Vapor Annealing Research Articles

Solvent-vapor-assisted crystallization of covalent organic framework films for CO2/CH4 separation

Abstract Covalent organic framework (COF) films were prepared using alternating vapor deposition of precursors followed by solvent vapor annealing. When an as-deposited film was annealed in appropriate solvent vapors, the prepared COF film exhibited an “onion-like” structure. This result opens a way to synthesize COF films with highly ordered planar layers through the interaction between layers. The CO2/CH4 separation performance of COF film was significantly improved due to the decrease in defects caused by crystallization.

Gradual Optimization of Molecular Aggregation and Stacking Enables Over 19% Efficiency in Binary Organic Solar Cells.

Volatile solid additive is an effective and simple strategy for morphology control in organic solar cells (OSCs). The development of environmentally friendly new additives which can also be easily removed without high-temperature thermal annealing treatment is currently a trend, and the working mechanism needs to be further studied. Herein, a highly volatile and non-halogenated solid additive 1-benzothiophene (BBT) is reported to regulate molecular aggregation and stacking of active layer components. According to the film-forming kinetics process, a momentary intermediate phase is formed during spin-coating, which slows down the film-forming process and leads to more ordered molecular stacking in the solid film after introducing solid additive BBT. Subsequently, after solvent vapor annealing (SVA) further treatment, the resultant blend films exhibit a tighter and more ordered molecular stacking. Consequently, the synergistic effect of solid additive BBT and SVA treatment can effectively control morphology of active layer and improve carrier transport characteristics, thereby enhancing the performance of OSCs. Finally, in D18-Cl:N3 system, an impressive power conversion efficiency of 19.53% is achieved. The work demonstrates that the combination of highly volatile solid additives and SVA treatment is an effective morphology control strategy, guiding the development of efficient OSCs.

Molecules as Lubricants at the Nanoscale:Tunable Growth of Organic Structures from Nano- to Millimeter-Scale Using Solvent Vapour Annealing.

The creation of ordered structures of molecules assembled from solution onto a substrate is a fundamental technological necessity across various disciplines, spanning from crystallography to organic electronics. However, achieving macroscopic order poses significant challenges, since the process of deposition is inherently impacted by factors like solvent evaporation and dewetting flows, which hinder the formation of well-organized structures. Traditional methods like drop casting or spin coating encounter limitations due to the rapid kinetics of solvent evaporation, leading to limited control over final uniformity and order. In response to these challenges, Solvent Vapour Annealing (SVA) has emerged as a promising solution for realizing ordered molecular structures at scales ranging from nano- to milli- meters. SVA decouples the self-assembly stage from the deposition stage by utilizing solvent vapours which can enable rearrangement, movement, and diffusion of large molecules on the surface even on a macroscopic scale. Essentially acting as "molecular lubricants," solvent vapours enable the formation of well-ordered molecular films. This review discusses the advancements, obstacles, and promising strategies associated with utilizing SVA for the development of innovative nanostructured thin films, and emphasizes the originality and effectiveness of molecular assembly on substrates achieved through this approach.

Mechanism of quantum yield enhancement in Si quantum dots by high-pressure water vapor annealing from single-dot studies

High-pressure water vapor annealing (HWA) was recently demonstrated as a method that can substantially improve the photoluminescence quantum yield (PLQY) of silicon quantum dots (Si QDs) with the oxide shell. In this Letter, the mechanism of this enhancement is studied optically on a single-dot level. HWA treatment is performed on Si QDs formed on a silicon-on-insulator wafer, and their photoluminescence (PL) properties were examined before and after the treatment. Our experiments show a 2.5 time enhancement in the average blinking duty cycle of Si QDs after 2.6 MPa HWA treatment without changing the average ON-state PL intensity. This observation proves the carrier trapping process is suppressed on the HWA-built Si/SiO2 interface. We also discussed the mechanism behind the PLQY enhancement of HWA-treated Si QDs by comparing single-dot-level data to reported ensemble PL Si QDs results. HWA treatment is found to mainly brighten “grey” (not 100% efficient) QDs, a mechanism different from changing dark (non-emitting) to bright (100% efficient) Si QDs by ligand passivation.

