Characteristics Of Organic Thin-film Transistors Research Articles

Analysis of electrical and hysteresis characteristics of flexible OTFT using solution-processable DPP-DTT polymer and Parylene-C

Organic thin-film transistors (OTFTs) fabricated on Parylene-C substrates have the advantages of a simple process, low cost, and flexible characteristics. This study introduces the manufacturing method and electrical characteristics of an OTFT using organic materials as the substrate, gate dielectric, and channel material. PDPP2T-TT-OD(DPP-DTT) is used as the channel material, which is a highly mobile p-type polymer with good air stability. The proposed OTFT device has flexible characteristics because it is fabricated on a Parylene-C substrate and can be used even in a curved state. Furthermore, the manufacturing process was largely achieved via a simple, low-cost solution process using spin-coating and photolithography with a photo-curable material. Under flat conditions, the threshold voltage (VTH) is approximately –3 V, the average ION/IOFF ratio is approximately 105, and the mobility is 0.84 cm2/Vs.

A study of bottom-contact organic thin-film transistors based on source/drain tunneling structure

During recent years, OLED display has become one of the important products for consuming electronics and industrial domains. For AMOLED display, thin film transistor (TFT) is necessary for driving OLED devices. As the polysilicon and amorphous silicon TFTs have become mainstreams, organic thin film transistors (OTFTs) have also been proposed and shown their advantages in low-temperature process compatible with flexible substrates. However, OTFTs applicable to drive OLED generally adopt bottom-contact configuration. The performance of bottom-contact OTFTs are often inferior relative to top-contact OTFTs, due primarily to the high contact resistance of bottom-contact OTFTs. To overcome this disadvantage, the source/drain tunneling structure is proposed in this article. This tunneling structure can not only decorate the electrode/organic semiconductor interface to obtain a good metal-semiconductor contact, but also can form a MOS configuration at source/drain contact, so as to increase the carrier concentration at the contact to further reduce contact resistance. In this paper, a typical P-type high-mobility small molecule material of Pentacene is employed as the active layer of the OTFT, while CuO film, Polystyrene (PS) film is separately selected as the tunneling layer at source/drain contact. The diverse parameters, especially field effect mobility, were analyzed by analyzing electrical characteristics of the top-contact and bottom-contact OTFTs. In addition, the contact resistances were confirmed by line transfer method. The bottom-contact OTFTs incorporating tunneling layer own a maximum hole mobility of 0.143cm2/V·s.This suggested that the source/drain tunneling structure can indeed improve the electrical characteristics of bottom-contact OTFT, and further improvement is necessitated to achieve the high performance comparable to top-contact device.

The Effect of Alkyl Chain Length in Organic Semiconductor and Surface Polarity of Polymer Dielectrics in Organic Thin-Film Transistors (OTFTs).

The interface between dielectric and organic semiconductor is critically important in determining organic thin-film transistor (OTFT) performance. Surface polarity of the dielectric layer can hinder charge transport characteristics, which has restricted utilization of polymeric dielectric materials containing polar functional groups. Herein, the electrical characteristics of OTFTs are analyzed depending on the alkyl chain length of organic semiconductors and surface polarity of polymer dielectrics. High-performance dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene (DBTTT) and newly synthesized its alkylated derivatives (C6-DBTTT and C10-DBTTT) are utilized as organic semiconductors. As dielectric layers, non-polar poly(1,3,5-trimethyl-1,3,5-trivinylcyclitrisiloxane) (pV3D3) and poly(2-cyanoethyl acrylate-co-diethylene glycol divinyl ether) [p(CEA-co-DEGDVE)] with polar cyanide functionality are utilized. The fabricated OTFTs with pV3D3 commonly exhibit the excellent charge transport characteristics. In addition, the OTFT performance is improved with lengthening the alkyl chain in organic semiconductors, which can be attributed to the molecular orientation of semiconductors. On the other hand, non-alkylated DBTTT OTFTs with polar p(CEA-co-DEGDVE) show relatively poor electrical characteristics, while their performance is drastically enhanced with the alkylated DBTTTs. The ultraviolet photoelectron spectroscopy (UPS) reveals that surface polarity of the dielectric layer can be abated with alkyl chain in organic semiconductors. It is believed that this study can provide a useful insight to optimize dielectric/semiconductor interface to achieve high-performance OTFTs.

