Power/Performance Bits: June 28

Mimicking roses for solar

Scientists from the Karlsruhe Institute of Technology (KIT) and the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) reproduced the epidermal cells of rose petals and integrated the transparent replicas into an organic solar cell, with an efficiency gain of 12%.

The epidermis of rose petals consists of a disorganized arrangement of densely packed microstructures, with additional ribs formed by randomly positioned nanostructures, providing the rose with stronger color contrasts and thus increased chance of pollination.

In order to exactly replicate the structure of these epidermal cells over a larger area, the scientists transferred it to a mold made of polydimethylsiloxane, a silicon-based polymer, and pressed the resulting negative structure into optical glue which was finally left to cure under UV light.

The epidermis of a rose petal is replicated in a transparent layer which is then integrated into the front of a solar cell. (Source: Guillaume Gomard/KIT)

They then integrated the transparent replica of the rose petal epidermis into an organic solar cell, resulting in power conversion efficiency gains of 12% for vertically incident light. At very shallow incidence angles, the efficiency gain was even higher. In addition, as examinations using a confocal laser microscope showed, every single replicated epidermal cell works as a microlense, extending the optical path within the solar cell, enhancing the light-matter-interaction, and increasing the probability that the photons will be absorbed.

“Our method is applicable to both other plant species and other PV technologies,” said Guillaume Gomard of KIT. “Since the surfaces of plants have multifunctional properties, it might be possible in the future to apply multiple of these properties in a single step.” The results of this research lead to another basic question: What is the role of disorganization in complex photonic structures? Further studies are now examining this issue with the perspective that the next generation of solar cells might benefit from their results.

Grid-scale zinc battery

Researchers from Stanford and Toyota Central R&D Labs proposed a new battery design for grid-scale energy storage with electrodes made of inexpensive zinc and nickel.

A variety of zinc-metal batteries are available commercially, but few are rechargeable, because of tiny fibers called dendrites that form on the zinc electrode during charging. Theses dendrites can grow until they finally reach the nickel electrode, causing the battery to short circuit and fail.

The research team solved the dendrite problem by simply redesigning the battery. Instead of having the zinc and nickel electrodes face one another, as in a conventional battery, the researchers separated them with a plastic insulator and wrapped a carbon insulator around the edges of the zinc electrode.

A conventional zinc (Zn) battery (left) short circuits when dendrites growing from the zinc anode make contact with the metal cathode. Stanford scientists redesigned the battery (right) using plastic and carbon insulators to prevent zinc dendrites from reaching the cathode. (Source: Shougo Higashi)

“With our design, zinc ions are reduced and deposited on the exposed back surface of the zinc electrode during charging,” said Shougo Higashi, a scientist from Toyota Central R&D Labs. “Therefore, even if zinc dendrites form, they will grow away from the nickel electrode and will not short the battery.”

To demonstrate stability, the researchers successfully charged and discharged the battery more than 800 times without shorting.

Bending photovoltaics

Scientists at the Gwangju Institute of Science and Technology in South Korea have made ultra-thin photovoltaics about 1 micrometer thick and flexible enough to wrap around the average pencil. The bendy solar cells could power wearable electronics like fitness trackers and smart glasses.

The solar cells were constructed from gallium arsenide. The researchers stamped the cells directly onto a flexible substrate without using an adhesive that would add to the material’s thickness. The cells were then cold welded to the electrode on the substrate by applying pressure at 170 degrees Celcius and melting a top layer of photoresist material that acted as a temporary adhesive. The photoresist was later peeled away, leaving the direct metal to metal bond.

The metal bottom layer also served as a reflector to direct stray photons back to the solar cells. The researchers tested the efficiency of the device at converting sunlight to electricity and found that it was comparable to similar thicker photovoltaics. They performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters.

Ultra-thin solar cells flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide, as shown here. (Source: Juho Kim, et al/APL)

The team also performed numerical analysis of the cells, finding that they experience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick.

A few other groups have reported solar cells with thicknesses of around 1 micrometer, but have produced the cells in different ways, for example by removing the whole substrate by etching. By transfer printing instead of etching, the new method developed by Lee and his colleagues may be used to make very flexible photovoltaics with a smaller amount of materials.