Effect of Passivating Molecules and Antisolvents on Lifetime of Green Dion-Jacobson Perovskite Light-Emitting Diodes.

We investigated the influence of two passivating molecules containing a P═O group on the performance of quasi-2D Dion-Jacobson halide perovskite light-emitting diodes, namely, triphenylphosphine oxide (TPPO) and diphenyl-4-triphenylsilylphenyl phosphine oxide (TSPO1). We found that both passivating molecules lead to increased efficiency compared to control devices, while they had opposite effects on device lifetime, with a decrease observed for TPPO and an increase observed for TSPO1. The two passivating molecules resulted in differences in energy-level alignment, electron injection, film morphology and crystallinity, and ion migration during operation. While TPPO resulted in improved photoluminescence decay times, overall higher maximum external quantum efficiency (EQE) and device lifetime were obtained for TSPO1 compared to TPPO (14.4% vs 12.4% EQE, 341 min vs 42 min T50).

Dual Surface Modifications of NiOx/Perovskite Interface for Enhancement of Device Stability.

Various phosphonic acid based self-assembled monolayers (SAMs) have been commonly used for interface modifications in inverted perovskite solar cells. This typically results in significant enhancement of the hole extraction and consequent increase in the power conversion efficiency. However, the surface coverage and packing density of SAM molecules can vary, depending on the chosen SAM material and underlying oxide layer. In addition, different SAM molecules have diverse effects on the interfacial energy level alignment and perovskite film growth, resulting in complex relationships between surface modification, efficiency, and lifetime. Here we show that ethanolamine surface modification combined with [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) results in significant improvement in device stability compared to devices with 2PACz modification only. The significantly smaller size of ethanolamine enables it to fill any gaps in 2PACz coverage and provide improved interfacial defect passivation, while its different chemical structure enables it to provide complementary effects to 2PACz passivation. Consequently, the perovskite films are more stable under illumination (slower photoinduced segregation), and the devices exhibit significant stability enhancement. Despite similar power conversion efficiencies (PCE) between 2PACz only and combined ethanolamine-2PACz modification (PCE of champion devices ∼21.6-22.0% for rigid and ∼20.2-21.0% for flexible devices), the T80 lifetime under simulated solar illumination in ambient is improved more than 15 times for both rigid and flexible devices.

Encapsulation and Stability Testing of Perovskite Solar Cells for Real Life Applications.

With the progress in the development of perovskite solar cells, increased efforts have been devoted to enhancing their stability. With more devices being able to survive harsher stability testing conditions, such as damp heat or outdoor testing, there is increased interest in encapsulation techniques suitable for this type of tests, since both device architecture compatible with increased stability and effective encapsulation are necessary for those testing conditions. A variety of encapsulation techniques and materials have been reported to date for devices with different architectures and tested under different conditions. In this Perspective, we will discuss important factors affecting the encapsulation effectiveness and focus on the devices, which have been subjected to outdoor testing or damp heat testing. In addition to encapsulation requirements for these testing conditions, we will also discuss device requirements. Finally, we discuss possible methods for accelerating the testing of encapsulation and device stability and discuss the future outlook and important issues, which need to be addressed for further advancement of the stability of perovskite solar cells.

Hydrophobic Surface Coating Can Reduce Toxicity of Zinc Oxide Nanoparticles to the Marine Copepod Tigriopus japonicus.

