Organic Semiconductor Layer Research Articles

The Impact of Non‐Monolithic Semiconductor Capacitance on Organic Electrochemical Transistors Performance and Design

AbstractThe existing device models for organic electrochemical transistors (OECTs) fail to provide any device design guidelines for optimized performance parameters such as transconductance that are pivotal for the applications OECTs in sensing. Moreover, the current models are based on the questionable assumption of a homogenous organic semiconductor layer, and all predict a linear behavior of the resistance with the OECT channel length. Consequently, the experimentally observed nonlinear resistance behavior in OECTs has been overlooked thus far. Here, an OECT device model is developed that accurately describes the nonlinear behavior of the OECT channel resistance and offers the first guidelines for maximizing transconductance. The model is inherently nonlinear and the nonlinearity stem from the non‐monolithic capacitance of the organic semiconductor layer. Moreover, the model provides a consistent and reliable estimations for the contact resistance in OECTs. The success of the model in accurately describing and providing predictions of the OECT operation by relating the device's geometrical parameters with electrochemical parameters of the semiconductor layer paves the way toward unlocking OECT potentials in diverse applications, from biosensing to neuromorphic computing and flexible electronics.

Design and analysis of central gate organic thin film transistor

Abstract This paper presents a novel central gate organic thin-film transistor (OTFT) with electrodes in top-bottom configuration and its comparative performance analysis with conventional single and dual gate OTFTs. The central gate OTFT is comprised of a single gate electrode configured to be the center thin film. A pair of identical dielectric layers viz. top dielectric layer and a bottom dielectric layer on both sides of gate electrode to facilitate the further deposition top and bottom organic semiconductor layer over them respectively. The organic semiconductor layers (OSC) are configured as top OSC layer and a bottom OSC to accommodate two independent conduction channels. The source and drain electrodes are also deposited in pairs in top and bottom configurations. Due to the subtle modifications in the architecture the resultant output drain current of the central gate (CG) OTFT has increased manifold. It has been found in the study that the CG OTFT produces 94%, 76.36%, 80.58% higher drain current than bottom gate bottom contact (BGBC), top gate bottom contact (TGBC) and bottom gate top contact (BGTC) OTFT respectively of similar size, operating under identical voltage regime. Similarly in the case of dual gate (DG) OTFT an improvement of 5.10% has been recorded without any additional gate electrode at the same operating voltages.

Revealing two-dimensional electric field crowding effect in breakdown performance of DPPT-TT polymer-based OFETs

The diketopyrrolopyrrole-based polymer (DPPT-TT) has been employed in organic power field effect transistors due to its exceptional off-state breakdown performance. The impact of organic semiconductor layer thickness on the breakdown performance has not been explored. In this study, we investigate the impact of DPPT-TT layer thickness on the breakdown voltage (BV) by fabricating organic field effect transistors (OFETs) with various DPPT-TT layer thicknesses. Our findings reveal that the devices' BV is a strong function of DPPT-TT layer thickness, and reducing the DPPT-TT layer thickness from 68 to 15 nm results in a decrease in BV from 291 to 86 V, attributed to the two-dimensional (2D) electric field crowding effect. An analytical model utilizing the 2D Poisson equation reveals an electric field at the DPPT-TT layer's surface. Thinner DPPT-TT layer exhibits larger electric field peak, leading to premature breakdown near the drain electrode. The relationship between breakdown electric field and DPPT-TT layer thickness was established by fitting the experimental data to the model, revealing an average BV error of only 8.8%. This phenomenon is validated to be ubiquitous in polymer based OFETs via DPPT-TT-based and P3HT-based devices. According to the proposed model, this 2D electric field crowding effect can be mitigated by adjusting the dielectric layer thickness (tD) and/or the dielectric material.

Molecular Design of Hole Transport Materials to Immobilize Ion Motion for Photostable Perovskite Solar Cells.

