTThe effects of substitute side chain were investigated using donor-acceptor (D-A) conjugated copolymers consisting of a cyclopentadithiophene (CPDT) derivative and dithienyl-benzothiadiazole (DTBT). The intrinsic properties of the copolymers were significantly altered by perturbations of the intramolecular charge transfer (ICT). The absorption of PCPDT-ttOTBTOT (P2), which assumed a tail-tail configuration, tended to blue-shift relative to the absorption of PCPDT-TBTT (P1). The absorption of PCPDT-hhOTBTOT (P3), which assumed a head-head configuration, was blue-shifted relative to that of P2. The electrical transport properties of field-effect transistors (FETs) were sensitive to the side chain position. The field-effect mobility in P2 (μ2=1.8×10–3 cm2/V·s) was slightly lower that in P1 (μ1=4.9×10–3 cm2/V·s). The mobility of P3, however, was very low (μ3=3.8×10–6 cm2/V·s). Photoexcitation spectroscopy showed that the charge generation efficiency (shown in transient absorption spectra) and polaron pair mobility in P1 and P2 were higher than in P3, yielding P1 and P2 device performances that were better than the performance of devices based on P3.

Electrical transport properties of fullerene peapods interacting with light

In this paper, we investigate the transport properties of field-effect transistors (FETs) based on various kindsof fullerene (C60, C70,and C84) peapods. The encapsulation of various fullerenes inside single-walled carbonnanotubes (SWNTs) is characterized by TEM and Raman spectroscopy. Ourresults indicate that the transport characteristics of p-type SWNTs are extremelysensitive to the encapsulation of various kinds of fullerene. In the absence of lightillumination, the threshold voltage of p-type SWNTs shifts towards positive values afterC60 and C70 encapsulation, and ambipolar transport characteristics are observed forC84 peapods. The photoinduced electron transfer phenomenon is observedfor fullerene peapods under light illumination. The optical response forC60 and C70 peapod FET devices is reflected in a shift of threshold voltage towardsnegative values, and a recoverable characteristic is observed when lightis off. After a long period of light illumination, in contrast to p-typeC60 and C70 peapods, an n-type transport characteristic is observed onC84 peapods.

Photoinduced electron transfer in C60 encapsulated single-walled carbon nanotube

The transport properties of field-effect transistors based on C60 fullerene peapods have been investigated and our findings indicate that the transport characteristics of single-walled carbon nanotubes (SWNTs) are highly sensitive to the encapsulated fullerenes due to the charge-transfer effect. Under light illumination, the photoinduced electron transfer characteristics are found in C60 peapods, which is different from those observed for empty SWNTs. The response characteristic of peapod devices to light is reflected in a shift of threshold voltage toward negative values, indicating an electron-doping process. After removing light the photoresponse is fully recoverable, indicating complete restoration of transferred electrons.

Transport properties of field-effect transistor with Langmuir-Blodgett films of C60 dendrimer and estimation of impurity levels

Field-effect transistor (FET) device has been fabricated with Langmuir-Blodgett films of C60 dendrimer. The device showed n-channel normally off characteristics with the field-effect mobility of 2.7×10−3cm2V−1s−1 at 300K, whose value is twice as high as that (1.4×10−3cm2V−1s−1) for the FET with spin-coated films of C60 dendrimer. This originates from the formation of ordered π-conduction network of C60 moieties. From the temperature dependence of field-effect mobility, a structural phase transition has been observed at around 300K. Furthermore, the density of states for impurity levels was estimated in the Langmuir-Blodgett films.

Transport properties of field-effect transistors with thin films of C 76 and its electronic structure

The C 76 field-effect transistor (FET) showed n-channel normally-off like behavior with n-channel field-effect mobility, μ n, of 3.9 × 10 −4 cm 2 V −1 s −1, and the highest on–off ratio, 125, among higher fullerenes FETs. The carrier transport in the C 76 FET followed a thermally-activated hopping transport model. The normally-off like properties of C 76 FET could be reasonably explained in terms of the electronic structure of thin films determined by photoemission spectroscopy.