Organic Self-assembled Monolayers Research Articles

The impact of interface and heterostructure on the stability of perovskite-based solar cells

Perovskite solar cells have made significant progress in achieving high power conversion efficiency (>26%) in the past decade. However, achieving long-term stability comparable to established silicon solar cells is still a significant challenge, requiring further investigation into degradation mechanisms and continued exploration of interface engineering strategies. Here we review stability at the interfaces between perovskite and charge transport layers. These interfaces are particularly vulnerable to defects and degradation under external stresses such as heat, light, and bias, further compounded by their ionic nature and thermal expansion mismatch. To address these issues, strategies such as the use of additives, organic self-assembled monolayers, and low-dimensional perovskites have been developed to improve interface stability. These approaches enhance crystallinity, reduce defect-related recombination, and improve mechanical toughness.

Determining the stopping power of low kinetic energy Ne+ projectiles in self-Assembled monolayers

Neutral impact collision ion scattering spectroscopy (NICISS) is used to measure the energy loss in organic self-assembled monolayers (SAMs) on Au using Ne+ with low kinetic energies from 3 to 5 keV. With increasing film thickness, the energy loss of the projectiles increases because the projectile experiences more collisions with target atoms.Through comparing Monte-Carlo simulations with the NICISS experiments, it was found that contributions from nuclear stopping for Ne+ were significantly larger than for He+ mainly due to the stronger contribution of small-angle scattering of Ne+ making Ne NICISS unsuitable for depth profiling at energies of 5 keV or lower. The measured Ne+ electronic stopping in SAMs is small despite the large atomic number of Ne. Comparing experiments and DFT calculations shows that the latter accurately reproduce stopping powers for Ne+, while SRIM overestimates the stopping power. This contrasts He+ ions, where DFT and SRIM align closely with experiments.

Double-side 2D/3D heterojunctions for inverted perovskite solar cells.

Defects at the top and bottom interfaces of three-dimensional (3D) perovskite photoabsorbers diminish the performance and operational stability of perovskite solar cells owing to charge recombination, ion migration and electric-field inhomogeneities1-5. Here we demonstrate that long alkyl amine ligands can generate near-phase-pure 2D perovskites at the top and bottom 3D perovskite interfaces and effectively resolve these issues. At the rear-contact side, we find that the alkyl amine ligand strengthens the interactions with the substrate through acid-base reactions with the phosphonic acid group from the organic hole-transporting self-assembled monolayer molecule, thus regulating the 2D perovskite formation. With this, inverted perovskite solar cells with double-side 2D/3D heterojunctions achieved a power conversion efficiency of 25.6% (certified 25.0%), retaining 95% of their initial power conversion efficiency after 1,000 h of 1-sun illumination at 85 °C in air.

Efficient Silicon Solar Cells through Organic Self‐Assembled Monolayers as Electron Selective Contacts

AbstractEffective charge carrier‐selective contacts are a crucial component of high‐performance crystalline silicon (c‐Si) solar cells. Organic materials deposited via self‐assembly on the c‐Si surface are promising candidates for simplified, scalable, and cost‐effective processing of charge extraction layers. This study investigates the application of nPACz self‐assembled monolayers (SAMs), based on carbazole and phosphonic acid groups, where n (= 2, 4, or 6) is the aliphatic chain length, to facilitate electron extraction in c‐Si solar cells by tuning the work function of aluminum (Al) at the rear contact. So far, these SAM molecules are mainly applied as the hole‐selective layer in state‐of‐the‐art perovskite and organic solar cells, via anchoring on a metal oxide electrode. Here, by inserting 2PACz between amorphous silicon passivated c‐Si and Al, an electron‐selective contact with a contact resistivity of 65 mΩ cm2 is achieved and a power conversion efficiency of 21.4% with an open‐circuit voltage of 725 mV and a fill factor of 79.2% is demonstrated. Although the 2PACz displays some instability in this study, its initial performance is comparable to those achieved with conventionally used n‐type amorphous silicon. This study highlights the potential of solution‐processable organic SAMs in forming carrier‐selective contacts for c‐Si heterojunction solar cells.

Excited-State Odd-Even Effect in Through-Space Interactions.

