Bottom-contact Organic Field-effect Transistors Research Articles

Tuning Electrode Work Function and Surface Energy for Solution Shearing High-Performance Organic Field-Effect Transistors.

A bottom-contact organic field-effect transistor (OFET) is easily adaptable to the standard lithography process because the contact electrodes are deposited before the organic semiconductor (OSC). However, the low surface energy of bare electrodes limits utilizing solution-processed single-crystal OSCs. Additionally, the bare electrode usually leads to a significant charge injection barrier, owing to its relatively low work function (WF). Here, we simultaneously improved the surface energy and WF of gold electrodes by conducting oxygen plasma treatment to achieve high-performance OFET based on solution-processed organic single crystals. We cultivated a thin layer of gold oxide on Au electrodes to increase the WF by ∼0.7 eV. The surface energy of Au electrodes was enhanced to the same as AlOx dielectric surface, enabling the seamless growth of large-area C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene) organic single-crystal thin films via solution shearing. This technique facilitates the production of high-performance OFETs with the highest carrier mobility of 6.7 cm2 V-1 s-1 and sharp switching characterized by a subthreshold swing of 63.6 mV dec-1. The bottom-contact OFETs exhibited a lower contact resistance of 1.19 kΩ cm than its F4-TCNQ-doped top-contact control device. This method offers a straightforward and effective strategy for producing high-quality single-crystal OFETs, which are potentially suitable for commercial applications.

Influence of thin-film processing on the performance of organic field-effect transistors

Due to their functional and processing versatility, organic semiconductors have gained much interest in recent years. Tailoring the charge transport in a polymeric semiconductor system is imperative for developing optimized devices. Factors such as selection of solvent species and annealing temperature play a significant role in deciding the morphology of the polymer semiconductor and hence the device performance. Bottom-gate bottom-contact organic field-effect transistors were fabricated with regioregular poly(3-hexylthiophene) (P3HT) as the active material. The effects of using chlorobenzene and o-dichlorobenzene as the solvents for P3HT and annealing temperatures on the device performance were evaluated. The devices made using chlorobenzene showed greater field-effect mobility compared to those fabricated with o-dichlorobenzene. This behavior can be attributed to the disentanglement of polymer chains in a solvent with better matching solubility parameters and subsequent ease of arranging in ordered structures during processing. Furthermore, annealing the o-dichlorobenzene devices at temperatures closer to the crystallization temperature of P3HT showed an increase in field-effect mobility. At temperatures closer to the crystallization temperature, a polymer film in a semidry state can allow increased ordering of the chains leading to enhanced charge transportation.

New metallophthalocyanines including benzylphenoxy groups and investigation of their organic-field effect transistor (OFET) features

In this study, metal and metal-free novel phthalocyanines containing peripheral and non-peripheral tetra 2-benzylphenoxy groups were synthesized. The compounds were characterized by UV–Vis, FT-IR, 1H NMR, and MALDI-TOF mass spectrometry as well as elemental analysis. These new phthalocyanines exhibited excellent solubility in most organic solvents, and their redox behavior was investigated in different solvents such as dimethyl sulfoxide (DMSO) and dichloromethane (DCM). The redox behavior of the peripheral and non-peripheral phthalocyanine compounds 1a-c and 2a-c was determined by cyclic voltammetry and in situ spectroelectrochemistry. According to organic field-effect transistors (OFETs) measurements, the peripheral and non-peripheral phthalocyanine-cobalt complexes which have higher mobility than others were utilized top-gate bottom-contact OFETs fabrication. The output characteristics of the device show that its mobility is approximately 5 × 10−2 cm2/Vs with p-type accumulation.

ESD Breakdown of Parylene OFETs With Varying Contact Overlap Capacitance

We examine the electrostatic discharge (ESD) behavior of bottom-gate bottom-contact pentacene organic field-effect transistor (OFET) devices with a parylene-C gate dielectric. A comparison between devices with gate and contact overlap geometries of −2 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \text{m}$ </tex-math></inline-formula> are analyzed. In particular, the relationship between contact overlap, the protective effect of the increased overlap, and the electric field in the dielectric and their effect on ESD resilience is detailed. We find a markedly increased tolerance for negative contact overlaps and a small decrease in tolerance for more positive overlaps. We also report on a highly resilient effect where OFET devices continue to operate but with a reduction in performance after multiple dielectric breakdown. A negative overlap length should be carefully considered when designing OFET devices in a tradeoff between decreased performance and higher ESD resilience.

