190
S. Leu and D. Sontag
to generate additional charge carriers. The reflection of the ground naturally depends
on its composition; it is quantified by the «albedo».
30 (Fresh snow has for example
an albedo of 80%, whereas green grass has an albedo of only 25%.
31 ) HJT cells can
be manufactured up to 92% bifaciality.
32 With an albedo of 20%, for example, 18%
of surplus energy (18% = 20% × 92%) is generated from the back side, and adds to
the energy generated from the front side. A bifacial HJT module with, for example,
20% module efficiency
33 from the front side, 92% bifaciality and with 20% albedo,
has an overall efficiency of 20% × (1 + 0.18) = 23.6%.
7.4.5 Metallisation and Contacting
The electrical conductance of the TCO layer of 15 at 100 nm thickness is not
sufficient to collect the current over the entire cell without large losses. It is therefore
not sufficient to pick up the current only at the edge of the solar cell; rather, it
is necessary to provide thin metal fingers to support the transport of the charge
carriers to the edge of the cell. Of course, this is again a compromise between low
electrical (resistance) losses obtained with many and thick metal fingers and low
optical (shading) losses obtained with few and thin metal fingers. There are four
different technologies for contacting HJT cells:
• Busbar Technology
• Multi wire technology
• SmartWire Connection Technology (SWCT)
• Adhesive Technology.
All four technologies are illustrated in Fig. 7.17. As a rule, about 60–90 fingers
with a width of 35–50 μm and a height of about 5–20 μm are used; they are applied by
screen printing onto the solar cell.
34 In HJT cells, the high temperature technology like
for homojunction solar cells does not work because the amorphous layer cannot
endure more than 200 °C. For the HJT technology so-called low-temperature pastes
given by polymer-based screen printing pastes are used with a curing temperature
30 The «albedo» is a dimensionless quantity indicating the ratio between the reflected and the incident
global irradiance (see also Chaps. 2 and 10).
31 These values are applicable in the case where no object obstructs the incoming sunlight. In a PV
system values of albedo are considerably lower and depend on the layout of the system.
32 This means that the conversion efficiency for the light coming in from the back side is 92% of
the conversion efficiency of light coming in, from the front side.
33 which is equivalent to a world-record monofacial module of 20% module efficiency.
34 The Busbar technology is often realized with high temperature pastes, at which temperatures of
approx. 800 °C are used to melt the silver flakes in the paste. This is how the fingers and the busbars
are applied. High temperature pastes are also used in multiwire applications. Instead of the busbars,
pads are printed to fix the wires. The conductivity of such pastes is high. The five ribbons (busbar
technology) or the 2 × 7 wires (multi wire technology) are soldered crosswise to the fingers at
approx. 240 °C.
S. Leu and D. Sontag
to generate additional charge carriers. The reflection of the ground naturally depends
on its composition; it is quantified by the «albedo».
30 (Fresh snow has for example
an albedo of 80%, whereas green grass has an albedo of only 25%.
31 ) HJT cells can
be manufactured up to 92% bifaciality.
32 With an albedo of 20%, for example, 18%
of surplus energy (18% = 20% × 92%) is generated from the back side, and adds to
the energy generated from the front side. A bifacial HJT module with, for example,
20% module efficiency
33 from the front side, 92% bifaciality and with 20% albedo,
has an overall efficiency of 20% × (1 + 0.18) = 23.6%.
7.4.5 Metallisation and Contacting
The electrical conductance of the TCO layer of 15 at 100 nm thickness is not
sufficient to collect the current over the entire cell without large losses. It is therefore
not sufficient to pick up the current only at the edge of the solar cell; rather, it
is necessary to provide thin metal fingers to support the transport of the charge
carriers to the edge of the cell. Of course, this is again a compromise between low
electrical (resistance) losses obtained with many and thick metal fingers and low
optical (shading) losses obtained with few and thin metal fingers. There are four
different technologies for contacting HJT cells:
• Busbar Technology
• Multi wire technology
• SmartWire Connection Technology (SWCT)
• Adhesive Technology.
All four technologies are illustrated in Fig. 7.17. As a rule, about 60–90 fingers
with a width of 35–50 μm and a height of about 5–20 μm are used; they are applied by
screen printing onto the solar cell.
34 In HJT cells, the high temperature technology like
for homojunction solar cells does not work because the amorphous layer cannot
endure more than 200 °C. For the HJT technology so-called low-temperature pastes
given by polymer-based screen printing pastes are used with a curing temperature
30 The «albedo» is a dimensionless quantity indicating the ratio between the reflected and the incident
global irradiance (see also Chaps. 2 and 10).
31 These values are applicable in the case where no object obstructs the incoming sunlight. In a PV
system values of albedo are considerably lower and depend on the layout of the system.
32 This means that the conversion efficiency for the light coming in from the back side is 92% of
the conversion efficiency of light coming in, from the front side.
33 which is equivalent to a world-record monofacial module of 20% module efficiency.
34 The Busbar technology is often realized with high temperature pastes, at which temperatures of
approx. 800 °C are used to melt the silver flakes in the paste. This is how the fingers and the busbars
are applied. High temperature pastes are also used in multiwire applications. Instead of the busbars,
pads are printed to fix the wires. The conductivity of such pastes is high. The five ribbons (busbar
technology) or the 2 × 7 wires (multi wire technology) are soldered crosswise to the fingers at
approx. 240 °C.