Effects of drying time on the formation of merged and soft MAPbI3 grains and their photovoltaic responses.

The grain sizes of soft CH3NH3PbI3 (MAPbI3) thin films and the atomic contact strength at the MAPbI3/P3CT-Na interface are manipulated by varying the drying time of the saturated MAPbI3 precursor solutions, which influences the device performance and lifespan of the resultant inverted perovskite photovoltaic cells. The atomic-force microscopy images, cross-sectional scanning electron microscopy images, photoluminescence spectra and absorbance spectra show that the increased short-circuit current density (J SC) and increased fill factor (FF) are mainly due to the formation of merged MAPbI3 grains. Besides, the open-circuit voltage (V OC) of the encapsulated photovoltaic cells largely increases from 1.01 V to 1.15 V, thereby increasing the power conversion efficiency from 17.89% to 19.55% after 30 days, which can be explained as due to the increased carrier density of the MAPbI3 crystalline thin film. It is noted that the use of the optimized drying time during the spin coating process results in the formation of merged MAPbI3 grains while keeping the contact quality at the MAPbI3/P3CT-Na interface, which boosts the device performance and lifespan of the resultant perovskite photovoltaic cells.

Irradiation-Induced Synthesis of Ag/ZnO Nanostructures as Surface-Enhanced Raman Scattering Sensors for Sensitive Detection of the Pesticide Acetamiprid.

Detecting pesticides using techniques that involve simple fabrication methods and conducting the detection at very low levels are challenging. Herein, we report the detection of acetamiprid at the quadrillionth level using surface-enhanced Raman scattering (SERS). The SERS chip comprises Ag nanoparticles deposited on a tetrapod structure of ZnO coated onto indium tin oxide glass (denoted as Ag@ZnO-ITO). Controlled Ag decoration of ZnO occurs via irradiation-induced synthesis. The morphology of the surface plays a significant role in achieving an enhanced SERS performance for acetamiprid detection. 4,4'-Dipyridyl (DPY) is used to investigate synthesis conditions for the chip, leading to an optimal irradiation time of 60 min. Furthermore, the enhancement factor for acetamiprid on Ag@ZnO-ITO is higher than 107. These results demonstrate that SERS sensors have the potential for practical use in acetamiprid detection.

Modulation of the carrier balance of lead-halide perovskite nanocrystals by polyelectrolyte hole transport layers for near-infrared light-emitting diodes.

An alternative material, methylamine (MA)-doped poly[3-(4-carboxymethyl)thiophene-2,5-diyl] (P3CT) as hole transport layer (HTL) was investigated for efficient solution-processed near-infrared perovskite light-emitting diodes (NIR PeLEDs). The best NIR PeLEDs performance was achieved with an optimized composition ratio of the MA-doped P3CT (1:1) due to the balance of the electron and hole carrier in the active layer. The charge-balanced NIR PeLEDs exhibit the highest radiance of 858.37 W sr-1 m-2, a low turn-on voltage of 1.82 V, and an external quantum efficiency of 7.44%. Our findings show that using P3CT as an alternative HTL has the potential to significantly improve PeLED performance, allowing it to play a role in the development of practical applications in high-power NIR LEDs.

Corrosion Resistance and Thermal Conductivity Enhancement of Reduced Graphene Oxide-BaSO4-Epoxy Composites.

The results of studies on the corrosion protectiveness and thermal conductivity of reduced graphene oxide-BaSO4 epoxy composites are reported here. A commercial epoxy resin and reduced graphene oxide (rGO) were blended with a hardening reagent and then mixed with prepared BaSO4-epoxy resin (B-epoxy). The reduced graphene oxide-BaSO4-epoxy composite (rGO-B-epoxy) paste was used to coat the surfaces of Al 7205 alloy and the corrosion and thermal properties were investigated. A corrosion test in a 3.5 wt% synthetic sea water solution showed that the composite coating containing BaSO4 had the best corrosion resistance. Moreover, the rGO-B-epoxy composite showed better protection against corrosion than the epoxy alone. The rGO-B-epoxy composite with 5 wt% BaSO4 had an in-plane coefficient of thermal conductivity of approximately 165.0 W/m K, and the in-plane thermal diffusivity was 71.38 mm2/s. In standard thermal conductivity tests, all three samples had values below 40 W/m K. The rGO-B-epoxy composites showed good surface corrosion protection and in-plane thermal conductivity.

