Control of the resistive switching voltage and reduction of the high-resistive-state current of zinc oxide by self-assembled monolayers.

We investigated the effect of self-assembled monolayer (SAM) modification of ZnO on the resistive switching behaviour by fabricating electrode-sandwiched devices (ITO/ZnO-SAM/Al). The resistive switching voltages of SAM-modified ZnO films were shifted from that of bare ZnO depending on the surface dipole induced by the SAMs. In particular, methylaminopropyl-substituted SAM-modified ZnO showed lower switching voltage (1.6 V) than bare ZnO (2.9 V). Moreover, the on/off ratio was also improved by SAM modification (from 102 to 104).

Investigation of Degradation Mechanism of Y6-Based Inverted Organic Solar Cells and Their Utilization in Durable Near-Infrared Photodetection.

The device durability of inverted organic solar cells (OSCs) is investigated based on Y6, which is an effective nonfullerene acceptor for high-performance OSCs. The durability of Y6-based inverted OSCs is poor and it can be caused by aggregation of Y6 in the bulk-heterojunction layer due to heating by continuous photo-irradiation (≈65 °C, 100 mW cm-2 , and 72 h). It is found that the aggregation of Y6 is suppressed at a low temperature (≈50 °C), and that the Y6-based devices can be useful as a photodurable near-infrared detector upon continuous laser irradiation.

Local Cross-Coupling Activity of Azide-Hexa(ethylene glycol)-Terminated Self-Assembled Monolayers Investigated by Atomic Force Microscopy.

Azide-oligo(ethylene glycol)-terminated self-assembled monolayers (N3-OEG-SAMs) are promising interfacial structures for surface functionalization. Its many potential applications include chemical/bio-sensing and construction of surface models owing to its cross-coupling activity that originates from the azide group and oligo(ethylene glycol) (OEG) units for non-specific adsorption resistance. However, there are only a few studies and limited information, particularly on the molecular-scale structures and local cross-coupling activities of N3-OEG-SAMs, which are vital to understanding its surface properties and interfacial molecular design. In this study, molecular-scale surface structures and cross-coupling activity of azide-hexa(ethylene glycol)-terminated SAMs (N3-EG6-SAMs) were investigated using frequency modulation atomic force microscopy (FM-AFM) in liquid. The N3-EG6-SAMs were prepared on Au(111) substrates through the self-assembly of 11-azido-hexa(ethylene glycol)-undecane-1-thiol (N3-EG6-C11-HS) molecules obtained from a liquid phase. Subnanometer-resolution surface structures were visualized in an aqueous solution using a laboratory-built FM-AFM instrument. The results show a well-ordered molecular arrangement in the N3-EG6-SAM and its clean surfaces originating from the adsorption resistance property of the terminal EG6 units. Surface functionalization by the cross-coupling reaction of copper(I)-catalyzed azide-alkyne cycloaddition was observed, indicating a structural change in the form of fluctuating structures and island-shaped structures depending on the concentration of the alkyne molecules. The FM-AFM imaging enabled to provide information on the relationship between the surface structures and cross-coupling activity. These findings provide molecular-scale information on the functionalization of the N3-EG6-SAMs, which is helpful for the interfacial molecular design based on alkanethiol SAMs in many applications.

Paste Aging Spontaneously Tunes TiO2 Nanoparticles into Reproducible Electrosprayed Photoelectrodes.

In this study, the spontaneous microstructure tuning of TiO2 was observed by aging the ethanol/water TiO2 paste for up to 20 days at ambient conditions. A dynamic light scattering study reveals that it formed the outstanding reproducible TiO2 microstructure with a ∼200 nm average particle size and stabilizes in 6 to 20 days under an ambient atmosphere. Interestingly, the as-deposited day 15 sample spontaneously changed its crystallinity upon keeping the paste at ambient conditions; meanwhile the day 0 sample showed an amorphous structure. A dense, uniform, and stable TiO2 electrode was cast on a fluorine doped-tin oxide substrate using the electrospray technique. We exploit the spontaneous evolution of the TiO2 nanopowder to revisit the fabrication procedure of the TiO2 photoelectrode for dye-sensitized solar cells (DSSCs). The controlled microstructure TiO2 film was used in DSSCs, which, to the best of our knowledge, achieved the highest power conversion efficiency of 9.65% using N719 dye in sensitizing the TiO2 photoanode.

