Self-assembled Monolayer Treatment Research Articles

Influence of Substrate Modification with Dipole Monolayers on the Electrical Characteristics of Short-Channel Polymer Field-Effect Transistors

This study investigates the influence of self-assembled monolayer treatment of gate insulators on the electrical characteristics of bottom-gate/bottom-contact organic field-effect transistors (OFETs) with short channel lengths of 5 μm to 30 nm. The treatment of 3-chloropropyltrichlorosilane (CPTS) with large dipoles produces a high built-in electric field perpendicular to the SiO2 gate insulator surface, which results in a threshold voltage shift and enhanced hole injection compared to the treatment of phenethyltrichlorosilane (PETS) with small dipoles. Pronounced parabolic drain current‒voltage (ID‒VD) characteristics due to a space-charge limited current are observed in short-channel OFETs based on poly(3-hexylthiophene) with CPTS-treated gate insulators. CPTS treatment on short-channel OFETs based on poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) suppresses the nonlinear ID increase in the low VD region caused by the voltage drop at the Au/F8T2 contact. The influence of the increase in the net source-drain electric field associated with the reduced voltage drops on the channel-length dependence of the field-effect mobility of short-channel F8T2 FETs is also discussed.

Fabrication of ultrathin low-voltage-driven printed organic circuits with anodized gate islands

Abstract Anodization of gate metals can form thin and robust oxide dielectrics of low-voltage-driven organic thin-film transistors (TFTs) for low-cost, flexible electronics. However, anodic oxide dielectrics have rarely been applied to circuit fabrication with complex layouts because separate gate electrode islands cannot be anodized at the same time. To overcome this limitation, we devise a method to simultaneously anodize multiple aluminum gate islands using removable interconnects. The anodic oxide properties, including dielectric constant, film thickness, and leakage current, were thoroughly investigated by varying anodization voltages from 5 to 50 V and a self-assembled monolayer treatment. By printing p-type polymer ink on top of the pattern-grown anodic oxide dielectrics, p-type organic TFTs were fabricated on a 2-μm-thick Parylene substrate. The printed TFTs exhibited subthreshold swing of 200 mV·dec−1, carrier mobility of 0.3 cm2 V−1 s−1, and threshold voltage of 0.17 V on average. Even when the substrate film was crumpled, the TFT characteristics did not substantially changed. Finally, a 3 V differential amplifier with multiple gate islands was successfully demonstrated. These findings suggest that high-quality anodic metal oxide films, when fully incorporated into a low-cost, large-area manufacturing process, can be applied to the fabrication of complex, low-voltage-driven TFT circuits on flexible substrates.

Improving performance of OFET by tuning occurrence of charge transport based on pentacene interaction with SAM functionalized contacts

Optimized pentacene thin film transistors have been achieved by two technological self-assembled monolayers (SAM) treatments, with surface modification of 3-octadecyltrichlorosilane (OTS) monolayer on SiO2 dielectric layer and 2,3,4,5,6-pentafluorobenzenethiol (PFBT) monolayer on gold source/drain electrodes, respectively. Higher mobility and on/off ratio with 0.36 cm2 V−1 s−1 and >106 are consistently investigated for films deposited on PFBT compared to on OTS, which could be explained by the different occurrence charge transport near the semiconductor/dielectric and semiconductor/electrodes interfaces. It can be further confirmed by XPS and electrical property measurements. In the end, a high mobility increased up to 0.68 cm2 V−1 s−1 was obtained by cumulative treatments of the two SAM. In particular, the improving performance of the organic thin films transistor devices through tuning the charge transport with SAM layers highlighting the utility of surface modulations and controlling of charge transport of interfaces in nanoscale materials.

