Minhuan Wang

Sunlight-induced resistance changes and their effects on the semiconductor–metal transition behavior of VO2 film

High-quality VO2 films with precisely controlled thicknesses were grown on sapphire substrates by plasma-assisted oxide molecular beam epitaxy (MBE). To evaluate the degradation of semiconductor–metal transition (SMT) behavior of VO2 films under solar radiation, the temperature-driven SMT was investigated by measuring the electrical resistance during heating and cooling processes under solar simulator AM1.5, which provided illumination approximately matching the natural sunlight. The distinct reversible SMTs were observed for all the samples, whereas a remarkably conflicting trend in resistance change for extremely thin and thick samples was observed after exposure to the sunlight soaking system. The corresponding mechanism was proposed based on sunlight-induced resistance changes due to the transformation in the electron correlation and structural symmetry. The results might be especially attractive for some specific applications of VO2 films where solar radiation was inevitable.

VO2 Thermochromic Films on Quartz Glass Substrate Grown by RF-Plasma-Assisted Oxide Molecular Beam Epitaxy

Vanadium dioxide (VO2) thermochromic thin films with various thicknesses were grown on quartz glass substrates by radio frequency (RF)-plasma assisted oxide molecular beam epitaxy (O-MBE). The crystal structure, morphology and chemical stoichiometry were investigated systemically by X-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. An excellent reversible metal-to-insulator transition (MIT) characteristics accompanied by an abrupt change in both electrical resistivity and optical infrared (IR) transmittance was observed from the optimized sample. Remarkably, the transition temperature (TMIT) deduced from the resistivity-temperature curve was reasonably consistent with that obtained from the temperature-dependent IR transmittance. Based on Raman measurement and XPS analyses, the observations were interpreted in terms of residual stresses and chemical stoichiometry. This achievement will be of great benefit for practical application of VO2-based smart windows.

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.

Engineered Multifunctional Fluorinated Film Based on Semicontinuous Emulsion Polymerization Using Polymerizable Quaternary Ammonium Emulsifiers

Along with society’s progress, high-quality coatings are widely used. Although fluorinated polymers were successfully prepared by semicontinuous emulsion polymerization with surfactants, chlorotrifluoroethylene (CTFE), and acrylate monomers, the optimization collocation of surfactants still has room for improvement. The traditional emulsifiers are physically absorbed onto the surface of latex particles. The latex film generated by latex particles is unstable in water, which limits its application. Herein, a novel series of cationic quaternary ammonium polymerizable surfactant was selected because it can react with CTFE and acrylate monomers and can become a part of the polymers. We also studied the effects of emulsifier type on resultant emulsion properties. In addition, wonderful weatherability, water resistance, and antibacterial and antifouling of the multifunctional fluorinated films were observed, which would open up a bright future for coating industries.

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 (100 days), 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 (100 days), 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...

Bromine Doping as an Efficient Strategy to Reduce the Interfacial Defects in Hybrid Two-Dimensional/Three-Dimensional Stacking Perovskite Solar Cells

Solar-to-electricity conversion efficiency, power conversion efficiency (PCE), and stability are two important aspects of perovskite solar cells (PSCs). However, both aspects are difficult to simultaneously enhance. In the recent two years, two-dimensional (2D)/three-dimensional (3D) stacking structure, designed by covering the 3D perovskite with a thin 2D perovskite capping layer, was reported to be a promising method to achieve both a higher PCE and improved stability simultaneously. However, when reducing the surface defects of 3D perovskite, the thin 2D capping layer itself may probably introduce additional interfacial defects in a 2D/3D stacking structure, which is thought to be able to trigger trap-assisted nonradiative recombination or ion migration. Thus, efforts should be paid to reduce the interfacial defects of 2D hybrid perovskite when serving as a modification layer in a 2D/3D stacking structure PSCs. Here, we demonstrate that bromine (Br) doping of the 2D perovskite capping layer is an efficient strategy to passivate interfacial defects robustly, by which the photoluminescence lifetime is enhanced notably, whereas the interfacial charge recombination is suppressed a lot. As a result, the PCE is enhanced from 18.01% (3D perovskite) to 20.07% (Br-doped 2D/3D perovskite) along with improved moisture stability.