Achieving ideal transistor characteristics in conjugated polymer semiconductors.

Organic thin-film transistors (OTFTs) with ideal behavior are highly desired, because nonideal devices may overestimate the intrinsic property and yield inferior performance in applications. In reality, most polymer OTFTs reported in the literature do not exhibit ideal characteristics. Supported by a structure-property relationship study of several low-disorder conjugated polymers, here, we present an empirical selection rule for polymer candidates for textbook-like OTFTs with high reliability factors (100% for ideal transistors). The successful candidates should have low energetic disorder along their backbones and form thin films with spatially uniform energetic landscapes. We demonstrate that these requirements are satisfied in the semicrystalline polymer PffBT4T-2DT, which exhibits a reliability factor (~100%) that is exceptionally high for polymer devices, rendering it an ideal candidate for OTFT applications. Our findings broaden the selection of polymer semiconductors with textbook-like OTFT characteristics and would shed light upon the molecular design criteria for next-generation polymer semiconductors.

Role of the Alkylation Patterning in the Performance of OTFTs: The Case of Thiophene-Functionalized Triindoles

Organic semiconductors have emerged as potential alternatives to conventional inorganic materials due to their numerous assets and applications. In this context, the star-shaped triindole core stands as a promising system to design new organic materials with enticing charge-transporting properties. Herein, we present the synthesis of three thiophene-containing triindole derivatives that feature N-alkyl chains of different lengths, from methyl to decyl. The impact of the alkylation patterning on the crystallinity of the thin films and their resultant performance as semiconductor have been analyzed. All derivatives displayed p-type semiconductor properties, as demonstrated via both TOF measurements and integration in organic thin-film transistor (OTFT) devices. The attachment of longer alkyl chains and the functionalization of the silicon substrate with octadecyltrichlorosilane (OTS) prompted better OTFT characteristics, with a hole mobility value up to 5 × 10−4 cm2 V−1 s−1. As elucidated from the single crystal, this core is arranged in a convenient cofacial packing that maximizes the π-overlapping. The analysis of the thin films also corroborates that derivatives possessing longer N-alkyl chains confer a higher degree of order and a more adequate morphology.

Surface engineering of zinc phthalocyanine organic thin-film transistors results in part-per-billion sensitivity towards cannabinoid vapor

Phthalocyanine-based organic thin-film transistors (OTFTs) have been demonstrated as sensors for a range of analytes, including cannabinoids, in both liquid and gas phases. Detection of the primary cannabinoids, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD), is necessary for quality control and regulation, however, current techniques are often not readily available for consumers, industry, and law-enforcement. The OTFT characteristics, X-ray diffraction (XRD) spectra, and grazing incident wide angle x-ray scattering (GIWAXS) spectra of two copper and three zinc phthalocyanines, with varying degrees of peripheral fluorination, were screened to determine sensitivity to THC vapor. Unsubstituted ZnPc was found to be the most sensitive material and, by tuning thin-film morphology, crystal polymorphs, and thickness through altered physical vapor deposition conditions, we increased the sensitivity to THC by 100x. Here we demonstrate that deposition conditions, and the resulting physical film characteristics, play a significant role in device sensitization.

