Environmental Engineering Reference
In-Depth Information
The i-ZnO layer has a three-fold function in this case. Since this
is an insulating layer, this material fills all pinholes in the structure
andreducesshortingofdevices,increasingtheproductionyieldand
initiale
ciencyofthedevice.Thesecondfunctionistocontributeto
the compound insulating layer to form MIS-type electrical contact,
improving
V
oc
. The third function is the most interesting one. It
should be noted that at the interfaces of these layers, intermixing
and in- and out-diffusion take place during growth, processing, and
ageing. Therefore, their effective thicknesses are much less than the
initially deposited values. The layer (c) is, therefore, thin enough for
optically generated and accelerated electrons to tunnel through but
acts as a barrier layer for the back diffusion of photo-generated and
separated electrons in the front metal contact. Therefore, this layer
can be labelled as an electron back diffusion barrier (ebdb) layer.
The presence of this layer reduces the back diffusion of electrons
over the potential barrier to recombine with photo-generated and
separated holes present in the back metal contact. This prevention
willimprovetheopencircuitvoltageofthedeviceand,hence,
J
sc
and
other device parameters. A thicker ebdb layer will hamper electron
transport to the front contact and add to series resistance, causing
the deterioration of device parameters.
In a similar way to an ebdb layer, both (b) n-CdS and (d) n-
ZnO:Allayersareactingasholebackdiffusionbarrier(hbdb)layers.
Their presence in the device, therefore, minimises the hole transfer
to the front contact and the recombination of photo-generated and
separatedchargecarriersbythermionicemissionoverthepotential
barrier.
5.7.2
Type-II CIGS-Based Solar Cell
There is another type of device developed at NREL in the United
States and several other laboratories. The band diagram for this
second type is shown in Fig. 5.6. This type-II device has a Cu-rich
electrodeposited CIGS (
∼
2.2
μ
m) on the Mo back contact [1]. The
work reported by the author's group in 2004 and 2007 proves
without ambiguity that this material with Cu richness is p-type in
electrical conduction [24, 47, 48]. Note that there are at least two
papersreportingCu-richmaterialasn-type[49,50],butmorerecent