Improved characteristics of conventional and inverted polymer photodetectors using phosphonic acid-based self-assembled monolayer treatment for interfacial engineering of Ga-doped ZnO electrodes

The characteristics of conventional and inverted polymer photodetectors based on a blend of a donor, poly(3-hexylthiophene) (P3HT), and an acceptor, fullerene derivative [6,6]phenyl-C61-butyric acid methyl ester (PCBM) using Ga-doped ZnO (GZO) electrodes modified by phosphonic acid-based self-assembled monolayer (SAM) treatment in a short time are investigated. Fluoroalkyl SAM, 1H,1H,2H,2H-perfluorooctane phosphonic acid (FOPA) treatment leads to efficient hole extraction from the active layer. The characteristics of the conventional device with GZO modified by FOPA treatment are almost the same as those with indium tin oxide modified by FOPA. Cs2CO3 and aminoalkyl SAM, 11-aminoundecylphosphonic acid (11-AUPA) treatments suppress the hole injection from GZO to the organic layer. For the inverted devices with GZO cathodes using Cs2CO3 and 11-AUPA, the dark current decreases, which results in the improved photodetector detectivity. An inverted device with both Cs2CO3 and 11-AUPA exhibits incident-photon-to-current conversion efficiency (IPCE) of approximately 65% (80%) at 0 V (−6 V) under light irradiation (λ = 500 nm), high on/off ratio, and improved durability. Improved open-circuit voltage and IPCE at low voltages are achieved by these treatments, which are related with the improved internal built-in field, the reduction of recombination probability in the vicinity of GZO, and the modified charge collection efficiency.

Solution-Processed and Self-Assembled Monolayer-Treated High-Voltage Organic Thin Film Transistors for Flexible MEMS Integration

ABSTRACTWe report a 6,13-Bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) solution-processed high-voltage organic thin film transistor (HVOTFT) with a large breakdown voltage of |VDS| > 450 V, a three-fold increase from previous results. An offset channel architecture and an organosilane-based self-assembled monolayer (SAM) treatment were used to achieve large breakdown voltages. Solution-processed HVOTFTs will enable novel high-voltage and flexible applications. Reliability of the HVOTFT under high-field stress was studied in the context of threshold and onset to conduction voltages.

Injection-modulated polarity conversion by charge carrier density control via a self-assembled monolayer for all-solution-processed organic field-effect transistors

We demonstrated modulation of charge carrier densities in all-solution-processed organic field-effect transistors (OFETs) by modifying the injection properties with self-assembled monolayers (SAMs). The all-solution-processed OFETs based on an n-type polymer with inkjet-printed Ag electrodes were fabricated as a test platform, and the injection properties were modified by the SAMs. Two types of SAMs with different dipole direction, thiophenol (TP) and pentafluorobenzene thiol (PFBT) were employed, modifying the work function of the inkjet-printed Ag (4.9 eV) to 4.66 eV and 5.24 eV with TP and PFBT treatments, respectively. The charge carrier densities were controlled by the SAM treatment in both dominant and non-dominant carrier-channel regimes. This work demonstrates that control of the charge carrier densities can be efficiently achieved by modifying the injection property with SAM treatment; thus, this approach can achieve polarity conversion of the OFETs.

Metal-oxide assisted surface treatment of polyimide gate insulators for high-performance organic thin-film transistors.

We developed a facile method for treating polyimide-based organic gate insulator (OGI) surfaces with self-assembled monolayers (SAMs) by introducing metal-oxide interlayers, called the metal-oxide assisted SAM treatment (MAST). To create sites for surface modification with SAM materials on polyimide-based OGI (KPI) surfaces, the metal-oxide interlayer, here amorphous alumina (α-Al2O3), was deposited on the KPI gate insulator using spin-coating via a rapid sol-gel reaction, providing an excellent template for the formation of a high-quality SAM with phosphonic acid anchor groups. The SAM of octadecylphosphonic acid (ODPA) was successfully treated by spin-coating onto the α-Al2O3-deposited KPI film. After the surface treatment by ODPA/α-Al2O3, the surface energy of the KPI thin film was remarkably decreased and the molecular compatibility of the film with an organic semiconductor (OSC), 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), was increased. Ph-BTBT-C10 molecules were uniformly deposited on the treated gate insulator surface and grown with high crystallinity, as confirmed by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The mobility of Ph-BTBT-C10 thin-film transistors (TFTs) was approximately doubled, from 0.56 ± 0.05 cm2 V-1 s-1 to 1.26 ± 0.06 cm2 V-1 s-1, after the surface treatment. The surface treatment of α-Al2O3 and ODPA significantly decreased the threshold voltage from -21.2 V to -8.3 V by reducing the trap sites in the OGI and improving the interfacial properties with the OSC. We suggest that the MAST method for OGIs can be applied to various OGI materials lacking reactive sites using SAMs. It may provide a new platform for the surface treatment of OGIs, similar to that of conventional SiO2 gate insulators.

