Tiny high-performance solar cells go sideways to generate power

A team of researchers has come up with high-performance, micro-scale solar cells that outshine comparable devices by making sideways swipes.

University of Wisconsin-Madison engineers' miniature solar panels could power myriad personal devices - wearable medical sensors, smartwatches, even autofocusing contact lenses.

Large, rooftop photovoltaic arrays generate electricity from charges moving vertically. The new, small cells capture current from charges moving side-to-side, or laterally. And 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 arduous 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 corresponding author Hongrui Jiang. "Top-down structures need to be made in multiple steps and then aligned, which is very challenging at small scales."

Lateral solar cells also offer engineers greater flexibility in materials selection.

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. And very few substances perform both tasks well.

Instead of building its solar cell sandwich one layer at a time, Jiang's group 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 percent of incoming light into useful electricity. Jiang's group nearly tripled that measure, achieving up to 5.2 percent efficiency.

Jiang and colleagues are working to make their solar cells even smaller and more efficient by exploring materials that further optimize 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 study appears in the journal Advanced Materials Technologies.