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ECN publication
Passivating Multi Cristalline Si Solar Cells using SiNx:H
Published by: Publication date:
ECN Solar Energy 1-8-2005
ECN report number: Document type:
ECN-RX--05-134 Conference Paper
Number of pages: Full text:
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Presented at: 15th Workshop on Crystalline Silicon Solar Cells & Modules: Materials and Processes, Vail CO, USA, 7-10 augustus 2005.


Passivating solar cells with SiNx:H has been so far a scarcely understood effect that can only be optimized for cell production in an empirical way. In this study we determine the structural properties of SiNx:H layers with Fourier Transform Infrared (FTIR) measurements and relate these to both the deposition parameters and its passivating qualities for solar cells. Furthermore we determined the relations between the hydrogen diffusion in the SiNx:H and the structural properties of these layers.

      The Si-N, Si-H and N-H bond densities for layers deposited with either nitrogen (N2), ammonia (NH3) or deuterated ammonia (ND3|) and silane (SiH4) are affected by the N/Si flow ratio and the pressure p in a similar way although the differences in dissociation energy and rate cause different deposition mechanisms. Comparing the Si-N and Si-H bond densities found for NH3 and ND3 grown layers, we find that roughly 25% of the hydrogen in the SiNx:H layers stems from the ammonia precursor gas, while 75% stems from the silane.

      We show that Si-N bond density is an important parameter governing both the bulk and surface passivation of the SiNx:H layers. In spite of the different deposition mechanisms, the same relations hold between the H-diffusion coefficient, Si-N bond density and passivating qualities of SiNx:H layers deposited with either N2 or NH3. The best bulk and surface passivating layers have a relatively low hydrogen diffusion coefficient due to a high Si-N bond density. We find optimum values for bulk and surface passivation for Si-N bond densities of 1.3*1023 cm-3, regardless of the type of SiNx:H used and regardless of the starting wafer quality. Lower Si-N bond densities result in layers with a more open structure and this will probably lead to H2 formation during annealing. These H2 molecules will effuse into the ambient during firing, and do not contribute to the passivation of solar cells. Higher Si-N bond densities result in a too dense structure, prohibiting an effective diffusion of H-atoms into the bulk of the solar cells. This study further indicates that FTIR analysis gives us a quick and reliable tool to check the quality and properties of SiNx:H layers. This will allow optimization of SiNx:H deposition systems without having to make complete solar cells.

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