Page 47 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells
P. 47
Stability of Cu(In,Ga)Se 2 Solar Cells
2.3.2.4 Role of the sodium on the absorber stability
Large variations in composition and structure of the CIGS absorber can be observed,
which are often related to the sodium content. In this chapter, the impact of the alkali
supply - typically sodium - on the stability of the CIGS absorber was studied. Sodium
can be added during growth or in a post-treatment and is a key to achieve high-effi-
ciency CIGS solar cells. Additionally, it is known that sodium is an important factor in
the long-term stability of CIGS solar cells [49-52]. In this chapter, only studies on the
individual absorber are included, while more information about full cells can be found
in chapters 2.4.
According to Braunger et al. [49], the oxidation of CIS or CIGS is accelerated in the
presence of sodium. The oxidation of the absorber at elevated temperatures barely
influences the efficiency, but oxidation of the absorbers in humid air at room tem-
perature results in a drastic decrease in electrical performance. They [49] describe that
reactions of sodium with selenium species are possible at very low temperatures. For
o
temperatures under 500 C, sodium polyselenides (NaSe ) form, while above 500°C,
x
2
sodium monoselenides (Na Se) are formed. CIGS absorbers grown on soda lime glass,
2
therefore in the presence of sodium, showed less indium or gallium oxides at the sur-
face, but more elemental selenium and selenium oxides, compared to CIGS grown on
sodium-free substrates. As humidity was found to promote the degradation, water
catalysed reactions for the incomplete oxidation of Na Se , which is present as precip -
2
X
itate / intergranular segregation in the polycrystalline CIGS film, in air were suggested:
• Na Se + 1/2 O Na O + x Se (2.2)
2
X
2
2
• Na Se + 3/2 O Na SeO + (x-1) Se (2.3)
X
2
2
3
2
• Na Se + 1/2 O +CO Na CO + x Se (2.4)
3
2
2
2
X
2
Accordingly, a water and sodium catalysed reaction pathway of the absorber was sug -
gested:
• 2 Cu(In Ga )Se + 3/2 O 2 ´Cu(I)´ + (1-x) In O + x Ga O + 4 Se (2.5)
3
2
2
3
1-X
X
2
2
Since Braunger et al. detected a partial oxidation of selenium in their degraded sam-
ples, they assumed another reaction pathway: Se + O SeO [49]. According to the
2
2
authors, the presence of water leads to the hydrolysis of the oxides, i.e. the formation
of In(OH) , Ga(OH) , H SeO and NaOH. In conclusion, Braunger et al. found sodium to
2
3
3
3
promote an enhanced removal of selenium from the absorber layer via the formation
45
2.3.2.4 Role of the sodium on the absorber stability
Large variations in composition and structure of the CIGS absorber can be observed,
which are often related to the sodium content. In this chapter, the impact of the alkali
supply - typically sodium - on the stability of the CIGS absorber was studied. Sodium
can be added during growth or in a post-treatment and is a key to achieve high-effi-
ciency CIGS solar cells. Additionally, it is known that sodium is an important factor in
the long-term stability of CIGS solar cells [49-52]. In this chapter, only studies on the
individual absorber are included, while more information about full cells can be found
in chapters 2.4.
According to Braunger et al. [49], the oxidation of CIS or CIGS is accelerated in the
presence of sodium. The oxidation of the absorber at elevated temperatures barely
influences the efficiency, but oxidation of the absorbers in humid air at room tem-
perature results in a drastic decrease in electrical performance. They [49] describe that
reactions of sodium with selenium species are possible at very low temperatures. For
o
temperatures under 500 C, sodium polyselenides (NaSe ) form, while above 500°C,
x
2
sodium monoselenides (Na Se) are formed. CIGS absorbers grown on soda lime glass,
2
therefore in the presence of sodium, showed less indium or gallium oxides at the sur-
face, but more elemental selenium and selenium oxides, compared to CIGS grown on
sodium-free substrates. As humidity was found to promote the degradation, water
catalysed reactions for the incomplete oxidation of Na Se , which is present as precip -
2
X
itate / intergranular segregation in the polycrystalline CIGS film, in air were suggested:
• Na Se + 1/2 O Na O + x Se (2.2)
2
X
2
2
• Na Se + 3/2 O Na SeO + (x-1) Se (2.3)
X
2
2
3
2
• Na Se + 1/2 O +CO Na CO + x Se (2.4)
3
2
2
2
X
2
Accordingly, a water and sodium catalysed reaction pathway of the absorber was sug -
gested:
• 2 Cu(In Ga )Se + 3/2 O 2 ´Cu(I)´ + (1-x) In O + x Ga O + 4 Se (2.5)
3
2
2
3
1-X
X
2
2
Since Braunger et al. detected a partial oxidation of selenium in their degraded sam-
ples, they assumed another reaction pathway: Se + O SeO [49]. According to the
2
2
authors, the presence of water leads to the hydrolysis of the oxides, i.e. the formation
of In(OH) , Ga(OH) , H SeO and NaOH. In conclusion, Braunger et al. found sodium to
2
3
3
3
promote an enhanced removal of selenium from the absorber layer via the formation
45