The odd-even effect is a fantastic phenomenon in nature, which has been applied in diverse fields such as organic self-assembled monolayers and liquid crystals. Currently, the origin of each odd-even effect remains elusive, and all of the reported odd-even effects are related to the ground-state properties. Here, we discover an excited-state odd-even effect in the through-space interaction (TSI) of nonconjugated tetraphenylalkanes (TPAs). The TPAs with an even number of alkyl carbon atoms (C2-TPA, C4-TPA, and C6-TPA) show strong TSI, long-wavelength emission, and high QY. However, the odd ones (C1-TPA, C3-TPA, C5-TPA, and C7-TPA) are almost nonexistent with negligible QY. Systematically experimental and theoretical results reveal that the excited-state odd-even effect is synthetically determined by three factors: alkyl geometry, molecular movability, and intermolecular packing. Moreover, these flexible luminescent TPAs possess tremendous advantages in fluorescent information encryptions. This work extends the odd-even effect to photophysics, demonstrating its substantial importance and universality in nature.

Selective Deposition of N-Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array.

The incorporation of organic self-assembled monolayers (SAMs) in microelectronic devices requires precise spatial control over the self-assembly process. In this work, selective deposition of N-heterocyclic carbenes (NHCs) on specific electrodes within a two-microelectrode array is achieved by using pulsed electrodeposition. Spectroscopic analysis of the NHC-coated electrode arrays reveals that each electrode is selectively coated with a designated NHC. The impact of NHC monolayers on the electrodes' work function is quantified using Kelvin probe force microscopy. These measurements demonstrate that the work function values of each electrode can be independently tuned by the adsorption of a specific NHC. The presented deposition method enables to selectively coat designated microelectrodes in an electrode array with chosen NHC monolayers for tuning their chemical and electronic functionality.

Sidewall patterning of organic–inorganic multilayer thin film encapsulation by adhesion lithography

A simple sidewall patterning process for organic–inorganic multilayer thin-film encapsulation (TFE) has been proposed and demonstrated. An Al2O3 thin film grown by atomic layer deposition (ALD) was patterned by adhesion lithography using the difference in interfacial adhesion strength. The difference in interfacial adhesion strength was provided by the vapor-deposited fluoro-octyl-trichloro-silane self-assembled monolayer (SAM) patterns formed by a shadow mask. The sidewall patterning of multilayer TFE was shown possible by repeating the adhesion lithography and the vapor deposition of organic polymer and SAM patterns using shadow masks. The proposed process has the advantage of being able to pattern the blanket ALD-grown Al2O3 thin films by adhesion lithography using a SAM pattern that can be more accurately predefined with a shadow mask.

DFT Study of Cl− Ingress into Organic Self-Assembled Monolayers on Aluminum

We address the mechanism by which organic layers on aluminum substrate hinder the penetration of Cl− toward the metal substrate. Localized corrosion by chlorides on Al and its alloys is a major problem, and organic molecules that form self-assembled monolayers on metal substrates may provide efficient corrosion protection. In one of our previous works, we established experimentally that long-chain n-alkyl carboxylic acids form protective layers against Cl− corrosion on Al substrates. In a different work, we identified, using implicit models of the organic layer and metal substrate, two essential effects by which organic layers hinder the penetration of Cl− ions toward the metal substrate. The first effect is due to the inferior solvation of ions in the organic layer compared to that in an aqueous solvent. The second effect is due to the electric field at the electrochemical interface, and the extent to which it affects the penetration of Cl− depends on the electrode potential and the thickness of the organic layer. Both effects are related to a low dielectric constant of the self-assembled monolayer. In the present study, we continue our investigation and explicitly model the organic monolayer and Al substrate using density-functional-theory calculations. To this end, we consider organic monolayers consisting of either dodecanoic- or hexanoic-acid molecules. Current calculations confirm the findings of the simplified implicit models, i.e. the energy barrier for the Cl− penetration increases with the thickness of the organic monolayer and with Cl− concentration in the monolayer. Furthermore, we propose a new mechanism by which Cl− penetrates the organic monolayer. Due to the considerably inferior solvation of Cl− in the organic layer compared to that in water, calculations suggest that it is energetically easier to locally “open” the organic monolayer by creating a hole large enough to accommodate water molecules and Cl−. The presence of water molecules ensures a stronger Cl− solvation and a better electrostatic screening between anions. While the energy barrier for the Cl− penetration via the local “opening” mechanism is suggested to be smaller than for the penetration of Cl− into dense homogeneous organic monolayer, it is still significant enough to pose a considerable kinetic barrier for the penetration of Cl− from the aqueous solution into the organic monolayer at room temperature.

