Mobility Of Organic Field-effect Transistors Research Articles

Reduction of charge injection barrier by 1-nm contact oxide interlayer in organic field effect transistors

The enhancement of the charge injection process by the insertion of an ultrathin (∼1 nm) contact oxide interlayer (COI) at the metal/organic material interface in organic field effect transistors (OFETs) is reported. Six different oxides were used as COI, and Al2O3 was found to exhibit the highest OFET mobility with a reduction in the average contact resistance (Rc) from 19.9 to 1.9 kΩ·cm. Photoelectron yield spectroscopy analysis revealed that the insertion of COI increases the work function of an Au contact and reduces the charge injection barrier at the interface, which lowers Rc and, therefore, results in enhanced device performance.

Progress of the improved mobilities of organic field-effect transistors based on dielectric surface modification

The surface property of the dielectric has a significant influence on growth, morphology, order of the organic semiconductor, and charge carrier transport. The relevant research shows that the mobility of organic field-effect transistor could be effectively improved via ameliorating the surface property of the dielectric. The purpose of this review is to introduce the main factors, including the roughness and the surface energy of dielectric, which exert a tremendous influence on the field effect mobility of OFET, and chiefly describe the progress of the two common methods used for the dielectric modification, viz., the self-assembled monolayer modification and the polymer modification. Finally, the novel applications at present are summarized in this review and some perspectives on the research trend are proposed.

Nanofiber growth and alignment in solution processed n-type naphthalene-diimide-based polymeric field-effect transistors

The nanofiber growth of an n-type conjugated polymer poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}, P(NDI2OD-T2), was studied during solution processing. Highly reproducible fibrous nanostructures obtained at various dip coating speeds provide an insight into the growth mechanism of the polymer nanofibers during solution deposition and into the correlation between their morphology and charge-carrier mobility of organic field-effect transistors (OFETs). Remarkably, the nanofibers orient parallel to the dip coating direction on reaching 4 layers leading to a significantly improved charge transport between source and drain electrodes. We have observed that the morphology, thickness and orientation have great impacts on the charge carrier transport. We prove that the first fully covered fibrous layer is of minimum thickness for a charge carrier migration to happen in the transistor, whereas the mobility strongly rises in the subsequent 3–4 fibrous layers, which are considered to provide alternative pathways for charge carriers. The strong increase in the thickness of 3–4 layers is also attributed to the additional film alignment taking place during the dip coating process. Therefore, the type of fibrous nanostructures does not change with film thickness.

Carrier mobility in organic field-effect transistors

A study of carrier transport in top-gate and bottom-contact TIPS-pentacene organic field-effect transistors (OFETs) based on mobility is presented. Among three mobilities extracted by different methods, the low-field mobility obtained by the Y function exhibits the best reliability and ease for use, whereas the widely applied field-effect mobility is not reliable, particularly in short-channel transistors and at low temperatures. A detailed study of contact transport reveals its strong impact on short-channel transistors, suggesting that a more intrinsic transport analysis is better implemented in relatively longer-channel devices. The observed temperature dependences of mobility are well explained by a transport model with Gaussian-like diffusivity band tails, different from diffusion in localized states band tails. This model explicitly interprets the non-zero constant mobility at low temperatures and clearly demonstrates the effects of disorder and hopping transport on temperature and carrier density dependences of mobility in organic transistors.

Independently driven four-probe method for local electrical characteristics in organic thin-film transistors under controlled channel potential

We describe an independently driven four-probe method to investigate local channel mobility in organic field-effect transistors (OFETs). In OFET devices, probe-organic contact resistance affects device characteristics even in four-probe measurement because a change in contact resistance at the source probe induces a change in channel potential, resulting in different local carrier density. To overcome this problem, we introduced a feedback circuit between the source probe and a channel voltage probe to keep the channel potential constant. We demonstrate four-probe I-V measurement on a pentacene thin film (50 nm thick) under controlled channel potential. The feedback successfully enables us to separate contact resistance and channel resistance even under different contact conditions. We also measured four-probe resistance as a function of gate bias and channel probe position. The present results were in good agreement with two-dimensional model calculation by arranging four probes in a defect-free area; the mobility of the pentacene single grain was evaluated to be 0.25 cm(2)/(V s).

