170
S. Leu and D. Sontag
3. those between Copper-Indium-Gallium-Diselenide and Cadmium Sulfide
(CIGS/CdS),
4. those relating to Group
7 III–V tandem cells (GaAs, etc.).
These other contacts are not described in this chapter: (1) has been mentioned
in Chap. 6, (2) and (3) will be mentioned in Chap. 8—whereas (4) will not at all
be treated in this book. Indeed, although III–V tandem cells give the highest cell
efficiencies ever obtained (over 40%), their fabrication cost is at present prohibitively
high, so that they are only used in special situations (like concentrating photovoltaics,
CPV, see Chap. 10, Sect. 10.3.2) and in space applications.
(b) Band Diagram and Tunnelling
In Fig. 7.5 the construction of the band diagram of a heterojunction cell is shown
[6]—for the sake of simplicity this is done only for the front side, here for the p-side,
where the pn-junction is. The band diagram is essential to understand the benefits
of passivated contacts, which ultimately characterize the HJT cell. Let us first look
only at the individual layers—the individual components of the heterojunction before
joining them together. Individual layers: The bandgap of the p-doped, amorphous
silicon a-Si:H(p) layer is shown hatched on the left in Fig. 7.5, drawing A. Just next
to it (drawing B), is the bandgap of the n-doped silicon c-Si (n) layer (wafer). This
bandgap, dotted in drawing B, is within the first bandgap («type 1» heterojunction).
When building the heterojunction, e.g. putting in contact the (p) amorphous silicon
layer and the (n) crystalline silicon wafer, equilibration occurs, which will determine
the band diagram as discussed below.
As in the dark—without sunlight—the potential in a material has to be balanced, the two Fermi levels will be adjusted to the same potential. Let us look at
what happens in detail: When we bring the two individual materials A and B into
contact,
8 the respective majority charge carriers begin to diffuse to the other side.
In the n-doped silicon c-Si (n) layer, the electrons are the majority carriers. The
electrons flow from the right side in Fig. 7.6 (position ➁) to the left side (position
➀).
This flow charges the right side (β) positively and the left side (α) negatively, see
Fig. 7.6. This charge carrier exchange flows until the two Fermi levels have become
equal and an equilibrium state for E F prevails. In Fig. 7.6 the two Fermi levels are
therefore on the same line.
Let us take a closer look to the region around the interface. The band bending and
the gradient caused thereby in the Conduction Band Edge E C and in the Valence
Band Edge E V constitute an electrical potential for the charge carriers that drives
7 The term «Group» here refers to the numbering of the columns in the periodic system.
8 The idea that we start with individual layers, which we afterwards join together is, of course, just
a “Gedankenexperiment”, an artifice we imagine in our minds, so as to understand better what
happens. In reality the two individual layers are joined together right from the beginning—as the
HJT cell is fabricated.
S. Leu and D. Sontag
3. those between Copper-Indium-Gallium-Diselenide and Cadmium Sulfide
(CIGS/CdS),
4. those relating to Group
7 III–V tandem cells (GaAs, etc.).
These other contacts are not described in this chapter: (1) has been mentioned
in Chap. 6, (2) and (3) will be mentioned in Chap. 8—whereas (4) will not at all
be treated in this book. Indeed, although III–V tandem cells give the highest cell
efficiencies ever obtained (over 40%), their fabrication cost is at present prohibitively
high, so that they are only used in special situations (like concentrating photovoltaics,
CPV, see Chap. 10, Sect. 10.3.2) and in space applications.
(b) Band Diagram and Tunnelling
In Fig. 7.5 the construction of the band diagram of a heterojunction cell is shown
[6]—for the sake of simplicity this is done only for the front side, here for the p-side,
where the pn-junction is. The band diagram is essential to understand the benefits
of passivated contacts, which ultimately characterize the HJT cell. Let us first look
only at the individual layers—the individual components of the heterojunction before
joining them together. Individual layers: The bandgap of the p-doped, amorphous
silicon a-Si:H(p) layer is shown hatched on the left in Fig. 7.5, drawing A. Just next
to it (drawing B), is the bandgap of the n-doped silicon c-Si (n) layer (wafer). This
bandgap, dotted in drawing B, is within the first bandgap («type 1» heterojunction).
When building the heterojunction, e.g. putting in contact the (p) amorphous silicon
layer and the (n) crystalline silicon wafer, equilibration occurs, which will determine
the band diagram as discussed below.
As in the dark—without sunlight—the potential in a material has to be balanced, the two Fermi levels will be adjusted to the same potential. Let us look at
what happens in detail: When we bring the two individual materials A and B into
contact,
8 the respective majority charge carriers begin to diffuse to the other side.
In the n-doped silicon c-Si (n) layer, the electrons are the majority carriers. The
electrons flow from the right side in Fig. 7.6 (position ➁) to the left side (position
➀).
This flow charges the right side (β) positively and the left side (α) negatively, see
Fig. 7.6. This charge carrier exchange flows until the two Fermi levels have become
equal and an equilibrium state for E F prevails. In Fig. 7.6 the two Fermi levels are
therefore on the same line.
Let us take a closer look to the region around the interface. The band bending and
the gradient caused thereby in the Conduction Band Edge E C and in the Valence
Band Edge E V constitute an electrical potential for the charge carriers that drives
7 The term «Group» here refers to the numbering of the columns in the periodic system.
8 The idea that we start with individual layers, which we afterwards join together is, of course, just
a “Gedankenexperiment”, an artifice we imagine in our minds, so as to understand better what
happens. In reality the two individual layers are joined together right from the beginning—as the
HJT cell is fabricated.