208
A. Romeo
8.3.2.4 Back Contact
The back contact is the part of the solar cell, which is opposite to the sunny side. With
many other technologies, as in the case of CuInGaSe 2 , a metal such as molybdenum
would be sufficient for contacting the cell. On the contrary, for CdTe the back contact
is a complicated issue, due to the high electron affinity
10 of this material: this requires
a metal with a work function
11 greater than 5.7 eV. No metals available have this
value and different solutions have to be applied.
Generally, CdTe forms a Schottky contact with metals, presenting an energy barrier. The height of this can be reduced by generating a heavily p-doped CdTe surface
and by applying a buffer layer with a high carrier concentration.
This has been obtained in the past by applying a Cu/metal double layer; but it
leads to a high instability of the device due to copper diffusion.
On the other hand, applying copper above a diffusion barrier, with a molybdenum
layer on top of this stack, makes a more stable back contact. In this way, copper
has the predominant task of contacting the cell and reducing the series resistance.
A large variety of combinations have been proposed with this structure: ZnTe:Cu,
As 2 Te 3 :Cu, Bi 2 Te 3 :Au. ZnTe is, at the moment, the variant, which is the most used.
8.3.3 Performance and Degradation
Nowadays, CdTe modules are commercially produced by a few companies, mainly
First Solar (USA), which is the largest producer of thin-film modules in the market,
and by Calyxo-TS Solar (Germany).
Efficiencies of finished modules can exceed 18% (by First Solar with an area
of 7038 cm
2 ) [1]. In the past there has been a problem with stability of the solar
cell efficiency; this was connected with the diffusion of copper, which is needed
to provide a suitable back contact, as previously explained. However, in the last
years the embedding of Cu into a ZnTe layer has practically solved this problem, as
demonstrated by the high reliability of First Solar modules [8].
8.4 Flexible Thin-Film Solar Cells
As mentioned above, conventional polycrystalline thin-film solar cells are usually
manufactured on (TCO-coated) glass substrates.
10 The electron affinity of an atom or molecule is defined as the amount of energy released or spent
when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion.
11 In solid-state physics, the work function is the minimum energy needed to remove an electron
from a metal or semiconductor and place in the space (vacuum) immediately outside the solid
surface.
A. Romeo
8.3.2.4 Back Contact
The back contact is the part of the solar cell, which is opposite to the sunny side. With
many other technologies, as in the case of CuInGaSe 2 , a metal such as molybdenum
would be sufficient for contacting the cell. On the contrary, for CdTe the back contact
is a complicated issue, due to the high electron affinity
10 of this material: this requires
a metal with a work function
11 greater than 5.7 eV. No metals available have this
value and different solutions have to be applied.
Generally, CdTe forms a Schottky contact with metals, presenting an energy barrier. The height of this can be reduced by generating a heavily p-doped CdTe surface
and by applying a buffer layer with a high carrier concentration.
This has been obtained in the past by applying a Cu/metal double layer; but it
leads to a high instability of the device due to copper diffusion.
On the other hand, applying copper above a diffusion barrier, with a molybdenum
layer on top of this stack, makes a more stable back contact. In this way, copper
has the predominant task of contacting the cell and reducing the series resistance.
A large variety of combinations have been proposed with this structure: ZnTe:Cu,
As 2 Te 3 :Cu, Bi 2 Te 3 :Au. ZnTe is, at the moment, the variant, which is the most used.
8.3.3 Performance and Degradation
Nowadays, CdTe modules are commercially produced by a few companies, mainly
First Solar (USA), which is the largest producer of thin-film modules in the market,
and by Calyxo-TS Solar (Germany).
Efficiencies of finished modules can exceed 18% (by First Solar with an area
of 7038 cm
2 ) [1]. In the past there has been a problem with stability of the solar
cell efficiency; this was connected with the diffusion of copper, which is needed
to provide a suitable back contact, as previously explained. However, in the last
years the embedding of Cu into a ZnTe layer has practically solved this problem, as
demonstrated by the high reliability of First Solar modules [8].
8.4 Flexible Thin-Film Solar Cells
As mentioned above, conventional polycrystalline thin-film solar cells are usually
manufactured on (TCO-coated) glass substrates.
10 The electron affinity of an atom or molecule is defined as the amount of energy released or spent
when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion.
11 In solid-state physics, the work function is the minimum energy needed to remove an electron
from a metal or semiconductor and place in the space (vacuum) immediately outside the solid
surface.