Printing solar panels with ‘electronic mayonnaise’
12/01/2006
A new class of complex copper indium gallium selenide (CIGS) thin-film materials, discovered and developed by HelioVolt, self-assembles into two interpenetrating phases-as if it were an ‘electronic mayonnaise’-with constant electrical properties despite slight changes in composition. Those changes alter only the percentage of each phase present in the final thin film, while the charge transport properties remain essentially constant within each phase. Initial results suggest that CIGS can be printed across large areas without vacuum chambers, to enable formation of low-cost photovoltaic (PV) structures on architectural glass.
Building on more than two years of collaboration with the US National Renewable Energy Laboratory (NREL), HelioVolt is continuing a cooperative R&D agreement at NREL’s facilities to develop non-vacuum nanomaterial deposition processes optimized for HelioVolt’s proprietary manufacturing technology.
One of the most basic concepts of materials engineering is that manufacturing complex structures requires the control of complex manufacturing processes. Atmospheric deposition processes offer a combination of low costs, process simplicity, and reduced manufacturing times-but most complex thin-film materials require the additional control of a vacuum system to create proper nanostructures. CIGS thin films seem to violate this manufacturing principle, as they can self-assemble into complex structures without requiring the control and expense of vacuum processing.
HelioVolt’s CIGS crystal structure, self-assembling percolation network, and electrostatic printing process can create high-quality films over meters of area in atmospheric conditions. CIGS PV material can be printed directly onto traditional construction materials including architectural glass, steel, roofing, and polymers, in >20% of the time required for conventional processes, according to HelioVolt. B.J. Stanbery, company president and CEO, explains that the currently favored technique for atmospheric printing is an electrostatic press. Thickness control may be forgiving since ~80% of the optical absorption and conversion occurs in the top 0.25µm of material.
“Our collaborative efforts with NREL over the years have formed the groundwork for a viable new solar paradigm-large scale production of building materials that are durable, versatile, visually appealing, and capable of economically harvesting energy from the sun,” said Stanbery.
How CIGS can function in this way was a mystery until a complex defect structure was discovered that combines two copper vacancies with an indium anti-site defect in nearest neighbor sites in the tetrahedral lattice to form a dipole (see figure), although there is no net charge. “After these neutral-defect structures had been discovered, I realized that if it were a two phase material it would explain the electronic properties,” Stanbery told SST.
The second phase retains the same crystallographic orientation as the first phase, differing only in defect structures, so the difference doesn’t show up in x-ray diffraction or in any other standard metrology tests. Only the invariance of the thin film’s electrical properties with compositional changes, unlike nearly all other complex materials, led Stanbery to consider the existence of two very similar phases in the material.
Under the proper conditions, CIGS spontaneously arranges itself at the nanoscale in a manner similar to mayonnaise, in which oil and water form a micron-scale colloidal emulsion due to the influence of egg yolk. Mayonnaise emulsions form due to competition between short-range and long-range forces, and CIGS percolation networks are likewise thought to form due to competition between such forces. In CIGS, the unique second phase, composed of nanoscale defectivities, forms due to competition between long-range strain and short-range dipole interactions. This phase grows to interpenetrate the first phase in a manner consistent with “percolation theory”-neither diffusion nor conduction, but a sort of random walk along random paths. -E.K.