On the Mechanism of Hydrogen Diffusion in Si Solar Cells Using PECVD SiN:H
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On the Mechanism of Hydrogen Diffusion in Si Solar Cells Using PECVD SiN:H B.L. Sopori,1 Y. Zhang,2 R. Reedy,1 K. M. Jones,1 Y. Yan,1 M. M. Al-Jassim,1 J. Kalejs,3 and B. Bathey3 1 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 2 Bioarray Solutions, Warren, NJ 07059 3 RWE Schott Solar, Billerica, MA 01821 ABSTRACT A mechanism for transport and diffusion of H in a silicon solar cell by PECVD SiN:H process is proposed. Plasma-induced surface damage “stores” H during the nitride deposition, which is driven into the bulk of the solar cell during metal-contact firing. Theoretical and experimental results are given that verify this mechanism. INTRODUCTION Commercial Si solar cells are fabricated on low-cost wafers that contain high concentrations of impurities and defects. Most of these impurities and defects are removed or annihilated during solar cell fabrication by the gettering action associated with process steps such as phosphorous diffusion and Al alloying—processes used for formation of N/P junction and metallization contacts, respectively [1]. However, significant concentrations of impurities and defects continue to remain in the cell in electrically active states. Hydrogen has been used very successfully to passivate these residual defects and impurities, leading to improvements in cell performance. In some cases, cell efficiency can increase from 10% -11% to 14% – 15%. Until recently, H passivation was done as a separate process step in which H was diffused at temperatures of 300°– 400°C by a plasma process. However, a new approach is now being used in the photovoltaic (PV) industry in which H passivation is a byproduct of a multifunctional process [2,3]. This process consists of deposition of a SiN:H film as an antireflection (AR) coating, and its subsequent processing to form the front metal contact by firing a screen-printed ink-pattern directly through this layer. Although this process is now used universally in the PV industry, a basic understanding of various mechanisms involved in it is severely lacking. To date, the process optimization is done empirically, for each material type or each batch of wafers, simply by optimizing the cell performance through trial and error. However, detailed studies are needed to understand each function of this process separately and to evaluate their synergistic effects. This understanding will help in optimum process design for higher cell efficiencies. One of the issues that need to be investigated is how H is transported from plasma into the bulk of the solar cell. In this paper, we describe the mechanism for H transport based on our theoretical model, and report experimental details for its verification. SiN:H DEPOSITION AND SUBSEQUENT PROCESSING In a typical commercial SiN:H-processing sequence, a thin layer (about 750 Å) of SiN:H is deposited on an N/P device by a plasma-enhanced chemical vapor deposition (PECVD) technique, using NH3 and SiH4 as process gases. The deposition is done in a temperature range of 300° - 400°C. The paramet
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