Analytic Analysis of the Features of GaAs/Si Radial Heterojunctions: Influence of Temperature and Concentration

In this article, the electrophysical characteristics of GaAs/Si radial heterojunctions are studied analytically over a temperature range of 50 K to 500 K in increments of 50 K, considering various doping concentrations. The analysis encompasses band gap narrowing (BGN), built-in potential, the difference in band gap between GaAs and Si, and capacitance-voltage (C-V) characteristics. In particular, we focus on shell radii of 0.5 μm and 1 μm within the structure. We found that the thickness of the depletion region of the GaAs/Si radial heterojunction increases with rising temperature. When the doping concentration changes from 2∙1015 to 2∙1018 BGN decreases by 2 MeV. The charge capacity of the GaAs/Si radial heterojunction increases by 3 nF as the temperature rises from 50 K to 500 K. Additionally, the built-in potential of the GaAs/Si radial heterojunction decreases by 1.5 volts with increasing temperature.

Band gap widening and narrowing in moderately and heavily doped n-ZnO films

Enhanced photocatalytic and antibacterial activity of acridinium-grafted g-C3N4 with broad-spectrum light absorption for antimicrobial photocatalytic therapy

Lead-free perovskite ferroelectric thin films with narrow direct band gap suitable for solar cell applications

New multicomponent equiatomic rare earth oxides (ME-REOs) containing 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiatomic proportions are synthesized using nebulized spray pyrolysis. All the systems crystallized as a phase pure fluorite type (Fm3[combining macron]m) structure in spite of the high chemical complexity. A nominal increase in the lattice parameter compared to CeO2 is observed in all ME-REOs. X-ray photoelectron spectroscopy performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidation state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidation state, respectively. The presence of Ce4+ contributes substantially to the observed stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concentration rapidly increases and then saturates with the number of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the composition or thermal treatments. Considering the high level of oxygen vacancies present and the observed low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable.

Sb<sub>2</sub>(S,Se)<sub>3</sub> thin film solar cells have been developed rapidly in recent years due to their abundant raw materials, simple preparation method, stable performance, etc. In this study, based on the characteristic of tunable band gap of Sb<sub>2</sub>(S,Se)<sub>3</sub> light absorption layer, wx-AMPS software is used to simulate and design the Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cell with narrowing band gap structure, and compared with the Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cell with constant band gap (50% selenium content). The results show that the additional electric field formed by the narrowing band gap can promote the holes’ transport and inhibit the carrier’s recombination. Compared with the constant band gap structure, the narrowing band gap structure can increase the short-circuit current density of Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cells from 19.34 to 22.94 mA·cm<sup>–2</sup>, the filling factor from 64.34% to 77.04%, and the photoelectric conversion efficiency from 12.03% to 14.42%. Then, the effect of electron mobility on the performance of Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cells with narrowing band gap is studied. It is found that when the hole mobility is 0.1 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>, the advantage of narrowing band gap can gradually appear after the electron mobility is higher than 0.25 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The performance of Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cell is enhanced with the electron mobility further increasing. However, when the electron mobility is higher than 5 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>, the device performance is saturated. Moreover, we demonstrate that the degradation caused by thick or high defect state of Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cell can be effectively alleviated by applying the narrowing band gap due to the suppression of the carrier recombination. When the thickness is 1.5 μm and the defect density is 10<sup>16</sup> cm<sup>–3</sup>, the photoelectric conversion efficiency of Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cell with narrowing band gap is 6.34% higher than that of the constant bandgap. Our results demonstrate that the band gap engineering of the light absorption layer is one of the effective technical routes to optimizing the performance of Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cells. Since the photo-absorption material such as amorphous/microcrystalline silicon germanium, Copper indium gallium selenide and perovskite have the characteristic of tunable band gap. The design of the gradient band gap structure can also be applied to the optimization of the above alloy or compound solar cells.

Two laser-based methods were used for the deposition of narrow band gap semiconductor films based on iron oxides. The first one is laser chemical vapor deposition (LCVD) from iron carbonyl Fe(CO)₅ vapors using Ar⁺ laser radiation. The width of the band gap of the films depends on the film thickness and the applied electrical field. The film thickness was varied from 10 to 18 nm and consequently the band gap width (E_g) varied from 0.46 to 0.66 eV. The longer the exposure time of the Si substrate to Ar⁺ laser radiation, the higher the content of iron oxides, the larger the width of the band gap in the deposited semiconductor materials. The second method is the reactive pulsed laser deposition (RPLD) of an iron target in low pressure oxygen atmosphere using a KrF excimer laser (&lgr;=248 nm, &tgr; congruent to 30 ns) at the fluence of 4 J/cm² and the repetition rate of 10 Hz. A number of pulses (3000-8500) increasing with oxygen ambient pressure (0.05-1 Pa) was used for each deposition with the aim of depositing films with almost equal thickness (~100 nm). The width of the band gap (E_g) varied in the range 0.13-0.34 eV, depending on the oxygen pressure: the higher the oxygen pressure, the higher the iron oxide content in the deposited film, the larger the width of the band gap in the deposited semiconductor material. It is shown that both LCVD and RPLD methods are appropriate technologies for the deposition of narrow variable band gap semiconductor thin films.