(Digital Presentation) Effect of Active Layer Thickness on Organic Thin-Film Transistors

ABSTRACT: Thin-Film Transistors (TFTs) are a substantial technological advancement in recent decades for various applications. The source/drain electrode layer and organic active layer thicknesses of Organic Thin-Film Transistors (OTFTs) should be optimized for better device performance. Organic electronics have been used for a wide range of applications, including flexible screens, smart digital devices, and photovoltaic cells, because of their flexibility and environmental sensitivity. The OTFT has the capability to match the efficiency of thin-film amorphous Silicon transistors, while also being companionable with low-temperature solution/printing-processed fabrication on flexible coupling substrates. The OTFTs are a valuable tool for determining unipolar carrier transport parameters in various situations.In this present research work, authors have utilized the concept of OTFTs in the assessment of bipolar transport properties in active layer blends. It offers a strategy to improve the precision of the assessment. Thereafter, in this this research work, impacts of active layer thickness on physical parameters of OTFT device performance have been realized. The 2D numerical device simulators have been used to examine the proposed OTFT structures. The discussion focuses on the various characteristics and parameters of OTFTs. The OTFTs are transistors that manage electric current flow using organic semiconductors as an active layer. The output and transfer characteristics of various OTFT structures have been used to calculate the performance characteristics of OTFTs. Optimizing the OTFT's organic active layer thickness is critical for high device performance. Both photo-current and photo-responsivity exhibit the same variation trend with increasing organic active layer thicknesses, increasing rapidly for a while and then tending to saturate at high values. The research findings demonstrate the impact of these parameters on device performance and temperature and the need to optimize these variables in the device. The carrier mobility of the high-performance P3HT: PCBM-based OTFT structure was approximately 10 cm2/Vs, and an ON/OFF current ratio of ~103. These results are compatible with those OTFTs fabricated previously. Research Methodology, Results, and Discussions A schematic structure is shown in Figure 1(a) proposed structure top view, and 1(b) a side view of the proposed structure used for simulation. Top-gate OTFT and bottom-gate OTFT geometrical structures have been used in this OTFT. The device's contact resistance, field mobility, and threshold voltage degrade subjected to bias stress. Since top-gate OTFT has a higher device degradation, the authors used bottom-gate OTFT. Due to the distinct properties here in this work the authors utilized conventional materials such as a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the active layer. A P3HT: PCBM mix is simulated with an electronic device simulator and analyses the variation in active layer thickness that affects the device's performance and the factors that influence the performance of the device in this research work. The output and transfer characteristics of OTFTs have been realized using proper boundary conditions and OTFT physics. The proposed OTFT structures' output and transfer characteristics have been evaluated, and the simulated results show that the optimized structure's drain current (Id, max) is ~2.85 μA. The obtained results show that the device has good transfer and output characteristics at lower gate voltages. With increasing organic active layer thicknesses, both photo-current and photo-responsivity exhibit the same variation trend of increasing sharply at first and then tending to saturate at high values. Conclusion The simulation results examined the performance of devices built with bottom-gate OTFTs. A detailed analysis of the influence of active layer thickness on the OTFT configuration was realized. The device has the advantage of operating at a lower gate voltage, transforming it into a gate efficient control device. The operation at a lower voltage improves electrical stability. The electric field of the OTFT is 1.4x106 V/cm is obtained. The ON/OFF current ratio is higher. The outcomes of this analysis have been optimized and extracted after being tested in various contexts. The findings demonstrated that the suggested concept might be used to make rollable active-matrix displays, nonvolatile memory, sensors, and printed electronic devices. This work also reduces the SCEs and shows more efficient performance than traditional MOSFETs. Together with all application areas, the most important research area has been argued to be the simple manufacturing technique of OTFT with low production cost and non-breakable impacts that can be bent and folded. This device will be optimized with fabrication using these materials in the future, and thereafter, its characteristic will be verified with the simulated results. Figure 1

Self-Consistent Extraction of Mobility and Series Resistance: A Hierarchy of Models for Benchmarking Organic Thin-Film Transistors

