Organic Thin-film Transistors Research Articles

Conjugated Polymer Heteroatom Engineering Enables High Detectivity Symmetric Ambipolar Phototransistors.

Solution-processed high-performing ambipolar organic phototransistors (OPTs) can enable low-cost integrated circuits. Here, a heteroatom engineering approach to modify the electron affinity of a low band gap diketopyrrolopyrole (DPP) co-polymer, resulting in well-balanced charge transport, a more preferential edge-on orientation and higher crystallinity, is demonstrated. Changing the comonomer heteroatom from sulfur (benzothiadiazole (BT)) to oxygen (benzooxadiazole (BO)) leads to an increased electron affinity and introduces higher ambipolarity. Organic thin film transistors fabricated from the novel PDPP-BO exhibit charge carrier mobility of 0.6 and 0.3 cm2Vs⁻1 for electrons and holes, respectively. Due to the high sensitivity of the PDPP-based material and the balanced transport in PDPP-BO, its application as an NIR detector in an OPT architecture is presented. By maintaining a high on/off ratio (9 × 104), ambipolar OPTs are shown with photoresponsivity of 69 and 99 AW⁻1 and specific detectivity of 8 × 107 for the p-type operation and 4 × 109 Jones for the n-type regime. The high symmetric NIR-ambipolar OPTs are also evaluated as ambipolar photo-inverters, and show a 46% gain enhancement under illumination.

Cross-Linked Interfacial Layer for Enhanced Performance in n-Channel Organic Thin Film Transistors

Cross-Linked Interfacial Layer for Enhanced Performance in n-Channel Organic Thin Film Transistors

Axial Phenoxylation of Aluminum Phthalocyanines for Improved Cannabinoid Sensitivity in OTFT Sensors.

Cannabis producers, consumers, and regulators need fast, accurate, point-of-use sensors to detect Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) from both liquid and vapor source samples, and phthalocyanine-based organic thin-film transistors (OTFTs) provide a cost-effective solution. Chloro aluminum phthalocyanine (Cl-AlPc) has emerged as a promising material due to its unique coordinating interactions with cannabinoids, allowing for superior sensitivity. This work explores the molecular engineering of AlPc to tune and enhance these interactions, where a series of novel phenxoylated R-AlPcs are synthesized and integrated into OTFTs, which are then exposed to THC and CBD solution and vapor samples. While the R-AlPc substituted molecules have a comparable baseline device performance to Cl-AlPc, their new crystal structures and weakened intermolecular interactions increase sensitivity to THC. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) are used to investigate this film restructuring, where a significant shift in the crystal structure, grain size, and film roughness is detected for the R-AlPc molecules that do not occur with Cl-AlPc. This significant crystal reorganization and film restructuring are the driving force behind the improved sensitivity to cannabinoids relative to Cl-AlPc and demonstrate that analyte-semiconductor interactions can be enhanced through chemical modification to create more responsive OTFT sensors.

Large‐Area Deposition of Highly Crystalline F4‐Tetracyanoquinodimethane Thin Films by Molecular Step Templates

Theoretical studies have unequivocally determined the exceptional electron transport properties of the fluorinated tetracyanoquinodimethane (Fx‐TCNQ) family, presenting a promising avenue for the realization of high‐performance n‐channel organic thin‐film transistors (OTFTs). However, owing to the intrinsic low crystallinity of this class of materials, Fx‐TCNQ‐based n‐channel OTFTs have not been experimentally achieved so far. Herein, a molecular step template (MST)‐assisted method that dramatically improves the crystallinity of F4‐TCNQ thin films is reported. The MST not only lowers the nucleation barrier of F4‐TCNQ molecules along the in‐plane direction but also reduces the nucleation density. This approach facilitates the realization of compact, oriented, and highly crystalline F4‐TCNQ thin films, resulting in impressive electron mobility of up to 2.58 cm2 V−1 s−1. Notably, this achievement surpasses the electron mobility of F4‐TCNQ thin films fabricated without the MST by a factor of 107. Furthermore, the incorporation of the p‐type MST provides a novel pathway for constructing complementary inverters, showcasing a high voltage gain of 112.6 V V−1 and a substantial noise margin of 89.3% with exceptional uniformity. In this work, a general and efficient route is paved to produce high‐performance n‐channel OTFTs toward organic complementary circuits.

