182
S. Leu and D. Sontag
lead to the LID (light induced degradation, see Chap. 10) effect, which in its turn
causes degradation in the performance of a solar cell of 5–15%. LID effects occur
above all in monocrystalline p-type silicon, which is doped with boron and only to a
lesser extent in multicrystalline solar cells; they hardly occur, at all in n-type silicon.
When we bring more hydrogen into the bulk, we can reduce the LID effect and we
have less oxygen in silicon.
23 LID has according to recent investigations [14] been
observed to occur especially in solar cells that have undergone high temperature
cell processes (see Chap. 10). Another way to reduce the LID effect is to dope with
gallium (Ga). Because of patent restrictions, p-type gallium-doped wafers could not
be offered commercially until 2018; it is at the moment not clear whether Ga-doped
material will ever come onto the market.
24
With hydrogen we can reduce the LID effect. But the higher the hydrogen content,
the higher the LETID (light and elevated Temperature Induced Degradation)
effect. LETID occurs mainly in multicrystalline material when solar cells are exposed
to ambient temperatures higher than 50 °C and is less pronounced in monocrystalline
solar cells. It is enhanced because of the high firing temperature (800–1000 °C) used
for the metallisation paste during cell manufacturing (see Chap. 5). Due to these high
temperatures, more hydrogen penetrates into the cell and, thus, the higher the LETID
effect will be. The thinner the wafer, the less pronounced are LID and LETID.
If both effects, LID and LETID, occur simultaneously, this can lead to degradation,
which is as high as 15%. Since HJT cells are manufactured with n-type material and
additionally very low process temperatures are applied, LID and LETID effects are
negligible.
(e) Gettering According to the above explanations a–d, n-type material has significant advantages over p-type material when both have the same density of impurities.
This is also the reason why n-type material does in general not require gettering
processes. On the other hand, it is not at all possible to apply gettering processes
to HJT cells: Due to the temperature sensitivity of the amorphous layers, process
temperatures for HJT cells must not reach more than approximately 200 °C, whereas
for gettering one requires temperatures of more than 500 °C. Because of this limitation, gettering
25 cannot be used during the production of HJT cells, as it is used for
the production of “standard” (homojunction) p-type crystalline silicon solar cells.
Indeed, gettering is not at all necessary for HJT cells with n-type material. This is
because today’s n-type silicon is so pure that a high temperature gettering step (even
if it could be applied) would no longer significantly contribute to cell improvement.
23 Oxygen penetrates the silicon during crystallization and cannot be completely avoided.
24 This is because it is very difficult to obtain homogeneous doping of silicon with Gallium (see
segredation coefficient Chap. 5).
25 When silicon is cooled after the crystal pulling process, thermal donors (TD) may be formed by
oxygen clusters. These are negatively charged. Thermal donors influence the resistivity. In the ntype material the resistivity decreases, in the p-type material it increases. TD dissolve at over 500 °C
during a gettering process. However, the influence on the improvement of the cell efficiency is not
economically meaningful (0.2% abs.); this is the reason why gettering processes are not carried out
with n-type material.
S. Leu and D. Sontag
lead to the LID (light induced degradation, see Chap. 10) effect, which in its turn
causes degradation in the performance of a solar cell of 5–15%. LID effects occur
above all in monocrystalline p-type silicon, which is doped with boron and only to a
lesser extent in multicrystalline solar cells; they hardly occur, at all in n-type silicon.
When we bring more hydrogen into the bulk, we can reduce the LID effect and we
have less oxygen in silicon.
23 LID has according to recent investigations [14] been
observed to occur especially in solar cells that have undergone high temperature
cell processes (see Chap. 10). Another way to reduce the LID effect is to dope with
gallium (Ga). Because of patent restrictions, p-type gallium-doped wafers could not
be offered commercially until 2018; it is at the moment not clear whether Ga-doped
material will ever come onto the market.
24
With hydrogen we can reduce the LID effect. But the higher the hydrogen content,
the higher the LETID (light and elevated Temperature Induced Degradation)
effect. LETID occurs mainly in multicrystalline material when solar cells are exposed
to ambient temperatures higher than 50 °C and is less pronounced in monocrystalline
solar cells. It is enhanced because of the high firing temperature (800–1000 °C) used
for the metallisation paste during cell manufacturing (see Chap. 5). Due to these high
temperatures, more hydrogen penetrates into the cell and, thus, the higher the LETID
effect will be. The thinner the wafer, the less pronounced are LID and LETID.
If both effects, LID and LETID, occur simultaneously, this can lead to degradation,
which is as high as 15%. Since HJT cells are manufactured with n-type material and
additionally very low process temperatures are applied, LID and LETID effects are
negligible.
(e) Gettering According to the above explanations a–d, n-type material has significant advantages over p-type material when both have the same density of impurities.
This is also the reason why n-type material does in general not require gettering
processes. On the other hand, it is not at all possible to apply gettering processes
to HJT cells: Due to the temperature sensitivity of the amorphous layers, process
temperatures for HJT cells must not reach more than approximately 200 °C, whereas
for gettering one requires temperatures of more than 500 °C. Because of this limitation, gettering
25 cannot be used during the production of HJT cells, as it is used for
the production of “standard” (homojunction) p-type crystalline silicon solar cells.
Indeed, gettering is not at all necessary for HJT cells with n-type material. This is
because today’s n-type silicon is so pure that a high temperature gettering step (even
if it could be applied) would no longer significantly contribute to cell improvement.
23 Oxygen penetrates the silicon during crystallization and cannot be completely avoided.
24 This is because it is very difficult to obtain homogeneous doping of silicon with Gallium (see
segredation coefficient Chap. 5).
25 When silicon is cooled after the crystal pulling process, thermal donors (TD) may be formed by
oxygen clusters. These are negatively charged. Thermal donors influence the resistivity. In the ntype material the resistivity decreases, in the p-type material it increases. TD dissolve at over 500 °C
during a gettering process. However, the influence on the improvement of the cell efficiency is not
economically meaningful (0.2% abs.); this is the reason why gettering processes are not carried out
with n-type material.