Organic thin-film transistors (OTFTs) are usually benchmarked across different devices and technologies based on the threshold voltage, charge carrier mobility, and series resistance. However, conventional parameter extraction established for silicon transistors frequently lead to misleading results when applied to OTFTs. Some of the peculiarities of OTFTs can be addressed by the virtual-source (VS) emission–diffusion (ED) theory. Using published data and own measurements, we show that the electrical characteristics of OTFTs partially limited by mobile charge supply to the channel, thermic emission, or by velocity saturation can be successfully predicted by an organic VSED model. Simplifying the current-voltage dependence obtained from the VSED theory modified for organic materials (OVSED), we introduce a hierarchy of benchmark models only employing one additional parameter with a specific electrical signature for each of these limitations. As a special case, the hierarchy includes the widely applied Shichman–Hodges (SH) model. In combination with standard nonlinear least-squares solvers, such benchmark models can replace conventional extraction methods and are applicable to a wider range of OTFT technologies.

Study and Analysis of Simple and Precise of Contact Resistance Single-Transistor Extracting Method for Accurate Analytical Modeling of OTFTs Current-Voltage Characteristics: Application to Different Organic Semiconductors

Contact resistance (Rc) characterizes the interface of source-drain electrodes/organic semiconductors and controls the injection efficiency of carriers in organic thin-film transistors (OTFTs). This research paper presents and assesses two methods for extracting the value of the contact resistance from the measured current-voltage characteristics of OTFTs made with various p-type organic semiconductors as active layers. These two methods are the transition voltage method (TVM) and the transfer line method (TLM). The obtained Rc values by the TVM method are in fair agreement with those obtained by TLM, with a maximum percentage of difference around 10%, demonstrating the accuracy of the used transition-voltage method. An analytical model was employed to calculate output characteristics in the linear regime of OTFTs made with various organic semiconductors using the contact resistance values obtained by the transition voltage method. The calculated results are in reasonably good agreement with the experimental ones of each fabricated device, which affirms the ability of the used model to characterize the charge transport correctly in these types of devices. It can be concluded that the used TVM method is not only an easy and practical method, but also a precise way for extracting Rc in OTFTs produced using different organic semiconductor materials.

Fabrication, characterization, numerical simulation and compact modeling of P3HT based organic thin film transistors

This paper presents the fabrication, characterization and numerical simulation of poly-3-hexylthiophene (P3HT)-based bottom-gate bottom-contact (BGBC) organic thin film transistors (OTFTs). The simulation is based on a drift diffusion charge transport model and density of defect states (DOS) for the traps in the band gap of the P3HT based channel. It combines two mobility models, a hopping mobility model and the Poole–Frenkel mobility model. It also describes the defect density of states (DOS) for both tail and deep states. The model takes into account all the operating regions of the OTFT and includes sub-threshold and above threshold characteristics of OTFTs. The model has been verified by comparing the numerically simulated results with the experimental results. This model is also used to simulate different structure in four configurations of OTFT e.g. bottom-gate bottom-contact (BGBC), bottom-gate top-contact (BGTC), top-gate bottom-contact (TGBC) and top-gate top-contact (TGTC) configurations of the OTFTs. We also present the compact modeling and model parameter extraction of the P3HT-based OTFTs. The extracted compact model has been further applied in a p-channel OTFT-based inverter and three stage ring oscillator circuit simulation.

FABRICATION OF A NORMALLY-ON ORGANIC THIN FILM TRANSISTOR FOR ACTIVE SENSOR CONSTRUCTION

In organic active pressure sensor, a reduction in supply voltage of transistor is an effective way to decrease the power consumption. Up to now, for the development of pressure sensor based on normally-on OTFT (organic thin-film transistor), the OTFT where the conductive channel is formed without gate voltage supply, is still challenging. In this paper, we propose an approach to fabricate normally-on OTFT based on floating gate, photoactive gate dielectric layer and programming process using external UV source. After fabrication, measurements of OTFT characteristics, including transfer and output, were performed. Estimation of the critical parameters including the threshold voltage and field effect mobility was also described. Our fabricated OTFT shows good performance with a low threshold voltage of -4 V and high mobility of 0.893 cm2/Vs. Before programming, the OTFT is at normally-off state with drain current of 10-13 A at 0 V gate voltage. After programming, the OTFT changes to normally-on state with drain current of 10-6 A at 0 V gate voltage. OTFT fabrication is the essential step to construct an organic active pressure sensor in the future work.

