IF SEVERAL NEW technology developments prove out, one day utilities, commercial building owners and homeowners may be able to spray a solar paint onto a surface such as a building's roof or glass windows to convert the sun's rays into electricity.

Long envisioned as a way to cut the cost of solar cells, researchers have looked for materials that could be sprayed onto different surfaces to produce electricity. The general idea is to have a material that, once applied to a surface, would dry to form interconnected, microscopic solar cells.

Nanotechnology is at the core of today's efforts to develop what many refer to as solar paint. Some recent developments have advanced the technology and provide a glimpse into the potential benefits solar paint might one day deliver.

A National Science Foundation-funded effort at the University of California-Berkeley is working with nanocrystals based on copper, indium, gallium and selenide. Researchers at the university are making small particles of this material that can be dispersed in a solvent, creating ink or paint that generates electricity when exposed to the sun.

Many of the national labs are also working on solar paint. For example, the National Renewable Energy Laboratory is developing a silicon-based solar ink.

And researchers from the University of Toronto, the King Abdullah University of Science and Technology in Saudi Arabia, and Pennsylvania State University have created a solar cell based on what is called colloidal quantum dots. These dots are nanoscale semiconductors whose electronic characteristics can be varied by changing the size and shape of the individual crystals. Solar cells based on this work have been among the most efficient ever developed.

Solar cells based on solar paint will likely offer a trade-off in performance versus cost. Solar-paintbased cells are expected to have fairly low solar conversion efficiency rates, perhaps in the single-digit range at best. Currently, the highest touted efficiency rates for small experimental samples are in the 6 percent range.

Some in the industry believe that solar paint technology can be improved over time. Current thinking is that the cells would have to have efficiencies above 10 percent to be suitable for commercialization. That is still significantly lower than today's best photovoltaic cells, which deliver in the low to mid-20 percent efficiency.

However, the trade-off is that a solarpaint- based approach will offer cost savings in several areas, thus making them a viable alternative in some deployment situations. Many of the solar paint technologies being developed today would offer lower manufacturing costs. The materials could theoretically be applied under more common conditions such as at room temperature in an ordinary facility. This would yield a much lower cost than using clean rooms and silicon fabrication facilities, as are needed to produce the current generation of photovoltaic solar cells.

The expected flexibility afforded by spraying a paint- or ink-like substance on a surface offers other potential benefits. For example, the ability to apply the material to a variety of surfaces will enable things like roll-up solar panels, which could potentially offer much larger surface areas for collection. Additionally, if the solar paint approach allows application to common building surfaces, that would open up the technology to much larger arrays like a panel covering the entire side or roof of a building.

Unfortunately, solar paint also has several potential obstacles to success.

Nanotechnology-based solar cell approaches have been under development for nearly a decade with little movement from the lab to production environments.

Solar paints might also be rejected for use based on the potential health risks of nanoparticles.