Self-assembled Monolayer Material Research Articles

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications.

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

Ultrafast Quasi‐2D/3D Perovskite Photodetectors Conferred Using Interfacial Engineering of Self‐Assembled Monolayers

AbstractQuasi‐2D (q‐2D) perovskites are extensively studied due to their hybrid properties of 3D and 2D phases and have proven to be applied to a wide range of optoelectronics. Multiple device engineering methods for perovskite photodetectors are developed. However, the treatment of self‐assembled monolayer (SAM) is rarely discussed. Herein, a series of silane‐coupling agents of (3‐chloropropyl)trichlorosilane (CPTS), phenyltrichlorosilane, and (3‐aminopropyl)triethoxysilane is adopted as SAM materials and elucidated their influence on q‐2D/3D CsPbBr3 films and their photodetector performance. The optical properties and morphology results reveal that perovskite film on CPTS‐modified surface can prolong excitons’ lifetime, produce a larger grain size and better size distribution, while also present better crystallinity and united orientation. As for the fabricated photodetectors, the device on a CPTS‐modified surface has the highest photocurrent of 5.46 × 10−8 A, a responsivity of 0.16 A W−1, and an 18‐times‐higher specific detectivity than the pristine counterpart, which comes from the higher surface energy and favorable energy level alignment. The photodetector on the CPTS‐modified surface also shows better stability than that on the pristine surface. This study pioneers by using SAMs on q‐2D perovskite photodetectors and provides a facile route of device fabrication for perovskite photodetectors.

Dual-Use Self-Assembled Monolayer Controlling Charge Carrier Extraction in Organic Solar Cells.

The development and use of interface materials are essential to the continued advancement of organic solar cells (OSCs) performance. Self-assembled monolayer (SAM) materials have drawn attention because of their simple structure and affordable price. Due to their unique properties, they may be used in inverted devices as a modification layer for modifying ZnO or as a hole transport layer (HTL) in place of typical poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in conventional devices. In this work, zinc oxide (ZnO) is modified using five structurally similar SAM materials. This resulted in a smoother surface, a decrease in work function, a suppression of charge recombination, and an increase in device efficiency and photostability. In addition, they can introduced asfor hole extraction layer between the active layer and MoO3 , enabling the use of the same material at several functional layers in the same device. Through systematic orthogonal evaluation, it is shown that some SAM/active layer/SAM combinations still offered device efficiencies comparable to ZnO/SAM, but with improved device' photostability. This study may provide recommendations for future SAM material's design and development as well as a strategy for boosting device performance by using the same material across both sides of the photoactive layer in OSCs.

Tailored Self-Assembled Monolayer using Chemical Coupling for Indium-Gallium-Zinc Oxide Thin-Film Transistors: Multifunctional Copper Diffusion Barrier.

Controlling the contact properties of a copper (Cu) electrode is an important process for improving the performance of an amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) for high-speed applications, owing to the low resistance-capacitance product constant of Cu. One of the many challenges in Cu application to a-IGZO is inhibiting high diffusivity, which causes degradation in the performance of a-IGZO TFT by forming electron trap states. A self-assembled monolayer (SAM) can perfectly act as a Cu diffusion barrier (DB) and passivation layer that prevents moisture and oxygen, which can deteriorate the TFT on-off performance. However, traditional SAM materials have high contact resistance and low mechanical-adhesion properties. In this study, we demonstrate that tailoring the SAM using the chemical coupling method can enhance the electrical and mechanical properties of a-IGZO TFTs. The doping effects from the dipole moment of the tailored SAMs enhance the electrical properties of a-IGZO TFTs, resulting in a field-effect mobility of 13.87 cm2/V·s, an on-off ratio above 107, and a low contact resistance of 612 Ω. Because of the high electrical performance of tailored SAMs, they function as a Cu DB and a passivation layer. Moreover, a selectively tailored functional group can improve the adhesion properties between Cu and a-IGZO. These multifunctionally tailored SAMs can be a promising candidate for a very thin Cu DB in future electronic technology.

