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10
CdTe Solar Cells
Tom Baines, Thomas P. Shalvey, Jonathan D. Major
STEPHENSON INSTITUTE FOR RENEWABLE ENERGY, THE UNIVERSITY OF LIVERPOOL,
LIVERPOOL, UNITED KINGDOM
jon.major@liverpool.ac.uk
10.1 Introduction
Cadmium telluride (CdTe) solar cells have quietly established themselves as a mass market
PV technology. Despite the market remaining dominated by silicon, CdTe now accounts
for around a 7% market share [1] and is the first of the second generation thin film tech-
nologies to effectively make the leap to truly mass deployment. Blessed with a direct 1.5 eV
3
bandgap, good optical absorption ∼1 × 10 cm [2], and a simple binary phase chemistry,
−1
CdTe has been shown to be an eminently scalable technology. Device efficiencies on the
lab scale have now exceed 22% [3] and modules have reached 18.6%, in excess of multi-
crystalline silicon modules (Fig. 10.1). It is also now purported to be the lowest cost per
watt technology, have the shortest energy payback time and be the least carbon intensive
in production. CdTe solar cells clearly have a lot going for them but there remains a num-
ber of key technological challenges that, if overcome, could push the conversion efficien-
cies closer to the theoretical maximum of >30%. This chapter will discuss the current state
of play for this technology, reviewing each of the key cell1s in turn before briefly looking at
areas that are a focus for future development.
10.2 The CdTe Solar Cell: History, Layers, and Processes
Although early research focused on CdTe homojunction cells, that is n-CdTe/p-CdTe, in
the mold of silicon equivalents, this structure was quickly abandoned owing to the high
rates of surface recombination and strong unwanted optical absorption in the n-CdTe
layer. What is now thought of as the “standard” CdTe “superstrate” structure n-CdS/p-
CdTe, Fig. 10.2A, first emerged in the 1970s from the work of Bonnet and Rabenhorst [4].
As with many of the PV technologies, it has progressed in the intervening decades via a
series of empirically arrived at process improvements such as the chloride treatment,
improved back-contacting, and improved window layers. By 2001 device efficiencies
A Comprehensive Guide to Solar Energy Systems. http://dx.doi.org/10.1016/B978-0-12-811479-7.00010-5 215
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