7 Crystalline Silicon Solar Cells: Heterojunction Cells
167
with openings in the aluminium oxide through which a conductive aluminium paste
is used to contact the crystalline silicon (see Chap. 5). A high-temperature step in
the range of 850 °C forms, as in the «standard Al-BSF solar cell», an aluminiumsilicon «eutectic
2 » resulting in an Ohmic contact between aluminum and silicon, but
this time only at the openings of the aluminum oxide layer. At these contact points
not only Ohmic resistance losses appear but, like in standard Al-BSF solar cells, an
inferior passivation occurs, which results in reduced efficiency η.
The question now arises as to what an optimal contact might look like—a contact that has no or only very little recombination and no Ohmic losses. A reduction
of the recombination losses can be achieved, for example, by making the openings
even smaller; but this increases resistive losses. Very high doping of the silicon at
the openings reduces resistivity losses but increases recombination. (Auger recombination, see Chap. 4.) Laser-Fired Contacts [3] constitute another way to reduce
Ohmic losses. However, all these processes have additional process steps—a fact
that renders cell production more expensive.
(b) Passivated Contacts
Another, very elegant possibility to avoid direct metallic contact to silicon is to use
thin passivation layers with a thickness of only a few nanometres. Charge carriers
can «tunnel
3 » through the passivation layer and recombination effects cannot occur
since direct Ohmic contacts are not present. In addition, such passivated contacts can
form selective contacts, e.g. they conduct either only electrons or only holes. This
possibility is implemented in the heterojunction cell.
(c) The Amorphous Double-Layer Construction of the HJT Cell Leads to the
Highest Passivation
The first silicon-based heterojunction cells used doped amorphous material directly
applied on the silicon and achieved satisfactory passivation properties. However,
it has been shown that in this structure the charge carrier density is very high, so
that increased surface recombination takes place [4]. Finally, in 1992, an intrinsic,
undoped amorphous interlayer sandwiched between the silicon crystal and the
doped amorphous layer was found to provide even better passivation properties [5].
The recombination rate could be reduced and the fill factor FF increased. The cell
was named «HIT», which stands for «Heterojunction with Intrinsic Thin Layer
© ».
Sanyo achieved 23% cell efficiency with this structure in 2009. In 2014, Panasonic
obtained another breakthrough by applying passivated contacts on both sides, e.g.
on the front and on the back side. The efficiency η increased to 25.6%. In 2017,
2 The term «eutectic» designates a homogeneous mixture of substances that melts or solidifies at a
single temperature that is lower than the melting point of either of the constituents.
3 The term «tunneling» comes from quantum physics: If an electron encounters a potential barrier,
it can pass through the barrier with a certain probability, even if its energy is lower than that of
the barrier. That would not be possible according to classical physics. This probability is called the
«residence probability»—it is given by the wave function representing the electron. The electron
is described in quantum mechanics by a wave: as a wave it can also extend to the other side of the
barrier. OR, in other words, it can just «tunnel» through the barrier.
167
with openings in the aluminium oxide through which a conductive aluminium paste
is used to contact the crystalline silicon (see Chap. 5). A high-temperature step in
the range of 850 °C forms, as in the «standard Al-BSF solar cell», an aluminiumsilicon «eutectic
2 » resulting in an Ohmic contact between aluminum and silicon, but
this time only at the openings of the aluminum oxide layer. At these contact points
not only Ohmic resistance losses appear but, like in standard Al-BSF solar cells, an
inferior passivation occurs, which results in reduced efficiency η.
The question now arises as to what an optimal contact might look like—a contact that has no or only very little recombination and no Ohmic losses. A reduction
of the recombination losses can be achieved, for example, by making the openings
even smaller; but this increases resistive losses. Very high doping of the silicon at
the openings reduces resistivity losses but increases recombination. (Auger recombination, see Chap. 4.) Laser-Fired Contacts [3] constitute another way to reduce
Ohmic losses. However, all these processes have additional process steps—a fact
that renders cell production more expensive.
(b) Passivated Contacts
Another, very elegant possibility to avoid direct metallic contact to silicon is to use
thin passivation layers with a thickness of only a few nanometres. Charge carriers
can «tunnel
3 » through the passivation layer and recombination effects cannot occur
since direct Ohmic contacts are not present. In addition, such passivated contacts can
form selective contacts, e.g. they conduct either only electrons or only holes. This
possibility is implemented in the heterojunction cell.
(c) The Amorphous Double-Layer Construction of the HJT Cell Leads to the
Highest Passivation
The first silicon-based heterojunction cells used doped amorphous material directly
applied on the silicon and achieved satisfactory passivation properties. However,
it has been shown that in this structure the charge carrier density is very high, so
that increased surface recombination takes place [4]. Finally, in 1992, an intrinsic,
undoped amorphous interlayer sandwiched between the silicon crystal and the
doped amorphous layer was found to provide even better passivation properties [5].
The recombination rate could be reduced and the fill factor FF increased. The cell
was named «HIT», which stands for «Heterojunction with Intrinsic Thin Layer
© ».
Sanyo achieved 23% cell efficiency with this structure in 2009. In 2014, Panasonic
obtained another breakthrough by applying passivated contacts on both sides, e.g.
on the front and on the back side. The efficiency η increased to 25.6%. In 2017,
2 The term «eutectic» designates a homogeneous mixture of substances that melts or solidifies at a
single temperature that is lower than the melting point of either of the constituents.
3 The term «tunneling» comes from quantum physics: If an electron encounters a potential barrier,
it can pass through the barrier with a certain probability, even if its energy is lower than that of
the barrier. That would not be possible according to classical physics. This probability is called the
«residence probability»—it is given by the wave function representing the electron. The electron
is described in quantum mechanics by a wave: as a wave it can also extend to the other side of the
barrier. OR, in other words, it can just «tunnel» through the barrier.