Perovskite Solar Cells with an SbX3 (X = Cl, I) Interface Dipole Layer

Forming an interface dipole moment pointing from the absorber to the oxide substrate can improve the open-circuit voltage (VOC) of perovskite solar cells (PSCs) without affecting photocurrent. In past studies, however, interface dipole layers were generally formed using organic self-assembled monolayers or polyelectrolytes, which raises concerns about intrinsic stability. Meanwhile, perovskites’ substrate-dependent electronic properties question the apparent correlation between the oxide’s work function and the built-in voltage. In this work, we propose an interface dipole layer based on solution-processable metal halides, which increases the built-in voltage of the n–i–p diode by simultaneously reducing the electron transport layer’s work function and strengthening the n-type characteristics of the perovskite absorber. The n–i–p-structured PSCs with the SbX3 dipole layer achieve substantially enhanced VOC (1.171 V) and power conversion efficiency (23.11%). Further experiments and calculations confirm that the proposed dipole model can explain the VOC enhancement qualitatively and quantitatively.

Interfacial Molecular Compatibility for Programming Organic-Metal Oxide Superlattices.

Artificially programming a sequence of organic-metal oxide multilayers (superlattices) by using atomic layer deposition (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here, we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy, in situ quartz crystal microbalance measurements, and Fourier-transformed infrared spectroscopy. These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the -OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive -OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, and Zr oxides) and their multilayered structures.

Controlling surface effects in extremely high aspect ratio gold plasmonic electrodes

Nanofabrication is key to many technological advances, especially the challenge of merging nanophotonics with electronics. Here, we investigate the fabrication process of plasmonic interdigitated gold electrodes having a very high aspect ratio (i.e. long and thin geometries) and a large surface area. Stringent stability issues that arise when these structures are fabricated using inorganic adhesion layers, such as titanium or chromium, on silica substrates are highlighted. We ascribe these problems to thermodynamical non-equilibrium states of freshly deposited gold and, in particular, discuss the role of surface energy in determining the structural properties of high aspect ratio gold nanostructures. We then show that the use of organic silane self-assembled monolayers improves the long term stability of these structures and, finally, characterize the fabricated electrodes. This technology can unleash the potential of hybrid optoelectronic circuits where current and light are manipulated with the same component.

Organic self-assembled monolayers on superconducting NbSe2: interfacial electronic structure and energetics* *This article is dedicated to the memory of Professor Enrico Clementi, our mentor, friend, and a scientific giant on whose shoulders we stand.

While organic self-assembled monolayers (SAMs) have been widely used to modify the work function of metal and metal-oxide surfaces, their application to tune the critical temperature of a superconductor has only been considered recently when SAMs were deposited on NbSe2 monolayers (Calavalle et al 2021 Nano Lett. 21 136–143). Here, we describe the results of density functional theory calculations performed on the experimentally reported organic/NbSe2 systems. Our objectives are: (i) to determine how the organic layers impact the NbSe2 work function and electronic density of states; (ii) to understand the possible correlation with the experimental variations in superconducting behavior upon SAM deposition. We find that, upon adsorption of the organic monolayers, the work-function modulation induced by the SAM and interface dipoles is consistent with the experimental results. However, there occurs no significant difference in the electronic density of states near the Fermi level, a consequence of the absence of any charge transfer across the organic/NbSe2 interfaces. Therefore, our results indicate that it is not a SAM-induced tuning of the NbSe2 density of states near the Fermi level that leads to the tuning of the superconducting critical temperature. This calls for further explorations, both experimentally and theoretically, of the mechanism underlying the superconducting critical temperature variation upon formation of SAM/NbSe2 interfaces.

Effect of organic self-assembled monolayers on the deposition and adhesion of hydroxyapatite coatings on titanium

AbstractOrganic self-assembled monolayers with nitrogen-containing surface functionalities (amine or alkylammonium) were deposited on acid-etched titanium substrates. These were then coated with apatite deposited from 150% simulated body fluid at 37 °C and pH = 7.6 for 2, 4, and 6 days (after a 2-day “nucleation” step in contact with bioactive glass in 100 % simulated body fluid). Scanning electron microscopy showed that precipitates were numerous on amine- and alkylammonium-modified titanium surfaces after nucleation for 2 days. X-ray diffraction at grazing incidence detected crystalline hydroxyapatite on the modified and unmodified substrates after growth for 2 days. Tape peel tests followed by surface chemical analysis showed that the adhesion of the hydroxyapatite coating to the substrates was improved by amine and by alkylammonium modification of the etched titanium surfaces.

