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The effects of the sintering temperature on microstructures, electrical properties, and dielectric response of 1%Cr3+/Ta5+ co-doped TiO2 (CrTTO) ceramics prepared using a solid-state reaction method were studied. The mean grain size increased with an increasing sintering temperature range of 1300–1500 °C. The dielectric permittivity of CrTTO ceramics sintered at 1300 °C was very low (ε′ ∼198). Interestingly, a low loss tangent (tanδ ∼0.03–0.06) and high ε′ (∼1.61–1.9 × 104) with a temperature coefficient less than ≤ ±15% in a temperature range of −60 to 150 °C were obtained. The results demonstrated a higher performance property of the acceptor Cr3+/donor Ta5+ co-doped TiO2 ceramics compared to the Ta5+-doped TiO2 and Cr3+-doped TiO2 ceramics. According to a first-principles study, high-performance giant dielectric properties (HPDPs) did not originate from electron-pinned defect dipoles. By impedance spectroscopy (IS), it was suggested that the giant dielectric response was induced by interfacial polarization at the internal interfaces rather than by the formation of complex defect dipoles. X-ray photoelectron spectroscopy (XPS) results confirmed the existence of Ti3+, resulting in the formation of semiconducting parts in the bulk ceramics. Low tanδ and excellent temperature stability were due to the high resistance of the insulating layers with a very high potential barrier of ∼2.0 eV.

The charge compensation mechanism and dielectric properties of the (Al

In this work, the colossal dielectric properties and Maxwell—Wagner relaxation of TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 (x = 0–0.2) ceramics prepared by a solid-state reaction method are investigated. A single phase of Na1/2Y1/2Cu3Ti4O12 is achieved without the detection of any impurity phase. The highly dense microstructure is obtained, and the mean grain size is significantly reduced by a factor of 10 by increasing Ti molar ratio, resulting in an increased grain boundary density and hence grain boundary resistance (Rgb). The colossal permittivities of ε′ ~ 0.7–1.4 × 104 with slightly dependent on frequency in the frequency range of 102–106 Hz are obtained in the TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 ceramics, while the dielectric loss tangent is reduced to tanδ ~ 0.016–0.020 at 1 kHz due to the increased Rgb. The semiconducting grain resistance (Rg) of the Na1/2Y1/2Cu3Ti4+xO12 ceramics increases with increasing x, corresponding to the decrease in Cu+/Cu2+ ratio. The nonlinear electrical properties of the TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 ceramics can also be improved. The colossal dielectric and nonlinear electrical properties of the TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 ceramics are explained by the Maxwell–Wagner relaxation model based on the formation of the Schottky barrier at the grain boundary.

Organic-inorganic halide perovskites have been used as light absorbers in photovoltaic cells. The energy band gap of these perovskites can be tuned by mixing various halides at different concentrations. In this work, mixed iodide-bromide perovskite films at various concentrations were deposited on fluorine-doped tin oxide glass substrates via a spin coating technique. The films were characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy and synchrotron-radiation photoemission spectroscopy. The XRD results suggest that the lattice spacing decreased with increasing bromide concentration, which also relates to an increase in the energy band gap. The mixed halide films showed less crystallinity than that of pure halide films. The photoemission results showed that the iodide concentration at the surface was higher than expected. This could have been due to the higher solubility of iodide perovskite in the dimethylformamide solvent than that of bromide perovskite. These results suggest that the energy levels at the surface of the films can be different from those of the bulk material.

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

To improve efficiency of ZnO-based perovskite solar cells, a mixture iodide-bromide (I-Br) of FA0.4MA0.6Pbl3-2xBr2x perovskite structure was prepared by two-step sequential deposition method and used as photo absorber layer. Solar cells with structure of ITO/ZnO/PCBM/perovskite/P3HT/Au were fabricated and characterized. The perovskite films were prepared by mixing precursor PbI2 and PbBr2 at various molar ratio. It was found that the grain sizes and crystallite sizes of perovskite crystal were comparable for different Br doping. The existence of Br ions in perovskite layer was confirmed. In addition, the increase in band gap energy was observed. The highest average power conversion efficiency (PCE) of 2.17% was achieved from the device with FA0.4MA0.6PbI2.8Br0.2 layer. This PCE value is higher than the FA0.4MA0.6PbI3 device which exhibited 1.53%-efficiency. The enhancement in PCE could be explained by the development of light absorption and charges transport.