Samir Mahmmod Ahmad
@uomosul.edu.iq
University of Mosul/ Physics/ Science
University of Mosul
EDUCATION
Ph.D in Renewable Energy/Solar Cell
RESEARCH INTERESTS
Renewable energy, Silicon solar cell
Scopus Publications
Scopus Publications
Samir Mahmmod Ahmad, Cheow Siu Leong, Richard W. Winder, K. Sopian, and Saleem H. Zaidi
Springer Science and Business Media LLC
The electrical, morphological, and compositional characteristics of screen-printed Al paste contacts on p-doped Si wafers have been investigated in horizontal and vertical thermal annealing configurations over a wide temperature range. The horizontal configuration refers to an industrial six-zone conveyor belt rapid thermal annealing furnace. The vertical configuration refers to a modified three-zone quartz tube furnace with vertically stacked wafers. The contact resistivity was measured by using the transmission line method. In the horizontal configuration, the resistivity exhibited a pronounced minimum at temperature of ∼ 870°C, while higher temperatures resulted in a rapid increase in the contact resistivity. In contrast, the resistivity variation in the vertical configuration was linear. The lowest contact resistivities measured were 136 mΩ cm2 in the horizontal and 103 mΩ cm2 in the vertical configuration, demonstrating a 24% reduction with the latter approach. The surface morphology and composition of the Al/Si contact interface were determined by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy. The measured elemental concentrations were curve-fit to accurately measure the width of the interface regions. The Al/Si contact region was observed to consist of five parts: (a) a top sintered paste layer of Al/Si spheres, (b) voids between the Al/Si spheres, (c) an Al/Si eutectic region, (d) an epitaxially grown Al-doped Si layer, and (e) the lightly Al-diffused Si substrate. Sintered Al/Si spheres were observed to consist of a solid core of Al embedded in a thin shell of Al, Al2O3, SiO2, and Si. The rapid rise in resistivity at high temperatures is attributed to enhanced oxidation of Al and Si islands, resulting in thicker Al2O3/SiO2 films between metallic Al spheres. The lower resistivity observed in the vertical configuration was attributed to larger, more uniform Al–Si eutectic regions, higher density of Al/Si films within the paste region, and transformation of sintered Al spheres into larger pseudosquare islands. The proposed Al/Si interface model was further supported by the higher resistance measured for the pulsed laser-based Al/Si contact with high Si concentrations in the Al/Si eutectic region. An approximately linear reduction in resistivity as a function of time over a broad range varying from microseconds to seconds reinforced the proposed model and suggests that longer, steady-state annealing is the preferred approach to achieve the lowest contact resistivity.
Samir Mahmmod Ahmad, Cheow Siu Leong, Richard W. Winder, K. Sopian, and Saleem H. Zaidi
Springer Science and Business Media LLC
A phenomenological model of screen-printed silver contact to an n-doped, p-type multi-crystalline Si wafer, based on extensive electrical, morphological, and compositional evaluations, has been developed. Rapid and quasi steady state heating configurations over broad (150–925°C) temperature ranges were investigated. Conventional rapid thermal annealing (RTA) with a conveyor belt was used for a rapid and custom-designed three-zone quartz tube furnace (QTF) for slow temperature variations. Lowest contact resistivity at 0.15 mΩ cm2 was observed in RTA horizontal configuration which was 25 times smaller than the same in QTF. RTA contact resistivity measurements revealed a minimum at 870°C while linear reduction in contact resistance was observed for the QTF configuration. The silver/silicon contact was based on three physical mechanisms: (1) migration of Si into glass and silver regions of the paste, (2) intermixing of silver and silicon (nano- and micrometer scale), and (3) epitaxial growth of silver/silicon crystallites. Experimental evidence of silicon migration was supported through extensive phosphorous concentration measurements from silicon and silver/silicon regions. The glass film with a colloidal distribution of randomly-distributed silver/silicon crystallites leads to lower contact resistance. Rapid temperature fluctuations facilitate development of Ag/Si crystallites. The higher contact resistance in quasi steady state thermal configuration was attributed to glass films with reduced density of Ag/Si crystallites. This disadvantage may be eliminated through post-contact, forming gas annealing at lower temperatures.
