Researchers in the US have developed a new kind of organic solar cell that converts a small but significant fraction of the sunlight that falls onto it into electricity, while still allowing most of the visible part of that light to pass through. Thanks to this transparency, the team says that the cell could be mounted onto windows in buildings or cars in order to tap a currently under-exploited source of energy.

Most of today's commercial solar cells are made from the semiconductor silicon. When photons with sufficient energy strike the silicon they create pairs of electrons and holes. An electric field created by adding impurities to the silicon splits the electron–hole pairs apart, which results in an electric current. However, the costs involved in processing the silicon mean that photovoltaic cells remain very expensive compared to other forms of electricity generation.

One alternative is plastic – or organic – semiconductors, which are much cheaper to work with and are also flexible and lightweight. However, in plastic solar cells the liberated electrons and holes bind strongly to one another, forming particle-like entities known as excitons. These excitons only break apart when they reach a "heterojunction", which is the interface formed by making cells from two different organic materials. But because excitons tend to travel only very short distances before they self-annihilate, cells must be very thin for significant numbers of excitons to reach the heterojunction and generate a measurable current. This need for thinness makes the cells inefficient.

Exploiting excitons

Now, Richard Lunt and Vladimir Bulovic of Massachusetts Institute of Technology have turned the exciton problem on its head. They exploit the fact that the formation of excitons alters a material's absorption properties. So rather than absorbing wavelengths more or less equally across a broad spectrum, as is the case in silicon, their prototype cell instead displays distinct absorption peaks. By combining the organic molecules chloroaluminium phthalocyanine and carbon-60, their cell absorbs light at infrared and ultraviolet wavelengths but has limited absorption at visible wavelengths. In other words, it is able to extract energy from the non-visible parts of the spectrum while leaving most of the visible light free to propagate.

The fact that the new device does not absorb appreciably at visible wavelengths makes it less efficient than opaque organic cells. However, say Lunt and Bulovic, it is more efficient that other kinds of transparent cell that absorb across the spectrum. As they point out, these other cells must be made very thin if they are not to become opaque and as a result have efficiencies of less than 1% when at least partially transparent. In contrast, they achieved efficiencies of up to 1.3% when transmitting at least 65% of the incident visible light and up to 1.7% for transparencies greater than 55%.

While these efficiencies are very low compared to the 22% of the best commercial silicon cells, the MIT researchers claim that they should be able to up the efficiency of their cell to around 12% by increasing the length of the heterojunction interface. They will do this by blending the two organic materials, and also by stacking a series of cells together, each absorbing at a slightly different position within the infrared spectrum (ultraviolet absorption provides a very small fraction of the cell's output).

Rolling onto existing windows

They say that their cell could be coated directly onto the glass in new windows or onto a flexible substrate that is then rolled onto existing windows, pointing out that exploitation of existing window structures would lower installation costs compared with conventional solar cells. They estimate that they will need between five and ten years to commercialize their technology because uncertainties remain regarding the durability of organic cells.

Lunt acknowledges that while the new cell could allow private individuals and companies to exploit more of the sunlight that falls on their buildings, it will not avoid the need for other energy sources. "It will be one of the tools in the clean-energy tool box," he says.

Martin Green, of the University of New South Wales in Australia, believes that organic photovoltaic cells will be used in niche applications, but he does not think that they can compete with mainstream cell technologies even if they are cheaper to manufacture. He argues that the savings obtained in the manufacturing process will be nullified by the extra costs, common to all photovoltaic technologies, needed to "field-safe, professional systems with a long field life."

Green's position is backed up by a recent report from US technology analysts Lux Research, which concluded that the low efficiencies and short lifetimes of organic photovoltaic cells will make them uncompetitive with crystalline silicon and inorganic thin-film technologies over the next decade.

The work is described in App. Phys. Lett. 98 113305.