Atteq ur Rehman

Review of the Potential of the Ni/Cu Plating Technique for Crystalline Silicon Solar Cells

Developing a better method for the metallization of silicon solar cells is integral part of realizing superior efficiency. Currently, contact realization using screen printing is the leading technology in the silicon based photovoltaic industry, as it is simple and fast. However, the problem with metallization of this kind is that it has a lower aspect ratio and higher contact resistance, which limits solar cell efficiency. The mounting cost of silver pastes and decreasing silicon wafer thicknesses encourages silicon solar cell manufacturers to develop fresh metallization techniques involving a lower quantity of silver usage and not relying pressing process of screen printing. In recent times nickel/copper (Ni/Cu) based metal plating has emerged as a metallization method that may solve these issues. This paper offers a detailed review and understanding of a Ni/Cu based plating technique for silicon solar cells. The formation of a Ni seed layer by adopting various deposition techniques and a Cu conducting layer using a light induced plating (LIP) process are appraised. Unlike screen-printed metallization, a step involving patterning is crucial for opening the masking layer. Consequently, experimental procedures involving patterning methods are also explicated. Lastly, the issues of adhesion, back ground plating, process complexity and reliability for industrial applications are also addressed.

Advancements in n-Type Base Crystalline Silicon Solar Cells and Their Emergence in the Photovoltaic Industry

The p-type crystalline silicon wafers have occupied most of the solar cell market today. However, modules made with n-type crystalline silicon wafers are actually the most efficient modules up to date. This is because the material properties offered by n-type crystalline silicon substrates are suitable for higher efficiencies. Properties such as the absence of boron-oxygen related defects and a greater tolerance to key metal impurities by n-type crystalline silicon substrates are major factors that underline the efficiency of n-type crystalline silicon wafer modules. The bi-facial design of n-type cells with good rear-side electronic and optical properties on an industrial scale can be shaped as well. Furthermore, the development in the industrialization of solar cell designs based on n-type crystalline silicon substrates also highlights its boost in the contributions to the photovoltaic industry. In this paper, a review of various solar cell structures that can be realized on n-type crystalline silicon substrates will be given. Moreover, the current standing of solar cell technology based on n-type substrates and its contribution in photovoltaic industry will also be discussed.

Crystalline silicon thin-film solar cells on ceramic substrates

We provide a review and analysis of research on crystalline silicon thin-film solar cells (CSiTFSCs) on ceramic substrates. The use of foreign substrates (non-silicon materials) for the processing of crystalline silicon solar cells could potentially decrease solar-grade silicon consumption and significantly reduce module costs. In order to enhance the efficiency potential of CSiTFSCs on ceramic substrates, high-temperature silicon film deposition is favored. High-quality electronic-grade silicon films are intended to be deposited at higher temperature as it can help increase both deposition rates and grain sizes. The potential low-cost ceramic substrates have some major restrictions in terms of cell processing technology at high temperatures. In this paper, an overview of the research on thin-film solar-cell technologies on ceramic substrates is presented. Major processing steps for CSiTFSC such as substrate/intermediate layer requirements and silicon thin-film deposition at high temperatures will be discussed. So far, devices have been demonstrated with efficiencies up to 13.4% on graphite, 8.2% on mullite, and 9.4% on silicon nitride (Si3N4) ceramic substrates.

Study of Annealing Temperature for Ni/Cu/Ag Plated Front Contact Single Crystalline Solar Cells

Ni/Cu/Ag contact formed by plating has continuously been studied as a future metallization technique of solar cells due to the lower cost of material and better electrical performance compared with the screen-printed Ag contact. For the metallization of samples, native oxide on a laser-patterned Si area was etched with a buffered oxide etch solution for uniform plating. A Ni seed layer for Cu plating was deposited by using alkaline electroless plating. Afterward, a Cu-Ag metal stack was plated by light-induced plating followed by annealing in a tube furnace with an N2 gas atmosphere. This annealing process forms NiSix, which enhances contact resistance and adhesion. However, Ni penetration through the emitter layer can produce shunting paths, which decrease cell performance. In this experiment, 2.6 N/mm was obtained as the highest adhesion result. In addition, voids that can degrade adhesion were observed at the interface of Cu-Ag due to the Kirkendall effect. According to the experiments results, the suggestion of annealing condition was discussed to have good electrical and physical properties of plated Ni/Cu/Ag front contact.