Poor operational stability is a crucial factor limiting the further application of perovskite solar cells (PSCs). Organic semiconductor layers can be a powerful means for reinforcing interfaces and inhibiting ion migration. Herein, two hole-transporting molecules, pDPA-SFX and mDPA-SFX, are synthesized with tuned substituent connection sites. The meta-substituted mDPA-SFX results in a larger dipole moment, more ordered packing, and better charge mobility than pDPA-SFX, accompanying with strong interface bonding on perovskite surfaces and suppressed ion motion as well. Importantly, mDPA-SFX-based PSCs exhibit an efficiency that has significantly increased from 22.5 % to 24.8 % and a module-based efficiency of 19.26 % with an active area of 12.95 cm2. The corresponding cell retain 94.8 % of its initial efficiency at maximum power point tracking (MPPT) after 1,000 h (T95=1,000 h). The MPPT T80 lifetime is as long as 2,238 h. This work illustrates that a small degree of structural variation in organic compounds leaves considerable room for developing new HTMs for light stable PSCs.

(Invited) Transferable Organic Semiconductor Nanosheets for Van Der Waals Heterojunction Devices

The combination of organic semiconductors (OSCs) and two-dimensional materials provides a new basis to harness the advantages of both material systems, for example to fabricate unipolar pentacene thin-film transistors using graphene contacts [1] and p-n junctions using pentacene and molybdenum disulfide [2]. However, the fabrication of such devices by vapor deposition of OSCs is a challenge because growth is limited by the surface-dependent diffusion of small molecules. In this talk, I will present a novel method for the preparation of transferable OSC nanosheets that overcomes this problem. Highly crystalline, wafer-scale (3") nanosheets as thin as 20 nm can be obtained by precisely delaminating the OSC layer from a water-soluble sacrificial film in an appropriate wetting geometry. These free-floating films are then deposited on arbitrary substrates while maintaining morphology and crystallinity. Consequently, we demonstrate functional OSC nanosheets on top of prefabricated electrodes and monolayer MoS2, resulting in devices such as unipolar, ambipolar, and anti-ambipolar field effect transistors [3].

Direct-Written Silver Electrodes for All-Solution-Processed Low-Voltage Organic Thin Film Transistors Towards Flexible Electronics Applications

Recent progress inprinted electronics has offered the possibility of fabricatingvarious organic-based electronic devices,such as organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs).As one of the important deposition methods inprinted electronics, an inkjet printing technique offers the deposition of solution-processable materials onto a variety of substrates using simpler fabrication steps at lower processing temperatures, which is suitable for flexible electronic applications. Despite being the leading choice in OTFT fabrication,the clogging issues that frequently occurred at the printhead nozzle havenot only limitedthe material selection but also restrained the efforts to bring organic electronic devices to the market. Apart from that, although remarkable progress has been made to enhance the performance of the OTFT, the high operating voltage resulting from the low gate capacitance density of the inorganic oxide-based dielectric layer remains a critical limitation that hinders the practical application of the OTFT. Hence, in this paper, we propose a simple solution-based method to develop a low-voltageOTFT. This work utilized a direct-write printing technique to print silver source/drain and gate electrodes incorporated into a bottomgate bottom contact OTFT structure, a spin-coating deposition method to deposit both small molecule TIPS-pentacene organic semiconducting layer and high-kPVA dielectric layer. Notably, the proposed OTFT achieved a micrometre channel length with a saturation mobility of 4.49 × 10-1cm2/Vs, a threshold voltage of -1.5V, an on/off current ratio of 108, and a subthreshold swing of 66.8mV/dec whilethe overall fabrication temperature and operating voltage arekept below 150 °Cand -15 V, respectively.The direct ink writing technology incorporated into the high-kdielectric layer provides a new strategy to fabricate organic-related components,particularly the OFTFs at lower manufacturing cost and temperature towardsflexible and low-operating voltage electronic devices.

Evaluation of Industrial Poly(tert-butyl acrylate) insulated A p-channel Organic Field-Effect Transistor (PtBA-p-OFET)

Poly(tert-butyl acrylate) (PTB-p-A) has been investigated as a promising insulator layer for p-channel organic field effect transistors (p-OFETs) using the p-type semiconductor Poly(3-hexylthiophene-2,5-diyl (P3HT) due to its favorable insulating properties, good film-forming ability and electrical charge separation properties. Top-gate, bottom-contact PTBA-p-OFET devices are fabricated with Indium Thin Oxide (ITO) source/drain electrodes and a P3HT organic semiconductor layer. The frequency-dependent capacitance of the PTBA-p-OFETs was studied through a plot to determine the key parameters, including the threshold voltage (VTh), field-effect mobility (μFET), and the current on/off ratio (Ion/off) of the device. The PTB-p- OFETs exhibit field-effect mobility value of 6.13x10-4 (cm2/V.s), an on/off current ratio of 1.11x102, and a threshold voltage of -15.8 V. The capacitance-frequency characteristics of the capacitor structure were analyzed and found to have as 7.6 nF/cm2 per unit area. This work presents PTBA as a promising for high-performance p-OFET applications.

