Background of the CdTe Solar Cell and the New Device Concept - Advances in Thin-Film Solar Cells - page 86

4.6.2 Improvements to Back Contact Using MIS-Type
Structures
For an e cient CdTe-based solar cell device, the Schottky barrier
height should be either ∼ 0.96 eV or ∼ 1.18 eV for this system
and the latter value is the most beneficial. The formation of lower
barriers results in poor performance due to weak band bending
and, hence, a weak internal electric field. The selection of a metal
contact containing a small amount of p-type dopant such as Cu, Sb,
or As further supports the Fermi-level pinning close to the valence
bandproducingane cientsolarcell.Hence,theCu/Au-,Cu/Ni-,Sb-
or As- containing contacts are good candidates to produce highest
quality rectifying contacts to this device structure. However, the
in-diffusion of a p-type dopant into the n-type CdTe layer should
be avoided in order to eliminate the rapid degradation due to the
formationofaveryresistiveelectricalcontactwithaging.Whenthis
happens and the e ciency drops down considerably, it should be
possible to remove the entire back contact using chemical etching
and reproduce the working device again with new metal contacts if
there is no damage to the thin-film structure during etching. This
procedure should produce high-e ciency devices again, and the
preliminary work has indeed confirmed this observation [30].
Introduction of a p-type thin semiconducting layer with a
bandgap larger than that of CdTe (e.g., p-ZnTe) or Sb-containing
layers such as Sb 2 Te 3 , will protect the p-type top surface layer
from reactions with metal contact, improving both e ciency and
durability of the device. Romeo et al. in 1999 [39] have indeed
shownthepositiveeffectsofSb 2 Te 3 incorporationatthebackmetal
contact.Asuitableconductingpolymerlayerwithp-typeconduction
and a bandgap greater than that of CdTe will be an ideal candidate
forthispurposetoproduceinorganic/organichybriddevicesandto
increase both e ciency and lifetime of this solar cell.
Another way of improving both performance e ciency and
lifetime is by forming an MIS-type contact with an ultra-thin
insulating layer [40]. Inorganic compounds such as CaF 2 and SrF 2
or any other insulator will be suitable for this purpose. There is an
added value of this approach in the associated pinhole plugging of
the device structure, improving the yield of the device fabrication