Copper Indium Diselenide. From virtual obscurity as a semiconductor material, CIS solar cells
have seen remarkable progress in efficiencies (see Figure 3) with 19.3% efficiency achieved
recently in CIGS (with gallium added), nearly rivaling the best polycrystalline silicon laboratory
devices. Commercial-size modules with >13% efficiency have been fabricated, and early
commercial products are 9–11% efficient. The layer sequence for the device structure is
substrate/Mo/CIGS/CdS/ZnO. High-efficiency devices (18.6%) have also been fabricated by
replacing the CdS with ZnS (“cadmium-free” devices).
The many elements in CIGS solar cells can form a great variety of compounds during film
growth and cell processing, making the CIGS system very complicated. On the other hand, it is
also very tolerant of defects and impurities because the chemistry, as well as the structure, can
adjust in many possible ways. The most striking feature of CIGS is the tolerance of the electrical
properties to deposition approaches (and hence manufacturing processes).
The substrate may be either glass or a flexible material (e.g., stainless steel or polyimide) in a
roll-to-roll arrangement. Flexible, lightweight CIGS products are being sold for consumer and
some military applications. The other process involves sputtering of the metals with prescribed
conditions, followed by a selenization (and occasionally sulfurization) step at high temperature
(~500oC, ~1 hr) in a H 2 Se (and H 2 S) atmosphere. Several megawatts of PV products are being
fabricated by this process.
Materials Supply for Present PV Systems. The issue of available future supplies of various
elements used in present PV cells is summarized in Table 2.
Polycrystalline Thin-film Multijunctions. The successes to date with CdTe and CIGS recently
generated efforts to develop possible routes, combinations of materials, and device structures
toward demonstrating a multijunction polycrystalline thin-film solar cell with an efficiency
25% (and, ultimately, module efficiencies >20%). The materials selected for the initial studies
are based on CIGS and CdTe and related alloys, but other materials are also being investigated
(Symko-Davies 2004).
Thin Crystalline Silicon. An emerging thin-film technology area is thin-film crystalline silicon
deposited on low-cost substrates. This possibility could combine the inherent advantages of
silicon (abundance and device stability) with that of thin films (low materials use and cell
interconnection during film deposition). The efforts in this area fall into two general categories.
The first is to develop microcrystalline silicon bottom cells for dual- or triple-junction a-Si:H
devices. The second area involves thin crystalline silicon films (thickness typically from <10 μm
to a few tens of microns) deposited on low-cost and preferably insulating substrates to allow the
integral interconnection of solar cells. Major issues to be resolved include developing methods of
“light trapping” to compensate for the reduced thickness of the silicon film and selecting a
substrate that is not only low cost but also compatible with film deposition and processing.
Approaches range from “lift-off” techniques using reusable, single-crystal substrates to epitaxial
growth on low-cost metallurgical-grade silicon wafers to recrystallizing amorphous or small-
grained films (either by melting or in the solid phase).