Electron Donating Functional Polymer Dielectrics to Reduce the Threshold Voltage of n‐Type Organic Thin‐Film Transistors

AbstractLow‐cost and high‐performance electronics based on synthetically simple materials are required to fuel the deployment of smart packaging and wearable electronics. Metal phthalocyanines (MPcs) are promising semiconductors for use in n‐type organic thin film transistors (OTFTs) but often require high operating voltages. The first silicon phthalocyanine‐based OTFT with a polymer dielectric is reported as an alternative to traditional metal oxide dielectrics. Incorporating poly(methyl methacrylate) (PMMA) as the dielectric successfully reduces the threshold voltage (VT) of bispentafluorophenoxy SiPc (F10‐SiPc) from 14.9V to 7.3V while retaining high mobility. Further reduction in VT is obtained by using copolymers and blends of PMMA and dimethylamino ethyl methacrylate (DMAEMA)‐containing polymers, where a higher molar fraction of DMAEMA leads to a consistent drop in VT to ‐0.7 V. The electron‐donating groups of the tertiary amines in the DMAEMA show clear interfacial doping of the semiconductor, reducing the voltage required to populate the dielectric/semiconductor interface with charge carriers and turn on the device. Blending trace amounts of DMAEMA‐containing copolymers with PMMA proves to be an effective strategy for reducing the VT while keeping the charge mobility high, unlike when using pure copolymers with elevated DMAEMA content. Time of flight secondary ion mass spectroscopy (ToF‐SIMS) and X‐ray photoelectron spectroscopy (XPS) demonstrate that the DMAEMA‐containing copolymer is floating to the surface of the PMMA blend at the dielectric–semiconductor interface, which explains the reduced VT. Synchrotron scanning transmission X‐ray microscopy (STXM) demonstrates that PMMA promotes a more edge‐on orientation of F10‐SiPc films, compared to the more face‐on orientation when deposited on the DMAEMA containing copolymer. This study demonstrates a straightforward process for designing dielectric polymers and their blends for the reduction in VT for n‐type OTFTs.

A high-performance organic thin-film transistor with Parylene/PMMA bilayer insulation based on P3HT

This work introduces a pioneering approach in the development of organic thin-film transistors (OTFTs),featuring a double-layer dielectric structure that combines poly(para-xylylene)s (Parylene) and poly(methyl methacrylate) (PMMA) to leverage the high insulation properties and high surface polarity of Parylene with the low insulation properties and low surface polarity of PMMA. This combination results in devices that showcase significantly enhanced electrical performance, including superior charge carrier mobility, increased current on/off ratios, and greater transconductance. Utilizing poly(3-hexylthiophene) (P3HT) for the active layer, the study demonstrates the advantage of the dual dielectric layers in minimizing hysteresis in the transfer curve, thereby facilitating the systematic growth of the organic active layer and enhancing electrical conductivity over single-layer alternatives. The superior performance of the Parylene/PMMA double-layer insulating structure opens new avenues for the advancement of organic electronics, presenting methodologies for performance optimization and expanding the application spectrum of OTFTs.

Poling induced changes in polarization domain structure of polymer ferroelectrics for application in organic thin film transistors

While electrical poling of organic ferroelectrics has been shown to improve device properties, there are challenges in visualizing accompanying structural changes. We observe poling induced changes in ferroelectric domains by applying differential phase contrast (DPC) imaging in the scanning transmission electron microscope, a method that has been used to observe spatial distributions of electromagnetic fields at the atomic scale. In this work, we obtain DPC images from unpoled and electrically poled polyvinylidene fluoride trifluorethylene films and compare their performance in polymer thin film transistors. The vertically poled films show uniform domains throughout the bulk compared to the unpoled film with a significantly higher magnitude of the overall polarization. Thin film transistors comprising a donor–acceptor copolymer as the active semiconductor layer show improved performance with the vertically poled ferroelectric dielectric film compared with the unpoled ferroelectric dielectric film. A poling field of 80–100 MV/m for the dielectric layer yields the best performing transistors; higher than 100 MV/m is seen to degrade the transistor performance. The results are consistent with a reduction in deleterious charge carrier scattering from ferroelectric domain boundaries or interfacial dipoles arising from electrical poling.