Tunable doping in PbS nanocrystal field-effect transistors using surface molecular dipoles

We study the effect of self-assembled monolayer (SAM) treatment of the SiO2 dielectric on the electrical characteristics of PbS transistors. Using SAMs, we observe threshold voltage shifts in the electron transport, allowing us to tune the electrical properties of the devices depending on the SAM molecule used. Moreover, the use of a specific SAM improves the charge carrier mobility in the devices by a factor of three, which is attributed to the reduced interface traps due to passivated silanol on the SiO2 surface. These reduced traps confirm that the voltage shifts are not caused by the trap states induced by the SAMs.

Synthesis and characterisation of liquid crystal molecules based on thieno [3,2-b] thiophene and their application in organic field-effect transistors

ABSTRACTA thieno [3,2-b] thiophene-based planar liquid crystal molecule, C12-PTTT, which possesses the characteristics of an organic field-effect transistor (OFET), was synthesised and its mesomorphic properties were studied by polarising optical microscopy and differential scanning calorimetry. A wide temperature range for a highly oriented smectic phase was observed. OFET devices made under various conditions, such as self-assembled monolayer treatment and alignment treatment, and with micro-channel structures, were fabricated to measure the electrical properties. Among different OFET device conditions, the best electric carrier mobility for C12-PTTT was 0.038 cm2 V−1 s−1.

Improved Sensitivity and Durability of Poly(3‐Hexylthiophene)‐Based Polymeric Photodetectors Using Indium Tin Oxide Modified by Phosphonic Acid–Based Self‐Assembled Monolayer Treatment

SUMMARYOrganic photodetectors based on poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) blends with indium tin oxide (ITO) modified by phosphonic acid–based self‐assembled monolayer (SAM) treatment in a short time are investigated. The mixed SAMs serve to tune the surface free energy and the work function of ITO by varying the blend ratio. The phosphonic acid–based SAM treatment results not only in lowering of the injection barrier at the ITO/organic layer interface but also in the lowering of the contact resistance between ITO and the organic layer, as shown by impedance spectroscopy in P3HT hole‐only device with SAMs. P3HT:PCBM device with ITO modified by short treatment time of 1H,1H,2H,2H‐perfluorooctane‐phosphonic acid exhibits an incident‐photon‐to‐current conversion efficiency of above 50% at –2 V, a high on/off ratio, and improved durability.

자기조립단층과 농축 기술을 이용한 저농도 내분비계 장애물질 검출용 미소유체채널 기반 전기화학 센서

This paper demonstrates a microfluidic electrochemical sensor for detecting endocrine disruptor such as estradiol at a very low concentration by using preconcentration technique. In addition, self-assembled monolayer(SAM) was also employed on the working electrode of the electrochemical sensor in order to increase the estradiol capture efficiency of the sensor. SAM treatment on the working electrode enhanced the specific binding between the surface of the working electrode and the estradiol antibody. The estradiol antibody was applied on the working electrode at different concentrations(10, 20, 50, 100, 200 pg/ml) for observing the concentration dependency. The measured electrochemical redox current changed with the amount of the bound estradiol on the Au working electrode surface and the sensor can detect all the target material when the immobilized antibody amount is more than the estradiol amount in the water. The elecrochemical estradiol sensor without SAM treatment showed a low current of 7.79 nA, while the sensor treated with SAM resulted in 339 nA at 200 pg/ml, which is more than 40 fold higher output current. When combining the preconcentration technique and the SAM-treated electrode, the measured current became more than 100 fold higher than that of the sensor without neither SAM treatment nor preconcentration technique. The combination of these two techniques can would enable the proposed microfluidic electrochemical sensor to detect a very low concentration endocrine disruptor.

