Bingye Zhang

Evidence of cation vacancy induced room temperature ferromagnetism in Li-N codoped ZnO thin films

Room temperature ferromagnetism (RTFM) was observed in Li-N codoped ZnO thin films [ZnO:(Li, N)] fabricated by plasma-assisted molecular beam epitaxy, and p-type ZnO:(Li, N) shows the strongest RTFM. Positron annihilation spectroscopy and low temperature photoluminescence measurements indicate that the RTFM in ZnO:(Li, N) is attributed to the defect complex related to VZn, such as VZn and Lii-NO-VZn complex, well supported by first-principles calculations. The incorporation of NO can stabilize and enhance the RTFM of ZnO:(Li, N) by combining with Lii to form Lii-NO complex, which restrains the compensation of Lii for VZn and makes the ZnO:(Li, N) conduct in p-type.

Origin of breakdown mechanism in multicrystalline silicon solar cells

The local breakdown behavior of multicrystalline silicon solar cells occurring at reverse bias voltages of −10 V has been investigated by means of electroluminescence images and temperature dependent current density-voltage (J-V) measurements. Identification of temperature coefficient of breakdown current indicates that Zener effect is the dominating mechanism of the local breakdown (so-called type II breakdown). Investigations of the carrier transport mechanism under forward bias voltage suggest that there exist a large amount of defects in depletion region. The origin of type II breakdown is attributed to the defects in depletion region.

Stability and heating rate dependent metal–insulator transition properties of VO 2 film grown by MBE

High quality VO2 films with various thicknesses were grown on sapphire substrate by molecular beam epitaxy. X-ray diffraction and atomic force microscopy measurements indicated that high quality single phase VO2 films with dense and smooth surface as well as free of cracks were achieved. Via adjusting the thickness of VO2 films, the crystalline quality and the surface morphology of VO2 films are significantly improved. The stability and heating rate dependent metal–insulator transition property were investigated. The decreasing of transition temperature and the degeneracy of the stability of VO2 is mainly due to the surface degradation from air exposure and the interdiffusion between the substrate and VO2 film.

Enhanced efficiency of Cu2ZnSn(S,Se)4 solar cells via anti-reflectance properties and surface passivation by atomic layer deposited aluminum oxide

Reducing interface recombination losses is one of the major challenges in developing Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, we propose a CZTSSe solar cell with an atomic layer deposited Al2O3 thin film for surface passivation. The influence of passivation layer thickness on the power conversion efficiency (PCE), short-circuit current density (Jsc), open-circuit voltage (Voc) and fill factor (FF) of the solar cell is systematically investigated. It is found that the Al2O3 film presents notable antireflection (AR) properties over a broad range of wavelengths (350–1000 nm) for CZTSSe solar cells. With increasing Al2O3 thickness (1–10 nm), the average reflectance of the CZTSSe film decreases from 12.9% to 9.6%, compared with the average reflectance of 13.6% for the CZTSSe film without Al2O3. The Al2O3 passivation layer also contributes to suppressed surface recombination and enhanced carrier separation. Passivation performance is related to chemical and field effect passivation, which is due to released H atoms from the Al–OH bonds and the formation of Al vacancies and O interstitials within Al2O3 films. Therefore, the Jsc and Voc of the CZTSSe solar cell with 2 nm-Al2O3 were increased by 37.8% and 57.8%, respectively, in comparison with those of the unpassivated sample. An optimal CZTSSe solar cell was obtained with a Voc, Jsc and η of 0.361 V, 33.78 mA and 5.66%. Our results indicate that Al2O3 films show the dual functions of AR and surface passivation for photovoltaic applications.

