Khoa Vật lý kỹ thuật và Công nghệ nano, Trường Đại học Công nghệ, Đại học Quốc Gia Hà Nội

Khủng hoảng năng lượng trên toàn cầu hiện nay đã gây ra một hệ lụy vô cùng lớn, giá xăng, dầu tăng cao buộc chính quyền các nước cần quan tâm đặc biệt tới lĩnh vực này. Gần đây, giá xăng, dầu tại Việt Nam tăng kỷ lục trên 30.000 VNĐ/lít, buộc người dân phải thay đổi nhu cầu, thói quen đi lại của mình. Với chi phí vận hành phù hợp, xe ô tô điện (EC – Electric Car) có thể được xem như là một giải pháp hứa hẹn nhằm thay thế cho các loại xe có động cơ chạy bằng xăng, dầu. Trong nghiên cứu này, chúng tôi đã thiết kế và tính toán công suất cho trạm sạc pin mặt trời nạp ô tô điện (VFe34) và xe đạp điện (dùng cho hộ gia đình có công suất P = 9 kWp, DVốn = 179.672.000 VNĐ). Thời gian thu hồi vốn là 5,2 năm và lợi nhuận ước lượng là 34.197.878 VNĐ/năm. Đối với trạm sạc pin mặt trời cho 12 ô tô điện có vốn đầu tư thêm cho các mô hình trạm sạc độc lập, trạm sạc pin mặt trời hòa lưới có tích trữ và không có tích trữ cho thấy hiệu quả là lợi nhuận của từng mô hình. Chi phí nguyên vật liệu 759.176.000 VNĐ cho một trạm sạc pin mặt trời hòa lưới thì sau 4,1 năm đã hoàn vốn (kể từ năm thứ ba trở đi mỗi năm thu về hơn 180 triệu). Thay vì thiết kế một trạm sạc ô tô thông thường, nghiên cứu này đã đưa ra các mô hình trạm sạc thông minh tích hợp pin mặt trời nhằm chứng minh hiệu quả và mức độ ứng dụng thực tế cao.

The crystal structure, phonon vibration, microstructure, and magnetic properties have been investigated in multiferroics Bi0.9Sm0.1Fe1-xMnxO3 for \(x = 0.02 – 0.1\). The structural analysis by XRD and Rietveld refinement suggest that Mn doping compounds crystallize in the polar R3c rhombohedral symmetry (isostructural with BiFeO3). Raman analysis confirms no structural transformation but the change of line widths and peak intensities reveal the lattice distortion in Mn-substitution samples. The study of microstructure shows no obvious change of grain size and shape. The magnetic properties of the as-prepared samples show the linear magnetic field dependence of magnetization, suggesting the antiferromagnetic feature of polycrystalline ceramics. The field dependence of magnetization measured after two-years synthesis and after applying an electric field reveal a decrease of maximum magnetization but the hysteresis loops retain the antiferromagnetic behavior. The implication of these results is that the magnetic properties of single structural phase compound, including coercivity and remanent magnetization, do not show the aging behavior as observed in the morphotropic phase boundary systems.

In this work, we carried out survey on magnetic field strength and gradient in space around arrays of micro-sized parallelepipedic magnets by simulation and calculation. Magnetic field distributions are a function of magnet’s size and position with respect to magnet’s surface. Our purpose is to explain how magnetic interactions evolve while dimensions of magnetic sources are reduced. Firstly, the simulations and calculations were executed for a magnet with a large surface size of 1,000×1,000 µm2, a thickness of 5 µm, and a residual magnetism of 1.6T perpendicular to its surface. Then, the similar works were also performed for arrays of magnets with smaller surface sizes, e.g. 1,000×500 µm2; 1,000×200 µm2; 1,000×100 µm2; 1,000×50 µm2 and 1,000×10 µm2. Consequently, both the magnetic field strength and gradient in the space which is above and near the surface of the magnets, particularly, the space from the surface of the magnets to the height of 100 µm far from the surface of the magnets, were enhanced when the magnets’ size were appropriately reduced. This suggests that the application field of the magnets will be expanded and their integration into microsystems will be grown as the size of the magnets is reduced.  

Lead-free Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT) particles were synthesized by using  sol-gel method. The samples were investigated by X-ray diffractometry (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), and vibrating sample magnetometer (VSM). The absorption of microwaves of the lead-free BNKT powders is determined from the magnetic (permeability) and dielectric (permittivity) properties at the frequency range from 2 to 18 GHz. Absorption characteristics of paraffin(wax)-mixed BNKT compounds at different sample thicknesses were also investigated. The microwave absorption properties show that the maximum reflection loss is -21.72 dB (99.9%) at 13.66 GHz with a thickness of 3.2 mm. BNKT composites are thought to be used as a promising microwave absorption material.  

This article presents optical simulation results of planar CH3NH3PbI3 solar cells using a MATLAB script developed by McGehee’s group (Stanford University). The device structure is composed of FTO/HEL/AL/EEL/LiF/Al, where HEL is the hole-extraction layer, AL is the active layer (CH3NH3PbI3), and EEL is the electron-extraction layer. In this MATLAB script, the transfer matrix method was used, where transmission and reflection were calculated for each interface in the stack as well as attenuation in each layer. The wavelength-dependent optical constants (n and k) of each layer were measured by spectroscopic ellipsometry (SE). The exciton generation rates within the active layer were calculated based on the data of optical constants, as well as the thickness of each layer. Considering the Internal Quantum Efficiency (IQE) equal to 100% at all wavelengths, the predicted short-circuit currents (JSC) were also estimated. The obtained results show a good agreement with the experimental values of JSC measured on real devices.

In this work, electrical simulations of planar CH3NH3PbI3 solar cells using a SCAPS-1D (a Solar Cell Capacitance Simulator) simulation tool were performed to determine the density of defects based on the performance parameters and J-V characteristics of actual experimental planar CH3NH3PbI3 solar cells. Two types of defects (bulk and interface) were introduced into the simulation model. The densities of those defects were found by fitting the J-V characteristics and performance parameters including , , and FF to the experimental data. The methods and results reported in this paper showed a close relationship between the parameters of defects in planar perovskite solar cells.      

Organometal halide perovskite materials have shown high potential as light absorbers for photovoltaic applications. In this work, perovskite planar solar cells were fabricated on corning substrates with the structure as follows: the first layer made of tantalum-doped tin oxide as transparent contact material, followed by sputtering niobium-doped titanium oxide as the compact electron transport layer; covered with perovskite CH3NH3PbI3 as the light harvester by combination between spin-coating and dipping methods; CuSCN was evaporated as the hole transport layer; the final thin Al/Ag electrodes were deposited. This configuration is shortly described as Al/TTO/NTO/CH3NH3PbI3/CuSCN/Ag. Such heterojunctions are expected to be suitable for the development of efficient hybrid solar cells. The fabricated cells were measured under the air mass 1.5 illumination condition, showed the rectification effect and exhibited a power conversion efficiency of 0.007%, with a open circuit voltage of 53.2 mV, a short circuit current of 0.36 mA/cm2, and a form factor of 37%. The power conversion efficiency will be further optimized in near future.