The Dye is the Thing
Harvesting sunlight before turning it into electricity could become easier thanks to an exotic organic dye developed in the US.
Coated onto an ordinary sheet of glass, the dye traps light inside the glass allowing it to be channelled to photovoltaic cells placed along the edges of the sheet.
The technique, say its inventors, could turn up to 20% of incident light into electricity at a fraction of the cost of conventional photovoltaic cells.
One way to reduce the cost of photovoltaic power is to focus light from a large area onto a small cell. In that way, a small cell can harvest light from a larger area. But the collecting optics must track the Sun’s path across the sky, requiring expensive machinery and control systems.
The dye-covered glass works differently. The dye molecules absorb sunlight over a wide range of visible wavelengths and then emit light at a longer wavelength.
About 80% of the emitted light then becomes trapped within the glass by an effect called total internal reflection, which guides the light within the sheet in the same way it guides light through optical fibres.
Solar cells along the edges of the glass that are designed to work most efficiently at the longer wavelength then convert this trapped light into electricity, says MIT electrical engineer Marc Baldo.
Baldo says the idea was first developed in the 1970s, but failed to get off the ground because the dyes at that time were quickly broken down by sunlight. Another problem was that the dyes absorbed light at the same frequency at which they emitted it, and this prevented much of the light from reaching the reaching the solar cells at the edge.
To reduce absorption, Baldo’s team used dyes that absorb light only weakly at the frequencies at which it is emitted.
They were also able to increase the range of light absorbed by using two dyes in seperate layers. While the upper layer absorbs shorter wavelength light, the lower layer absorbs longer wavelengths.
The team says that together, the dyes can absorb light across the visible spectrum and emit it at the longer frequencies needed for optimal conversion.
Based on experiments, the team calculates that the power conversion of the prototype would be about 6.8% efficient, about the same as commerical cells made from amorphous silicon. But Baldo believes that with various improvements, the technique could eventually reach an efficiency of more than 20%.
The collectors might double as windows, the researchers say, or could be used in place of standard solar panels.
Matching dye emission to solar-cell absorption and stacking concentrators “are good ideas,” says Allen Barnett of the University of Delaware. But issues remain.
Although the new dyes are not as fragile as the ones used in the 1970s, they still have a lifetime of only about three months, Baldo told New Scientist. “That by itself is not good enough, but it’s on the way.”