174
S. Leu and D. Sontag
J 02 denotes the leakage current caused by the space charge zones. In homojunction
cells a distinct space charge zone is present. In heterojunction cell, the p-region is
separated from the n-region by a layer of intrinsic amorphous silicon, and the space
charge region is not so pronounced, because of lower doping efficiencies and thinner
layers, as compared to homojunction cells.
(d) Structure of Amorphous Layer
Amorphous silicon consists of the same atoms as pure, crystalline silicon. However,
it does not show a periodic crystalline structure. Although at the interface with the
crystalline silicon it takes over the “crystalline” structure but already three to four
bond lengths further, e.g. already after 0.5 nm, the bond angles deviate strongly, so
that no periodic correlation as in crystalline silicon is recognizable. This leads to two
effects: first, due to the absence of a crystalline structure, the electrical conductivity
decreases and, secondly, many open bonds occur, e.g. many unsaturated defects, socalled “dangling bonds” (see also Chap. 6). These lead to increased recombination of
the charge carriers. The dangling bonds can be neutralized with hydrogen; this step is
called “saturation” in technical jargon. This is why hydrogen is added to amorphous
silicon. The electrical conductivity of the amorphous material can be increased by
doping, but remains always relatively low, due to its amorphous character.
In HJT cells the low conductivity of the amorphous layer is not an obstacle provided the amorphous layer is thin. The charge carriers can then cross this layer
vertically. Since the layer is very thin, there is no appreciable loss due to electrical
resistance. However, the low conductivity is not sufficient to enable the charge carriers to move laterally to the metal contacts. It is therefore necessary to cover the
amorphous layer with an additional, highly conductive layer. This layer must also be
transparent to light.
(e) TCO Layer
Transparent Conductive Oxides (TCO) are suitable candidates, which incorporate all
required properties. The charge carriers only have to flow now vertically through the
two thin amorphous layers i-p or i-n to the cell surface. The lateral current conduction
to the metal fingers is then taken over by the TCO layer. In addition to its function as
a conductive layer, the TCO layer also takes over the function of an Anti-Reflection
Coating (ARC) layer. Such an ARC layer reduces reflection losses—more light can
thereby be absorbed by the solar cell. The TCO layer must hence be highly transparent
and should ideally not absorb any light itself (parasitic absorption). As explained in
Chap. 4, this layer has an optimal thickness of 80 nm, if it is used (as is the case here)
as a single anti-reflective layer.
The two objectives that the TCO layer has to meet contradict each other: Either the
TCO layer is very transparent but does not conduct well electrically, or it conducts
very well electrically and is less transparent because it is thicker or it is doped to a
higher extent. In order to determine the optimum for the cell, a compromise has to be
found in which the mutual distance of the metal fingers and their thickness must also
be taken into account. An optimum is usually found with a relatively low conductivity
of the TCO layer combined with fine and narrow metal fingers, which are, typically,
S. Leu and D. Sontag
J 02 denotes the leakage current caused by the space charge zones. In homojunction
cells a distinct space charge zone is present. In heterojunction cell, the p-region is
separated from the n-region by a layer of intrinsic amorphous silicon, and the space
charge region is not so pronounced, because of lower doping efficiencies and thinner
layers, as compared to homojunction cells.
(d) Structure of Amorphous Layer
Amorphous silicon consists of the same atoms as pure, crystalline silicon. However,
it does not show a periodic crystalline structure. Although at the interface with the
crystalline silicon it takes over the “crystalline” structure but already three to four
bond lengths further, e.g. already after 0.5 nm, the bond angles deviate strongly, so
that no periodic correlation as in crystalline silicon is recognizable. This leads to two
effects: first, due to the absence of a crystalline structure, the electrical conductivity
decreases and, secondly, many open bonds occur, e.g. many unsaturated defects, socalled “dangling bonds” (see also Chap. 6). These lead to increased recombination of
the charge carriers. The dangling bonds can be neutralized with hydrogen; this step is
called “saturation” in technical jargon. This is why hydrogen is added to amorphous
silicon. The electrical conductivity of the amorphous material can be increased by
doping, but remains always relatively low, due to its amorphous character.
In HJT cells the low conductivity of the amorphous layer is not an obstacle provided the amorphous layer is thin. The charge carriers can then cross this layer
vertically. Since the layer is very thin, there is no appreciable loss due to electrical
resistance. However, the low conductivity is not sufficient to enable the charge carriers to move laterally to the metal contacts. It is therefore necessary to cover the
amorphous layer with an additional, highly conductive layer. This layer must also be
transparent to light.
(e) TCO Layer
Transparent Conductive Oxides (TCO) are suitable candidates, which incorporate all
required properties. The charge carriers only have to flow now vertically through the
two thin amorphous layers i-p or i-n to the cell surface. The lateral current conduction
to the metal fingers is then taken over by the TCO layer. In addition to its function as
a conductive layer, the TCO layer also takes over the function of an Anti-Reflection
Coating (ARC) layer. Such an ARC layer reduces reflection losses—more light can
thereby be absorbed by the solar cell. The TCO layer must hence be highly transparent
and should ideally not absorb any light itself (parasitic absorption). As explained in
Chap. 4, this layer has an optimal thickness of 80 nm, if it is used (as is the case here)
as a single anti-reflective layer.
The two objectives that the TCO layer has to meet contradict each other: Either the
TCO layer is very transparent but does not conduct well electrically, or it conducts
very well electrically and is less transparent because it is thicker or it is doped to a
higher extent. In order to determine the optimum for the cell, a compromise has to be
found in which the mutual distance of the metal fingers and their thickness must also
be taken into account. An optimum is usually found with a relatively low conductivity
of the TCO layer combined with fine and narrow metal fingers, which are, typically,