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MECHANISM OF ENHANCED HYDROGEN DIFFUSION IN SOLAR CELL SILICON

BHUSHAN L. SOPORI National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401 ABSTRACT Experimental results that support our previously reported observations of enhanced diffusivity of hydrogen in some solar cell silicon are presented. The diffusivities of hydrogen, implanted at low energies, were measured for several commercial silicon substrates. It is shown that the diffusivity of hydrogen in some solar cell substrates is about two orders of magnitude higher than that in Float Zone silicon in a temperature range of 1000 - 3000 C. This value of diffusivity is also close to that observed along some grain boundaries in polycrystalline silicon. It is determined that hydrogen in-diffusion is accompanied by an out-diffusion of boron, and that the hydrogen diffusivity is retarded by high concentrations of oxygen. A mechanism for enhanced hydrogen diffusion is proposed which invokes formation of a highly mobile vacancy-hydrogen complex. INTRODUCTION The technological interest in the passivation of defects by hydrogen spans over many electronic devices. A major application of hydrogen passivation is expected in solar cell fabrication. In particular, hydrogenation of low-cost solar cells is expected to improve their conversion efficiencies and, hence, reduce the cost of photovoltaic energy. Although improvements in silicon solar cell efficiencies due to hydrogen passivation have been demonstrated in the laboratory, commercial application of hydrogen passivation is somewhat hampered by a lack of information on many critical aspects of hydrogen in silicon. One such area that is only vaguely understood is related to the diffusivity of hydrogen in silicon and its dependence on the material parameters. The hydrogen diffusivity was first measured by Van Wieringen and Warmoltz by diffusing hydrogen through a silicon cylinder at high temperatures, from 970 0 to 1200 °C (1). They determined diffusivity, Dh could be described by: Dh

2 = 9.4 x 10-3 exp ( -0.48eV/kT ) cm . S1

Later results, obtained on single crystal wafers, showed that the diffusion profile did not match a traditional complementary error function, and that the estimated lowtemperature diffusivity was considerably less than that extrapolated using the above equation (2,3). These results are being explained on the basis of a trapping mechanism in which defects, both self-defects as well as other defects, and other impurities can act as sinks for hydrogen interstitials. This mechanism is believed to be dominant at lower temperatures and can explain, for example, the lower diffusivity of hydrogen observed for lower resistivity silicon. Recently, we have observed that the diffusivity of hydrogen in some solar grade silicon can be quite high, several orders of magnitude higher than Float Zone silicon Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society

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(4,5). In this paper we report additional results of such an enhanced diffusivity and propose a mechanism that