Environmental Engineering Reference
In-Depth Information
develop these solar cells. A literature search on this subject clearly
shows different views expressed from various research groups
active in this field. The following sections present three different
models proposed to date to describe the PVaction ofthis device.
5.4.1
p-CIGS/n-CdS Hetero-Junction
The most widely assumed concept is the hetero-junction formed
between p-type CIGS material and the n-type CdS layer. Most of the
research and analysis carried out to date are based on this hetero-
junctionconcept.Theseexplanationsarerelevantiftheenergyband
diagram shown in Fig. 5.2 is accurate.
5.4.2
p-CIGS/n-CIGS Homo-Junction
In the above device structure, the thickness of n-CdS layer varies
in a wide range, from 10 nm [27] to 85 nm [28]. In all cases,
the device shows PV activity, and the best performance is usually
given by devices with extremely thin layers (10-85 nm) of CdS.
It is well established that the surface and interface chemistry of
electronic devices observed using soft-XPS work show that these
interfaces are highly intermixed due to chemical reactions and in-
and out-diffusion of semiconductor elements across the boundary.
Therefore, about 80 nm thick layer of CdS is not su
cient to form a
CdS crystal lattice after consumption within interfaces and, hence,
to form a well-defined energy bandgap. For these reasons, some
groups[29]havesuggestedthatthen-CdSlayercanonlybeasurface
passivation layer and that the rectifying interface may be a buried
homo-junctionwithinthethickCIGSlayer(ofabout3
μ
m).However,
this proposed idea has also not been proved by experimental or
theoretical means. This property cannot be seen from Fig. 5.2 since
a much thicker CIGS layeris not shown in this diagram.
5.4.3
p-CIGS/n-ODC Hetero-Junction
As discussed in section 5.2.3, the formation of a thin layer of Cu-
deficient or In-rich surface layer is due to a natural phenomenon
relatedtoCudiffusionawayfromthefreeCIGSsurface.Thisordered
