New tiny solar cells may power personal devices

Scientists have created high-performance, micro-scale solar cells that outshine comparable devices and could power personal devices such as wearable medical sensors, smartwatches and autofocusing contact lenses.

Large, rooftop photovoltaic arrays generate electricity from charges moving vertically.

The new, small cells, developed by researchers at the University of Wisconsin-Madison in the US capture current from charges moving side-to-side, or laterally.

They generate significantly more energy than other sideways solar systems.

New-generation lateral solar cells promise to be the next big thing for compact devices because arranging electrodes horizontally allows engineers to sidestep a traditional solar cell fabrication process - the task of perfectly aligning multiple layers of the cell's material atop one another.

"From a fabrication point of view, it is always going to be easier to make side-by-side structures," said Hongrui Jiang, professor at UW-Madison.

"Top-down structures need to be made in multiple steps and then aligned, which is very challenging at small scales," said Jiang.

Top-down photovoltaic cells are made up of two electrodes surrounding a semiconducting material like slices of bread around the meat in a sandwich.

When light hits the top slice, charge travels through the filling to the bottom layer and creates electric current.

In the top-down arrangement, one layer needs to do two jobs: It must let in light and transmit charge.

Therefore, the material for one electrode in a typical solar cell must be not only highly transparent, but also electrically conductive. Very few substances perform both tasks well.

Instead of building its solar cell sandwich one layer at a time, researchers created a densely packed, side-by-side array of miniature electrodes on top of transparent glass.

The resulting structure separates light-harvesting and charge-conducting functions between the two components.

Existing top-of the-line lateral new-generation solar cells convert merely 1.8% of incoming light into useful electricity. Jiang's group nearly tripled that measure, achieving up to 5% efficiency.

"In other structures, a lot of volume goes wasted because there are no electrodes or the electrodes are mismatched," said Jiang.

"The technology we developed allows us to make very compact lateral structures that take advantage of the full volume," he said.

Researchers are working to make their solar cells even smaller and more efficient by exploring materials that further optimise transparency and conductivity.

Ultimately they plan to develop a small-scale, flexible solar cell that could provide power to an electrically tunable contact lens.

The research was published in the journal Advanced Materials Technologies.