Color-tunable phosphor of Sr3YNa(PO4)3F:Tb3+ via interionic cross-relaxation energy transfer

A series of color-tunable Sr3YNa(PO4)3F:Tb3+ phosphors with a fluorapatite structure were synthesized by a traditional high-temperature solid state reaction. The emitting color tuning from blue to green can be observed by gradually increasing Tb3+ concentrations, which is attributed to the enhanced cross-relaxation (CR) between Tb3+ ions, as described by (5D3, 7F6)–(5D4, 7F0). The CR process is analyzed based on the Dexter and Inokuti–Hirayama model, which is assigned to the electric dipole–dipole interaction. The energy transfer critical distance between Tb3+ ions is evaluated to be 18.1 A. In addition, the thermal quenching mechanism of Sr3YNa(PO4)3F:Tb3+ is also investigated. At the general working temperature of an LED (423 K), the luminescence intensity still maintains 81% and 92% with the Tb3+ concentration of 10 and 30 mol%, respectively, indicating an excellent thermal quenching performance of Tb3+. Due to the good optical and thermal properties, the Sr3YNa(PO4)3F:Tb3+ phosphor can be used as a promising green emitting phosphor candidate in the field of white light applications.

Enhanced stability of perovskite solar cells using hydrophobic organic fluoropolymer

Hydrophobic organic fluoropolymers (HOFPs) with excellent hydrophobic, heat-resistant, and sunlight-transparent properties were synthesized by emulsion polymerization. The HOFP layer was inserted between a (FAMA)Pb(IBr)3 active layer and a hole transport layer in perovskite solar cells (PSCs). The performance of the resulting PSC devices depends highly on the thickness of the HOFP layer. Under optimized HOFP thickness, a moderate steady power conversion efficiency (PCE) of 16.9% was achieved. Remarkably, the optimized PSCs without any encapsulation exhibit outstanding shelf stability under ambient conditions, and the PCE could maintain 80% of its initial value after 2400 h (100u2009days), which was among the ever reported best stability whereas, the reference device without HOFP shows rapid severe degradation after only a few days. The significantly improved stability of PSCs was mainly ascribed to the impermeable barrier properties of the HOFP layer, which protect the perovskite active layer against moisture and oxygen from the ambient atmosphere.Hydrophobic organic fluoropolymers (HOFPs) with excellent hydrophobic, heat-resistant, and sunlight-transparent properties were synthesized by emulsion polymerization. The HOFP layer was inserted between a (FAMA)Pb(IBr)3 active layer and a hole transport layer in perovskite solar cells (PSCs). The performance of the resulting PSC devices depends highly on the thickness of the HOFP layer. Under optimized HOFP thickness, a moderate steady power conversion efficiency (PCE) of 16.9% was achieved. Remarkably, the optimized PSCs without any encapsulation exhibit outstanding shelf stability under ambient conditions, and the PCE could maintain 80% of its initial value after 2400 h (100u2009days), which was among the ever reported best stability whereas, the reference device without HOFP shows rapid severe degradation after only a few days. The significantly improved stability of PSCs was mainly ascribed to the impermeable barrier properties of the HOFP layer, which protect the perovskite active layer against moisture...

Theoretical analysis on quenching mechanisms for Lu2O3: Eu3+ nanospheres

Lu2O3 nanospheres with various concentrations of Eu3+ are synthesized via urea homogeneous precipitation method. The influence of the Eu3+ doping concentration on the structural, optical properties was systemically studied. The emission spectra of Lu2O3: Eu3+ nanospheres consist of a group of sharp lines from blue to red light originating from 5D1, 0→7FJ transitions of Eu3+. The photoluminescence intensities and decay times varies with the increasing Eu3+ concentrations, which are attributed to the energy transfer among the Eu3+ ions. The fluorescent quenching is mainly due to the D–D interaction by using the Van Uitert’s model and Dexter’s model. The Lu2O3: Eu3+ phosphor exhibits good thermal characteristics. The emission intensity of Lu2O3: Eu3+ phosphor only decreases 8% with the increasing of temperature from 303 to 403xa0K. The temperature quenching behavior of Lu2O3: Eu3+ is attributed to the crossover process via the charge transfer band with high activation energy of 0.264xa0eV. Moreover, the Judd–Ofelt parameters

Low Temperature Surface Passivation of Black Silicon Solar Cells by High-Pressure O2 Thermal Oxidation

Omega_{lambda}