Studies on Backside Al-Contact Formation in Si Solar Cells: Fundamental Mechanisms
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Studies on Backside Al-Contact Formation in Si Solar Cells: Fundamental Mechanisms Bhushan Sopori,1 Vishal Mehta,1 Przemyslaw Rupnowski,1 Helio Moutinho,1 Aziz Shaikh,2 3 Chandra Khadilkar,2 Murray Bennett,3 and Dave Carlson 1 National Renewable Energy Laboratory, Golden, CO 80401, USA 2 Ferro Electronic Materials, Vista, CA 92083, USA 3 BP Solar, Frederick, MD 21703, USA ABSTRACT We have studied mechanisms of back-contact formation in screen-printed Si solar cells by a fire-through process. An optimum firing temperature profile leads to the formation of a P-Si/P+Si/ Si-Al eutectic/agglomerated Al at the back contact of a Si solar cell. Variations in the interface properties were found to arise from Al-Si melt instabilities. Experiments were performed to study melt formation. We show that this process is strongly controlled by diffusion of Si into Al. During the ramp-up, a melt is initiated at the Si-Al interface, which subsequently expands into Al and Si. During the ramp-down, the melt freezes, which causes the doped region to grow epitaxially on Si, followed by solidification of the Si-Al eutectic. Any agglomerated (or sintered) Al particles are dispersed with Si. Implications on the performance of the cell are described. INTRODUCTION The contact formation of most commercial Si solar cells is done by co-firing the cells with screen-printed front and back contacts. Typically, a gridded front-contact pattern of Ag-based ink is directly applied on the antireflection coating of SiN:H, whereas the backside has a blanket Al-based contact. The firing process, also called fire-through contact metallization, performs the following functions: (i) On the front contact, the glass frit dissolves the antireflection coating of SiN:H and allows Ag to react with Si. This reaction involves participation of a solvent metal that leaches from the glass frit and helps to lower the eutectic point of Si-Ag alloy; (ii) Diffuses hydrogen from the SiN:H-Si interface into the bulk of the cell and passivates impurities and defects; and (iii) Produces an interaction of Si and Al to form a deep back-surface field (BSF) and sinters unreacted Al to form a low-resistance contact. It is important to understand how each of these functions of this complex process can be optimized. This paper describes our investigations on the formation of the back contact, and how most of the requirements of a good back contact can be met by a suitable process. We will first describe requirements of a back contact and then discuss our experiments and results. Requirements of a Good Back Contact These requirements may be divided into electronic and optical categories. The electronic properties include: (i) a BSF for minority-carrier reflection, (ii) a uniform low-resistance ohmic contact, and (iii) a smooth, dimple-free surface of Al. Optically, the back contact must be reflecting with very little absorption so that it contributes effectively to light trapping. In addition to these, the process of making the back contact should be compatible with efficie
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