Layer-dependent anisotropic frictional behavior in two-dimensional monolayer hybrid perovskite/ITO layered heterojunctions.

Two-dimensional (2D) organic-inorganic hybrid perovskites, which possess outstanding optical and electrical properties, are promising semiconductor materials that have attracted significant interest in widespread applications. The frictional behavior of 2D perovskite materials with other transparent conductive materials, such as indium tin oxide (ITO), offers promising developments in optoelectronic devices. Therefore, the understanding of this frictional behavior is essential. Atomic force microscopy (AFM) is employed here to measure the frictional behavior between the (001) plane of the 2D organic-inorganic hybrid (C4H9NH3)2PbBr4 perovskite and the (111) plane of the ITO. The experimental analyses characterizing the nature of the friction in a single-crystalline heterojunction are reported. Based on the results of the analyses of interfaces between 2D monolayer perovskites and ITO, a strong anisotropy of friction is clearly demonstrated. The anisotropy of friction is observed as a four-fold symmetry with low a frictional coefficient, 0.035, in misaligned contacts, and, 0.015, in aligned contacts in the heterojunction configuration. In addition, atomistic simulations reveal underlying frictional mechanisms in the dynamical regimes. A new phenomenon discovered in the studies establishes that the measured frictional anisotropy surprisingly depends on the number of atomic layers in the 2D perovskite. The frictional anisotropy decreases significantly with the increase in the number of layers up to 16 layers, and then it becomes independent of the thickness. Our results are predicted to be of a general nature and should be applicable to other 2D hybrid perovskite heterojunction configurations, and thus, furthers the development of adaptive and stretchable optoelectronic nanodevices.

Size-dependent Young's modulus in ZnO nanowires with strong surface atomic bonds.

The mechanical properties of size-dependent nanowires are important in nano-electro-mechanical systems (NEMSs), and have attracted much research interest. Characterization of the size effect of nanowires in atmosphere directly to broaden their practical application instead of just in high vacuum situations, as reported previously, is desperately needed. In this study, we systematically studied the Young's modulus of vertical ZnO nanowires in atmosphere. The diameters ranged from 48 nm to 239 nm with a resonance method using non-contact atomic force microscopy. The values of Young's modulus in atmosphere present extremely strong increasing tendency with decreasing diameter of nanowire due to stronger surface atomic bonds compared with that in vacuum. A core-shell model for nanowires is proposed to explore the Young's modulus enhancement in atmosphere, which is correlated with atoms of oxygen occurring near the nanowire surface. The modified model is more accurate for analyzing the mechanical behavior of nanowires in atmosphere compared with the model in vacuum. Furthermore, it is possible to use this characterization method to measure the size-related elastic properties of similar wire-sharp nanomaterials in atmosphere and estimate the corresponding mechanical behavior. The study of the size-dependent Young's modulus in ZnO nanowires in atmosphere will improve the understanding of the mechanical properties of nanomaterials as well as providing guidance for applications in NEMSs, nanogenerators, biosensors and other related areas.

Promoting Large-Area Slot-Die-Coated Perovskite Solar Cell Performance and Reproducibility by Acid-Based Sulfono-γ-AApeptide.

Homogeneous and pinhole-free large-area perovskite films are required to realize the commercialization of perovskite modules and panels. Various large-area perovskite coatings were developed; however, at their film coating and drying stages, many defects were formed on the perovskite surface. Consequently, not only the devices lost substantial performance but also their long-term stability deteriorated. Here, we fabricated a compact and uniform large-area MAPbI3-perovskite film by a slot-die coater at room temperature (T) and at high relative humidity (RH) up to 40%. The control slot-die-coated perovskite solar cell (PSC) produced 1.082 V open-circuit voltage (Voc), 24.09 mA cm-2 short current density (Jsc), 71.13% fill factor (FF), and a maximum power conversion efficiency (PCE) of 18.54%. We systematically employed a multi-functional artificial amino acid (F-LYS-S) to modify the perovskite defects. Such amino acids are more inclined to bind and adhere to the perovskite defects. The amino, carbonyl, and carboxy functional groups of F-LYS-S interacted with MAPbI3 through Lewis acid-base interaction and modified iodine vacancies significantly. Fourier transform infrared spectroscopy revealed that the C═O group of F-LYS-S interacted with the uncoordinated Pb2+ ions, and X-ray photoelectron spectroscopy revealed that the lone pair of -NH2 coordinated with the uncoordinated Pb2+ and consequently modified the I- vacancies remarkably. As a result, the F-LYS-S-modified device demonstrated more than three-fold charge recombination resistance, which is one of the primary requirements to fabricate high-performance PSCs. Therefore, the device fabricated employing F-LYS-S demonstrated remarkable PCE of 21.08% with superior photovoltaic parameters of 1.104 V Voc, 24.80 mA cm-2 Jsc, and 77.00%. FF. Concurrently, the long-term stability of the PSCs was improved by the F-LYS-S post-treatment, where the modified device retained ca. 89.6% of its initial efficiency after storing for 720 h in air (T ∼ 27 °C and RH ∼ 50-60%).

