Influence of Halide Substitution and External Stimuli on Ion Transport in Inverted MAPb(I1-xBrx)3 Perovskite Solar Cells

The coupled electronic-ionic response in various MAPb(I1-xBrx)3-based inverted perovskite solar cells (PSCs) is studied in-operando by impedance spectroscopy (IS) under varied AM1.5G light intensities and electrical biases. We show that the concentration of Br- in the composition significantly alters the capacitance and resistive response of the PSC under external stimuli. For example, we observed that the low frequency capacitance does not increase proportionally with light intensity, instead it is highly dependent on the amount of Br- in the composition. We found that the recombination resistance (Rrec) has a linear inverse relationship with light intensity in MAPbI3 and MAPbBr3 whereas, the mixed compositions show deviation. Interestingly, the deviation of Rrec from linearity also scales with the increase in Br- concentration. Upon applying an electrical bias, a large deviation of Rrec from linearity was observed all mixed halide compositions exhibited a non-linear inverse trend. We further report the diffusion coefficient (D) for each MAPb(I1-xBrx)3 composition under different light intensity. Notably, the D values decreased on changing the composition from MAPbI3 (10-7 cm2 s-1) to MAPb(I0.8Br0.2)3 and MAPbBr3 (10-8 cm2 s-1). On the other hand, mixed compositions containing more than 20% Br- concentration show faster diffusion kinetics. Overall, our results emphasize on the complex and intertwined nature of electronic and ionic response in PSC that is tunable by changing the halide composition.

AlGaN-Based 1.55 µm Phototransistor as a Crucial Building Block for Optical Computers

An optically activated, enhancement mode heterostructure field effect transistor is proposed and analytically studied. A particular feature of this device is its gate region, which is made of a photovoltaic GaN/AlN-based superlattice detector for a wavelength of 1.55 µm. Since the inter-subband transition in this superlattice does normally not interact with TE-polarized (or vertically incoming) radiation, a metallic second-order diffraction grating on the transistor gate results in a re-orientation of the light into the horizontal direction—thus providing the desired TM-polarization. Upon illumination of this gate, efficient inter-subband absorption lifts electrons from the ground to the first excited quantized state. Due to partial screening of the strong internal polarization fields between GaN quantum wells and AlN barriers, this slightly diagonal transition generates an optical rectification voltage. Added to a constant electrical bias, this optically produced gate voltage leads to a noticeable increase of the transistor’s source-drain current. The magnitude of the bias voltage is chosen to result in maximal transconductance. Since such a phototransistor based on high-bandgap material is a device involving only fast majority carriers, very low dark and leakage currents are expected. The most important advantage of such a device, however, is the expected switching speed and, hence, its predicted use as an optical logic gate for photonic computing. In the absence of a p-n-junction and thus of both a carrier-induced space charge region, and the parasitic capacitances resulting thereof, operation frequencies of appropriately designed, sufficiently small phototransistors reaching 100 GHz are envisaged.

Simulation and analysis of a Single Mode Indium Gallium Arsenide Phosphide Vertical Cavity Surface Emitting Laser for Optical Communication

This present work is about simulating and analysing a Vertical Cavity Surface Emitting Laser (VCSEL) structure used in optical fibre communication systems. In this paper a VCSEL structure made of seven Quantum Wells of Indium Gallium Arsenide Phosphide (InGaAsP) emitting at 1550 nm is simulated. The device is analysed looking at the following characteristics: Direct current current and voltage (IV) characteristics, light power against electrical bias, optical gain against electrical bias, light distribution over the structure, output power and threshold current. Specification of material characteristics, ordinary physical models settings, initial VCSEL biasing, mesh declarations, declaration of laser physical models, their optical and electrical parameters were defined using Atlas syntax. Mirror ratings and quantum wells are the two main parameters that were studied and analysed to come up with structure trends. By determining important device parameters such as proper selection of the emission wavelength and choice of material; a VCSEL with an output power of 9.5 mW was simulated and compared with other structures.

Magnetoresistance Recovery in the Amorphous Dielectric Material SiCOH

The use of a magnetoresistance in the characterization of transport properties in the amorphous low-k dielectric material SiCOH is demonstrated. The double occupancy of charge carriers in trap states within the dielectric material can only exist in spin singlet formation due to Pauli Exclusion. The trap-assisted negative magnetoresistance (MR) in amorphous SiCOH, driven by an applied electric field that results in an observed increase in magnitude of the current in the conduction band is due to singly occupied trap spin-mixing suppression of carriers with the application of an external magnetic field. The material MR decays with time under electrical bias and temperature stress as traps are filled by charge carriers and from space charge accumulation. The MR can be reinstated by the ionization of these traps via the conduction mechanisms of nonthermally activated tunneling and thermal ionization with the assistance of an applied coulombic potential barrier lowering electric field. In this work a direct correlation is shown between a material MR and the trapping, de-trapping, and trap avoidance of singly occupied traps in the transport of charge carriers in the amorphous low-k dielectric material SiCOH (a-SiCOH).

A Review of Photoelectrocatalytic Reactors for Water and Wastewater Treatment

The photoexcitation of suitable semiconducting materials in aqueous environments can lead to the production of reactive oxygen species (ROS). ROS can inactivate microorganisms and degrade a range of chemical compounds. In the case of heterogeneous photocatalysis, semiconducting materials may suffer from fast recombination of electron–hole pairs and require post-treatment to separate the photocatalyst when a suspension system is used. To reduce recombination and improve the rate of degradation, an externally applied electrical bias can be used where the semiconducting material is immobilised onto an electrically conducive support and connected to a counter electrode. These electrochemically assisted photocatalytic systems have been termed “photoelectrocatalytic” (PEC). This review will explain the fundamental mechanism of PECs, photoelectrodes, the different types of PEC reactors reported in the literature, the (photo)electrodes used, the contaminants degraded, the key findings and prospects in the research area.

Solar photoelectrochemical synthesis of electrolyte-free H2O2 aqueous solution without needing electrical bias and H2

An external bias-free photoelectrochemical system containing solid polymer electrolytes achieves efficient and durable synthesis of pure (electrolyte-free) aqueous H2O2 solution.

Condensing water vapor to droplets generates hydrogen peroxide

It was previously shown [J. K. Leeet al.,Proc. Natl. Acad. Sci. U.S.A., 116, 19294–19298 (2019)] that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water–air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2production in condensate microdroplets showed that H2O2was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H2O2was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2generation. We also found that the H2O2production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.