Solar Cells Based on Single PN Junctions - Nanophysics of Solar and Renewable Energy - page 147

beam evaporation. This is a complicated and expensive process but led to the most
ef cient CIGS cells, 14.1%, ever prepared on a flexible polyimide substrate.
The traditional process for forming the PN junction, the use of a chemical bath to
deposit a layer of CdS, is objectionable because Cd is toxic. An alternative means of
making the CIGS PN junction without Cd has also been demonstrated [77] achieving
16% ef ciency. In this method, the CIGS absorber layer is contacted by a CIGS:Zn
junction-forming layer, Zn 0.9 Mg 0.1 O insulator layer, ITO (indium tin oxide)
conductive window layer, and current collecting grid.
In this work [77], the CIGS absorber was deposited on Mo-coated glass by physical
vapor deposition. The PN junction was formed by evaporating Zn onto the exposed
surface of the CIGS, held at 300 C. The zinc is believed to diffuse to a depth of about
50 nm in an annealing time about 5min. This forms an internal PN homojunction
within the CIGS. The Zn 0.9 Mg 0.1 O insulator layer was grown on the CIGS:Zn
surface by cosputtering the oxides from separate sources with adjustable rf power to
establish the desired ratio Zn/Mg

9. This ratio was found to adjust the conduction
band position in the new layer to match that in the CIGS:Zn layer, to allow
photoelectrons to pass out to the ITO electrode without scattering. The resulting
physical interface was studied by transmission electronmicroscopy, TEM, and found
to be epitaxial. The ITO transparent conductor about 100 nm thick was prepared by
sputtering, followed by metal grid electrodes applied by evaporation.
High-ef

¼

ciency CIGS cells are compared in Figure 6.14. (The same group has
recently reported a record ef ciency for this type of cell as 19.9%.) These cells use a
coevaporation method for the CIGS layer. The traditional wet process is used to form
the PN junction. This is described as growing 50 - 60 nmCdS films on the CIGS layer

Figure 6.13 Current voltage curve [78] of

CIGS cell grown using a dry process and
avoiding use of cadmium, on Mo-coated glass
substrate. Efficiency of 16.2% was achieved in
cells of this type after applying a MgF 2

antireflection coating. Layers, bottom to top:
glass, Mo back contact, CIGS absorber layer,
CIGS:Zn junction-forming layer, Zn 0.9 Mg 0.1 O
insulator layer, and ITO (indium tin oxide)
conductive window layer.

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