Ni/Cu/Ag plated contacts: A study of resistivity and contact adhesion for crystalline-Si solar cells

Ni/Cu/Ag plated contacts were examined as an alternate to Ag screen printed contacts for silicon (Si) solar cell metallization. To realize a reliable contact for industrial applications, the contact resistance and its adhesion to Si substrates were evaluated. Si surface roughness by picosecond (ps) laser ablation of silicon-nitride (SiNx) antireflection coating (ARC) was done in order to prepare the patterns. The sintering process after Ni/Cu/Ag full metallization in the form of the post-annealing process was applied to investigate the contact resistivity and adhesion. A very low contact resistivity of approximately 0.5 mΩcm2 has been achieved with measurements made by the transfer length method (TLM). Thin finger lines of about 26 μm wide and a line resistance of 0.51 Ω/cm have been realized by plating technology. Improved contact adhesion by combining the ps-laser-ablation and post-annealing process has been achieved. We have shown the peel-off strengths >1 N/mm with a higher average adhesion of 1.9 N/mm. Our pull-tab adhesion tests demonstrate excellent strength well above the wafer breakage force.

Past and future of graphene/silicon heterojunction solar cells: a review

Graphene/silicon (Gr/Si) Schottky junction solar cells represent an alternative low-cost, easy fabrication structure in photovoltaic devices. After graphenes emergence in 2004, the first Gr/Si solar cell was fabricated in 2010, and was able to achieve upto 15% efficiency in less than a decade. This breakthrough in cell efficiency was realized by the fact that Gr has tremendous electrical and optical properties for photovoltaic applications. In this review, we highlight some of the recent progress in Gr/Si heterojunction solar cells. The growth processes of 2D graphene using the CVD process are discussed in detail. Afterwards, the key parameters that help to enhance the power conversion efficiency (PCE) of solar cells are considered. The interface of Gr/Si and the effects of chemical doping on the cell parameters were studied. Lastly, the challenges and limitations along with the future developments for Gr/Si solar cells are discussed in detail.

High Temperature Crystallization Process of a-Si Thin-films on Aluminum Nitride Substrates

Crystallization of silicon (Si) from amorphous silicon (a-Si) on foreign substrates has been studied by various research institutes. Crystallization of silicon thin-films on foreign substrates acts as an active layer in silicon thinfilm solar cells. In this research, due to the compatibility of thermal stability and expansion coefficient with Si, we used an aluminum nitride (AIN) substrate as an alternative candidate to glass and other ceramic substrates. P-type amorphous Si 5 μm thin-film was deposited using an ebeam evaporator directly on AIN substrates. The deposited layer was annealed at high temperature (°C) with N2 environment in a conventional tube furnace. Optical characterization was done using an optical microscope to investigate the surface morphology of as-grown and annealed samples. A smoother surface with an average grain size of about 3–4 μm was formed after annealing. Reflectance parameters were measured by UV-vis spectrometry. UV-vis-NIR was studied on as-grown and annealed samples to calculate the quality factor of the Si thin-film which was about 84.4 %. X-ray diffraction (XRD) was used to determine the phase direction of the Si thin-film before and after thermal annealing. It was observed that FWHM varied from 7.73 to 9.30 cm−1, Raman shift was from 520 to 522 cm−1, and the stressed level also changed from 180 to 540 Mpa after annealing at high temperature for a long time. Interestingly, a crystallinity fraction was achieved of about 90 % at 1100 °C.

Microstructural surface analysis of Ni/Cu front contact after peel force test to improve contact adhesion

ABSTRACT Ni/Cu two-step plating has been applied to solar cells as a metallization technique since it has low contact resistance after nickel sintering process, and is suitable for fabricating narrow fingers. However, a reliable adhesion of the Ni/Cu contact is still one of the remaining challenges. In this experiment, a peel force test was used to investigate the dependence of varied sintering temperatures on the adhesion of the Ni/Cu front contact. The surface of the bus bars was observed by field emission scanning electron microscope (FE-SEM) to analyse the high and low adhesion regions. When the adhesion result was high, silicon chunks from the substrate were often observed on the nickel layer (ribbon side). Also, the influence of oxidation on the nickel surface during sintering was discussed with the oxygen atomic ratio, which was measured by energy dispersive spectroscopy (EDS). Decent adhesion values up to 1.16 N/mm average with 2.47 N/mm maximum have been achieved with good soldering process.