Polymeric semiconductor/graphene hybrid field-effect transistors

Solution processed organic field-effect transistors (OFETs) usually have high on/off ratios but suffer from low mobilities, while transistors based on graphene usually exhibit very high mobilities but low on/off ratios. We demonstrate a straightforward route to solve the challenging problem of enhancing the effective mobility in OFETs while keeping the on/off ratio sufficiently high. We achieve this by developing hybrid FETs incorporating both organic semiconductors and graphene. Compared against OFETs with only pure organic semiconductors, our hybrid FETs exhibit up to 20 times higher effective mobilities, and yet they keep the on/off ratios comparable or better. P3HT/graphene hybrid FETs exhibit mobility as high as 0.17 cm 2 V −1 s −1, and PQT-12/graphene hybrid FETs show effective mobility up to 0.6 cm 2 V −1 s −1. We expect that incorporating graphene is a general route to enhance the performance of OFETs, providing a low-cost avenue for enhancing OFET performance.

Theoretical analysis of carrier mobility in organic field-effect transistors

A theoretical analysis of the carrier mobility in organic transistors is presented. We noticed that the assumption of zero potential at open/quasi-free surface may cause a large deviation of the areal charge density in the organic film, greater in thinner-film transistors. Taking into account this effect, the effective mobility is obtained using the Kubo–Greenwood integral, which provides the total conductivity in the band and thus in the whole organic film. The mobility is studied with respect to gate voltage and temperature, for various disorder and transport diffusivity levels, enabling a better insight of the carrier mobility in organic transistors.

Indacenodithiophene and Quinoxaline-Based Conjugated Polymers for Highly Efficient Polymer Solar Cells

Two new low-band-gap conjugated polymers based on the polymerization between indacenodithiophene and 2,3-diphenylquinoxaline or phenanthrenequinoxaline were synthesized. Due to the fused phenanthrenequinoxaline unit, the polymer (PIDT-phanQ) possesses better planarity than PIDT-diphQ, resulting in an improved hole mobility in organic field-effect transistors and power conversion efficiency in polymer solar cells.

A solution processable fluorene-benzothiadiazole small molecule for n-type organic field-effect transistors

We report an n-type organic semiconductor [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (herein referred to as K12) for use in organic field-effect transistors (OFETs). K12 can be processed by spin-coating from solution or by vacuum deposition, organizing into highly orientated microcrystalline structures at modest (75 °C) annealing temperatures. OFETs with n-octyltrichlorosilane or hexamethyldisilazane monolayers, or poly(propylene-co-1-butene) (PPCB) modified dielectric surfaces were prepared. The mobility, ON/OFF ratio, threshold voltage, and current hysteresis were found to be dependent on the thermal history of the film and surface onto which it was deposited. The highest OFET mobility achieved was 2.4×10−3 cm2/V s, for spin-coated films with a PPCB modified silicon dioxide dielectric.

Large Memory Effect and High Carrier Mobility of Organic Field-Effect Transistors Using Semiconductor Colloidal Nano-Dots Dispersed in Polymer Buffer Layers

Large Memory Effect and High Carrier Mobility of Organic Field-Effect Transistors Using Semiconductor Colloidal Nano-Dots Dispersed in Polymer Buffer Layers

Large Memory Effect and High Carrier Mobility of Organic Field-Effect Transistors Using Semiconductor Colloidal Nano-Dots Dispersed in Polymer Buffer Layers

We fabricated organic memory field-effect transistors (FETs) using PbS colloidal nano-dots (NDs) dispersed in thin poly(methyl methacrylate) (PMMA) layers inserted between gate insulators (SiO2) and pentacene active layers as floating gates. The colloidal NDs were dispersed in chloroform solution with PMMA, and spin-coated on SiO2 surfaces. The fabricated memory FETs showed significantly large threshold voltage shifts of 64.5 V at maximum after a writing voltage of 100 V was applied to their control gates, and a maximum carrier mobility of 0.36 cm2 V-1 s-1, which was comparable to that of reference pentacene FETs without colloidal NDs, was obtained because of the improved crystallinity of the pentacene films.

