Imagine solar cells so thin, flexible and lightweight they could be placed on almost any material or surface – including your hat, shirt, smartphone, or even on a sheet of paper or a helium balloon.
Researchers at MIT have recently demonstrated just such a technology: the thinnest, lightest solar cells ever produced. Though it might take years to develop into a commercial product, the proof-of-concept shows a new approach to making solar cells that could help power the next generation of portable electronic devices.
MIT Professor Vladimir Bulović said the key to the new approach is to make the solar cell, the substrate that supports it and a protective overcoating to shield it from the environment all in one process. The substrate is made in place and never needs to be handled, cleaned or removed from the vacuum during fabrication, thus minimising exposure to dust or other contaminants that could degrade the cell’s performance.
In this initial proof-of-concept experiment, the team used a common flexible polymer called parylene as both the substrate and the overcoating, and an organic material called DBP as the primary light-absorbing layer. Parylene is a commercially available plastic coating used widely to protect implanted biomedical devices and printed circuit boards from environmental damage.
The entire process takes place in a vacuum chamber at room temperature and without the use of any solvents, unlike conventional solar-cell manufacturing, which requires high temperatures and harsh chemicals. In this case, both the substrate and the solar cell are ‘grown’ using established vapour deposition techniques.
“The innovative step is the realisation that you can grow the substrate at the same time as you grow the device,” Bulović said.
To demonstrate just how thin and lightweight the cells are, the researchers draped a working cell on top of a soap bubble – without popping the bubble. The researchers acknowledge that this cell might be too thin to be practical, but parylene films of thicknesses of up to 80 microns can be deposited easily using commercial equipment without losing the other benefits of in-line substrate formation.
A flexible parylene film, similar to kitchen cling-wrap but only one-tenth as thick, is first deposited on a sturdier carrier material – in this case, glass. Figuring out how to cleanly separate the thin material from the glass was a key challenge. The researchers lift the entire parylene/solar cell/parylene stack off the carrier after the fabrication process is complete, using a frame made of flexible film.
The final ultra-thin, flexible solar cells, including substrate and overcoating, are just one-fiftieth of the thickness of a human hair and one-thousandth of the thickness of equivalent cells on glass substrates (about two micrometers thick), yet they convert sunlight into electricity just as efficiently as glass-based counterparts. Bulović says that like most new inventions, it all sounds very simple once it’s been done. But actually developing the techniques to make the process work required years of effort.
While they used a glass carrier for their solar cells, it could be something else. The substrate and solar cells could be deposited directly on fabric or paper, for example.
While the solar cells in this demonstration device are not especially efficient, because of its low weight its power-to-weight ratio is among the highest ever achieved. That’s important for applications where weight is important, such as on spacecraft or on high-altitude helium balloons used for research. Whereas a typical silicon-based solar module, whose weight is dominated by a glass cover, might produce about 15 W of power per kilogram of weight, the new cells have already demonstrated an output of 6 W per gram – about 300 times higher.
“It could be so light that you don’t even know it’s there, on your shirt or on your notebook,” Bulović says.
“These cells could simply be an add-on to existing structures.”
It is early, laboratory-scale work, and developing it into a manufacturable product will take time, the team said. Yet while commercial success in the short term might be uncertain, this work could open up new applications for solar power in the long term.
The next question is, “How many miracles does it take to make it scalable? We think it’s a lot of hard work ahead, but likely no miracles needed,” Bulović said.