Coated zinc oxide nanoparticles (ZnO-NPs) are more commonly applied in commercial products but current risk assessments mostly focus on bare ZnO-NPs. To investigate the impacts of surface coatings, this study examined acute and chronic toxicities of six chemicals, including bare ZnO-NPs, ZnO-NPs with three silane coatings of different hydrophobicity, zinc oxide bulk particles (ZnO-BKs), and zinc ions (Zn-IONs), toward a marine copepod, Tigriopus japonicus. In acute tests, bare ZnO-NPs and hydrophobic ZnO-NPs were less toxic than hydrophilic ZnO-NPs. Analyses of the copepod's antioxidant gene expression suggested that such differences were governed by hydrodynamic size and ion dissolution of the particles, which affected zinc bioaccumulation in copepods. Conversely, all test particles, except the least toxic hydrophobic ZnO-NPs, shared similar chronic toxicity as Zn-IONs because they mostly dissolved into zinc ions at low test concentrations. The metadata analysis, together with our test results, further suggested that the toxicity of coated metal-associated nanoparticles could be predicted by the hydrophobicity and density of their surface coatings. This study evidenced the influence of surface coatings on the physicochemical properties, toxicity, and toxic mechanisms of ZnO-NPs and provided insights into the toxicity prediction of coated nanoparticles from their coating properties to improve their future risk assessment and management.

Structure-Dependent Photoluminescence in Low-Dimensional Ethylammonium, Propylammonium, and Butylammonium Lead Iodide Perovskites.

Hybrid organic-inorganic perovskites have attracted great attention as the next generation materials for photovoltaic and light-emitting devices. However, their environment instability issue remains as the largest challenge for practical applications. Recently emerging two-dimensional (2D) perovskites with Ruddlesden-Popper structures are found to greatly improve the stability and aging problems. Furthermore, strong confinement of excitons in these natural quantum-well structures results in the distinct and narrow light emission in the visible spectral range, enabling the development of spectrally tunable light sources. Besides the strong quasi-monochromatic emission, some 2D perovskites composed of the specific organic cations and inorganic layer structures emit a pronounced broadband emission. Herein, we report the light-emitting properties and the degradation of low-dimensional perovskites consisting of the three shortest alkylammonium spacers, mono-ethylammonium (EA), n-propylammonium (PA), and n-butylammonium (BA). While (BA)2PbI4 is known to form well-oriented 2D thin films consisting of layers of corner-sharing PbI6 octahedra separated by a bilayer of BA cations, EA with shorter alkyl chains tends to form other types of lower-dimensional structures. Nevertheless, optical absorption edges of as-prepared fresh EAPbI3, (PA)2PbI4, and (BA)2PbI4 are obviously blue-shifted to 2.4-2.5 eV compared to their 3D counterpart, methylammonium lead iodide (MAPbI3) perovskite, and they all emit narrow excitonic photoluminescence. Furthermore, by carefully optimizing deposition conditions, we have achieved a predominantly 2D structure for (PA)2PbI4. However, unlike (BA)2PbI4, upon exposure to ambient environment, (PA)2PbI4 readily transforms to a different crystal structure, exhibiting a prominently broadband light from ∼500 to 800 nm and a gradual increase in intensity as structural transformation proceeds.

Novel Molecular Doping Mechanism for n-Doping of SnO2 via Triphenylphosphine Oxide and Its Effect on Perovskite Solar Cells.

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n-type doping of oxide semiconductors. Herein, successful n-type doping of SnO2 is demonstrated by a simple, air-robust, and cost-effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P+ O- σ-bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi-photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO2 , which enlarges the built-in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO2 /perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

Generation of highly reactive oxygen species on metal-supported MgO(100) thin films.

The formation of highly reactive oxygen species (ROS) on metal oxide surfaces has attracted considerable interest due to their diverse applications. In this work, we have performed density functional theory calculations to investigate the co-adsorption of oxygen and water on ultrathin MgO(100) films deposited on a Mo(100) substrate. We reveal that molecular oxygen can be completely decomposed stepwise with the assistance of water. Consequently, a series of highly ROS including superoxide, hydroperoxide, hydroxyl and single oxygen adatoms are formed on Mo(100) supported MgO(100) thin films. The reaction barriers accompanied by the generation of ROS are reported, and the influence of the thickness of MgO(100) films is also discussed. The promising routes to produce these species provide valuable information to understand the importance of synergy effects between the substrate, the co-adsorbed species, and the film thickness in multiphase catalyst design.

Encapsulation of Perovskite Solar Cells for High Humidity Conditions.

