Hydrogenated silicon-nitride thin films as antireflection and passivation coatings for multicrystalline silicon solar cells

Hydrogenated silicon-nitride thin films were deposited by varying the silane-to-ammonia ratio in a plasma enhanced chemical vapor deposition (PECVD) system under relatively low temperature. This paper mainly investigates the SiN_x deposition and the effect of rapid thermal processing (RTP) on the surface passivation and on the anti-reflection coating. Also, an extensive study has been carried out on the effect of the rapid thermal processing on the carrier lifetime, reflectance, chemical composition, refractive index, and interface states, which decides the final output of the cell. By varying the silane-to-ammonia ratio in the plasma gas, it was possible to modify the index of refraction (from 1.9 to 2.3) and the silicon surface state passivation properties of the films. The results indicate that the mid-gap surface state density in silicon can be reduced down to 1.1 × 10¹⁰ cm ^-2eV^-1 for the SiN_x:H layer deposited under an optimized silane-to-ammonia ratio. A noticeable feature from the C-V studies on these films is that the interface state density [D_it) decreases as the firing temperature is increased. An improvement in the multicrystalline silicon solar cell parameters was observed, for cells with optimal SiN _x:H layers as compared to those with non-optimum SiN_x:H layers.