Samir Mahmmod Ahmad, Cheow Siu Leong, K. Sopian, and Saleem H. Zaidi
Springer Science and Business Media LLC
Formation of an Ohmic contact requires a suitable firing temperature, appropriate doping profile, and contact dimensions within resolution limits of the screen-printing process. In this study, the role of the peak firing temperature in standard rapid thermal annealing (RTA) six-zone conveyor belt furnace (CBF) and two inexpensive alternate RTA systems [a custom-designed, three-zone, 5″-diameter quartz tube furnace (QTF) and a tabletop, 3″-diameter rapid thermal processing (RTP)] has been investigated. In addition, the role of sheet resistance and contact area in achieving low-resistance ohmic contacts has been examined. Electrical measurements of ohmic contacts between silver paste/n+-emitter layer with varying sheet resistances and aluminum paste/p-doped wafer were carried out in transmission line method configuration. Experimental measurements of the contact resistivity (ρc) exhibited the lowest values for CBF at 0.14 mΩ cm2 for Ag and 100 mΩ cm2 for Al at a peak firing temperature of 870°C. For the QTF configuration, lowest measured contact resistivities were 3.1 mΩ cm2 for Ag and 74.1 mΩ cm2 for Al at a peak firing temperature of 925°C. Finally, for the RTP configuration, lowest measured contact resistivities were 1.2 mΩ cm2 for Ag and 68.5 mΩ cm2 for Al at a peak firing temperature of 780°C. The measured contact resistivity exhibits strong linear dependence on sheet resistance. The contact resistivity for Ag decreases with contact area, while for Al the opposite behavior is observed.
Samir Mahmmod Ahmad, Siu Leong Cheow, Norasikin A. Ludin, K. Sopian, and Saleem H. Zaidi
Elsevier BV
Abstract Solar cell industrial manufacturing, based largely on proven semiconductor processing technologies supported by significant advancements in automation, has reached a plateau in terms of cost and efficiency. However, solar cell manufacturing cost (dollar/watt) is still substantially higher than fossil fuels. The route to lowering cost may not lie with continuing automation and economies of scale. Alternate fabrication processes with lower cost and environmental-sustainability coupled with self-reliance, simplicity, and affordability may lead to price compatibility with carbon-based fuels. In this paper, a custom-designed formulation of phosphoric acid has been investigated, for n-type doping in p-type substrates, as a function of concentration and drive-in temperature. For post-diffusion surface passivation and anti-reflection, thermally-grown oxide films in 50–150-nm thickness were grown. These fabrication methods facilitate process simplicity, reduced costs, and environmental sustainability by elimination of poisonous chemicals and toxic gases (POCl3, SiH4, NH3). Simultaneous fire-through contact formation process based on screen-printed front surface Ag and back surface through thermally grown oxide films was optimized as a function of the peak temperature in conveyor belt furnace. Highest efficiency solar cells fabricated exhibited efficiency of ∼13%. Analysis of results based on internal quantum efficiency and minority carried measurements reveals three contributing factors: high front surface recombination, low minority carrier lifetime, and higher reflection. Solar cell simulations based on PC1D showed that, with improved passivation, lower reflection, and high lifetimes, efficiency can be enhanced to match with commercially-produced PECVD SiN-coated solar cells.
Adnan Ali, Ayu Wazira, Samir Ahmad, Kamaruzzaman Sopian, and Saleem H. Zaidi
IEEE
Inability to achieve complete optical absorption in thin-film crystalline Si (TF c-Si) solar cells fundamentally limits efficiency. Therefore, materials with high absorption across the entire solar spectrum are highly desirable. In this study, several TF solar cell configurations based on Silicon Germanium (Si<sub>1-x</sub>Ge<sub>x</sub>) alloys have been investigated. With the help of PC1D software, TF c-Si solar cells with varying composition of Si<sub>1-x</sub>Ge<sub>x</sub> layers with x in the range of 4 % to 50 % have been modeled. In all cases, Si<sub>1-x</sub>Ge<sub>x</sub> solar cells performed better than c-Si TF solar cells; the optimized Ge concentration was determined to be 20%. For example, in case of 10-μm thick TF solar cell, crystalline Si/Si<sub>1-x</sub>Ge<sub>x</sub> exhibits simulated efficiency of ~ 22 % in contrast with ~ 17 % for same thickness c-Si solar cell. Optimization of the alloy composition, doping concentration of the Si<sub>1-x</sub>Ge<sub>x</sub>, thickness of the alloy layer and placement of the layer in the cell structure will be reported at the conference.
RECENT SCHOLAR PUBLICATIONS
Publications
"A Phenomenological Model of the Screen-Printed, Silver Paste Contact to Si Substrate"
"In-depth investigation of spin-on doped solar cells with thermally grown oxide passivation"
"Role of Firing Temperature, Sheet Resistance, and Contact Area in Contact Formation on Screen-Printed Metal Contact of Silicon Solar Cell"
"PC1D analysis of thin-film crystalline SixGe1−x/Si solar cells"
"Electrical, Morphological, and Compositional Characterization of Screen-Printed Al Contacts Annealed in Horizontal and Vertical Configurations"
"Performance evaluation of screen-printed C-Si solar cells fabricated by the simple and low-cost process"
"INVESTIGATION OF SCREEN-PRINTED METAL CONTACTS TO n-DOPED MULTI-CRYSTALLINE Si SUBSTRATE"