Environmental Engineering Reference
In-Depth Information
only be identified separately for good devices with high e
ciencies.
If the device performance is poor due to various other reasons,
V
oc
values will merge together, making it hard to identify different
groups. It is also apparent that there are four closely spaced defect
levels, as observed by the PAS technique, available in the vicinity of
groups A' and B'. This may produce a continuous range of
V
oc
values
between groups A and B instead of producing discrete
V
oc
values.
The Fermi-level pinning effect tends to produce discrete barrier
heights and, hence, different groups of
V
oc
values. However, the
presenceofMISstructurebroadenseachgroupandmakesithardto
identifydiscretenature.Onlycarefulobservationsofhigh-e
ciency
solar cells will help in identifying this pattern in CIGS solarcells.
5.8 Discussion on Further Improvements of CIGS
Solar Cells
Based on this concept, it is now possible to propose ways of
improving the device e
ciency. There are several different ap-
proaches, and they are discussed under the followingheadings.
5.8.1
Optimisation of Growth, Doping, and Bandgap
Engineering
In both situations, the first part of the absorber material (CIGS)
shouldbegrownwithCurichness.ItiswellknownthatCu-richCIGS
have large grains and high electrical conduction. The large grains
provide a better substrate for subsequent material layers to grow
well, and high electrical conduction helps in the formation of a low-
resistance ohmic contact to Mo back contact. Gradual increase of
the bandgap can be obtained by adding In, Ga and S. These graded
bandgap structures are shown in both Figs. 5.5 and 5.6 for effective
charge carrier creation, separation, and collection. The addition of
In makes the CIGS material n-type, and the inclusion of Ga makes
the CIGS layers p-type. Therefore, by varying the In and Ga content,
the doping concentration can be optimised in principle, although
challenging. In order to produce a depletion region of
2-3
μ
m, to
coverthefulldepthofthedevice,thedopingconcentrationneedsto
∼