Enhanced multiferroicity and narrow band gap in B-site Co-doped Aurivillius Bi5FeTi3O15

The broad tunability of the energy band gap through size control makes colloidal quantum dots (QDs) promising for the development of photovoltaic devices. Large-size lead sulfide (PbS) QDs, exhibiting a narrow energy band gap, are particularly interesting as they can be used to augment perovskite and c-Si solar cells due to their complementary NIR absorption. However, their complex surface chemistry makes them difficult to process for the development of solar cells. The shape of the QDs transformed from octahedron to cuboctahedron as their size increases, a phenomenon guided by surface energy minimization. As a result, the surface properties change drastically for large-size QDs, which exhibit nonpolar (200) facets and polar (111) facets, as opposed to only (111) facets in small-size QDs. Recent advancements in solution-phase surface passivation strategies, used for the development of high-performance solar cells using the small size and wide band gap QDs, failed to translate a similar enhancement in the case of large-size and narrow band gap QDs. Here, we report a hybrid passivation strategy for large-size and narrow band gap QDs to passivate both (111) and (200) facets, respectively, using inorganic lead triiodide (PbI3-) and organic 3-chloro-1-propanethiol (CPT). By employing charge balance calculation, we identified the desired narrow band gap for QDs to complement the perovskite and c-Si absorption. The distinct choice of the organic ligand CPT enhances the colloidal stability of QDs in the solution phase and improves surface passivation to stop QD fusion in solid films. Photophysical properties show narrower excitonic and emission peaks and a reduction in the Stokes shift. Hybrid passivation leads to a 94% increase in the power conversion efficiency of solar cells and a 74% increase in the external quantum efficiency at the excitonic peak.

Al-doped Zn1−x Mg x O and Zn1−y Cd y O thin films were prepared on glass substrates by sol–gel method. The codoping thin films showed preferential c-axis orientation, and the lattice constant c evaluated from the shift of the position of (002) peak displayed an increasing evolution from x = 8 at.% to y = 8 at.%, indicating a roughly statistical substitution of Mg2+ and Cd2+ for Zn2+ in their solid solution. The effects of narrowing and widening band gap (E g) on conductivity of (Cd, Al) and (Mg, Al) codoped ZnO thin films were simultaneously investigated using transmission spectra and electrical measurements. The transmittances of these films are obviously decreased by vacuum annealing to 50–60%. However, the carrier concentration and Hall mobility both increase, and resistivity decreases with narrowing band gap in 1 at.% Al-doped Zn1−x Mg x O and Zn1−y Cd y O thin films from x = 8 at.% to y = 8 at.%. It is revealed that the conductivity of Al-doped ZnO thin films could be enhanced by this simple band gap modification.

Band gap change in doped ZnO is an observed phenomenon that is very interesting from the fundamental point of view. This work is focused on the preparation of pure and single phase nanostructured ZnO and Cu as well as Mn-doped ZnO for the purpose of understanding the mechanisms of band gap narrowing in the materials. ZnO, Zn0.99Cu0.01O and Zn0.99Mn0.01O materials were prepared using a wet chemistry method, and X-ray diffraction (XRD) results showed that all samples were pure and single phase. UV-visible spectroscopy showed that materials in the nanostructured state exhibit band gap widening with respect to their micron state while for the doped compounds exhibited band gap narrowing both in the nano and micron states with respect to the pure ZnO materials. The degree of band gap change was dependent on the doped elements and crystallite size. X-ray photoelectron spectroscopy (XPS) revealed that there were shifts in the valence bands. From both UV-visible and XPS spectroscopy, it was found that the mechanism for band gap narrowing was due to the shifting of the valance band maximum and conduction band minimum of the materials. The mechanisms were different for different samples depending on the type of dopant and dimensional length scales of the crystallites.

Influence of temperature and nickel catalyst on the structural and optical properties of indium oxide nanostructured films synthesized by chemical vapor deposition technique

Mn1-xCuxO2/ reduced graphene oxide nanocomposites: Synthesis, characterization, and evaluation of visible light mediated catalytic studies

Pure and In-doped Tin Oxide (SnO2) nanoparticles with different doping concentration were prepared by hydrothermal method. The prepared samples were characterized by XRD, TEM, and UV–vis absorption spectroscopy. The crystal lattice spacing of In-doped SnO2 nanoparticles decreases compared with pure SnO2. The crystal lattice spacing ratios for (110), (101) and (211) crystal face shrinks acutely with 1.9% doping in SnO2. Crystallite size calculated by the FWHM of the first major XRD peaks (110) decreases from 3.945 to 3.281 nm as the doping concentration increases from 0 to 10%, whereas the strain increases. XRD study reveals that crystal lattice spacing of SnO2 nanoparticles shrinks due to the effect of In element doping. Optical band-gap of SnO2 nanoparticles decreases from 3.76 to 3.33 eV first, and then increases to 3.55 and 3.52 eV, respectively, as the doping concentration changes from 1.9, 3.8 to 10%. Band-gap narrowing originates from the effect of crystal lattice spacing shrinkage.

Anomalies in wide band gap SnO2 nanostructures