Surface and bulk passivation of multicrystalline silicon solar cells by ...

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damage or stress in the silicon lattice in such a way that extended defects needed for
trapping impurities are formed. These chemically reactive trapping sites are usually
located at the wafer backside.
Solar cells are minority-carrier devices and use nearly the entire bulk of the
device. It is more attractive to apply external gettering techniques to clean up the bulk
of the material. Phosphorous diffusion and Al alloying are some of the processes that
have worked well for efficient gettering of solar cells. Because these processes are
used extensively in solar-cell manufacturing for junction and contact formation, all Si
solar cells experience a certain degree of gettering.
1.7 Passivation of Residual Impurities and Defects
It should be noted that not all impurities can be completely gettered during solar-cell
processing. Even impurities which are readily getterable remain in the solar cell at
significant levels and introduce detrimental effects on solar-cell performance. In
addition to the residual impurities, many crystallographic defects are stable at the
processing temperatures. It is often observed that defect concentrations remain
essentially unaltered by solar-cell processing. Therefore, it is important to identify
methods of dealing with residual impurities and defects. Passivation has been applied
to deal with residual impurities and defects. .
It is known that the hydrogen passivation yields very good results in terms of
passivation. One of the most promising methods is application of the hydrogenated
amorphous silicon nitride layers (a-SiΝ :Η) deposited by plasma-enhanced chemical
vapor deposition (PECVD). These layers are used as very effective antireflective
coatings [37]. The major interest in these films, however, is attributed to the bulk and
surface defect passivation of silicon. It has been shown that hydrogen released from