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Chapter 10 • CdTe Solar Cells 221

10.2.4 The Chloride Process

What is widely referred to as the chloride treatment step is typically essential to the pro-
duction of functional CdTe solar cells. Indeed, barring single crystal cell exceptions [40]
(which may to an extend be considered a separate technology entirely), no cell reported has
exceeded 10% without some form of chloride treatment. It was first developed by Basol [41]
using an electrochemical process, and the use of chlorine appears to have emerged from
its prior use to photosensitize CdS films. otherwise it is hard to understand the logical leap
to applying Cl which, being a group VII element, one would initially assume would be an
n-type dopant, to achieve p-type character while alternative treatments based of mgCl 2 [42]
or ChF 2 Cl [43] have been identified, cadmium chloride (CdCl 2 ) has long been established,
and as such remains the research and industrial standard process. There a number of meth-
odological variations in the manner of application but the principle remains the same; the
free CdTe “back surface” is coated with a thin layer of CdCl 2 , typically deposited via either
thermal evaporation [44] or from a solution via spray or drop casting [45]. The stack struc-
ture is then annealed somewhere in the 380–450°C temperature range, usually in an air or
oxygen containing ambient [46] (although some oxygen free processes have been reported
as successful [30]). Following this annealing the cell is typically rinsed in water to remove
any excess CdCl 2 remaining on the surface prior to whatever contacting procedure is being
applied while the practical application of the CdCl 2 treatment was quickly established, the
understanding of what the CdCl 2 treatment was actually doing to the device has taken longer
to develop and has changed in recent times. This is primarily due to the multifaceted nature
of its influences being hard to disentangle. on a structural level it has been widely demon-
strated to mediate recrystallization in the CdTe and CdS layers [47], the level of recrystalli-
zation being partly dependent upon the starting grain structure of the films. For CdTe films
deposited by low temperature methods, such as thermal evaporation or sputtering, which
have a small as-deposited grain structure, CdCl 2 treatment induces near-complete recrys-
tallization of the film to a significantly larger final grain structure [48] (Fig. 10.6). For higher
temperature methods such as CSS, the as-deposited grain structure is large and thus more
thermodynamically stable meaning recrystallization is only seen at the near CdS interface
region where the grain structure is smaller and more defective [49]. Another standard result
has been to observe an increase in carrier concentration following chloride treatment [42].
This has been widely attributed to the formation of the chlorine A-center V Cd -Cl i while the
chloride treatment does undoubtedly have an effect on the doping level seen in measured
devices, more recent work suggests its primary role may be to pacify grain boundaries. It has
been demonstrated via high-resolution electron microscopy that the incorporated chlorine
is predominately located at the grain boundaries, with their being little incorporated in the
grain interiors [50]. This in turn has been shown to have a pronounced effect on the grain
boundaries electrical behavior when analyzed by techniques such as eBIC [50]. hence it
may be considered that the process is in effect a passivation treatment rather than a dop-
ing step. There have also been suggestions that the chloride treatment may have inherent
limits and that alternative processes need to be developed to overcome the current voltage
limited performance of the technology (Section 10.3.2).