Scientists seek self-assembling photovoltaic molecules for high-tech solar cells

The researchers have previously pioneered breakthroughs in organic light-emitting diodes and holographic storage technologies. They are now applying their expertise to designing, synthesising and characterising molecules that will form highly ordered ‘liquid crystal’ coatings 100 nanometres in thickness – about one-thousandth the diameter of a human hair – that will efficiently transport electrical charge.

“What you’d really like is a solar panel array that would come on a flexible plastic substrate which would be extremely inexpensive and which you could roll out on your roof like wallpaper,” said Professor Neal R Armstrong, one of the researchers. “It would be efficient enough at energy conversion to economically generate power.”

According to the US Department of Energy statistics, electricity customers in the United States currently pay six to seven cents per kilowatt hour for conventionally generated electricity, and 20 to 30 cents per kilowatt hour for solar generated electricity. “That sounds discouraging, but the cost of solar generated power was about 90 cents per kilowatt hour 10 to 15 years ago,” said Armstrong, and the cost is expected to drop further with the introduction of efficient new inorganic thin films such as cadmium telluride, he said. However, the heavy metals in these substances raise environmental concerns, a problem that would not arise with organic solar cells, said Armstrong.

One of the challenges faced by the researchers is the poor electrical efficiency of organic films which, until recently, were only one to one-and-a-half percent efficient at converting solar power to electrical power. “The discouraging thing was that these materials could be tailored to absorb most of the solar spectrum, they were very cheap, and they were easy to spread as thin film on a transparent electrode,” said Armstrong. “But their electrical properties were very bad. Even if you could generate a lot of electrical charge inside a thin film you couldn’t transport it, and you couldn’t harvest it efficiently.” However, theoretically, there is now no reason why solar cells with 20% efficiency cannot be developed, he said. “But we feel a 10% conversion efficiency is a realistic goal, based on our own recent work and the work of several other groups in Europe and Japan.”

The researchers will also be working on making networks of nanoparticles of the semiconductor titanium dioxide more porous in order to allow photoactive dye to cover a larger area, so increasing a solar panel’s light absorption potential. “Further, dyes currently used do not absorb the full spectrum of the sun, only the visible part, and only about 45% of the light is being harvested,” said Optical Sciences Associate Professor Bernard Kippelen, one of the researchers. “So we will also work on making dyes that absorb more of the infrared part of the spectrum.”