Spray Pyrolyzed TiO2 Embedded Multi-Layer Front Contact Design for High-Efficiency Perovskite Solar Cells.

The photovoltaic performance of perovskite solar cells (PSCs) can be improved by utilizing efficient front contact. However, it has always been a significant challenge for fabricating high-quality, scalable, controllable, and cost-effective front contact. This study proposes a realistic multi-layer front contact design to realize efficient single-junction PSCs and perovskite/perovskite tandem solar cells (TSCs). As a critical part of the front contact, we prepared a highly compact titanium oxide (TiO2) film by industrially viable Spray Pyrolysis Deposition (SPD), which acts as a potential electron transport layer (ETL) for the fabrication of PSCs. Optimization and reproducibility of the TiO2 ETL were discreetly investigated while fabricating a set of planar PSCs. As the front contact has a significant influence on the optoelectronic properties of PSCs, hence, we investigated the optics and electrical effects of PSCs by three-dimensional (3D) finite-difference time-domain (FDTD) and finite element method (FEM) rigorous simulations. The investigation allows us to compare experimental results with the outcome from simulations. Furthermore, an optimized single-junction PSC is designed to enhance the energy conversion efficiency (ECE) by > 30% compared to the planar reference PSC. Finally, the study has been progressed to the realization of all-perovskite TSC that can reach the ECE, exceeding 30%. Detailed guidance for the completion of high-performance PSCs is provided.

Selective Extraction of Nonfullerene Acceptors from Bulk-Heterojunction Layer in Organic Solar Cells for Detailed Analysis of Microstructure.

Detailed analyses of the microstructures of bulk-heterojunction (BHJ) layers are important for the development of high-performance photovoltaic organic solar cells (OSCs). However, analytical methods for BHJ layer microstructures are limited because BHJ films are composed of a complex mixture of donor and acceptor materials. In our previous study on the microstructure of a BHJ film composed of donor polymers and fullerene-based acceptors, we analyzed donor polymer-only films after selectively extracting fullerene-based acceptors from the film by atomic force microscopy (AFM). Not only was AFM suitable for a clear analysis of the morphology of the donor polymers in the BHJ film, but it also allowed us to approximate the acceptor morphology by analyzing the pores in the extracted films. Herein we report a method for the selective extraction of nonfullerene acceptors (NFAs) from a BHJ layer in OSCs and provide a detailed analysis of the remaining BHJ films based upon AFM. We found that butyl glycidyl ether is an effective solvent to extract NFAs from BHJ films without damaging the donor polymer films. By using the selective extraction method, the morphologies of NFA-free BHJ films fabricated under various conditions were studied in detail. The results may be useful for the optimization of BHJ film structures composed of NFAs and donor polymers.

Ionic Liquid-Assisted MAPbI3 Nanoparticle-Seeded Growth for Efficient and Stable Perovskite Solar Cells.

With the rapid improvement of perovskite solar cells (PSCs), long-life operational stability has become a major requirement for their commercialization. In this work, we devised a pristine cesium-formamidinium-methylammonium (termed as CsFAMA) triple-cation-based perovskite precursor solution into the ionic liquid (IL)-assisted MAPbI3 nanoparticles (NPs) through a seeded growth approach in which the host IL-assisted MAPbI3 NPs remarkably promote high-quality perovskite films with large single-crystal domains, enhancing the device performance and stability. The power conversion efficiency (PCE) of the MAPbI3 NP-seeded growth of MAPbI3 NPs/CsFAMA-based PSCs is as high as 19.44%, which is superior to those of MAPbI3 NPs and pristine CsFAMA films as the photoactive layer (9.52 and 17.33%, respectively). The long-term light-soaking and moisture stability of IL-aided MAPbI3 NPs/CsFAMA-based devices (non-encapsulated) remain above 90 and 80%, respectively, of their initial output after 2 h of light illumination (1 sun) and 6000 h storage at ambient with a relative humidity range of 30-40%. The use of the IL-assisted MAPbI3 NP-seeded growth for PSCs is a significant step toward developing stable and reliable perovskite photovoltaic devices.

Naphthalene diimide-incorporated helical thienoacene: a helical molecule with high electron mobility, good solubility, and thermally stable solid phase.