Rapid Electrohydrodynamic Jet Printing for Self-Aligned Organic Semiconducting Layers and Printed Organic Electronics

Rapid Electrohydrodynamic Jet Printing for Self-Aligned Organic Semiconducting Layers and Printed Organic Electronics

Comparison of p-n and p-i-n vertical diodes based on p-PMItz/n-Si, p-PMItz/n-4HSiC and p-PMItz/i-SiO2/n-Si heterojunctions

In this paper, we present a comprehensive comparison study between p-n and p-i-n vertical diodes employing diverse heterojunction configurations under dark conditions. The diodes are fabricated utilizing p-PMItz as the organic semiconductor layer interfacing with different inorganic substrates, including n-Si (n-type silicon), n-4HSiC (n-type 4H silicon carbide), and incorporating an intrinsic SiO2 (silicon dioxide) layer in the p-PMItz/i-SiO2/n++-Si configuration. The current–voltage and dielectric characteristics are analyzed to discern the performance discrepancies among these diode configurations. The influence of heterojunction interfaces and band alignments on device behavior is investigated, shedding light on the charge transport mechanisms within these structures. Our findings reveal distinct trends in device characteristics for p-n and p-i-n diodes, highlighting the significance of heterojunction design in optimizing device performance. This comparative analysis offers valuable insights for the development of efficient organic–inorganic hybrid diodes tailored for various optoelectronic applications.

Enhancing Sensitivity in Gas Detection: Porous Structures in Organic Field-Effect Transistor-Based Sensors.

Gas detection is crucial for detecting environmentally harmful gases. Organic field-effect transistor (OFET)-based gas sensors have attracted attention due to their promising performance and potential for integration into flexible and wearable devices. This review examines the operating mechanisms of OFET-based gas sensors and explores methods for improving sensitivity, with a focus on porous structures. Researchers have achieved significant enhancements in sensor performance by controlling the thickness and free volume of the organic semiconductor layer. Additionally, innovative fabrication techniques like self-assembly and etching have been used to create porous structures, facilitating the diffusion of target gas molecules, and improving sensor response and recovery. These advancements in porous structure fabrication suggest a promising future for OFET-based gas sensors, offering increased sensitivity and selectivity across various applications.

A minimalist approach to 3D photoemission orbital tomography: algorithms and data requirements

Photoemission orbital tomography provides direct access from laboratory measurements to the real-space molecular orbitals of well-ordered organic semiconductor layers. Specifically, the application of phase retrieval algorithms to photon-energy- and angle-resolved photoemission data enables the direct reconstruction of full 3D molecular orbitals without the need for simulations using density functional theory or the like. However, until now this procedure has remained challenging due to the need for densely-sampled, well-calibrated 3D photoemission patterns. Here, we present an iterative projection algorithm that completely eliminates this challenge: for the benchmark case of the pentacene frontier orbitals, we demonstrate the reconstruction of the full orbital based on a dataset containing only four simulated photoemission momentum measurements. We discuss the algorithm performance, sampling requirements with respect to the photon energy, optimal measurement strategies, and the accuracy of orbital images that can be achieved.

Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment.

The interface between a metal electrode and an organic semiconductor (OS) layer has a defining role in the properties of the resulting device. To obtain the desired performance, interlayers are introduced to modify the adhesion and growth of OS and enhance the efficiency of charge transport through the interface. However, the employed interlayers face common challenges, including a lack of electric dipoles to tune the mutual position of energy levels, being too thick for efficient electronic transport, or being prone to intermixing with subsequently deposited OS layers. Here, we show that monolayers of 1,3,5-tris(4-carboxyphenyl)benzene (BTB) with fully deprotonated carboxyl groups on silver substrates form a compact layer resistant to intermixing while capable of mediating energy-level alignment and showing a large insensitivity to substrate termination. Employing a combination of surface-sensitive techniques, i.e., low-energy electron microscopy and diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy, we have comprehensively characterized the compact layer and proven its robustness against mixing with the subsequently deposited organic semiconductor layer. Density functional theory calculations show that the robustness arises from a strong interaction of carboxylate groups with the Ag surface, and thus, the BTB in the first layer is energetically favored. Synchrotron radiation photoelectron spectroscopy shows that this layer displays considerable electrical dipoles that can be utilized for work function engineering and electronic alignment of molecular frontier orbitals with respect to the substrate Fermi level. Our work thus provides a widely applicable molecular interlayer and general insights necessary for engineering of charge injection layers for efficient organic electronics.