Surface Structure Analysis of Initial High-Temperature Oxidation of SS441 Stainless Steel.

Chromia-forming ferritic stainless steel (FSS) is a highly promising interconnect material for application in solid oxide fuel cells. In this study, initial oxidation of chromium oxides was performed at 500-800 °C to understand the evolution of materials at an early stage. The structural variations in oxide scales were analyzed through scanning electron microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffractometry (XRD), laser confocal microscopy (LSCM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Surface electrochemical properties were investigated through electrochemical impedance spectroscopy to understand how the heat treatment temperature affected surface impedance. Treatment temperatures higher than 700 °C facilitate the diffusion of Cr and Mn, thus allowing ferritic spinels to form on the surface and leading to high electrical conductivity.

19% Efficient P3CT-Na Based MAPbI3 Solar Cells with a Simple Double-Filtering Process.

A high-efficiency inverted-type CH3NH3PbI3 (MAPbI3) solar cell was fabricated by using a ultrathin poly[3-(4-carboxybutyl)thiophene-2,5-diyl]-Na (P3CT-Na) film as the hole transport layer. The averaged power conversion efficiency (PCE) can be largely increased from 11.72 to 18.92% with a double-filtering process of the P3CT-Na solution mainly due to the increase in short-circuit current density (JSC) from 19.43 to 23.88 mA/cm2, which means that the molecular packing structure of P3CT-Na thin film can influence the formation of the MAPbI3 thin film and the contact quality at the MAPbI3/P3CT-Na interface. Zeta potentials, atomic-force microscopic images, absorbance spectra, photoluminescence spectra, X-ray diffraction patterns, and Raman scattering spectra are used to understand the improvement in the JSC. Besides, the light intensity-dependent and wavelength-dependent photovoltaic performance of the MAPbI3 solar cells shows that the P3CT-Na thin film is not only used as the hole transport layer but also plays an important role during the formation of a high-quality MAPbI3 thin film. It is noted that the PCE values of the best P3CT-Na based MAPbI3 solar cell are higher than 30% in the yellow-to-near infrared wavelength range under low light intensities. On the other hand, it is predicted that the double-filtering method can be readily used to increase the PCE of polymer based solar cells.

Investigation of random lasing from all-inorganic halide perovskite quantum dots prepared under ambient conditions.

Random lasing from CsPbBr3 quantum dots (QDs) prepared by the hot injection method under ambient conditions has been investigated. The lasing characteristics and performance were related to the thickness and aggregation of the QDs film on a glass substrate. The perovskite emitted linear polarized ASE from the edge of the prepared sample as pump energy above a certain threshold, owing to the gain guiding effect. In comparison to the Q-switched Nd:YAG laser, the prepared perovskite random lasers produced a speckle reduced image with a lower contrast of around 0.051. Through temperature-dependent measurements under a surface normal emission configuration, the photon energy of ASE revealed a red shift as the temperature increased and showed a larger characteristic temperature of around 230 K. This result illustrates that the perovskite prepared under ambient conditions can be a promising material for a microcavity laser in the near future.

Bright and fast-response perovskite light-emitting diodes with an ICBA:modified-C60 nanocomposite electrical confinement layer.

Bright and fast-response CH3NH3PbBr3 perovskite light-emitting diodes (PeLEDs) are realized by using ICBA:modified C60 (MC60) nanocomposites as the hole blocking layer (HBL) and electron transport layer (ETL). The photoluminescence spectrum shows that the use of hydrophilic MC60 in the ETL helps the surface passivation of the perovskite layer. In addition, the photoelectron spectra and water-droplet contact angle images show that the use of the ICBA:MC60 nanocomposite ETL can simultaneously confine the electrons and holes in the perovskite layer, which boosts the injected electron-hole radiative recombination efficiency and thereby increases the electroluminescence from 1 cd m-2 to 2080 cd m-2 at 6 V when the ICBA:3,5OEC60 nanocomposite ETL is used. In addition, the operational frequency of the optimal PeLED is up to 1.5 MHz.

Origins of the s-shape characteristic in J-V curve of inverted-type perovskite solar cells.

