الفهرس 
English CoverOnly 14 pages are availabe for public view
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Abstract
Conventional energy sources such as natural gas, oil, coal, or nuclear are limited and highly depleted. Today, energy based on ‘green’ resources has attracted considerable interest and investment worldwide, as a viable alternative to the use of polluting fossil fuels. Solar energy is the most abundant source of such ‘green’ renewable energy, coming in its two forms: light and heat. The photovoltaic (PV) solar cell (SC) is one of the predominant solar energy harvesting devices to convert light directly to electricity. Currently, bulk crystalline silicon (C-Si) photovoltaic modules have ” " ~ " ”90 % of the global PV market. Silicon (Si) is one of the largest broadband light absorbing materials, where the power conversion efficiency (PCE) of a planar C-Si SC reaches 22 %. However, C-Si SC suffers from high cost. The second generation SC technology, based on low-purity thin-film (TF) materials, has emerged to reduce the high cost of the traditional C-Si SC and yet, the conversion efficiency is only ” " ~ " ”12 %, due to the small optical path length. The third generation of solar cell technology has improved the light absorption in TF-SC using light trapping techniques. This approach increases the optical path length and promotes the generation of e-h carriers, which elevates the efficiency of TF SCs. Consequently, an efficient TF SC can be designed using less active material with reduced cost. Nanowires (NW) are highly promising nanostructures that have unique optical and electrical characteristics compared to TF SCs. Such NWs have a number of merits, such as reduction in reflection, improvement in trapping, and consumption of less material.
In this Thesis, a novel design of quad crescent-shaped silicon NW SC is introduced and numerically studied. The suggested NW has quad crescent shapes which create a cavity between any adjacent NWs. Such a cavity will permit multiple light scattering with improved absorption. Additionally, new modes will be excited along the NWs which are highly coupled with the incident light. Further, the surface reflection from the crescent NWs is decreased due to the reduced surface filling ratio. The finite difference time domain method is utilized to analyze the optical characteristics of the reported structure. The proposed NW offers short circuit current density (Jsc) of 27.8 mA/cm2 and ultimate efficiency (ɳul) of 34٪ with an enhancement of 14٪ and volume reduction of 40٪ compared to the conventional NWs. The Jsc and ɳul are improved to 35.8 mA/cm2 and 43.7٪ by adding a Si substrate and back reflector to the suggested crescent NWs. The electrical characteristics of quad-crescent-shaped silicon nanowire (NW) solar cells (SCs) are numerically analyzed and as a result their performance optimized. The structure discussed consists of four crescents, forming a cavity that permits multiple light scattering with high trapping between the NWs. Additionally, new modes strongly coupled to the incident light are generated along the NWs. As a result, the optical absorption has been increased over a large portion of light wavelengths and hence the power conversion efficiency (PCE) has been improved. The electron-hole (e-h) generation rate in the design reported has been calculated using the 3D finite difference time domain method. Further, the electrical performance of the SC reported has been investigated through the finite element method, using the Lumerical charge software package. In this investigation, the axial and core-shell junctions were analyzed looking at the reported crescent and, as well, conventional NW designs. Additionally, the doping concentration and NW-junction position were studied in this design proposed, as well as the carrier-recombination-and-lifetime effects. This study has revealed that the high back surface field layer used improves the conversion efficiency by ” " ~ " ”80 %. Moreover, conserving the NW radial shell as a low thickness layer can efficiently reduce the NW sidewall recombination effect. The PCE and short circuit current were determined to be equal to 18.5٪ and 33.8 mA/cm2 for the axial junction proposed. However, the core-shell junction shows figures of 19٪ and 34.9 mA/cm2. The suggested crescent design offers an enhancement of 23٪ compared to the conventional NW, for both junctions. For a practical surface recombination velocity of 102 cm/s, the PCE of the proposed design, in the axial junction, has been reduced to 16.6٪, with a reduction of 11٪. However, the core-shell junction achieves PCE of 18.7٪ with a slight reduction of 1.6٪. Therefore, the optoelectronic performance of the core-shell junction was marginally affected by the NW surface recombination, compared to the axial junction.