Non-Centrosymmetric Single Crystalline Biomolecular Nano-Arrays for Responsive Electronics.

Herein, a novel strategy is reported for synthesizing libraries of single crystalline amino acid (AA) nanocrystals with control over size, anisotropy, and polymorphism by leveraging dip-pen nanolithography (DPN) and recrystallization via solvent vapor annealing. The crystals are prepared by first depositing nanoreactors consisting of a solvent with AAs, followed by water vapor-induced recrystallization. This leads to isotropic structures that are non-centrosymmetric with strong piezoelectric (g33 coefficients >1000mVmN-1), ferroelectric, and non-linear optical properties. However, recrystallizing arrays of isotropic DL-alanine nanodot features with a binary solvent (water and ethanol) leads to arrays of 1D piezoelectric nanorods with their long axis coincident with the polar axis. Moreover, positioning nanoreactors containing AAs (the nanodot features) between micro electrodes leads to capillary formation, making the reactors anisotropic and facilitating piezoelectric nanorod formation between the electrodes. This offers a facile route to device fabrication. These as-fabricated devices respond to ultrasonic stimulation in the form of a piezoelectric response. The technique described herein is significant as it provides a rapid way of investigating non-centrosymmetric nanoscale biocrystals, potentially pivotal for fabricating a new class of stimuli-responsive devices such as sensors, energy harvesters, and stimulators.

Solvent Vapor Annealing and Plasma Treatment Stabilize Silver Nanowire Layers

AbstractSilver nanowires (AgNW) find use in transparent conductive electrodes with applications in solar cells, touch screens, and wearables. Unprotected AgNW are prone to atmospheric corrosion and lose conductivity over time. Known passivation techniques either require submersion of pre‐deposited AgNW in liquid compounds or the modification of AgNW inks prior to deposition, which alters viscosity and complicates deposition. Here, new possibilities for stabilization of pre‐deposited AgNW networks without need for submersion are explored. It is demonstrated that AgNW networks can be stabilized either by argon or hydrogen plasma treatment or by solvent vapor annealing with ethanol, methanol, or ethyl acetate. These treatments yielded stable electrical resistance over at least nine weeks, whereas untreated or thermally annealed AgNW layers quickly lost conductivity. The potential of solvent vapor annealing is further explored by demonstrating a new processing technique for stable polymer matrix composites containing AgNW. Co‐deposited layers of AgNW with polystyrene microbeads are annealed in ethyl acetate vapor to stabilize the AgNW while at the same time merging polymer beads into a closed film around the AgNW. The resulting composites maintained stable resistance and transmittance for at least seven weeks.

Lithium iodide transport in double gyroid nanochannels formed by zwitterion‐containing solid polymer electrolytes

AbstractThe fabrication of nanostructured block copolymer (BCP) films endowed with lithium ion‐conducting gyroid (GYR) nanochannels is an appealing solution to build solid polymer electrolytes (SPEs) that combines high ionic conductivity (IC) with suitable mechanical properties. However, the formation of a well‐developed GYR structure remains challenging to achieve from the self‐assembly of polyelectrolyte BCP chains. To overcome this issue, large ion conducting gyroid grains were produced within freestanding polystyrene‐block‐poly(2‐vinylpyridine)‐block‐poly(ethylene oxide) (PS‐b‐P2VP‐b‐PEO) films by combining a solvent vapor annealing (SVA) treatment with an infiltration process. Here, the SVA treatment enabled the manufacture of SPEs entirely composed of a GYR structure while the infiltration process allowed for the incorporation of an appropriate Li salt (i.e., lithium iodoacetate, LiIAc) within the 3D‐interconected nanochannels via a Menshutkin reaction. By using this SVA‐Infiltration strategy, it has been demonstrated that the creation of ion conducting GYR nanochannels enhances the ion transportation capacity of pyridine‐containing SPEs since substantially lower ICs were measured from analog PS‐b‐P2VP‐b‐PEO/LiIAc films having a nominally disordered as‐cast state. Remarkably, zwitterionic pyridinium‐based moieties formed inside the interpenetrated nanochannels enable an efficient migration of Li+ and I3− species, leading to an IC as high as 10−4 S cm−1 at 70°C.

