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1210-Q01-01

Hydrogen Passivation of Defects in Crystalline Silicon Solar Cells M. Stavola1, F. Jiang1, S. Kleekajai1, L. Wen1, C. Peng1, V. Yelundur2, A. Rohatgi2, G. Hahn3, L. Carnel4 and J. Kalejs5 1 Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015 USA 2 School of Electrical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 USA 3 Department of Physics, University of Konstanz, 78457 Konstanz, Germany 4 REC Wafer AS, NO-3908 Porsgrunn, Norway 5 American Capital Energy, N. Chelmsford, MA 01863 USA

ABSTRACT Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. We have developed strategies by which hydrogen in silicon can be detected by IR spectroscopy with high sensitivity. The introduction of hydrogen into Si by the post-deposition annealing of a hydrogen-rich, SiNx coating has been investigated to determine hydrogen's concentration and penetration depth. Different hydrogenation processes were studied so that their effectiveness for the passivation of bulk defects could be compared. The best conditions investigated in our experiments yielded a hydrogen concentration near 1015 cm-3 and a diffusion depth consistent with the diffusivity of H found by Van Wieringen and Warmoltz.

INTRODUCTION Crystalline-Si, solar-cell materials are fabricated by a variety of methods (Czochralski, cast, ribbon …), giving rise to materials with different defect content and properties [1-3]. The Si substrates that are often used for the fabrication of solar cells to reduce cost give rise to defect issues that must be addressed. These Si materials typically have higher concentrations of structural defects, carbon, and transition-metal impurities than can be tolerated for microelectronics applications. To improve the performance of solar cells fabricated from lowcost Si materials, hydrogen is introduced to passivate defects in the Si bulk. A process that is used by industry to introduce hydrogen is by the post-deposition annealing of a hydrogen-rich SiNx layer that is used as an antireflection (AR) coating [4-7]. A number of questions about this hydrogen introduction process and hydrogen’s subsequent interactions with defects have proved difficult to address because of the low concentration of hydrogen that is introduced into the Si bulk. The present paper contains a survey of recent experiments that have been performed to detect and quantify the small concentrations of hydrogen in Si that are typically introduced by the post-deposition annealing of a SiNx AR coating. A schematic of a simple design for a Si solar cell is shown in Fig. 1 [8]. A shallow, phosphorus-diffused junction is fabricated on a p-type Si substrate. Multicrystalline Si with a grain size of order 1 cm2 is often used for the substrate material as a compromise between quality and cost. In addition to the front and back contacts, an antireflection coating is deposited onto the solar-cell front surface to increase the absorption of incident sunlight.