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of processing — see Fig. 4.7(b). The observation of very similar
behaviour in Schottky barrier formation at metal contacts to bulk
crystal CdTe and thin-film CdTe layers is a unique property of
CdTe [8].
4.4 New Concept for CdS/CdTe Solar Cell
This new device concept follows the work on metal contacts to
II-VI semiconductors, as reported in a review article in 1998 [7]
and as summarised in section 4.3. As a result of a large body of
information, it has been shown that a Schottky barrier forms at
the metal/n-CdTe interface, as shown in Fig. 4.6. This work was
carried out on various n-type bulk CdTe materials, and the Schottky
barrier formation is found to be governed by Fermi-level pinning
at one of the five possible discrete levels. These experimentally
identified defect levels are situated in the bandgap at 0.40
±
0.04,
0.65
0.02 eV below
the conduction band minimum. Depending on the growth history
of the material, fabrication process, and the metal contact used, the
Fermi-level pinning will take place at one of the above five levels.
The high density of these local defect states can be found in the top
surface layer with a thickness of a few 100 A, and some of these
defects coincide with the native defects found in the bulk material.
The thickness of this modified surface layer depends on the surface
treatmentandetchingpriortometallisation.Althoughthisworkwas
carriedoutinthepastonbulkCdTecrystals,thesubsequentworkon
electrodepositedCdTethinlayershasproducedanidenticalpicture,
asdescribedinsection4.3.ItisanastonishingpropertyofCdTethat
the same picture emerges from most of the published work in the
literature,irrespectiveofthegrowthmethodusedfortheproduction
of CdTe layers. The above behaviour was observed for solar cells
based on electrodeposited CdTe layers, showing e
ciencies above
10% for lab-scale devices. Therefore, the following descriptions
are relevant only for best devices fabricated out of the CdS/CdTe
material system.
When a glass/CG/CdS/CdTe/metal structure is fabricated using
CBD-CdS and electrodeposited CdTe (ED-CdTe) the structure and
±
0.02, 0.73
±
0.02, 0.96
±
0.04, and 1.18
±
