202
A. Romeo
In the three-stage process bandgap grading is obtained both at the back contact
and at the front contact, resulting in a double bandgap grading. In this way, the
gallium concentration increases both at the back and at the front of the CIGS layer
with a minimum bandgap in the CIGS bulk. This structure was formed spontaneously
delivering high efficiencies and reaching the conclusion that a front Ga-rich layer is
beneficial for device performance.
Sputtering + selenization (+sulfurization) of precursor materials
This kind of process consists of the subsequent deposition of a precursor material
(that can be an element or a compound), followed by thermal annealing in a selenium
and/or a sulphur environment, for the final formation of the absorber.
The most used precursor materials are metallic and metal selenide layers.
Precursors can be deposited by thermal evaporation, sputtering or electrodeposition. However, the most successful process is the deposition of elemental layers by
DC magnetron sputtering: well established for the production of large-area solar modules up to 60 × 120 cm
2 size [3]. In this particular case the chalcogenide
3 formation
is made by the so-called SAS process: sulfurization after selenization by sequential
selenization in H 2 Se (99.99%) and subsequent sulfurization in H 2 S (99.99%) with
Argon (Ar) in a furnace.
The addition of sulphur into the CIGS matrix is made here, in order to increase
the open-circuit voltage of the solar cell and to tune the bandgap-grading.
In sputtering deposition, a plasma
4 is generated with an inert gas (typically Argon),
and then the ions are accelerated on to a target material by a potential difference that
can be provided by a direct (DC) or radiofrequency (RF) current. The collisions
generated on the target transfer enough energy to the target atoms or molecules,
which are then able to move in form of neutral particles and reach the substrate.
In RF sputtering, the accelerating voltage on the target is typically being switched
with a frequency of a few kHz; while in DC sputtering, the voltage is constant. For
semiconductors or insulators, RF sputtering is the natural choice to avoid electrical
charging of the target. However, it is also possible to use a DC pulsed technique
where the voltage is fixed but pulsed, to avoid charging and consequently arcing.
5
It has been found that the beneficial effect of using soda-lime glass substrates
could be assigned to the supply of sodium. Similar or even better results can be
obtained by using an Na-precursor layer, either deposited before/after or during the
CIGS process. This leads in its turn to higher values of open-circuit voltage and
higher fill factors. All these effects could be explained by a higher doping density, as
Na, K and other alkali elements enhance substantially the performance by increasing
the absorber doping.
3 Chalcogenides are materials containing one or more chalcogen elements (e.g. S, Se or Te) as a
substantial constituent.
4 Plasma consists of a gas of ions and atoms which have some of their orbital electrons removed,
and free electrons.
5 For arcing we mean the formation of an electric arc, that would shunt the DC circuit.
A. Romeo
In the three-stage process bandgap grading is obtained both at the back contact
and at the front contact, resulting in a double bandgap grading. In this way, the
gallium concentration increases both at the back and at the front of the CIGS layer
with a minimum bandgap in the CIGS bulk. This structure was formed spontaneously
delivering high efficiencies and reaching the conclusion that a front Ga-rich layer is
beneficial for device performance.
Sputtering + selenization (+sulfurization) of precursor materials
This kind of process consists of the subsequent deposition of a precursor material
(that can be an element or a compound), followed by thermal annealing in a selenium
and/or a sulphur environment, for the final formation of the absorber.
The most used precursor materials are metallic and metal selenide layers.
Precursors can be deposited by thermal evaporation, sputtering or electrodeposition. However, the most successful process is the deposition of elemental layers by
DC magnetron sputtering: well established for the production of large-area solar modules up to 60 × 120 cm
2 size [3]. In this particular case the chalcogenide
3 formation
is made by the so-called SAS process: sulfurization after selenization by sequential
selenization in H 2 Se (99.99%) and subsequent sulfurization in H 2 S (99.99%) with
Argon (Ar) in a furnace.
The addition of sulphur into the CIGS matrix is made here, in order to increase
the open-circuit voltage of the solar cell and to tune the bandgap-grading.
In sputtering deposition, a plasma
4 is generated with an inert gas (typically Argon),
and then the ions are accelerated on to a target material by a potential difference that
can be provided by a direct (DC) or radiofrequency (RF) current. The collisions
generated on the target transfer enough energy to the target atoms or molecules,
which are then able to move in form of neutral particles and reach the substrate.
In RF sputtering, the accelerating voltage on the target is typically being switched
with a frequency of a few kHz; while in DC sputtering, the voltage is constant. For
semiconductors or insulators, RF sputtering is the natural choice to avoid electrical
charging of the target. However, it is also possible to use a DC pulsed technique
where the voltage is fixed but pulsed, to avoid charging and consequently arcing.
5
It has been found that the beneficial effect of using soda-lime glass substrates
could be assigned to the supply of sodium. Similar or even better results can be
obtained by using an Na-precursor layer, either deposited before/after or during the
CIGS process. This leads in its turn to higher values of open-circuit voltage and
higher fill factors. All these effects could be explained by a higher doping density, as
Na, K and other alkali elements enhance substantially the performance by increasing
the absorber doping.
3 Chalcogenides are materials containing one or more chalcogen elements (e.g. S, Se or Te) as a
substantial constituent.
4 Plasma consists of a gas of ions and atoms which have some of their orbital electrons removed,
and free electrons.
5 For arcing we mean the formation of an electric arc, that would shunt the DC circuit.