Organic solar power efficiency gets a boost
Few would argue with the attractiveness of solar as an alternative energy source, but the cost of conventional photovoltaics has long been a stumbling block on the path to making it a viable option. This is changing rapidly. Grid parity, as the target for equaling coal burning production costs is called, has recently been claimed by solar manufacturers and research dedicated to improving solar systems continues on many fronts. Photovoltaics using organic molecules is one of them. This technology promises cells that are cheap, easy to make and flexible, and this flexibility makes them suitable for a diverse range of applications like powering your mobile phone, or lining your backpack or window shades. The problem is that currently they only last a few thousand hours and are inefficient, converting less than 6 percent of light into electricity. Work by researchers at the National Institute of Standards and Technology (NIST) could help change this.
Rigid silicon cells are around 15% efficient, and much higher have been reached in multi-junction systems - a clear advantage over current organic technology. But it's a balance between between efficiency and production costs, so if these new type of cells - which use organic molecules to capture sunlight and convert it into electricity - reach 10% they will become competitive according to NIST's David Germack.
"The industry believes that if these cells can exceed 10 percent efficiency and 10,000 hours of life, technology adoption will really accelerate," says Germack. "But to improve them, there is critical need to identify what's happening in the material, and at this point, we're only at the beginning."
The breakthrough is a new way to control the formation of the most common class of organic photovoltaics. The advantage of these materials is that they use a type of 'ink' as their base layer which can be printed or coated on flexible substrates, but getting it to harden in an optimal way is a problem. This ink is made up of a polymer that absorbs electrons and ball-shaped carbon molecules called fullerenes that collect electrons. Ideally the polymer network reach the bottom of the film and the fullerene channels should reach the top, but this cannot be reliably controlled when the ink hardens.
The team has learned a way to control this process. By applying X-ray absorption measurements to the film interfaces they found fullerenes were repulsed and the polymer attracted - the right result for getting the electrons to flow in the right direction. The method could also improve the photovoltaic's lifetime.
"We've identified some key parameters needed to optimize what happens at both edges of the film, which means the industry will have a strategy to optimize the cell's overall performance," Germack says. "Right now, we're building on what we've learned about the edges to identify what happens throughout the film. This knowledge is really important to help industry figure out how organic cells perform and age so that their life spans will be extended."