Power/Performance Bits: May 19

3D microbatteries for large-scale on-chip integration

By combining 3D holographic lithography and 2D photolithography, researchers from the University of Illinois at Urbana-Champaign created a high-performance 3D microbattery suitable for large-scale on-chip integration with microelectronic devices.

According to Paul Braun, professor of materials science and engineering at Illinois, “Micro-scale devices typically utilize power supplied off-chip because of difficulties in miniaturizing energy storage technologies. A miniaturized high-energy and high-power on-chip battery would be highly desirable for applications including autonomous microscale actuators, distributed wireless sensors and transmitters, monitors, and portable and implantable medical devices.”

Enabled by a 3D holographic patterning technique—where multiple optical beams interfere inside the photoresist creating a desirable 3D structure—the battery possesses well-defined, periodically structured porous electrodes, that facilitates the fast transports of electrons and ions inside the battery, offering supercapacitor-like power.

The holographically patterned microbattery. (Source: University of Illinois)

“Although accurate control on the interfering optical beams is required to construct 3D holographic lithography, recent advances have significantly simplified the required optics, enabling creation of structures via a single incident beam and standard photoresist processing. This makes it highly scalable and compatible with microfabrication,” said materials science and engineering professor John Rogers, who worked with Braun and his team to develop the technology.

“Micro-engineered battery architectures, combined with high energy material such as tin, offer exciting new battery features including high energy capacity and good cycle lives, which provide the ability to power practical devices,” stated William King, professor of mechanical science and engineering and co-author of the work.

Two black silicon breakthroughs

Rice University scientists have found a way to simplify the manufacture of solar cells by using the top electrode as the catalyst that turns plain silicon into valuable black silicon.

Black silicon is silicon with a highly textured surface of nanoscale spikes or pores that are smaller than the wavelength of light. The texture allows the efficient collection of light from any angle, at any time of day. The team has been fine-tuning the creation of black silicon for some time, but an advance in the manufacturing technique could push it closer to commercialization.

An electron microscope image from earlier research shows the nanoscale spikes that make up the surface of black silicon used in solar cells. (Source: Barron Group/Rice University)

Rice chemist Andrew Barron noted the new work has two major attractions. “One, removing steps from the process is always good. Two, this is the first time in which metallization is a catalyst for a reaction that occurs several millimeters away.”

Barron said the metal layer used as a top electrode is usually applied last in solar cell manufacturing. The new method, called contact-assisted chemical etching, applies the set of thin gold lines that serve as the electrode earlier in the process, which also eliminates the need to remove used catalyst particles.

The researchers discovered that etching in a chemical bath takes place a set distance from the lines. That distance appears to be connected to the silicon’s semiconducting properties.

An extremely thin layer of gold atop titanium, which bonds well with both gold and silicon, should be an effective electrode that also serves for catalysis, according to Barron. “The trick is to etch the valleys deep enough to eliminate the reflection of sunlight while not going so deep that you cause a short circuit in the cell.”

The electrode’s ability to act as a catalyst suggests other electronic manufacturing processes may benefit from a bit of shuffling. “Metal contacts are normally put down last,” Barron said. “It begs the question for a lot of processes of whether to put the contact down earlier and use it to do the chemistry for the rest of the process.”

In another advance for the material, researchers from Finland’s Aalto University and the Polytechnic University of Catalonia obtained a record-breaking efficiency of 22.1% on nanostructured silicon solar cells. The result, certified by Fraunhofer ISE CalLab as an almost 4% absolute increase to their previous record, was achieved by applying a thin passivating film on the nanostructures using atomic layer deposition, and by integrating all metal contacts on the back side of the cell.

The surface recombination has long been the bottleneck of black silicon solar cells and has so far limited the cell efficiencies to only modest values. The new cells consist of a thick back-contacted structure that is known to be highly sensitive to the front surface recombination. The certified external quantum efficiency of 96% at 300nm wavelength demonstrated that the increased surface recombination problem no longer exists and for the first time the black silicon is not limiting the final energy conversion efficiency.

The surface area of the best cells in the study was already 9 cm2. This is a good starting point for upscaling the results to full wafers and all the way to the industrial scale. (Source: Aalto University)

“The energy conversion efficiency is not the only parameter that we should look at,” says Professor Hele Savin from Aalto University, who coordinated the study. Due to the ability of black cells to capture solar radiation from low angles, they generate more electricity already over the duration of one day as compared to the traditional cells.

“This is an advantage particularly in the north, where the sun shines from a low angle for a large part of the year. We have demonstrated that in winter Helsinki, black cells generate considerably more electricity than traditional cells even though both cells have identical efficiency values,” she adds.

In the near future, the goal of the team is to apply the technology to other cell structures, particularly thin and multi-crystalline cells.