Effect of substrates on covalent surface modification of graphene using photosensitive functional group

Abstract Graphene has gained much significance for its potential applications in optics and electronics owing to its unique physical and chemical properties. Nevertheless, its gapless band structure greatly limits its wider application in optoelectronic devices. The present study seeks to explore chemical functionalization as an effective method to tune the properties of graphene. Covalent modification of graphene by aryl diazonium salt of a photosensitive functional group (azobenzene) has been used to achieve this goal. This is based on the fact that graphene is a two-dimensional, atomically thin lattice of sp2-bonded carbon atoms, therefore, its properties can be modulated by modifying the underlying dielectric surface with a self-assembled monolayer resulting in doping control. In the study, a clear difference in the rate of electron-transfer reactions with the photosensitive functional group is shown for monolayer graphene supported on SiO2/Si substrates and organic molecule functionalized SiO2/Si substrates. Graphene supported on SiO2/ Si is more reactive towards functionalization than graphene on organic molecule functionalized surfaces, as shown by Raman spectroscopy. The transport characteristics of functionalized graphene on conventional SiO2/Si substrates as well as substrates modified with organic molecule octadecyltrichlorosilane self-assembled monolayers are also explored and compared.

Controlling Heterogeneous Catalysis with Organic Monolayers on Metal Oxides.

ConspectusA key theme of heterogeneous catalysis research is achieving control of the environment surrounding the active site to precisely steer the reactivity toward desired reaction products. One method toward this goal has been the use of organic ligands or self-assembled monolayers (SAMs) on metal nanoparticles. Metal-bound SAMs are typically employed to improve catalyst selectivity but often decrease the reaction rate as a result of site blocking from the ligands. Recently, the use of metal oxide-bound organic modifiers such as organophosphonic acid (PA) SAMs has shown promise as an additional method for tuning reactions on metal oxide surfaces as well as modifying oxide-supported metal catalysts. In this Account, we summarize recent approaches to enhance catalyst performance with oxide-bound monolayers. These approaches include (1) modification of metal oxide catalysts to tune surface reactions, (2) formation of SAMs on the oxide component of supported metal catalysts to modify sites at the metal-support interface, and (3) enhancement of catalyst performance (e.g., stability) through modification of sites remote from the active sites.Both the headgroups and organic tail groups of PA SAMs or other ligands can influence reactions on metal oxide surfaces. Binding of the headgroup can selectively poison certain active sites, altering the selectivity in a manner analogous to metal-bound ligands (at the expense of active site quantity). Moreover, tail groups can be functionalized to interact favorably with reactants and intermediates, for instance through dipole-dipole interactions. On supported metal catalysts like Pt/Al2O3, PA SAMs can selectively form on the oxide support. This selective deposition allows for modification of the metal-support interface with minimal blockage of metal sites. PA headgroups were shown to provide tunable acid sites at the interface, dramatically improving hydrodeoxygenation rates of various alcohols. Additionally, organic tail functionality was used to activate or stabilize specific reactants at the interface, such as with the use of amine-functionalized PAs to stabilize chemisorption of CO2 during the reverse water gas shift reaction. PAs have also been found to affect the electronic properties of bulk metal sites through long-range electron withdrawal via the oxide, providing an additional avenue to tune catalytic behavior. Finally, organic modifiers were shown to enhance catalytic performance without directly modifying the active site. For instance, in biphasic liquid environments the modification of catalyst particles with hydrophobic or hydrophilic SAMs shifts the selectivity of multipath reactions on the basis of the hydrophobicities of different intermediates and products. As another "long-range" effect, the deposition of ligands on oxide supports improved catalyst stability through both improved resistance to sintering and suppression of active site poisoning. The recent contributions discussed in this Account demonstrate the versatility and significant potential for the approach of modifying catalysts with oxide-bound organic monolayers.

5,5′-dithiobis-(2-nitrobenzoic acid) self-assembled monolayer for corrosion inhibition of copper in sodium chloride solution

In this paper, 5,5′-Dithiobis-(2-nitrobenzoic acid) (DTNB) was used as an organic self-assembled monolayer on copper surface for the first time. The inhibition performance in 3.5 wt% NaCl solution was studied via electrochemical techniques, surface analysis methods and theoretical calculations. The influence of concentration on inhibition performance of DTNB was investigated by electrochemical impedance spectroscopy and potentiodynamic polarization. It is found that the inhibition efficiency of the self-assembled monolayer first increases and then decreases with the addition of DTNB, and the maximum inhibition efficiency is 92.4% at the DTNB concentration of 1 × 10-3 mol·L-1. The inhibition effect of DTNB was also evaluated by morphology observation of copper using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Raman spectroscopy, X-ray photoelectron spectroscopy and quantum chemical calculations were utilized to determine that DTNB is well bound to copper surface through chemical bonds.

