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The impact of alkali elements
7.1 Introduction
This chapter focuses on the identification of the mechanisms that are involved in CIGS
solar cell degradation. More information about long term stability of CIGS absorbers
and solar cells can be found in chapters 2.3.2 and 2.4 respectively. The impact of the
alkali elements sodium and potassium on the degradation behaviour of CIGS solar
cells was studied. These elements are used in CIGS solar cells in order to obtain high
conversion efficiencies [1,2]. However, earlier studies demonstrated that either or both
elements could have a negative impact on the long-term stability of CIGS modules [3-
6]. More information can be found in chapter 2.3.2.4.
7.1.1 Impact of sodium and potassium
Cu(In,Ga)Se based thin film solar cells require incorporation of a small amount of es-
2
pecially sodium and potassium into the absorber layer to obtain high efficiencies. This
increase in efficiency is obtained by an increase in hole carrier density, which results
in the improvement of p-type conductivity and therefore an improved open circuit
voltage (V ) and fill factor (FF). Several models have been proposed to account for
oc
the increase in the hole carrier density. The models proposed in the literature are, for
example:
(i) Minimisation of compensating antisite In donor defects by sodium.
Cu
(ii) Increase in acceptor density due to antisite Na defects.
In
(iii) the neutralisation of donor-like selenium vacancy defects by oxygen atoms
accompanied by the presence of sodium during the growth [7].
Furthermore, sodium is also known to affect the CIGS growth orientation and enhance
(112) texture formation. It is also reported that sodium addition leads to enhanced
grain size [8], although in this work, the opposite effect has been observed.
Similar effects have been observed from other alkali elements including potassium,
but knowledge about the effect of this element is even more limited. However, recent
efficiency records have been obtained [2] by the introduction of potassium. The main
reason for enhanced efficiency was the possibility to add more gallium to the CIGS
layer when potassium is present, thereby allowing higher bandgap CIGS. This again
allows the deposition of CIGS material with a higher V .
oc
The average concentration of sodium in CIGS films is approximately 0.1%. Recent stud -
ies [9,10] have shown that the concentration varies a lot within the CIGS layer: sodium
contents are very high at the grain boundaries (around 1%) compared to the CIGS bulk
(30-150 ppm) (Figure 7.1).
215
7.1 Introduction
This chapter focuses on the identification of the mechanisms that are involved in CIGS
solar cell degradation. More information about long term stability of CIGS absorbers
and solar cells can be found in chapters 2.3.2 and 2.4 respectively. The impact of the
alkali elements sodium and potassium on the degradation behaviour of CIGS solar
cells was studied. These elements are used in CIGS solar cells in order to obtain high
conversion efficiencies [1,2]. However, earlier studies demonstrated that either or both
elements could have a negative impact on the long-term stability of CIGS modules [3-
6]. More information can be found in chapter 2.3.2.4.
7.1.1 Impact of sodium and potassium
Cu(In,Ga)Se based thin film solar cells require incorporation of a small amount of es-
2
pecially sodium and potassium into the absorber layer to obtain high efficiencies. This
increase in efficiency is obtained by an increase in hole carrier density, which results
in the improvement of p-type conductivity and therefore an improved open circuit
voltage (V ) and fill factor (FF). Several models have been proposed to account for
oc
the increase in the hole carrier density. The models proposed in the literature are, for
example:
(i) Minimisation of compensating antisite In donor defects by sodium.
Cu
(ii) Increase in acceptor density due to antisite Na defects.
In
(iii) the neutralisation of donor-like selenium vacancy defects by oxygen atoms
accompanied by the presence of sodium during the growth [7].
Furthermore, sodium is also known to affect the CIGS growth orientation and enhance
(112) texture formation. It is also reported that sodium addition leads to enhanced
grain size [8], although in this work, the opposite effect has been observed.
Similar effects have been observed from other alkali elements including potassium,
but knowledge about the effect of this element is even more limited. However, recent
efficiency records have been obtained [2] by the introduction of potassium. The main
reason for enhanced efficiency was the possibility to add more gallium to the CIGS
layer when potassium is present, thereby allowing higher bandgap CIGS. This again
allows the deposition of CIGS material with a higher V .
oc
The average concentration of sodium in CIGS films is approximately 0.1%. Recent stud -
ies [9,10] have shown that the concentration varies a lot within the CIGS layer: sodium
contents are very high at the grain boundaries (around 1%) compared to the CIGS bulk
(30-150 ppm) (Figure 7.1).
215