Organic Electronics: 1 GHz Pentacene Diode Rectifiers Enabled by Controlled Film Deposition on SAM‐Treated Au Anodes (Adv. Electron. Mater. 2/2016)

A gigahertz-operable pentacene rectifier enabled by controlled film formation using a self-assembled monolayer (SAM) is demonstrated by Kang et al. in article number 1500282. Pentacene deposited on SAM-treated Au forms an ordered structure, thereby exhibiting much improved electrical characteristics, in contrast with a disordered pentacene structure on bare Au. Moreover, SAM treatment lowers the charge injection barrier, which further improves the device performance.

Surface conductivity of InAs/GaSb superlattice infrared detectors treated with thiolated self assembled monolayers

The surface conductivity of InAs/GaSb based type II superlattice (T2SL) long wavelength infrared material following the deposition of thiolated self-assembled monolayers (SAMs) of cysteamine, octadecanethiol, dodecanethiol, and hexanethiol are reported. Quantitative mobility spectrum analysis (QMSA) was employed to study the mobility and to isolate and identify surface carriers following SAM treatments on planar samples. QMSA data collected following the deposition of the SAMs on InAs/GaSb material correlates well with dark current measurements, demonstrating the usefulness of QMSA as a tool for evaluating surface conductivity and predicting device performance. All samples displayed a reduction in surface conductivity and dark current density following thiol treatment. Dark current densities were reduced to 1.1 × 10−5, 1.3 × 10−5, 1.6 × 10−5, and 5 × 10−6 A/cm2 for hexanethiol, dodecanethiol, octadecanethiol, and cysteamine, respectively, from 5.7 × 10−4 A cm2 for unpassivated devices.

Low-Temperature Solution-Processed Gate Dielectrics for High-Performance Organic Thin Film Transistors.

A low-temperature solution-processed high-k gate dielectric layer for use in a high-performance solution-processed semiconducting polymer organic thin-film transistor (OTFT) was demonstrated. Photochemical activation of sol-gel-derived AlOx films under 150 °C permitted the formation of a dense film with low leakage and relatively high dielectric-permittivity characteristics, which are almost comparable to the results yielded by the conventionally used vacuum deposition and high temperature annealing method. Octadecylphosphonic acid (ODPA) self-assembled monolayer (SAM) treatment of the AlOx was employed in order to realize high-performance (>0.4 cm2/Vs saturation mobility) and low-operation-voltage (<5 V) diketopyrrolopyrrole (DPP)-based OTFTs on an ultra-thin polyimide film (3-μm thick). Thus, low-temperature photochemically-annealed solution-processed AlOx film with SAM layer is an attractive candidate as a dielectric-layer for use in high-performance organic TFTs operated at low voltages.

Interfaces analysis by impedance spectroscopy and transient current spectroscopy on semiconducting polymers based metal–insulator–semiconductor capacitors

Impedance and transient current measurements on metal–insulator–semiconductor (MIS) capacitors are used as tools to thoroughly investigate the bulk and interface electronic transport properties of semiconducting polymers, i.e. poly(3-hexylthiophene) (P3HT). Distinct features were observed at both interfaces, i.e. metal–semiconductor and semiconductor–insulator. The results revealed a dispersive transport in the bulk due to the band tail of the localized states, presence of interface states at the interface between the insulator and the semiconductor and formation of a less conductive small layer at the interface semiconductor–metal contact due to intrusions of sputtered Au particles. Effects of self-assembled monolayers (SAMs) treatments of the gate insulating dielectric were investigated showing that treating the gate dielectric with either ozone or hexamethyldisilazane (HMDS) or octyltrichlorosilane (OTS) alter not only the interface semiconductor–insulator but the bulk properties as well. An exponential density of states with a width parameter of 38–58meV depending on the surface treatment was found to be representative of the band tail of P3HT. Though both OTS and HMDS treatments slightly increase the density of interface states, only OTS treated samples showed a decrease in disorder parameter of the bulk. The latter fact can be attributed to an increase of the grain size due to a favored π-π stacking film growth. An outcome explaining the already reported increase of the lateral mobility and decrease of the vertical mobility observed upon OTS treatment of the gate insulating dielectric in poly(3-hexylthiophene) based devices.

