Page 222 - A Comprehensive Guide to Solar Energy Systems

Chapter 10 • CdTe Solar Cells 225

- For initial optimization of the chloride treatment, ensure an excess of the chloride
(>100 nm) being used is present on the back surface. The treatment temperature will
almost always fall in the 380–450°C range, and the time can simply be optimized by
tracking cell performance as a function of treatment.
- Following chloride treatment wash the surface thoroughly in deionized water to
remove any excess. Typically there will be oxide or chloride phases remaining at the
surface [64], which this washing will not remove. etching can aid removal of these
phases with either nitric–phosphoric acid [65] or bromine methanol being standard.
- numerous contact process of varying complexity can be employed and these will
have a significant influence on the performance. however, a good basic contact is a
few nanometer of Cu followed by a gold back-contact being applied. This will give
consistently good results for most cells.
- To make a good front-contact to the solar cell the CdTe layer can be easily removed
by manual scribing with a scalpel blade or similar. The CdS layer also needs to be
removed, and this can be done by swabbing with hCl which will remove the CdS but
not damage the TCo (unless it is Zno based e.g., Zno:Al).
- Contact size is an overlooked issue but must be considered carefully. owing to
the sheet resistance of TCo larger contacts will significantly reduce the FF of the
measured cell. The temptation to use particularly small contacts should be avoided
however as this can lead to significant error in estimation of the current density via
even minor variations in the contact size. This size variation can occur readily if the
back surface is conductive following doping. Defining the contact area by manual
scribing or measuring using an optical aperture is advisable. A suitable contact size is
2
considered to be in the 0.20–1.0 cm range.
- Additional FF losses may also be seen if there is a large physical distance between the
front and back-contacts, again due to the nonnegligible resistance of the TCo. This
can be compensated for by the use of bus-bars across the cell which can be as simple
as strips of conductive paint which will minimize parasitic shunting.

10.3 Looking Forward—Voltage, Doping, and Substrate Cells

Despite CdTe becoming established as an industrial technology, there remain a number of
key areas still requiring development. The aim is not only to continue the upward trajec-
tory of CdTe solar cells in an attempt to more closely match the 30% efficiency potential,
but also to investigate alternative device structures such as the substrate cell, as this may
have additional benefits for production. This section will discuss the development of the
alternative substrate geometry solar cells and the current voltage limitation problems.

10.3.1 Substrate Cells
one of the more radical approaches to CdTe solar cell development has been the idea of
inverting the cell structure to use the “substrate” geometry (Fig. 10.8). This mimics the