Page 169 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells

Degradation mechanisms of the molybdenum back contact

six oxygen atoms as its basic element. The MoO is formed by bilayers consisting of
3
two interleaved sheets of MoO piled up along the b direction and held together by
6
weak Van der Waals forces. Ions can be inserted into and extracted from this van der
Waals gap by redox reactions with the host MoO matrix:
3

+
-
MoO + xA + xe → A MoO 3 (5.2)
x
3
Na + Na + Na + Na +
O O O O O O O O O O O O O O O O O O O O
VI
Mo Mo Mo Mo Mo Mo Mo VI Mo Mo Mo Mo Mo Mo Mo V
VI
VI
VI
V
V
VI
VI
VI
VI
V
VI
O O O O O O O O O O O O O O O O O O O O O O
+
Van der Waals gap + Na + e - Na Na + Na Na Na +
+
+
+
O O O O O O O O O O O O O O O O O O O O
VI
V
VI
V
V
VI
VI
VI
VI
V
Mo Mo Mo Mo Mo Mo Mo VI Mo Mo Mo Mo Mo Mo Mo V
VI
VI
O O O O O O O O O O O - Na + e - O O O O O O O O O O O
+
Van der Waals gap Na + Na + Na +
O O O O O O O O O O O O O O O O O O O O
VI
VI
V
VI
V
VI
Mo Mo Mo Mo Mo Mo Mo VI Mo Mo Mo Mo Mo Mo Mo VI
VI
VI
VI
V
VI
VI
O O O O O O O O O O O O O O O O O O O O O O
Figure 5.32
+
Simplified schematic representation of intercalation of Na into a MoO 3 material. The process involves the reduction from
Mo to Mo [36]. All depicted oxygen atoms have a charge of -2, which is not shown.
5+
6+
With A=H, Li, K, Na, …
This reversible inclusion of a molecule or ion into a compound is called ‘intercalation’
[35].
+
This intercalation process of Na into MoO can lead to the formation of Na Mo O ,
0.9
3
17
6
which is often referred as ´purple bronze´, while ´red´ and ´blue´ bronzes with different
alkali concentrations also exist [37]. It should be noticed that intercalation of K and
+
+
Li in the MoO layered structure also leads to the formation of bronzes. We have
3
+
only found Na in the analyses, so Na is certainly the dominant ion here, but it is
+
possible that a small quantity of K is also present. The physical properties of Na(K)
MoO purple bronzes as well as the blue bronzes AMoO with A= K, Rb, Tl and Cs
3
x
0.3
have been extensively studied because of their charge density wave phenomenon
at low temperatures arising from their low-dimensional metallic character [38,39].
As this specific transition is related to the electron properties at room temperature,
several studies of the electronic properties of these materials through photoemission
measurements of the band structure have been performed [40].
For several molybdenum bronzes, including KMo O [31], K Mo O [40] and
17
17
0.9
0.3
6
6
KMo O [41], a small peak near the Fermi level has been detected. This peak has been
17
6
167