A Naphthalene diimide (NDI)-incorporated helical thienoacene was developed. The compound has high electron mobility (1.4 cm2 V-1 s-1) thanks to its two-dimensional π-π interaction assisted by the intermolecular C-HO hydrogen bonding of the NDI moieties. Moreover, its bow-shaped π-skeleton reduces molecular fluctuation and gives the compound a thermally stable solid phase, which enables us to fabricate thermally stable organic devices.

Low-Temperature Processed TiOx Electron Transport Layer for Efficient Planar Perovskite Solar Cells.

The most frequently used n-type electron transport layer (ETL) in high-efficiency perovskite solar cells (PSCs) is based on titanium oxide (TiO2) films, involving a high-temperature sintering (>450 °C) process. In this work, a dense, uniform, and pinhole-free compact titanium dioxide (TiOx) film was prepared via a facile chemical bath deposition process at a low temperature (80 °C), and was applied as a high-quality ETL for efficient planar PSCs. We tested and compared as-deposited substrates sintered at low temperatures (< 150 °C) and high temperatures (> 450 °C), as well as their corresponding photovoltaic properties. PSCs with a high-temperature treated TiO2 compact layer (CL) exhibited power conversion efficiencies (PCEs) as high as 15.50%, which was close to those of PSCs with low-temperature treated TiOx (14.51%). This indicates that low-temperature treated TiOx can be a potential ETL candidate for planar PSCs. In summary, this work reports on the fabrication of low-temperature processed PSCs, and can be of interest for the design and fabrication of future low-cost and flexible solar modules.

Metal Oxide Compact Electron Transport Layer Modification for Efficient and Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs) have appeared as a promising design for next-generation thin-film photovoltaics because of their cost-efficient fabrication processes and excellent optoelectronic properties. However, PSCs containing a metal oxide compact layer (CL) suffer from poor long-term stability and performance. The quality of the underlying substrate strongly influences the growth of the perovskite layer. In turn, the perovskite film quality directly affects the efficiency and stability of the resultant PSCs. Thus, substrate modification with metal oxide CLs to produce highly efficient and stable PSCs has drawn attention. In this review, metal oxide-based electron transport layers (ETLs) used in PSCs and their systemic modification are reviewed. The roles of ETLs in the design and fabrication of efficient and stable PSCs are also discussed. This review will guide the further development of perovskite films with larger grains, higher crystallinity, and more homogeneous morphology, which correlate to higher stable PSC performance. The challenges and future research directions for PSCs containing compact ETLs are also described with the goal of improving their sustainability to reach new heights of clean energy production.

Temperature-dependent changes in the molecular orientation and visible color of phthalocyanine films.

A simple phthalocyanine zinc complex exhibits a visible color change in response to weak external stimuli, i.e., changes in solvent and temperature. Its chromism was attributed to its controlled aggregation via weak interactions between the central metal and peripheral oxygen atoms. In solution, intense absorption and fluorescence bands appeared in both the longer-wavelength and NIR region in non-coordinating solvents, while a simple sharp Q band was observed in coordinating solvents. Variable-temperature absorption spectra and fluorescence lifetime measurements were used to characterize the aggregation-induced absorption and emission in non-coordinating solvents. A selective aggregation-disaggregation process was also observed in thin films of this phthalocyanine zinc complex, and the optical properties of the film depend on the annealing temperature. The changes in the NIR region lead to a sizable visible color change that is recognizable by the naked human eye. The temperature-dependent control of the aggregation process in the thin film was confirmed using operando spectroscopy techniques.

Oblique Electrostatic Inkjet-Deposited TiO2 Electron Transport Layers for Efficient Planar Perovskite Solar Cells.

In this study, a new, simple, and novel oblique electrostatic inkjet (OEI) technique is developed to deposit a titanium oxide (TiO2) compact layer (CL) on fluorine-doped tin oxide (FTO) substrate without the need for a vacuum environment for the first time. The TiO2 is used as electron transport layers (ETL) in planar perovskite solar cells (PSCs). This bottom-up OEI technique enables the control of the surface morphology and thickness of the TiO2 CL by simply manipulating the coating time. The OEI-fabricated TiO2 is characterized tested and the results are compared with that of TiO2 CLs produced by spin-coating and spray pyrolysis. The OEI-deposited TiO2 CL exhibits satisfactory surface coverage and smooth morphology, conducive for the ETLs in PSCs. The power-conversion efficiencies of PSCs with OEI-deposited TiO2 CL as the ETL were as high as 13.19%. Therefore, the present study provides an important advance in the effort to develop simple, low-cost, and easily scaled-up techniques. OEI may be a new candidate for depositing TiO2 CL ETLs for highly efficient planar PSCs, thus potentially contributing to future mass production.

