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Chapter 10 • CdTe Solar Cells 219

layers [21] (Fig. 10.4). A number of different materials have been used such as a buffer, pri-
marily Zno [22], or Sno 2 , but also a number of other such as Zn 1−x Sn x o [23]. These buffer
layers have demonstrated a route to greater CdS thickness reduction without compromise
of performance [21]. how the buffer layer achieves this has never been comprehensively
established and it may in fact have more than one role. Clearly there is some involvement
in minimizing the deleterious effects of CdS pinholes and associated localized reduc-
tion in diode quality, but it additionally appears it may be able to aid the band alignment
between the TCo and CdS. An oxide/CdS bilayer has therefore become more standard as
the window layer structure for CdTe cells than a CdS layer in isolation.
Despite the improvements offered by the introduction of a buffer layer, recent devel-
opments have shown CdTe solar cells evolve away from CdS layers all together, toward
something with a higher bandgap, and thus greater degree of transparency. The first ini-
tial move in this direction was by the use of an oxidized CdS:o layer, which has a greatly
increased bandgap compared to CdS owing to quantum confinement effects in the nano-
meter scale grain structure [24]. Despite the success of this method it never resulted in
device improvement beyond 16.7%. The initial instinct to eliminate the CdS would be to
simply replace it with a high bandgap oxide, with the likes of Zno or Sno 2 seeming obvious
choices. unfortunately owing to the lattice mismatch problems such layers are inherently
unsuited and in isolation produce devices with low performance. Recent work has identi-
fied mg 1−x Zn x o (mZo) as being a suitable alternative [25] and the research field is quickly
moving toward adopting this as a widespread alternative to CdS, coupled to the use of a
CdSe interfacial layer to effect bandgap grading [26] while this may or may not establish
itself as new standard window layer, as the technology continues to develop, we will see
evolution away from the traditional CdS layer.

10.2.3 CdTe Absorber Layer
CdTe is a polycrystalline thin film which can be doped either n or p type. The deposited
layers tend to be weakly p-type, with doping being increased via key postgrowth processing
such as the chloride treatment (Section 10.2.4) or the copper back-contacting step (Sec-
tion 10.2.5). The quality of the absorber layer can be highly dependent on the deposition
technique used, with lower temperature deposition routes producing smaller grained and
typically lower performing material (although recrystallization following chloride treat-
ment will modify the grain structure significantly). numerous deposition routes have been
established, electrodeposition [27], sputtering [28], thermal evaporation [29], moCVD [30],
and CSS [31], all of which have been demonstrated of forming functional devices. Among
these techniques it is CSS, which has been most readily adopted at the industrial level and
indeed the majority of recent “champion” cells have used this technique. Irrespective of
the deposition technique used however the CdTe layer itself is between 1 and 8 µm thick.
Cells with layers thinner than 1 µm show reduced performance owing to incomplete opti-
cal absorption [32] or shunting due to pinholes [33], while layers thicker than this show
performance loss owing to additional resistivity of the overly thick absorber. one would not
expect to encounter losses at such a relatively low thickness such as 8 µm, when dealing