Achieving low-voltage operating high-mobility organic thin-film transistors by a multi-layered gate dielectric

Organic thin-film transistors (OTFTs) have attracted great attention for their inherent advantages and promising applications in emerging fields. Simultaneously realizing low-voltage operation and high-mobility in one OTFT is one of prerequisites for the commercialization, which is a huge challenge so far. An important route to address this challenge is to develop an ideal gate dielectric with a high capacitance and a low interfacial trap density at the dielectric/channel. In this Letter, we demonstrate the low-voltage operating high-mobility OTFTs by elaborately designing and processing a multi-layered gate dielectric. The gate dielectric consists of a high permittivity polymer film, a polymer buffing layer with a high surface energy, and an ultrathin long-chain alkane buffer layer. The effects of both the structures and the processes of gate dielectrics on the performances of OTFTs are investigated in detail. In addition, the relevant physical mechanisms are discussed. Finally, the optimal OTFTs exhibit high mobilities with the average and maximum values up to 5.62 and 6.74 cm2/V s, respectively, at low operating voltages below −5 V. Our findings reveal that designing and processing a reasonable multi-layered gate dielectric is a promising strategy to achieve both high-mobility and low-voltage operation in OTFTs, thereby fostering their development.

Non-exponential transients and the evaluation of stress in (disordered) organic thin film transistors

In this work, the transient techniques for disordered organic thin film transistors are analyzed. A special emphasis is made on stress. Stress in this work is the continuous increase of the threshold voltage upon applying a gate bias. In this work a new stress evaluation method is presented that allows for a rapid determination of stress. Moreover, a figure-of-merit is proposed that can be applied to non-exponential transients, including those of stressing. The transients are compared to the empirical transient functions reported in literature that range from power-law to stretched exponential and logarithmic.

Significant Reduction of 1/f Noise in Organic Thin-Film Transistors With Self-Assembled Monolayer: Considerations of Density-of-States

Significant Reduction of 1/<i>f</i> Noise in Organic Thin-Film Transistors With Self-Assembled Monolayer: Considerations of Density-of-States

Compact modeling of hysteresis in organic thin-film transistors

In this work, we propose a model that describes the temporal evolution of the threshold voltage and trapped charge density in Thin-Film Transistors (TFTs) under dynamic conditions, paving the way for the characterization and modeling of memory transistors. The model is expressed as a first-order differential equation for the trapped charge density, which is controlled by a time constant and an independent term proportional to the drain current. The time-dependent threshold voltage is introduced in a previously developed compact model for TFTs with special consideration to the contact effects. The combination of both models and the use of an evolutionary parameter extraction procedure allow for reproducing the experimental dynamic behavior of TFTs. The results of the model and the evolutionary procedure have been validated with published experimental data of pentacene-based transistors. The procedure is able to simultaneously reproduce three kinds of experiments with different initialization routines and constraints in each of them: output and transfer characteristics with hysteresis and current transients characteristics.

Reliability aspects in repulsive steric hindrance approach: Selectivity and sensitivity investigations

Dual metal regulated bottom gate top contact structured Organic Thin Film Transistor based biosensor (Bio-OTFT) for recognition of breast cancer cell lines along with the non-tumorigenic epithelial cell line is presented here. A feasible fabrication scheme for Bio-OTFT has been demonstrated. Repulsive steric hindrance (RSH) consideration inside the cavity zone directs more practical and realistic sensing mechanisms. The insightful electrostatic analysis regarding electric field, energy band diagram, transfer characteristics (IDS-VGS), and transconductance (gm) has been described. Alteration of the dielectric constant (k) of these cancerous and healthy cell lines stated the recognition scheme in this reported Bio-OTFT. The efficiency of the recommended model has been assessed thoroughly with the reported OTFT literatures. The sensitivity is examined respecting drain current (IDS) in (A/μm), gm in (μS/μm), threshold voltage (VTH) in Voltage and sub-threshold swing (SS) in (mV/dec) and reaches to the maximum value of 2.3 × 108, 1.85 × 1010, 0.68, and 0.08 for T-47D cell line at VGS = −1.25 V. The recommended model thus suggests an enormous reliability and possibility for biosensing purposes.

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.