Morphological Characteristics of Biopolymer Thin Films Swollen-Rich in Solvent Vapors.

Biopolymers exhibit a large variety of attractive properties including biocompatibility, flexibility, gelation ability, and low cost. Therefore, especially in more recent years, they have become highly suitable for a wider and wider range of applications stretching across several key sectors such as those related to food packaging, pharmaceutic, and medical industries, just to name a few. Moreover, biopolymers' properties are known to be strongly dependent on the molecular arrangements adopted by such chains at the nanoscale and microscale. Fortunately, these arrangements can be altered and eventually optimized through a plethora of more or less efficient polymer processing methods. Here, we used a space-confined solvent vapor annealing (C-SVA) method to subject various biopolymers to rich swelling in solvent vapors in order to favor their further crystallization or self-assembly, with the final aim of obtaining thin biopolymer films exhibiting more ordered chain conformations. The results obtained by atomic force microscopy revealed that while the gelatin biopolymer nucleated and then crystallized into granular compact structures, other biopolymers preferred to self-assemble into (curved) lamellar rows composed of spherical nanoparticles (glycogen and chitosan) or into more complex helix-resembling morphologies (phytagel). The capability of the C-SVA processing method to favor crystallization and to induce self-assembly in various biopolymeric species or even monomeric units further emphasizes its great potential in the future structuring of a variety of biological (macro)molecules.

Improving efficiency and flexibility of non-halogenated solvent-processed dual-layer organic solar cells through solvent vapor annealing

Dual-layer organic solar cells (OSCs), fabricated through sequential-casting with separately dissolved donor and acceptor materials, offer simplified solution preparation and morphology control. However, the poor solubility of organic materials in non-halogenated solvents often results in undesirable vertical component distribution and insufficient donor/acceptor interfaces in non-halogenated solvent-processed dual-layer OSCs, adversely affecting photovoltaic performance and flexibility. In this study, we applied a solvent vapor annealing (SVA) method using CS2 solvent in o-xylene solvent-processed dual-layer OSCs. The SVA method effectively adjusted the vertical component distribution of the active layer and increased the donor/acceptor interfaces, leading to an improved power conversion efficiency (PCE) of 17.24 %. Additionally, SVA films exhibited superior tensile properties, with a crack onset strain of 5.07 %, surpassing that of the as-cast films (4.32 %), attributed to the stronger interaction between the donor and acceptor layers with more donor/acceptor interfaces. Consequently, large-area (1 cm2) flexible devices achieved a significant efficiency of 14.20 % and maintained excellent mechanical flexibility, with 80 % of the initial efficiency retained after 1000 bending cycles. This work presents an effective approach for fabricating high-performance non-halogenated solvent-processed flexible dual-layer OSCs.