Fullerene derivative thin films have been widely used in inverted-type perovskite solar cells as the electron transport layer (ETL) and hole blocking layer. However, the smooth contact at the interface between the hydrophobic [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hydrophilic CH3NH3PbI3 (MAPbI3) thin film has not yet been completely understood. The contact at the PCBM/MAPbI3 interface strongly influences the photovoltaic performance. The photovoltaic devices were characterized by measuring the light intensity-dependent current density-voltage (J-V) curves and impedance spectra, which show that the contact at the PCBM/MAPbI3 interface simultaneously influences the shunt resistance (carrier recombination) and series resistance (interfacial contact). In addition, x-ray diffraction patterns, atomic force microscopic images, absorbance spectra and photoluminescence spectra were used to explore the contact at the PCBM/MAPbI3 interface. The experimental results show that the flat MAPbI3 thin film cannot be completely covered by a PCBM thin film and thereby results in the s-shape characteristic in the J-V curve of the resultant solar cells. The s-shaped J-V curve can be suppressed by increasing the crystallinity and surface roughness of the MAPbI3 thin film. With the use of an interface modification layer in between the PCBM thin film and Ag cathode, the power conversion efficiency of MAPbI3 solar cells can be increased from 10.50% to 13.71%.

Performance Enhancement of CdS/CdSe Quantum Dot-Sensitized Solar Cells with (001)-Oriented Anatase TiO2 Nanosheets Photoanode.

CdS/CdSe quantum dot-sensitized solar cells (QDSSCs) were fabricated on two types of TiO2 photoanodes, namely nanosheets (NSs) and nanoparticles. The TiO2 NSs with high (001)-exposed facets were prepared via a hydrothermal method, while the TiO2 nanoparticles used the commercial Degussa P-25. It was found that the pore size, specific surface area, porosity, and electron transport properties of TiO2 NSs were generally superior to those of P-25. As a result, the TiO2 NS-based CdS/CdSe QDSSC has exhibited a power conversion efficiency of 4.42%, which corresponds to a 54% improvement in comparison with the P-25-based reference cell. This study provides an effective photoanode design using nanostructure approach to improve the performance of TiO2-based QDSSCs.

Functional graded fullerene derivatives for improving the fill factor and device stability of inverted-type perovskite solar cells.

A graded fullerene derivative thin film was used as a dual-functional electron transport layer (ETL) in CH3NH3PbI3 (MAPbI3) solar cells, to improve the fill factor (FF) and device stability. The graded ETL was made by mixing phenyl-C61-butyric acid methyl ester (PCBM) molecules and C60-diphenylmethanofullerene-oligoether (C60-DPM-OE) molecules using the spin-coating method. The formation of the graded ETLs can be due to the phase separation between hydrophobic PCBM and hydrophilic C60-DPM-OE, which was confirmed by XPS depth-profile analysis and an electron energy-loss spectroscope. Comprehensive studies were carried out to explore the characteristics of the graded ETLs in MAPbI3 solar cells, including the surface properties, electronic energy levels, molecular packing properties and energy transfer dynamics. The elimination of the s-shape in the current density-voltage curves results in an increase in the FF, which originates from the smooth contact between the C60-DPM-OE and hydrophilic MAPbI3 and the formation of the more ordered ETL. There was an improvement in device stability mainly due to the decrease in the photothermal induced morphology change of the graded ETLs fabricated from two fullerene derivatives with distinct hydrophilicity. Consequently, such a graded ETL provides dual-functional capabilities for the realization of stable high-performance MAPbI3 solar cells.

Saturation and beating of acoustic phonon oscillations excited near the exciton resonance of strained polar ZnO/Zn0.8Mg0.2O multiple quantum wells.

Saturation and beating of coherent acoustic phonon (CAP) oscillations were observed and attributed to the screening of a built-in electric field with increasing pump power using degenerate pump-probe measurements near the exciton resonance of polar ZnO/Zn0.8Mg0.2O multiple quantum wells (MQWs). After purifying the CAP signals by using an empirical mode decomposition, we found not only that the CAP amplitude follows the trend of the band gap renormalization (BGR) and shows saturation at high pump power, but also that the CAP oscillation period coincides with that of the MQWs, consistent with the XRD and TEM results. An additional low-frequency oscillation modifying the CAP signal is revealed due to the negative change in refractive index caused by BGR as the pump power increases.

