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CdTe THIN-FILM SOLAR CELLS 643
CdTe
1.0
CSS PVD Spray
CdTe 0.95 S 0.05
1
Spray
0.8
Intensity
[AU]
0.6
0.4
0.2
Quantum efficiency
0.1
0.01
CSS
PVD
0.0
76.0 76.2 76.4 76.6
2-q
[deg]
76.8 77.0 77.2
825 850 875 900
Wavelength
[nm]
Figure 14.19
Correlation of XRD (511)/(333) reflection with long-wavelength quantum efficiency
copper-containing material. Copper will react with Te to form a p + -layer that can then
be contacted with a metal or with graphite. The subtelluride Cu 2 Te has been directly
measured at the back surface using GIXRD methods [151]. Also, Cu acts as a relatively
shallow donor in CdTe and can be diffused into CdTe from a doped contact material such
as graphite paste [34] or ZnTe:Cu [205].
There are a variety of surface treatments that have been used, prior to formation of
the copper layer, to reduce the back barrier. Table 14.3 gives a summary of these surface
treatments and the corresponding materials that have typically been used for the back
contact. Although fabrication laboratories tend to utilize a single surface treatment and
contact material, there is little evidence that the preferred contact process is dependent on
the deposition technique.
The high bulk-diffusion coefficient for Cu in CdTe, 3 × 10 −12 cm 2 /s at 300 K [156],
coupled with its multiple valence states and weak Cu–Te bond give rise to potential stability
issues related to its use as discussed below. Alternatives to the use of copper in the back
Table 14.3 Back-contact formation methods (NP = nitric + phosphoric acid mixture, BDH = sequential
reaction in bromine, acidic dichromate, and hydrazine)
CdTe deposition
method
Surface
treatment
Primary
contact
Thermal
treatment
Additional
contact
Reference
PVD Te + H 2 Cu 200 ◦ C/Ar C [152]
ED BDH Cu None Ni or Au [153]
Spray Etch C + dopant None None [154]
Screen None C + Cu dopant 400 ◦ C/N 2 None [155]
VTD BDH Cu 200 ◦ C/Ar C [151]
CSS NP Etch C + HgTe + Cu 200 ◦ C/He Ag paste [84]
Sputter Br Etch ZnTe:N In situ Metal [156]
MOCVD Br Etch ZnTe:Cu In situ Metal [157]