Environmental Effects on the Performance of Small‐Molecule Organic Thin‐Film Transistors

AbstractElectrical measurements are performed on a bottom‐gate bottom‐contact organic thin‐film transistor with 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES ADT) (diF‐TES ADT) as the active layer, varying the relative humidity of the environment surrounding the transistor. The results strongly indicate that water negatively impacts the electrical performance of the transistor and that the ingress of water is dynamic provided that the organic semiconductor layer is not encapsulated. It is found that the change can be reversed and the performance restored by removing the source of water. The drain current and threshold voltage of the transistor varied linearly when the relative humidity is changed from 35% to 7%, suggesting that the transistor can be used as a humidity sensor.

Effect of source and drain electrode positions on the performance of bottom gate organic thin film transistor

In recent years several novel technologies have emerged on commercial level. Organic electronics is one such technology that has created new avenues for the development of flexible, robust, and cost-effective electronic devices. Transistor developed using organic materials are called organic thin film transistors (OTFT). Transistor is elementary component of any electronic device, any variation in the size or performance of the transistors reflects directly on the overall performance of the electronic devices. Bottom gate bottom contact (BGBC) OTFT is commonly employed single gate organic transistor owing to its simple architecture and easy fabrications. Since in organic semiconductor material carrier mobility of charges is lower, hence its overall performance declines. As a result, its application in electronic circuits becomes limited. Therefore, to enhance the performance of BGBC OTFT and facilitate the optimal charge mobilisation inside organic semiconductor layer i.e., OSC, improvement in the architecture of BGBC is deemed necessary. In this research article the performance of BGBC OTFT is analysed for different positions of electrodes viz. source and drain embedded inside the OSC layer. The study signifies the effect of electrode positions on performance optimization of BGBC OTFT without changing the materials, dimensions, or the operating voltage regime. It has been found that for 30 nm thick OSC layer, the electrodes should ideally be placed between 5 nm to 10 nm above the junction of OSC and dielectric layer inside OSC layer for optimal results.

Stability of ternary interfaces and its effects on ideal switching characteristics in inverted coplanar organic transistors

Inverted coplanar or bottom-gate bottom-contact (BGBC)-type thin-film transistors (TFTs) present several advantages for the manufacture and application of organic TFTs, although serious difficulties are encountered when trying to achieve sufficiently high performance. Recently, it was demonstrated that both high mobility and ideal switching are attainable in BGBC-type printed organic TFTs with highly clean semiconductor-gate dielectric interfaces. However, an unknown channel material dependence in the device performance is found. Here, we show that the stability of semiconductor/metal/dielectric ternary interfaces is a crucial factor in the operation of BGBC-type organic TFTs. We fabricate single-crystal organic semiconductor (OSC) films with various numbers of layers using two different materials (phenyl/alkyl-substituted benzothieno[3,2-]benzothiophene and phenyl/alkyl-substituted benzothieno[3,2-]naphtho[2,3-]thiophene) on highly lyophobic Cytop gate dielectric surfaces. The transfer characteristics exhibit notable time-dependent degradation, which clearly depends on the material, layer number, and encapsulation. Kelvin-probe force microscopy measurements reveal that the degradation is ascribed to contact resistance at the source electrodes, while it can be more suppressed in multilayer (two or more layers) OSCs. Atomic force microscopy and in-plane x-ray diffraction profiles present signs of the transformation in single molecular bilayer OSCs laid on the electrodes. The results suggest the importance of the quality of the OSC layer at ternary interfaces, providing a clue for improving the performance of BGBC-type organic TFTs. Published by the American Physical Society 2024

Buried‐Contact Organic Field‐Effect Transistor: The Way of Alleviating Drawbacks from Interfacial Charge Transfer