Understanding of multi-level resistive switching mechanism in GeOx through redox reaction in H2O2/sarcosine prostate cancer biomarker detection.

Formation-free multi-level resistive switching characteristics by using 10 nm-thick polycrystalline GeOx film in a simple W/GeOx/W structure and understanding of switching mechanism through redox reaction in H2O2/sarcosine sensing (or changing Ge°/Ge4+ oxidation states under external bias) have been reported for the first time. Oxidation states of Ge0/Ge4+ are confirmed by both XPS and H2O2 sensing of GeOx membrane in electrolyte-insulator-semiconductor structure. Highly repeatable 1000 dc cycles and stable program/erase (P/E) endurance of >106 cycles at a small pulse width of 100 ns are achieved at a low operation current of 0.1 µA. The thickness of GeOx layer is found to be increased to 12.5 nm with the reduction of polycrystalline grain size of <7 nm after P/E of 106 cycles, which is observed by high-resolution TEM. The switching mechanism is explored through redox reaction in GeOx membrane by sensing 1 nM H2O2, which is owing to the change of oxidation states from Ge0 to Ge4+ because of the enhanced O2- ions migration in memory device under external bias. In addition, sarcosine as a prostate cancer biomarker with low concentration of 50 pM to 10 µM is also detected.

Negative voltage modulated multi-level resistive switching by using a Cr/BaTiOx/TiN structure and quantum conductance through evidence of H2O2 sensing mechanism.

Negative voltage modulated multi-level resistive switching with quantum conductance during staircase-type RESET and its transport characteristics in Cr/BaTiOx/TiN structure have been investigated for the first time. The as-deposited amorphous BaTiOx film has been confirmed by high-resolution transmission electron microscopy. X-ray photo-electron spectroscopy shows different oxidation states of Ba in the switching material, which is responsible for tunable more than 10 resistance states by varying negative stop voltage owing to slow decay value of RESET slope (217.39 mV/decade). Quantum conductance phenomenon has been observed in staircase RESET cycle of the memory devices. By inspecting the oxidation states of Ba+ and Ba2+ through measuring H2O2 with a low concentration of 1 nM in electrolyte/BaTiOx/SiO2/p-Si structure, the switching mechanism of each HRS level as well as the multi-level phenomenon has been explained by gradual dissolution of oxygen vacancy filament. Along with negative stop voltage modulated multi-level, current compliance dependent multi-level has also been demonstrated and resistance ratio up to 2000 has been achieved even for a thin (<5 nm) switching material. By considering oxidation-reduction of the conducting filaments, the current-voltage switching curve has been simulated as well. Hence, multi-level resistive switching of Cr/BaTiOx/TiN structure implies the promising applications in high dense, multistate non-volatile memories in near future.

High-quality and Large-size Topological Insulator Bi2Te3-Gold Saturable Absorber Mirror for Mode-Locking Fiber Laser.

A novel high-quality, large-size, reflection-type topological insulator Bi2Te3-Gold (BG) film-based nonlinear optical modulator has been successfully fabricated as a two-dimensional saturable absorber mirror (SAM) by pulsed laser deposition (PLD). This BG-SAM possesses saturation fluence of 108.3 µJ/cm2, modulation depth (ΔR) of 6.5%, non-saturable loss of 38.4%, high damage threshold above 1.354 mJ/cm2 and excellent uniformity providing for the generation of passive mode-locked (ML) pulses for erbium-doped fiber lasers (EDFLs) on a large sample area. Under 124 mW 976 nm pumping, We obtained 452-fs continuous-wave ML pulses with pulse energy of 91 pJ and full width at half-maximum (FWHM) of 6.72-nm from this EDFL. The results clearly evidence that the PLD is an efficient method for fabricating BG-SAM that is suitable for a compact ultrafast ML fiber laser system.

Detection of pH and Enzyme-Free H2O2 Sensing Mechanism by Using GdO x Membrane in Electrolyte-Insulator-Semiconductor Structure.

