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Degradation mechanisms of the molybdenum back contact
5.1 Introduction
While all other layers within Cu(In,Ga)Se (CIGS) photovoltaics (PV) are varied in
2
composition and deposition processes, sputtered molybdenum (Mo) is used in almost
all CIGS solar cell and module designs as back contact (see chapter 1.3). Mo films show
good conductivity, acceptable adhesion and are reasonably smooth. Furthermore,
sputtered Mo can withstand the temperatures required for CIGS production processes,
allows beneficial sodium diffusion and while it is inert to the metals in CIGS, it reacts
with selenium. This results in the beneficial formation of molybdenum diselenide
(MoSe ) layers, which function as an ohmic contact between molybdenum and CIGS
2
[1]. The composition of the Mo/CIGS interface has a large impact on the performance
of the solar cell. It has also been shown that the grain structure of the molybdenum
back contact influences the structure and thus on the efficiency of the CIGS layers
[2,3]. The main drawback of molybdenum is the relatively low reflectance compared
to other metals.
However, the characteristics of molybdenum are not stable with time. It was shown
by reference [4] that storage of molybdenum in an inert atmosphere led to a change
in cell parameters, but the impact of humidity is even larger.
It has been observed that damp heat exposure leads to degradation of molybdenum,
which has been observed in two locations of CIGS modules [5,6]:
· Scribe P3 – Corrosion of molybdenum in scribe P3 can become significant when
the electrical connection between neighboring cells is completely destroyed.
(chapter 2.5.1.3)
· Scribe P2: Corrosion can occur in this scribe, which is filled with zinc oxide
and molybdenum and allows the transport of current. The surface area of this
scribe is relatively small and reduction in conductivity might influence the
module greatly (chapter 2.5.1.2). Wennerberg et al. [6] reported an increase in
the contact resistance in P2 from 1.5x10 to 5.5x10 Ωcm , corresponding to a
2
-3
-2
fill factor decrease from 74% to 57%.
Molybdenum degradation occurs mainly at places in CIGS solar cells with strong
damage – these include scratches from zinc oxide particles due to scribing and
scratches from sand blasting during edge preparation. However, extensive knowledge
about the stability of molybdenum and the impact of the deposition conditions on
the degradation stability is still largely unknown, while this is required to obtain more
stable and cost-effective CIGS modules. For more a more extensive overview on the
literature about molybdenum degradation, the reader is referred to chapter 2.3.1. To our
127
5.1 Introduction
While all other layers within Cu(In,Ga)Se (CIGS) photovoltaics (PV) are varied in
2
composition and deposition processes, sputtered molybdenum (Mo) is used in almost
all CIGS solar cell and module designs as back contact (see chapter 1.3). Mo films show
good conductivity, acceptable adhesion and are reasonably smooth. Furthermore,
sputtered Mo can withstand the temperatures required for CIGS production processes,
allows beneficial sodium diffusion and while it is inert to the metals in CIGS, it reacts
with selenium. This results in the beneficial formation of molybdenum diselenide
(MoSe ) layers, which function as an ohmic contact between molybdenum and CIGS
2
[1]. The composition of the Mo/CIGS interface has a large impact on the performance
of the solar cell. It has also been shown that the grain structure of the molybdenum
back contact influences the structure and thus on the efficiency of the CIGS layers
[2,3]. The main drawback of molybdenum is the relatively low reflectance compared
to other metals.
However, the characteristics of molybdenum are not stable with time. It was shown
by reference [4] that storage of molybdenum in an inert atmosphere led to a change
in cell parameters, but the impact of humidity is even larger.
It has been observed that damp heat exposure leads to degradation of molybdenum,
which has been observed in two locations of CIGS modules [5,6]:
· Scribe P3 – Corrosion of molybdenum in scribe P3 can become significant when
the electrical connection between neighboring cells is completely destroyed.
(chapter 2.5.1.3)
· Scribe P2: Corrosion can occur in this scribe, which is filled with zinc oxide
and molybdenum and allows the transport of current. The surface area of this
scribe is relatively small and reduction in conductivity might influence the
module greatly (chapter 2.5.1.2). Wennerberg et al. [6] reported an increase in
the contact resistance in P2 from 1.5x10 to 5.5x10 Ωcm , corresponding to a
2
-3
-2
fill factor decrease from 74% to 57%.
Molybdenum degradation occurs mainly at places in CIGS solar cells with strong
damage – these include scratches from zinc oxide particles due to scribing and
scratches from sand blasting during edge preparation. However, extensive knowledge
about the stability of molybdenum and the impact of the deposition conditions on
the degradation stability is still largely unknown, while this is required to obtain more
stable and cost-effective CIGS modules. For more a more extensive overview on the
literature about molybdenum degradation, the reader is referred to chapter 2.3.1. To our
127