Tackling P3HT:Y‐Series Miscibility Through Advanced Processing for Tunable Aggregation

AbstractPolymer and small molecule blend thin films are of strong interest for organic electronics and particularly organic solar cells. The high miscibility in blends of ordinary P3HT and state‐of‐the‐art Y‐series non‐fullerene acceptors (NFAs) suppresses phase separation and aggregation challenging successful charge separation and transport. In a recent work, current‐induced doping (CID) is introduced, a method to precisely control the aggregation of Poly(3‐hexylthiophene) (P3HT) in solution. The highly ordered pre‐aggregation in solution is used here to control the P3HT aggregation in neat films and blends with Y12 (BTP‐4F‐12). This results in a 25‐fold increase in hole mobility in P3HT organic field‐effect transistor (OFET) devices and tunability of the P3HT aggregate quality in the presence of Y12 over large ranges. At the same time, particularly the Y12 long‐range ordering is heavily suppressed by increasing P3HT aggregation. However, solvent vapor annealing (SVA) leads to an extraordinarily high Y12 ordering, changes in the crystal orientation of Y12, and a further improvement of P3HT aggregation. A broad range of different degrees of aggregation of both materials can therefore be obtained in the final thin films solely by changing processing parameters without changing the composition of the material system.

Binary Organic Solar Cells with Exceeding 19% Efficiency via the Synergy of Polyfluoride Polymer and Fluorous Solvent.

Rational molecular design and suitable device engineering are two important strategies to boost the efficiencies in organic solar cells (OSCs). Yet these two approaches are independently developed, while their synergy is believed to be more productive. Herein, a branched polyfluoride moiety, heptafluoroisopropoxyl group, is introduced into the side chains of conjugated polymers for the first time. Compared with the conventional alkyl chain, this polyfluoride chain can endow the resulting polymer namely PF7 with highly packing order and strong crystallinity owing to the strong polarization and fluorine-induced interactions, while good solubility and moderate miscibility are retained. As a result, PF7 comprehensively outperforms the state-of-the-art polymer PM6 in photovoltaic properties. More importantly, based on the solubility of heptafluoroisopropoxyl groups in fluorous solvents, a new post-treatment denoted as fluorous solvent vapor annealing (FSVA) is proposed to match PF7. Differing from the existing post-treatments, FSVA can selectively reorganize fluoropolymer molecules but less impact small molecules in blend films. By employing the synergy of fluoropolymer and fluorous solvent, the device achieves a remarkable efficiency of 19.09%, which is among the best efficiencies in binary OSCs. The polymer PF7 and the FSVA treatment exhibit excellent universality in various OSCs with different material combinations or device architectures.

Vacancy modulation dramatically enhances the thermoelectric performance of InTe single crystal

InTe single crystals have demonstrated great promise in the field of thermoelectric materials, particularly when oriented along the [110] direction. This specific crystal orientation exhibits higher electronic conductivity and lower thermal conductivity compared to other orientations of InTe. Through first-principles calculations, we identified the anisotropic valence band and phonon dispersion as the underlying factors. Moreover, reducing the density of In+ vacancies in InTe was found to lower the band effective mass and modulate carrier scattering, enhancing the material quality factor (B). To explore these findings, we systematically grew InTe single crystals, achieving exceptional thermoelectric performance. A record-breaking power factor of 12.0 μW·cm−1·K−2 and a dimensionless figure of merit (zT) of 0.5 at room temperature were obtained. Notably, InTe crystals oriented along [110] with low In+ vacancy density exhibited the highest average zT of 0.63 among InTe-based thermoelectric materials within the 300–473 K temperature range. Furthermore, we introduced an effective method of reducing In+ vacancies through Indium vapor annealing, resulting in the highest reported carrier mobility of 182 cm2·V−1·s−1 for InTe. Our study highlights the potential for improving InTe's thermoelectric performance near room temperature through vacancy modulation and crystal orientation.

Constructing Efficient ASM Organic Solar Cells Based on DRCN5T

All-small-molecule (ASM) donor-acceptor materials have the advantages of good molecular reproducibility and relatively simple synthesis operations. In this paper, the small molecule donor material DRCN5T was synthesized and its related properties were investigated. The HOMO result of DRCN5T was determined to be -5.27 eV. The LUMO energy level was -3.62 eV. In terms of UV-visible absorption, the DRCN5T thin film exhibited a redshift due to π-π stacking as compared to CF solution. Finally, DRCN5T:IDIC-based ASM devices were prepared. After the solvent vapor annealing treatment, the device performances were significantly enhanced, with an open-circuit voltage (VOC) of 0.95 V, a short-circuit current density (JSC) of 12.66 mA cm-2 and a fill factor (FF) of 61.40%, and the power conversion efficiency (PCE) was 7.40%.

