Higher H diffusivities were observed in some solar-cell materials than in Czochralski or float-zone wafers. Secondary ion mass spectroscopic profiles of H or D, which had been implanted at low energies and at temperatures ranging from 100 to 300C, were compared for various types of Si substrate. The data (table 25) could be described by the expressions:

polycrystalline:     D (cm2/s) = 1 x 10-4exp[-0.50(eV)/kT]

Czochralski:     D (cm2/s) = 1 x 10-4exp[-0.58(eV)/kT]

float-zone:     D (cm2/s) = 1 x 10-4exp[-0.56(eV)/kT]

It was found that the presence of O seemed to lower the grain-boundary diffusivities. The diffusivity in Czochralski material was lower than that in float-zone material. It was suggested that the lower diffusivity in the former case was also related to O. A new technique, which exploited the H decoration of dislocations was used to verify directly the large diffusion depths in some solar-cell material. The higher H diffusivity permitted reverse-side hydrogenation of solar cells to be carried out in less than 0.5h; with a significant improvement in cell performance.

Observation of Enhanced Hydrogen Diffusion in Solar Cell Silicon. B.L.Sopori, K.Jones, X.J.Deng: Applied Physics Letters, 1992, 61[21], 2560-2

Table 25

Diffusivity of H in Various Types of Si