Analysis of ITO surface modified with aromatic-based self-assembled molecules

This study focuses on characterization and enhancement of indium tin oxide (ITO) surface by using a self-assembled monolayer (SAM) technique. It is generally used to establish a compatible interface and a good charge injection from ITO surface. New generation organic semiconductor molecules such as dibenzo[a,c]phenazine-11-carboxylic acid, 4-(9H-carbazol-9-yl) benzoic acid, 4-(2,5-di-2-thienyl-1H-pyrrol-1-yl) benzoic acid, 5-[(diphenyl) amino]isophthalic acid and 5-[(3-methylphenyl)(phenyl)amino]isophthalic acid were coated on ITO. The surface of ITO was characterized by scanning spreading resistance microscopy, Kelvin probe force microscopy, scanning tunnelling microscopy, X-ray photoelectron spectroscopy and cyclic voltammetry techniques to ensure the chemical bonding of SAM materials on it.

Precise control of nanoscale spacing between electrodes using different natured self-assembled monolayers

Herein, we introduce an interdigitated horizontal electrode (IHE) structure with a metal-based electron-collecting (or -injecting) electrode and a hole-collecting (or -injecting) electrode composed of a conductive polymeric material that has a nanoscale distance and is horizontally separated. In the IHE, a metal electrode is fabricated on a silicon-oxide substrate, and a self-assembled monolayer (SAM) is selectively bonded to the metal and the oxide to form a conductive polymer electrode by dip coating. Each of the SAM materials is composed of a head part bonded to the substrate surface and a tail part that is hydrophilic or hydrophobic. This inherent property makes the metal electrode hydrophobic and the oxide substrate hydrophilic. Ag is used as a metal electrode material and is combined with alkanethiol SAMs. The alkylsilane SAMs are combined with the silicon oxide substrate to make them hydrophilic, using poly (3, 4-ethylenedioxythiophene)-poly (PEDOT: PSS) as the conductive polymer material. In this study, we have found that there is a difference in the spacing between the two electrodes that depends on the combination of SAM materials. Each interval was spaced from a minimum of 140 nm to a maximum of 385 nm.

Effects of interface energy modification in solution-processed In2O3 thin film transistors for sensing applications

Thin film transistors (TFTs) based on metal oxides have high mobility, transparency, and excellent stability, and are considered to be a key part of the next generation transparent electronic devices. Solution-processed metal oxide thin films using the sol-gel method has enabled the deposition of metal oxide thin films without a vacuum system, and it allows metal oxide TFTs to be fabricated more efficiently at low cost and over a large area through a simple process. In this study, we fabricated In2O3 using the sol-gel method and adopted various self-assembled monolayers (SAMs) to modify the interface between the semiconductor and dielectric layer. We used octadecyltrichlorosilane as a hydrophobic surface and partially removed it through exposure to UV radiation to form a patterned semiconductor using a simple spin-coating procedure that remarkably reduced the leakage current. 3-aminopropylsilane, (3-mercaptopropyl)trimethoxysilane, and cyclopentadienyltrimethylsilane as SAM materials and toluene and water as solvents to dilute them were used to modify the surface of the SiO2. We measured the contact angle under various conditions and the electrical characteristics of the TFTs annealed in various temperatures of 230, 250, and 300°C to confirm the effect of interface energy modification, and concluded that surface energy affected the mobility of the In2O3 transistors and the lower surface energy enhanced the mobility of the TFTs.

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.

Evaluating the use of self-assembled monolayer (SAM) release coatings for replicated optics