Impact of crystallinity on thermal conductivity of RF magnetron sputtered MoS2 thin films

This study investigates the effects of sulfur atomic defects and crystallinity on the thermal conductivity of MoS2 thin films. Utilizing scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and Raman spectroscopy, we examined MoS2 films, several nanometers thick, deposited on Si/SiO2 substrates. These films were prepared via a combination of RF magnetron sputtering and sulfur vapor annealing (SVA) treatment. Structural analyses, including cross-sectional STEM and in-plane and out-of-plane XRD measurements, revealed an increase in the S/Mo ratio and grain size of the MoS2 films following SVA treatment. Notably, the in-plane thermal conductivity of MoS2 films treated with SVA was found to be at least an order of magnitude higher than that of films without SVA treatment. This research suggests that the in-plane thermal conductivity of MoS2 thin films can be significantly enhanced through crystallinity improvement via SVA treatment.

Optimizing Alkyl Side Chains in Difluorobenzene-Rhodanine Small-Molecule Acceptors for Organic Solar Cells.

A series of small molecules, T-2FB-T-ORH, T-2FB-T-BORH, and T-2FB-T-HDRH, were synthesized to have a thiophene-flanked difluorobenzene (T-2FB-T) core and alkyl-substituted rhodanine (RH) end groups for their use as nonfullerene acceptors (NFAs) in organic solar cells (OSCs). Octyl, 2-butyloctyl (BO), and 2-hexyldecyl (HD) alkyl side chains were introduced into RHs to control the material's physical properties based on the length and size of the alkyl chains. The optical properties of the three NFAs were found to be almost the same, irrespective of the alkyl chain length, whereas the molecular crystallinity and material solubility significantly differed depending on the alkyl side chains. Owing to the sufficient solubility of T-2FB-T-HDRH, OSCs based on PTB7-Th and T-2FB-T-HDRH were fabricated. A power conversion efficiency of up to 4.49% was obtained by solvent vapor annealing (SVA). The AFM study revealed that improved charge mobility and a smooth and homogeneous film morphology without excessive aggregation could be obtained in the SVA-treated film.

In Situ Monitoring the Face-on to Bimodal Texture Transition of P(NDI2OD-T2) during Side-Chain Selective Solvent Vapor Annealing

In Situ Monitoring the Face-on to Bimodal Texture Transition of P(NDI2OD-T2) during Side-Chain Selective Solvent Vapor Annealing

Processing of Thin Films Based on Cellulose Nanocrystals and Biodegradable Polymers by Space-Confined Solvent Vapor Annealing and Morphological Characteristics.

L-poly(lactic acid), poly(3-hydroxybutyrate), and poly-hydroxybutyrate-co-hydroxyvalerate are biodegradable polymers that can be obtained from renewable biomass sources. The aim of this study was to develop three types of environmentally friendly film biocomposites of altered microstructure by combining each of the above-mentioned polymers with cellulose nanocrystal fillers and further processing the resulting materials via space-confined solvent vapor annealing. Cellulose was previously obtained from renewable biomass and further converted to cellulose nanocrystals by hydrolysis with the lactic acid. The solutions of biodegradable polymers were spin-coated onto solid substrates before and after the addition of cellulose nanocrystals. The obtained thin film composites were further processed via space-confined solvent vapor annealing to eventually favor their crystallization and, thus, to alter the final microstructure. Indeed, atomic force microscopy studies have revealed that the presence of cellulose nanocrystals within a biodegradable polymer matrix promoted the formation of large crystalline structures exhibiting fractal-, spherulitic- or needle-like morphologies.

