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2. Master Thesis / Tesis Master
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Title:	Development of amorrphous oxygenated cadmium sulfide (a-CdS:O) window material for higher efficiency ultra thin CdTe solar cell
Authors:	Islam Mohammad Aminul. (P49137)
Supervisor:	Nowshad Amin, Associate Professor Dr.
Keywords:	Solar cells. CdS/CdTe cell
Issue Date:	2-Oct-2011
Description:	Thin film cadmium telluride (p-type CdTe) absorbers with cadmium sulphide (n-type CdS) hetero-junction partner are promising candidates for high efficiency low cost solutions of solar energy harvesting devices. These devices have band gaps well-suited for effective absorption of sunlight. Most importantly, the materials used in these devices can be deposited in a variety of industry-friendly ways, so that the cost associated with manufacturing is generally lower than other available technologies. Although poly-crystalline CdS has been found to be the best suited heterojunction partner for CdTe solar cell, the conventional polycrystalline CdS/CdTe cell has three main issues that limit device performance. At first, poly-CdS film has a band gap of 2.42 eV, which causes considerable optical absorption at the blue region to reduce the current (Jsc) by lowering quantum efficiency (QE). Then, a lower band gap (1.3 eV) CdS1-yTey can be formed due to the inter diffusion between CdTe and CdS layer at the P-N junction which also affects the performance. Finally, there exists nearly 10% lattice mismatch between poly CdTe and poly CdS, which causes high defect density around the junction region. Lattice mismatch can be reduced by device fabrication in high temperature but it enhances the inter-diffusion again and forms intermixed layers (CdS1-yTey and CdTexS1-x) causing pinholes and limits the improvement of Voc and FF. In order to overcome these problems, this research proposed the introduction of poly-CdS to amorphous oxygenated CdS (a-CdS:O) as window layer. This a-CdS:O window material has higher optical band gap (2.5-3.1 eV), better lattice match with CdTe absorber materials and reduced inter-diffusion tendency of CdS and CdTe layers as found from studies. This dissertation systematically studies the conversion process of poly CdS to a-CdS:O and develops a strategy for the fabrication of suitable a-CdS:O layer to be applied in CdTe solar cells. The effects of the a-CdS:O layer on CdTe solar cell performance has been studied through simulation, experimental fabrication and characterizations. Numerical simulation is used to evaluate how combinations of TCO front contact, a-CdS:O window materials and back surface field effect the cell performance. The stress analysis on the effects of light intensity and temperature was also performed. Degradation in specific parameters like Voc, Jsc, FF, Rs, Rsh has been found and an attempt has been made to explain these phenomena. Strategies to improve efficiency and cell performance are explored from simulation works. Experimentally, to fabricate an ultra thin CdTe solar cell structure, CdTe layer of 1 μm was deposited on top of a-CdS:O as the absorber layer aiming to attain reduced material and energy input. The cell was activated using CdCl2 treatment at 380 0 C for 15 minutes. Different transparent conducting oxides (TCO) and back surface field (BSF) layer materials have been investigated for best performance. RF sputtering has been used to deposit all the layers (ITO/a-CdS:O/CdTe/BSF/BC) under various conditions, during which the process has been optimized. The conversion of poly CdS to a-CdS:O is observed by XRD spectra, UV-Vis spectrometry, AFM images and EDX confirmed the structural transition of CdS to a-CdS:O. From the UV-Vis absorbance line, the band gap of a-CdS:O have been found to be 2.33 eV - 3.03 eV. The SEM images of a-CdS:O and CdTe showed large grain sizes of the thin film, which also pave the way for better performance by reducing grain boundary recombination area. In conclusion, this study has explored and exposed the possibility of introducing amorphous oxygenated CdS to the conventional CdTe solar cells for better conversion efficiency.,Master
Pages:	131
Call Number:	TK2960 .I845 2011 3
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

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