The evaluation of electron trapping parameters from conductivity glow curves in cadmium sulphide

Physics of defects in metal halide perovskites

Metal halide perovskites are widely used in optoelectronic devices, including solar cells, photodetectors, and light-emitting diodes. Defects in this class of low-temperature solution-processed semiconductors play significant roles in the optoelectronic properties and performance of devices based on these semiconductors. Investigating the defect properties provides not only insight into the origin of the outstanding performance of perovskite optoelectronic devices but also guidance for further improvement of performance. Defects in perovskites have been intensely studied. Here, we review the progress in defect-related physics and techniques for perovskites. We survey the theoretical and computational results of the origin and properties of defects in perovskites. The underlying mechanisms, functions, advantages, and limitations of trap state characterization techniques are discussed. We introduce the effect of defects on the performance of perovskite optoelectronic devices, followed by a discussion of the mechanism of defect treatment. Finally, we summarize and present key challenges and opportunities of defects and their role in the further development of perovskite optoelectronic devices.

A coating strategy to achieve effective local charge separation for photocatalytic coevolution

Semiconductors of narrow bandgaps and high quantum efficiency have not been broadly utilized for photocatalytic coevolution of H₂ and O₂ via water splitting. One prominent issue is to develop effective protection strategies, which not only mitigate photocorrosion in an aqueous environment but also facilitate charge separation. Achieving local charge separation is especially challenging when these reductive and oxidative sites are placed only nanometers apart compared to two macroscopically separated electrodes in a photoelectrochemical cell. Additionally, the driving force of charge separation, namely the energetic difference in the barrier heights across the two type of sites, is small. Herein, we used conformal coatings attached by nanoscale cocatalysts to transform two classes of tunable bandgap semiconductors, i.e., CdS and GaInP₂, into stable and efficient photocatalysts. We used hydrogen evolution and redox-mediator oxidation for model study, and further constructed a two-compartment solar fuel generator that separated stoichiometric H₂ and O₂ products. Distinct from the single charge-transfer direction reported for conventional protective coatings, the coating herein allows for concurrent injection of photoexcited electrons and holes through the coating. The energetic difference between reductive and oxidative catalytic sites was regulated by selectivity and local kinetics. Accordingly, the charge separation behavior was validated using numerical simulations. Following this design principle, the CdS/TiO₂/Rh@CrO_x photocatalysts evolved H₂ while oxidizing reversible polysulfide redox mediators at a maximum rate of 90.6 μmol⋅h^-1⋅cm^-2 by stacking three panels. Powered by a solar cell, the redox-mediated solar water-splitting reactor regenerated the polysulfide repeatedly and achieved solar-to-hydrogen efficiency of 1.7%.

Origin of surface trap states in CdS quantum dots: relationship between size dependent photoluminescence and sulfur vacancy trap states

Trap state emission mainly originates from deep trapped electrons at surface Cd with sulfur vacancy sites of CdS quantum dot.

Thermoluminescence study of X-ray and UV irradiated natural calcite and analysis of its trap and recombination level

Thermoluminescence (TL) of natural light-orange color calcite (CaCO3) mineral in micro-grain powder form was studied at room temperature X-ray and UV irradiation under various irradiation times. TL was recorded in linear heating rate (2 K/s) from room temperature (300 K) to 523 K. Trapping parameters such as activation energy, order of kinetics, frequency factor have been evaluated by Computerized Glow Curve Deconvolution technique. Three electron trap centers had been estimated at depth 0.70, 1.30 and 1.49 eV from the conduction band. Investigation of emission spectra recorded at various temperatures showed single recombination center at depth 2.74 eV from the conduction band. Due to thermally assisted tunneling of electron and subsequent center-to-center recombination, a distinct peak of lower activation energy (0.60 eV) was observed at relatively higher temperature (~360 K) for X-ray irradiated sample. In UV excitation, there was an indication of photo-transfer phenomenon, where low TL intensity might have been observed; but due to simultaneous excitation of electrons from valence band to the trap level, TL intensity was found to increase with UV irradiation time. The results obtained within temperature range 300-523 K were explained by considering a band diagram.

Colour-causing defects and their related optoelectronic transitions in single crystal CVD diamond

Defects causing colour in nitrogen-doped chemical vapour-deposited (CVD) diamond can adversely affect the exceptional optical, electronic and spintronic properties of the material. Several techniques were used to study these defects, namely optical absorption spectroscopy, thermoluminescence (TL) and electron paramagnetic resonance (EPR). From our studies, the defects causing colour in nitrogen-doped CVD diamond are clearly not the same as those causing similar colour in natural diamonds. The brown colour arises due to a featureless absorption profile that decreases in intensity with increasing wavelength, and a broad feature at 360 nm (3.49 eV) that scales in intensity with it. Another prominent absorption band, centred at 520 nm (2.39 eV), is ascribed to the neutral nitrogen-vacancy-hydrogen defect. The defects responsible for the brown colour possess acceptor states that are 1.5 eV from the valence band (VB) edge. The brown colour is removed by heat treatment at 1600 ° C, whereupon new defects possessing shallow (<1 eV) trap states are generated.

Photosynthetic energy conservation investigated by thermoluminescence. Activation energies and half-lives of thermoluminescence bands of chloroplasts determined by mathematical resolution of glow curves

Thermoluminescence of isolated chloroplasts was analysed by a computer-assisted multicomponent curve fitting procedure to determine the activation energies, the free energies of activation, frequency factors and half-lives of the component bands of the glow curve. Optimal fit was obtained in the temperature region from -80 degrees C to +80 degrees C by the resolution of the glow curve into seven bands with peak positions at -24, -12, +12, +17, +28, +44, and +69 degrees C. All of the activation free energies of the thermoluminescence bands were much higher than 0.59 eV, the minimum free energy of activation required for the back reaction of the primary charge separation as calculated on the basis of the theory of Ross and Calvin (Ross, R.T. and Calvin, M. (1967) Biophys. J. 7, 595-614). The high free energies of activation and long half-lives (longer than 50 ms) of the thermoluminescence bands suggest that thermoluminescence in the temperature region from -80 degrees C to 80+ C does not reflect the change recombination of primary products but represent the reversal of subsequent stabilization steps of the charge separation process which proceed along the acceptor and donor sides of Photosystem II.