Environmental Engineering Reference
In-Depth Information
work on electrodeposition of CIGS material [24, 25]. For example,
GaSe is always a p-type semiconductor [26]; therefore, Ga must
be producing acceptor-like native defects in the material instead
of acting as a simple substitutional n-type dopant. However, the
amount of Ga added to the material is helpful in reducing n-doping
duetoInadditionandincreasingthebandgapofthecompoundalloy.
BycontrollingtheatomicconcentrationofInandGa,suitabledoping
levels fordevices and the energy bandgap can be achieved.
5.3 Summary of Accumulated Knowledge on CIGS-Based
Solar Cells
Mostresearch groups followtheconventionalsubstrate-typedevice
structuretodevelopCIGS-basedsolarcells.Thissectionsummarises
the device structure, current understanding of the physics behind
thisdevice,reporteddeviceperformance,andsomenotablefeatures
leading to a new understanding of the solid-state principles behind
these devices.
5.3.1
Conventional Device Structure
The CIGS solar cell device, currently under intense research has
a glass/Mo/CIGS/n-CdS/i-ZnO/n-ZnO:Al/metal-grid structure. A
schematicdiagramofthecompletestructureforthisdeviceisshown
in Fig. 5.1. The device is gradually built up starting from the back
metalcontactofMo,sputteredontotheglasssubstrate.Thegrowth
process of CIGS varies, and the most common techniques used are
co-evaporation of elements or sputtering to deposit individual Cu,
In, and Ga layers and then selenise at temperatures above 550
◦
C
using H
2
Se gas [19]. Some groups introduce S also on to the surface
by the sulphidation process at 600
◦
C using H
2
S gas [19]. After the
completionofthegrowthoftheCIGSlayer,athinlayerofn-CdS(
∼
80
nm) is incorporated usually using the chemical bath deposition
(CBD) technique. Intrinsic ZnO (
∼
70 nm) and an n-type Al-doped
ZnO (
∼
100 nm) layers are then deposited using the sputtering
technique. A grid type Al/Ni front metal contact is finally deposited
by either thevacuum evaporation orsputtering technique.