Development of a Novel Nanoarchitecture of the Robust Photosystem I from a Volcanic Microalga Cyanidioschyzon merolae on Single Layer Graphene for Improved Photocurrent Generation.

Here, we report the development of a novel photoactive biomolecular nanoarchitecture based on the genetically engineered extremophilic photosystem I (PSI) biophotocatalyst interfaced with a single layer graphene via pyrene-nitrilotriacetic acid self-assembled monolayer (SAM). For the oriented and stable immobilization of the PSI biophotocatalyst, an His6-tag was genetically engineered at the N-terminus of the stromal PsaD subunit of PSI, allowing for the preferential binding of this photoactive complex with its reducing side towards the graphene monolayer. This approach yielded a novel robust and ordered nanoarchitecture designed to generate an efficient direct electron transfer pathway between graphene, the metal redox center in the organic SAM and the photo-oxidized PSI biocatalyst. The nanosystem yielded an overall current output of 16.5 µA·cm−2 for the nickel- and 17.3 µA·cm−2 for the cobalt-based nanoassemblies, and was stable for at least 1 h of continuous standard illumination. The novel green nanosystem described in this work carries the high potential for future applications due to its robustness, highly ordered and simple architecture characterized by the high biophotocatalyst loading as well as simplicity of manufacturing.

Impermeable Graphene Oxide Protects Silicon from Oxidation.

The presence of a natural silicon oxide (SiOx) layer over the surface of silicon (Si) has been a roadblock for hybrid semiconductor and organic electronics technology. The presence of an insulating oxide layer is a limiting operational factor, which blocks charge transfer and therefore electrical signals for a range of applications. Etching the SiOx layer by fluoride solutions leaves a reactive Si-H surface that is only stable for few hours before it starts reoxidizing under ambient conditions. Controlled passivation of silicon is also of key importance for improving Si photovoltaic efficiency. Here, we show that a thin layer of graphene oxide (GOx) prevents Si surfaces from oxidation under ambient conditions for more than 30 days. In addition, we show that the protective GOx layer can be modified with molecules enabling a functional surface that allows for further chemical conjugation or connections with upper electrodes, while preserving the underneath Si in a nonoxidized form. The GOx layer can be switched electrochemically to reduced graphene oxide, allowing the development of a dynamic material for molecular electronics technologies. These findings demonstrate that 2D materials are alternatives to organic self-assembled monolayers that are typically used to protect and tune the properties of Si and open a realm of possibilities that combine Si and 2D materials technologies.

Effect of cavitation intensity control on self-assembling of alkanethiols on gold in room temperature ionic liquids

This study investigates the effect of cavitation intensity on self-assembling of alkanethiol molecules on gold in room temperature ionic liquids (RTILs) under low frequency ultrasound irradiation (20 kHz). The use of RTILs, with low vapor pressure, enabled cavitation activity to be controlled up to quenching through pressure decrease within an argon-saturated atmosphere. This control possibility was used to acquire deeper insights into the role of cavitation on self-assembling processes. It was shown by electrochemical, contact angles and Polarization Modulation - Infrared Reflection Absorption Spectroscopy (PM-IRRAS) measurements that cavitation activates orientation and organization of self-assembled monolayers (SAM). X-ray Photoelectron Spectroscopy (XPS) revealed that, even if chemical adsorption of molecules is highly activated under ultrasound irradiation, it is not dependent on acoustic cavitation intensity.

Wet Chemical Cleaning of Organosilane Monolayers

Thin organic self-assembled monolayer films are used to promote adhesion and seal the pores of metal oxides as well as direct the deposition of layers on patterned surfaces. Defects occur as the self-assembled monolayer forms, and the number and type of defects depend on surface preparation, deposition solvent, temperature, time and other parameters. Particles commonly deposit during organosilane self-assembly on metal oxide surfaces. The particles are defects because they are prone to react in subsequent processing, which may not be desirable if the organosilane serves as a pore sealant or passivation layer. Cleaning the organosilane by solvent extraction to remove non-polar agglomerates followed by an aqueous mixture of ammonium hydroxide and hydrogen peroxide, which is Standard Clean 1, a common particle removal step for silicon surfaces, produced monolayers with few agglomerates based on atomic force microscopy without etching the layer. The combined cleaning sequence contained fewer particles than separate cleaning steps, showing that the cleans removed particles with different compositions. The thickness and contact angle of cleaned monolayers was comparable to those made using a costlier solvent.