7 Crystalline Silicon Solar Cells: Heterojunction Cells
175
less than 40 μm thick. These approximately 60–80 metal fingers consist of highly
conductive silver. Usually the metal fingers are printed on the TCO using a silver
paste. In most cases, a screen-printing process is used.
The most common TCO material currently used in heterojunction cells is Indium
Tin Oxide (ITO). Depending on the application, the tin content varies between 5
and 10%. A high tin content increases conductivity, but—at the same time—it limits
transparency. Further in ITO sputtering, one can have a whole range of transparency
versus conductivity by just varying the oxygen content in the film. Production is about
choosing the optimal parameters for the most suitable TCO layer. Since indium is
a scarce raw material, efforts are currently being made to reduce its use—or in the
medium term, even to replace it completely.
7.2.3 The Heterojunction Cell
(a) HJT Cells with the pn-Junction on the Front Side
So far we have described the physical properties of the heterojunction cell; in the
following we want to investigate the different possibilities for the cell structure.
Basically we need three layers on both sides of the cell:
• an Intrinsic-amorphous layer
• a doped amorphous layer and
• a TCO layer.
All we have to do in order to create a Heterojunction cell is to coat both sides of the
wafer with a double layer of intrinsic and doped amorphous silicon and then finish it
on either side with a TCO layer. However, we have to make sure that the doping on
the front side is opposite to the one used on the back side. We obtain a symmetrical
cell in terms of the number and type of layers. Figure 7.9 illustrates this in detail. This
is the structure, where the p-n junction (emitter)
12 , e.g. the separation of electrons
and holes is located on the front side. This can be recognized by the fact that the
doping of the crystalline bulk material (c-Si) is n-type, whilst the doped amorphous
layer on the front side (a-Si:H(p
+ )) is p-type. The amorphous layer (a-Si:H(n
+ )) on
the back side is here n-type like the bulk. In short: the cell is p-type on the front side
and n-type on the back side. On the back side there are the regions n (bulk c-Si:H(n)
and a-Si:H(n
+ )
13 : As can be seen in Fig. 7.9, through the peak E
C situated at the
back side, the electrons can tunnel through, but not the holes.
If the barriers (peaks) are narrow enough, there is a certain probability of residence
beyond the barrier and the charge carriers ‘see’ an energetically more favourable
12 In photovoltaics, the term emitter is defined as the region where the minority carriers leave the
solar cell. In n-type material, the minority carriers are the holes and the holes leave the solar cell
on the p side (positive electrode). The reverse applies to p-type material. In p-type material the
minority carriers are the electrons and these leave the solar cell on the n-side (negative electrode).
13 n + means more doped than n and correspondingly p + means more doped than p.
175
less than 40 μm thick. These approximately 60–80 metal fingers consist of highly
conductive silver. Usually the metal fingers are printed on the TCO using a silver
paste. In most cases, a screen-printing process is used.
The most common TCO material currently used in heterojunction cells is Indium
Tin Oxide (ITO). Depending on the application, the tin content varies between 5
and 10%. A high tin content increases conductivity, but—at the same time—it limits
transparency. Further in ITO sputtering, one can have a whole range of transparency
versus conductivity by just varying the oxygen content in the film. Production is about
choosing the optimal parameters for the most suitable TCO layer. Since indium is
a scarce raw material, efforts are currently being made to reduce its use—or in the
medium term, even to replace it completely.
7.2.3 The Heterojunction Cell
(a) HJT Cells with the pn-Junction on the Front Side
So far we have described the physical properties of the heterojunction cell; in the
following we want to investigate the different possibilities for the cell structure.
Basically we need three layers on both sides of the cell:
• an Intrinsic-amorphous layer
• a doped amorphous layer and
• a TCO layer.
All we have to do in order to create a Heterojunction cell is to coat both sides of the
wafer with a double layer of intrinsic and doped amorphous silicon and then finish it
on either side with a TCO layer. However, we have to make sure that the doping on
the front side is opposite to the one used on the back side. We obtain a symmetrical
cell in terms of the number and type of layers. Figure 7.9 illustrates this in detail. This
is the structure, where the p-n junction (emitter)
12 , e.g. the separation of electrons
and holes is located on the front side. This can be recognized by the fact that the
doping of the crystalline bulk material (c-Si) is n-type, whilst the doped amorphous
layer on the front side (a-Si:H(p
+ )) is p-type. The amorphous layer (a-Si:H(n
+ )) on
the back side is here n-type like the bulk. In short: the cell is p-type on the front side
and n-type on the back side. On the back side there are the regions n (bulk c-Si:H(n)
and a-Si:H(n
+ )
13 : As can be seen in Fig. 7.9, through the peak E
C situated at the
back side, the electrons can tunnel through, but not the holes.
If the barriers (peaks) are narrow enough, there is a certain probability of residence
beyond the barrier and the charge carriers ‘see’ an energetically more favourable
12 In photovoltaics, the term emitter is defined as the region where the minority carriers leave the
solar cell. In n-type material, the minority carriers are the holes and the holes leave the solar cell
on the p side (positive electrode). The reverse applies to p-type material. In p-type material the
minority carriers are the electrons and these leave the solar cell on the n-side (negative electrode).
13 n + means more doped than n and correspondingly p + means more doped than p.