Page 221 - A Comprehensive Guide to Solar Energy Systems

224 A ComPRehenSIVe GuIDe To SolAR eneRGy SySTemS

as a p-type dopant has been most successful contacting technique and as a result the most
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widespread, reaching hole densities of 10 cm . however, due to the fast-diffusing nature
of copper in CdTe this has also been widely demonstrated to lead to long-term stability
issues. A heat treatment is often required for copper to form the desired Cu 2 Te complexes
but this can aid migration of copper atoms from the back-contact toward the main junc-
tion along grain boundaries, resulting in shunting pathways that reduce long-term per-
formance. upon reaching the main junction, copper can dope the CdS layer, increasing
its resistivity, and is known to contribute defect states that aid recombination and lower
performance [56]. This is not solely a problem at the point of contact fabrication as nu-
merous studies have shown Cu-contacted devices to be less stable even under standard
operating conditions. Additional layers placed between the CdTe and metal contact have
also been used to reduce the contact resistance by choosing an appropriate material that
forms an ohmic contact to each adjacent layer. ZnTe has demonstrated the effectiveness
of this method [57] and is now used in commercial modules due to its similar lattice pa-
rameter and favorable band positions. When doped with copper it forms a pseudo-ohmic
contact; however, doping densities must be kept low to avoid excessive copper migration
but the overall stability of the contact is improved. other interface layers that have been
studied that could achieve higher doping densities while remaining stable include Sb 2 Te 3
[58], FeS 2 [59,60], moo 3 [61], niP [62], and As 2 Te 3 [63], which have shown an ability to form
ohmic contacts to varying degrees. The current state of the art when it comes to contact-
ing is now a copper doped layer of either ZnTe or Te coupled to an aluminum contact. This
apparently gives sufficient ohmic contact coupled to stability suitable enough for module
production where a 20+ year lifespan is essential.

10.2.6 General CdTe Solar Cell Production Notes

There are numerous subtleties to CdTe solar cell fabrication that, while useful to know, are
never reported in journal articles as they are not particularly novel or exciting. They are
however essential to producing working solar cells, this section is intended to give a brief
selection of useful tips for fabrication intended to help CdTe solar cell novices.
- It is preferable to use a commercial TCo-coated glass, in particular FTo as this is
stable to all but the most extreme of deposition processes. ITo may be applicable for
some processes but for others it will break down.
- Deposit CdS at a thickness of >200 nm. This will be thick enough, irrespective of
deposition technique to ensure good coverage. Some optical losses will result but the
thickness can be reduced in subsequent depositions.
- The thickness of the CdTe required will depend on the deposition method used.
For techniques such as sputtering, buried junctions (i.e., depletion region located
away from the CdS interface) may occur for thicknesses much greater than1.5 µm so
layers will need to be ∼1 µm. For higher temperature deposition techniques due to
the increased grain size thicker films may be required to ensure good coverage of the
substrate and thus minimize shunting.