Revealing the Kinetic Phase Behavior of Block Copolymer Complexes Using Solvent Vapor Absorption-Desorption Isotherms.

Controlling the self-assembled morphologies in block copolymers heavily depends on their molecular architecture and processing conditions. Solvent vapor annealing is a versatile processive pathway to obtain highly periodic self-assemblies from high chi (χ) block copolymers (BCPs) and supramolecular BCP complexes. Despite the importance of navigating the energy landscape, controlled solvent vapor annealing (SVA) has not been investigated in BCP complexes, partly due to its intricate multicomponent nature. We introduce characteristic absorption-desorption solvent vapor isotherms as an effective way to understand swelling behavior and follow the morphological evolution of the polystyrene-block-poly(4-vinylpyridine) block copolymer complexed with pentadecylphenol (PS-b-P4VP(PDP)). Using the sorption isotherms, we identify the glass transition points, polymer-solvent interaction parameters, and bulk modulus. These parameters indicate that complexation completely screens the polymer interchain interactions. Furthermore, we established that the sorption isotherm of the homopolymer blocks serves to deconvolute the intricacy of BCP complexes. We applied our findings by developing annealing pathways for grain coarsening while preventing macroscopic film dewetting under SVA. Here, grain coarsening obeyed a power law and the growth exponent revealed a kinetic transition point for rapid self-assembly. Overall, SVA-based sorption isotherms have emerged as a critical method for understanding and developing annealing pathways for BCP complexes.

Impact of water vapor annealing treatments on Al2O3/diamond interface

Our group developed the first inversion-type p-channel diamond metal–oxide–semiconductor field-effect transistor, which featured normally off properties by employing water vapor annealing treatments for the oxygen-terminated diamond surface. Despite the comprehensive device-grade characterization, the impact of water vapor annealing treatments on the Al2O3/diamond interface has not been investigated in detail. In this work, we fabricated four diamond metal–oxide–semiconductor (MOS) capacitors without and with water vapor annealing treatments for various times of 30 min, 1 h, and 2 h and conducted the cycle capacitance–voltage (C–V) and simultaneous C–V measurements. The large cycle C–V shift existed in the sample without water vapor annealing treatment, whereas it was significantly suppressed by water vapor annealing treatments, indicating the effective passivation of the traps with long time constants. The simultaneous C–V results showed a similar trend that the frequency dispersion of the simultaneous C–V was dramatically reduced with water vapor annealing treatments, and the interface quality of Al2O3/diamond had a slight dependence on the water vapor annealing times. Based on simultaneous C–V measurements, the interface state density (Dit) at an energy level of 0.2–0.6 eV from the valence band edge of diamond was extracted for the different MOS capacitors. The Dit was reduced by one order of magnitude with water vapor annealing treatments, and it almost did not change with the water vapor annealing times. Besides, the flat band voltage shift and effective fixed charge were also dramatically reduced by water vapor annealing. The possible physical reason for the interface improvement by water vapor annealing treatments was discussed.

Sequential Deposition Processing to Control Vertical Composition Distribution toward High‐Performance and Stable Inverted Organic Solar Cells

AbstractSequential deposition (SD) is widely applied in the fabrication of high‐performance organic solar cells (OSCs). Nevertheless, the feasibility of SD in inverted photovoltaic devices is rarely mentioned. Moreover, the vertical component distribution in SD‐processed devices is still under debate. This study demonstrates that SD can achieve the entire bulk heterojunction morphology by precisely controlling the morphology of the active layer. To further promote photovoltaic performance, two post treatments, thermal annealing (TA) and solvent vapor annealing (SVA), are conducted to compare their effects on SD‐processed inverted photovoltaic devices. The results verify that SD can significantly enhance the thermal stability of devices. This work provides a valuable insight for systematically understanding SD processing and the preparation of stable inverted OSCs with superior performance.