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Breakthrough solar cell lighter, thinner than soap bubbles

Posted: 02 Mar 2016     Print Version  Bookmark and Share

Keywords:MIT  solar cell  portable electronic device  parylene  polymer 

A team of researchers from the Massachusetts Institute of Technology (MIT) has created what they say are the thinnest, lightest solar cells that could be placed on almost any surface such as a shirt, smartphone, or even on a sheet of paper or a helium balloon. Though it may take years to develop into a commercial product, the laboratory proof-of-concept shows a novel approach to making solar cells that could help power the next generation of portable electronic devices.

The process is described in a paper by MIT professor Vladimir Bulovic, research scientist Annie Wang, and doctoral student Joel Jean, in the journal Organic Electronics.

Bulovic, MIT's associate dean for innovation and the Fariborz Maseeh (1990) Professor of Emerging Technology, 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.

"The innovative step is the realisation that you can grow the substrate at the same time as you grow the device," Bulovic said.

Lightweight solar cell

Figure 1: 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. (Photo: Joel Jean and Anna Osherov)

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 PCBs 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.

One process, many materials

The team emphasises that these particular choices of materials were just examples, and that it is the in-line substrate manufacturing process that is the key innovation. Different materials could be used for the substrate and encapsulation layers, and different types of thin-film solar cell materials, including quantum dots or perovskites, could be substituted for the organic layers used in initial tests.

But already, the team has achieved the thinnest and lightest complete solar cells ever made, they claimed. 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 may be too thin to be practical—"If you breathe too hard, you might blow it away," said Jean—but parylene films of thicknesses of up to 80μm 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, explained Wang, who has spent many years working with parylene.

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