Page 167 - Mirjam-Theelen-Degradation-of-CIGS-solar-cells

Degradation mechanisms of the molybdenum back contact

① Damp heat treatment
H 2 O/O 2 H 2 O/O 2
NaOH Na (aq) + OH (aq)
-
+
④ The newly exposed Mo can be oxidised again to
yield multilayer stacks of (Na)MoO x Molybdenum

Soda lime glass (incl. sodium)
Grain boundary
((Na)MoO x ) allowing
migration of sodium
Water film
② A: Needle formation ② B: Mo(VI) formation
MoSe 2 Mo(s) + O 2 (g)  MoO 3 (s)
+ 2- 2- -
Na (aq) + O /CO 3 /NO x
Mo (2/15 mtorr) (Na)MoO x and MoSe 2
(Na)MoO x
Mo (25 mtorr) Molybdenum Molybdenum
(Na)MoO x Mo with (Na)MoO x
Soda lime glass (incl. sodium) Soda lime glass
Mo with (Na)MoO x
(Na)MoO x and MoSe 2
Needle NaX
2-
NaX (X=O , CO 3 , NO x ) - Removal from damp heat
2-
③
(Na)MoO x and MoSe 2
Molybdenum

Mo with (Na)MoO x
Soda lime glass

Cracking of the Mo
Figure 5.31
Simplified schematic representation of the degradation of a thin bilayer molybdenum film obtained by the lift-off process on
soda lime glass

The top layer was divided into multiple regions: the darker ‘spots’, of which the surface

was rich in MoO and ‘foreign’ species, like hydrogen, carbon, cyanide, sulfur, chloride
x
and oxygen, and poor in MoSe, while the lighter regions contained more MoSe.
2
2
Deeper in the film, foreign species as well as MoSe and MoO are still present, but

x
2
in decreasing quantities. Positive species, like sodium, can also be present, but are
not detected with SIMS in a negative mode. In order to schematically represent these
reactions, Figure 5.30 shows the molybdenum films before and after degradation.
This representation is based on SIMS measurements and on references [8,16].

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