Picene and PTCDI based solution processable ambipolar OFETs

Facile and efficient solution-processed bottom gate top contact organic field-effect transistor was fabricated by employing the active layer of picene (donor, D) and N,N′-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (acceptor, A). Balanced hole (0.12 cm2/Vs) and electron (0.10 cm2/Vs) mobility with Ion/off of 104 ratio were obtained for 1:1 ratio of D/A blend. On increasing the ratio of either D or A, the charge carrier mobility and Ion/off ratio improved than that of the pristine molecules. Maximum hole (µmax,h) and electron mobilities (µmax,e) were achieved up to 0.44 cm2/Vs for 3:1 and 0.25 cm2/Vs for 1:3, (D/A) respectively. This improvement is due to the donor phase function as the trap center for minority holes and decreased trap density of the dielectric layer, and vice versa. High ionization potential (− 5.71 eV) of 3:1 and lower electron affinity of (− 3.09 eV) of 1:3 supports the fine tuning of frontier molecular orbitals in the blend. The additional peak formed for the blends at high negative potential of − 1.3 V in cyclic voltammetry supports the molecular level electronic interactions of D and A. Thermal studies supported the high thermal stability of D/A blends and SEM analysis of thin films indicated their efficient molecular packing. Quasi-π–π stacking owing to the large π conjugated plane and the crystallinity of the films are well proved by GIXRD. DFT calculations also supported the electronic distribution of the molecules. The electron density of states (DOS) of pristine D and A molecules specifies the non-negligible interaction coupling among the molecules. This D/A pair has unlimited prospective for plentiful electronic applications in non-volatile memory devices, inverters and logic circuits.

Direct-patterned copper/poly(ethylene oxide) composite electrodes for organic thin-film transistors through cone-jet mode by electrohydrodynamic jet printing

Direct-patterned Cu-based conductive electrodes were printed through electrohydodynamic (EHD) jet printing via cone-jet mode. The introduction of a capping agent in the synthesis of the Cu ink limited excessive conductivity, which enabled the pristine Cu ink to achieve the various printing modes of EHD jet printing: dripping, micro-dripping, cone-jet, and multi-jet. We also obtained optimal printing conditions by adjusting the viscosity by adding poly(ethylene oxide) (PEO) to the pristine Cu inks, resulting in well-defined printing patterns. The optimized PEO content in the ink was determined to be ∼10 wt%, which gave us a stable cone-jet mode and well-defined Cu/PEO composite electrode lines with sharp edges. We utilized the Cu/PEO composite electrode lines as source and drain and a triisopropylsilylethynyl (TIPS)–pentacene/PS blend as the active layer for bottom-gate bottom-contact organic field-effect transistors (OFETs). The resulting devices exhibited an average field-effect mobility (μFET), threshold voltage (Vth), and on/off current ratio (Ion/Ioff) of 0.253 cm2/V, 0.253 V, and ∼106, respectively.

Influence of Backbone Regioregularity on High-Mobility Conjugated Polymers Based on Alkylated Dithienylacrylonitrile.

We designed and synthesized two donor-acceptor type conjugated polymers, the regioirregular polymer RI-PDPP-CNTVT-6 and its regioregular counterpart RR-PDPP-CNTVT-6, based on diketopyrrolopyrrole (DPP) and alkylated dithienylacrylonitrile (CNTVT) units. Among them, the 2-decyltetradecyl side chain on the DPP acceptor unit and the hexyl side chain on the CNTVT donor unit were used to ensure enough solubility for them. The backbone regioregularity was used to tune electronic structures and carrier transport of the conjugated system. The two conjugated polymers were characterized for their thermal, photophysical, electrochemical, and solution-processable properties, thin-film microstructures, and morphologies. The top-gate bottom-contact configuration organic field-effect transistor (OFET) devices based on these two conjugated polymers showed excellent ambipolar performances. Remarkably, the regioirregular polymer RI-PDPP-CNTVT-6 exhibited higher charge-carrier mobilities than the regioregular counterpart polymer RR-PDPP-CNTVT-6 did, as their highest hole/electron mobilities (μhmax/μemax) were 1.48/1.27 and 0.48/0.052 cm2 V-1 s-1, respectively. Moreover, the influence of backbone regioregularity on its thermal stability, electrochemical and photophysical properties, solution processability, and charge-carrier mobility was intensively studied. Our results afforded a promising pathway toward the development of excellent ambipolar OFETs with high performance, good solution processability, and thermal stability.