Low voltage organic devices and circuits with aluminum oxide thin film dielectric layer

Low voltage operating organic devices and circuits have been realized using atomic layer deposition deposited aluminum oxide thin film as dielectric layer. The dielectric film has per unit area capacitance of 165 nF/cm2 and leakage current of 1 nA/cm2 at 1 MV/cm. The devices and circuits use the small-molecule hydrocarbon pentacene as the active semiconductor material. Transistors, inverters, and ring oscillators with operating voltage lower than 5 V were obtained. The mobility of organic field-effect transistors was extracted to be 0.16 cm2/Vs in saturation range, the threshold voltage is 0.3 V, and the on/off current ratio is larger than 105. The gain of inverters is estimated to be 12 at −5 V supply voltage, and the propagation delay is 0.25 ms per stage in 5-stage ring oscillators.

Backbone Curvature in Polythiophenes

A series of five polymers containing different benzodithiophene isomers copolymerized with alkylated dithiophene have been synthesized and characterized in terms of their semiconducting properties. Because of the different bonding geometry of the benzodithiophene monomers, a varying degree of curvature is introduced into the polymer backbone chain. The influence of this curvature on the solubility, the electronic levels, the morphology in a film, and the charge carrier mobility in organic field-effect transistors has been investigated. It turns out that an increased degree of curvature improves the solubility, but decreases the order in the film. As a result, the polymer with an intermediate degree of curvature yields the highest charge-carrier mobilities. These findings shall serve as guideline for the rational design of other semiconducting polymers.

Dependence of Meyer–Neldel energy on energetic disorder in organic field effect transistors

Meyer–Neldel rule for charge carrier mobility was studied in C60-based organic field effect transistors (OFETs) fabricated at different growth conditions which changed the degree of disorder in the films. The energetic disorder in the films was found to correlate with a shift in the Meyer–Neldel energy, which is in excellent agreement with the predictions of a hopping-transport model for the temperature dependent OFET mobility in organic semiconductors with a Gaussian density-of-states (DOS). Using this model the width of the DOS was evaluated and it was found to decrease from 88 meV for the films grown at room temperature to 54 meV for films grown at 250 °C.

Thickness dependence of surface morphology and charge carrier mobility in organic field-effect transistors

With the aim of understanding the relationships between organic small molecule field-effect transistors (FETs) and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly (3-hexylthiophene) (RR-P3HT) field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies (average size and average roughness) of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

Improved Morphology and Performance from Surface Treatments of Naphthalenetetracarboxylic Diimide Bottom Contact Field-Effect Transistors

We report bottom contact organic field-effect transistors (OFETs) with various surface treatments based on n-channel materials, specifically, 1,4,5,8-naphthalene-teracarboxylic diimides (NTCDIs) with three different fluorinated N-substituents, systematically studied with a particular emphasis on the interplay between the morphology of the organic semiconductor films and the electrical device properties. The morphological origins of the improvements were directly and dramatically visualized at the semiconductor-contact interface. As a result of a series of treatments, a large range of performances of bottom contact side-chain-fluorinated NTCDI OFETs (mobility from 1 x 10(-6) to 8 x 10(-2) cm(2)/(V s), on/off ratio from 1 x 10(2) to 1 x 10(5)) were obtained. The surface treatments enabled systems that had shown essentially no OFET activity without electrode modification activity to perform nearly as well as top contact devices made from the same materials. In addition, for the fresh bottom contact NTCDI device, the effect of gate bias stress on the tens-of-minutes time scale, during which the threshold voltage (V(t)) shifted and relaxed with similar time constants, was observed.