Optimizing the plasma oxidation of aluminum gate electrodes for ultrathin gate oxides in organic transistors

A critical requirement for the application of organic thin-film transistors (TFTs) in mobile or wearable applications is low-voltage operation, which can be achieved by employing ultrathin, high-capacitance gate dielectrics. One option is a hybrid dielectric composed of a thin film of aluminum oxide and a molecular self-assembled monolayer in which the aluminum oxide is formed by exposure of the surface of the aluminum gate electrode to a radio-frequency-generated oxygen plasma. This work investigates how the properties of such dielectrics are affected by the plasma power and the duration of the plasma exposure. For various combinations of plasma power and duration, the thickness and the capacitance of the dielectrics, the leakage-current density through the dielectrics, and the current–voltage characteristics of organic TFTs in which these dielectrics serve as the gate insulator have been evaluated. The influence of the plasma parameters on the surface properties of the dielectrics, the thin-film morphology of the vacuum-deposited organic-semiconductor films, and the resulting TFT characteristics has also been investigated.

Multifunctional Interfacial Layers from a One-Step Process for Effective Charge Capturing and Erasing in Low-Voltage-Driven Organic Thin-Film Transistors

The microstructure-dependent charge capturing and releasing behavior of dielectrics influences the memory characteristics of organic thin-film transistors (OTFTs). We use polyimide (PI) as the char...

Compact DC Modeling of Organic Thin-Film Transistors Including Their Parasitic Non-Linear Contact Effects Based on a Novel Extraction Method

This article presents a novel generic method for the extraction of the current–voltage characteristics of organic thin-film transistors (TFTs) that specifically accounts for the individual contributions of the source and drain contacts and their nonlinear behavior. Based on this extraction method, an analytical model has been derived that describes the behavior of the intrinsic TFT and of the source and drain contacts both in the linear and the saturation regimes. The essential feature of the proposed extraction method is that it does not depend on or assume any physical phenomena underlying the electrical characteristics of the contacts. Instead, it extracts the current–voltage characteristics of the contacts which can then be fit to any physics-based TFT model. This is beneficial both for circuit designers and for those developing physics-based TFT models. The proposed extraction method has been applied to organic TFTs fabricated on flexible plastic substrates in the staggered and coplanar device architectures using two different small-molecule organic semiconductors. The compact dc model has been used to simulate organic TFTs with channel lengths ranging from 4 to $100~\mu \text{m}$ and a transimpedance amplifier circuit based on TFTs with different channel lengths, and the results are in good agreement with the measurements.

Versatile model for the contact region of organic thin-film transistors

Abstract Contact effects in organic thin film transistors (OTFTs) remain an important problem to be solved in these devices. Therefore, the correct physio-chemical modeling of the contact regions in OTFTs is necessary. In this work, a versatile model for the contact region of OTFTs is proposed, since it describes the current-voltage characteristics of different kinds of contacts. It reproduces the behavior of Schottky barrier or space-charge limited contacts. It is a simple unified model since only a single parameter is necessary in order to distinguish between both kinds of contacts. The model is easily integrated in a generic compact model for the current-voltage characteristics of OTFTs. The resulting compact model, used in combination with an evolutionary parameter extraction procedure, allows to extract the intrinsic parameters and the current-voltage curves at the contact of single short-channel transistors. There is no need to use transistors with multiple channel lengths to accurately characterize the contact region or the active channel of the transistor. The model is tested with published experimental data of OTFTs with Schottky barrier or space-charge limited contacts. Finally, the method has been used as a diagnostic tool to analyze how an ammonia sensor reacts to different concentrations of the ammonia gas. Interestingly, alterations in the contact region have been detected when the gas concentration varies, transforming the space-charge limited contact of a pristine OTFT into a Schottky barrier contact under the exposure of gas.