A 15-nm-thick GdO x membrane in an electrolyte-insulator-semiconductor (EIS) structure shows a higher pH sensitivity of 54.2 mV/pH and enzyme-free hydrogen peroxide (H2O2) detection than those of the bare SiO2 and 3-nm-thick GdO x membranes for the first time. Polycrystalline grain and higher Gd content of the thicker GdO x films are confirmed by transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS), respectively. In a thicker GdO x membrane, polycrystalline grain has lower energy gap and Gd(2+) oxidation states lead to change Gd(3+) states in the presence of H2O2, which are confirmed by electron energy loss spectroscopy (EELS). The oxidation/reduction (redox) properties of thicker GdO x membrane with higher Gd content are responsible for detecting H2O2 whereas both bare SiO2 and thinner GdO x membranes do not show sensing. A low detection limit of 1 µM is obtained due to strong catalytic activity of Gd. The reference voltage shift increases with increase of the H2O2 concentration from 1 to 200 µM owing to more generation of Gd(3+) ions, and the H2O2 sensing mechanism has been explained as well.

Temperature-Dependent Non-linear Resistive Switching Characteristics and Mechanism Using a New W/WO3/WOx/W Structure.

Post-metal annealing temperature-dependent forming-free resistive switching memory characteristics, Fowler-Nordheim (F-N) tunneling at low resistance state, and after reset using a new W/WO3/WOx/W structure have been investigated for the first time. Transmission electron microscope image shows a polycrystalline WO3/WOx layer in a device with a size of 150 × 150 nm(2). The composition of WO3/WOx is confirmed by X-ray photo-electron spectroscopy. Non-linear bipolar resistive switching characteristics have been simulated using space-charge limited current (SCLC) conduction at low voltage, F-N tunneling at higher voltage regions, and hopping conduction during reset, which is well fitted with experimental current-voltage characteristics. The barrier height at the WOx/W interface for the devices annealed at 500 °C is lower than those of the as-deposited and annealed at 400 °C (0.63 vs. 1.03 eV). An oxygen-vacant conducting filament with a diameter of ~34 nm is formed/ruptured into the WO3/WOx bilayer owing to oxygen ion migration under external bias as well as barrier height changes for high-resistance to low-resistance states. In addition, the switching mechanism including the easy method has been explored through the current-voltage simulation. The devices annealed at 500 °C have a lower operation voltage, lower barrier height, and higher non-linearity factor, which are beneficial for selector-less crossbar memory arrays.

Annealing Effect on (FAPbI3)1-x(MAPbBr3)x Perovskite Films in Inverted-Type Perovskite Solar Cells.

This study determines the effects of annealing treatment on the structure and the optical and electronic behaviors of the mixed (FAPbI3)1-x(MAPbBr3)x perovskite system. The experimental results reveal that (FAPbI3)1-x(MAPbBr3)x (x ~ 0.2) is an effective light-absorbing material for use in inverted planar perovskite solar cells owing to its large absorbance and tunable band gap. Therefore, good band-matching between the (FAPbI3)1-x(MAPbBr3)x and C60 in photovoltaic devices can be controlled by annealing at various temperatures. Accordingly, an inverted mixed perovskite solar cell with a record efficiency of 12.0% under AM1.5G irradiation is realized.

Passive mode locking of ytterbium- and erbium-doped all-fiber lasers using graphene oxide saturable absorbers.

Broadband graphene oxide/PVA films were used as saturable absorbers (SAs) for mode locking erbium-doped fiber laser (EDFL) and ytterbium-doped fiber laser (YDFL) at 1.06 µm and 1.55 µm. They provide modulation depths of 3.15% and 6.2% for EDFL and YDFL, respectively. Stable self-starting mode-locked pulses are obtained for both lasers, confirming that the graphene oxide is cost-effective. We have generated mode-locked pulses with spectral width, repetition rate, and pulse duration of 0.75 nm, 9.5 MHz, and 2.7 ps. This is the shortest pulse duration directly obtained from an all-normal-dispersion YDFL with graphene-oxide saturable absorber.

Broadband charge transfer dynamics in P3HT:PCBM blended film.

Broadband exciton dynamics in P3HT:PCBM blended film was observed by the femtosecond time-resolved photoluminescence sum-frequency technique. Onsager-Braun theory is applied to analyze the distribution of charge transfer radius at different energy levels. In our evaluation, the optimal diameter of P3HT fiber is about 14.3 nm for achieving the best exciton dissociation in P3HT:PCBM blended films. This technique can be readily used in the optimization of high-efficiency organic photovoltaics.