Bottom-contact organic thin film transistors with transparent Ga-doped ZnO source-drain electrodes

In this study, transparent Ga-doped ZnO (GZO) films grown by magnetron sputtering were used as the source-drain (S-D) electrodes of bottom-contact organic field effect transistors (OTFTs). The GZO films exhibited good transmittance (beyond 80 %) in the visible region and the work functions of GZO films were found to be gradually increased with increasing the annealing temperature from 500 to 700 °C, leading to the reduction of contact barriers and improvement of device performances. OTFT fabricated by GZO S-D electrodes exhibited higher field-effect mobility of 0.0101 cm2/Vs and threshold voltage of −7.7 V when the GZO electrodes were annealed at 700 °C. To further increase the carrier injection, an ultra-thin CuPc film was used as the buffer layer prior to the deposition of Pentacene, and an increased mobility of 0.0257 cm2/vs was obtained. The results indicate that optimized transparent GZO films prepared by magnetron sputtering have great potential in the S-D electrodes of OTFTs.

Influence of solvent additives on the morphology and electrical properties of diF-TES ADT organic field-effect transistors

Abstract Solution processing of organic semiconductors offers key advantages, including low-cost and compatibility with large-area flexible substrates. Controlling film crystallization from solution, however, is not trivial. This is particularly important since the device properties are highly dependent on the film microstructure, which, in turn, can vary significantly with processing. In this work, we tuned the small molecule organic semiconductor film microstructure by using a binary solvent consisting of a host solvent and a high-boiling point solvent additive and studied the effect of additive content on charge transport. Spin coated 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene films processed from a mixture of chlorobenzene and dichlorobenzene, in various ratios, were investigated by using Polarized Optical Microscopy, Scanning Electron Microscopy and Atomic Force Microscopy analysis and their electrical properties were evaluated from bottom-contact bottom-gate organic field-effect transistors. We found that the mobility increased by three times for a 8% dichlorobenzene content as a result of reducing the density of the grain boundaries within the device channel.

Sequential Fluorination on Napthaleneamide-Based Conjugated Polymers and Their Impact on Charge Transport Properties

Improving π-conjugated polymer electron transport and injection efficiency are important to realize high performance n-channel organic field-effect transistors (OFETs). This paper reports a series of naphthalene diimide (NDI-) based n-type π-conjugated polymers, NDI-T-1FP-T, NDI-T-2FP-T, and NDI-T-4FP-T, with varioulis amounts of fluorine atoms (1F, 2F, and 4F) in their backbones. We found notable differences in energy levels, ground and excited state dipole moments (Δμge), exciton lifetimes, dihedral angles, and charge transport properties by varying the fluorine content: Δμge and charge transport properties reduced with increasing fluorine content. Electron mobility of optimized top gate bottom contact OFETs using the n-type π-conjugated polymers were 0.35, 0.18, and 0.16 cm2 V–1 s–1, respectively. OFET performances were further improved further by low concentration polyethylenimine (PEI) doping, significantly improving field-effect mobility to (maximum) 0.51, 0.34, and 0.26 cm2 V–1 s–1, respectively. D...

Structural Evaluation of 5,5'-Bis(naphth-2-yl)-2,2'-bithiophene in Organic Field-Effect Transistors with n-Octadecyltrichlorosilane Coated SiO2 Gate Dielectric.