Nanoscale visualization of fast carrier dynamics in organic thin-film transistors by time-resolved electrostatic force microscopy

Fast carrier dynamics in organic thin-film transistors (OTFTs) was investigated by time-resolved electrostatic force microscopy (tr-EFM). We found that the carrier diffusion in the OTFTs proceeded in two stages: fast diffusion and slow diffusion. By applying the instantaneous frequency method to EFM, the temporal evolution of the spatial distribution of fast carriers in the channel region of the OTFTs, which took place on the timescale of several hundreds of nanoseconds, was evaluated. The inhomogeneous distribution of the local decay time constant showed that the carrier diffusion of the OTFTs was limited by the grain boundaries between each crystalline region. The quantitative capability of the method was verified by comparing the values of the carrier mobility estimated by the tr-EFM measurement and a numerical simulation. The mobility estimated from the experiment and the simulation showed good agreement, showing the possibility of the tr-EFM to evaluate the time evolution of dynamic phenomena in semiconductor devices.

On the Aging of OTFTs and Its Impact on PUFs Reliability.

Given the current maturity of printed technologies, Organic Thin-Film Transistors (OTFT) still show high initial variability, which can be beneficial for its exploitation in security applications. In this work, the process-related variability and aging of commercial OTFTs have been characterized to evaluate the feasibility of OTFTs-based Physical Unclonable Functions (PUFs) implementation. For our devices, ID-based PUFs show good uniformity and uniqueness. However, PUFs' reliability could be compromised because of the observed transient and aging effects in the OTFTs, which could hinder the reproducibility of the generated fingerprints. A systematic study of the aging of OTFTs has been performed to evaluate the PUFs' reliability. Our results suggest that the observed transient and aging effects could be mitigated so that the OTFTs-based PUFs' reliability could be improved.

Improving the detecting sensitivity of transistor-based DNA sensor by the optimization of channel thicknesses

Organic thin-film transistor (OTFT) has been adopted as a promising sensing platform to detect DNA target molecules by analyzing their electrical parameters. The source/drain current ratio of OTFT (with and without DNA molecules) as the sensor sensitivity have been widely utilized to detect and identify DNA molecules. In this paper, pentacene as a typical organic semiconductor with various thin-film thicknesses have been adopted as the active layer of OTFT that induce to the changeable sensor sensitivity for the DNA molecules. Importantly, the sensitivity of the device shows the remarkable dependence in the applied gate voltages (VG) and an improved trend is observed with the lowering of VG. Importantly, a ten-fold improvement in the sensor sensitivity is achieved by the optimization of channel thicknesses at the low VG condition. Furthermore, the variable-temperature measurements are carried out to explore the charge mechanism of OTFT induced by the DNA molecules. Our studies indicate the sensor sensitivity is mainly determined by the charge-injection process of OTFT arisen in the linear region. Current work will be helpful for deeply understanding the operating mechanism of OTFT-based biosensors.

Experimental Characterisation of BGBC OTFT for Indoor CO&lt;sub&gt;2&lt;/sub&gt; Gas Sensing

Global warming is a concern nowadays due to excessive release of harmful gasses to the environment, leading to greenhouse effect phenomena worldwide. Based on the data provided by global pollution agencies, the release of greenhouse gasses to the atmosphere is the main cause of pollution and the increase in atmospheric temperature due to warming. Greenhouse gasses (GHGs) contents released to the environment is worrying, with carbon dioxide (CO2) is reported at the highest concentration compared to other gasses. There are many studies conducted to develop and evaluate the performance of harmful gas sensors incorporating inorganic and organic semiconductive materials. Organic semiconductors (OSCs) are environmentally friendly materials, relatively cheaper technology, and comprised of a wide range of materials with good carrier mobility. Therefore, in this work, Organic Thin Film Transistor (OTFT) is developed for gas sensor application. As global warming is becoming more serious, this solution is instead a sustainable solution to the environment, as organic molecules which are held together via Van der Waals bond are easily processed via low-temperature deposition and solution processing as compared to more complicated processes involved in conventional inorganic counterpart. In addition, the developed sensor is generally robust due to the ability to withstand high humidity conditions and can be fabricated on flexible substrates. In this work, suitable materials are identified in basic OTFT construction, which are the electrodes, dielectric and substrate. The scope is mainly focusing on the development of bottom gate OTFT construction, incorporating p-type active material which are Trisisopropylsilylethynyl Pentacene (TIPS Pentacene), Aluminium (Al) as drain and source electrodes, PEDOT: PSS as gate electrode and Polyvinyl alcohol (PVA) as gate dielectric. The materials in bottom gate bottom contact (BGBC) configuration, fabricated via screen printing technique is experimentally tested towards CO2 detection. CO2 is initially detected at 1618 ppm with contact resistance of 15 kΩ, and at 10 ml/minute flow rate, the developed configuration is demonstrated able to achieve sensitivity of 2.069 Ω/ppm. In conclusion, the studied BGBC OTFT has demonstrated suitability and applicability in CO2 gas sensing for sustainable environmental condition monitoring, that could lead to safer environment for the living things on earth. With the proposed dimensions, in the future it is possible to proceed with this work to be fabricated by using more advanced techniques such as photolithography and many others.