Carrier injection and transport in organic field-effect transistor investigated by impedance spectroscopy

In this research, impedance spectroscopy (IS) has been applied on top-contact pentacene organic field-effect transistors (OFETs) for characterization of the contact resistance and carrier transport properties. The various relaxation processes were analyzed based on the Maxwell–Wagner model, assuming pentacene as a dielectric material. The mobilities obtained from the IS analysis correspond well with that obtained from the current–voltage (I–V) analysis. This alternative method by IS provides an additional advantage of isolating the effect of carrier concentration when evaluating the mobility of OFETs.

Interdependence of contact properties and field- and density-dependent mobility in organic field-effect transistors

The current characteristics of organic field-effect transistors (OFET) often show a disadvantageous nonlinearity at low drain voltages. It has been shown recently [J. Appl. Phys. 102, 054509 (2007)] that in top contact (TOC) OFETs this effect can be caused by trap recharging if the contacts are of Schottky type. For bottom contact (BOC) OFETs, in spite of controversial discussions, Schottky contacts as origin of the nonlinearity are often stated. At first, it is shown here by a mixed mode simulation that for large ideality factors a Schottky contact only at drain leads to such a nonlinearity. However, with the same Schottky contacts at drain and source the effect is covered by the high resistance of the contact at source. Next, the different influences of Schottky contacts on BOC OFETs and TOC OFETs with varying overlap of the source/drain contacts with the gate are clarified. Further, it is demonstrated with detailed two-dimensional simulations that the combination of the presence of Schottky contacts with a field dependence of the mobility can cause the nonlinearity. For the mobility we use the field dependent Pool/Frenkel model, and the models of Limketai et al.[Phys. Rev. B 75, 113203 (2007)] and Pasveer/Coehoorn et al.[Phys. Rev. Lett. 94, 206601 (2005)], which depend in addition on the carrier concentration. Their influence on the device performance has been clarified by the simulations. Simulated profiles of concentrations and fields lead to the understanding of the mechanism causing the nonlinearity. This mechanism is especially effective for the Pasveer/Coehoorn model. The field dependence of the mobility is a consequence of the energetic distribution of the hopping states and can hardly be avoided in solution based deposition of the active polymer layer. A strategy to prevent the nonlinearity is therefore an optimization of the contact-polymer interface such that the contacts become Ohmic.

Dependence of Electrical and Time Stress in Organic Field Effect Transistor with Low Temperature Forming Gas Treated Al2O3 Gate Dielectrics

We report the characteristics of the organic field effect transistor (OFET) after electrical and time stress. Aluminum oxide (Al2O3) was used as a gate dielectric layer. The surface of the gate oxide layer was treated with hydrogen (H2) and nitrogen (N2) mixed gas to minimize the dangling bond at the interface layer of gate oxide. According to the two stress parameters of electrical and time stress, threshold voltage shift was observed. In particular, the mobility and subthreshold swing of OFET were significantly decreased due to hole carrier localization and degradation of the channel layer between gate oxide and pentacene by electrical stress. Electrical stress is a more critical factor in the degradation of mobility than time stress caused by H2O and O2 in the air.

Performance and Fabrication of Organic Field Effect Transistor with Active Layer Prepared by Soluble Pentacene

An organic field-effect transistor (OFET) was prepared with thin film fabricated by soluble pentacene as the semiconductor layer, polyimide thin film as the gate insulating layer, and Au as the drain, source and gate electrodes. The pentacene and polyimide thin films were fabricated by the cast and spin coat methods. The carrier mobility of the organic field-effect transistor was 1.2×10-2 cm2/Vs. The ON/OFF ratio was 3×102. As compared with the carrier mobility of OFET prepared by the evaporation method, their values were the same as that of the spin-coating method. However, the ON/OFF ratio obtained was lower. This indicates that it is either the leakage current in polyimide thin film that is large or it is that of the off-current of the pentacene thin film.