Commercially applicable, solution-processed organic TFT and its backplane application in electrophoretic displays

We report a video graphics array (VGA) electrophoretic display (EPD) that adopts commercially applicable process technologies for its solution-processed organic thin-film transistor (OTFT). For source/drain (S/D) engineering, a copper (Cu) electrode was employed instead of high-work-function materials and to obtain a low-resistance signal line on the array that is functionalized by self-assembled monolayer (SAM) treatment on the electrode surface. For gate engineering, a gate electrode with Cu was successfully formed onto the organic gate insulator (OGI) by using sputter deposition, thus providing an industrially suitable technology to replace evaporation processes even on frail organic material. Competitive device performance driving the electrophoretic display was obtained on a TFT derived from anthradithiophene that was fabricated at low temperatures of less than 140°C and that has the stable electrical feature of ΔVT of 3V for induced bias stress with a long time span. Up until now, electrode engineering using low-cost and low-work-function materials has been too problematic for practical use of OTFT backplanes, but we have successfully applied it to produce a 6-in. electrophoretic display based on a large substrate of 370×470mm2 for the backplane process. Thus, as far as mass production is concerned, the solution-processed organic transistor shows promise as a backplane unit with low-temperature processing.

Enhanced self-assembled monolayer treatment on polymeric gate dielectrics with ultraviolet/ozone assistance in organic thin film transistors

Chemical vapor deposition (CVD) is utilized to form self-assembled monolayers on polymeric insulators. Ultraviolet/ozone (UVO) treatment is used to enhance the alignment of HMDS monolayer on polymeric insulator surface and a time dependent effect is observed for UVO.

Highly enhanced charge injection and bulk transport in organic gap‐type diodes via one‐pot treatment process: experiment and simulation

The gap-type diode is a basic component of organic field-effect transistors. In the critical voltage range above 10 V, the gap-type diode works in the same way as a sandwich-type diode adapted for organic light-emitting diodes and organic photovoltaic cells. However, the gap-type diode is rarely studied, and compact modelling is not often found in previous reports that deal with this type of an organic diode. Enhanced bulk transport is exhibited using the one-pot treatment of self-assembled monolayers (SAMs) for an SiO2 surface and an Au contact metal in the gap-type organic diode. Charge-injection improvement via the SAM treatment on Au induces higher bulk transport. This phenomenon is analysed mainly through compact modelling.

Processing follows function: pushing the formation of self-assembled monolayers to high-throughput compatible time scales.

Self-assembled monolayers (SAMs) of organic molecules can be used to tune interface energetics and thereby improve charge carrier injection at metal-semiconductor contacts. We investigate the compatibility of SAM formation with high-throughput processing techniques. Therefore, we examine the quality of SAMs, in terms of work function shift and chemical composition as measured with photoelectron and infrared spectroscopy and in dependency on molecular exposure during SAM formation. The functionality of the SAMs is determined by the performance increase of organic field-effect transistors upon SAM treatment of the source/drain contacts. This combined analytical and device-based approach enables us to minimize the necessary formation times via an optimization of the deposition conditions. Our findings demonstrate that SAMs composed of partially fluorinated alkanethiols can be prepared in ambient atmosphere from ethanol solution using immersion times as short as 5 s and still exhibit almost full charge injection functionality if process parameters are chosen carefully. This renders solution-processed SAMs compatible with high-throughput solution-based deposition techniques.

Regulating charge injection in ambipolar organic field-effect transistors by mixed self-assembled monolayers.

We report on a technique using mixed self-assembled monolayers (SAMs) to finely regulate ambipolar charge injection in polymer organic field-effect transistors. Differing from the other works that employ single SAM specifically for efficient charge injection in p-type and n-type transistors, we blend two different SAMs of alkyl- and perfluoroalkyl thiols at different ratios and apply them to ambipolar OFETs and inverter. Thanks to the utilization of ambipolar semiconductor and one SAM mixture, the device and circuit fabrications are facile with only one step for semiconductor deposition and another for SAM treatment. This is much simpler with respect to the conventional scheme for the unipolar-device-based complementary circuitry that demands separate deposition and processing for individual p-channel and n-channel transistors. Our results show that the mixed-SAM treatments not only improve ambipolar charge injection manifesting as higher hole- and electron-mobility and smaller threshold voltage but also gradually tune the device characteristics to reach a desired condition for circuit application. Therefore, this simple but useful approach is promising for ambipolar electronics.