Functionalized Phenanthrene Imide-Based Polymers for n-Type Organic Thin-Film Transistors.

High-performing n-type polymers are crucial for the advance of organic electronics field, however strong electron-deficient building blocks with optimized physicochemical properties for constructing them are still limited. The imide-functionalized polycyclic aromatic hydrocarbons (PAHs) with extended π-conjugated framework, high electron deficiency and good solubility serve as promising candidates for developing high-performance n-type polymers. Among the PAHs, phenanthrene (PhA) features a well-delocalized aromatic π-system with multiple modifiable active sites . However, the PhA-based imides are seldom studied, mainly attributed to the synthetic challenge. Herein, we report two functionalized PhAs, CPOI and CPCNI, by simultaneously incorporating imide with carbonyl or dicyanomethylene onto PhA. Notably, the dicyanomethylene-modified CPCNI exhibits a well stabilized LUMO energy level (-3.84 eV), attributed to the synergetic inductive effect from imide and cyano groups. Subsequently, based on CPOI and CPCNI, two polymers PCPOI-Tz and PCPCNI-Tz were developed. Applied to organic thin-film transistors, owing to the strong electron-deficiency of CPCNI, polymer PCPCNI-Tz shows an improved electron mobility and largely decreased threshold voltage compared with PCPOI-Tz. This work affords two structurally novel electron-deficient building blocks and highlights the effectiveness of dual functionalization of PhAs with strong electron-withdrawing groups for devising n-type polymers.

All Solution Processable OTFT-Based on Direct-Written Printing Method Towards Flexible Electronics Applications

Development of organic electronics, particularly organic thin film transistors (OTFTs), have been centred of discussion among researchers due to their potential uses in flexible electronics applications. Conventionally, an inkjet printing method has been deployed to fabricate the OTFTs due to its simplicity in fabrication steps and applicability to diverse substrates and solution-processable materials. Nonetheless, this technique has a major drawback which requires low viscosity materials to prevent clogging issue at the printer’s nozzle. This in return limits the material selections and requires additional steps i.e. modification of the selected materials to fit the printer’s requirement or in other words, to avoid clogging at the nozzle. Therefore, this paper proposes a method to fabricate a bottom gate bottom contact (BGBC) OTFT by using a direct-write printing technique which is compatible with a commercial conductive ink that can be directly used without any further modification. This technique does not compromise the fabricated devices overall performance and can fabricate the devices up to micrometre scale. The proposed OTFT achieved a saturation mobility of 4.28x10-5 cm2/Vs, a threshold voltage of -0.4 V, an on/off current ratio of 102, and a subthreshold swing of 10 V/dec with overall fabrication temperature is less than 150 ℃, hence, makes it suitable for flexible electronics applications.

Enhanced performance in doped micro-nano porous organic thin-film transistors

Molecular doping, as an effective technique for controlling the electrical property of organic semiconductors (OSCs) by introducing additional charges, has been proven to adjust important device parameters in organic thin-film transistors (OTFTs). Doping highly crystalline OSCs without disrupting structural order is a crucial challenge, as it significantly affects the charge carrier mobility. Here, we demonstrate a molecular doping method without disrupting the molecular ordering to improve the charge carrier mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) based OTFTs via a simple thermal spin-coating method. The key is to introduce micro-nano pores into C8-BTBT thin-film for channel doping, which is achieved by mixing with the unsubstituted BTBT as it can be easily removed from the thin-film through an ordinary annealing process. Micro-nano pores allow the dopant molecules (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, F4-TCNQ) to access the conductive channel of OTFT, which is beneficial for charge injection. Indeed, we further discover that F4-TCNQ doped porous C8-BTBT thin-films exhibit better charge mobility than those of neat and F4-TCNQ doped C8-BTBT films in OTFTs. This work proposes an effective way to expose OSC conjugated core to the dopant, which not only improves the charge transfer reaction between organic/dopant semiconductor through cofacial stacking, but also reduces the trap density and contact resistance.