5 Crystalline Silicon Solar Cells: Homojunction Cells
113
5.2.4 Passivation
(a) Bulk Passivation by Saturation of Impurities with Hydrogen
In Chap. 4, we learned that impurities will be embedded into silicon—becoming, thus,
recombination centres (defects), leading to Shockley-Read-Hall recombination. With
the help of atomic hydrogen, it is possible to electrically deactivate (passivate) the
impurities and to prevent electrons, which were generated as electron-hole pairs, from
recombining there. Additionally, the antireflection layer composed of silicon nitride
serves also as a source of hydrogen. On one hand, silicon nitride is a nearly ideal
antireflection coating, and, on the other hand, it also contains a large proportion of
hydrogen
18 —a proportion that can be adapted via the process control. The hydrogen
atoms diffuse many micrometres into the silicon, even at temperatures around 200 °C
and, thus, lead to a passivation of the defects. At temperatures > 800 °C, as used in the
production of standard solar cells, the complete bulk is slowly flooded with hydrogen.
(b) Bulk Passivation by Gettering
In the production of the silicon crystal, foreign atoms can be incorporated in the
silicon, defects can occur and dislocations generate stress in the crystal structure.
All of these effects can cause recombination centres to be formed. In Fig. 5.7 some
typical defects are illustrated.
The gettering process reduces contaminants in a wafer and increases the carrier
lifetime. This process is not fully understood. One may just state the following: With
the application of additional layers such as aluminium on the back and phosphorus
on the front, regions are formed that act like a sink for foreign atoms [7]. Especially
at high process temperatures, foreign atoms are easily released from the crystal
compound and are pushed to the surface via substitution by P or Al. On the front
side, so-called phosphorus glass is etched away after the phosphorus diffusion and,
thus, the impurities are also removed. On the back, the foreign atoms are trapped in
the aluminium. As a result, a part of the recombination centres can be eliminated.
(c) Surface Passivation
Fig. 5.7 Some typical defects in monocrystalline silicon. a a foreign atom (e.g. a dopant atom)
occupies the place of a lattice atom (silicon atom); b a foreign atom lying in an interstitial space; c a
substituted impurity generating stress in the crystal lattice structure; d precipitates (e.g. a group of
oxygen atoms forming “lumps”; e vacancy (a position in the crystal lattice which is not occupied);
f dislocation (crystallographic defect) in the lattice structure [6]
18 The hydrogen enters into the silicon nitride layer during its fabrication process, which is based
on a plasma (activated gas) containing silane, ammonia and hydrogen.
113
5.2.4 Passivation
(a) Bulk Passivation by Saturation of Impurities with Hydrogen
In Chap. 4, we learned that impurities will be embedded into silicon—becoming, thus,
recombination centres (defects), leading to Shockley-Read-Hall recombination. With
the help of atomic hydrogen, it is possible to electrically deactivate (passivate) the
impurities and to prevent electrons, which were generated as electron-hole pairs, from
recombining there. Additionally, the antireflection layer composed of silicon nitride
serves also as a source of hydrogen. On one hand, silicon nitride is a nearly ideal
antireflection coating, and, on the other hand, it also contains a large proportion of
hydrogen
18 —a proportion that can be adapted via the process control. The hydrogen
atoms diffuse many micrometres into the silicon, even at temperatures around 200 °C
and, thus, lead to a passivation of the defects. At temperatures > 800 °C, as used in the
production of standard solar cells, the complete bulk is slowly flooded with hydrogen.
(b) Bulk Passivation by Gettering
In the production of the silicon crystal, foreign atoms can be incorporated in the
silicon, defects can occur and dislocations generate stress in the crystal structure.
All of these effects can cause recombination centres to be formed. In Fig. 5.7 some
typical defects are illustrated.
The gettering process reduces contaminants in a wafer and increases the carrier
lifetime. This process is not fully understood. One may just state the following: With
the application of additional layers such as aluminium on the back and phosphorus
on the front, regions are formed that act like a sink for foreign atoms [7]. Especially
at high process temperatures, foreign atoms are easily released from the crystal
compound and are pushed to the surface via substitution by P or Al. On the front
side, so-called phosphorus glass is etched away after the phosphorus diffusion and,
thus, the impurities are also removed. On the back, the foreign atoms are trapped in
the aluminium. As a result, a part of the recombination centres can be eliminated.
(c) Surface Passivation
Fig. 5.7 Some typical defects in monocrystalline silicon. a a foreign atom (e.g. a dopant atom)
occupies the place of a lattice atom (silicon atom); b a foreign atom lying in an interstitial space; c a
substituted impurity generating stress in the crystal lattice structure; d precipitates (e.g. a group of
oxygen atoms forming “lumps”; e vacancy (a position in the crystal lattice which is not occupied);
f dislocation (crystallographic defect) in the lattice structure [6]
18 The hydrogen enters into the silicon nitride layer during its fabrication process, which is based
on a plasma (activated gas) containing silane, ammonia and hydrogen.