8 CdTe and CuInGaSe 2 Thin-Film Solar Cells
201
8.2.2.1 Back Contact and Absorber Layer
CIGS solar cells can be fabricated on soda lime glass; the first layer, grown on the
supporting substrate, is the back contact and typically molybdenum (Mo) is the most
common material for this purpose.
Mo is generally grown by sputtering or by e-beam evaporation and its influence
on the Ohmic contact behaviour at the CIGS/Mo interface makes MoSe 2 formation
an important issue. MoSe 2 is generated after molybdenum deposition during CIGS
growth, especially when co-evaporation of the elements in a selenium atmosphere is
done. It was for a long time considered as a side effect, but it has been proven that it
improves the performance of the back contact.
The CIGS absorber material can be prepared with a large variety of deposition
techniques that go well beyond the scope of this chapter. In this sense we will
concentrate on the two most successful processes [2]:
(a) Co-evaporation.
(b) RF-sputtering of precursors and subsequent heating of the stack in a selenium
atmosphere (selenization).
Co-evaporation
It consists of evaporating each single element (Cu, In, Ga and Se) at the same
time, by using a crucible for each element and controlling the evaporation flux.
Using co-evaporation it is possible to reach very high efficiencies at the laboratory
scale, particularly by the application of an evaporation profile with constant rates.
In fact by varying evaporation rates of the single elements it is possible to profile the
bandgap and/or to adjust the composition.
For example by varying the Cu flux, the bulk copper content can exceed the
standard composition in a part of the process, enhancing grain growth. Moreover it
is possible to control the process by varying the composition from Cu-rich to Cu-poor
or vice versa.
On one hand, excess copper is beneficial during the deposition process but on the
other hand, a Cu-rich CIGS film also leads to segregation of Cu x Se that shunts the
cell.
Already during the early eighties, the design of a deposition process with a balance
between Cu-rich and Cu-poor was known and introduced by Boeing (the so-called
Boeing process). This has subsequently resulted in a co-evaporation process divided
into three-stages: a sequential process which starts with a (In,Ga) 2 Se 3 growth stage,
continues with a deposition of Cu and selenium and ends with another (In,Ga) 2 Se 3
stage.
A very important feature of co-evaporation is the possibility to control in situ
2 the
ratio between gallium and indium. Changing the Ga/(Ga+In) ratio during the process
is one of the key factors for high efficiency, since a high Ga content near the back
contact improves the minority carrier lifetimes, reducing recombination.
2 In situ: in the same evaporation chamber and during the evaporation process.
201
8.2.2.1 Back Contact and Absorber Layer
CIGS solar cells can be fabricated on soda lime glass; the first layer, grown on the
supporting substrate, is the back contact and typically molybdenum (Mo) is the most
common material for this purpose.
Mo is generally grown by sputtering or by e-beam evaporation and its influence
on the Ohmic contact behaviour at the CIGS/Mo interface makes MoSe 2 formation
an important issue. MoSe 2 is generated after molybdenum deposition during CIGS
growth, especially when co-evaporation of the elements in a selenium atmosphere is
done. It was for a long time considered as a side effect, but it has been proven that it
improves the performance of the back contact.
The CIGS absorber material can be prepared with a large variety of deposition
techniques that go well beyond the scope of this chapter. In this sense we will
concentrate on the two most successful processes [2]:
(a) Co-evaporation.
(b) RF-sputtering of precursors and subsequent heating of the stack in a selenium
atmosphere (selenization).
Co-evaporation
It consists of evaporating each single element (Cu, In, Ga and Se) at the same
time, by using a crucible for each element and controlling the evaporation flux.
Using co-evaporation it is possible to reach very high efficiencies at the laboratory
scale, particularly by the application of an evaporation profile with constant rates.
In fact by varying evaporation rates of the single elements it is possible to profile the
bandgap and/or to adjust the composition.
For example by varying the Cu flux, the bulk copper content can exceed the
standard composition in a part of the process, enhancing grain growth. Moreover it
is possible to control the process by varying the composition from Cu-rich to Cu-poor
or vice versa.
On one hand, excess copper is beneficial during the deposition process but on the
other hand, a Cu-rich CIGS film also leads to segregation of Cu x Se that shunts the
cell.
Already during the early eighties, the design of a deposition process with a balance
between Cu-rich and Cu-poor was known and introduced by Boeing (the so-called
Boeing process). This has subsequently resulted in a co-evaporation process divided
into three-stages: a sequential process which starts with a (In,Ga) 2 Se 3 growth stage,
continues with a deposition of Cu and selenium and ends with another (In,Ga) 2 Se 3
stage.
A very important feature of co-evaporation is the possibility to control in situ
2 the
ratio between gallium and indium. Changing the Ga/(Ga+In) ratio during the process
is one of the key factors for high efficiency, since a high Ga content near the back
contact improves the minority carrier lifetimes, reducing recombination.
2 In situ: in the same evaporation chamber and during the evaporation process.