The Modares Journal of Electrical Engineering
Volume:14 Issue: 4, 2015

Single-walled carbon nano tube (SWCNT) is used as a buffer layer to improve the performance of polymer solar cells. The buffer layer is located between the active layer and the cathode electrode in the solar cell structure such as PET/ITO/PEDOT:PSS/MEH-PPV:C60 (1:2)/SWCNT/Al. SWCNTs act as tunnels to help electron collection by cathode electrode, prevent the randomly movement of electrons, leading to the reduction of recombination at the interfaces after dissociation. By implementing this method, the short circuit current (Jsc) of fabricated solar cells increases from 20% to 40% depending on the solvent and the open circuit voltage (Voc) reduces slightly. Since the type of solvent has an important effect on the properties of fabricated solar cells, the active layer was spin coated using three different solvents which were Chlorobenzene, 1, 2-Dichlorobenzene and Toluene. The best performance was achieved for the device fabricated using Chlorobenzene. The resulted output powers using Chlorobenzene, 1, 2-Dichlorobenzene and Toluene were 10.04 µW/cm2, 7.59 µW/cm2 and 1.74 µW/cm2 respectively.

Design and optimization of a single crystalline silicon (c-Si) solar cell is performed to achieve the maximum light conversion efficiency. Various parameters such as doping concentration and thicknesses, and geometrical dimension of surface pyramids are studied. The inverted surface pyramid is used to increase the efficiency of the solar cell, and engineered oxide layer is used as the passivation and anti-reflect layer. Semi-analytic modeling of the output parameters of the solar cell, and numerical simulation for the structures are performed for the optimization. Fill factor (FF) of 85.4%, open circuit voltage (VOC) of 761 mV, short circuit current density (JSC) of 40.1 mA/cm2 and the overall cell efficiency of 26.3% are achieved. The optimization procedure leads to 1.5% efficiency increase in compare with the similar works.

We propose a new generation of unbiased (bias-free) antenna-less CW THz photomixer emitters array made of asymmetric MSM gratings with subwavelength pitch, operating in optical near-field regime. We take advantage of the size effects in near-field optics and electrostatics to demonstrate the possibility of enhancing the THz power by four orders of magnitude, as compared with a similar array of the same size that operates in the far-field regime. We show that with an appropriate choice of grating parameters in such THz sources the first plasmonic resonant cavity mode in nano-slit between two adjacent MSM can enhance the optical near-field absorption and hence the photo carriers’ generation under the slit in the active medium. These photo carriers, on the other hand, are accelerated by the large built-in electric field sustained under the nano-slits by two dissimilar Schottky barriers to create the desired large THz power that is mainly radiated downward. A hybrid numerical simulation method is used to model and analyze these phenomena.

In this paper, we have presented a theoretical model to investigate the modulation response, the relative intensity noise and the frequency noise characteristics of 1.55µm tunneling injection quantum dot (TI-QD) lasers. Using a small signal model based on the carriers and photon rate equations, the modulation and noise behaviour of these lasers are simulated. Our simulations imply that the modulation response of QD laser enhances in tunneling heterostructures, which is in agreement with the reported experimental results. In fact, it is demonstrated that due to tunneling of carriers directly to the ground state across a barrier, the modulation response enhances and the 3dB bandwidth of laser increases. Furthermore, the calculations indicate that the relative intensity noise level of this type of lasers increases due to decrease in the carrier population at the excited and wetting layer states. Meanwhile, the frequency noise level and linewidth show similar values to those in conventional QD lasers.

In this manuscript a compact high speed optical Mach-Zehnder modulator with very low energy consumption based on hybrid plasmonic waveguide is presented. Compared to dielectric waveguide-based structures, large propagation constant of optical modes in hybrid plasmonic waveguides reduces the propagation length required for attaining necessary phase shift for proper operation of Mach-Zehnder interferometer. On the other hand, strong light confinement in hybrid plasmonic waveguide facilitates realization of relatively short bends (with small bend radius) and compact Y-junctions which in turn reduces the overall footprint of modulator. Our simulations show theoretical modulation speed of more than 1 THz and very low energy consumption about 17 fJ/bit. In addition, the modulation depths as high as 25 dB are achievable by applying voltages between 3 V and -3 V, in a push-pull configuration. Apart from electrodes, the overall footprint is about ~120 mm2 which is, based on our knowledge, very smaller than that of common Mach-Zehnder modulators.

In this paper, a new configuration for delta conversion UPS is proposed, which could help the system for better utilization of the UPS inverters. In this configuration, parallel and series inverters of the conventional delta conversion UPS, under the grid fault conditions, are connected in parallel which help to share the load power and this results in utilizing the series inverter during fault mode. Therefore, using this configuration not only decreases the total capacity of the UPS inverters, but also decreases the system total cost. In the proposed configuration, two inverters have the same sizes and specifications which results in the system modularity that simplifies its implementation and maintenance and reduces the manufacturing and life cycle cost of the UPS system. In addition, the proposed configuration increases the system reliability. To illustrate proper operation of the proposed configuration, some simulations are carried out under the different conditions. The given simulation results validate appropriate operation of the proposed configuration.