Modeling organic thin-film transistors based on the virtual source concept: A case study

We apply the virtual source concept to describe the DC current-voltage characteristics of organic thin-film transistors. The draining of charge carriers from the virtual source is calculated using emission-diffusion theory. The electrical characteristics of organic thin-film transistors are commonly analyzed employing a drift mobility that is enhanced by the transistor overdrive. One possible origin of such a mobility model is charge trapping. We show that the direct parametrization of the fraction of mobile to total charges including trapped ones allows for a straight forward adaptation of the virtual-source emission-diffusion theory to organic electronics capable of describing measured transfer and output curves with a small number of parameters. The resulting model offers an alternative parametrization of well-known bias and temperature dependences. Beyond others, a limited charge injection does not arise from the high-field bulk saturation velocity but from the unidirectional thermal velocity and the charge carrier mean free path is a critical model parameter. Moreover, diffusivity replaces the drift mobility as the current scaling factor, which would lead to slightly different predictions for the current-voltage curves in the case that the Einstein relation is not valid.

(Invited) Evolutionary Parameter Extraction for Organic TFT Compact Models Including Contact Effects

The development of accurate and computationally efficient models is critical to reduce the cycle time between design, manufacturing and characterization of electronic devices, circuits and systems. The models must include degrading effects in order to better describe the performance of manufactured device. This is the case of contact effects in the modelling of organic thin film transistors (OTFTs). In fact, much effort was made to include contact effects into compact models. In this work, we consider a generic analytical model for the current-voltage (ID-VD) characteristics of OTFTs, valid for all the operation regions of the transistor, including the subthreshold region [1]. This model was later redefined with the inclusion of a model for the current-voltage (ID-VC) curves of the contact region, and a parameter extraction procedure, in which a sequence of iterative steps is carried out until a good agreement between experimental and simulated current voltage curves is obtained [2]. The parameter extraction procedure was accelerated in [3] with the proposal of an evolutionary procedure to extract the parameters that best fit experimental current-voltage characteristics. Later in [4], improvements over this evolutionary procedure were presented. Due to the explosion of different applications of OTFTs such as phototransistors and a wide variety of physical and chemical sensors, novel materials, contacts, and designs for transistor were proposed and used. For these applications, more accurate models and better extraction parameter procedures are needed. These new structures impose more requirements to the extraction methods and models, and therefore to the extracted parameters. From this point of view, the evolutionary extraction procedures proposed in [3, 4] are a good option to be used in combination with the above mentioned generic analytical model for the current-voltage (ID-VD) characteristics of OTFTs [1, 2]. In order to adapt both the compact model and the extraction procedure to new applications of the OTFTs, rules are added in form of optimization objectives and constrains for the different parameters. In this work, we present such rules applied to different sets of thin film phototransistors and sensors. In the first place, we consider the multi-objective evolutionary algorithm (MOEA), NSGA-II [5]. It was used in [3, 4], and seeks values of the parameter in the compact model that best meet some user defined objectives. In [3], two objectives were optimized, while in [4], four objectives were optimized, along with some defined constraints. It is known that classic MOEAs, such as the NSGA-II, have serious limitations when coping with more than three objectives. These problems are referred as many-objective optimization problems (MaOPs). Among the limitations of MOEAs to treat MaOPs are the selection operators, computational cost, visualization of the Pareto optimal front (POF), and more importantly, the convergence to an optimal solution. Recently, a many-objective implementation of the NSGA-II, the NSGA-III, was released [5]. NSGA-III was designed to specially deal with MaOPs, incorporating different operators to the ones used by its predecessor. Thus, in a second part of this work, we substitute the NSGA-II algorithm with the NSGA-III one for the characterization of the same sets of OTFTs. Finally, the results of using both algorithms are compared and will be discussed. Acknowledgments This work was supported by projects MAT2016-76892-C3-3-R and TIN2015-67020-P funded by the Spanish Government, European Regional Development Funds (ERDF) and the Canada Research Chair Program.