We report on the structure and morphology of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2) films in bottom-contact organic field-effect transistors (OFETs) with octadecyltrichlorosilane (OTS) coated SiO2 gate dielectric, characterized by atomic force microscopy (AFM), grazing-incidence X-ray diffraction (GIXRD), and electrical transport measurements. Three types of devices were investigated with the NaT2 thin-film deposited either on (1) pristine SiO2 (corresponding to higher surface energy, 47 mJ/m2) or on OTS deposited on SiO2 under (2) anhydrous or (3) humid conditions (corresponding to lower surface energies, 20-25 mJ/m2). NaT2 films grown on pristine SiO2 form nearly featureless three-dimensional islands. NaT2 films grown on OTS/SiO2 deposited under anhydrous conditions form staggered pyramid islands where the interlayer spacing corresponds to the size of the NaT2 unit cell. At the same time, the grain size measured by AFM increases from hundreds of nanometers to micrometers and the crystal size measured by GIXRD from 30 nm to more than 100 nm. NaT2 on OTS/SiO2 deposited under humid conditions also promotes staggered pyramids but with smaller crystals 30-80 nm. The NaT2 unit cell parameters in OFETs differ 1-2% from those in bulk. Carrier mobilities tend to be higher for NaT2 layers on SiO2 (2-3 × 10-4 cm2/(V s)) compared to NaT2 on OTS (2 × 10-5-1 × 10-4 cm2/(V s)). An applied voltage does not influence the unit cell parameters when probed by GIXRD in operando.

Doped bottom-contact organic field-effect transistors

The influence of doping on doped bottom-gate bottom-contact organic field-effect transistors (OFETs) is discussed. It is shown that the inclusion of a doped layer at the dielectric/organic semiconductor layer leads to a significant reduction in the contact resistances and a fine control of the threshold voltage. Through varying the thickness of the doped layer, a linear shift of threshold voltage VT from −3.1 to −0.22 V is observed for increasing thickness of doped layer. Meanwhile, the contact resistance at the source and drain electrode is reduced from 138.8 MΩ at VGS = −10 V for 3 nm to 0.3 MΩ for 7 nm thick doped layers. Furthermore, an increase of charge mobility is observed for increasing thickness of doped layer. Overall, it is shown that doping can minimize injection barriers in bottom-contact OFETs with channel lengths in the micro-meter regime, which has the potential to increase the performance of this technology further.

Investigating the contact material influence on didodecyloxy-PPE OFETs

In this contribution we discuss the impact of the contact formation between semiconductor and source and drain contact on the device performance of bottom-contact top-gate organic field-effect transistors (OFETs) based on fibrillary semiconducting networks of poly(2,5-didodecyloxy-para-phenyleneethynylene) (D-OPPE). We show that the density and strength of the junctions between the D-OPPE fibers and the electrodes is likewise essential for the device performance as the electronic match of the involved materials and can even predominate the electronical effect. Due to the large bandgap of D-OPPE, high work function contacts are required to allow for efficient hole injection. MoO3 interlayers are known to improve the hole injection and thus commonly the device performance of p-channel OFETs. Indeed, photoelectron spectroscopy revealed a significant hole injection barrier reduction of 0.7 eV once the employed Cr/Au contacts are covered with MoO3. At first glance, it is thus not surprising that the device performance of p-channel OFETs with MoO3 modified contacts is superior. However, pure Cr/Au contacts can perform similarly once a smoothly applicated dielectric as Parylene C is used instead of PMMA. This cannot be explained by the above-mentioned electronical argument. To understand this finding, microscope techniques were used to address morphological effects rather than electronical properties. Our investigations showed that the semiconductor fibrils between channel and bottom contacts are thicker but the density of these interconnects between semiconductor fibrils and bottom contact is substantially lower for the bare Cr/Au contact. Depending on the successive deposition of the gate dielectric this density might be reduced further essentially destroying the injection pathway of carriers.

Influence of Simultaneous Tuning of Molecular Weights and Alkyl Substituents of Poly(thienoisoindigo-alt-naphthalene)s on Morphology and Change Transport Properties.