We examined different encapsulation strategies for perovskite solar cells by testing the device stability under continuous illumination, elevated temperature (85 °C) and ambient humidity of 65 %. The effects of the use of different epoxies, protective layers and the presence of desiccant were investigated. The best stability (retention of ∼80 % of initial efficiency on average after 48 h) was obtained for devices protected by a SiO2 film and encapsulated with a UV-curable epoxy including a desiccant sheet. However, the stability of ZnO-based cells encapsulated by the same method was found to be inferior to that of TiO2 -based cells. Finally, outdoor performance tests were performed for TiO2 -based cells (30-90 % ambient humidity). All the stability tests were performed following the established international summit on organic photovoltaic stability (ISOS) protocols for organic solar cell testing (ISOS-L2 and ISOS-O1).

Metal oxide nanoparticles with low toxicity.

A number of different nanomaterials produced and incorporated into various products are rising. However, their environmental hazards are frequently unknown. Here we consider three different metal oxide compounds (SnO2, In2O3, and Al2O3), which have not been extensively studied and are expected to have low toxicity. This study aimed to comprehensively characterize the physicochemical properties of these nanomaterials and investigate their toxicity on bacteria (Escherichia coli) under UV illumination and in the dark, as well as on a marine diatom (Skeletonema costatum) under ambient illumination/dark (16-8h) cycles. The material properties responsible for their low toxicity have been identified based on comprehensive experimental characterizations and comparison to a metal oxide exhibiting significant toxicity under illumination (anatase TiO2). The metal oxide materials investigated exhibited significant difference in surface properties and interaction with the living organisms. In order for a material to exhibit significant toxicity, it needs to be able to both form a stable suspension in the culture medium and to interact with the cell walls of the test organism. Our results indicated that the observed low toxicities of the three nanomaterials could be attributed to the limited interaction between the nanoparticles and cell walls of the test organisms. This could occur either due to the lack of significant attachment between nanoparticles and cell walls, or due to their tendency to aggregate in solution.

In situ synthesis of CuxO/SnOx@CNT and CuxO/SnOx@SnO2/CNT nanocomposite anodes for lithium ion batteries by a simple chemical treatment process.

SnO2-based electrodes for lithium ion batteries (LIBs) typically exhibit high initial specific capacity but poor cycling performance. A possible strategy to improve the cycling performance is to prepare nanocomposites containing SnO2. Here we demonstrate a straightforward method to prepare composites containing SnOx and CuxO by a simple chemical treatment of the LIB electrode on copper foil. The in situ formation of a multiphase composite results in a dramatic improvement in the cycling performance, so that specific capacities exceeding 580 and 800 mA·h/g can be obtained after 70 charge/discharge cycles for CuxO/SnOx@CNT and CuxO/SnOx@SnO2/CNT electrodes, respectively (compared to <100 mA·h/g for pure SnO2). The capacity retention achieved at the 70th cycle compared to the 2nd cycle was 96% for the CuxO/SnOx@CNT electrode. The mechanisms responsible for the formation of a composite material and the improvement in the performance are discussed.

Antibacterial and photocatalytic activity of TiO2 and ZnO nanomaterials in phosphate buffer and saline solution.

We studied antibacterial and photocatalytic activity of anatase TiO2 and ZnO in phosphate buffer and saline solution. We found that the different anions in the suspension medium (chloride and phosphate) significantly affected the following suspension properties: the stability of nanoparticle suspension, the release of metal ions from the nanoparticles, and the production of the reactive oxygen species by the nanoparticles. As a result, antibacterial activity and photocatalytic dye degradation were also affected. However, the effect of the suspension medium was different for ZnO and TiO2. Obtained results are discussed.

Effect of ZnO nanoparticle properties on dye-sensitized solar cell performance.

We have investigated the effect of ZnO nanoparticle properties on the dye-sensitized solar cell performance. Nanoparticles with different sizes and optical properties were considered. We found that there is a complex relationship between native defects, dye adsorption, charge transport and solar cell performance. The presence of a high concentration of nonradiative defects was found to be detrimental to photovoltaic performance, whereas for radiative defects, samples displaying orange-red defect emission exhibited better performance compared to samples with green defect emission (when the samples had similar emission intensities). Detailed discussion of the nanoparticle properties and their relationship with dye adsorption, electron injection, electron lifetime, electron transport time, and solar cell performance is given.