AbstractFacile charge transfer between source/drain (S/D) electrodes and organic semiconductor (OSC) channel is crucial for high‐mobility organic field‐effect transistors (OFETs). Herein, a novel OFET geometry is developed by modifying a top‐contact bottom‐gate device structure, termed a buried‐contact OFET, enabling close proximity between the S/D‐OSC interface and conducting channel, consequently decreasing the access contact resistance (RC,acc) and overall contact resistance (RC). Conventional post‐thermal annealing is combined with a burying pressure (pressure‐thermal annealing (PTA)). The synergistic effect of thermal and pressure annealings leads to the softened OSC layer enabling metal electrodes to bury inward by applied pressure. This process induces structural transitions from a top‐contact to buried‐contact configuration, as verified by atomic force microscopy and finite element simulations. Transfer line method and 4‐probe measurements revealed that PTA reduces the contact by 1/3 (65 kΩ cm) and the source‐to‐drain voltage waste due to charge injection from 52% to 31%. Consequently, the field‐effect mobility is four times higher than that of a conventional thermally annealed top‐contact OFET. The density of deep traps (Ntr) is mainly distributed in the OSC bulk responsible for charge injection. Remarkably, the Ntr decreased 30‐fold using PTA, resulting in a shallow sub‐threshold region and a threshold voltage close to zero.

The role of spontaneous orientation polarization on charge storage behavior at an interface between organic semiconductor layers

Optically generated carriers are pinned at the organic/organic interface if the two organic layers have a large polarization difference, underlining that the combination of polar molecules is crucial to extend the carrier lifetime.

Enhanced Performance of Organic Field‐Effect Transistor with Bi‐Functional N‐Type Organic Semiconductor Layer

AbstractOrganic field‐effect transistors (OFETs) hold great promise for applications in non‐volatile memories, detectors, and artificial synapses due to the good flexibility and biocompatibility. However, certain drawbacks such as high operating voltages and significant degradation in endurance characteristics have hindered their practical implementations. Herein, a novel approach is proposed to enhance the performance of OFETs by incorporating a bi‐functional n‐type polymer semiconductor interlayer, Poly‐{[N,N'‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (N2200), into a pentacene OFET structure. The device exhibits remarkable improvements, with reliable P/E operation cycles of over than 104 and a retention time of more than 10 years. On one hand, the inclusion of N2200 as an n‐type semiconductor effectively reduces the height of hole‐injection barrier for trapping and thus reducing the working voltage based on the electrostatic induction theory. On the other hand, n‐type semiconductor N2200 serves as a native hole‐consumption (or hole‐trapping) dielectric, and its narrower bandgap restrains the formation of deep hole‐traps, thus favoring the endurance characteristics of the OFET.

Selective Detection of Functionalized Carbon Particles based on Polymer Semiconducting and Conducting Devices as Potential Particulate Matter Sensors.

This paper reports a new mechanism for particulate matter detection and identification. Three types of carbon particles are synthesized with different functional groups to mimic the real particulates in atmospheric aerosol. After exposing polymer-based organic devices in organic field effect transistor (OFET) architectures to the particle mist, the sensitivity and selectivity of the detection of different types of particles are shown by the current changes extracted from the transfer curves. The results indicate that the sensitivity of the devices is related to the structure and functional groups of the organic semiconducting layers, as well as the morphology. The predominant response is simulated by a model that yielded values of charge carrier density increase and charge carriers delivered per unit mass of particles. The research points out that polymer semiconductor devices have the ability to selectively detect particles with multiple functional groups, which reveals a future direction for selective detection of particulate matter.

Plexcitonic system for high photonic spin Hall effect

The photonic spin Hall effect (PSHE) has been intensively studied and widely applied, especially in spin photonics. In this paper, we present the analysis of PSHE in the Plexcitonic system. The signature of the Plexciton is studied by implanting an organic semiconductor layer over the plasmon active metal. We report a high spin-dependent transverse shift of ∼ 60 μm, which is ∼200-fold larger than that of the pure Plasmonic counterpart. The underlying mechanism of PSHE is explained in the circular-polarization basis using the concept of superposition of abnormal and normal modes. It is observed that the transverse shift is highly sensitive to the alterations in the environmental refractive indices leading to its application as a highly sensitive PSHE-based refractometer, with a high sensitivity of 81°/RIU. We believe these results may open pathways for modulating the PSHE and developing nanophotonic applications such as advanced sensors and next-generation optical devices.