7 Crystalline Silicon Solar Cells: Heterojunction Cells
183
(f) Thin Wafer With a share of more than 50% in the total production costs, the
production of wafer material is the biggest cost driver in the manufacturing of a solar
cell. Therefore, wafer production also offers the greatest potential for savings. Due
to the low process temperatures and the symmetrical cell structure of heterojunction
cells, the use of very thin wafers becomes possible here. A thick metal layer (Aluminium) on the back side, as is usual with monofacial homojunction cells, leads to a
warping of the cells—this is due to the different thermal expansion coefficients α of
the metal and silicon; these differences in the values of α have a negative effect—in
conventional cell production—because of the high firing temperatures (>800 °C) and
the subsequent rapid cooling. This can lead to major problems during cell production
such as warping and cell breakage during screen printing. In the case of the heterojunction cell, the metal layer is completely omitted, so that thinner wafers can be
used for cell production. This leads to two opposite effects: A thinner wafer means
that more light passes through the solar cell without being absorbed, so less light
contributes to carrier generation. This reduces the short-circuit current density J sc of
the solar cell. At the same time, passivation of the wafer surface is becoming increasingly important, as more charge carriers reach the surfaces and do not recombine in
the bulk. A high degree of passivation, as is the case with heterojunction cells, leads
to an increase in the open circuit voltage V oc . To a certain extent, these two effects
(lower J sc and higher V oc ) cancel each other out and cell efficiency remains the same.
Nevertheless, in addition to the cost advantage, there is also a performance advantage
for thin wafers. When the solar cells are integrated into a module, series resistance
losses occur in accordance with (7.4).
P loss = I
2 R
(7.4)
Equation (7.4) means that the power loss P loss increases quadratically with the total
current in the module. A lower current means, thus, a reduction in module losses.
It can be seen that with the same power class of individual cells, the power of the
modules from thin cells is higher than that of the modules from thicker cells [15].
According to Fig. 7.12 the optimum wafer thickness for a heterojunction cell is
80–100 μm whereas for homojunction cells the minimum cell thickness is about
>140 μm.
For the reasons a–f given above, we may conclude that the design of the silicon
heterojunction cell optimally complements the properties of n-type material, and that
n-type wafers are ideally suited for the production of highly efficient HJT cells.
183
(f) Thin Wafer With a share of more than 50% in the total production costs, the
production of wafer material is the biggest cost driver in the manufacturing of a solar
cell. Therefore, wafer production also offers the greatest potential for savings. Due
to the low process temperatures and the symmetrical cell structure of heterojunction
cells, the use of very thin wafers becomes possible here. A thick metal layer (Aluminium) on the back side, as is usual with monofacial homojunction cells, leads to a
warping of the cells—this is due to the different thermal expansion coefficients α of
the metal and silicon; these differences in the values of α have a negative effect—in
conventional cell production—because of the high firing temperatures (>800 °C) and
the subsequent rapid cooling. This can lead to major problems during cell production
such as warping and cell breakage during screen printing. In the case of the heterojunction cell, the metal layer is completely omitted, so that thinner wafers can be
used for cell production. This leads to two opposite effects: A thinner wafer means
that more light passes through the solar cell without being absorbed, so less light
contributes to carrier generation. This reduces the short-circuit current density J sc of
the solar cell. At the same time, passivation of the wafer surface is becoming increasingly important, as more charge carriers reach the surfaces and do not recombine in
the bulk. A high degree of passivation, as is the case with heterojunction cells, leads
to an increase in the open circuit voltage V oc . To a certain extent, these two effects
(lower J sc and higher V oc ) cancel each other out and cell efficiency remains the same.
Nevertheless, in addition to the cost advantage, there is also a performance advantage
for thin wafers. When the solar cells are integrated into a module, series resistance
losses occur in accordance with (7.4).
P loss = I
2 R
(7.4)
Equation (7.4) means that the power loss P loss increases quadratically with the total
current in the module. A lower current means, thus, a reduction in module losses.
It can be seen that with the same power class of individual cells, the power of the
modules from thin cells is higher than that of the modules from thicker cells [15].
According to Fig. 7.12 the optimum wafer thickness for a heterojunction cell is
80–100 μm whereas for homojunction cells the minimum cell thickness is about
>140 μm.
For the reasons a–f given above, we may conclude that the design of the silicon
heterojunction cell optimally complements the properties of n-type material, and that
n-type wafers are ideally suited for the production of highly efficient HJT cells.