Modifying Electrical and Magnetic Properties of Single-Walled Carbon Nanotubes by Decorating with Iron Oxide Nanoparticles.

In this work, we report a facile approach to modify the magnetic and electrical property of Hipco single-walled carbon nanotubes (SWCNTs) by attaching iron oxide nanoparticles (FeNPs). The Raman spectra of FeNP-SWCNTs revealed an apparent intensity reduction of metallic peaks in Radial breathing mode (RBM) region, which indicates the ratio change between the metallic and semiconducting component of SWCNTs after modification. Distinctive magnetic and electric resistance properties of FeNP-SWCNTs were found to be associated with the different shapes of FeNPs. The specific FeNP examples as showcased in this work can be applied to tune the electric and magnetic properties of other nanostructured materials in general and shed light on various electronic and magnetic applications.

Performance enhancement by vertical morphology alteration of the active layer in organic solar cells.

Bulk heterojunction organic solar cells (OSCs) have attracted worldwide attention due to their great potential as a green, flexible and low-cost renewable energy source. A vertical configuration in the active layer due to the aggregation of donor and acceptor molecules and the influence on the performance of OSCs deserve an in-depth study. In this study, five different vertical configurations of the active layer in OSCs were built up. The absorbance and indexes of the devices were theoretically analyzed. It was found that the configuration with the donor and acceptor molecules distributed equally exhibits the highest power conversion efficiency, followed by the configuration with the donor closer to the anode and the acceptor closer to the cathode, which matches experimental results well. Further analyses present the recombination, resistance, quantum efficiency and current leakage of all the configurations. It is anticipated that our results will promote the better understanding and development of the OSC field.

Effect of Donor-Acceptor Vertical Composition Profile on Performance of Organic Bulk Heterojunction Solar Cells.

In organic bulk heterojunction solar cells (OSCs) donor-acceptor vertical composition profile is one of the crucial factors that affect power-conversion efficiency (PCE). In this simulation study, five different kinds of donor-acceptor vertical configurations, including sandwich type I and type II, charge transport favorable, charge transport unfavorable, and uniform vertical distribution, have been investigated for both regular and inverted OSC structures. OSCs with uniform and charge transport favorable vertical composition profiles demonstrate the highest efficiencies. High PCE from charge transport favorable configuration can be attributed to low recombination because of facilitated charge transport in active layer and collection at electrodes, while high PCE from uniform structure is due to sufficient interfaces for efficient exciton dissociation. OSCs with sandwich and charge transport unfavorable structures show much lower efficiencies. The physical mechanisms behind simulation results are explained based on energy band diagrams, dark current-voltage characteristics, and comparison of external quantum efficiency. In conclusion, experimental optimization of vertical composition profile should be directed to either uniform or charge transport favorable vertical configurations in order to achieve high-performance OSCs.

Modelling of segmented high-performance thermoelectric generators with effects of thermal radiation, electrical and thermal contact resistances.

In this study, segmented thermoelectric generators (TEGs) have been simulated with various state-of-the-art TE materials spanning a wide temperature range, from 300 K up to 1000 K. The results reveal that by combining the current best p-type TE materials, BiSbTe, MgAgSb, K-doped PbTeS and SnSe with the strongest n-type TE materials, Cu-Doped BiTeSe, AgPbSbTe and SiGe to build segmented legs, TE modules could achieve efficiencies of up to 17.0% and 20.9% at ΔT = 500 K and ΔT = 700 K, respectively, and a high output power densities of over 2.1 Watt cm(-2) at the temperature difference of 700 K. Moreover, we demonstrate that successful segmentation requires a smooth change of compatibility factor s from one end of the TEG leg to the other, even if s values of two ends differ by more than a factor of 2. The influence of the thermal radiation, electrical and thermal contact effects have also been studied. Although considered potentially detrimental to the TEG performance, these effects, if well-regulated, do not prevent segmentation of the current best TE materials from being a prospective way to construct high performance TEGs with greatly enhanced efficiency and output power density.