To simultaneously assess the impact of molecular weight (Mn) and alkyl substituent variations of polymers on the structural and optoelectronic properties, herein, we conduct a systematic study of a series of poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np)-based polymers containing different alkyl substituents (2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT) chains) and Mn's (low (L) and high (H)). All of the polymers produce almost identical energy levels, whereas their optical spectra show a clear dependence on Mn's and the alkyl substituents. Interestingly, increasing the alkyl substituent sizes of the polymers steadily increases the lamellar d-spacings (d100), ultimately leading to a densely packed lamellar structure for PTIIGHD-Np. In addition, both H-PTIIGOD-Np and H-PTIIGDT-Np exhibit larger π-stacking crystallites than the corresponding low-Mn polymers, while for PTIIGHD-Np, their size increases in the low-Mn batch. Ultimately, L-PTIIGHD-Np shows the best hole mobility of 1.87 cm2 V-1 s-1 in top-gate and bottom-contact organic field-effect transistors (OFETs) with a poly(methyl methacrylate), which is nearly 1 order of magnitude higher than other polymers tested in this study. Our results demonstrate that the simultaneous Mn and alkyl substituent engineering of the polymers can optimize their film morphology to produce high-performance OFETs.

Reducing the contact resistance in bottom-contact-type organic field-effect transitors using an AgOx interface layer

Bottom-contact organic field-effect transistors (OFETs) are prepared by inserting an AgOx layer between a pentacene layer and the source–drain electrodes. The contact resistance in the device is ∼8.1 kΩ·cm with an AgOx layer oxidized for 60 s but reaches 116.9 kΩ·cm with a non-oxidized Ag electrode. The drain current and mobility in the OFETs with the AgOx layer increase with the oxidization time and then gradually plateau, and this trend strongly depends on the work function of the Ag surface. Further, the hole injection is enhanced by the presence of Ag2O but inhibited by the presence of AgO.

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm2/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm2/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.

Diketopyrrolopyrrole-based conjugated polymer for printed organic field-effect transistors and gas sensors

We synthesize a diketopyrrolopyrrole (DPP)-based conjugated polymer (LGC-D148) that includes thiophene and difluorophenylenedithiophene as donors for the active layer of solution processable organic field-effect transistors (OFETs) and printed ammonia gas sensors. Typical ambipolar characteristics with reasonably high field-effect mobility (μFET) of 0.48 ± 0.12 cm2/V s for the holes and μFET of 0.079 ± 0.02 cm2/V s for the electrons are achieved by optimizing the top-gate bottom-contact OFETs. The LGC-D148 OFETs show a reliable and reversible gas sensing property upon exposure to ammonia gas. Specifically, a high sensitivity of over 90% is attained upon exposure to 1000 ppm ammonia gas, by a repeatable and gradual negative shift in the transfer curves.

A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors

We report on the reduction of contact resistance in solution-processed TIPS-pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) and PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) top-gate bottom-contact organic field-effect transistors (OFETs) by using different contact-modification strategies. The study compares the contact resistance values in devices that comprise Au source/drain electrodes either treated with 2,3,4,5,6-pentafluorothiophenol (PFBT), or modified with an evaporated thin layer of the metal-organic molecular dopant molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), or modified with a thin layer of the oxide MoO3. An improved performance is observed in devices modified with Mo(tfd)3 or MoO3 as compared to devices in which Au electrodes are modified with PFBT. We discuss the origin of the decrease in contact resistance in terms of increase of the work function of the modified Au electrodes, Fermi-level pinning effects, and decrease of bulk resistance by electrically doping the organic semiconductor films in the vicinity of the source/drain electrodes.

Low-voltage printable OFETs for sub-ppm detection of ammonia under humid conditions

This report describes the development of a gas sensor platform based on all solution-processed bottom-gate bottom-contact organic field-effect transistors that operate at &#x2264; -2 V. The sensor was able to detect ammonia as low as 600 ppb under high relative humidity (RH=80%).

(Invited) A New Materials Concept for High Performance Organic Thin Film Transistors

We have established a new materials concept for organic field effect transistors (OFETs) solution-processed, i.e., utilization of highly ordered smectic liquid crystals, which exhibit self-organization and solid-like nature, in order to solve the problems such as film uniformity and thermal durability in small molecule OFET materials. 2-Phenyl-7-decyl-benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) exhibiting SmE phase was designed and synthesized to apply OFETs. Bottom-gate and bottom-contact OFETs fabricated with its polycrystalline thin films exhibited high mobility of over 10cm2/Vs with its small variation from device to device and high thermal durability up to 200oC . In addition, we succeeded in operating the OFETs at a few volt with a low-dielectric gate insulator material of SiO2.