Cone-jet printing of aligned silver nanowire/poly(ethylene oxide) composite electrodes for organic thin-film transistors

Abstract Aligned silver nanowire (AgNWs) conductive electrodes are printed through an electrohydrodynamic (EHD) jet printing via cone-jet mode. We investigate effects of poly(ethylene oxide) (PEO) addition to AgNW inks in terms of surface energy, ink viscosity, and electrical conductivity. The optimized PEO content in the ink is determined to be ∼10 wt%, at which well-defined AgNWs/PEO composite electrodes aligned along the printing direction can be prepared by cone-jet mode of EHD jet printing. We fabricate bottom contact organic thin film transistors (OTFTs) with TIPS-pentacene/PS blend as a semiconducting layer to take advantage of printed AgNWs/PEO electrodes. The resulting devices exhibit stable transfer characteristics for 12 h even in ambient air with average field-effect mobility (μFET), threshold voltage (Vth) and on/off current ratio (Ion/Ioff) of 0.51 cm2/V, −2.0 V, and ∼106, respectively. To understand the characteristics of OTFTs based on AgNWs/PEO composite electrodes in detail, we examine crystalline morphology and molecular orientation of TIPS-pentacene/PS blend on various substrates through field emission scanning electron microscopy and two-dimensional grazing incidence X-ray diffraction, respectively.

An analytical modeling approach to the electrical behavior of the bottom-contact organic thin-film transistors in presence of the trap states

In this paper, a complete analytical model of the conduction behavior of organic thin-film transistors (OTFTs) within the framework of trap states has been presented. The aim of the work is to develop modeling equations which can be evolved into compact models generic to OTFTs rather than relying on the MOSFET-based models for the modeling and simulation of organic-based devices. The exponential density of trap states and the trap decay rate has been included in the modeling equations of the bottom-contact OTFTs with pentacene as the active layer. In this work, we propose for the first time a modeling approach to mobility. The mobility model takes into account the low-field mobility enhancement as well as high-field mobility degradation because of the presence of defect states. The current–voltage equations in the linear and saturation regimes have been modified to include the effect of trap states. The influence of trap states on the device capacitance has been modeled, and it has been found that the high density of trap states leads to deviation in the capacitance characteristics of OTFTs. Results obtained from the modeling equations of surface potential, total charge density, mobility and current–voltage relations show performance degradation with the increase in the density of trap states. The results predicted from the analytical modeling equations are in excellent agreement with the reported electrical behavior of the OTFTs, thus proving the feasibility of the proposed model. The proposed analytical model is valid for OTFTs in the linear and saturation regimes even for short-channel length transistors.

Electrical and Optical Characteristics of PQT-12-Based Organic TFTs Fabricated by Floating-Film Transfer Method

The electrical and optical characteristics of bottom-gate top-contact poly (3, 3’’’- dialkylquaterthiophene) (PQT-12) polymer-based organic thin film transistors (OTFTs) fabricated by floating-film transfer method (FTM) have been investigated in this paper. The atomic force microscopy, UV-Vis spectroscopy, and photoluminance characteristics of the FTM-based PQT-12 films have been compared with the PQT-12 films deposited by the conventional spin-coating method. The improved electrical characteristics of FTM-based OTFT have been found as compared to those of the spin-coated OTFTs. Due to better properties of the FTM-based PQT-12 films over the spin coated films, the electrical and optical characteristics of the FTM-based OTFT have been compared under dark and illuminated conditions. The FTM film-based OTFT shows the respective values of field effect mobility and threshold voltage of 7.8 × 10−2 cm2/Vs and –8.1 V under dark and 8.9 × 10 −2 cm2/Vs and –5.3 V under illumination of 200 μW/cm2 at 540 nm. The maximum responsivity of 11.3 A/W is found at the light intensity of 5 μW/cm2 at 540 nm.