Carbon fiber-reinforced composite replicated mirrors offer weight savings, higher stiffness, tailorable CTEs, higher thermal conductivities, and excellent damage tolerant mechanical properties in comparison to traditionally processed glass mirrors. These mirrors can also be rapidly manufactured by replicating the surface of a high-precision glass mandrel multiple times. The mold release coating used is critical and must be on the molecular scale, uniform, and exhibit low adhesion in order to produce high-fidelity replication. This study investigates the use of a self-assembled monolayer (SAM) as the mold release agent for the manufacture of replicated mirrors. We have synthesized and tested perfluoropolyether (PFPE) coatings formed on glass substrate via self-assembly. Ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy were used to characterize the morphology, thickness uniformity and the chemical profile of the coatings. The release force necessary to remove the replicated surfaces from the mandrel is critical and was directly related to the surface distortion of the replica. The release force increased with each successive replication due to pinholes and defects. Incorporating a secondary, lower molecular weight SAM reagent significantly reduced both surface agglomeration and transfer of SAM material to the replicated resin surface. A 60% reduction in the replica release force and a 50% improvement in the replica surface figure were achieved due to reductions in the defect density of the SAM coating.

Study on Work Function of Modified Indium Tin Oxide and Performance of Organic Electroluminescent Devices.

ABSTRACTThe work function of the indium tin oxide electrode (ITO) modified by a self-assembled monolayer (SAM), which is used as an electrode in organic electroluminescent (EL) devices, was investigated in this study. It is revealed that chemical modification of ITO with p-substituted with different terminal groups (NH2−, Cl−, and CF3−) benzoic acids as a SAMs material with carboxyl binding group is caused to increase the work function of the ITO electrode. Through a self-assembly process, the transmittance of the ITO with a SAM was not changed. The work function change of ITO with various SAM was measured by using cyclic voltammetry. Characteristics of EL devices were increased because the energy barrier was decreased in an interface between the ITO and an organic layer in the EL devices. The correlation of the work function change and the performance of the chemically modified EL devices was estimated.

Direct deformation study of AFM probe tips modified by hydrophobic alkylsilane self-assembled monolayers

The in-use wear of atomic force microscopy (AFM) probe tips is crucial for the reliability of AFM measurements. Increase of tip size for several nanometers is difficult to monitor but it can already taint subsequent AFM data. We have developed a method to study the shape evolution of AFM probe tips in nanometer scale. This approach provides direct comparison of probe shape profiles, and thus can help in evaluation of the level of tip damage and quality of acquired AFM data. Consequently, the shape degradation of probes modified by hydrophobic alkylsilane self-assembled monolayers (SAMs) was studied. The tip wear length and wear volume were adopted to quantitatively verify the effectiveness of hydrophobic coatings. When compared with their silicon counterparts, probes modified by SAM materials exhibit superior wear-resistant behavior in tapping mode scans.

Electrical Properties of Organic Light-Emitting Diodes with an Insertion of CMTS Self-Assembled Monolayers

Electrical properties of organic light-emitting diodes were studied in a device with an insertion of chloromethyl-trichlorosilane self-assembled monolayer (SAM) between the ITO anode and organic layer. Self-assembled monolayer is generally used for a surface reformation of ITO substrate to enhance the charge injection at the anode/organic interface. For a formation of SAM onto the ITO anode, the ITO substrate was immersed in a 20 mM solution of SAM material. SAM-treatment time on the ITO substrate was varied to be 0, 10, 15, 20, and 25 minutes. And then, the device was made in a structure of ITO(170 nm)/SAM/TPD(50 nm)/Alq3(70 nm)/LiF(0.5 nm)/Al(100 nm). A device with properly treated self-assembled monolayer at the anode/organic interface gives an improvement in turn-on voltage, luminance, and efficiency compared to the one without SAM layer. A current efficiency and an external quantum efficiency of the device with 15 minutes treated SAM layer were increased by 2.6 times compared to the ones without the SAM layer. Fowler-Nordheim tunneling conduction mechanism was applied to the current density-voltage characteristics to obtain a hole injection barrier height. It was found that there is a reduction in barrier height by 25% for the device with 15 minutes SAM-treated compared to the one without SAM-treated.

The surface engineering of top electrode in inverted polymer bulk-heterojunction solar cells

We present surface engineering of the top electrode in organic solar cells through soft contact lamination. A polymer conductor and a self-assembled monolayer material are utilized for the surface engineering of gold electrode in a polymer bulk-heterojunction solar cell. Approximately a threefold increase in the overall power conversion efficiency is shown to be realized with the surface engineering, which is made possible by the lamination.