Basic Research Needs for Solar Energy Utilization - Office of ...

Non-ingot-based Technologies. Silicon ribbon or sheet technologies avoid the costs and material
losses associated with slicing ingots. The current commercial approaches in the field are the
edge-defined, film-fed growth (EFG) of silicon ribbons and the string ribbon process.
In the current EFG process, an octagonal cylinder, with 12.5-cm-wide flat faces, is pulled
directly from the melt by using a graphite die to define its shape. The octagons are cut into
wafers with an automated laser system. The ribbon thickness is typically 250–300 μm, and the
growth rate is typically 2–3 cm/min.
The string ribbon process has evolved rapidly from conception to commercial production. In this
simple, very stable process, two high-temperature string materials are brought up through small
holes in the bottom of a shallow graphite crucible containing the silicon melt. The strings
stabilize the ribbon edges and result in continuous growth (with melt replenishment using
granular silicon feedstock). The strings are nonconductive and are left in the ribbon through
cell/module processing. The ribbon thicknesses and growth rates are similar to those in EFG; the
ribbon width is currently 8 cm. The material quality of EFG and string ribbons is similar to the
multicrystalline wafers, and 14%-efficient cells are routinely fabricated.
Other Approaches under Development. Full-scale production of silicon modules based on
micron-sized silicon spheres was recently announced. In this process, sub-millimeter-size silicon
spheres are bonded between two thin aluminum sheets, processed into solar cells, and packaged
into flexible, lightweight modules. Another approach uses a micromachining technique to form
deep narrow grooves perpendicular to the surface of a 1- to 2-mm-thick single-crystal silicon
wafer. This results in large numbers of thin (50 μm), long (100 mm), narrow (nearly the original
wafer thickness) silicon strips that are processed into solar cells just prior to separation from the
wafer. In a final technique, a carbon foil is pulled through a silicon melt, resulting in the growth
of two thin silicon layers on either side of the foil. After the edges are scribed and the sheet is cut
into wafers, the carbon foil is burned off resulting in two silicon wafers (150 μm thick) for
processing into solar cells.
Flat-plate Thin Films. Thin-film technologies have the potential for substantial cost advantages
over wafer-based crystalline silicon because of factors such as lower material use (due to direct
band gaps), fewer processing steps, and simpler manufacturing technology for large-area
modules. Many of the processes are high throughput and continuous (e.g., roll-to-roll); they
usually do not involve high temperatures, and, in some cases, do not require high-vacuum
deposition equipment. The process of module fabrication, involving the interconnection of
individual solar cells, is usually carried out as part of the film-deposition processes. The major
systems are amorphous silicon, cadmium telluride, and copper indium diselenide (CIS) and
related alloys. Figure 3 illustrates the significant progress in laboratory cell efficiencies in these
technologies. Future directions include multijunction thin films aimed at significantly higher
conversion efficiencies, better transparent conducting oxide electrodes, and thin polycrystalline
silicon films.
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