13.4.2
mixture of sulphur and selenium, Cu 2 ZnSn(SSe) 4 (CZTSS).
In contrast to CIGS, CZTS is based on non-toxic and abundantly available elements.
The current record efficiency is 12%. It is achieved with CZTSS solar cells on lab scale by
IBM [47].
Cadmium telluride solar cells
In this section we will discuss the cadmium telluride (CdTe) technology, which currently
is the thin-film technology with the lowest demonstrated cost per W p . We start with
discussing the physical properties of CdTe, which is a II-VI semiconductor because it
consists of the II valence electron element cadmium (Cd) and the VI valence electron
element tellurium (Te). Like the III-V semiconductors discussed in Section 13.2, CdTe
forms a zincblende lattice structure where every Cd atom is bonded to four Te atoms and
vice versa.
The bandgap of CdTe is 1.44 eV, a value which is close to the optimal bandgap for
single junction solar cells. CdTe is a direct bandgap material, consequently only a few
micrometres of CdTe are required to absorb all the photons with an energy higher than the
bandgap energy. In order for the light-excited charge carriers to be collected efficiently at
the contacts, their diffusion coefficient has to be in the order of the thickness.
CdTe can be n-doped by replacing the II-valence electron atom Cd with a III-valence
electron atom like aluminium, gallium or indium. n doping can be achieved as well by
replacing a VI-valence tellurium atom with a VII-valence electron element like fluorine,
chlorine, bromine and iodine atoms. The III- and VII-valence atoms act as shallow donors.
A tellurium vacancy also acts like a donor.
p-doping of CdTe can be achieved by replacing Cd with a I-valence electron atom
like copper, silver or gold. It can also be achieved by replacing Te atoms with V-valence
electron elements such as nitrogen, phosphorus or arsenic. These elements act as shallow
acceptors. But a Cd vacancy also acts as an acceptor. In solar cells, p-doped CdTe is used.
However, it is difficult to obtain CdTe with a high doping level.
Figure 13.22 (a) shows the structure of a typical CdTe solar cell. First, transparent
front contact is deposited onto the glass superstrate. This can be tin oxide or cadmium
stannate, which is a Cd-Sn-oxide alloy. On top of that the n layer is deposited, which is a
cadmium sulphide layer, similar to the n-buffer layer in CIGS solar cells (Section 13.4.1).
Then, the p-type CdTe absorber layer is deposited with a typical thickness of a few
micrometres. Making a good back contact on CdTe is rather challenging because the
material properties of CdTe restrict the choice of acceptable metals. Heavily doping the
mixture of sulphur and selenium, Cu 2 ZnSn(SSe) 4 (CZTSS).
In contrast to CIGS, CZTS is based on non-toxic and abundantly available elements.
The current record efficiency is 12%. It is achieved with CZTSS solar cells on lab scale by
IBM [47].
Cadmium telluride solar cells
In this section we will discuss the cadmium telluride (CdTe) technology, which currently
is the thin-film technology with the lowest demonstrated cost per W p . We start with
discussing the physical properties of CdTe, which is a II-VI semiconductor because it
consists of the II valence electron element cadmium (Cd) and the VI valence electron
element tellurium (Te). Like the III-V semiconductors discussed in Section 13.2, CdTe
forms a zincblende lattice structure where every Cd atom is bonded to four Te atoms and
vice versa.
The bandgap of CdTe is 1.44 eV, a value which is close to the optimal bandgap for
single junction solar cells. CdTe is a direct bandgap material, consequently only a few
micrometres of CdTe are required to absorb all the photons with an energy higher than the
bandgap energy. In order for the light-excited charge carriers to be collected efficiently at
the contacts, their diffusion coefficient has to be in the order of the thickness.
CdTe can be n-doped by replacing the II-valence electron atom Cd with a III-valence
electron atom like aluminium, gallium or indium. n doping can be achieved as well by
replacing a VI-valence tellurium atom with a VII-valence electron element like fluorine,
chlorine, bromine and iodine atoms. The III- and VII-valence atoms act as shallow donors.
A tellurium vacancy also acts like a donor.
p-doping of CdTe can be achieved by replacing Cd with a I-valence electron atom
like copper, silver or gold. It can also be achieved by replacing Te atoms with V-valence
electron elements such as nitrogen, phosphorus or arsenic. These elements act as shallow
acceptors. But a Cd vacancy also acts as an acceptor. In solar cells, p-doped CdTe is used.
However, it is difficult to obtain CdTe with a high doping level.
Figure 13.22 (a) shows the structure of a typical CdTe solar cell. First, transparent
front contact is deposited onto the glass superstrate. This can be tin oxide or cadmium
stannate, which is a Cd-Sn-oxide alloy. On top of that the n layer is deposited, which is a
cadmium sulphide layer, similar to the n-buffer layer in CIGS solar cells (Section 13.4.1).
Then, the p-type CdTe absorber layer is deposited with a typical thickness of a few
micrometres. Making a good back contact on CdTe is rather challenging because the
material properties of CdTe restrict the choice of acceptable metals. Heavily doping the