Ruthenium complex containing block copolymer for the enhancement of carbon nanotube photoconductivity.

We report the synthesis of a multifunctional block copolymer incorporated with pyrene and ruthenium terpyridyl thiocyanato complex moieties by reversible addition-fragmentation chain transfer polymerization. The pyrene block in the copolymer facilitates the dispersion of multiwalled carbon nanotubes in DMF solution because of the strong π-π interaction between the pyrene moieties and nanotube surface. On the other hand, the ruthenium complexes greatly enhance the photosensitivity of the functionalized nanotubes in the visible region. The photocurrent responses of the nanotubes at different wavelength measured by conductive AFM spectrum strongly agree with the absorption spectrum of the ruthenium complex. The results demonstrate a new and versatile approach in enhancing and fine-tuning the photosensitivity or other opto-electronic properties of carbon nanotubes by multifunctional block copolymers.

Indium tin oxide nanorod electrodes for polymer photovoltaics.

We have deposited indium tin oxide (ITO) nanorods on glass and glass/ITO substrates by DC sputtering and by e-beam deposition. The properties of the nanorods deposited by different methods and on different substrates have been investigated. The ITO nanorods were also used as an electrode in bulk heterojunction polymer solar cells. We found that the nanorod morphology and sheet resistance had a significant effect on the solar cell performance, with significant improvements in the efficiency compared to commercial ITO film substrates in all cases except for e-beam deposited nanorods on glass that had high sheet resistance. The best power conversion efficiency achieved was 3.2 % (for sputtered ITO nanorods on ITO), compared to 2.1 % for commercial ITO substrates.

Multifunctional Poly(N-vinylcarbazole)-Based Block Copolymers and their Nanofabrication and Photosensitizing Properties.

The synthesis of poly(N-vinylcarbazole)-based block copolymers functionalized with rhenium diimine complexes or pendant terpyridine ligands is reported. The copolymers are synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, and they exhibit interesting morphological properties as a result of the phase separation between different blocks. The rhenium complex polymer block may function as a photosensitizer, while the terpyridine-containing polymer block can be used as the template for nanofabrication by selective deposition of zinc complexes.

Tuning the absorption, charge transport properties, and solar cell efficiency with the number of thienyl rings in platinum-containing poly(aryleneethynylene)s.

The synthesis, characterization, and photophysics of a series of solution-processable and strongly visible-light absorbing platinum(II) polyynes containing bithiazole-oligo(thienyl) rings were presented. Tuning the polymer solar cell efficiency, as well as optical and charge transport properties, in soluble, low-band gap PtII-based conjugated poly(heteroaryleneethynylene)s using the number of oligothienyl rings is described. These materials are highly soluble in polar organic solvents due to the presence of solubilizing bithiazole moieties and show strong absorptions in the solar spectra, rendering them excellent candidates for bulk heterojunction polymer solar cells. Their photovoltaic responses and power conversion efficiencies (PCEs) depend to a large extent on the number of thienyl rings along the main chain, and some of them can be used to fabricate highly efficient solar cells with PCEs of up to 2.7% and a peak external quantum efficiency to 83% under AM1.5 simulated solar illumination, which is comparable to that of poly(3-hexylthiophene)-based devices fabricated without additional processing (annealing or TiO(x) layer). The influence of the number of thienyl rings and the metal group on the performance parameters and optimization of solar cell efficiency was evaluated and discussed in detail. At the same blend ratio of 1:4, the light-harvesting ability and PCE increase sharply as the thienyl chain length increases. The present work provides an attractive approach to developing conjugated metallopolymers offering broad solar absorptions and tunable solar cell efficiency and demonstrates the potential of metalated conjugated polymers for efficient power generation.