Environmental Engineering Reference
In-Depth Information
collectionandbasedonthecombinationofap-nhomo-junction
and an MIS-type front contact. In both cases, the I-layer is a
compound layer made out of n-ODC, n-CdS, i-ZnO, and n-ZnO:Al
layers. The n-ODC layer is naturally supporting both devices for
Fermi-level pinning at the E
4
level and creating required band
bending.
(g) Because of the four main Fermi-level pinning positions, the
φ
b
can vary from batch to batch or device to device depending on
the material quality and processing steps. Therefore, the open
circuit voltages of good devices can show discrete values as
observed and reported in the literature. For poor devices,
V
oc
values will be merged together, making it impossible to identify
this pattern.
(h) With this improved understanding, the device can be further
developedbyaddingebdbandhbdblayersintothedevicestruc-
ture, optimising the doping concentration and engineering the
bandgap of material layers used. In addition, the identification
of defect levels responsible for Fermi-level pinning and removal
or passivation of undesirable defect levels will lead to high
performance CIGS solar cells.
5.10 Summary
This review chapter summarises the knowledge accumulated in
the literature on CIGS material and solar cells based on CIGS.
After reviewing the present use of solid-state physics principles to
describe thin-film solar cells based on CIGS, an improved concept is
proposed with the aid of advances in the field on electrical contacts
to CIGS material. It has been shown that the Fermi-level pinning
takes place at one of the few experimentally observed defect levels.
The main levels observed are at 0.77, 0.84, 0.93, and 1.03 eV with
±
0.02 eV error and are situated above the top of the valence band.
Asaresult,discretevaluesofopencircuitvoltagesareobservedand
thesituationisverysimilartothatofCdS/CdTesolarcellsdescribed
in chapter 4. Based on this new concept, different ways for further
development of CIGS solar cells are proposed and this device has
high potential to achieve much higher conversion e
ciencies.