High performance photoanodic catalyst prepared from an active organic photovoltaic cell - high potential gain from visible light.

Photoelectrochemical oxidation of thiols was enhanced with a threshold potential of -0.35 V vs. Ag/AgCl by the use of a ZnPc/PCBM:P3HT/ZnO electode, which was prepared by removing the PEDOT:PSS/Au electrode of an inverted OPV device and coating it with ZnPc. A co-photocatalysis property of ZnPc was observed in the photoelectrochemistry and scanning Kelvin probe microscopy.

One-Step Synthesis of One-Dimensional Supramolecular Assemblies Composed of Helical Macromolecular Building Blocks.

Living systems achieve sophisticated functions using supramolecular protein assemblies, in which the protein building blocks possess a specific secondary structure and are noncovalently arranged in a preprogrammed manner. Herein, we demonstrate the one-step synthesis of one-dimensional macromolecular assemblies by simply mixing a glycine-based isocyanide with a nickel catalyst, in which helical constituent polymers are linked end-to-end through multiple hydrogen bonds. The applicable scope of this approach is not confined to a particular monomer bearing a specially designed pendant, but covers a wide range of glycine-based isocyanides with or without aromatic and other functional groups. Surprisingly, copolymerization with an analogous chiral isocyanide (1 mol %) afforded an almost perfect one-handed helical supramolecular fiber owing to intramolecular/intermolecular dual chiral amplifications. The simplicity and broad applicability of this approach, which can also afford exquisite chiral amplification, enable the creation of a wide variety of functional supramolecular assemblies and provide access to new supramolecular materials.

Low-Temperature-Processed Brookite-Based TiO2 Heterophase Junction Enhances Performance of Planar Perovskite Solar Cells.

In the design of electron-transport layers (ETLs) to enhance the efficiency of planar perovskite solar cells (PSCs), facile electron extraction and transport are important features. Here, we consider the effects of different titanium oxide (TiO2) polymorphs, anatase and brookite. We design and fabricate high-phase-purity, single-crystalline, highly conductive, and low-temperature (<180 °C)-processed brookite-based TiO2 heterophase junctions on fluorine-doped tin oxide (FTO) as the substrate. We test and compare single-phase anatase (A) and brookite (B) and heterophase anatase-brookite (AB) and brookite-anatase (BA) as ETLs in PSCs. The power-conversion efficiencies (PCEs) of PSCs with low-temperature-processed single-layer FTO-B as the ETL were as high as 14.92%, which is the highest reported efficiency of FTO-B-based single-layer PSC. This implies that FTO-B serves as an active phase and can be a potential candidate as an n-type ETL scaffold in planar PSCs. Moreover, the surface of highly crystalline brookite TiO2 exhibits a tendency toward interparticle necking, leading to the formation of compact scaffolds. Furthermore, PSCs with heterophase junction FTO-AB ETLs exhibited PCEs as high as 16.82%, which is superior to those of PSCs with single-phase anatase (FTO-A) and brookite (FTO-B) as the ETLs (13.86% and 14.92%, respectively). In addition, the PSCs with FTO-AB exhibited improved efficiency and decreased hysteresis compared with those with FTO-BA (13.45%) due to the suitable band alignment with the perovskite layer, which resulted in superior photogenerated charge-carrier extraction and reduced charge accumulation at the interface between the heterophase junction and perovskite. Thus, the present work presents an effective strategy by which to develop heterophase junction ETLs and manipulate the interfacial energy band to further improve the performance of planar PSCs and enable the clean and eco-friendly fabrication of low-cost mass production.

Lignocellulose nanofibers prepared by ionic liquid pretreatment and subsequent mechanical nanofibrillation of bagasse powder: Application to esterified bagasse/polypropylene composites.

