Flexible solar cells as light as soap bubbles developed

MIT scientists have developed the thinnest, lightest flexible solar cells ever produced that could be placed on almost any material or surface, including clothes, smartphones or even a soap bubble.

The laboratory proof-of-concept shows a new approach to making solar cells that could help power the next generation of portable electronic devices, researchers 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, said Vladimir Bulovic, professor from Massachusetts Institute of Technology (MIT) in US.

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.

Researchers 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 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.

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, 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.

The researchers lift the entire solar cell 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 micrometres thick - yet they convert sunlight into electricity just as efficiently as their glass-based counterparts.

The study was published in the journal Organic Electronics.