Screen-Printed Al Back Contacts on Si Solar Cells: Issues and Some Solutions
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1210-Q05-02
Screen-Printed Al Back Contacts on Si Solar Cells: Issues and Some Solutions Vishal Mehta,1, 2 Bhushan Sopori,1 Robert Reedy,1 Bobby To,1 Helio Moutinho,1 and N.M. Ravindra2 1 National Renewable Energy Laboratory, Golden, CO 80401, USA 2 New Jersey Institute of Technology, Newark, NJ 07102, USA ABSTRACT This paper identifies some mechanisms that lead to problems in back Al contact formation. Major issues are related to a basic problem that the Al melt has a large surface tension and tries to ball up during the firing step. Other issues arise from dissolution of the Si-Al interface and entrapment of glass within the Si-Al alloy. Si diffusion into Al can be applied to control the melt, while cooling rate can help improve the structure of various regions of the back contact for a favorable series resistance. We also discuss a modified time-temperature profile that can lead to a deep and uniform back-surface field. INTRODUCTION The fire-through contact metallization technique (also known as co-firing) is widely used to fabricate metal contacts in commercial crystalline and multicrystalline silicon (c-Si) solar cells. Contact firing requires preparation of N+/P wafers, which are then coated with about 750 Å of SiN:H by a plasma-enhanced chemical vapor deposition process. Next, screen-printed contacts consisting of gridded, Ag-based front contacts and continuous, Al-based back contacts are applied. The firing of both contacts is performed simultaneously, typically in a belt furnace using a profile that peaks at about 800°C. In this process, silver paste on the front side etches SiN:H, which reacts with Si and makes a near-ohmic contact with the N+-Si layer, and hydrogen diffuses into the Si substrate and passivates defects and impurities. On the back side, Al alloys with silicon to produce a back contact with a P+ back-surface field (BSF). Considerable amount of work has been done to determine various reactions that occur during a firing process. In particular, the Ag-Si contact formation and hydrogen diffusion have been studied in much detail. However, in spite of the fact that Al-Si reactions are well understood, the formation of a good back contact is difficult to achieve. A back contact must meet many requirements as discussed in the next section. All of them necessitate the formation of a strong alloy between Si and Al. However, strong alloying of the Si-Al contact is accompanied by many issues that continue to prevent high-efficiency solar cell fabrication. These issues include the following: (i) Molten Al do not stick uniformly to the silicon surface because of its high surface tension and tendency to ball up (Figure 1). This figure also shows another problem—discoloration—it is often observed after extended firing; (ii) Strong contact formation is often accompanied by a variety of problems such as increased shunting and formation of holes in the cell; (iii) Glass (which is necessary for improving the mechanical properties of the contact) can be trapped within Al particles, thus preventing them from re
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