In the present study, we examined the efficacy of choline acetate (ChOAc, a cholinium ionic liquid))-assisted pretreatment of bagasse powder for subsequent mechanical nanofibrillation to produce lignocellulose nanofibers. Bagasse sample with ChOAc pretreatment and subsequent nanofibrillation (ChOAc/NF-bagasse) was prepared and compared to untreated control bagasse sample (control bagasse), bagasse sample with nanofibrillation only (NF-bagasse) and with ChOAc pretreatment only (ChOAc-bagasse). The specific surface area was 0.83m2/g, 3.1m2/g, 6.3m2/g, and 32m2/g for the control bagasse, ChOAc-bagasse, NF-bagasse, and the ChOAc/NF-bagasse, respectively. Esterified bagasse/polypropylene composites were prepared using the bagasse samples. ChOAc/NF-bagasse exhibited the best dispersion in the composites. The tensile toughness of the composites was 0.52J/cm3, 0.73J/cm3, 0.92J/cm3, and 1.29J/cm3 for the composites prepared using control bagasse, ChOAc-bagasse, NF-bagasse, and ChOAc/NF-bagasse, respectively. Therefore, ChOAc pretreatment and subsequent nanofibrillation of bagasse powder resulted in enhanced tensile toughness of esterified bagasse/polypropylene composites.

Identifying Molecular Orientation in a Bulk Heterojunction Film by Infrared Reflection Absorption Spectroscopy.

The molecular orientation of organic molecules of zinc phthalocyanine (ZnPc) in single-component films on copper iodide (CuI) substrates can be controlled to achieve a molecular orientation lying flat on the substrate (flat-on) owing to π-d orbital interactions between the ZnPc molecules and the CuI. A 3-fold enhancement in the performance of organic photovoltaic cells has been reported by introducing a CuI interlayer between a ZnPc:fullerene (C60) bulk heterojunction (BHJ) film and the substrate. However, the mechanism underpinning the resultant solar cell performance enhancement was unclear. Herein, we report on the results of using in situ reflection absorption spectroscopy measurements during the vacuum deposition of coevaporated ZnPc:C60 BHJ films on various substrates to investigate the ZnPc molecular orientation. Our results revealed that the flat-on molecular orientation of ZnPc molecules in ZnPc:C60 BHJ films on CuI interlayers and flat-on ZnPc substrates can be successfully identified via the strong π-π interactions between the BHJ film and the substrate. The π-π interactions between individual ZnPc molecules are stronger than the π-d interactions between ZnPc molecules and CuI in coevaporated ZnPc:C60 films, as is evident from the molecular orientation of ZnPc, as determined by in situ reflection absorption spectroscopy. Our findings demonstrate that precisely controlling the molecular orientations of the films could enhance organic photovoltaic (OPV) performance. The present work provides important insights that will enable the design of higher performance OPV cells.

Mechanism of Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase.

In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage (V oc) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT•+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of V oc. From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation.

Improved Reproducibility and Intercalation Control of Efficient Planar Inorganic Perovskite Solar Cells by Simple Alternate Vacuum Deposition of PbI2 and CsI.

Vacuum deposition is a simple and controllable approach that aims to form higher-quality perovskite films compared with those formed using solution-based deposition processes. Herein, we demonstrate a novel method to promote the intercalation control of inorganic cesium lead iodide (CsPbI3) perovskite thin films via alternate vacuum deposition. We also investigated the effect of layer-by-layer deposition of PbI2/CsI to fabricate efficient planar heterojunction CsPbI3 thin films and solar cells. This procedure is comparatively simple when compared with commonly used coevaporation techniques; further, precise intercalation control of the CsPbI3 thin films can be achieved by increasing the number of layers in the layer-by-layer deposition of PbI2/CsI. The best control and the highest reproducibility were achieved for the deposition of four double layers owing to the precise intercalation control during the deposition of the CsPbI3 thin film. A power conversion efficiency of 6.79% was obtained via alternating vacuum deposition of two double layers with a short-circuit current density (J sc) of 12.06 mA/cm2, an open-circuit voltage (V oc) of 0.79 V, and a fill factor (FF) of 0.72. Our results suggest a route for inorganic precursors to be used for efficient perovskite solar cells via alternating vacuum deposition.

Efficient small-molecule photovoltaic cells using a crystalline diindenoperylene film as a nanostructured template.

A cascade-type small-molecule organic photovoltaic cell using a crystalline diindenoperylene film as a nanostructured template is demonstrated. This cell architecture simultaneously realizes organic nanostructure and cascade energy concepts, which significantly improves the photocurrent generation and fill factor, leading to a power conversion efficiency of 5.2±0.3%.