Page 224 - A Comprehensive Guide to Solar Energy Systems

Chapter 10 • CdTe Solar Cells 227

inclusion of carefully selected buffer layers. This has largely been using the approaches for
superstrate cells with highly doped layers at the back surface such as Cu, Cu x Te, and Sb 2 Te 3
or layers that have a higher work function than CdTe to promote favorable band bending
at the interface such as moo 3 and Wo 3 [73]. To date the most successful back-contact has
utilized a combination of moo 3 as the high work function buffer layer (6.5 eV) and Te as
the back-contact buffer layer with 10% [70] being achieved for Cu free devices and over
13% with controlled Cu doping [67].
Despite the reduced performance there have been a few studies, which show that ad-
vantage of using the substrate configuration to analyze cell processing such as the role of
Cu in degrading CdTe PV and the influence Cl processing the cells at different stages of cell
fabrication. To date though the amount of research undertaken on substrate CdTe devices
is vastly inferior to that of their superstrate counterpart. however, with recent develop-
ments in efficiencies, the potential for in depth analysis of the key step related to process-
ing and the potential industrial advantages means that substrate CdTe may yet generate a
substantial amount of research interest in future years.

10.3.2 Open Circuit Voltage Limitations
The impressive uplift in CdTe solar cell efficiency of recent years (Fig. 10.1) has been predi-
cated on increased current, to the extent that the current champion cells now have short
circuit current values close to the theoretical limit. This has been achieved by maximizing
the optics of the cell; removing the CdS layer in favor of an oxide such as mg 1−x Zn x o [25]
to improve short wavelength collection, coupled to grading the CdTe bandgap via sele-
nium incorporation to form lower bandgap CdTe 1−x Se x phases and improved long wave-
length collection [26]. As the current is effectively maxed out any further improvement
will therefore need to come via improvements in the open circuit voltage, something that
has proved far more challenging to achieve. In the prior 20 years there has been less than
30 mV increase in the V oC of champion cells. Improving beyond the current limit of 876 mV
[3] (Fig. 10.9) to closer to the theoretical limit of >1100 mV seems a challenge as would
require an increase in both minority carrier lifetime and doping density of the material
[74], a considerable challenge. Recent work on single crystal CdTe absorber cells has
demonstrated what may be possible though while the use of single crystals is obviously
impractical for mass production owing to cost and deposition time considerations, they
do provide tremendous insight into what may be possible. Work from national Renew-
able energy laboratory demonstrated that by using arsenic doped single crystal absorbers
voltages in excess of 1 V were achievable [40]. These devices also removed the chloride and
copper steps, which are considered standard practice. Similarly work which utilized single
crystal absorbers but in a double heterojunction arrangement with cadmium magnesium
telluride also yielded voltages in excess of 1 V [75]. The path toward similar voltage levels
for polycrystalline equivalent cells would therefore appear to be centered around trying to
replicate similar materials properties. This may mean abandoning the established chlo-
ride treatments process in search of alternatives which can yield higher doping, focusing